A micromechanics-based fatigue life prediction approach with octagonal fiber model for thermoplastic composites

초록

Fatigue tests were carried out on [45/-45]2s laminated composites, made of E-glass with sizing SE4740 and thermoplastic resin, in order to find its fatigue behavior. To theoretically predict the laminate response, a micromechanics-based fatigue life prediction approach, called MMFatigue, was proposed in this work, which was composed of two steps: macro stress analysis and micromechanics-based fatigue analysis. In the macro stress analysis, the ply stresses were derived from the laminate stresses using classical laminate theory. For the second step, an octagonal fiber model (OFM), was proposed to describe the micro structure, such that the micro fatigue stress in each constituent (fiber, matrix and interface) were calculated from ply stress. To convert multiaxial micro stresses to a uniaxial equivalent stress, a fatigue model was employed for each constituent. Then, a modified Goodman formulation of constant life diagram (CLD) was utilized to define constituent effective stress, which is a function of the mean value and amplitude of equivalent stress, as well as constituent strengths. Finally, the effective stress and fatigue life were correlated through experimentally obtained S-N curve for each constituent. To verify the accuracy of proposed approach, the prediction results were compared with test data of [45/-45]2s laminates. As an application, MMFatigue was applied to predict the thermoplastic wind turbine blade fatigue life, by comparing to the prediction using GL method to demonstrate the capability of the proposed approach. Besides, the fatigue tests and prediction of [45/-45]2s thermoset laminate composites were also performed to investigate the difference between thermoplastic and thermoset composites.

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