Piecewise non-linear constant life diagram formulation for FRP composite materials

A new formulation for predicting an asymmetric, piecewise non-linear constant life diagram (CLD) is introduced in this paper. The development of the new model is based on the representation of the fatigue data on the stress ratio (R)–stress amplitude (σa) plane instead of the conventional representation on the mean stress (σm)–stress amplitude (σa) plane. The advantage of this new CLD formulation is the accuracy of the derived CLD for a wide range of composite materials, even when only a limited amount of data is available. In addition it is not based on any assumptions and no complicated optimization processes have to be followed for the estimation of the model parameters. Although the idea of plotting constant amplitude fatigue data in CLDs in order to account for the mean stress effect is not new, it is the first time that a CLD formulation is derived based on the interpretation of the available fatigue data on the R–σa plane. The validity of the proposed CLD formulation is evaluated by comparing predicted and experimental results for a wide range of composite materials. As proved herein, the proposed CLD formulation offers a better performance than most conventional CLD models found in the literature.