X Peyrard
email: xavier.peyrard@irstea.fr

L Liger

C Guillemain

V Gouy

A trench study to assess transfer of pesticides in subsurface lateral flow for a soil with contrasting texture on a sloping vineyard in Beaujolais

Keywords: subsurface lateral flow, pesticide transfers, instrumented trench, hillslope hydrology, texture-contrast soil, flowdependent monitoring, viticulture

Subsurface lateral flow in both texture-contrast soils and catchments with shallow bedrock is suspected to be a nonpoint source of contamination of watercourses by pesticides used in agriculture. As a case study, the north of the Beaujolais region (eastern France) provides a favorable environment for such contamination due to its agro-pedoclimatic conditions. Environments seen in the Beaujolais region include intense viticulture, permeable and shallow soils, steep hillslopes, and storms that occur during the periods of pesticide application. Watercourse contamination by pesticides has been widely observed in this region, and offsite pesticide transport by subsurface lateral flow is suspected to be involved in diffuse and chronic presence of pesticides in surface water. In order to confirm and quantify the potential role of such processes in pesticide transfer, an automated trench system has been designed. The trench was set up on a steep farmed hillslope in a texture-contrast soil. It was equipped with a tipping bucket flowmeter and an automatic sampler to monitor pesticide concentrations in lateral flow at fine resolution, by means of a flow-dependent sampling strategy. Four pesticides currently used in vine growing were studied to provide a range of mobility properties: one insecticide (chlorpyrifos-methyl) and three fungicides (spiroxamine, tebuconazole, and dimethomorph). With this system, it was possible to study pesticide concentration dynamics in the subsurface lateral flow, generated by substantial rainfall events following pesticide applications. The experimental design ascertained to be suitable method in which to monitor subsurface lateral flow and related transfer of pesticides.

Introduction

The use of pesticides in agriculture has led to extensive contamination of rivers in France (SOeS, 2011), and other countries [START_REF] Schulz | Field studies on exposure, effects, and risk mitigation of aquatic nonpoint-source insecticide pollution: a review[END_REF]. The contaminations threaten fresh water resources and have been shown to directly impact aquatic ecosystem's diversity and biogeochemical functions [START_REF] Schulz | Field studies on exposure, effects, and risk mitigation of aquatic nonpoint-source insecticide pollution: a review[END_REF]Schäfer et al. 2007). Both chronic and acute pesticide exposures have been shown to have impacts [START_REF] Tlili | Impact of chronic and acute pesticide exposures on periphyton communities[END_REF]. In order to reduce this contamination, it is necessary to identify and quantify the processes responsible for the migration of agrochemicals from farmland to surface water. Surface runoff and drainage that occur soon after pesticide application have been identified as major processes resulting in fast and intense contamination peaks in rivers [START_REF] Bach | Input pathways and river load of pesticides in Germany--a national scale modeling assessment[END_REF][START_REF] Louchart | Herbicide transport to surface waters at field and watershed scales in a Mediterranean vineyard area[END_REF][START_REF] Dabrowski | Runoff-related pesticide input into the Lourens River, South Africa: basic data for exposure assessment and risk mitigation at the catchment scale[END_REF][START_REF] Brown | Pesticide transport via sub-surface drains in Europe[END_REF]). The role of subsurface lateral flow (SSLF) in the transport of pesticides into surface water has been much less studied. It is likely to generate chronic transfers in contexts of soils with a shallow impeding layer [START_REF] Brown | Movement of pesticides to surface waters from a heavy clay soil[END_REF][START_REF] Truman | Fenamiphos transport, transformation, and degradation in a highly weathered soil[END_REF][START_REF] Kahl | Micro-trench experiments on interflow and lateral pesticide transport in a sloped soil in northern Thailand[END_REF]. Indeed, when water infiltrating into the soil reaches a less permeable layer, it may accumulate if the incoming flow rate is higher than the permeability of this layer. Depending on the slope, this accumulated water may flow down to be collected in ditches and quickly reach rivers. This lateral diversion of water flow occurring in the soil is known as SSLF, throughflow, and interflow. Different types of soil layers impeding vertical infiltration have been reported: bedrock, fragipan, plough pan, argillic horizon of texture-contrast soils [START_REF] Hardie | Subsurface lateral flow in texture-contrast (duplex) soils and catchments with shallow bedrock[END_REF][START_REF] Tang | A review of rapid transport of pesticides from sloping farmland to surface waters: processes and mitigation strategies[END_REF].

As reviewed by [START_REF] Hardie | Subsurface lateral flow in texture-contrast (duplex) soils and catchments with shallow bedrock[END_REF], SSLF in hillslope hydrology has been investigated in the last decades, by mainly focusing on the understanding of streamflow generation, or agricultural soil management. SSLF transient occurrence in soils, as well as its spatiotemporal variability, requires custom monitoring techniques and protocols for measurement and sampling. Different instrumentations have been described in the literature to measure SSLF. For example, [START_REF] Lehman | Interflow of water and tracer chemical on sloping field plots with exposed seepage faces[END_REF] used a pit to intercept SSLF and a volumetric method with calibrated receptacles to measure discharge in a shallow bedrock context. [START_REF] Anderson | Automatic monitoring of soil-moisture conditions in a hillslope spur and hollow[END_REF] as well as [START_REF] Ritsema | Evaluation of vertical and lateral flow through agricultural loessial hillslopes using a two-dimensional computer simulation model[END_REF] monitored SSLF with tensiometer networks in respectively shallow bedrock and impeding soil layer contexts. Whipkey and Ronald (1965), [START_REF] Webster | The measurement of soil water tension in the field[END_REF][START_REF] Weyman | Measurements of the downslope flow of water in a soil[END_REF] studied SSLF in a shallow bedrock context using multiple instruments: trenches, tensiometers and piezometers, and also used a manual volumetric method to measure SSLF discharge in trenches. [START_REF] Mccord | Lateral moisture flow beneath a sandy hillslope without an apparent impeding layer[END_REF] monitored soil moisture content on a sandy hillslope that was subject to SSLF, but absent of any apparent impeding layer. Among these various methods, only trenches measured direct subsurface lateral water fluxes, and this measurement was mainly achieved by volumetric methods.

