Modelling the fatigue life of polymer–matrix fibre-composite components

A methodology has been proposed for predicting the fatigue life of fibre-composite components and structures which combines relatively short-term fracture mechanics data, obtained from experimental measurements, with a finite-element analysis (FEA) of the component or structure. The approach has been used to study the growth of damage in, and the cyclic fatigue life of I-beams. The beams were manufactured using carbon-fibre-reinforced-plastic (CFRP) and contained a 60 mm diameter notch in the web. Experimental work has shown that the development of significant damage was limited to the region of material in the web around the 60 mm diameter notch. A significant amount of matrix micro-cracking damage occurred within the first 0.5×106 fatigue cycles, mainly in the +45° plies and 0° plies, in which the fibres are orientated at 90° and at 45°, respectively, to the local tensile stress. This matrix cracking eventually led to some limited delamination after about 0.5×106 cycles had elapsed and this occurred mainly along the global +45°/−45° ply interfaces, with some delamination also occurring along the +45°/0° ply interface. The growth of these two types of damage eventually led to fibre fracture which was the final cause of structural failure of the web material, and hence of the I-beam. Thus, the model has concentrated upon modelling these types of damage mechanisms. The agreement between the results from the theoretical model and the experiments is good, especially when it is considered that there are no ‘adjustable factors’ involved in the modelling studies. For example, the theoretical model predicts the number of cycles, Nf, for the I-beam to fail structurally to be about 4.9×106 cycles. The experimentally measured value was 4.78×106 cycles.