domain: spi phys chim

text:
F Coussa

T Fourest

TWENTY-SECOND INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS (ICCM22) EXPERIMENTAL CHARACTERIZATION OF STRAIN-RATE SENSITIVITY ON FAILURE PROPERTIES OF CARBON/EPOXY COMPOSITE

Keywords: laminate composites, dynamic, off-axis failure, strain-rate effects

Due to their interesting strength-to-weight ratio, composites materials are now widely used in several fields of transportation industry. The strain rates of interest in aeronautic applications typically range from creep loading at = 10 -6 s -1 to dynamic condition at = 10 5 s -1 . When dealing with a crash situation the rates of interest are usually reduced from quasi-static at = 10 -4 s -1 to = 10 3 s -1 . The use of composite materials for such dynamically loaded structures requires the knowledge and understanding of composite materials response to high strain-rates. Therefore, reliable design of the composite components requires the detailed response of the composite material at high strain-rates to be known. This research focuses on the experimental investigation of a unidirectional carbon/epoxy laminate (T700/M21), under static and dynamic loadings, in order to study the strain-rate sensitivity of failure properties for combined transverse and inplane shear loadings in tension as well as in compression. The approach which is considered in this study relies on the use of off-axis specimens. Servo-hydraulic jack are classically used to cover strain rates of interest. The main advantage of offaxis specimens lies in the apparent simplicity of tests that permit to induce coupled transverse/shear stress states by using a single actuator and simple specimen geometries. However, their drawback is that they could induce unwanted transverse load on the dynamic device, which are usually not dimensioned to sustain high transverse loads.

Concerning tensile tests, oblique tabs will be used in this study to reduce transverse loads with off-axis specimens. This solution has already been mentioned in others studies [START_REF] Pierron | The 10 off-axis tensile test: a critical approach[END_REF][START_REF] Sun | An oblique end-tab design for testing off-axis composite specimens[END_REF] where an homogenous stress state in gauge section had been ensured for the characterization of shear modulus on 10° off-axis specimens. Few authors have used offaxis specimens in tension to investigate the failure of composite materials. The tests conducted in [START_REF] Leong | Interlaminar/interfiber failure of unidirectional glass fiber reinforced composites used for wind turbine blades[END_REF] using dog-bone shape specimens, for which the failure was not contained in the reduced section for small off-axis angles, may introduce a lack of consistency for the computation of the failure stress. Rectangular shape specimens are therefore preferred in this study. In order to fully characterize the failure envelop of the studied material, combined transverse/shear specimens with fibre orientation angles Ɵ = 15°, 30°, 45°, 60°, 75° and transverse specimens Ɵ = 90° are considered for the experimental campaign. A numerical analysis has been performed using the Zebulon FE software to define the angle of oblique tabs ϕ with respect to each Ɵ° orientation. Additionally, keeping in mind that the smaller the specimen length, the higher the reachable strain-rate, three specimen lengths (150 mm, 75 mm and 40 mm) have been tested in quasistatic regime in order to evaluate the influence of geometry reduction on the recorded macroscopic failure stress. No difference and low discrepancy between each specimen were reported. At the end, the smaller geometry was selected and the tensile tests have been conducted under three different loading speeds (Fig. 1). It was noticed that for most specimens the failure does not occur at the junction with the tabs which means that stress concentration at the junction has been successfully avoided by using oblique tabs.

