Effect of processing induced defects on the failure characteristics of graphite epoxy angles

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Virginia Polytechnic Institute and State University


The objective of this study was to investigate the bending strength and failure characteristics of AS4/3501-6 and AS4/1806 graphite/epoxy angles sections as a function of processing induced defects and porosity. The angle sections were removed from 30-inch long angles fabricated at Lockheed Georgia Company with two quasi-isotropic stacking sequences, (± 45/90₂/ ∓ 45/0₂), and (± 45/90₂ ∓ 45/0₂)₃. Various degrees of porosity were introduced into the angles using four processing techniques: a standard lay-up, a solvent wipe during lay-up, moisture introduction between plies during lay-up, and a low pressure cure cycle. Two 2.5-inch wide angle sections, each with a 1.5-inch short leg and a 3.0-inch long leg, were bonded together along their long leg to form a T-shaped specimen. Bending of the T-specimen was introduced by pressing up on the underside of the flanges while holding the base of the specimen fixed. The experimental results have shown a significant effect of the processing induced defects on the failure load and bending stiffness for AS4/3501-6 specimens, but not for AS4/1806 specimens.

An anisotropic analysis of the angle curved section was performed using Lekhnitskii's stress function approach. Stress and strain fields were studied and two failure criteria (Dual maximum stress and Tsai-Wu) were investigated in order to predict T-specimen failure load and failure mode. Reasonable correlation between prediction and experiments was found for the AS4/3501-6 (± 45/90₂/ ∓ 45/0₂)₃ T-specimens, but both failure criteria were found to be too conservative in predicting failure for the AS4/3501-6 (± 45/O₂/ ∓ 45/90₂)₃, T-specimens. The predicted failure modes were in good agreement with the experimental observations for both Iaminates.