Although SSLF in hillslope hydrology is well documented, the subsurface lateral transfer of pesticides has received less attention. It is a concern especially in mountainous and hilly farmlands, because they are usually underlain by shallow bedrock or fragipan. [START_REF] Kahl | Micro-trench experiments on interflow and lateral pesticide transport in a sloped soil in northern Thailand[END_REF] measured pesticide transfers via SSLF in a shallow bedrock context with clay loam to clay soils -characterized by an important amount of macropores -occurring downslope on a litchi orchard in Northern Thailand. In this study, [START_REF] Kahl | Micro-trench experiments on interflow and lateral pesticide transport in a sloped soil in northern Thailand[END_REF] focused on macroporal SSLF which were collected by several wick samplers that were arranged on the upslope side of trenches. Macroporal SSLF discharge and pesticide concentrations were calculated by collecting sampling containers every one to two days. These authors observed simultaneous recoveries of tracers injected at different distances from the trenches during tracing experiments, and concluded that SSLF and related lateral pesticide transfers were caused by long preferential flows. In a highly

Author-produced version of the article published in Environmental Sciences and Pollution Research (2016), vol. 23, n° 1, pp 14-22 The final publication is available at http://link.springer.com/article/10.1007/s11356-015-4917-5 doi:10.1007/s11356-015-4917-5

weathered soil with both a moderate slope (0-8%) and a shallow impeding layer, [START_REF] Truman | Fenamiphos transport, transformation, and degradation in a highly weathered soil[END_REF] measured a large export of Fenamiphos in SSLF using a daily grab sampling strategy. However, these daily approaches could not capture the dynamics at the finer scale of a sub-event, which is necessary to accurately describe flow processes involved in pesticide transfers via SSLF. [START_REF] Brown | Movement of pesticides to surface waters from a heavy clay soil[END_REF] monitored discharge and isoproturon concentration in SSLF with a fine temporal resolution and a flow-dependent strategy in a drained clay soil. However, the specific contribution of SSLF could not be assessed because of its mixing with surface runoff in the experimental setup. [START_REF] Johnson | Preferential flow pathways and their capacity to transport isoproturon in a structured clay soil[END_REF] performed a thirty minute resolution monitoring of SSLF and isoproturon concentrations in a mole-drained clay soil. They showed that even without an apparent impeding layer, a substantial amount of pesticides could be transferred by lateral flow on a moderately sloped (5-10%) heavy clay soil.

Finally, it appears that knowledge about pesticide transfers by lateral flow is still scarce and incomplete. As highlighted by [START_REF] Tang | A review of rapid transport of pesticides from sloping farmland to surface waters: processes and mitigation strategies[END_REF], additional field data in various agro-pedo-climatic contexts are needed to better assess the role of lateral flow in pesticide transfers. No field measurement of pesticide transfers via total SSLF (both macroporal and matrix flows) is available for sloping farmed soils with a shallow impeding layer. This gap is an issue, as such contexts are suspected to be very favorable for these transfers and because they may concern a large portion of agricultural soils [START_REF] Phillips | Geogenesis, pedogenesis, and multiple causality in the formation of texture-contrast soils[END_REF][START_REF] Hardie | Subsurface lateral flow in texture-contrast (duplex) soils and catchments with shallow bedrock[END_REF].

Hence, the objectives of this paper are 1.) to describe an in situ experimental setup designed to monitor and sample the total SSLF to calculate induced pollutant fluxes, 2.) to verify the suitability of this design by obtaining measurements at the fine time scale of a SSLF event, 3.) to highlight the importance of short time step resolution monitoring strategy to study the dynamics of these transfers over a SSLF event. A sloping texture-contrast soil that is subject to subsurface lateral pesticide transfer, on a farmed plot in the Beaujolais region in France, has been used as a case study to reach these objectives.

Material and methods

Site description

The experimental site is located on a vineyard hillslope (longitude 46.16937 N, latitude 4.64828 E) in the Morcille river watershed (8 km²) within the northern Beaujolais region (eastern France) [START_REF] Rabiet | Assessing pesticide concentrations and fluxes in the stream of a small vineyard catchment--effect of sampling frequency[END_REF]). This watershed is representative of the climate, soils and agricultural practices of this region. The climate is temperate with Mediterranean and oceanic influences, having a mean annual rainfall of 859.3 mm. The soils are mainly sandy, with the parent material being granite. More than 70% of the surface of the watershed is dedicated to vine growing, leading to a strong modification of the landscape and a large use of pesticides (annual organic pesticide application means in 2011 were 10.6 kg.ha -1 , 5.3 kg.ha -1 , and 0.14 kg.ha -1 , respectively for fungicides, herbicides, and insecticides active ingredients) during the growing season. The slopes range from 10 to 35% (average 20%) on the 100 m (length) by 60 m (width) of the studied plot (Fig 1). In this watershed, cultivated soils have poor organic matter content of about 2% at the soil surface and less than 1% below ground surface (bgs) (Table 1). An increase in clay content is observed between the topsoil and the subsoil. The topsoil-subsoil transition is generally abrupt and is never more than 10 cm thick. This boundary is located between 30 and 80 cm bgs, depending on the location on the plot. The topsoil and subsoil saturated hydraulic conductivities (K s ) have been determined with tension infiltrometer measurements and completed by the Müntz double-ring approach [START_REF] Smith | Bilan de présence des micropolluants dans les milieux aquatiques continentaux, Période 2007-2009[END_REF] to better characterize macropore contribution (Table 1). K s determined from the tension infiltrometer experiments were calculated by extrapolating the stationary infiltration flux measured at the following successive potentials: -10, -3, and -0.5 cm of the water column [START_REF] Ankeny | Simple field method for determining unsaturated hydraulic conductivity[END_REF][START_REF] Reynolds | Determination of Hydraulic Conductivity Using a Tension Infiltrometer[END_REF]. For the topsoil, K s obtained with double ring infiltration experiments is at least one order of magnitude higher than with a tension infiltrometer. This difference could be explained by the relatively coarse texture of the topsoil; by the presence of small vine roots; and by worm holes (all having been visually confirmed). These measurements show that the subsoil K s can be two orders of magnitude lower than those in the topsoil. Furthermore it must be kept in mind that the observed topsoil K s are underestimations, since they do not take into account the direct effect of vine trunks and main roots on infiltration because of the impossibility to meet good experimental conditions at (or near) them.

