Tuesday  |  Salon 9  |  10:20 AM–10:40 AM

#15965, Damage Quantification in Multi-directional Laminates using Thermoelastic Stress Analysis

During in-service operation, laminated composite components are exposed to complex thermo-mechanical loading scenarios causing damage initiation, accumulation and evolution. Hence comparing inspection techniques capabilities for a full-scale diagnosis of these materials possible defects and damage evolution has become a point of discussion. Thermoelastic Stress Analysis (TSA) is a full field infra-red imaging technique, which is used to determine the stress in component under cyclic loading. Traditionally TSA has been used to obtain only the stresses on surface of the material. The paper describes recent research demonstrating that TSA carried out on laminated composites at low frequencies allows ply-by-ply heat transfer and hence opens the possibility of acquiring information from inner plies (subsurface). It is shown that the stacking sequence plays a significant role in the thermoelastic response depending on the mechanical and thermal properties of each ply in the stack. Digital Image Correlation (DIC) is a full field surface measurement technique which employs white light imaging to obtain surface displacements and hence strains. In contrast to TSA, DIC relies on the kinematics of the surface and therefore is independent of heat diffusion effects. In the paper a damage parameter, D, is introduced that is directly calculated from a combination of both the thermoelastic response and the material stiffness derived from DIC. It is shown that D can quantify the degree of damage in Carbon Fibre Reinforced Polymer (CFRP) [90,0]3S and [0,90]3S coupons. The coupons are loaded under uniaxial tension designed so that damage evolves in the subsurface plies. The non-adiabatic thermoelastic response at low loading frequencies is used to reveal damage and then higher frequencies are used to quantify the effect of damage on the surface ply response when adiabatic conditions prevail. Simultaneously, DIC is used to obtain the stiffness degradation at different damage levels. It is demonstrated that damage quantification is possible in laminated composites by using TSA at different loading frequencies (3Hz - 20Hz) in combination with DIC measurements from the surface.

Rafael Ruiz Iglesias   University of Bristol
Geir Ólafsson   University of Bristol
Ole Thybo Thomsen   University of Bristol
Janice Dulieu-Barton   University of Bristol