Composite Materials Strength Determination Within the Current Certification Methodology for Aircraft Structures

Although several composite failure criteria have been proposed over the years to predict the static strength of polymer composites, including micromechanical and first-ply failure theories, none of them has demonstrated an ability to predict the onset of damage across the wide range of load conditions and structural configurations. For that reason, current aerospace practice uses empirically derived laminate-based allowables to demonstrate compliance with regulatory requirements for static strength. Some examples of certification by analysis supported by test evidence as well as certification by test are given, and the fundamental differences are highlighted in the context of the building block philosophy. The lack of accepted failure criteria and general material-degradation models for the prediction of damage initiation and propagation provides the background for the current methodology, which relies either on testing alone or on analysis supported by extensive test evidence.