Issue 29

M. Scafè et alii, Frattura ed Integrità Strutturale, 29 (2014) 399-409; DOI: 10.3221/IGF-ESIS.29.35 406 Figure 1 : Stress-Strain curves of sample E5 (normalized results). Figure 2 : %Bending - Average Strain curve of sample E5 (normalized results). The following Fig. 3 - 6 show some optical microscope photographs of the samples failure. Images were taken by a WILD HEERBRUGG optical microscope. The Fig. 3 and 4 show a cross-ply laminate failure (overview and detail). The failure mode (brooming) of this specimen (B4) is in agreement with the acceptable failure modes reported in ASTM D6641 [2]-. The brooming failure was frequently observed in our cross-ply and angle-ply specimens. Welsh and Adams indicated that this failure mode is probably a post-failure phenomenon and that a true compressive failure was achieved prior to the brooming effect [3]. The conventional 0° unidirectional composite specimens, typically fail in a fibre micro-buckling mode before the compressive strength of individual fibres is achieved. Fig. 5 and 6 show a 0° UD laminate failure (overview and detail), where a specimen premature failure caused by fibre micro-buckling can be observed (typical kink band – fig. 7 of [8]). Low magnification Scanning Electron Microscope (SEM) images of the samples failure are shown in the Fig. 7 – 10. Images were taken by a SEM LEO 438-VP. Fig. 7 and 8 clearly show the failure of a specimen taken from 0° UD laminate produced by micro-buckling of individual carbon fibres. In fact, Fig. 7 shows the kink-band reported in Fig. 5 - 6, while Fig. 8 details some bundles of fibres broken in buckling within the same kink-band of Fig. 7. In Fig. 9 a cross-ply sample was photographed (lateral view of brooming failure); it is observable a 0° ply between two 90° plies. The effect of the presence of the 90° plies, which limits locally the buckling of the fibres in a sample subjected to compression, is highlighted by the absence of kink-bands throughout the sample. In Fig. 10 is visible a detail of compression failure of some 0° fibres bundles, within the same sample of Fig. 9.