T. Chartier. T rouxel Fig 8. SEM micrographs of the fracture surfaces from CN specimens of (a)A/A and(b)AZIO/AZIO Highcr magnification vicws of the fracture path(c)A/A and (d)AZIO/AZIO A cross-head speed of 0.01 mm min was Youngs moduli of A and AZ10 in order to com selected to compare the materials of the second pare with the effective moduli of the diferent series by Chevron-Notched (CN)tests(Fig. 7b). composites(Fig. 10). It is observed that Youngs Under the chosen testing condition, the CN speci- modulus values measured parallel to the layering mens of this series failed in purely brittle mode (which should be close to the Voigt expression) (unstable crack propagation) with maximum load agree well with the prediction, whereas values peaks slightly higher than in the case of AZ10. In measured normal to the layering (corresponding addition, the incidence of the layer orientation to the reuss model) fall far outside the bounds with respect to the propagation direction was This statement confirms the presence of an interfa investigated on the 2/2/2/10/2/2/2 grade, by testing cial effect, affecting the elastic properties, i.e. the a V-notched CN specimen oriented in such a way ultrasonic wave propagation in the present case that the crack propagated parallel to the layer Possible sources for this effect include composition interfaces. This figure shows that the reinforcement heteroge es within the layer surfaces(chemical is much more significant normal to the layering alteration)and changes in the interatomic bond yalues for Kic are summarized in Fig 9, together ing distances(stored elastic energy or thermally ith the values previously obtained on the same induced residual stresses materials by the Single Edge Notched Bear (SENB) method. CN tests give lower values fo Kc than SENB tests as is expected. This is tenta- 4 Conclusion tively attributed to a sharper crack tip in the first case. Poissons ratios being identical for all stud- Laminar composites proved to have promising d materials, the theoretical prediction of Voigt mechanical properties with fracture strength and and Reuss(Hill bounds) were drawn from the toughness up to 1 5 to 2 times higher for the306 T. Chartier, T. ROLLWI 1 mm (4 20 urn (b) (4 Fig. 8. SEM micrographs of the fracture surfaces from CN specimens of (a) A/A and (b) AZlOiAZlO. Higher magnification views of the fracture path (c) A/A and (d) AZlOiAZlO. A cross-head speed of 0.01 mm min ’ was selected to compare the materials of the second series by Chevron-Notched (CN) tests (Fig. 7b). Under the chosen testing condition, the CN speci￾mens of this series failed in purely brittle mode (unstable crack propagation) with maximum load peaks slightly higher than in the case of AZlO. In addition, the incidence of the layer orientation with respect to the propagation direction was investigated on the 2/2/2/10/2/2/2 grade, by testing a V-notched CN specimen oriented in such a way that the crack propagated parallel to the layer interfaces. This figure shows that the reinforcement is much more significant normal to the layering. Values for K,, are summarized in Fig. 9, together with the values previously obtained on the same materials by the Single Edge Notched Beam (SENB) method.’ CN tests give lower values for K,, than SENB tests as is expected. This is tenta￾tively attributed to a sharper crack tip in the first case. Poisson’s ratios being identical for all stud￾ied materials, the theoretical prediction of Voigt and Reuss (Hill bounds) were drawn from the Young’s moduli of A and AZ10 in order to com￾pare with the effective moduli of the different composites (Fig. 10). It is observed that Young’s modulus values measured parallel to the layering (which should be close to the Voigt expression) agree well with the prediction, whereas values measured normal to the layering (corresponding to the Reuss model) fall far outside the bounds. This statement confirms the presence of an interfa￾cial effect, affecting the elastic properties, i.e. the ultrasonic wave propagation in the present case. Possible sources for this effect include composition heterogeneities within the layer surfaces (chemical alteration) and changes in the interatomic bond￾ing distances (stored elastic energy or thermally induced residual stresses). 4 Conclusion Laminar composites proved to have promising mechanical properties with fracture strength and toughness up to 1.5 to 2 times higher for the