A R Boccaccini et al. / Composites Science and Technology 65(2005)325-333 linearity for specimen ck6. This particular behaviour for each sample is connected with the different micro- structural phases at the Cn tip, i.e. at the moment of crack initiation and/or at the critical crack length, acr, (when the crack reaches the length corresponding to the transition from stable to unstable propagation). In addition, the intersection of the matrix/fibre interface with the major crack plane results in very high tendency for delamination. In the case that the crack tip is still lo- ated in the Cn area(triangle), the driving force and CN 1 mm geometry inhibit the delamination development, this being an advantage of the cn ge Fig4.Optical macrograph of a chevron notched specimen after the SiC fibre reinforced-glass matrix composites in a previ test, showing microcracks in direction perpendicular to the maj ous investigation [24]. Once the major crack tip has the opportunity to intersect the fibre/ matrix interface after departure from the Cn triangle, very extensive delani- Based on the analysis of load-time curves, calculation nation takes place and the major crack plane is thus of critical crack length and analysis of fracture morphol- no longer kept ogy, the fracture behaviour of the composites can be The results of AE analysis support well the previous nterpreted. Fig. 5 shows an optical macrograph of the explanation. In cases where"pop-in"events are ob- pecimens fracture plane. The Cn tip and the direction served on the load-time(and load-deflection)curves,a he crack development are marked by arrows. The rapid increase of AE events is detected, as registered ack length, acr, corresponding to the minimum value by the curve of cumulative number of AE events against on the calibration function, Ymin, as reported in time. This can be seen in Fig 3(a) on both AE curves, Table 1, corresponds to the transition of crack propaga- chl and ch2, generated by two-channel E events ion from controlled to uncontrolled regimes. From the registration analysis of crack initiation and propagation in different When a tensile stress is applied to a composite, sev- samples, it follows that, due to the complex microstruc ral fracture phenomena can occur in the material ture of the material, different possible regions exist with including matrix microcracking, fibre-matrix debonding the sample where crack initiation at the Cn tip may and delamination along the matrix/fibre interface [13]. occur. This leads to significant variability in the material The aE technique used here indicated that individual response and thus to the high scatter of fracture tough- fracture events in the present composites started at ess values found, as reported in Table 1. Fractographic about 1/2 to 2/3 of the maximum load(arrow"a"in observations support this explanation, as elaborated Fig 3(a)). Microstructural changes responsible for this below AE events should be matrix microcracking and partial Support of the above follows also from Fig. 3(b), i.e. local fibre/matrix interfacial decohesion. Deflection very different fracture behaviour may be expected for from linearity of the load-time trace was typical for this each specimen. It is possible to identify the crack devel- stage of CN specimen loading, as well as at loads close opment at the CN tip for example by "pop-ins "present to the maximum load (arrow "bin Fig. 3(a)). This on traces of specimens ckl and ck5, or deflection from was associated with a strong increase in the cumulative number of AE events. Although not conclusively proved in this study, the pronounced increase in AE events is thought to be due to the major crack propagation through the mullite matrix as well as due to fibre/ matrix debonding and fibre fracture. An increase in the number of AE events observed at the end of the linear part of the load-deflection trace indicates therefore that the actual crack has developed at the Cn tip. In thes se cases measurements of Kle can be taken to be valid, 1 mm the unstable fracture (at maximum force) occurs from a propagating crack perpendicular to the fibre axis A typical fracture surface of a CN sample is shown in Fig. 5. Optical macrograph of the fracture plane of a chevron notched st specimen, showing the chevron notch tip and crack Fig. 6. Extensive fibre pull-out, typical of this kind of direction as marked by the arrow. The calculated value of the critical composites [26]is observed. However, due to the previ- crack length aer is also shown. A section of the fracture surface on the ously discussed effects of heterogeneous microstructure right side of the image is not focused because of fracture relief of the composite on Klc values, it is not possible toBased on the analysis of load–time curves, calculation of critical crack length and analysis of fracture morphol￾ogy, the fracture behaviour of the composites can be interpreted. Fig. 5 shows an optical macrograph of the specimens fracture plane. The CN tip and the direction of the crack development are marked by arrows. The crack length, acr, corresponding to the minimum value on the calibration function, Y
