Defect characterisation in FRP shell components using derivative-based optimisation methods

• Applicants: Dr. Jens Prager; Professorin Dr. Andrea Walther
• Subject Area: Measurement Systems ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 428590437

Non-destructive testing of components is in many cases performed using ultrasonic techniques. Standardised inspection techniques generally allow drawing conclusions on the reflectivity or sound attenuation of a defect, rather than providing information about its size and shape. To overcome this restriction imaging techniques are applied to reconstruct defects in voluminous components. These techniques, however, can not necessarily be applied to modern plate-like or shell-shaped components made of fibre-reinforced plastics (FRP) which exhibit anisotropic acoustic properties. The application of inverse techniques for defect reconstruction so far fails due to the lack of efficient techniques to simulate the propagation of ultrasound (forward models) in these structures. The reason for this is the unfavourable ratio of wavelength to the components’ dimensions which brings well-established numerical techniques to their limits since these discretise the whole inspection volume (e.g. FEM). The proposed research project applies novel semi-analytical approaches for efficient simulation of guided wave ultrasound propagation. Analytical optimisation strategies will complement these. The approach will result in an inverse technique which allows for the reconstruction of defect geometries, sizes, and positions using ultrasonic data acquired in shell-shaped anisotropic components. To achieve this goal a cooperative project is planned to interconnect various approaches. Algorithmic Differentiation (AD) to solve the inverse problem will be applied as well as the Scaled Boundary Finite Element Method (SBFEM) for simulating the wave propagation. Using different discretisation scenarios SBFEM provides benefits for the extremely efficient simulation of transducer-generated ultrasonic wave propagation in defect-free areas, as well as wave field interaction with defects. To empower the technique for the addressed issue, it has to be expanded and modified accordingly (e.g. by considering anisotropy and damping as well as optimising the meshing). The AD procedures for calculating derivative information for the optimisation process also have to be expanded and adapted to the simulation code. Both techniques will be adapted separately and then be combined. Experiments will support the investigation. In the first stage of the project, the development and the validation will be performed for isotropic plates. During a second stage - to be applied for after successful validation of the first phase - the project gradually will be expanded to cope with anisotropic shell-shaped structures with variable cross-sections. Sensitivity studies will evaluate the reliability of the inverse approach. The primary goal is the development of a reliable imaging technique for defect reconstruction for the inspection of FRP structures, but moreover, the scientific weight of the project also links to the further development of the numerical simulation and optimisation techniques under concern.

• DFG Programme: Research Grants