Development of a methodology for determination of interface fracture toughness of laminate composites-the 0°/90° pair

Experiments are carried out to determine the delamination toughness for a crack along the interface between two transversely isotropic materials. The material chosen for study consists of carbon fibers embedded within an epoxy matrix. A crack is introduced between two layers of this material, with fibers in the upper layer along the 0°-direction and those in the lower layer along the 90°-direction. The Brazilian disk specimen is employed in the testing. Finite element analyses are carried out to determine stress intensity factors arising from the applied load. Residual stresses resulting from the curing process create transverse cracks in the 90°-layer. Stress intensity factors pertaining to this stress field are obtained, as well. These stress intensity factors are superposed with those from the applied load to obtain the total stress intensity factor. From the load at fracture, the critical interface energy release rate Gic as a function of phase angle ψ is determined, and results are compared to a fracture criterion. Two lay-ups of the carbon fiber/epoxy material are examined.