Percolation of fatigue damage in plain-weave textile reinforced composites

Several studies have investigated the mechanical response of textile reinforced composites under cyclically repeated loadings (fatigue). However, there is still a need for accurate quantitative evaluation of the fatigue damage accumulation and accurate predictive models to ensure the safe design of textile composite components. In this study, a thermography based method has been developed to detect the evolution of the fatigue damage in plain-weave carbon fiber/epoxy composites. The method adopts the unit cell as the unit of damage evolution (damaged unit cell). The fatigue damage has been related to the development of clusters of damaged unit cells and their distribution on the surface of the tensile fatigued specimen, considering the thermoelastic damage analysis (TDA). Percolation theory was also applied to demonstrate how the number and properties of the clusters, formed by irregularly distributed damaged unit cells, affect the entire system. The method showed that the size ratio of the maximum damaged unit cell cluster, Cmax, increased rapidly after the damaged unit cell ratio exceeded the ‘percolation threshold’ pc. This threshold announced the fatigue final failure, which occurred with rapid accumulation of transverse cracks, wider local delamination and fibers breakage. The arrangement of the damaged unit cells was also quantitatively evaluated using the fractal concept. The fractal dimension D of the damaged unit cells cluster increased up to a steady state value Dc. Beyond this value, the fatigue damage had a faster evolution when p exceeded pc. Finally, the fatigue damage evolution was predicted using the concept of percolation and mutual interference between damaged units.