Fig

Tension infiltrometer 1.06×10 -6 m.s -1 (6.27×10 -7 -2.22×10 -6 m.s -1 ) n = 6 2.07×10 -7 m.s -1 (4.31×10 -8 -1.04×10 -6 ) n = 7 Mean K s (min -max) Double ring infiltrometer 4.33×10 -5 m.s -1 (7.07×10 -6 -2.31×10 -4 m.s -1 ) n = 4 4.59×10 -7 m.s -1 (3.09×10 -8 -6.81×10 -6 m.s -1 ) n = 2
The studied plot is hydrologically isolated from the surrounding farmed plots at its north, west, and south boundaries. In order to collect the flow seeping from the permeable topsoil, two 1.5 m long zinc gutters with a "V" profile were fitted in the subsoil on the upslope side of the trench. When fitted into the soil, the two gutters were bent and soldered together, to provide an outlet at middle of the trench. Water flows through this outlet, which was a full PVC cylinder fitted hermetically through the gutters. The cylinder was perforated on its upper part (to allow the water to flow inside) and embedded into a V-shaped PVC pipe acting as a siphon to ensure a minimal sampling volume of 0.5 L. Water flowed through a PTFE pipe in a 0.1 L tipping bucket (max discharge: 300 L.h -1 ), which was wired to a CR800 datalogger (Campbell Scientific), and was finally drained downslope through an underground pipe.

On the upslope side of the trench, the soil is propped with fiberglass netting supported by a hard steel grating, while the other sides of the trench were held with wood supports. The materials of the elements that intended to be in contact with the pesticide solution were selected to minimize potential substance adsorption for an acceptable cost.

The whole structure of the trench was kept in place by a wooden frame, and steel plates covered the surface to avoid rainfall entry and to allow tractor passage. Upslope from the trench, a plastic edging impeded the entrance of surface runoff into the trench. Precipitations were measured using a tipping bucket rain gauge wired to the CR800 datalogger. An alternative strategy that isolates a subplot to better control the source area has been studied. This strategy was rejected because it induces flow modifications (due to local soil structure destruction) and it implies cutting vine roots while they are suspected to play a major role in subsurface lateral preferential flows.

Pesticide application and characteristics, sample treatment and analysis

Four pesticides currently used in vine growing were studied: one insecticide (chlorpyrifos-methyl) and three fungicides (spiroxamine, tebuconazole, and dimethomorph). They were applied at different rates and dates (Table 2) by the farmer using a tractor with a canon sprayer. These pesticides were selected because of their range of physico-chemical properties and mobility (Table 3), and also because they are an ecological concern in the Morcille watershed due to their widespread use and detection in the Morcille River. Limit of quantification (µg.L -1 ) 0.004 0.0002 0.0005 0.002 The automatic sampler has been used in a 24-glass bottle configuration and samples were kept cool (below 10°C for a maximum of 24 h) with ice blocks in the sampler. During their transport to the laboratory, samples were kept below 5°C in a dark and cool box. They were filtered using 0.7 µm glass fiber filter (GF/F) with a vacuum filtration glass system. Filtered samples were frozen and stored at -18°C prior to analysis. Sample freezing has been shown not to affect studied pesticide concentrations [START_REF] Margoum | Stabilité des substances organiques dans les échantillons d'eau entre le prélèvement et la prise en charge analytique[END_REF]. The pesticide concentrations were measured by solid phase extraction (Oasis HLB 3 cc 60 mg cartridges, Waters) followed by liquid chromatography coupled with tandem mass spectrometry (SPE-LC-MS/MS). Extractions were performed with an Autotrace 280 Dionex (Thermo Scientific). Cartridges were pre-conditioned using 3 mL of methanol, then with 3 mL of ultrapure water, and finally with 3 mL of acetonitrile. After the percolation of the 250 mL filtered sample, the cartridge was washed using 3 mL of ultrapure water and eluted with 6 mL of acetonitrile. Finally, the collected elution solvent was evaporated under a nitrogen stream for 30 min, and the dried extract was dissolved in 250 µL solution of ultrapure water and acetonitrile

Author-produced version of the article published in Environmental Sciences and Pollution Research (2016), vol. 23, n° 1, pp 14-22 The final publication is available at http://link.springer.com/article/10.1007/s11356-015-4917-5 doi:10.1007/s11356-015-4917-5

(80/20). Chromatographic separation was performed with a Waters HSS T3 column (C18 1.7 µm, 100 × 2.1 mm) at 40°C, injection volume was 10 µL. Acetonitrile and ultrapure water both with 0.1% (v/v) formic acid were used in an analytical gradient of 15 min. Limits of quantification are given in Table 3 for all studied substances, and analytical uncertainties varied from 20 to 60% at the quantification level, depending on the compound. Concentrations of undetected compounds are considered to be zero and concentrations of unquantified detected compounds are considered to be equal to half their respective limit of quantification.

Results

Subsurface lateral flow

Although Between the beginning of March and the end of June 2014, the soil was dry and the cumulative rainfall amounts were lower than during the July events. Moreover, the main features of the previous rainfall events (10, 23, and 28 June) were high rainfall intensities, leading a large part of rainwater to flow over the soil surface rather than to infiltrate.

  Incoming surface runoff is deflected by a pipe network and SSLF coming from the west plot is stopped by a bank, and then evacuated by the surface pipe network. Subsurface water level monitoring (2013-2014) by piezometers located next to the west boundary, demonstrated that no water had crossed it. The north and south boundaries were not physically isolated because the SSLF stream lines obtained from monitoring wells proved to have a steady direction similar to the surface gradient (i.e. west-east), ensuring minimal water exchanges with these adjacent plots. Instrumentation: wells, rain gauge, trench, automatic sampler and sampling strategy In 2012, 18 monitoring wells were set up on the plot (Fig 1) in holes dug down to the boundary between the sandy topsoil and the more clay-rich subsoil. Monitoring wells were 4 cm in diameter and 1 m long full PVC tubes, which were slotted on the 20 cm base of their casing. Annular space was filled with gravel and cemented with bentonite at the soil surface to avoid runoff of flow inside. Six monitoring wells were equipped with water level dataloggers. The other wells were regularly monitored manually with a contact gauge during and after rainfall events. Data from the monitoring wells highlighted the presence of a temporary perched water table, located in the downslope part of the plot (Fig 1). In the upslope area, monitoring wells revealed the existence of a very transient saturation of the topsoil. Both this transient saturation and the presence of the perched water table were suspected to account for subsurface lateral flows (data not shown) generated in the plot and gathering downslope. In March 2014, a trench (Fig 1) was set up to collect the subsurface lateral flow occurring above the impeding subsoil (Fig 2). The trench was 3 m long, 0.6 m wide and 0.8 m deep (the subsoil was found between 50 and 60 cm bgs during the excavation).

