Crack tip damage analysis in fatigue fracture of epoxy resin

The application of the crack layer theory to fatigue crack propagation (FCP) in epoxy is discussed. A crack tip damage evolution coefficient μ is introduced to assess the extent of damage as a fraction of the damage associated with critical crack propagation. The results can be expressed in the form\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{d\ell }}{{dN}} = \frac{{\beta G_1^2 }}{{\mu G_{1c} - G_1 }} $$\end{document}where dl / dN is the rate of FCP, G 1 is the energy release rate whose critical value is G 1 c , and β is a phenomenological constant. Although no damage was detected from microscopic analyses, μ increases fivefold during stable crack propagation. Fractal analysis of fracture surface profiles provides a quantitative measure of the roughness associated with crack advance. The fractural measure d is found to evolve in a similar fashion as μ, suggesting the applicability of d to quantify crack tip damage evolution.