min, as reported in Table 1, corresponds to the transition of crack propaga￾tion from controlled to uncontrolled regimes. From the analysis of crack initiation and propagation in different samples, it follows that, due to the complex microstruc￾ture of the material, different possible regions exist with￾in the sample where crack initiation at the CN tip may occur. This leads to significant variability in the material response and thus to the high scatter of fracture tough￾ness values found, as reported in Table 1. Fractographic observations support this explanation, as elaborated below. Support of the above follows also from Fig. 3(b), i.e. very different fracture behaviour may be expected for each specimen. It is possible to identify the crack devel￾opment at the CN tip, for example by ‘‘pop-ins’’ present on traces of specimens ck1 and ck5, or deflection from linearity for specimen ck6. This particular behaviour for each sample is connected with the different micro￾structural phases at the CN tip, i.e. at the moment of crack initiation and/or at the critical crack length, acr, (when the crack reaches the length corresponding to the transition from stable to unstable propagation). In addition, the intersection of the matrix/fibre interface with the major crack plane results in very high tendency for delamination. In the case that the crack tip is still lo￾cated in the CN area (triangle), the driving force and CN geometry inhibit the delamination development, this being an advantage of the CN geometry as proven for SiC fibre reinforced–glass matrix composites in a previ￾ous investigation [24]. Once the major crack tip has the opportunity to intersect the fibre/matrix interface after departure from the CN triangle, very extensive delami￾nation takes place and the major crack plane is thus no longer kept. The results of AE analysis support well the previous explanation. In cases where ‘‘pop-in’’ events are ob￾served on the load–time (and load–deflection) curves, a rapid increase of AE events is detected, as registered by the curve of cumulative number of AE events against time. This can be seen in Fig. 3(a) on both AE curves, ch1 and ch2, generated by two-channel AE events registration. When a tensile stress is applied to a composite, sev￾eral fracture phenomena can occur in the material, including matrix microcracking, fibre–matrix debonding and delamination along the matrix/fibre interface [13]. The AE technique used here indicated that individual fracture events in the present composites started at about 1/2 to 2/3 of the maximum load (arrow ‘‘a’’ in Fig. 3(a)). Microstructural changes responsible for this AE events should be matrix microcracking and partial local fibre/matrix interfacial decohesion. Deflection from linearity of the load–time trace was typical for this stage of CN specimen loading, as well as at loads close to the maximum load (arrow ‘‘b’’ in Fig. 3(a)). This was associated with a strong increase in the cumulative number of AE events. Although not conclusively proved in this study, the pronounced increase in AE events is thought to be due to the major crack propagation through the mullite matrix as well as due to fibre/matrix debonding and fibre fracture. An increase in the number of AE events observed at the end of the linear part of the load–deflection trace indicates therefore that the actual crack has developed at the CN tip. In these cases, the measurements of KIc can be taken to be valid, since the unstable fracture (at maximum force) occurs from a propagating crack perpendicular to the fibre axis. A typical fracture surface of a CN sample is shown in Fig. 6. Extensive fibre pull-out, typical of this kind of composites [26] is observed. However, due to the previ￾ously discussed effects of heterogeneous microstructure of the composite on KIc values, it is not possible to Fig. 4. Optical macrograph of a chevron notched specimen after the test, showing microcracks in direction perpendicular to the major crack. Fig. 5. Optical macrograph of the fracture plane of a chevron notched test specimen, showing the chevron notch tip and crack propagation direction as marked by the arrow. The calculated value of the critical crack length acr is also shown. A section of the fracture surface on the right side of the image is not focused because of fracture relief. A.R. Boccaccini et al. / Composites Science and Technology 65 (2005) 325–333 329