R. Bermejo et aL Composites Science and Technology 67(2007)1930-1938 Indentations 200 a ATL 12 Displacement(mm) ATZ Engineering stress vs displacement for the indented laminates and monolith showing the fracture steps in the laminates. The indenta- position, load forces and layer distribution in the bending test an values for the characteristic points in the two multilayers and in the b monolith recorded for comparative purposes Material Stress(MPa) First step Second step Fractur TZ monolith Crack bifurcation AMZ thin layer, where they are arrested due to the high compressive stresses. This behaviour has been observed by other authors for other multilayered systems, showing :150 the effectiveness of the internal compressive layers in hin dering the crack propagation [17,37]. As it can be inferred from Fig. 7, the stress level for the first step is around Fig. 8. Optical micrograph of (a) system B and (b)system C laminates 40 MPa for the laminate B and 85 MPa for the laminate after the bending test, showing the crack bifurcation in the first which are coincident with the failure stress measured compressive layer for the ATZ monolith, i.e. 140 MPa, minus the correspond ng tensile stress in the AtZ layers calculated for each sys- regard, it should be emphasised that crack bifurcation was m,i.e. 102 MPa and 60 MPa respectively. In addition, discerned at fracture for both layered systems. Such a find unstable failure does not follow this first step of crack ing could be expected for samples type B, since edge crack growth in these materials. As a matter of fact, after this ing had been previously developed in this layered system first stepwith three"pop-in"events for each laminate cor- along the centre of the AMz compressive layers responding to the initial crack growth from each indenta-(Fig 5a). On the other hand, evidence of crack bifurcation tion, as shown in Fig. 7) the stress continues increasing was also encountered in systems type C(Fig. 8b), where with a different slope. Although further steps in the curve edge cracking had not been previously detected(Fig 5b) are not as clearly seen as the first one, a change in the slope This is a relevant observation since bifurcation has always of the curve can be appreciated in the referred figure at been associated with the presence of an edge crack in the stress levels of 130 MPa and 175 MPa for laminates b compressive layer and, consequently, related to the thick- and C, respectively; the latter showing a third step at ness of such layer [13, 21]. Although some authors have 92 MPa before the catastrophic failure takes place. This shown that, when the load is applied parallel to the layer experimental finding may be related to the stable crack plane, bifurcation may occur even for such cases where growth of the impinging crack when entering the thin edge crack does not appear at the surface of the compres- AMZ layer, suggesting an overall increase of the crack sive layers, this phenomenon has been proved to extend resistance provided by the compressive internal layers only few microns inside the layer; failure being initiated Experimental observations of the fracture surfaces show by straight propagation of the crack within the bulk [38]. a crack-microstructure interaction during the bending test In our case, Fig &b points out that partial bifurcation in the two laminates associated with the first compressive observed even for a layer thickness thin enough to avoid layer seen by the propagating crack(Fig. &a and b). In this the edge crack formation, being the stepwise fracture dueAMZ thin layer, where they are arrested due to the high compressive stresses. This behaviour has been observed by other authors for other multilayered systems, showing the effectiveness of the internal compressive layers in hin￾dering the crack propagation [17,37]. As it can be inferred from Fig. 7, the stress level for the first step is around 40 MPa for the laminate B and 85 MPa for the laminate C, which are coincident with the failure stress measured for the ATZ monolith, i.e. 140 MPa, minus the correspond￾ing tensile stress in the ATZ layers calculated for each sys￾tem, i.e. 102 MPa and 60 MPa respectively. In addition, unstable failure does not follow this first step of crack growth in these materials. As a matter of fact, after this first step (with three ‘‘pop-in’’ events for each laminate cor￾responding to the initial crack growth from each indenta￾tion, as shown in Fig. 7) the stress continues increasing with a different slope. Although further steps in the curve are not as clearly seen as the first one, a change in the slope of the curve can be appreciated in the referred figure at stress levels of 130 MPa and 175 MPa for laminates B and C, respectively; the latter showing a third step at 192 MPa before the catastrophic failure takes place. This experimental finding may be related to the stable crack growth of the impinging crack when entering the thin AMZ layer, suggesting an overall increase of the crack resistance provided by the compressive internal layers. Experimental observations of the fracture surfaces show a crack-microstructure interaction during the bending test in the two laminates associated with the first compressive layer seen by the propagating crack (Fig. 8a and b). In this regard, it should be emphasised that crack bifurcation was discerned at fracture for both layered systems. Such a find￾ing could be expected for samples type B, since edge crack￾ing had been previously developed in this layered system along the centre of the AMZ compressive layers (Fig. 5a). On the other hand, evidence of crack bifurcation was also encountered in systems type C (Fig. 8b), where edge cracking had not been previously detected (Fig. 5b). This is a relevant observation since bifurcation has always been associated with the presence of an edge crack in the compressive layer and, consequently, related to the thick￾ness of such layer [13,21]. Although some authors have shown that, when the load is applied parallel to the layer plane, bifurcation may occur even for such cases where edge crack does not appear at the surface of the compres￾sive layers, this phenomenon has been proved to extend only few microns inside the layer; failure being initiated by straight propagation of the crack within the bulk [38]. In our case, Fig. 8b points out that partial bifurcation is observed even for a layer thickness thin enough to avoid the edge crack formation, being the stepwise fracture due Fig. 7. Engineering stress vs. displacement for the indented laminates and ATZ monolith showing the fracture steps in the laminates. The indenta￾tion position, load forces and layer distribution in the bending test are schematised. Table 4 Stress values for the characteristic points in the two multilayers and in the ATZ monolith recorded for comparative purposes Material Stress (MPa) First step Second step Fracture Laminate B 40 130 160 Laminate C 85 175 195 ATZ monolith – – 140 Fig. 8. Optical micrograph of (a) system B and (b) system C laminates after the bending test, showing the crack bifurcation in the first compressive layer. 1936 R. Bermejo et al. / Composites Science and Technology 67 (2007) 1930–1938