Failure phenomena in two-dimensional multi-fibre microcomposites - 3. A Raman spectroscopy study of the influence of interfacial debonding on stress concentrations

Raman spectroscopy has been used to investigate the influence of fibre/matrix debonding on the stress situation in neighbouring fibres. Microcomposites containing five untreated Tenax(R) HM-40 carbon fibres aligned parallel in an epoxy matrix were produced. It was found that in the fibres neighbouring a broken and debonded fibre, three stress concentrations can be detected. One of the stress concentrations is located at the position of the fibre break, while the other two are located at the end of each of the debonded zones around the fibre break. Importantly, for inter-fibre spacings up to at least 9 fibre diameters, i.e. 65 mu m, these three stress concentrations govern not only the locus of failure of the fibres in the microcomposite, i.e. coordinated failure, but also the growth of the debonded zones around those fibre breaks. The debonding propagates in such a way that the stress concentrations are aligned and a stable stress situation is reached. Thus, it can be concluded that although the failure process in the case of poor fibre/matrix adhesion appears to be of a random nature, it is in fact a stress concentration controlled process. Finally, experimental evidence is given for skin/core failure, or splitting, of a strained untreated fibre. (C) 1998 Elsevier Science Ltd. All rights reserved.