The ability to predict stresses and failures caused by out-of-plane loads has gained importance as airframe manufacturers begin to use integral (co-cured or bonded) composite structures to minimize weight and maximize performance. Rapid and accurate analysis methods are needed to reduce the amount of testing required to ensure confidence in integral composite structures. Recently, simple two-dimensional analysis methods were developed to predict the out-of-plane failure strengths of composite airframe structures. These analyses were primarily derived to address induced stresses in laminate corner radii, direct stresses as a result of fuel pressure loads, induced stresses as a result of panel buckling, and induced stresses as a result of stiffener runouts or other load path changes.
The development and verification of analyses for out-of-plane failures have shown the need for out-of-plane material properties and strength allowables that are not traditionally determined for composite laminates. Accurate stress and failure predictions depend heavily on these experimentally determined values. The test methods and experience gained in this program indicate the need for new techniques to determine the interlaminar properties required for accurate stress and failure predictions. Test techniques are suggested for determining some of the values necessary for these predictions.