S. Baste / Composites Science and Technology 61(2001)2285-2297 2. Experimental velocity of the pulses is done by a signal processing method using Hilbert transform [9] An ultrasonic device [7 and an extensometer used To fully describe the elastic behaviour of an ortho imultaneously provide a useful method for carrying out tropic material, the nine elastic constants Cii are identi a strain partition under load [8]. Applied to CMCs, the fied by measuring the phase velocities of bulk waves contribution of the damage mechanisms to their highly transiting in two accessible principal planes [planes( non linear behaviour can be evaluated 2)and(1, 3), Fig. 3] and in a non principal plane [plane (1, 45] described by the bisectrix of axis 2 and 3)of the 2.. Ultrasonic method sample[10]. The identification in plane(1, 2)gives four elastic constants: Cll, C22, C66 and Cl and three others The ultrasonic device consists in an immersion tank are obtained in plane(1, 3): C33, Css and C13. The two associated to a tensile machine [7]. It allows to study the remaining coefficients C23 and C44 are identified by complete stifness tensor variation under load thus it is propagation in the non-principal plane(1, 45). The possible to know which coefficients are affected during confidence interval associated to each identified con- the damage process. stant is then estimated by a statistical analysis [11] Wave speed measurements are performed by using ultrasonic pulses which are refracted through the sample 2. 2. Under load strain partition immersed in water. The measurement of the phase measure the plastic strains. In ticity of metals, dislocations are the main faws lead to relative discomposition th unloading, and which found expression, from a macro scopic point of view, in permanent strains. The intro- duction of the term "inelastic"is lead by a possible part- recovery of the strains beyond the yield point. It is all the more justified for brittle matrix composites. The matrix microcracking does not modify a pre-existing elastoplastic behaviour but creates residual strains that could not exist otherwise. the inelastic strains in Cmcs are the macroscopic indication of the transverse crack matrix sliding [12]. This sliding direction of the load and thus partly reversible. A Fig. 2. Consequence of matrix cracking and fibre-matrix sliding on ultrasonic device used to investigate the stress-induced development of the stress/strain behaviour the damage of CMcs2. Experimental An ultrasonic device [7] and an extensometer used simultaneously provide a useful method for carrying out a strain partition under load [8]. Applied to CMCs, the contribution of the damage mechanisms to their highly non linear behaviour can be evaluated. 2.1. Ultrasonic method The ultrasonic device consists in an immersion tank associated to a tensile machine [7]. It allows to study the complete stiffness tensor variation under load thus it is possible to know which coefficients are affected during the damage process. Wave speed measurements are performed by using ultrasonic pulses which are refracted through the sample immersed in water. The measurement of the phase velocity of the pulses is done by a signal processing method using Hilbert transform [9]. To fully describe the elastic behaviour of an ortho￾tropic material, the nine elastic constants Cij are identi- fied by measuring the phase velocities of bulk waves transiting in two accessible principal planes [planes (1, 2) and (1, 3), Fig. 3] and in a non principal plane [plane (1, 45
] described by the bisectrix of axis 2 and 3) of the sample[10]. The identification in plane (1, 2) gives four elastic constants; C11, C22, C66 and C12 and three others are obtained in plane (1, 3): C33, C55 and C13. The two remaining coefficients C23 and C44 are identified by propagation in the non-principal plane (1, 45
). The confidence interval associated to each identified con￾stant is then estimated by a statistical analysis [11]. 2.2. Under load strain partition Unloading–reloading cycles are usually carried out to measure the plastic strains. In the plasticity of metals, dislocations are the main flaws. They lead to relative discompositions that remain stable after complete unloading, and which found expression, from a macro￾scopic point of view, in permanent strains. The intro￾duction of the term ‘‘inelastic’’ is lead by a possible part￾recovery of the strains beyond the yield point. It is all the more justified for brittle matrix composites. The matrix microcracking does not modify a pre-existing elastoplastic behaviour but creates residual strains that could not exist otherwise. The inelastic strains in CMCs are the macroscopic indication of the transverse crack opening displacement (COD) due to interfacial fibre￾matrix sliding [12]. This sliding is subordinate to the direction of the load and thus partly reversible. A Fig. 1. Damage mechanisms in CMCs. Fig. 2. Consequence of matrix cracking and fibre-matrix sliding on the stress/strain behaviour. Fig. 3. Sample instrumented for strain partition under load in the ultrasonic device used to investigate the stress-induced development of the damage of CMCs. 2286 S. Baste / Composites Science and Technology 61 (2001) 2285–2297