Lara Mahfouz

Basile Marchand

Vincent Guipont

Florent Coudon

Vincent Maurel
email: vincent.maurel@minesparis.psl.eu

Driving forces in thermal barrier coatings blistering

Keywords: TBC, EB-PVD, rumpling, growth strain, LASAT, LASDAM Highlights

Thermal barrier coating (TBC) systems are known to be extremely sensitive to the applied thermo-mechanical loading in terms of both microstructure evolution and of subsequent damage, leading to final failure by ceramic top-coat layer spallation. The analysis of the relationship between mechanical behavior and final failure is limited by the scatter in the experimental results. Based on a typical TBC system for columnar top-coat, this study focuses on blister evolution for pure thermal cycling. The blister is processed by LAser Shock Adhesion Test (LASAT) method. It is observed experimentally that prior to significant further interfacial debonding, the height of the blister increases with the number of applied cycles. To clarify this, a finite element analysis was developed to account for multilayer system behavior and growth strain associated to a thermal-grown oxide layer evolution. Finally, this study demonstrates that rough interfaces, elasto-viscoplastic behavior of the metallic bond-coat, and oxide growth strain are needed to model the experimentally observed evolution of the blister. Thus, the driving forces acting on bond-coat rumpling, are similar to driving forces for blister height increase and final failure of the TBC.

Introduction

The assessment of coating adhesion and its evolution in experimental conditions close to those in operation is a key challenge in order to screen the performance of new coating solutions or to estimate the lifetime before spalling of existing coatings. High pressure turbine blades are subjected to very high temperature and aggressive environment so that a thermal barrier coating (TBC) is required on the single crystal Ni-base superalloys. TBC are typically a multilayer system composed of (i) a bond-coat (BC) layer reservoir of Al to form (ii) a protective oxide layer called the thermally grown oxide (TGO) and (iii) the top-coat (TC) layer providing through its low thermal conductivity the thermal insulation. The TC is mostly made of partially stabilized zirconia or a gadolinum based layer to prevent CMAS infiltration [START_REF] Evans | Oxidation failure of tbc systems: An assessment of mechanisms[END_REF]. However, measurements of the adhesion of the TC in TBC systems are particularly difficult to address because of the extreme sensitivity of this adhesion to many parameters met in industrial applications. Among these parameters some are intrinsic [START_REF] Evans | The influence of oxides on the performance of advanced gas turbines[END_REF]: the aging of metallic material by phase transformations (BC and substrate) [START_REF] Swadźba | Microstructure degradation of eb-pvd tbcs on pd-pt-modified aluminide coatings under cyclic oxidation conditions[END_REF][START_REF] Sallot | Microstructure evolution of a platinum-modified nickel-aluminide coating during thermal and thermo-mechanical fatigue[END_REF], the evolution of the TGO (thickness, roughness, ...) and the nucleation of cracks at the TC/TGO interface [START_REF] Spitsberg | On the failure mechanisms of thermal barrier coatings with diffusion aluminide bond coatings[END_REF][START_REF] Evans | Oxidation failure of tbc systems: An assessment of mechanisms[END_REF]. The other parameters being extrinsic: the chemical composition of gases (lab air, combustion gases...), the external thermo-mechanical loading on the component, the sintering of the TC layer [START_REF] Ahrens | Sintering and creep processes in plasma-sprayed thermal barrier coatings[END_REF][START_REF] Zhu | Development and thermal fatigue testing of ceramic thermal barrier coatings[END_REF] and the extrinsic impact of CMAS infiltration [START_REF] Vidal-Setif | Microstructural characterization of the interaction between 8ypsz (eb-pvd) thermal barrier coatings and a synthetic cas[END_REF][START_REF] Naraparaju | Tailoring the eb-pvd columnar microstructure to mitigate the infiltration of cmas in 7ysz thermal barrier coatings[END_REF]. Among the evolution of microstructure of (Ni,Pt)Al metallic bond-coat, the so-called rumpling effect is assumed to govern to a large extent damage mechanisms in TBCs [START_REF] Evans | The influence of oxides on the performance of advanced gas turbines[END_REF]. This rumpling phenomenon consists in a large increase of the surface roughness of (Ni,Pt)Al, when used without top-coat, under cyclic air oxidation, due to the growth strain associated to TGO growth [START_REF] Tolpygo | On the rumpling mechanism in nickel-aluminide coatings. Part I : an experimental assesment[END_REF]. The phenomenon is inhibited when considering low partial pressure of oxygen, or accelerated when mechanical loading is applied in addition to thermal cycling for typical thermomechanical fatigue testing [START_REF] Balint | Anisotropic tgo rumpling in eb-pvd thermal barrier coatings under in-phase thermomechanical loading[END_REF][START_REF] Sallot | Microstructure evolution of a platinum-modified nickel-aluminide coating during thermal and thermo-mechanical fatigue[END_REF][START_REF] Maurel | Rumpling of nickel aluminide coatings: a reassessment of respective influence of thermal grown oxide and phase transformations[END_REF]. On the other hand, many authors assume that rumpling promotes interfacial decohesion of the top-coat to TGO interface, by associating local debonding and grooving to rumpling, whereas the rumpling is also inhibited by the presence of the ceramic layer top-coat [START_REF] Tolpygo | Surface rumpling of a (ni,pt)al bond coat induced by cyclic oxidation[END_REF][START_REF] Tolpygo | Oxidation-induced failure of EB-PVD thermal barrier coatings[END_REF][START_REF] Yanar | Failure characteristics during cyclic oxidation of yttria stabilized zirconia thermal barrier coatings deposited via electron beam physical vapor deposition on platinum aluminide and on nicocraly bond coats with processing modifications for improved performances[END_REF]. This point is still to some extent at the assumption level, where only growth strain without top-coat layer has been demonstrated to impact the surface roughness [START_REF] Karlsson | A numerical model for the cyclic instability of thermally grown oxides in thermal barrier systems[END_REF].

Recently, the interest in the LAser Shock Adhesion Test (LASAT) method applied to ceramic coatings increased due to a clear easiness in use, a high sensitivity to effective adhesion threshold and a limited scatter as compared to standard methods [START_REF] Berthe | State-of-the-art laser adhesion test (LASAT)[END_REF][START_REF] Guipont | Bond strength determination of hydroxyapatite coatings on ti-6al-4v substrates using the laser shock adhesion test (lasat)[END_REF][START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF]. Using this laser-assisted technique, a delaminated area of controlled size can be introduced as an artificial interface defect at the same location as the damage created naturally by thermal cycling, namely the TGO/TC interface [START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF]. It has already been shown that the damage rate in presence of this pre-defect is similar to the one observed without it [START_REF] Maurel | Thermal cycling damage monitoring of thermal barrier coating assisted with lasat (laser shock adhesion test)[END_REF]. Otherwise, such LASAT-induced blister evolves within room temperature mechanical testing [START_REF] Sapardanis | Influence of macroscopic shear loading on the growth of an interfacial crack initiated from a ceramic blister processed by laser shock[END_REF], thermal cycling [START_REF] Maurel | Thermal cycling damage monitoring of thermal barrier coating assisted with lasat (laser shock adhesion test)[END_REF], and in the presence of a thermal gradient using burner rig facility [START_REF] Maurel | Recent progress in local characterization of damage evolution in thermal barrier coating under thermal cycling[END_REF]. One of the more spectacular results of pre-defect introduced by LASAT in the as-received (AR) state is as follows: submitting the blister to thermal cycling, the blister height is observed to increase drastically (from 10 to 150 µm), without significant growth of the debonded area until a certain number of cycles [START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF]. Above this critical number of cycles, both height and debonded diameter of the blister increased with thermal cycling up to spallation occurrence. Thus, because air thermal cycling is known to promote rumpling, this blistering is assumed to be linked to rumpling as a driving force: the first assumption is that rumpling promotes damage at the crack tip, decreasing the local stiffness by increase of the process zone size [START_REF] Maurel | Coated single crystal superalloys: processing, characterization, and modeling of protective coatings[END_REF]. Another assumption could be made considering that the rumpling mechanism promotes blistering by modifying the stress state and finally residual stresses acting on the blister after cooling. These residual stresses play a key role in the torque applied on the blister, and then can modify its morphology, particularly its height [START_REF] Hutchinson | Growth and configurational stability of circular, buckling-driven film delaminations[END_REF]. On the other hand, the parallel could be achieved with the case of a pure mechanical loading on a blister: again, the blister height increases before interfacial crack growth [START_REF] Sapardanis | Influence of macroscopic shear loading on the growth of an interfacial crack initiated from a ceramic blister processed by laser shock[END_REF][START_REF] Maurel | Coated single crystal superalloys: processing, characterization, and modeling of protective coatings[END_REF]. This point is of major interest for TBC systems, it should be understood as the elaboration of increasing available energy with cycling to meet the interfacial toughness, this latter being known to decrease with cycling [START_REF] Thery | Spallation of two thermal barrier coating systems: experimental study of adhesion and energetic approach to lifetime during cyclic oxidation[END_REF][START_REF] Vaunois | Development of bending tests for near shear mode interfacial toughness measurement of eb-pvd thermal barrier coatings[END_REF][START_REF] Maurel | Thermal cycling damage monitoring of thermal barrier coating assisted with lasat (laser shock adhesion test)[END_REF].

The aim of this paper is to analyze the rumpling impact on blister driven delamination through experimental results and finite element analysis (FEA). The experimental analysis is managed according to the monitoring of blister evolution measured for different thermal cycling conditions. The FE model is based on a blister involving similar dimensions of the experimental one. Thus, different assumptions in the model are tested and compared to the actual blistering phenomenology under thermal cycling. From this dual experimental-numerical approach, driving forces involved in blister height increase are investigated.

Experimental methods

The chosen TBC system meets the standards for high pressure turbine blades, and is provided by Safran group: the substrate is made of Ni base single crystal superalloy of first generation, AM1 (see Table 1 for composition), the bond-coat is obtained by the APVS method by first electro-deposition of Pt and Al deposit by PVD to form a diffusion (Ni,Pt)Al coating. The TC layer is made of partially yttria stabilized zirconia processed by EB-PVD; see previous works for coating details [START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF]. The samples are designed as disks of 23 mm in diameter and 3 mm in thickness, the edge of each sample being rounded to limit edge effect.

