Experiments on shear deformation, debonding and local load transfer in a model graphite/glass/epoxy microcomposite

Abstract

Recent statistical theories for the failure of polymer matrix composites depend heavily on details of the stress redistribution around fibre breaks. The magnitudes and length scales of fibre overloads as well as the extent of fibre/matrix debonding are key components in the development of longitudinal versus transverse crack propagation. While several theoretical studies have been conducted to investigate the roles of these mechanisms, little has been substantiated experimentally about the matrix constitutive behaviour and mechanisms of debonding at the length scale of a fibre break. In order to predict the growth of transverse and longitudinal cracks using the same micromechanical model, we microscopically observed the epoxy shear behaviour around a single fibre break in a three-fibre microcomposite tape. The planar specimens consisted of a single graphite fibre placed between two larger glass fibres in an epoxy matrix. The interfibre spacing was less than one fibre diameter (< 6 I~m) in order to reflect the spacing between fibres found in typical composites. The epoxy constitutive behaviour was modelled using shear-lag theory where the epoxy had elastic, plastic, and debond zones. The criteria for debonding were modified from conventional shear-lag approaches to reflect the orientational hardening in the epoxy network structure. The epoxy, which is brittle in bulk, locally underwent a shear strain of about 60% prior to debonding from the fibre.

Hierarchical scaling law for the strength of composite fibre bundles

“…Figure 13b evidences a good agreement between predicted and measured strengths in the macro scale (cross sections up to 30 mm 2 , larger than a standard UD FRP tensile-strength specimen, are shown); this implied using a higher shear-lag strength, which is likely to be more representative of the true in-situ matrix behaviour (Gulino et al, 1991, Hobbiebrunken et al, 2007.…”

“…The in-situ response of the matrix / interface to this event is complex, as for instance epoxy is usually brittle in bulk, but actually ductile and much stronger in-situ (Gulino et al, 1991, Hobbiebrunken et al, 2007, de Morais, 2001). This, together with the lack of agreement in the literature on whether fibre failure should be modelled through energy or stress based approaches (Nairn, 1997, Zhandarov et al, 1998, supports the use of a perfectly-plastic shear-lag approach, for the sake of simplicity McMeeking, 1999, de Morais, 2006).…”

Investigating the transient angle in a unidirectional composite in tension and compression

“…Integrated analysis of tensile failure in UD composites. Much research effort has also been devoted to the study of fiber/ matrix unwinding mechanisms, [19][20][21][22][23][24][25] of which, in most studies, the energy release rate (ERR) has been considered the driving force behind the development of detachment. However, it is clear that fiber/matrix separation alone will not lead to severe failure plane formation.…”

The effect of temperature on stress transfer between a broken fibre and the adjacent fibres in unidirectional fibre composites

“…4 results in interfacial shear yielding at a fibre-break [10,[25][26][27][28][29][30]. This develops a plastic region as the interfacial shear stress in the matrix exceeds the yield strength of the bulk epoxy resin.…”