Publications

Experimental and computational investigations into the effect of process induced stresses on the mode I fracture toughness of composite materials

Nelson, Stacy M.; Werner, Brian T.

Process induced residual stresses commonly occur in composite structures composed of dissimilar materials. These residual stresses form due to differences in the composite materials' coefficients of thermal expansion as well as the cure shrinkage exhibited by polymer matrix materials. These residual stresses can have a profound effect on the measured performance of a loaded composite structure. A material property of particular interest when modeling the formation of damage in composite materials is the mode I fracture toughness. Currently, the standard method of measuring the mode I fracture toughness involves a double cantilever beam (DCB) experiment, where a pre-crack is introduced into a laminate and subsequently opened under tension. The resulting apparent fracture toughness from the DCB experiment may depend upon a coupled interaction between a material property, the mode I energy release rate, and the effect of residual stresses. Therefore, in this study, a series of DCB experiments are completed in conjunction with the solution of representative finite element models to quantify and understand the effect of process-induced residual stresses and temperature variations on the apparent fracture toughness. Specifically, double cantilever beam experiments are completed at three temperatures to characterize three types of specimens composed of carbon fiber/epoxy and glass fiber/epoxy materials: carbon bonded to carbon, glass bonded to glass, and carbon bonded to glass. The carbon-to-carbon and glass-to-glass specimens provide estimates of the composite's fracture toughness in the absence of significant residual stresses and the carbon-to-glass specimens indicate the effect of measurable process induced stresses. Upon completion of testing, the measured results and observations are used to develop high-fidelity finite element models simulating the residual stresses formed throughout the manufacturing process and the subsequent DCB testing of a laminate composed of the carbon/epoxy and glass/epoxy materials. The stress fields and delamination behavior predicted through simulation assist in understanding the trends observed during the DCB experiments and demonstrate the important relationship between experimental and computational efforts.