LASAT was processed using a Saga Thales system, with a typical 5.2 ns pulse. Using SEM cross-section highlights debonded area corresponding to obvious blistering of the top-coat layer, e.g. for LASAT applied in the as-received condition and further submitted to 2000 S-FCT (details will be addressed in the sequel) Figure 1(a). To analyse in a non destructive manner the initial size of the debonded area, the substrate is put on a heating plate set to 60 • C and the top-surface imaging is obtained by infra-red thermography (IRT), Figure 1(b). A clear measurement of the debonded area corresponding to local minimum surface temperature is due to the air gap between the ceramic layer and the TGO, see Figure 1(a). The interested reader can refer to previous work for further experimental details [START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF][START_REF] Maurel | Recent progress in local characterization of damage evolution in thermal barrier coating under thermal cycling[END_REF]. Then, an optical profilometer is used to measure blister shape and height.

To define blister morphology, a perfect blister could be assessed by its height, δ, and the diameter of the debonded area, D, assuming a circular debonded area, Figure 1 For a given coating/substrate system, or in our case for a given TBC system (setting processing route and prior aging and/or thermo-mechanical loading), the debonded diameter, D, is known to be a function of the applied laser fluency during LASAT, for power beyond a given threshold, Figure 1(c). The blister height is function of the debonded diameter when freezing all other parameters (material and previous loading). It is a result of the residual stress in the system associated to the behavior of each layer involved in this process, Figure 1(d). LASAT was achieved either in the as-received (AR) state or after 100 thermal cycles (from 100 to 1100 • C with 50 minutes dwell at maximum temperature; long furnace thermal cycling, L-FCT), see Figure 2. Heating and cooling being achieved in approximately 5 and 4 minutes respectively.

The applied L-FCT before LASAT promotes an initial debonding localised at the TGO/TC interface after LASAT, see details in [START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF][START_REF] Maurel | Thermal cycling damage monitoring of thermal barrier coating assisted with lasat (laser shock adhesion test)[END_REF]. By this way, further damage initiated from debonded area by LASAT will be consistent with damage localisation at the TC/TGO interface, Figure 1(e), as observed for thermal cycling of similar systems [START_REF] Yanar | Failure characteristics during cyclic oxidation of yttria stabilized zirconia thermal barrier coatings deposited via electron beam physical vapor deposition on platinum aluminide and on nicocraly bond coats with processing modifications for improved performances[END_REF][START_REF] Courcier | Interfacial damage based life model for eb-pvd thermal barrier coating[END_REF]. Further thermal cycling was applied by short furnace thermal cycling (S-FCT), set to 100 to 1100 • C and a dwell time at maximum temperature of less than 5 minutes (rate of 3.33 • C.s -1 ), Figure 2. To control temperature in a robust manner for S-FCT, two slopes have been applied with 430 and 35 • C/min during heating. Hence, no over heating of the sample was observed. S-FCT cycle is chosen here because it is more damaging than L-FCT for the same equivalent time spent at maximum temperature, and is closer to in service condition associated to take-off, e.g. see [START_REF] Maurel | Thermal cycling damage monitoring of thermal barrier coating assisted with lasat (laser shock adhesion test)[END_REF].

To summarize the different conditions tested in this study, the names of the tested specimens are as follows:

• AR: LASAT was processed in the as-received state,

• 100 L-FCT: 100 L-FCT were applied, then LASAT was processed,

• 100 L-FCT + N S-FCT: 100 L-FCT were applied, then LASAT was processed, then N S-FCT were applied, with N=871 or 2015 cycles.

Experimental results

First, the cross-section of a specimen subjected to 2000 short furnace cycles (S-FCT) shows that rumpling takes place in the pre-defect area (where LASAT has induced a decohesion). There, a large roughness amplitude of more than 10 µm can be observed, Figure 1(a). In areas where the TC is remaining adherent to the TGO, roughness is clearly less pronounced, Figure 1(e).

Here, for sake of clarity we only present a global analysis of the blistering effect, by plotting the evolution of the height of the blister, δ, as a function of the debonded diameter, D. In the as-received (AR) state, independently of the applied Laser shock condition, one can observe a direct correlation between debonded diameter and blister height, see red triangular marker points in Figure 3(a). After 100 L-FCT, the blisters processed by LASAT exhibit a similar correlation between debonded diameter and blister height, see yellow square marker points in Figure 3(a). Only a slight offset by lowering the height to diameter relationship is observed as compared to LASAT processed in the AR state. The differences are related to the modification of localisation of the interfacial crack: in the AR state the crack processed by LASAT is cohesive and located in the TC layer near the interface (dense part of columnar microstructure), whereas after thermal cycling the crack is adhesive and right at the TC/TGO interface [START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF]. For the smallest debonded diameter, one measurement corresponds to no buckling whereas one point has a slight but clear buckling measurement of about 5 µm. This might be related to the metastable state of some delaminated areas that do not buckle.

For further cycling and considering short dwell time at maximum temperature, low number of cycles (here 871 S-FCT) are of limited impact on the height to diameter relationship, see diamonds marker points associated to S-FCT, to be compared to yellow square marker points in Figure 3(a). However, a slight increase in height is observed as compared to the trends observed at LASAT introduction. For higher number of thermal cycles (here 2015 S-FCT), the blistering has clearly increased as compared to the blister introduction by LASAT, see green square marker points in Figure 3(b). LASAT processed in AR corresponds to [START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF], other data are from the current study (b) influence of low and high number of cycles (note that only fit has been reported for blisters processed in the AR)

As a concluding remark: for a given debonded diameter, results show the growth of the blister height with respect to the aging by thermal cycling. It is worthy that this result is far beyond the uncertainty level induced by both experimental facilities used for blister measurements (for diameter and height of the blister), and scatter from shock to shock, that could be related to local modification of the properties (due to either processing or damage). This evolution should be related to a strong evolution of the mechanical state in the TBC system. However, state-of-the-art mechanical analyses of blister do not propose a clear overview of the driving forces leading to this blistering effect.

On this basis, a sensitivity analysis is carried out through a full-field finite element (FE) model of the TBC system. Explicit model of the different layers and of the blister geometry in the TC layer is achieved. The idea of the next sections will be to understand what could be the mechanical driving forces at the local scale that generate an increase of the blister height (for a given debonded diameter).

Numerical methodology

The TBC model is composed of different layers as follows:

• the TC, assuming thermoelastic isotropic behavior;

• the TGO, assuming thermoelastic isotropic behavior and oxide growth strain;

• the BC, assuming isotropic behavior for thermo-elastic or elasto-viscoplastic model;

• the substrate, assuming thermoelastic isotropy.

The model is made of these four layers, assuming a perfect circular blister so as to consider axisymmetric condition, for a debonded radius of 1.5 mm, an initial blister height at high temperature of 25 µm and a total radius of the model of 4 mm. At the interface, the impact of an initial roughness was also studied. Based on cross-section analyses obtained by SEM, a representative initial roughness is introduced by using a sinusoidal shape of the interface with a period of 50 µm and an amplitude of 10 µm. This ondulation being homogeneous all along the interface, Figure 4(a). For comparison purpose, the same model was meshed ignoring the interfacial roughness but describing the same blister size (in terms of debonded diameter and height in the initial state). In addition to symmetry axis, set to the left end of the model, uniform radial displacement is prescribed at the right end of the model. Last boundary condition applied relates to the blistered TC and subjacent TGO surface : to avoid nodes interpenetration during thermal cycling, a mortar based penalty algorithm has been used to model contact condition between debonded TC and TGO surface [START_REF] Akula | Mortex method for contact along real and embedded surfaces: coupling x-fem with the mortar method[END_REF]. The mortar method is straightforward here because the TGO surface and the inner surface of the TC layer are not conformal meshes. For the sake of simplicity, no friction is modelled within the contact. All FEA conditions are recalled in table 2. Oxidation kinetic is described through a parabolic growth written in an incremental form, so as to account to temperature evolution along thermal cycling [START_REF] Courcier | Interfacial damage based life model for eb-pvd thermal barrier coating[END_REF]:

dh dt = 1 2h α 0 exp(-Q/RT ) (1)
where h corresponds to the oxide thickness, α 0 is a material parameter used to assess oxide growth kinetic associated to a classical Arrhenius sensitivity to temperature T , Q is the thermal activation and R the gas constant.

Growth strain is related to the transformation of BC into TGO, accounting for Pilling-Bedworth ratio. The growth strain, is represented by an anisotropic eigenstrain tensor, ε ∼gr , with the maximum of growth strain normal to the oxide [START_REF] Frachon | Multiscale approach to predict the lifetime of eb-pvd thermal barrier coatings[END_REF][START_REF] Busso | Effects of breakaway oxidation on local stresses in thermal barrier coatings[END_REF][START_REF] Hille | Oxide growth and damage evolution in thermal barrier coatings[END_REF]:

[ε ∼gr ] =   0.003 0 0 0 0.27 0 0 0 0.003   (2)
This growth strain corresponds to a volume change from oxidized volume of metal V m to oxidized volume of oxide

V ox , V ox /V m ≃ 1 + ε 1 gr + ε 2 gr + ε 3
gr , where ε i gr corresponds to each of the diagonal components of the growth strain tensor [START_REF] Frachon | Multiscale approach to predict the lifetime of eb-pvd thermal barrier coatings[END_REF]. This results in a volume change of V ox /V m ≃ 1.28. For sake of simplicity, only internal oxidation is accounted for.

