18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS THE EFFECT OF THERMO-OXIDATION ON MATRIX CRACKING OF CROSS-PLY [0/90] S COMPOSITE LAMINATES D.Q. Vu 1,* , M. Gigliotti 1 , M.C. Lafarie-Frenot 1 1 Institut P' - Département Physique et Mécanique des Matériaux, CNRS - ENSMA - Université de Poitiers - UPR 3346, France *Corresponding Author (dinhquy.vu@lmpm.ensma.fr) Keywords : thermo-oxidation, matrix cracking, cross-ply sample 1 General Introduction damage and oxygen diffusion can then accelerate the Composite materials have been increasingly kinetics of degradation and affect the durability of integrated within aerospace structures as they the laminate. possess very interesting specific mechanical In this article the effects of thermo-oxidation on properties (strength and stiffness) and very good matrix cracking of cross-ply [0/90 3 /0] IM7/977-2 fatigue resistance. In the next future composite composite specimens (size: 180 mm x 20 mm x 1.25 materials are expected to be employed in structural mm) were investigated. Samples were first subjected parts subjected to rather severe thermal conditions. to isothermal aging tests at 150°C under 1.7 bar of For instance, composite structures for aero – engines oxygen and then to quasi-static tensile tests up to can be exposed to oxidative environments at high rupture. The sample polished edges were observed at temperatures (>120°C); under such conditions regular intervals during the tensile test by a traveling oxidation reaction/diffusion phenomena take place optical microscope mounted on the tensile machine within the polymer material and threaten the in order to count the number of matrix transverse integrity of the structure. cracks appearing in the 90° layers. Thermo-oxidation of organic matrix composites has Replicas of the sample surface edges were also observed by Scanning Electron Microscopy (SEM) been the subject of specific studies over the past ten to establish a possible link between thermo- years [1-4]: the coupled oxygen diffusion/reaction oxidation damage at the microscopic scale (fiber- phenomenon leads, on one hand, to matrix chemical matrix debonding) and the onset of transverse matrix shrinkage strains due to the departure of volatile cracks in the 90° layers. species and, on the other hand, to the formation of 2 Experimental setup an oxidized layer in which the mechanical properties 2.1 Sample preparation of the material are degraded. Chemical shrinkage The samples (dimensions: 180 mm x 20 mm x 1.25 sums up to existent hydro, thermal, mechanical mm) were cut out from a [0/90 3 /0] IM7/977-2 strains and can contribute significantly to the composite plate and polished automatically by an development of internal stresses. At the microscopic optimized procedure, set up to minimize the impact scale (the scale of the fibre), the stress concentration of polishing (see Fig. 1). The material elastic existing close to the fiber-matrix interface can properties of the unidirectional laminate are given in induce microscopic damage, such as fiber-matrix Tab. 1. The samples were then subjected to isothermal aging tests at 150°C under 1.7 bars debonding. At the lamina/laminate scale, internal oxygen pressure in a climatic chamber [3] before and applied stresses may give rise to damage under undergoing quasi-static tensile tests. the form of matrix cracking and delamination: the development of such damage can be affected by thermo-oxidation; moreover the interaction between
matrix cracks under mechanical or thermal loading. Z 0° 0° 90° 90° Y Based on the experimental results, researchers have 0° 0° X 20 mm F F proposed models to predict these phenomena. The 1,25 mm mechanical analysis of damage evolution in the 180 mm composite consists of two main steps: 40 mm observation part 40 mm - The first step aims at describing the stress field in the [0/90] s laminate in the presence of damage. Fig. 1: Sample geometry and dimensions. - The second step involves using a failure criterion 20 o C Properties to predict damage evolution. In the literature, different methods for calculating E 11 157 GPa the stress field in [0/90] s cross-ply laminates have E 22 8,5 GPa been presented. Among them, the “shear-lag” model ν 12 0,29 is one of the most used. Nairn and Mendels [5] have G 12 5 GPa recently classified the “shear-lag”-based models by α 1 0,23 × 10 -6 °C developing elastic calculations of general multi-layer α 2 30 × 10 -6 °C systems in which matrix cracking in [0/90] s cross- ply laminates is a particular case. The authors show Tab. 1. Properties of unidirectional laminate. that, whatever the stress distributions along the 2.2 Testing machine thickness, all the approaches lead to a fundamental “shear-lag” equation: Tensile tests were carried out on a hydraulic Instron 4505 machine (maximal load 100 kN). τ ∗ d x (1) 2 ( ) ∗ xz − α τ x = 2 ( ) 0 xz dx 2 − ≤ ≤ + ∗ L x L ( Fig. 3 ) ; τ ( x is shear stress at layers ) xz long interface ; « α » is a « shear-lag » parameter: distance QUESTAR (2) h k h k microscope 1 1 R L α = + + Camera 2 1 2 h E h E G G 1 22 2 11 23 13 Fig. 2: “In situ” observation setup for crack counting E 11 , E 22 , G 23 and G 13 are the elastic mechanical during tensile test. properties of the unidirectional laminate; k R and k L are constants which depend on the distribution of In situ observations of polished samples edges were τ shear stress along the thickness. performed during the tensile test without xz dismounting the sample by means of a video camera For a material similar to ours, by using k R =0.3300 and a long distance Questar microscope (Fig. 2). The and k L =0.3070, Nairn and Mendels [5] showed that camera transfers the images to a computer screen the stresses calculated by “shear-lag” models are and transverse cracks can be counted at each stage of very close to finite element results. Using an the loading. approach proposed by Lee and Daniel [6], Berthelot et al. [7] found a good agreement with the finite 3 Numerical simulation of matrix cracking in [0/90] s cross-ply composite laminate under static element calculations for k R =1/3 and k L =1/3. The tensile test study by Nairn and Mendels shows the high efficiency and simplicity of “shear-lag” models. Matrix cracking in off-axis plies is usually the first In the present paper, we will take k R =1/3 and k L =1/3 type of damage appearing in composite laminates. A to calculate the stress field. lot of experimental work has been carried out on A failure criterion is necessary to predict damage [0/90] s cross-ply laminates in order to identify and evolution. In this study, we employ an energetic characterize the physical and geometrical parameters approach: we assume that a crack forms when the governing the onset, propagation and saturation of energy release rate due to the creation of new crack 2
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