No abstract

The application of thermoelastic stress analysis in composite materials is particularly complicated because of the anisotropy of the material, that determines the thermoelastic constant to be dependent on the direction of the fibers. A further difficulty depends on the constructive stratification of the material, whose mechanical properties vary with the depth from the surface and this causes thermoelastic constants to be dependent on the frequency of the load applied. By using an analytical two layer model, it has been possible to interpret experimental data, thus proposing an explanation of the dependence on the frequency of the measured thermoelastic constants. This has shown that the practical use of the thermoelastic effect for quantitative stress analysis on composites needs constants calibrated at the correct frequency, also considering the thin layer of superficial resin present in every composite material.

Abstract. Thermoelastic stress analysis (TSA) has been applied to measure the first stress invariant on a composite helicopter component under load. The component comprised inner mono-directional glass fiber layers with an outer central plate in glass fiber cloth, covered by an anti-fretting surface coating. In order to obtain quantitative results, a previous calibration of the thermoelastic constant obtained on a composite sample with a similar anti-fretting coating has been necessary.

Orthotropic materials show different thermo-elastic constants depending on the orientation of fibres. While most of materials undergo a positive elongation with an increasing temperature, carbon fibres present a heat-shrink behaviour, which in carbon fibre composites has an important consequence on thermoelastic constants. A decrease of the thermoelastic constant with the frequency has been already observed in glass fibre composites. Experiments made on uniaxial carbon fibre composites showed that the longitudinal thermoelastic constant increases with the frequency, while the transversal one decreases. Furthermore, due to the opposite sign of the thermoelastic constant of carbon fibres and that of the surrounding matrix, the absolute values of the longitudinal thermoelastic constant resulted to be in CFRP ten times lower than in GFRP. An analytic model could successfully reproduce the frequency dependence of the longitudinal thermoelastic constant, thus helping in explaining the reason for the observed behaviour. Two calibration samples were used to obtain the thermoelastic constants in longitudinal and transversal direction. The values of the thermoelastic constants were then applied on a test sample with fibres forming 10° with the direction of the load. The expected theoretical results were compared to the results experimentally obtained, showing a good agreement. A preliminary calibration of the longitudinal and transversal thermoelastic constants showed to be a useful approach to obtain the correct value of the thermoelastic constant in a generic direction