As previously developed in [START_REF] Frachon | Multiscale approach to predict the lifetime of eb-pvd thermal barrier coatings[END_REF], the oxidation consists of a progressive transformation of elements associated initially to BC to oxide, so as to account for both the transition in mechanical behavior and the contribution of growth strain. This progressive oxidation is a key point in the proposed analysis enabling to consider a continuous mechanical history [START_REF] Freborg | Modeling oxidation induced stresses in thermal barrier coatings[END_REF][START_REF] Sfar | Numerical investigation of residual stress fields and crack behavior in tbc systems[END_REF][START_REF] Wei | Dynamic multi-crack evolution and coupling tbc failure together induced by continuous tgo growth and ceramic sintering[END_REF], whereas most of the studies in TBC prefer to analyze separately different oxide thicknesses (e.g. [START_REF] Busso | The influence of bondcoat and topcoat mechanical properties on stress development in thermal barrier coating systems[END_REF]). Thus, the mesh is refined in an area where the BC is prone to be transformed into TGO, see Figure 4(b). The rate of transformation is associated to the oxide growth kinetics, eq. 1 by using a progressive growth of the oxide fraction within the line of elements associated to the oxide/BC interface, following the prescribed sinusoidal shape of the interface. In these elements, the global stress is defined using the local stresses evaluated for both oxide and BC behaviors, following:

σ ∼ = f v σ ∼ ox + (1 -f v )σ ∼ BC (3)
where f v corresponds to the volume fraction of the oxide within the considered element, σ ∼ ox and σ ∼ BC are the stress tensors related to the oxide and the BC behaviors respectively. The local stresses, within this line of elements in transformation, are estimated using the Voigt assumption of an uniform total strain. For the mechanical behavior of the oxide layer, the total strain tensor ε ∼t is decomposed into:

ε ∼t = ε ∼m + ε ∼th + ε ∼gr (4)
where ε ∼m and ε ∼th correspond to mechanical and thermal strain tensors respectively. The oxide layer is assumed to be elastic ε ∼m = ε ∼e . To summarize the model assumptions for elasto-viscoplastic behavior of the BC, the constitutive equations are recalled in Table 3. The chosen model associates non-linear isotropic hardening, so as to account for slight softening observed experimentally for large strain values [START_REF] Texier | High temperature micromechanical behavior of a pt-modified nickel aluminide bond-coating and of its interdiffusion zone with the superalloy substrate[END_REF], and kinematic hardening to account for ratcheting effect associated to rumpling [START_REF] Karlsson | The displacement of the thermally grown oxide in thermal barrier systems upon temperature cycling[END_REF].

The low temperature thermo-elastic properties of each layer are detailed in Table 4, accounting for coefficient of thermal expansion (CTE) for each layer. Besides, details of visco-plastic parameters of the bond-coat layer function of significative temperatures are given in Table 5. The drastic modification of the yield stress, σ Y , corresponds to observed transition into ductile behavior of typical (Ni,Pt)Al layer [START_REF] Pan | Evolution of a diffusion aluminide bond coat for thermal barrier coatings during thermal cycling[END_REF][START_REF] Alam | Tensile behavior of a free-standing pt-aluminide (ptal) bond coat[END_REF][START_REF] Texier | High temperature micromechanical behavior of a pt-modified nickel aluminide bond-coating and of its interdiffusion zone with the superalloy substrate[END_REF].

The applied loading corresponds to thermal cycling experienced by the TBC systems, but ignoring the thermal gradient within its thickness. It consists in: first cooling from the temperature applied during the top coat deposition, of 1100 • C to room temperature at a cooling rate of 3.33 • C.s -1 . Then heating to the maximum temperature of 1100 • C at a heating rate of 3.33 • C.s -1 , followed by dwell at this maximum temperature for 5 minutes and the same cooling rate down to 100 • C. The cycling from 100 • C to 1100 • C was applied 10 times. Strain partitioning [42]

ε ∼t = ε ∼th + ε ∼e + ε ∼p Yield function f = J 2 σ ∼ -X ∼ -R 0 -R Isotropic Hardening R = Q BC 1 -exp (-bp) Kinematic Hardening X ∼ = 2 3 Cα ∼ , α ∼ = ε ∼ p - 3 2 D C X ∼ ṗ Flow Function ε ∼ p = ṗn ∼ , ṗ = f K n and n ∼ = ∂ f ∂σ ∼
[37] [30] [START_REF] Frachon | Multiscale approach to predict the lifetime of eb-pvd thermal barrier coatings[END_REF] Table 5: Yield stress of the BC layer when used in this study, from [START_REF] Texier | High temperature micromechanical behavior of a pt-modified nickel aluminide bond-coating and of its interdiffusion zone with the superalloy substrate[END_REF] Temperature (°C) 100 650 1100 σ Y (MPa) 10 10 3 70 28

Last but not least, the chosen kinetic for oxide growth was artificially increased to limit CPU time: the oxide thickness reached 5 µm after the 10 cycles described before. The influence of this assumption will be detailed in the discussion.

Parameters influencing blistering

Firstly, the local out-of-plane stress is known to be directly linked to interfacial damage: this should be seen as a driving force for interfacial crack opening [START_REF] Busso | The influence of bondcoat and topcoat mechanical properties on stress development in thermal barrier coating systems[END_REF]. In our case, because the location of the crack tip is defined, several observations can be developed from the case combining interfacial roughness, elasto-viscoplastic BC and growth strain, case #4 Table 2, corresponding to the reference case. When observing the mechanical state in the debonded area after thermal cycling and at low temperature (Figure 5(a) and (b)), σ 22 is lower than in the area where the TC is adherent to the TGO. Also, the maximum values are reached at the very crack tip. Besides, local oscillations of σ 22 are observed along the sinusoidal profile, from compression at its peak to tension at its valley. This last point is consistent with literature observations [START_REF] Busso | A mechanistic study of oxydation-induced degradation in a plasma-sprayed thermal barrier coating system. Part I: model formulation[END_REF][START_REF] Hille | Microcrack nucleation in thermal barrier coating systems[END_REF]. However, in the presence of the interfacial crack as experimentally tested, this sheds a new light on the observed evolution of the blister: if the crack is located at a peak of the rough interface, the compressive value of out-of-plane stress, σ 22 , clarifies that the crack could reach a stable position. On the other hand, if the crack is located at the valley of the local rough interface, large positive values of σ 22 would induce a significant crack propagation. However, this confirms that the crack tip should be carefully analyzed through the process zone where damage will develop [START_REF] Maurel | Recent progress in local characterization of damage evolution in thermal barrier coating under thermal cycling[END_REF]: this process zone corresponds to the area where the TC is adherent to the TGO, in the vicinity of the crack tip and it is the place of maximum stress. Local evolutions of mechanical quantities are thus studied within the TC layer at the crack tip for the different modelling assumptions presented in Table 2, focusing on local stress values, Figure 6a. The out-of-plane component, σ 22 , reflects a driving force for local damage when positive values are reached, Figure 6a. It is worth noting that a "non-local" damage is effective in this configuration: the damage will be associated to positive values of σ 22 at the valley, beyond the crack tip location, see dashed lines Figure 6a. Further crack propagation can only come from coalescence between the crack tip and this potentially damaged area by microcracking.

However, the evolution of σ 22 at the crack tip when located at the top, reflects the coupling of roughness and growth strain, see continuous lines Figure 6a. For a flat interface with growth strain and elasto-viscoplastic BC, case # 1, no evolution is observed for the 10 cycles tested. In contrast, a drastic increase of absolute value of σ 22 is observed for both elastic and elasto-viscoplastic BC behavior accounting for growth strain with a rough interface, cases #2 and 4. Finally, the association of rough interface, and elasto-viscoplastic BC behavior without growth strain leads to no evolution of the σ 22 component, case #3. As a first result, the local damage seems to be driven by the combination of a rough interface and growth strain within the oxide layer, and to a lower extent by the elasto-viscoplastic behavior of the BC layer, case #4, as compared to an elastic assumption, case #2: the absolute value of σ 22 is observed to be higher with viscoplasticity, case #4, than with elasticity, case #2.

The in-plane component, σ 11 , is related to the build-up of residual stresses in the TBC system during thermal cycling. This quantity is generally used to asses the impact of CTE mismatch between layers, but is also driven by growth strain and creep. Thus by averaging σ 11 in the adherent TC layer, we can derive some conclusions, Figure 6b. Again, the evolution of σ 11 reflects coupling of roughness and growth strain. The intensity of the in-plane compressive stress decreases significantly with cycling for a rough interface and growth strain, cases #2 and 4, with a minor effect of the BC behavior. However, a slight decrease is also observed for the flat interface accounting for growth strain, case #1, whereas σ 11 is constant for a rough interface without TGO growth strain, case #3.

To summarize this sensitivity analysis, combining a rough interface and growth strain within the oxide layer is needed to model a significant stress evolution within the TC layer in the vicinity of the crack tip. The combination of growth strain to elasto-viscoplastic BC behavior impacts σ 11 even when modelling a flat interface.

The experiments carried out in this context only give access to the evolution of blister height, which is observed to first increase with thermal cycling without a significant evolution of the debonded area (interfacial crack size). The proposed FEA also provides a blister evolution for a given crack length. In particular, the evolution of the blister height was plotted in the figure 6c. This leads to a major conclusion: the only way to observe an increase in the blister height, consistent with experiments, is to associate a rough interface, growth strain within the TGO and elasto-viscoplastic behavior for the BC layer, case #4. Despite the similar trends on the local stresses evolution for both elastic and elasto-viscoplastic BC behavior shown in the figure 6a and6b, the blister height is seen to slightly increase for the test case with an elastic BC. For a flat interface, growth strain and elasto-viscoplastic BC behavior, case #1, the height of the blister decreases, exhibiting the need of a model including a local roughness. Finally, the model combining an elasto-viscoplastic BC and a rough interface while ignoring the growth strain, case #3, leads to a constant blister height.

The almost constant blister height observed during thermal cycling for an elastic BC, case #2, suggests that the increase in blister height is not simply related to the stress level at the crack tip. Thus, the plasticity level in the BC layer could clarify the impact of the chosen assumptions. In the BC layer, local maxima of σ 22 correspond clearly to local maxima of strain localisation as can be observed from the field of the cumulated plastic strain (called p BC ) in Figure 5(c). Also, maxima are reached close to the crack tip, see label 2 ⃝ Figure 5(c). But, in addition to this, a lower but non-negligible plasticity appears in the area of the BC layer corresponding to adherent ceramic layer, close to the substrate interface and limited to few undulations from the crack tip, see label 3

⃝ Figure 5(c). This slight plasticity is vanishing away from the crack tip and could correspond for the blister to a kind of plastic hinge. By comparing the evolution of plasticity in this area for the different modelling assumptions, the evolution of blister height is clearly correlated to this strain localisation in the BC layer, Figure 6d.

