As part of an overall effort to develop durability and damage tolerance methodology for graphite-epoxy composites, coupon specimens have been developed to measure the fundamental static fracture and subcritical growth behavior of delaminations. Two basic designs, one for the tensile opening mode (Mode I) and one for the forward shear mode (Mode II), are described. These specimens were used to characterize the behavior of two types of interfaces (0/0 and 0/90) for static fracture, constant amplitude fatigue, and spectrum fatigue. Fracture mechanics technology was applied through the principles of strain-energy release rate. Three-dimensional finite-element analyses were employed to interpret the experimental results. A simple growth law was shown to correlate the constant-amplitude and spectrum-growth data. It was found that the applied cyclic load must be nearly equal to the critical static load to obtain observable growth in the tensile opening mode. On the other hand, the graphite-epoxy delamination growth rate in the forward shear mode is comparable to the aluminum growth rate in tension, which suggests that shear is the chief subcritical growth mode for graphite-epoxy.

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The Mode II interlaminar fracture of unidirectional graphite/epoxy (AS4/2220-3) and graphite/polyetheretherketone (APC-2) composite laminates subjected to impact loading is investigated using the Center Notch Flexural (CNF) test specimen. Experimentally, instrumented impact testing of the CNF specimen enables the impact force history and the absorbed energy during delamination propagation to be estimated. Scanning electron microscopic examination is conducted to assess the influence of impact loading on fracture morphology in the crack initiation and propagation regions of the fracture surface. Data reduction for evaluating the Mode II interlaminar initiation toughness is based upon beam theory that includes kinetic energy effects and dynamic finite element analysis of the specimen in conjunction with virtual crack closure techniques. The initiation toughness under impact loading was approximately 20 and 28% lower than the static values for AS4/2220-3 and APC-2, respectively. Data reduction for evaluating the Mode II interlaminar propagation toughness is based upon the area method and a 24 and 44% reduction in fracture toughness is measured for the AS4/2220-3 and APC-2 composite materials, respectively.

Ten different orientation arrangements (in various panel thicknesses) were used in fabricating 72 panels. Eighty-four tests were performed, and the range of R/t studied was 100 to 400. All specimens had a 12-in. outside radius and were 13-in. long by 9-in. wide. The results included development of the moire grid-shadow technique and the South well plot method for nondestructive testing as well as observations of the effects of orientation and R/t on the buckling behavior and imperfection sensitivity of such shells. Satisfactory correlation was obtained with a laminated anisotropic curved panel analysis using the Rayleigh-Ritz energy method, which included a crude assumption that the thickness variations in the panels could be used to measure imperfection amplitude.