Identification of Subsurface Damage in Multidirectional Composite Laminates Using Full-Field Imaging

Detection of subsurface damage in composite structures is essential for assessing performance and identifying regions for repair. Thermoelastic stress analysis (TSA) is a full-field infrared imaging technique which uses the thermoelastic response of a material to infer the stress state on the surface of a component under cyclic loading. Similarly, digital image correlation (DIC) is a full-field surface measurement technique which employs white light imaging to obtain surface displacements and hence strains. This study combines TSA and DIC to identify subsurface damage evolution and the source of thermoelastic response under cyclic loading. The non-adiabatic thermoelastic response at low loading frequencies is used to reveal damage in multidirectional coupons of carbon fibre reinforced polymer (CFRP). The paper considers the effect of loading frequency and shows that increasing the loading frequency prevents internal heat diffusion and as a result achieves adiabatic conditions. It is shown that the heat diffusion that occurs at lower loading frequencies allows the thermoelastic response from the subsurface plies in the CFRP laminate to influence surface response. Hence, appropriate selection of loading frequency results in either surface or combined surface and subsurface thermoelastic responses. Conversely, DIC relies on the kinematics of the surface and therefore is independent of heat diffusion effects. Additionally, both composite components’ stacking sequence and ply fibre orientation perform an essential role in the thermoelastic emission since the mechanical and thermal properties’ directionality influences the result. The main purpose of the paper is to study the damage evolution of the CFRP laminate at different loading frequencies.