Discussion

To summarise the above analysis, the role of boundary conditions in blister buckling shoud be discussed. The starting point is the elastic analysis of a perfect blister clamped on its edge proposed by Hutchinson et al [START_REF] Hutchinson | Growth and configurational stability of circular, buckling-driven film delaminations[END_REF]. The buckling is known to be strongly correlated to the boundary conditions. For a perfectly clamped structure, Figure 7(a.1), the effective radius to be accounted for in buckling analysis, R e f f , corresponds to the geometrical radius of the blister R e f f = R, whereas for simple contact (without torque constraint), Figure 7(a.2), R e f f = 2R. This does impact firstly the critical stress to induce buckling, σ c , the higher the effective radius, the lower the critical stress:

σ c = 1.2235 E 1 -ν 2 h R e f f 2 (5)
where E and ν correspond respectively to the Young's modulus and the Poisson's ratio of the TC layer [START_REF] Hutchinson | Growth and configurational stability of circular, buckling-driven film delaminations[END_REF]. In addition, the height of the blister δ is assessed as a function of the critical stress at buckling:

δ h ≃ 1 c 1 σ σ c -1 1/2 (6)
where h corresponds to the height of the TC layer, σ is the in-plane stress component considering equibiaxial loading, and c 1 is a function depending of the Poisson's ratio ν, following c 1 = 0.2473(1 + ν) + 0.2231(1ν 2 ). The above set of equations corresponds to a blister perfectly clamped on its edge, however in a rough approximation, from a perfect blister to simple contact boundary conditions, Figures 7(a.1) to (a.2), the critical stress is divided by 4 and thus, the height of the blister could increase by a factor of two. The strain localization described above, and the plastic hinge, will thus induce larger buckling considering a transition from rigid boundary condition to less constraint in torque. However, the in plane stress level decreasing with the number of thermal cycles, Figure 6b, the height of the blister should decrease inconsistently with experimental observations. It is worth noting that the in-plane stress σ 11 along y direction, is a function of the ordinate y from the crack tip to the surface of the top-coat, Figure 7(b). This result differs to a large extent from pure bending, Figure 7(c). Besides, the sign of stress values σ 11 is strongly influenced by the chosen modelling assumption. This point may clarify the limit of the elastic analysis derived from Hutchinson et al.

Finally, the torque, M, applied to the blister can be assessed by the integration of σ 11 along the y direction from the crack tip to the surface of the top-coat as shown in Figure 7(b), using M = h TC 0 y × σ 11 dy. The evolution of M with the number of thermal cycles is plot for the different assumptions in Figure 7(d). Again, only case #4 exhibits a global increase of torque with cycling: ignoring the interfacial roughness yields to a slight decrease in M, case #1, ignoring the BC plasticity yields to a slight increase but a quick saturation effect, case #2, and ignoring the growth strain yields to a constant torque, case #3. This finally proves that blister height increase is induced by modification of boundary conditions of the blister, associated to strain localization in the BC, and by a large increase in torque driving the blister height.

Last but not least, one important assumption should be analyzed: what is the influence of chosen accelerated oxidation kinetic on the evolution of mechanical state and buckling? This point is crucial in the proposed analysis by the way two kinetics should be considered: the oxidation kinetic but also the kinetic associated to viscosity and creep of the BC layer. To clarify this point, realistic oxidation kinetic has been used without modification of other parameters based on case #4 associating rough interface, visco-plasticity of the BC layer and growth strain. This constitutes the case #5. The CPU time for case #5 and 1000 cycles corresponds to 16 days, to be compared to 4 hours for case #5.

As a first salient result, the cumulated field of plasticity for the same oxide thickness of 2.75 µm has been plot for both cases #4 and #5, Figures 8(a) and (b). For accelerated oxidation kinetic, this corresponds to 2 cycles as compared to Fig. 5(c) plot for 10 cycles. It is obvious that for the realistic oxidation kinetic, the level of cumulated plasticity is much higher than for the accelerated one. This could be associated to an increase of the stress relaxation during oxide growth considering the realistic case. The final blister height is higher in case #5 than observed in case #4 for a given oxide thickness, Figure 8(c). When using the realistic model of oxidation, the experimental points corresponding to 100 L-FCT + 871 S-FCT as described in Figure 3, can be added to this plot. Both models reach consistent blister height as compared to experiment, but with large differences in cycle number, 10 and 1000 cycles, and TGO height, 2.8 µm and 5.8 µm for accelerated and realistic oxidation rates respectively. This shows the capability of the method to describe experimental results, with additional tuning of the creep and oxidation kinetics. This paves the way to inverse analysis for mechanical behavior characterization from blistering measurement.

The blister height predicted by the proposed model is close to minimum blister height observed experimentally. Since all others model assumptions (cases #1 to 3) induced lower blistering, this reinforces the chosen reference model and validates the sensitivity analysis proposed in this study.

Conclusion

This study is based on an analysis of the evolution of a blister during thermal cycling in a TBC system. The initial blister is processed by a laser shock, thanks to the LASAT method. This is a way to monitor easily its evolution since blistering is observed in most of EB-PVD TBC systems before final TC spallation. This new laser-aided methodology developed for the characterization of TBCs properties is introduced here as LASDAM for LAser Shock for DAmage Monitoring. In the experimental part of this paper, a significant evolution of the blister height was shown with respect to the number of thermal cycles and for a fixed debonded diameter. This confirms experimentally that thermal cycling affects the mechanical state in a blister. Thanks to the sensitivity analysis achieved on a FE model, the major finding is that the growth of the blister height is due to the combination of growth strain within the oxide layer, partial stress relaxation in the plane of the interface and an increase of the strain localization in the BC layer correlated to the local interfacial roughness. The fact that the delamination is stable (no significant crack growth measured), while the blister height increases, can be the result of negative stress values at the crack tip considering local rough interface. The only way to obtain further delamination observed experimentally should be the coalescence between the large interfacial crack stopped in a crest (due to crack closure) and micro-cracking at the next valley of the rough interface. In a nutshell, the driving forces for rumpling are shown in this study to be also the driving forces for blistering, known to be a major damage mode in TBC systems, and refered to as blister driven delamination. The plastic hinge and the torque increase finally clarify the blister height increase.

As a clear perspective to this work, the temperature gradient in the TBC system and the role of local sintering of the TC layer observed in service due to surface extreme temperature associated to the flame or CMAS impact could be further analyzed.

  (a) and (b).

Figure 1 :

 1 Figure 1: TBC system for LASAT specimen exposed to thermal cycling from 100 to 1100 • C using S-FCT (a) SEM cross-section (b) temperature field on ceramic side after 3 shocks processed by LASAT (c) sketch of the debonded diameter evolution with respect to the laser fluency for LASAT, (d) sketch of the blister height evolution with respect to the debonded diameter and (e) SEM detail of the microstructure far away from the location of the LASAT debonding.

Figure 2 :

 2 Figure 2: Thermal cycles tested in this study: long-and short furnace cycling test (L-FCT and S-FCT respectively)

Figure 3 :

 3 Figure 3: Evolution of blister height as a function of debonded diameter for different states (a) influence of low number of short cycles (N<900);LASAT processed in AR corresponds to[START_REF] Guipont | Buckling and interface strength analyses of eb-pvd tbc combining laser shock adhesion test (lasat) to thermal cycling[END_REF], other data are from the current study (b) influence of low and high number of cycles (note that only fit has been reported for blisters processed in the AR)

Figure 4 :

 4 Figure 4: FE model (a) global view of the rough multilayer system (the substrate is only partially shown in thickness) (b) detail of the initial TGO layer and the BC prone to be transformed into TGO; the grey arrow corresponds to the location of the crack tip between TC and TGO layer; 1 ⃝, 2 ⃝ and 3⃝ correspond respectively to sets of FE elements used to average stress at top and valley in the TC layer, and strain in the BC layer (associated colors are only to point out the precise areas used for averaging).

Figure 5 :

 5 Figure 5: Out-of-plane stress component in the TC layer, σ 22 (a) and detail in (b), cumulated plasticity in the BC layer (c) (note that the color bars correspond respectively to (a) and (b), at the top and to (c) at the bottom and that the separation between TC and TGO is artificial for visualisation purpose; the arrow indicates the abscissa of the crack tip)

Figure 6 :

 6 Figure 6: (a) Evolution of σ 22 at the crack tip and at the valley (corresponding to 1 ⃝ and 2 ⃝ respectively in Figure 4); (b) Evolution of σ 11 averaged in the adherent part of the TC; (c) blistering and (d) cumulated plasticity in the BC (corresponding to 3 ⃝ in Figure 4) for different model assumptions. Values correspond to the end of the cooling stage associated to each cycle.

Figure 7 :

 7 Figure 7: (a) schematic of boundary conditions (b) force and moment acting on the blister (c) evolution of σ 11 as a function of y and (d) evolution of the torque with the number of cycles as a function of the different assumptions

Figure 8 :

 8 Figure 8: Cumulated plasticity in the BC layer (a) 10 cycles with accelerated oxidation kinetic (case #4) (b) 1000 cycles with realistic oxidation kinetic (case #5) and (c) comparison of blistering evolution for both cases #4 and #5

Table 1 :

 1 Chemical composition of substrate superalloy, AM1 in % except for Hf and S in ppm (from[START_REF] Perez | Kinetic oxidation model including the transient regime for a single crystal nickel-based superalloy over the temperature range 750-1300°c[END_REF])

	Ni	Ta	Cr	Co	W	Al	Mo	Ti	Hf (ppm)	S (ppm)
	wt% Bal. 7.97 7.54 6.61 5.49	5.2	2.01 1.2	460-490 0.104-0.123
	at% Bal. 2.65 8.74 6.76 1.80 11.61 1.26 1.51 155-165 0.195-0.231

Table 2 :

 2 Different model assumptions used for FEA ; ε ∼gr corresponds to the growth strain associated to TGO growth, oxidation kinetic accelerated (acc.) or realistic (real)

	Case Interface	BC	ε ∼gr	oxidation kinetic
	1	Flat	elasto-viscoplastic yes	acc.
	2		elastic	yes	acc.
	3 4	Rough	elasto-viscoplastic no elasto-viscoplastic yes	acc. acc.
	5		elasto-viscoplastic yes	real
	interface (here 22 in a local frame)

Table 3 :

 3 Set of equations used for the mechanical behavior of the BC layer. With ε ∼m , ε ∼e and ε ∼p the mechanical, elastic and plastic strain tensor respectively, J 2 the von Mises yield function, and p is the cumulated plastic strain[START_REF] Lemaitre | Mechanics of Solid Materials[END_REF]

Table 4 :

 4 thermo-elastic parameters at 100 °C used in this study; initial thickness of each layer, h 0

		Substrate BC	TC TGO
	E (GPa)	133.	161. 60.	395.
	ν	0.34	0.33 0.1	0.22
	α (10 -6 K -1 )	14.	15.	10.	8.5
	h 0 (µm)	2000	50	125	1
	Ref.