Off-axis compression tests at high stroke-rates have been performed by several authors to characterize the compressive strength of composite materials [START_REF] Koerber | High strain rate characterisation of unidirectional carbon-epoxy im7-8552 in transverse compression and in-plane shear using digital image correlation[END_REF][START_REF] Ninan | Use of split hopkinson pressure bar for testing off-axis composites[END_REF][START_REF] Tsai | Dynamic compressive strengths of polymeric composites[END_REF]. While relatively long specimens are suitable and stable for use in tensile tests, shorter and thicker specimens are more reliable for compressive tests in order to avoid buckling phenomena. Nevertheless, it leads to additional difficulties such as high transverse loads in off-axis tests, high load levels relatively with the use of servo-hydraulic jack and potential risks of failure of the specimen at the boundary conditions. In [START_REF] Bing | Specimen size effects in offaxis compression tests of fiber composites[END_REF], the effect of the size of the block specimen on the measured strength was studied. They concluded that the specimen endsurface friction can produce specimen size effect in off-axis compression tests due to a non-uniform state of stress in the specimen. Consequently, the approach in this study is divided in two steps. On one hand, an experimental comparison of two offaxis compression specimen geometries has been performed. On the other hand, a new and dedicated set-up for compression tests of off-axis specimens suitable for dynamic loadings has been developed. Firstly, small block specimens with a rectangular section of 12x12 mm², corresponding to the specimen size usually used for Hopkinson bars tests, were compared with specimens with a rectangular section of 8x8 mm². Quasi-static compression tests have been conducted with both specimen geometries for the same off-axis angles previously mentioned for tensile tests. From the results of these tests, it has been determined that the smaller section specimen is compatible with the maximum load capacity of the hydraulic jack and does not lead to inconsistencies on the behavior and the failure stress (except for Ɵ° = 0° orientation). Moreover, specific test set-up has been design for dynamic tests. In order to reduce transverse loads and to guarantee a uniform state of stress during dynamic off-axis experiments, the solution held in this study relies on the use of an original end-loaded compression technique. The upper end-surface of specimen is loaded through a punctual-contact by means of an intermediate hemispherical component which is solidarized with the specimen. This prevents of any friction and edge effect and guarantees a uniaxial load until failure. The lower end-surface of specimen is seated on a radial ball-bearing and allows transverse displacements. With this device the transverse load is successfully avoided. This technical solution has been validated for quasi-static conditions and an experimental campaign from quasi-static to dynamic strain-rates using servo-hydraulic jack is under progress.

Fig. 1 .

 1 Fig.1. Macroscopic failure stress and failure envelop in off-axis tension tests for three stroke-rates.


text:
Selection of alternative material for High velocity impact:

Numerical and experimental validation of performance and weight saving R. Ortiz 1 , Y. Toso 2 , G.Portemont 1 , N. Toso-Pentecôte 2 , D. Joly 1 1 Onera -The French Aerospace Lab F -59045, Lille, France e-mail : roland.ortiz@onera.fr, web page: http://www.onera.fr In the scope of the future certification of engine powered aircraft, aircraft manufacturers should address and assume the event of engine burst, including the release/loss of fragments. To minimize the size of reinforcement area on the aircraft, ONERA and DLR have proposed to determine ballistic limit of potential alternative materials [1, 2, 3, and 4]. The present document is the state of the common ONERA/DLR work performed in the European Project CLEANSKY 2, which aims to develop numerical tools and methodology for high velocity impacts using the explicit research code Europlexus [START_REF]EUROPLEXUS User's Manual[END_REF] as well as the commercial code Abaqus/Explicit [6]. For that purpose, a comparison with experimental high velocity impact on different panels made with promising material has been performed.

The different tests used in this study in order to validate the simulations are: The T700/M21 composite materials, manufactured by ONERA, have been chosen as reference for this study. The aluminium plates have been tested at DLR.

For the T700/M21 composite, the material characterization has been performed by ONERA in a former study [6] while the HVI tests have been conducted at DLR.

ONERA has also performed experimental HVI tests with triaxialy braided material procured by DLR.

Hybrid materials has been also tested by ONERA (Ultra High Molecular Weight Polyethylene, Dyneema, see figure 1 -numerical simulation of HVI on hybrid materials). Thus new potential material could be necessary to define reinforcement concepts. A proposition of different selection of material has been proposed. Some of them will be tested. This study shows relevant activities for TRL 2 evidences.

2 German

 2 Aerospace Center (DLR) Pfaffenwaldring 38-40, D -70569 Stuttgart

-

  Tests with steel balls on aluminium target (3 mm and/or 6 mm thick plates) -Tests with steel balls on T700/M21 composite target (3 mm thick plates) -Tests with steel balls on triaxialy braided and hybrid materials (3 mm thick plates)

Figure 1 :

 1 Figure 1: Comparison between the numerical simulation performed on T700/M21 and hybrid materials for a HVI performed with a steel ball (with Eurolpexus Code).

Figure 2 :

 2 Figure 2: Europlexus and Abaqus results vs C-Scans -Comparison between experimental and numerical damage extension for a steel ball impact on T700/M21 panel.