S. Bueno, C Baudin/Composites: Part A 40(2009)137-143 2 Casting times corresponding to the designed layer thicknesses of the laminated Measured thicknesses of layers 1 and 5 are not shown because the specimens were machined before polishing. igned layer Casting time s Measured layer 3 2100 1320-1360 360-375 A10-layer 5 A10a 0.01 0.02 0.03 0.04 d [mm] Fig. 5. Characteristic load-displacement curve of specimens notched with a relative notch length (a/w) of 0. 26. Semistable behaviour is shown with a sudden load decrease down to approximately 54 N. A30 A10 A10 心 A10 Fig. 6. Characteristic fracture features of the laminated specimens. FE-SEM micrograph of a fracture surface. The internal A30 layer shows different fracture planes forming steps that followed the orientation of the pre-existing microcrcaks the point of impingement into the crack to traverse the remaining part of the first A30 and the second Al0 layers. When the crack cemal A30 layers in the laminated reaches the second A 30 layer(Fig. 7b) it impinges also a pre-exist thermally etched surface (a)Low ing microcrack but in this case multiple cracks at different the interfaces between ks preferentially oriented parallel to distances of the point of impingement. Moreover, the same process of a characteristic microcrack. occurs with the emerging cracks. Thus, the microcracks present in the laminate play a role similar to that of the weak interfaces microcracks, which indicates that the main crack was deflected between layers in the crack-deflecting laminates [ 9, 28, 35-39 nd or branched along them; fracture of the A10 layers was flat while the delaminaton distances are limited to the microcrack relatively to that of the A30 ones. In Fig. 7 the crack paths in the lengths. The fact that no deflection along the central part of the lateral surfaces are shown. The propagation of the main cracks internal layers, were the residual compressive stresses would be as never completely straight due to the above mentioned crack the largest, was observed further support their low level. deflection and or branching processes, nevertheless, significant The aspect of the load-displacement curves can be inferred differences were found depending on the particular layer of the from the fracture features just discussed( Figs. 6 and 7). The initial laminate. The deflection distances through the first A10 layers ad drop observed, corresponding to the kinetic pr and part of the first A30 ones were very small and multiple crack- the main crack, will be associated to its propagation to reach the ing was not observed whereas significant deflection distances and second A30 layer. The observed crack deflection in the first A30 multiple cracking were observed in the other layers. Fig. yer(Fig. 7a) would not be enough as to produce significant strain how a main crack reaches a pre-existing microcrack, an in the specimens. Conversely, the multiple deflection and forma be arrested by it emerging at a certain distance(80- tion of secondary cracks perpendicular to the layers in the secondmicrocracks, which indicates that the main crack was deflected and/or branched along them; fracture of the A10 layers was flat relatively to that of the A30 ones. In Fig. 7 the crack paths in the lateral surfaces are shown. The propagation of the main cracks was never completely straight due to the above mentioned crack deflection and/or branching processes, nevertheless, significant differences were found depending on the particular layer of the laminate. The deflection distances through the first A10 layers and part of the first A30 ones were very small and multiple crack￾ing was not observed whereas significant deflection distances and multiple cracking were observed in the other layers. Fig. 7a shows how a main crack reaches a pre-existing microcrack, and seems to be arrested by it emerging at a certain distance (80–100 lm) of the point of impingement into the crack to traverse the remaining part of the first A30 and the second A10 layers. When the crack reaches the second A30 layer (Fig. 7b) it impinges also a pre-exist￾ing microcrack but in this case multiple cracks emerge at different distances of the point of impingement. Moreover, the same process occurs with the emerging cracks. Thus, the microcracks present in the laminate play a role similar to that of the weak interfaces between layers in the crack-deflecting laminates [9,28,35–39] while the delaminaton distances are limited to the microcrack lengths. The fact that no deflection along the central part of the internal layers, were the residual compressive stresses would be the largest, was observed further support their low level. The aspect of the load–displacement curves can be inferred from the fracture features just discussed (Figs. 6 and 7). The initial load drop observed, corresponding to the kinetic propagation of the main crack, will be associated to its propagation to reach the second A30 layer. The observed crack deflection in the first A30 layer (Fig. 7a) would not be enough as to produce significant strain in the specimens. Conversely, the multiple deflection and forma￾tion of secondary cracks perpendicular to the layers in the second Table 2 Casting times corresponding to the designed layer thicknesses of the laminated structure described in Fig. 3 and measured thicknesses in the sintered materials. Measured thicknesses of layers 1 and 5 are not shown because the specimens were machined before polishing. Designed layer thickness, lm Casting time, s Measured layer thickness, lm A10–layer 1 2100 415 – A30–layer 2 300 90 350–370 A10–layer 3 1200 640 1320–1360 A30–layer 4 300 150 360–375 A10–layer 5 2100 1930 – Fig. 4. Characteristic microstructure of the internal A30 layers in the laminated structure. FE-SEM micrograph of a polished and thermally etched surface. (a) Low magnification observation showing microcracks preferentially oriented parallel to the interfaces between the layers. (b) Detail of a characteristic microcrack. 0.00 0 25 50 75 100 125 150 175 200 P [N] d [mm] 0.01 0.02 0.03 0.04 Fig. 5. Characteristic load–displacement curve of specimens notched with a relative notch length (a/W) of 0.26. Semistable behaviour is shown with a sudden load decrease down to approximately 54 N. Fig. 6. Characteristic fracture features of the laminated specimens. FE-SEM micrograph of a fracture surface. The internal A30 layer shows different fracture planes forming steps that followed the orientation of the pre-existing microcrcaks (Fig. 4a). S. Bueno, C. Baudín / Composites: Part A 40 (2009) 137–143 141