Fig 2

 2 Fig 2Inside view of the trench showing the steel grate (top right), the fiber glass netting (right), the zinc gutter (from bottom right to top center), the PVC outlet pipe (center), the V-shaped PVC pipe (center, below the low-point outlet), the sampling pipe (from top to center, entering into the V-shaped PVC pipe through the PVC outlet), and the trench tipping bucket (center left). The upslope side is on the right and water evacuation is below the tipping bucket

  the monitoring started at the beginning of March 2014, no SSLF was recorded before July, due to precipitation during spring of 2014 in the Beaujolais region (Fig 3). Three events were measured in July 2014 (Fig 3). Event A (6-16 July) was triggered by a 29 mm cumulative rainfall in its first part (6-9 July), and by a 22 mm cumulative rainfall in its second part (9-16 July). Event B (21-25 July) was triggered by a 34 mm cumulative rainfall, and event C (30 July-2 August) by a 31 mm cumulative rainfall.

Fig 3 Fig

 3 Fig 3 Cumulative rainfall, SSLF 3-h averaged discharge from mid-May 2014 to the 4 August, 2014, application dates, and A-B-C SSLF event periods

  Author-produced version of the article published in Environmental Sciences and Pollution Research (2016), vol. 23, n° 1, pp 14-22The final publication is available at http://link.springer.com/article/10.1007/s11356-015-4917-5 doi:10.1007/s11356-015-4917-5

  Author-produced version of the article published in Environmental Sciences and Pollution Research (2016), vol. 23, n° 1, pp 14-22The final publication is available at http://link.springer.com/article/10.1007/s11356-015-4917-5 doi:10.1007/s11356-015-4917-5

		Clay	12 %		20 %
	Texture (FAO)	Sandy loam	Loam to clay loam
	Organic matter	< 1 %		< 1 %
	Porosity (Std.), n = 3	0.41 (0.035)		0.36 (0.009)
	Mean K s
	(min -max)
	Table 1 T Texture, organ nic matter cont tent, and poros sity of the tops soil and subso oil at trench lo cation, and sat turated
	conductiv vities (K s ) of th he topsoil and	subsoil at fou ur scattered loc cations in the p plot, estimated d either by dou uble-ring
	infiltration n or tension in nfiltrometers ( (when mention ned, n is the nu umber of mea	surements con nsidered for th he displayed
	mean valu ue)
			Sandy	topsoil		Subsoil
	Textu ure	Sand L Loam	63 25	% %		45 % 35 %
						5

. 1 Plot and perched water table boundaries, surface contour, rain gauge, and experimental trench positions Author-produced version of the article published in Environmental Sciences and Pollution Research (2016), vol. 23, n° 1, pp 14-22 The final publication is available at http://link.springer.com/article/10.1007/s11356-015-4917-5 doi:10.1007/s11356-015-4917-5

Table 2

 2 Studied pesticides application dates and rates

	Application rates (g.ha -1 )

Table 3

 3 Studied substance characteristics and analytical limits of quantifications. K oc , solubility and field DT50 values come from the SIRIS database[START_REF] Ineris | SIRIS-Pesticides, base de données substances actives[END_REF]

		Chlopyrifos-methyl	Spiroxamine	Tebuconazole	Dimethomorph
	K oc (mL.g -1 )	4645	1947	1022	408
	Solubility in water at 20°C (mg.L -1 )	2.74	405	36	290
	Field DT50 (days)	3	24.5	31	42.1

Acknowledgements

The financial support of the Water Agency Rhône Méditerranée & Corse is acknowledged. The authors thank M. Letey and C. Dutremble for their contribution to the field work; P. Bouland, vinegrower, for allowing our research on his farmed plot, and the staff of the "diffuse agricultural pollution" team for their help during the digging of the trench.

in our study due to the presence of many living and degrading vine roots developed horizontally above the impeding layer. Pit studies undertaken after rainfall events revealed that these roots actually conducted lateral flow preferentially, compared to matrix flow. According to Wallach et al. (1992), another phenomenon that could play a role in the early-event pesticide concentration drop is the availability of pesticides for infiltration, which has been verified to be higher at the beginning of an event (initial pesticide concentration being in equilibrium in the soil solution) and to decrease by dilution and according to pesticide kinetics of desorption.

Dynamics and concentrations of the different monitored pesticides are related to their respective adsorption coefficient and half-life in the field. This is consistent with the SSLF tracing experiments performed by [START_REF] Kahl | Micro-trench experiments on interflow and lateral pesticide transport in a sloped soil in northern Thailand[END_REF], who noticed a greater mass recovery for pesticides with lower adsorption coefficients.

Conclusions

High temporal variability of transfers of four pesticides via SSLF in a texture-contrast soil was shown through a suitable experimental setup and associated sampling strategy. The instrumented trench measured SSLF discharge range and variations at fine resolution using a tipping bucket connected to a datalogger. A combination of low adsorptive material collecting SSLF with a V-shaped PVC pipe ensured representative sampling of SSLF. During three SSLF events, flow-dependent sampling revealed the transfer dynamics of one insecticide (chlorpyrifos-methyl), and three fungicides (dimethomorph, spiroxamine, tebuconazole). Monitoring pesticides with various physicochemical properties underlined a concentration hierarchy in accordance with their adsorption coefficients and halflives in the field. During the three events monitored, lateral transfer behavior showed the possible occurrence of preferential flow processes and resulting pesticide transfers, which is in concordance with pre-existing literature on SSLF. Fine-resolution monitoring of more events is necessary to confirm or contradict observed tendencies and to identify more clearly related mechanisms and influence factors.

The experimental setup was used in 2014 and 2015 to better understand lateral pesticide transfers on the study site.

An SSLF tracing experiment would give the opportunity to further explore SSLF pathways and investigate pesticide fluxes and rates of transfer, in relation to their physico-chemical properties. Moreover, a coupled monitoring of subsurface lateral transfers and surface runoff transfers at the event, seasonal, and yearly scales would provide more information on these scarcely studied processes.

