Design and operation of multi-specimen fully reversed fatigue systems for advanced composite materials

Advanced composite materials are finding ever-increasing employment in aerospace structural applications. Generally, the composites appear as the ‘skins’ (surfacing elements) on sandwich components. A typical sandwich component consists of many subelements, i.e., skins, adhesive, core, sealants, etc. The degradation mechanisms for the overall sandwich components must take into account the aerospace environment including loads, temperatures and moisture as well as fabrication- and service-induced flaws in the overall sandwich construction. This paper describes an experimental system designed to investigate the static compressive strengths of high-strength graphite/epoxy composite sandwich structures with various defects, after exposure to combined moisture-saturation and elevated-temperature environment in the presence of fatigue stress cycling. The moisture, thermal exposure, and stress cycling were designed to simulate mission conditions. The equipment was designed to perform multiple-specimen testing of sandwich structures including various construction parameters and flaw types. Stress cycling is of the variable-amplitude type in a fully reversed mode.