D Raab et al. /Materials Science and Engineering A 417 (2006)341-347 Fig. 6. SEM micrographs of fracture surfaces of samples fractured in biaxial flexure test: Nextel M 440 short fiber reinforced composites with uncoated fibers(left) and with 150 nm BN-coated fibers(right). interface exhibited increased fracture strength. Fig. 5 shows typ- these brittle materials caused by the complex stress field. The ical load-displacement curves for different composites obtained fracture is of statistical nature and no correlation between the in biaxial flexural strength tests Comparing load-displacement number of fracture fragments and the measured strength values curves, non-brittle fracture behaviour was found for compos- could be determined ites with boron nitride-coated fibers(40 and 150 nm BN coating thickness). In contrast to this, continuous fiber reinforced com- 3.4. ZenTronm glass fiber reinforcement posites without fiber coating break in brittle manner(not shown in Fig. 5), whereas the boron nitride coating leads to a non-brittle In these composites, because of aF <aM(see Table 1) fracture behaviour, shown in Fig. 5 radial compression stresses develop in the matrix. Regardless As expected, the fracture surface analysis of the short fiber of any possible diffusion processes at the interface leading to reinforced composites by SEM imaging did not indicate crack strong fiber-matrix bonding, the presence of radial compres- deflection at the fiber-matrix interface for composites containing sion stresses should contribute to clamp the fiber, and thus fiber uncoated fibers. The crack runs from the matrix into the fiber pull-out should be prevented in these composites. The thermal (Fig. 6, left). Indication of crack deflection and fiber debond- expansion mismatch is thus unfavourable in these composites to was observed, however, on similar composites containing promote toughening by fiber pull-out 150 nm BN-coated fibers(Fig. 6, right) The desized ZenTron glass fiber reinforced composite co. The macroscopic crack patterns were optically analysed to showed comparable strength values for continuous fiber rein- mpare crack initiation, crack deflection and bifurcations on forcement in both three-point bending tests and biaxial flexural different samples Selected sample images are shown in Fig. 7. strength tests For the short fiber reinforcement, the biaxial flex In all cases, the crack started in the central zone of the disk. This ure strength value is significantly lower than the value measured is consistent with the equibiaxial stress distribution in biaxial for the uniaxial fiber reinforced composite. In all cases, brit- flexure strength tests[19]. In the case of the short fiber reinforced tle fracture was observed. In contrast to this, the three-point samples, more than one bifurcation centre were found. In these bending strength test and biaxial strength test data for the tin composites, crack deflection occurs and the crack propagation is oxide-coated fibers were similar for the short fiber reinforced non-linear. Besides the radial crack pattern, samples containing composites, whereas the continuous fiber reinforced composites uncoated fibers showed also a circumferential crack branching. as expected, showed a higher strength in three-point bending Contrary to these findings, the sample with uncoated continu- The fracture toughness did not essentially improve regardless of ous fiber reinforcement showed straight crack propagation from fiber coating and fiber architecture(short or continuous fibers) the centre of the disk(right image in Fig. 7). In most cases, Earlier investigations have shown the ability of tin oxide inter crack propagation was orthogonal to the unidirectional fibers faces to deflect cracks in glass matrix/alumina fiber composites and bifurcation started at fiber-matrix interfaces. This fracture [15]. However, this effect is more probable in composites with pattern occurs because of the great variety of failure modes in high fiber content and low coating thickness [16] Fig. 7. Crack 150 nm BN-coated short fibers(middle)and uncoated continuous fibers(right).D. Raab et al. / Materials Science and Engineering A 417 (2006) 341–347 345 Fig. 6. SEM micrographs of fracture surfaces of samples fractured in biaxial flexure test: NextelTM 440 short fiber reinforced composites with uncoated fibers (left) and with 150 nm BN-coated fibers (right). interface exhibited increased fracture strength. Fig. 5 shows typ￾ical load–displacement curves for different composites obtained in biaxial flexural strength tests. Comparing load–displacement curves, non-brittle fracture behaviour was found for compos￾ites with boron nitride-coated fibers (40 and 150 nm BN coating thickness). In contrast to this, continuous fiber reinforced com￾posites without fiber coating break in brittle manner (not shown in Fig. 5), whereas the boron nitride coating leads to a non-brittle fracture behaviour, shown in Fig. 5. As expected, the fracture surface analysis of the short fiber reinforced composites by SEM imaging did not indicate crack deflection at the fiber–matrix interface for composites containing uncoated fibers. The crack runs from the matrix into the fiber (Fig. 6, left). Indication of crack deflection and fiber debond￾ing was observed, however, on similar composites containing 150 nm BN-coated fibers (Fig. 6, right). The macroscopic crack patterns were optically analysed to compare crack initiation, crack deflection and bifurcations on different samples. Selected sample images are shown in Fig. 7. In all cases, the crack started in the central zone of the disk. This is consistent with the equibiaxial stress distribution in biaxial flexure strength tests[19]. In the case of the short fiber reinforced samples, more than one bifurcation centre were found. In these composites, crack deflection occurs and the crack propagation is non-linear. Besides the radial crack pattern, samples containing uncoated fibers showed also a circumferential crack branching. Contrary to these findings, the sample with uncoated continu￾ous fiber reinforcement showed straight crack propagation from the centre of the disk (right image in Fig. 7). In most cases, crack propagation was orthogonal to the unidirectional fibers and bifurcation started at fiber–matrix interfaces. This fracture pattern occurs because of the great variety of failure modes in these brittle materials caused by the complex stress field. The fracture is of statistical nature and no correlation between the number of fracture fragments and the measured strength values could be determined. 3.4. ZenTronTM glass fiber reinforcement In these composites, because of αF < αM (see Table 1) radial compression stresses develop in the matrix. Regardless of any possible diffusion processes at the interface leading to strong fiber–matrix bonding, the presence of radial compres￾sion stresses should contribute to clamp the fiber, and thus fiber pull-out should be prevented in these composites. The thermal expansion mismatch is thus unfavourable in these composites to promote toughening by fiber pull-out. The desized ZenTronTM glass fiber reinforced composite showed comparable strength values for continuous fiber rein￾forcement in both three-point bending tests and biaxial flexural strength tests. For the short fiber reinforcement, the biaxial flex￾ure strength value is significantly lower than the value measured for the uniaxial fiber reinforced composite. In all cases, brit￾tle fracture was observed. In contrast to this, the three-point bending strength test and biaxial strength test data for the tin oxide-coated fibers were similar for the short fiber reinforced composites, whereas the continuous fiber reinforced composites, as expected, showed a higher strength in three-point bending. The fracture toughness did not essentially improve regardless of fiber coating and fiber architecture (short or continuous fibers). Earlier investigations have shown the ability of tin oxide inter￾faces to deflect cracks in glass matrix/alumina fiber composites [15]. However, this effect is more probable in composites with high fiber content and low coating thickness [16]. Fig. 7. Crack patterns of borosilicate glass/NextelTM 440 fiber composites, reassembled after biaxial flexure test. Composites with: uncoated short fibers (left), 150 nm BN-coated short fibers (middle) and uncoated continuous fibers (right)