Consideration of Environmental Conditions for the Fatigue Evaluation of Composite Airframe Structure

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This paper reports on experimentally derived compensation factors to account for missing environmental history (temperature and humidity) during fatigue testing of large composite structures (components). Compensation factors are either factors by which the fatigue load level of component fatigue tests is increased or by which the required residual strength has to be increased if preloading did not include environmental history. Preconditioned coupon specimens were subjected to accelerated quasirealistic mechanical and environmental flight-by-flight loading, and other coupon specimens were loaded exactly like the composite airframe structure (that is, with less complete simulated environment). Temperature and moisture content in residual strength testing were the same for surviving coupon specimens and the composite airframe structure. Corresponding to composite airframe structures of the Tornado and the Airbus fin box, the materials involved were the 175°C curing systems Ciba 914C and Fiberite 1076 E, and the 125°C curing systems Ciba 913C and Hexcel F550 with T300 carbon fibers in all resins. Different types of laminates and specimens were used to generate predominantly matrix shear stresses or fiber normal stresses within the coupon specimens. At present design load levels, the fatigue life to be proven was reached without failures of the coupon specimens subjected to the flight-by-flight load conditions of the component test or to quasirealistic combined mechanical and environmental loading. Only small differences in residual strength were noticed among the surviving specimens. At higher fatigue load levels the superimposed more complete environmental test conditions decreased the life to failure significantly, by factors between 1.5 and 9.1.

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