Debonding of fibres

The interfacial shear stress t is concentrated at the fibre ends (see Figure 6.23). With increase in strain, these are the sites where the interfrice first fails and debonding of fibres and matrix or shear failure of the matrix begins. This occurs when T here reaches the interfiicial shear strength. The magnitude of this strength is determined by three factors ... 

As an initially translucent FRP specimen fails in fatigue, it gradually becomes opaque. When the opacity first occurs, it does so only when the specimen is stressed, but after further stress cycles it becomes progressively permanent and more intense. At final separation of matrix laminates in axial loading, the fracture surface is normal to the loading direction and fibrous in appearance, the resin matrix often being reduced to a white powder. The initial (non-permanent) opacity is caused by debonding of fibres transverse to the load direction, which later cause matrix cracks and permanent opacity in the specimen. 

At low densities of fibres, fracture proceeds by brittle failure or debonding of the fibres a fully linear elastic loading curve is observed (type I loading curve in Table II). 

Acoustic emission utilizes the fact that a material s response to stress is accompanied by the release of energy as sound or ultrasound. Different kinds of events - notably matrix cracking, debonding and fibre fracture -can be distinguished. Thus the mechanical condition of a composite can be examined. 

Tvergaard, V. (1990) Effect of fibre debonding in a whisker-reinforced metal. Material Science and Engineering A, 125, 203-213. 

Three test methods, the fragmentation, pull-out and microbond methods, were used to analyse the micromechanics for carbon fibre/epoxy composites and Raman spectroscopy was used to determine the variation of fibre strain with position along a carbon fibre in a resin. It was demonstrated that the latter technique was capable of revolutionising interpretation of composite micromechanics and the different micromechanical test methods. In particular, it was shown that the technique could be used to distinguish between elastic deformation, interfacial debonding and shear yielding of the matrix at the interface. 19 refs. (FRC 94, Institute of Materials, Newcastle, March 1994)... 

Schematic sketch of distributions of fibre tensile stress aiKl interfacial shear stress after fibre-matrix debonding has commenced. 

Here, T = maximum load measured prior to debonding of the fibre, d=diameter of used fibre, and I=length of embedded portion of fibre. 

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