Acknowledgements

This work was funded by Safran Tech and supported by Mark Harvey, whom we would like to thank. We would also like to acknowledge Gérard Brabant and Sylvain Gailliegue (Mines Paris, CDM) for their support during LASAT experiments and thermal cycling, respectively. Additionally, we acknowledge Dana Cosson for conducting thermal cycling at Safran Tech. The authors would like to express their gratitude to Arjen Roos (Safran Tech) for his careful review and valuable advice on the paper.


domain: sde sdv

text:
Biswaranjan Paital
email: biswaranjanpaital@gmail.com

Abhipsa Bal

Georgina A Rivera-Ingraham

Jehan-Hervé Lignot

Increasing frequency of large-scale die-off events in the Bay of Bengal: reasoning, perspectives and future approaches

Keywords: Aquatic environment, Bay of Bengal, climate change, conservation strategies, natural disasters, catastrophic events, satellite monitoring]

The Bay of Bengal has been suffering from increasing frequency of large-scale die-off events for the past decades. Most frequently, these events are attributed to high-speed human development and its harmful effects on environment, which is nevertheless, the biggest challenges currently faced by the world. Increasing urbanization, environmental pollution and climate change are leading to unsustainable ecosystem exploitation and raising health and disease management challenges. Considerable modulations in major ecosystems and major disturbances in the global food chain are some of the most significant consequences of this uncontrolled urbanization. Global warming and El Niño events are few particular phenomena that drive mass deterioration of terrestrial foliages and fauna as well as aquatic organisms, respectively. We here review and discuss the die-off events occurring in the Bay of Bengal for the last decades as well as all the data obtained from the analyses of such events to provide a future perspective on potential management and monitoring strategies directed towards the protection of the flora and fauna of several major ecosystems from such die-off events.

Introduction

High-speed human development is one of the biggest challenges faced by the world. Increasing urbanization and unsustainable lifestyle often leads to preoccupying levels of waste accumulation and alterations of the physico-chemical properties of the environment, challenging organisms at the lowest biological levels [START_REF] Paital | RE: Full speed ahead to the City on the Hill[END_REF] . The loss of sensitive species from impacted ecosystems leads to changes in biodiversity composition, major disturbances in the global food chain, opens windows to biological invasions and the development of pathogens and eventually leads to significant problems with human health and disease management [START_REF] Paital | RE: Full speed ahead to the City on the Hill[END_REF][START_REF] Welker | Role of redox metabolism for adaptation of aquatic animals to drastic changes in oxygen availability[END_REF][3][START_REF] Leaf | Potential health effects of global climate and environmental changes[END_REF][START_REF] Sing | Full speed ahead to the City on the Hill[END_REF] . Global warming in particular is a cause for considerable concern due to its multiple and diverse consequences, including the mass deterioration of terrestrial foliage and fauna as well as aquatic organisms [START_REF] Thomas | Extinction risk from climate change[END_REF][START_REF] Pörtner | Climate change affects marine fishes through the oxygen limitation of thermal tolerance[END_REF][START_REF] Tollefson | Heat waves blamed on global warming FREE[END_REF] .

As previously mentioned, change in environmental conditions challenges organisms by disturbing energetic balances and triggers changes in their molecular and biochemical pathways [9][START_REF] Valkama | Effects of elevated O 3 , alone and in combination with elevated CO 2 , on tree leaf chemistry and insect herbivore performance: a meta-analysis[END_REF][START_REF] Fan | Quantitative flux analysis reveals folate-dependent NADPH production[END_REF] . As a result of exceeding, for example, the thermal tolerance limits, ectotherms and other homeotherms lose control over the cellular mechanisms responsible for fueling vital processes. This causes organisms to be pushed to their physiological limits, limiting their capacity to withstand small additional shifts in their environmental conditions and eventually leading to unsustainable metabolic rates and finally, collapse [START_REF] Sambad | Die-off event of blue buttons in Astsrang Beach[END_REF] . The resulting, sudden large-scale mortality of animals in mega ecosystems such as oceans (including coastal ecosystems) and rivers is increasing in frequency across the world. Thus, identifying the reason(s) behind these events becomes a challenging issue for current ecophysiologists, whilst conservationists seek suitable monitoring programs allowing highlighting potential conditions of vulnerability. There is however, cases where the reason(s) behind large dieoff events remained unknown [START_REF] Sambad | Die-off event of blue buttons in Astsrang Beach[END_REF] , leaving ecophysiologists facing the open challenge of understanding the physiological factors determining the sensitivity of such species, compared to other closely related organisms from catastrophic death --------**Present address: Laboratoire Environnement de Petit Saut, HYDRECO. BP823, Kourou, Guyane Française. events [START_REF] Paital | High speed urbanization and its effects on aquatic food chain especially on fish in Bata river of Odisha, India[END_REF][START_REF] Paital | Role of cyclonic storm as natural disaster and other factors on vulture mortality in India[END_REF][START_REF] Paital | Biochemical and environmental insights of declining vulture population in some Asian countries[END_REF][START_REF] Paital | Oxidative stress and ageing in animals under thermal stress due to global warming: A perspective[END_REF][START_REF] Paital | Redox metabolism in fishes under thermal stress warrants more attention[END_REF] . In this work, we review the massive dieoff events that are increasingly occurring in the Bay of Bengal, and we discuss the possible reasons behind these episodes, which have been scarce until now [START_REF] Paital | High speed urbanization and its effects on aquatic food chain especially on fish in Bata river of Odisha, India[END_REF][START_REF] Paital | Role of cyclonic storm as natural disaster and other factors on vulture mortality in India[END_REF][START_REF] Paital | Biochemical and environmental insights of declining vulture population in some Asian countries[END_REF][START_REF] Paital | Oxidative stress and ageing in animals under thermal stress due to global warming: A perspective[END_REF][START_REF] Paital | Redox metabolism in fishes under thermal stress warrants more attention[END_REF] .

With all the compiled data, we aim to open discussion on the possible contribution of climatic factors for such death events along the Bay of Bengal, and the usefulness of environmental monitoring in decreasing, if not to arresting such catastrophic events [START_REF] Chainy | An Overview of Seasonal Changes in Oxidative Stress and Antioxidant Defence Parameters in some Invertebrate and Vertebrate Species[END_REF][START_REF] Samanta | Effects of seasonal variation on oxidative stress physiology in natural population of toad Bufo melanostictus; Clues for analysis of environmental pollution[END_REF] .

Materials and Methods

As previously mentioned, the present article focuses on the Bay of Bengal, present in northeast of Indian Ocean. It is the largest bay in the world [START_REF]Map of Bay of Bengal-World Seas, Bay of Bengal Map Location -World Atlas[END_REF] and is surrounded by India and Sri Lanka to the west, Bangladesh to the north, and Myanmar and the Andaman and Nicobar islands to the east. The total area of the bay is ~2,172,000 km [START_REF] Welker | Role of redox metabolism for adaptation of aquatic animals to drastic changes in oxygen availability[END_REF] . Most of the data on large-scale die off events were collected from the meteorological sites, Govt. of India, New Delhi. Particular attention was paid to the recent mortality event affecting blue button jellyfish (Porpita porpita) occurring on the 2 nd May 2016 [START_REF] Sambad | Die-off event of blue buttons in Astsrang Beach[END_REF][START_REF]Map of Bay of Bengal-World Seas, Bay of Bengal Map Location -World Atlas[END_REF] and affecting a 6 km stretch of coast near Astarang beach (19°51'37.1"N+86°13 '30.3"E), between the two Indian heritage sites of Chandravaga and Konark (19°52'04.6"N+86°07 '57.8"E 19). Just a day after of this specific event, physic-chemical parameters were recorded in water collected from both surface and bottom (average depth 1,500 m) of the sea. Dissolved oxygen (DO), salinity, turbidity and pH were measured in the collected water samples using specific electrodes in the field (μP based soil and water analysis kit, Esico. Co., New Delhi, India) [START_REF]Map of Bay of Bengal-World Seas, Bay of Bengal Map Location -World Atlas[END_REF] . NO 3 and chlorophyll-a content were measured using standard spectrophotometric methods [START_REF] Paital | Seasonal variability of antioxidant biomarkers in mud crabs (Scylla serrata)[END_REF] . The detailed protocols were described elsewhere [START_REF] Mohanty | Assessment of temporal variation in coastal water characteristics though multivariate statistics-A case study at Sothwestern Bay of Bengal[END_REF][START_REF] Grasshoff | Methods of seawater analysis[END_REF][START_REF] Parsons | A manual chemical and biological methods for seawater analysis[END_REF] . Satellite imaging of physical parameters at the Indian sites was done by Dr. Hervé Demarcq, Institut de Recherche pour le Développement, France. The blue button jelly (P. porpita) on a 6 km stretch of beach along the Bay of Bengal near Astarang area (19°51'37.1"N+86°13'30.3"E) were calculated manually by dividing the area randomly into different squares of 100 x 100 m 2 . Then, from the average value, the total number of carcasses of blue buttons was estimated for the whole stretch of 6 km area. Results corresponding to water analyses are presented as mean ± SEM of 10 samples. SAS version 9.3 was used to conduct PCA and Factor analysis.