Conflict of interest

Author-produced version of the article published in Environmental Sciences and Pollution Research (2016), vol. 23, n° 1, pp 14-22 The final publication is available at http://link.springer.com/article/10.1007/s11356-015-4917-5 doi:10.1007/s11356-015-4917-5

15

The authors declare that they have no conflict of interest.

Author-produced version of the article published in Environmental Sciences and Pollution Research (2016), vol. 23, n° 1, pp 14-22 The final publication is available at http://link.springer.com/article/10.1007/s11356-015-4917-5 doi:10.1007/s11356-015-4917-5


D W A G N E R

N Ranc

Fatigue crack initiation detection by an infrared thermography method

Keywords: infrared pyrometry, number of cycles at crack initiation, temperature recording, very high cycle fatigue

A C T In this paper, the study of the temperature variation during fatigue tests was carried out on different materials (steels and aluminium alloys). Tests were performed at ambient temperature using a piezoelectric fatigue system (20 kHz). The temperature field was measured on the surface of the specimen, by means of an infrared camera.

Just at the beginning of the test, it was observed that the temperature increased, followed by a stabilization which corresponds to the balance between dissipated energy associated with microplasticity and the energy lost by convection and radiation at the specimen surface and by conduction inside the specimen. At the crack initiation, the surface temperature suddenly increases (whatever the localization of the initiation), which allows the determination of the number of cycles at the crack initiation and the number of cycles devoted to the fatigue crack propagation. In the gigacycle fatigue domain, more than 92% of the total life is devoted to the initiation of the crack.

So, the study of the thermal dissipation during the test appears a promising method to improve the understanding of the damage and failure mechanism in fatigue and to determine the number of cycles at initiation.

N O M E N C L A T U R E

b = burger vector E = Young's modulus K eff = effective stress intensity factor K = stress intensity factor σ = stress amplitude σ y = yield stress σ y = cyclic yield stress

I N T R O D U C T I O N

In fatigue tests, according to the strain or stress level, three domains exist. For few number of cycles at fracture (N f < 10 4 cycles), it is the low cycle fatigue domain; for intermediate number of cycles at fracture (10 4 < N f < 10 7 cycles), it is the high cycle fatigue domain (megacycle domain) and for high number of cycles at fracture (N f > 10 7 cycles), it is the very high cycle fatigue domain (gigacycle domain). 1 The latter domain is now investigated with the development of devices (piezoelectric fatigue machines) working at high frequency (20 or 30 kHz), allowing to obtain 10 8 or more cycles in reasonable tests time.

Correspondence: D. Wagner. E-mail: daniele.wagner@u-paris10.fr

These tests have shown that fracture can occur at 10 9 or more cycles which is problematic because many components and structures in several industries require design fatigue life often superior to 10 8 cycles. At a macroscopic scale, according to the fatigue domain, different types of crack initiation occur in cylindrical samples with a polished surface depending on whether it is low cycle, mega or giga-cycle fatigue range. For the smallest number of cycles at failure, the initiation sites are multiple and located on the surface. For intermediate number of cycles at failure, there is only one surface initiation site, whereas in gigacycle fatigue domain, the initiation site may be located in an internal zone or at the surface.

At the microscopic level, Mugrahbi et al. [START_REF] Mughrabi | Specific features and mechanisms of fatigue in the ultrahigh-cycle regime[END_REF] show that the initiation of fatigue crack in the gigacycle fatigue regime can be described in terms of microstructurally irreversible portion of the cumulative cycle strain. It seems that there is no basic difference between fatigue mechanisms in low, mega and giga-cycle fatigue except for the strain localization. For the low cycle fatigue, an important plastic deformation of the specimen bulk governs the initiation. In the megacycle fatigue, the plastic deformation is governed by the plane stress effect and the presence of flaws at the surface which is the critical location of fatigue initiation. However, at lower stress, the plastic deformation in plane stress condition is vanishing and the macroscopic behaviour of the metal is fully elastic except around flaws, metallurgical defects or inclusions. As the probability of occurrence of a flaw is greater in a volume than at a surface, it is normal that the initiation site in gigacycle fatigue will be often in subsurface. In this case, the important parameters are the defect size and the position of the defect. When the initiation is located on an internal defect, the crack propagation leads to a fish-eye propagation around the defect. The geometry of the fish-eye initiation is a circle that collapses on reaching the surface of the specimen. In an internal initiation, it is difficult to determine the number of cycles at initiation. To predict the number of cycles to initiate a fatigue crack from an inclusion, several models are used more or less successfully. 1 In the gigacycle fatigue range, the integration of Paris' law [START_REF] Paris | The relationship of effective stress intensity factor, elastic modulus and Burger's-vector on fatigue crack growth as associated with "Fish Eye" gigacycle fatigue phenomena[END_REF][START_REF] Marines-Garcia | Fatigue crack growth from small to large cracks in gigacycle fatigue with fish-eyes failure[END_REF][START_REF] Marines-Garcia | Fatigue crack growth from small to long cracks in VHCF with surface initiations[END_REF] allows one to predict the number of cycles in the fish-eye growth and obtain the number of cycles at crack initiation. As the crack initiation appears, a short crack around the defect propagates followed by a long crack. In all cases, a cyclic plastic zone around the crack exists. During fatigue cycles, the irreversible part of the rate of the plastic work is responsible for the intrinsic dissipation. When the crack initiation occurs, the plastic deformation at the crack tip increases, and the recording of the surface temperature of the sample during the test allows to follow the crack propagation and to determine the number of cycles at the crack initiation. [START_REF] Ranc | Study of thermal effects associated with crack propagation during very high cycle fatigue[END_REF] In the literature, the infrared pyrometry was already used by many authors, [START_REF] Luong | Fatigue limit evaluation of metals using an infrared thermographic technique[END_REF][START_REF] Liaw | Thermographic detection of fatigue damage of pressure vessel steels at 1,000 Hz and 20 Hz[END_REF][START_REF] Yang | Thermographic investigation of the fatigue behavior of reactor pressure vessel steels[END_REF][START_REF] Fargione | Rapid determination of the fatigue curve by the thermographic method[END_REF][START_REF] Curà | A new iteration method for the thermographic determination of fatigue limit in steels[END_REF][START_REF] Boulanger | Calorimetric analysis of dissipative and thermoelastic effects associated with the fatigue behavior of steels[END_REF][START_REF] Meneghetti | Analysis of the fatigue strength of a stainless steel based on the energy dissipation[END_REF][START_REF] Morabito | Analysis of heat sources accompanying the fatigue of 2024 T3 aluminium alloys[END_REF][START_REF] Beghi | Temperature variations around the crack tip during fracture test[END_REF][START_REF] Harvey | Effects of sequential cyclic and monotonic loadings on damage accumulation in nickel 270[END_REF] but it is the dissipation before the crack initiation which was investigated. Often, the purpose was to get a rapid estimation of the fatigue limit by recording the temperature during the beginning of the test on fatigue machines working at low frequency. To further in number of cycles, the duration of the tests was prohibitive.

