The end notched flexure (ENF) specimen is employed in an investigation of the in terlaminar fracture toughness in Mode II (skew symmetric shear) loading of unidirectional graphite/epoxy and graphite/PEEK composites. Important experimental parameters such as the influence of precracking and the data reduction scheme for the Mode II toughness are discussed. Nonlinear load-deflection response is significant for the tough thermo plastic resin composite but is also present for the brittle thermoset composite. The ob served nonlinearities, which are highly rate dependent, are attributed to a combination of slow stable crack growth preceding unstable crack growth and material inelastic behavior in the process zone around the crack tip.
The end notched flexure (ENF) specimen is employed in an investigation of the interlaminar fracture of unidirectional graphite/epoxy, graphite/polyimide, and graphite/PEEK composites subjected to Mode II fatigue loading. Fatigue crack growth rates are determined for the stable delamination growth observed under fully reversed cyclic loading. Within the crack driving force range studied, cyclic crack growth data obeyed a power law dependency on the cyclic strain energy release rate. The response of the various material systems is compared to crack growth under static loading. The importance of the data reduction scheme for the crack growth resistance is discussed. Friction is examined as a potential energy absorbing mechanism in the test by finite element analysis. Suggestions for appropriate experimental geometries minimizing frictional effects are presented. Finally, fatigue fracture surfaces are analyzed with scanning electron microscopy to identify crack growth mechanisms.
The structural performance of thickness-tapered laminates has been investigated using an energy-based damage tolerance methodology. The geometry studied is a thin laminate with discontinuous internal plies and a through-width delamination embedded at the interface between continuous and discontinuous sublaminates. An analytic model, based on shear deformation plate theory and linear-elastic fracture mechanics is employed to determine the Mode I and Mode II components of strain energy release rate. A two-dimensional plane strain finite element analysis is conducted to confirm the accuracy of the analytic predictions. The resulting pure mode strain energy release rates are combined with a mixed-mode growth criterion to predict the axial load required to induce delamination growth. Finally, the analytic and numerical model were used to predict failure in a delamination critical test specimen. Reasonable agreement of the actual and predicted failure loads was observed.
The influence of induced interlaminar loads on the structural performance of thickness-tapered laminates is investigated using an energy-based damage tolerance methodology. An analytical model based on the assumptions of shear deformation plate theory is presented. The model employs linear-elastic fracture mechanics to determine the strain energy release rate characteristic of delamination growth from an embedded flaw. The solution is general in terms of taper thickness, delamination size, laminate fiber orientation and material interlaminar fracture toughness. As such, the model provides a preliminary design tool for the evaluation of interlaminar performance of symmetric tapered composite laminates.
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