Results and Discussion

The Bay of Bengal and its resources have been largely exploited and been source of economic importance since early history. For example, it is well documented how the British East India Company, merchants of London, was one of the first trading ventures that were installed in the Bay of Bengal as early as the 1600 [START_REF]Asiatic Society of Bangladesh[END_REF] . Gopalpur-on-Sea of Odisha (India) was one of their main trading centers. The English East India and French East India Companies were some of the trading companies who operated in India along the Bay of Bengal shorelines. The socioeconomic importance of this area is also presently represented by the establishment of the Bay of Bengal Initiative for Multi Sectoral Technical and Economic Cooperation (BIMSTEC), supporting international free trade among countries in the area such as Bangladesh, Bhutan, India, Myanmar, Nepal, Sri Lanka, and Thailand [START_REF]Bay of Bengal[END_REF] . Aiming to further develop trading and exploitation in the Bay of Bengal, the Indian Government has proposed "The Sethusamudram Shipping Canal Project", directed towards creating a navigable channel linking the Gulf of Mannar with the Bay of Bengal.

All these major development in the area have been accompanied by the construction of important ports such as Chennai, Paradip, Chittagong, Kolkata, Mongla, Tuticorin, Visakhapatnam and Yangon harbours [START_REF]Bay of Bengal[END_REF] . Another consequence of this development has been the fast and often uncontrolled growth of cities associated to some major large rivers such as the Ganges, the Brahmaputra, and other rivers such as the IrrawaddyRiver, Godavari, Mahanadi, Krishna and Kaveri, all discharging into the Bay of Bengal [START_REF]Asiatic Society of Bangladesh[END_REF] . Altogether, the large cities associated to these rivers and the high industrialization are known to result in important pollution episodes with disastrous consequences for local fauna and flora [START_REF] Paital | High speed urbanization and its effects on aquatic food chain especially on fish in Bata river of Odisha, India[END_REF] . However, the importance of preserving and understanding the processes through which the Bay of Bengal is being affected by such processes is testified by the fact that approximately 31% of the world's coastal fishermen exploit its waters and aquatic recourses [START_REF]Asiatic Society of Bangladesh[END_REF][START_REF]Bay of Bengal[END_REF][START_REF]Limits of Oceans and Seas[END_REF] . "Thoni" and catamaran fishing boats of fishing villages thrive along the Bay of Bengal shorelines. The Bay hosts 26 economically important and 44 abundantly marine fish species that are exploited throughout the year resulting into an average fish landing of two million tons [START_REF]Asiatic Society of Bangladesh[END_REF][START_REF]Bay of Bengal[END_REF][START_REF]Limits of Oceans and Seas[END_REF] .

Irrespective of the international importance, millions of large-scale mortality episodes observed frequently in this area have become worryingly high in the past two years, as it will be further discussed below. Particularly, the blue button jellyfish (P. porpita) may be considered of special relevance [START_REF] Gul | On the occurrence of Porpita porpita (Cnidaria: Hydrozoa) at Pakistan coast (North Arabian Sea)[END_REF] due to its magnitude, here estimated to have affected 6 ± 3.34 individuals per square meter (Fig. 1), thus resulting in about 1.8 x 10 5 carcasses.

The water quality analyses carried out by BRP on samples taken up to 3-5 kms from the shore (Fig. 2) indicate no major fluctuations in surface or bottom water quality, thus not likely being significant contributors to the observed mortality episode. Surface atmospheric conditions prior to, and after the event prevented us to access to satellite data (data not shown), such as chlorophyll concentrations or water temperature. Data analysis using SAS indicates that in the rotated factor pattern, turbidity, DO and salinity come as factor 5 and pH and NO 3 content contribute as factor 6 as a function of temperature change (Fig. 3A-C). Fig. 3D indicates that temperature is not the sole contributory factor but rather all the parameters vary at a time in a season. Scientists from the Central Institute of Fresh Water Aquaculture and from the College of Veterinary Science and Animal Husbandry (Orissa University of Agriculture and Technology, Bhubaneswar, India) sampled the carcasses and could not find any clear explanation for these mortality rates. P. porpita are offshore pelagic organisms, and although they have been reported in nearby coastal environments (e.g. along Pakistani coasts [START_REF] Gul | On the occurrence of Porpita porpita (Cnidaria: Hydrozoa) at Pakistan coast (North Arabian Sea)[END_REF] ), the presence of so many carcasses on the beach is unheard of in this area. Local experts and conservationist such as Prof. Sushil Kumar Dutta suggested possible pollution at deep sea levels, increased ocean temperature, or imbalances in the oceanic food chain and fishing pressure as possible causes of these recent deaths [START_REF] Sambad | Die-off event of blue buttons in Astsrang Beach[END_REF] .

The endangered olive ridley turtle (Lepidochely solivaceae) population in the Bay of Bengal has also suffered from significant impact in the last years, and in 2016 several die-off episodes were recorded [START_REF] Decodemind | Massive Animal Die-Offs Around the World[END_REF] . In January 2016, carcasses of 300 olive ridley turtles and a bottle nose dolphin (Tursiops aduncus) were found on the beach of Puri along the Bay of Bengal, Odisha state. In late February 2016, about 40 additional carcasses were found on the shore along Puri beach. Only two months after, another 80 turtles were found dead near Jatadhari river mouth on Paradip beach, close to Gahirmatha Marine Sanctuary [START_REF] Decodemind | Massive Animal Die-Offs Around the World[END_REF] . Though these events have most frequently been attributed to the activity of fish trawlers [START_REF]Olive Ridley sea turtle, 5-Year review: Summary and evaluation[END_REF] or animal impact with propeller blades [START_REF]32 Times of India, Turtles wash ashore dead on Chennai beaches in 20 days[END_REF] , the sudden deaths of such large groups of Fig. 1 -Overview of the most recent die-off events along the east-coast of Bay of Bengal in India, which highlights the increasing frequency of these events and the large diversity of species affected. For some of these events, mortalities were associated with alterations of the physic-chemical properties of their environment (e.g. shifts in dissolved oxygen (DO), pH, phosphate, nitrate or ammonia concentrations, etc) [START_REF] Decodemind | Massive Animal Die-Offs Around the World[END_REF] animals suggests that other reasons are behind these mortalities.

In January 2015, 314 dead olive ridley turtles were washed ashore, along the Marina and Elliot's beaches of Chennai 32 . It was considered that they were caught in the fishing nets of trawlers 32 . Within the following months, nearly a thousand turtles were found dead along Gahirmatha, Satyabhaya, Barunei, Pentha while another 15 animals were found dead on the beach of Marina and Elliot of Chennai. These events coincided with the nesting period of this species [START_REF]Olive Ridley turtles carcasses dot Odisha's Gahirmatha coast[END_REF] .

The year 2010 was also negative for L. olivaceae in the area, as nesting sites were damaged due to an oil spill from the Essar-owned coking coal carrying vessel Malavika near Gopalpur port of Odisha [START_REF] Mohanty | Oil-spill-damaged-turtle-nesting-sites: GreenPiece[END_REF] . Probably the most dramatic mortality episode of olive ridley turtles in the area occurred in December 1999, after a super cyclone hit Odisha. Around 1,524 dead turtles were found along the Ekakula and Chinchiri river mouths along the Gahirmatha coast [START_REF] Bhupathy | Conservation of olive ridley sea turtle Lepidochelysolivaceae along the Nagapattinam coast, southeast coast of India[END_REF] . It was a large-scale die-off as the super cyclone coincided with the nesting time of L. olivaceae. Comparing the frequency of die-off events with previous years, enumeration of dead turtles in year 1997 and 1998 was 159 and 59, respectively. The beat houses and wireless network set up along the sea coast to protect the breeding and nesting grounds during that super cyclone were damaged. The die off was due to intensive mechanized trawl fishing and official machineries performing relief operations in the coastal waters of Gahirmatha during the period [START_REF]Olive Ridley sea turtle, 5-Year review: Summary and evaluation[END_REF] .

There are several places in the Bay of Bengal (e.g. Nagapattinam, Tamil Nadu) that, due to its rich vegetation, are suitable place for nesting of these sea turtles, and are thus a focus of monitoring programs which help keeping track of these events. From a survey carried out in 2001, it was concluded that the turtle mortality rate was the highest at the beginning of the year, and as abovementioned, these are attributed to the impact of fishing nets or mechanized boats and cataramans 36 .

However, other reasons for such large-scale die-off and the massive decline in the population of this endangered turtle species have been attributed to different reasons by experts. On the one hand, the development of socio-economic growth of the increasingly growing human populations may be causing the immediate environment to deteriorate. The increasing sources of coastal artificial lights misdirected nearly 90% of the hatchlings in 2004-05 and 50% due to pollution of water [START_REF] Bhupathy | Conservation of olive ridley sea turtle Lepidochelysolivaceae along the Nagapattinam coast, southeast coast of India[END_REF] . The direct impact of El Niño events on the olive ridley turtles are unknown but the global warming impacts lower trophic levels as a result of which the zooplankton of the marine ecosystem is depleted and hence they change the migration pathways from an unproductive habitat to more productive habitats rich in food [START_REF]Olive Ridley sea turtle, 5-Year review: Summary and evaluation[END_REF] . Similarly, changing ecological niche may avoid exposure to warm water and the search of food could drive the turtles deep into the sea. Such behavioral changes could determine their exposure to adverse conditions, leading them to unsuitable habitats, predation and physiological restrictions.

A review of the literature evidences that large marine mammals have also been recently affected in the area. The most recent alarming case concerns a 42-ft long and 18-ft wide carcass of a baleen whale (Systematic name Mysticeti) found at Baidhara Pentha beach of Balukhanda (19°50'03.6"N+85°59'43.9"E) in Puri district of Odisha during the first week of December 2016. In January 2016, 80 short finned pilot whales (Globicephala macrorhynchus) beached in Tuticorin along the coasts of the Bay of Bengal [START_REF] Samaya | Dead whale found brahmagiri[END_REF] . Out of these, 45 whales died while the rest were pushed back to the sea by volunteers and rescuers. The reason of the whales being stranded was believed to be due to disorientation from their usual path, maybe due to the earthquakes under the sea which force them towards the shallow waters of the shore [START_REF] Gul | On the occurrence of Porpita porpita (Cnidaria: Hydrozoa) at Pakistan coast (North Arabian Sea)[END_REF] . Carcasses of whales were also found in districts of Puri, Kendrapada and Ganjam of Odisha as mentioned earlier (Fig. 1) [START_REF]Carcasses of three whales washed ashore[END_REF] . In March 2015, three sperm whales (Physeter macrocephalus) were found dead near Puducherry, Alambaraikuppam and Uyyalikuppam. The animal found near Uyyalikuppam was a 50 ft-long male weighting 4 tonnes while the animal found near Alambaraikuppam was a female whale of 35 ft which was nearly 3 tonnes. Both had injuries in their tail, which made experts conclude that the death was due to fishing nets in which they were accidentally caught. The carcass found near Uyyalikuppam measured 52 ft long and was an adult male. This carcass had some superficial injuries and oil globules were found on the dorsal side of its body [START_REF]Whale facts[END_REF] .

