Delamination Onset in Polymeric Composite Laminates Under Thermal and Mechanical Loads

This paper describes a fracture mechanics damage methodology to predict edge delamination. The methodology accounts for residual thermal stresses, cyclic thermal stresses, and cyclic mechanical stresses. The modeling is based on the classical lamination theory and a sublaminate theory. The prediction methodology determines the strain energy release rate, G, at the edge of a laminate and compares it with the mode I fatigue and fracture toughness of the composite. To verify the methodology, isothermal static tests at 23°C, 125°C, and 175°C and tension-tension fatigue tests at 23°C and 175°C were conducted on laminates. The material system used was a carbon/bismaleimide, IM7/5260. Two quasi-isotropic lay-ups were used, [45/-45/0/90]_s and [-45/45/90/0]_s. Also, 24-ply unidirectional double cantilever beam specimens were tested to determine the fatigue and fracture toughness of the composite at different temperatures. Raising the temperature had the effect of decreasing the value of G at the edge for these lay-ups and also to lower the fatigue and fracture toughness of the composite. Experimentally, the static stress to edge delamination was not affected by temperature but the number of cycles to edge delamination decreased as temperature increased. The ply interface for delamination was well predicted. The predicted static stress and the number of cycles to edge delamination was generally conservative for the room temperature tests because of the large mode II G component present. For the elevated temperature tests the mode II component is smaller and the static stress and number of cycles to the onset to delamination was generally well predicted.