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Failure of fiber reinforced composites – role of the matrix behaviour and the interface strength
(2016)
Failure of fiber reinforced composites initiates at microdefects in the matrix or at the interface. The interface strength is calculated from the stress field directly before crack initiation by nonlinear finite element simulations. Large deformations and plastic material behaviour are taken into account. Microscopical inspections of the fibers after failure show only small regions with adhering matrix material. This indicates that the interface strength is below the matrix strength.
The aim of the paper is to analyse the failure process taking place on microscale in fiber reinforced composites. The failure is initiated at microscopical defects presumingly in the interface. The circumferential propagation of interface cracks starting at microdefects in unidirectional plies under transverse loading is studied using a representative volume element. First a linear elastic fracture mechanical analysis of interface cracks in a carbon fiber reinforced epoxy resin is performed by calculating the mixed mode energy release rate using the virtual crack closure method. The influence of the fiber spacing is studied by varying the fiber volume fraction. The total energy release rate rapidly grows after crack initiation indicating an unstable crack propagation. The first phase is dominated by mode I. The mode II energy release rate starts slightly delayed but surpasses the mode I part after a short crack length. By increasing the fiber volume fraction the maximum of the energy release rate shifts to smaller crack angles.
The problems of the limits of continuum mechanics are shown on some examples. The fiber-matrix debonding and subsequent crack propagation in fiber reinforced composites is studied. While Finite Element Analyses predict clear crack surfaces, either following the interface or bridging to the neighbouring fiber forming planes, the actual surfaces are very cliffy. Due to the much larger fracture surfaces and the completely different stress states on the cliffy fracture surfaces the continuum mechanical prediction is far from the actual local behaviour. The phenomenon of local high stresses which re much higher than the macroscopically measured strength of the material is also encountered in general fracture mechanics.
The scale dependency of materials is shown on indentation tests conducted on different scales. From a continuum mechanical point of view it makes no difference if the indentation test is performed on a nano-, micro- or macroscale, because it is a matter of self-similarity. However, in nano- or microindentation tests on a pure polymer using e.g. a Berkovic indentor, the extremely high stress concentrations arising at the indentor tip are domination more or less the whole region around the indentor. In case of a macroscopical test, this zone covers only a small part of the material loaded by the indentor. Accordingly, the results of the different test cannot be scaled up.