Measurement of residual strains as a parameter of matrix cracking in CFRP laminates

Abstract

Matrix cracking in CFRP laminates results in degradation of mechanical properties of the material and appearance of residual strains. In this study, the residual strains are investigated experimentally and analytically for CFRP [0/75₆]_s laminates. The typical stress-strain curves of cracked CFRP always show the presence of residual strain, a nonrecoverable strain in the laminates as the strain does not return to 0% even after being unloaded back to 0 MPa. This is a phenomenon where the matrix cracking in laminates increases the residual strain due to relaxation of curing stresses. Initially, the coefficients of thermal efficiency (CTE) of the laminates is measured to be used in the analysis to predict the residual strain due to the matrix cracks. The strain gauges were used in this study to measure the strains. Due to very small residual strains at the unloading condition, the residual strains were also measured at different stress levels for laminates with different crack densities and are compared with theoretical predictions. Time-dependent viscoelastic behavior of the material is also considered to accurately measure the residual strains due to the occurrence of matrix cracks. This was done by using the strain recovery test when the loads were stopped for 1-1.5 hours during unloading and the strain changes during these times were recorded. The experimental results of the residual strains are in reasonably good agreement with the theoretical predictions. The fiber non-linearity properties of the laminates may cause some experimental data to shift above the analytical line.