In this study, we investigate the temperature evolution measured by an advanced infrared imaging system on various materials between 10 5 and 10 8 cycles on a piezoelectric fatigue system at the frequency of 20 000 Hz. In this case, the duration of the test is compatible with the storage of the data in the camera and allows the recording of the temperature during the crack propagation. 1). The cast aluminium alloys showed a characteristic dentritic microstructure with porosities and the low alloyed steels a martensitic microstructure. The experimental mechanical properties of these alloys and the fatigue test conditions (stress amplitude) of the materials are given in Table 1. These alloys have been tested at 20 kHz using a piezoelectric fatigue system with different R ratio (Table 1). The specimens from AISI 4240 were prepared to be transversal (T) with respect to the rolling direction.

Experimental fatigue tests

From an historical point of view, the first ultrasonic fatigue machine was constructed in 1950 by Mason. [START_REF] Mason | Piezoelectronic Crystals and their Application[END_REF] With the development of computer techniques, C. Bathias and co-workers [START_REF] Wu | Automatic in Ultrasonic Fatigue Machine to Study Low Crack Growth at Room and High Temperature[END_REF][START_REF] Bathias | Determination of Fatigue Limit between 10 5 and 10 9 cycles using an Ultrasonic Fatigue Device[END_REF] have built a fully computer-controlled piezoelectric fatigue machine working at 20 ± 0.5 kHz. Figure 1 shows the principal aspect of this machine in which the following three components are present:

-A power generator that transforms 50 or 60 Hz voltage signal into ultrasonic 20 kHz electrical sinusoidal signals. -A piezoelectric (or magnetostrictive) transducer excited by the power generator, which transforms the electrical signal into longitudinal ultrasonic waves and mechanical vibration of the same frequency. -An ultrasonic horn (in titanium alloy, portion EDC in Fig. 1) that amplifies the vibration coming from the transducer in order to obtain the required strain amplitude in the middle section of the specimen. This amplifier maintains automatically the intrinsic frequency of the mechanical system in the range 19.5-20.5 kHz. Depending on the specimen loading, the horn is designed so that the displacement is amplified between E and C usually three to nine times. It means that the geometry of the horn must be determined. The finite element method may be required when the geometrical shape is complex.

The resonant length of the specimen and concentrator is calculated using Finite Element Method (FEM) by Ansys code. The converter, horn and specimen compose a mechanical vibration system where there are four stress nodes (null stress) and three displacement nodes (null displacement for an intrinsic frequency of 20 kHz) (Fig. 1). During the test, these nodes don't move and remain around at the same position and even damage occurs, because the crack remains small compared to the specimen size.

In our machine, in order to keep the stress constant, during the test, via computer control, there is:

-a linear relation between the electric potential and the dynamic displacement amplitude of the ceramic. -a linear relation between dynamic displacement amplitude of the ceramic and displacement amplitude of the horn. -a linear relation between displacement amplitude of the horn and electric potential.

The dynamic displacement amplitude at the horn extremity and at the specimen extremity is measured by an optic fibre sensor, which permits measurements of the displacement from 1 to 199.9 μm, with a resolution of 0.1 μm. During ultrasonic fatigue tests, the maximum strain values can be measured directly using miniature strain gauges, suitably positioned on the sample surface.

The macroscopic pure homogeneous elastic behaviour during the test (except the case of plastic instabilities dues to phase transformations . . . , the change of yield stress during the test . . .) allows the determination of the dynamic Young's modulus of the tested material. To avoid this use of a load sensor, once the knowledge of the dynamic modulus is achieved, the stress in the mid-section of the specimen is computed from the displacement of the piezo-ceramics system after calibration. When a crack initiates, the dynamic modulus decreases and the device cannot maintain the required frequency (19.5-20.5 kHz), and the test is automatically stopped when the frequency falls below 19.5 kHz. The uncertainty is given by the ratio 0.5/20 which corresponds to minus 2.5% of error.

Temperature measurement

In order to determine the temperature field on the surface of the specimen, a nonintrusive measurement technique by infrared pyrometry was chosen. In the test, an advanced, high-speed and high-sensitivity infrared camera from Cedip Infrared Systems (made up of a matrix of 320 × 240 Mercury Cadmium Telluride detectors) was used to record the temperature changes during ultrasonic fatigue tests. The spectral range of the camera lies between 3.7 and 4.9 μm. This method allows the visualization of temperature cartography with very good time resolution. In this study, the spatial resolution is 0.17 mm and the aperture time can vary between 10 μs (two cycles) and 1500 μs (around 30 cycles). The refresh time of the camera is between 0.83 and 100 Hz. It corresponds respectively to a noise of 0.4 K and 20 mK. The pyrometer was calibrated on a blackbody reference. In pyrometry, the error in temperature is often related to uncertainties on the emissivity of the surface. To eliminate this problem, the specimen surface was covered with a strongly emissive black paint. Figure 2 shows our experimental device.

R E S U L T S

Thermal results

Figures 3a and4a show the temperature variation at the centre of the specimen as a function of the number of cycles for different stress levels in the case of a cast aluminium-silicon alloy (AlSi7Mg06) and a low alloyed steel (AISI 4240). The result for the AISI 5120 is given in Ref. [6]. The global behaviour is the same for all studied alloys.

