Shearography (speckle pattern shearing interferometry) is a non-destructive testing technique that provides full-field surface strain characterization. Although real-life objects especially in aerospace, transport or cultural heritage are not flat (e.g. aircraft leading edges or sculptures), their inspection with shearography is of interest for both hidden defect detection and material characterization. Accurate strain measuring of a highly curved or free form surface needs to be performed by combining inline object shape measuring and processing of shearography data in 3D. Previous research has not provided a general solution. This research is devoted to the practical questions of 3D shape shearography system development for surface strain characterization of curved objects. The complete procedure of calibration and data processing of a 3D shape shearography system with integrated structured light projector is presented. This includes an estimation of the actual shear distance and a sensitivity matrix correction within the system field of view. For the experimental part a 3D shape shearography system prototype was developed. It employs three spatially-distributed shearing cameras, with Michelson interferometers acting as the shearing devices, one illumination laser source and a structured light projector. The developed system performance was evaluated with a previously reported cylinder specimen (length 400 mm, external diameter 190 mmm) loaded by internal pressure. Further steps for the 3D shape shearography prototype and the technique development are also proposed.
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.questions concerning effects on crack and delamination initiation and growth due to different thermal expansion coefficients. Hence, quantification of the thermal strains and stresses is important in order to investigate the possible effects of thermal cycling, i.e. when frequently cooled and heated by flight conditions and embedded heater elements.
The development of smart materials for embedding in aerospace composites provides enhanced functionality for future aircraft structures. Critical flight conditions like icing of the leading edges can affect the aircraft functionality and controllability. Hence, anti-icing and de-icing capabilities are used. In case of leading edges made of fibre metal laminates heater elements can be embedded between composite layers. However this local heating causes strains and stresses in the structure due to the different thermal expansion coefficients of the different laminated materials. In order to characterize the structural behaviour during thermal loading full-field strain and shape measurement can be used. In this research, a shearography instrument with three spatially-distributed shearing cameras is used to measure surface displacement gradients which give a quantitative estimation of the in-and out-of-plane surface strain components. For the experimental part, two GLARE (Glass Laminate Aluminum Reinforced Epoxy) specimens with six different embedded copper heater elements were manufactured: two copper mesh shapes (straight and S-shape), three connection techniques (soldered, spot welded and overlapped) and one straight heater element with delaminations. The surface strain behaviour of the specimens due to thermal loading was measured and analysed. The comparison of the connection techniques of heater element parts showed that the overlapped connection has the smallest effect on the surface strain distribution. Furthermore, the possibility of defect detection and defect depth characterisation close to the heater elements was also investigated.
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