Towards diagnostics of aerospace structural defects using a novel physics-based post-processing scheme employing lock-in thermography

Abstract

During its service life, an aircraft experiences various loadings that affect its structural performance. Thin sheet riveted joints are ubiquitous in aircraft and require thorough inspections to arrest any defects that may cause catastrophic failure. Subsequently once flown off, loose and/or cracked rivets may get stuck in critical areas such as flight controls or get ingested into the engine and become a source of Internal Object Damage (IoD). While several inspection schemes are employed in the aviation industry for rivets-related defect detection, most of them are subjective and skill-dependent. Towards the aim of developing an efficient rivet defects diagnostic technique, in this research work, a novel physics-based post-processing scheme employing lock-in thermography (LIT) is proposed for the detection of such defects. For this purpose, a hierarchical experimental thrust has been adopted, where in the beginning the scheme was developed on the lab experiments which was further extended/refined to detect defects on actual aircraft. Two different post-processing techniques have been used for the interpretation of LIT, namely the Fast Fourier Transform (FFT) technique and an indigenously developed temporal temperature gradient-based approach to capture defects whereby localized thermal emissivity variance at the defect location affects the local thermal conductivity and the surface temperature profile. The study considers two common defect i.e., looseness and sub-surface cracks in rivets. The results obtained from the novel post-processing technique provide empirical understanding, better quantification, and clearer comprehension of the two types of rivets-related defects by making it more objective as compared to the FFT technique. The proposed novel methodology in this work exhibits higher fidelity than conventional FFT by accurately identifying the rivet defects thereby providing a promising alternative to in-vogue visual inspections.