At the first time, we observe an increase of the temperature just at the beginning of the test. In the same material, the temperature increases depending on the maximum stress amplitude for a given number of cycles. It should be noted that this increase in temperature at the beginning of the test is followed by a stabilization corresponding to a balance between the mechanical energy dissipated and the energy lost by convection and radiation at the specimen surface and by conduction inside the specimen. The temperature variation depends on the material and on stress amplitude level ( σ ) for a same material. Table 2 gives the maximum temperature variation ( T) obtained just before fracture, and N f the number of cycles at fracture. The number of cycles at fracture was between 3 × 10 5 and 8 × 10 7 cycles, which is in the mega and gigacycle fatigue domain. An enlargement of the experimental results at the failure is represented in Figs 3b (corresponding to the test with σ = 52.5 MPa) and 4b. From the thermographies of the specimens recorded by the camera during the fatigue tests, the point with the highest temperature is found. It is at this point that the evolution of the temperature variation versus the number of cycles is drawn. The behaviour is the same for all studied materials. At the end of the test, there is a local and abrupt increase of the temperature.

It is shown that this rapid increase in temperature before failure is related to a very local increase of the temperature (thermographies of Figs 3c and4c). The waypoints marked on the graph in Figs 3b and 4b correspond to the captured pictures in Figs 3c and4c.

Fracture surface analysis

Scanning Electron Microscopy (SEM) was performed for fracture surface analysis in order to determine the origin and localization of the initiation (Table 2). In all materials, the initiation of the crack was initiated on inclusions (steels) or porosities (cast aluminium alloys). Figures 5 and6 show the crack initiation zone on the fracture surface in the cast aluminium alloy AlSi5Cu3Mg and the bearing steel AISI 5120 where the initiation was subsurface. In the steel, the initiation was on a non-metallic inclusion and leads to a typical fish-eye propagation whereas in the cast aluminium alloy, it is on a porosity. In this case, the fracture crack growth is circular around the porosity and produces like a fish eye.

D I S C U S S I O N

H. Mughrabi 2 has studied for many years the microstructural evolution of different materials during fatigue tests. He proposed to distinguish two kinds of materials which correspond at two extreme materials: the type I material is a ductile single-phase material without inclusions and the type II material is high-strength steel containing non-metallic inclusions. For the two types, the S-N curve can be described in a multi-stage W öhler-type S-N plot where there are four ranges characterized by: (I) the LCF (Low Cycle Fatigue domain) Coffin Manson range. (II) the PSB (Permanent Slip Bands) threshold related to the HCF (High Cycle Fatigue or mega cycle domain) plastic strain fatigue limit. (III) the transition from HCF limit to the UHCF range (gigacycle fatigue domain) (IV) the irreversibility threshold corresponding to the UHCF limit.

Whatever the fatigue range, glide of dislocations are observed in the matrix. The microstructural evolution is always the same and leads to local plastic deformation and thus to mechanical energy dissipated into heat. When inclusions (or porosities are present) in the case of type II materials, subsurface fatigue crack initiation at inclusions is dominant in the UHCF range III, 1,2 provided that the inclusion density lies below a critical value. [START_REF] Mughrabi | Specific features and mechanisms of fatigue in the ultrahigh-cycle regime[END_REF] It is now accepted that in this case, 1 the total lifespan is mainly consumed by the process of crack initiation. 4.0x10 [START_REF] Ranc | Study of thermal effects associated with crack propagation during very high cycle fatigue[END_REF] 6.0x10 6 8.0x10 6 1.0x10 7 1.2x10 7 1.4x10 [START_REF] Luong | Fatigue limit evaluation of metals using an infrared thermographic technique[END_REF] Cycle number

(4) (3)
(2)

(1) When the crack is initiated, two plastic zones exist at the root of the crack: [START_REF] Bathias | Fatigue des matériaux et des structures[END_REF] the monotonic plastic zone r y = K 2 /6πσ 2 y and the reverse (cyclic) plastic zone r R = K 2 /24πσ 2 y in which the plastic deformation is more in-tense (with σ y , the yield stress, σ y , cyclic yield stress and K, the stress intensity factor). When the crack initiates from a defect, such as inclusions or pores, it is said that probably a relation must exist between the fatigue limit and the crack growth threshold; 1,2,21 and the propagation of the crack in the fish eye can be modelled by the Paris-Hertzberg law [START_REF] Paris | The relationship of effective stress intensity factor, elastic modulus and Burger's-vector on fatigue crack growth as associated with "Fish Eye" gigacycle fatigue phenomena[END_REF] da/dN = b( K eff /Eb 1/2 ) 3 with E Young's modulus, b the Burger vector, K eff effective stress intensity factor.

In the first approach, the integration of this law without transition short crack-long crack from a o to a the radius of the fish eye (Fig. 7) conducts [START_REF] Marines-Garcia | Fatigue crack growth from small to large cracks in gigacycle fatigue with fish-eyes failure[END_REF] to N prop = πE 2 /2 σ 2 with N prop number of cycles for the propagation of the fish eye, σ the experimental nominal stress and a o = a int /0.94 (a int , radius of the defect: inclusion, . . .).

In our experiments, the temperature fields on the specimen surface are measured by infrared thermography. Just before the fracture, thermographies show a significant and local increase in the temperature. In order to better understand these thermal effects and to make a connection with the initiation and the propagation of the fatigue crack, a thermo-mechanical model was developed for the bearing steel. [START_REF] Ranc | Study of thermal effects associated with crack propagation during very high cycle fatigue[END_REF] From the Paris-Hertzberg law, the evolution of crack length versus time can be obtained: a(t) = a 0 /(1-t/t c ) 2 with t c = 2a 0 /bf (with f the loading frequency). Besides, the energy dissipated at each cycle per unit of crack length noted ξ is proportional to the surface of the reverse plastic zone r R : ξ = ηr 2 R where η is a coefficient depending only on the material properties. By replacing the expression of r R , K and a(t), the dissipated power per unit length of crack P (P = ξ f ) can be calculated. The fatigue crack is modelled by a circular ring heat source located in the reverse plastic zone at the crack tip whose radius increases with time. From the heat transfer equation and considering adiabatic conditions on the surface of the specimen, and normalizing dimensions in the problem, the evolution of the non-dimensional temperature during the fatigue testing can be obtained. The numerical resolution of the thermal problem allows the determination of the evolution of the temperature field with time in the specimen. The comparison between test and model shows a good correlation (Fig. 8). In particular, the propagation duration of the crack is well estimated by the model.

So, the increase in temperature at the end of the experimental test corresponds to the fracture initiation, and the number of cycles at initiation can be determined accurately. The number of cycles at the crack initiation (Fig. 8) corresponds to a temperature increase of 0.07 • C (noise of camera after filtration of temperature signal for an aperture time of 10 μs).