The whales are deep-sea divers and excellent navigators. The reason behind the above mentioned dieoffs along the Indian coast may be multiple. However, some researchers have hypothesized that this may be due to animals getting disorientated by sonar waves, changes in weather and global warming, water pollution, diseases, abnormalities in earth's magnetic field etc. Despite the numerous reasons suggested, this subject is far from being fully understood [START_REF]Coral Reef Biodiversity Assessment of the Andaman Islands, Bay of Bengal[END_REF] .

Another important ecologically relevant event along the coast of Bay of Bengal is the reduction in the coral reefs of Palk Bay, Gulf of Mannar and Andamans. A survey carried out in 2001 by UNDP GEF staff in collaboration with the coral scientists from Zoological Survey of India estimated the total area of shallow reef around northern Andaman to extend over 520 km [START_REF] Welker | Role of redox metabolism for adaptation of aquatic animals to drastic changes in oxygen availability[END_REF] . This implied a reduction of 276 km 2 (34.7%) compared to 796 km 2 of previous estimation 41. About 197 species of coral within 58 genera were recorded during the survey [START_REF] Rajasurya | Status of coral reefs in South coral reefs in South Asia: Bangladesh, India, Maldives and Sri Lanka[END_REF] . The island of St. Martin's of Bangladesh has 66 hard coral species with coral cover of 7.6% while the Gulf of Mannar hosts 117 hard coral species. The decline in Bangladesh coral cover was due to high level of sedimentation, unchecked and illegal fisheries, cyclones, storm surges, pollution etc. The threats to the corals of Gulf of Mannar, the Andamans and Sri Lanka are mainly destructive fishing, sand mining, pollution, sedimentation due to deforestation and ornamental fishing [START_REF] Ellison | Vulnerability Assessment of Mangroves to Climate Change and Sea-Level Rise Impacts[END_REF] . Along with above mentioned reasons, the climate change and global warming has a great impact on the coral reefs [START_REF] Ellison | Vulnerability Assessment of Mangroves to Climate Change and Sea-Level Rise Impacts[END_REF] . Probably the best known case is the mass coral bleaching in Australia, with around 35% of corals on the northern and central sections of the Great Barrier reef damaged as a result of increased water temperature, according to reports by the Australian Research Council Centre of Excellence for Coral Reef Studies [START_REF] Ellison | Vulnerability Assessment of Mangroves to Climate Change and Sea-Level Rise Impacts[END_REF] . Coral reefs in the Bay of Bengal are not an exception and have suffered massive deterioration in the last decades [START_REF]Climate Hot Map[END_REF] . For example, the 1969 cyclone buried in sand the coral reefs of Gulf of Mannar near Rameshwaram along the Bay in India. The 1998 El Niño phenomenon led to coral bleaching and destroyed a significant extension of the coral reefs, an impact which is still today intensively investigated and which results may open important research perspectives for the near future [START_REF] Ellison | Vulnerability Assessment of Mangroves to Climate Change and Sea-Level Rise Impacts[END_REF] .

Mangroves, as corals, are ecosystems that are highly threatened by climate change [START_REF]Climate Hot Map[END_REF] . They are also key ecological systems, providing for example, feeding habitats for many adult demersal fish and also permanent habitat for numerous species gleaned for subsistence [START_REF] Ellison | Vulnerability Assessment of Mangroves to Climate Change and Sea-Level Rise Impacts[END_REF][START_REF]Climate Hot Map[END_REF] . Furthermore, mangroves constitute buffering systems preventing coast degradation during cyclonic storms [START_REF][END_REF][START_REF] Odishatv | Thousands Of Dead Fish Found Floating In River Water At Paradip[END_REF] . The mangrove vegetation of Sunderbans, located along the West Bengal and Bangladesh Coast was declared as a World Heritage site by UNESCO (1997). Cyclone "Sidr" (2007) and "Aila" (2009) caused extensive damage to this mangrove system, a process which was significantly enhanced by the intense human deforestation for exploitation and development of shrimp farming. However, global warming and climate change-related impacts (e.g. salinity and sea-level rise [START_REF][END_REF] ) will have adverse impacts on the mangrove vegetation and associated fauna.

Unfortunately, the mortality episodes are not only affecting marine organisms, but are also frequent in freshwater ecosystems, often due to human-induced alterations of the habitat. For example, barely one month after the blue button event, a large die-off episode occurred in the nearby area of Paradip (Fig. 1), close to one of the largest Indian ports along the Bay of Bengal: early on the 5 th June 2016, about 500,000 fish carcasses were found at the Bata River, near Balijhara (20.5136926N, 85.6629975E). The main species affected were Rohu (Labeo rohita), Indian carp (Catla catla) and Mrigal carp (Cirrhinus mrigala), which were quickly removed from the site by official authorities to ensure environmental safety and samples were taken for further analyses. A few weeks later, through an official press conference, the State Government of Odisha expressed the concern of the Central Institute of Freshwater Aquaculture of the Government of India, who collected the data. Sample analyses suggested that water pollution induced by the extremely fast urbanization that is taking place in an area of about 25 acres near Bata River may be responsible for these deaths [START_REF]Times of India, Cyclone Nada to make landfall tomorrow Tamil Nadu[END_REF] .

Coincidence or not, the events we report here occurred very close to Kendrapara coast, where the Department of Forest and Environment of the Indian Government found earlier in 2016 several dead fin whales (Balaenoptera physalus),. Here again, authorities faced difficulties in identifying the cause of the death since these are the first events of this type registered in this area of the Bay of Bengal.

Two years earlier, two similar events of similar magnitude, occurred in the space of a few days. The first occurred at the Deogarh area, around 300 km away from where the blue-button carcasses were found. The second took place on the evening of 18 th June 2014, thousands of fish carcasses were found floating (Fig. 1) across a 236 km 2 area at Gohira reservoir (21°28'42.9"N+84°33'03.0"E) on Gohira river, a tributary of the river Brahmani in the Tileibani area of Deogarh district, Odisha state. A team composed of technicians from the Regional Office of Odisha State Pollution Control Board and Government officials sampled the carcasses after advising the local population against using the reservoir water or touching the fish carcasses. The State Pollution Control Board (Govt. of Odisha, India) attributed the massive die-offs to an episode of severe hypoxia for at least one of the two events [START_REF] Paital | Role of cyclonic storm as natural disaster and other factors on vulture mortality in India[END_REF] . On 4 th August 2014, a similar event of large scale mortality of a wide varieties of marine fishes was observed in Bada Ayapalli Village shore (19.313740 "N, 84.977382"E) near Gopalpur port, along Bay of Bengal of Odisha, India. After ruling out a possible effects of increased water temperature (coinciding with the rainy season and, water temperature being below 25 °C in the area), pollution was hypothesized to be the causative factor in this case. It is therefore, not possible to speculate about a common cause for all the aforementioned events.

It is widely accepted that anthropogenicallyinduced climate change is responsible for the increase in frequency of extreme environmental conditions such as droughts or storms. In the last 16 years, about 33 major ecological disasters, mainly being cyclones, tsunami and floods have been recorded along the Bay of Bengal (Table 1-3) Loss has been estimated to be around 320 billion USD, and nearly 450,000 human lives have been lost as a result. Among the 1-3) .

Conclusions

It is clear, from the examples we describe in the Bay of Bengal as well as from the numerous other reports available, that these episodes are a) increasing in frequency and b) affecting a wide variety of aquatic species, both benthic and pelagic, ranging from cnidarians to fish and large marine mammals (Fig. 1). With this comprehensive review of the information regarding the factors leading to massive die-offs, we conclude that, while physical parameters such as increased temperature or hypoxia are affecting ecosystems across the world, pollution is probably the primary factor affecting the Bay of Bengal. Although such phenomena are not new, environmentalists are concerned and are joining their efforts to fully understand and realistically evaluate how to counteract the deleterious effects of increasing urbanization leading to pollution [START_REF] Paital | RE: Full speed ahead to the City on the Hill[END_REF][START_REF] Sing | Full speed ahead to the City on the Hill[END_REF][START_REF] Samanta | Effects of seasonal variation on oxidative stress physiology in natural population of toad Bufo melanostictus; Clues for analysis of environmental pollution[END_REF][START_REF] Paital | Diclofenac Induced Vulture Deaths in Odisha, India: Time to Debate or Conserve Them?[END_REF] , particularly in countries such as India, where rates of anthropization are high. And, it leads to several deleterious effects on different ecosystems [78][79][80][START_REF] Paital | Redox metabolism in fishes under thermal stress warrants more attention[END_REF][START_REF] Paital | Possible resons of vulture mortality and their remedial measures with special reference to Indian context[END_REF][START_REF] Cardinale | Biodiversisty loss and its impact on humanity[END_REF] . Despite these efforts, the information here compiled demonstrates that the reasons behind some of these mortality events are unclear. This highlights the need to investigate by further setting up monitoring protocols for given species/ecosystems along with satellite-based analyses of water parameters aiming to understand and predict large-scale alterations in water quality. Otherwise, human-induced alterations of aquatic environments are likely to continue, with an increasingly negative effect on medium to large-scale biodiversity levels and directly impacting human populations across the world 84 .

Fig. 2 -

 2 Fig. 2 -Physic-chemical characteristics of the oceanic environments during massive die-off event of Porpita porpita in the Bay of Bengal. For further details refer to the materials and methods section

Table 1 -

 1 List of the recent catastrophic events in 2016 affecting the coasts of India, Bangladesh and Myanmar and their estimate socioeconomic impacts. The values provided in USD were evaluated in the respective years.

