Fatigue of fibre composites

If the temperature increase during cyclic loading is sufficiently small so that no thermal fatigue occurs, the polymer fails by mechanical fatigue. Similar to metals, we can distinguish the stages of crack initiation and crack propagation. 

In fibre composites, the presence of the fibres can change the fatigue strength of the matrix in several ways. On the one hand, global effects due to load transfer and the corresponding change in the stress and strain fields can play a role, on the other hand, local effects can occur, especially at the fibre-matrix interface. 

The shear stress at the interface between fibre and matrix locally increases the strain in the matrix (see section 9.3.2). This increased strain may cause local damage in the matrix and initiate cracks. This is especially important for short fibres because shear stresses occur only near the fibre ends in long fibres. 

If there are tensile and compressive components in the cyclic load (i.e., if i 0), fatigue life can be significantly reduced compared to pulsating tensile loading. The reason is that those fibres whose matrix interface has failed are sensitive to failure by buckling [131[. 

Fatigue can also occur in ceramic matrix composites. If a crack propagates under tensile loading, it will be bridged by the fibres and partially unloaded (see section 9.3.3). This can cause stable crack growth under static loading, with the crack not propagating further unless the load is raised (see 

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Perreux D, Suri C (1997) A study of the coupling between the phenomena of water absorption and damage in glass/epoxy composite pipes. Compos Sci Technol 57(9-10) 1403-1413 Phillips DC, Scott JM, Buckley N (1978) The effects of moisture on the shear fatigue of fibre composites. In Noton B, SignorelU R, Street K, Phillips L (eds) Proceedings of the 1978 international conference on composite materials (ICCM/2), Toronto, 16-20 April 1978, pp 1544-1559... 

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