For our materials, the results of the number of cycles at initiation N i and the ratio of N i over N f (number of cycles at fracture) is given in Table 3. These results confirm that in the gigacycle domain, more than 92% of the total life is devoted to the initiation of the crack. Figure 9 shows the relation between N i /N f versus N f , and the portion of the total life devoted to the initiation increases with the number of cycles to failure. The greater the number of cycles to fracture, the larger the part of total life for initiation of the crack. Consequently, the number of cycles for the propagation of the fish eye is weak and the comparison (Table 3) of the Paris model with our experiments is good.

C O N C L U S I O N

The final goal of this research is to understand the damage mechanism inside the metal, around defects, when the fatigue life reaches 10 9 . Of course, it is experimentally difficult to catch observations at 10 9 cycles. In order to approach this objective, the experimental study has been carried out for a fatigue range between 10 6 and 10 9 . Further to this, an analytical model has been derived from the Paris law.

A temperature measurement with an infrared camera was performed during fatigue tests on a piezoelectric fatigue machine on cast aluminium-silicon alloys and three low alloyed steels. Different stress levels were applied. At the beginning of the test, the temperature rapidly increases, followed by a stabilization. The higher the applied stress, the larger the increase in temperature and the more significant the energy dissipated. At the end of the test, the temperature increases very rapidly until the fracture. The numerical resolution of the thermal problem allows the determination of the evolution of the temperature field with time in the specimen. The comparison between test and model shows a good correlation, and the sudden increase of the temperature allows to determine the cycle number at crack initiation and it proves that more than 92% of the total life is devoted to the initiation of the crack. The portion of the total life devoted to the initiation increases with the number of cycles to failure. When the initiation is subsurface and leads to a fish-eye formation, the simplified Paris model allows a prediction of the number of cycles to fish eye formation. To conclude, it is proven that the key damage in VHCF is initiation and not slow crack propagation.

Fig. 1

 1 Fig. 1 Scheme of the piezoelectric fatigue machine.

Fig. 2

 2 Fig. 2 Thermal measurement device.

FractureFig. 3

 3 Fig. 3 Evolution of the temperature variation for the AlSi7 Mg06 alloy.

4

  Evolution of the temperature variation during the test for AISI 4240 steel (T = 610 • C, σ = 345 MPa).

Fig. 5

 5 Fig. 5 (a) Crack initiation zone in the AlSi5Cu3 Mg alloy. (b) Detail of the crack initiation zone in the AlSi5Cu3 Mg alloy.

Fig. 6 Fig. 7

 67 Fig. 6 (a) Fracture surface of the AISI 5120 steel (with fish eye). (b) Non-metallic inclusion in the fish eye in the AISI 5120 steel 6 .

Fig. 8

 8 Fig.8Comparison between model and experiment for crack initiation detection (AISI 5120)[START_REF] Ranc | Study of thermal effects associated with crack propagation during very high cycle fatigue[END_REF] .

Fig. 9 N

 9 Fig. 9 N i /N f evolution versus N f for the studied materials.

Table 1

 1 Mechanical properties and fatigue test conditions

		UTS		Stress
	Alloy	(MPa)	R ratio	amplitude (MPa)
	AlSi7Mg06	344	0.01	52.5
				50
				45
	AlSi5Cu3Mg	222	-1	5 5
	AISI 4240 (T = 430 • C)	1508	-1	460
	AISI 4240 (T = 610 • C)	1035	-1	345
	AISI 5120	2000	-1	335
	E X P E R I M E N T A L C O N D I T I O N S
	Materials
	Two cast aluminium-silicon alloys (an AlSi7Mg06 and an
	AlSi5Cu3Mg) and three low alloyed steels (two 42CrMo4
	or AISI 4240 and a bearing steel AISI 5120) were studied.
	The aluminium alloys differed by the composition (silicon
	amount . . .), and the two AISI 4240 steels by the heat
	treatment. Both the low alloyed AISI 4240 steels were
	austenized at 830 • C during 30 min, oil quenched but one
	was tempered during 1 h at 430 • C and the second one at
	610 • C.
	The differences result in various Yield Stress and Ul-
	timate Tensile Stress (Table

Table 2

 2 Results of fatigue tests

		σ	T		Locali-
	Alloy	(MPa) ( • C) N f	zation
	AlSi7Mg06	52.5	5.2 8.6932 × 10 6 Surface
		50	3.8 8.3276 × 10 6
		45	1.3 1.25132 × 10 7
	AlSi5Cu3Mg	50	14	8.11392 × 10 7 Subsurface
	AISI 4240 (T = 430 • C) 460	28	3.3094 × 10 5 Surface
	AISI 4240 (T = 610 • C) 345	80	1.76197 × 10 7 Surface
	AISI 5120	335	230 8.3700 × 10 7 Subsurface

Table 3

 3 Number of cycles at initiation and comparison with Paris' model for fish-eye propagation

	Alloy	N i	N f	N i /N f	Localization	N f -N i	N prop
	AlSi7Mg06	8.1010 × 10 6	8.6932 × 10 6	0.932	Surface	5.92 × 10 5	-
		8.0826 × 10 6	8.3276 × 10 6	0.971	Surface	2.45 × 10 5	-
		1.15310 × 10 7	1.25132 × 10 7	0.922	Surface	9.82 × 10 5	-
	AlSi5Cu3Mg	7.89184 × 10 7	8.11392 × 10 7	0.973	Fish eye	2.23 × 10 6	2.73 × 10 6
	AISI 4240 (T = 430 • C)	3.05 × 10 5	3.3094 × 10 5	0.922	Surface	2.6 × 10 4	-
	AISI 4240 (T = 610 • C)	1.76155 × 10 7	1.76197 × 10 7	0.998	Surface	4.2 × 10 3	-
	AISI 5120	8.3620 × 10 7	8.3700 × 10 7	0.999	Fish eye	0.8 × 10 5	6.16 × 10 5
	E AlSi5Cu3Mg = 72560 MPa, E AISI5120 = 210000 MPa
			Evolution Ni/Nf en fct de Nf
		1,1
		1
		0,9
	Ni/Nf	0,8
		0,7
		0,6
		0,5
		1,E+05	1,E+06	1,E+07	1,E+08	1,E+09
				Nf