Thermoelastic Determination of KI and KII in an Orthotropic Graphite-Epoxy Composite

Motivated by difficulties in recording reliable data in the immediate vicinity of a crack tip, the general fracture mechanics concept often employed for isotropy of utilizing stresses which are valid away from the crack with distance measurements is extended to orthotropy. Specifically, stress intensity factors, K_I and K_II, for inclined cracks are determined here in a uniaxially-loaded orthotropic graphite-epoxy composite using measured temperatures and least-squares. An advantageous novel technique for evaluating the thermo-mechanical coefficients is also implemented by commingling numerical and measured information. The through cracks are inclined at either 85° or 45° with respect to the load, the latter also being the strong/stiff composite direction. Retaining higher-order terms in the stress representations enables using measured temperatures away from the crack. The present method of determining the two thermo-mechanical coefficients circumvents the challenges encountered when attempting to evaluate the largest thermo-mechanical coefficient from uniaxial coupons since this coefficient typically occurs in the weakest direction of the composite.