Thermal strains in heated Fiber Metal Laminates

Current trends in aircraft design go towards smart materials and structures including the use of multi-purpose materials. Fiber Metal Laminates (FML) with embedded electrical heater elements in leading edges of aircraft used for anti- or de-icing follow those trends. The laminated structure of FMLs with layers of different materials leads to anisotropic material characteristics. The FML used in this study is GLARE (Glass Laminate Aluminum Reinforced Epoxy). The anisotropic structure raises questions concerning possible effects on the material characteristics when frequently heated by embedded heater elements and cooled by flight conditions. In order to investigate those possible effects on FMLs, knowledge about the thermal strains and stresses is important. Furthermore, non-destructive techniques are likely to be a future requirement to detect defective heater elements and delaminations at heated leading edges. Thus, this research uses a shearography (speckle pattern shearing interferometry) instrument in order to investigate the surface strain components of FMLs during thermal loading with the embedded heater elements. Parallel to the experiments, numerical analyses were conducted in order to investigate the strain-stress state due to thermal loading with embedded heater elements. The results of both, the strain measurement with the shearography instrument and the numerical analyses were analyzed and compared. The numerical results show how the embedded heater element affects the residual stress-strain state and the stresses due to thermal loading.