A way forward for industry to determine valid cyclic-fatigue relationships for polymer-matrix fibre composites

In the present paper a novel methodology is proposed for predicting an ‘upper-bound’ (i.e. ‘worst case’) fatigue crack growth (FCG) rate curve from laboratory double-cantilever beam (DCB) test data which is representative of a composite laminate exhibiting no, or only very little, retardation (i.e. slowing) of the FCG rate under fatigue loading, as is the case with most real-life applications. The proposed methodology also takes into account the inherent scatter of such fatigue tests. To achieve this we have employed a variant of the Hartman-Schijve equation. The resulting conservative ‘upper-bound’ FCG rate curve represents a unique and valid ‘upper-bound’ fatigue relationship which may be used in industrial applications for (a) material development, characterisation and comparative studies and (b) design and lifing studies