Experimental and analytical studies on the damage initiation in composite laminates at cryogenic temperatures

This paper describes the development of damage in the form of transverse microcrack and interlaminar delamination within laminates under combined thermal and mechanical loadings. The material system investigated was a carbon fiber-reinforced polymer composite. The thermomechanical properties required for the analysis were obtained from tests on unidirectional laminates. The laminates investigated for damage initiation and growth were [30/−30/90]s and [0/±45/90]s. Mechanical testing was conducted at 23 °C and −196 °C. The microcrack and delamination were determined using acoustic emission and/or small incremental step loadings. The residual stresses due to difference in curing and test temperatures were calculated using classical laminated plate theory with thermo-mechanical properties generated, and their effect on the initiation of damage was assessed. The stress levels at the initiation of microcrack and delamination determined by experiment were compared with analytical predictions using the maximum stress criteria. The interlaminar free-edge stresses were analyzed using a global–local model. The results of this stress analysis were compared with the experimental results. While the residual stresses increase with decreasing test temperature, so do the composite transverse and shear strengths, and these two effects compete in their influence on the stress at onset of initial damage.