	Year	Areas	Event name and intensity	Human fatality Agricultural damage Other impacts
	2016	Sumatra, Andaman, Nicobar Islands, Thailand, Malaysia, South India	VARDAH, Very Severe cyclonic Storm, 140 km h -1	38	Banana, guava plantation and paddy fields.	Huge loss in fishing and tourism industry 47
	2016 2016 2016 2016	Sri Lanka, South India Andaman Islands, Myanmar, South India Sri Lanka, East-coast of India, Bangladesh, Myanmar Tirupati, Chennai, Nellore areas of India	NADA, Cyclonic storm, 75 km h -1 KYANT, Cyclonic storm, 85 km h -1 ROANU, Cyclonic storm, 85 km h -1 Flash flood due to Vardah Chennai-224mm Tirupati & Nellore-76 mm	12 --227 No fatality due to early evacuation	------Entire cultivated fields	--48 --49 $1.7 billion 50 --50

Table 2 -

 2 Catastrophic events and their estimate socio-economic impacts along Bay of Bengal from year 2010-2015. The values provided in USD were evaluated in the respective years.

	Year	Areas	name and intensity	Human fatality Agricultural damage	Other impacts
	2015 2015	Bangladesh, Myanmar, Northern India Tamil Nadu and Andhra Pradesh	KOMEN Cyclonic storm 75-85 km h -1 Torrential rains led to intensive flood	187-280 (mismatch between official and actual loss ~500	Leaving more than lakhs of people homeless in West Bengal and Bangladesh 100,000 acres of crop areas attributing to nearly $190 million loss	Heavy to very heavy rainfall induced damages in Odisha, West Bengal, Madhya Pradesh, Bihar etc. 51 Total industrial loss of about $2.18 billion 52
	2014 2013 2013 2012 2011 2010	Andhra Pradesh, Andaman and Nicobar Islands, Odisha, Andhra Pradesh, Chhatisgarh India and Myanmar Indonesia, Sri Lanka, Myanmar, Bangladesh Sri Lanka and South India Southern India and Sri Lanka Sri Lanka and India	HUDHUD Very Severe cyclonic storm 220-270 km h -1 PHAILIN Extremely severe cyclonic storm 215-260 km h -1 VIYARU 85 km h -1 NILAM Cyclonic storm 85-100 km h -1 THANE Cyclonic storm 140-165 km h -1 LAILA Cyclonic storm 100-120 km h -1	124 45 107 dead and 6 missing 75 48 65	Standing paddy, sugarcane, groundnut and sugarcane crops before harvested stages were lost. Damage of > 2, 00, 000 hectares of agriculture product in filed. Damage of > 3, 200 hectares of coconut plantation in Andhra Pradesh Damage of 128,000 hectares of crop fields, mainly sweet potato in Bangladesh. 739 hectares of crop lands in Andhra Pradesh were damaged. Floodwater inundated 51,486 hectares of directly sown land and13, 421 hectares transplanted land. 700 fishermen were reported to be stranded. --	ROANU $ 3.4 billion 53 Property worth $696 million was damaged along with 40 transmission towers and housing 54 Significant flooding in Sri Lanka. 50,000 of homes were damaged. Thousands of trees were uprooted. Loss of $5.24 million 55 Transportation, communication and power transmission were disrupted. Many houses were damaged. Loss of $56.7 million 56 Fishing Industry severely affected. Loss of $235 million 57 Loss of $117.49 million 58 (Contd.)

Table 2 -

 2 Catastrophic events and their estimate socio-economic impacts along Bay of Bengal from year 2010-2015. The values provided in USD were evaluated in the respective years (Contd.)

	Year	Areas	Event name and intensity	Human fatality Agricultural damage	Other impacts
	2010 2010	Myanmar and Bangladesh Malaysia, Sri Lanka and India	GIRI Extremely severe Cyclonic storm 195-250 km h -1 JAL Cyclonic storm 110-120 km h -1	157 direct and 10 cyclone induced 118 dead 12 missing	Catastrophic damage in parts of Myanmar Affected around 80,000 people Loss of $1.73 billion 60 Loss of $359 million 59

Table 3 -

 3 Disastrous events affecting the coasts of India, Bangladesh and Myanmar and their estimate socio-economic impacts from the year 1999-2009. The values provided in USD were evaluated in the respective years.

	Year 2009 2009 2008 2008 2008	Areas India and Bangladesh 315.7807 India, Bangladesh and Myanmar Sri Lanka and India Northeastern states of India. RASHMI, Cyclonic storm, Event name and intensity Human fatality Agricultural damage AILA, Severe cyclonic storm 110-120 km h -1 339 1 million people were homeless. >7,000 people were infected with diseases (i.e. diarrhoea) BIJLI, Cyclonic storm, 75-95 km h -1 07 Several esidents were lost. NISHA, 85-100 km h -1 204 90,000 people affected. Loss of $ 800 million 63 Other impacts Loss of $ 295.6 billion 61 --62 85 Km h -1 28 died, 50 missing. --Huge physical damages by the uprooted trees 64 Bangladesh, Myanmar, India and Sri Lanka NARGIS, Cyclonic storm, 165-215 Km h -1 1, 38, 366 1.50 million people were "severely affected". Loss of $10 billion 65
	2007	Bangladesh and West Bengal	SIDR, Cyclonic storm, 215-260 km h -1	~15, 000	Enormous physical damages in Bangladesh.	Loss of $ 1.7 billion 66
	2007 2006 2006 2005 2005 2004	Andaman Islands, Nicobar Islands, Bangladesh, Burma India Andaman Islands, Myanmar, and northern Thailand Andhra Pradesh Thailand Andhra Pradesh Indonesia,, Sri Lanka, India, Thailand, Maldives, Malaysia, Madagascar, Somalia, Kenya, Tanzania, South Africa.	AKASH, Cyclonic storm, 85-120 km h -1 OGNI, Cyclonic storm, 65 km h -1 MALA, Very severe cyclonic storm, 185-220 km h -1 BAAZ, Cyclonic storm, 83 km h -1 PYARR, Cyclonic storm, 65 km h -1 Sumatra-Andaman earthquake Tsunami, 9.0 magnitude quake epicentre 3.316°N 95.854°E3.316°N 95.854°E. Magnitude 9.1-9.3 M W	14 death, 3 fishermen were killed, 50 missed 24 37 died and 18 were drowned. 11 Officially death of 1 person was confirmed 230,000-280,000	Destroyed 205 houses and left an additional 845 damaged. 1, 99, 986 acres of crop area damaged. Mala brought heavy rains to northern Thailand. Heavy rains destroyed the crop fields. Crops over 4, 82, 000 hectares were damaged. Widespread damage of the infrastructure, shortages of food and water, and economic damage.	Moderate damages to crop land near coast and damage of 2 ha (4.9 acres) of shrimp farms. Loss of $ 982 million 67 3, 61, 553 livestock lost. 26, 853 houses damaged completely. Whereas 73,218 houses were partially damaged 68 Loss of $ 6.7 million 69 Heavy rain induced floods in some districts. Many villages were marooned 70 Death of 291 live stocks. > 12,000 houses were destroyed. Loss of INR 5, 030 lakhs 70 Total agricultural loss 71 (Contd.)

Table 3 -

 3 Disastrous events affecting the coasts of India, Bangladesh and Myanmar and their estimate socio-economic impacts from the year 1999-2009. The values provided in USD were evaluated in the respective years. Nargis" (2008) was the most deadly, which caused the loss of 140,000 human lives and resulted in extensive agricultural damages across Bangladesh, Myanmar, India and Sri Lanka. Cyclone "Sidr", on the other hand, affected Bangladesh and West Bengal resulting in the death of about 15,000 people. Irrespective of the loss, the frequency of cyclones is gradually increasing, with a new record established in 2016 (Table

	Year	Areas	Event name and intensity Human fatality Agricultural damage	Other impacts
	2003 2002 2001 2000 2000 1999	Andhra Pradesh Odisha and West Bengal Andhra Pradesh Sri Lanka and Tamil Nadu Tamil Nadu and Pondicherry Myanmar, Bangladesh, Indi a (particularly Odisha)	Severe cyclonic storm, 102 km h -1 Severe cyclonic, storm 102 km h -1 Cyclonic storm, 65 km h -1 Very severe, cyclonic storm 167 km h -1 Very severe cyclonic Storm, 189 Km h -1 Super cyclone, 260 km h -1	358 18 in Odisha and 2 in West Bengal 108 dead and 21 missing 9 in Sri Lanka 2 ~10, 000	Nearly 62, 000 of agricultural fields faced damage. Several acres of crop fields were damaged due to torrential rains and salt water. Damage to cultivated lands of around 125,000 hectares. Damage to paddy crops over 49,000 acres in Sri Lanka In Tamil Nadu 281 hectares of Paddy and 650 hectares of plantation cultivation was damaged. Damage to paddy crops, plantain, coconut plantations in Pondicherry and Tamil Nadu. Paddy crops in 16,17,000 hectares and other crops in 33,000 hectares were damaged.	Destroyed 1, 637 buildings fully, 7, 453 building destroyed partially. > 2, 000 villages were disconnected. Loss of $163 million 72 Disrupted transport and communication. Thousands houses were damaged. 4 fishing trawlers were missed 73 Several fishing coasts and 55,000 houses were damaged 73 Transportation and communication were disrupted. 83, 000 houses in Srilanka were destroyed. Near about 1000 of houses were destroyed in India in Tamil Nadu. The country faced heavy rainfall leading to floods 73 Nearly 30, 000 houses were damaged. The aftermath flood washed away the brick buildings also 74 Nearly 3.8 lakhs cattle heads were perished. Thousands of houses were damaged leaving people homeless. Loss of $4.44 billion 75
	cyclones, "

Acknowledgements

The authors are grateful to Dr. Hervé Demarcq (Institut de Recherche pour le Développement, France) for his assistance in the satellite imaging of physical parameters at the Indian sites mentioned in this study. BRP is highly thankful to the Science and Engineering Research Board, Department of Science and Technology, Govt. of India, New Delhi for providing funding (Grant Letter number ECR/2016/001984) and Department of Science and Technology, Government of Odisha (Grant letter number 1188/ST, Bhubaneswar, dated 01.03.17, ST-(Bio)-02/2017) for providing funding.