Research On Damage Imaging Methods Based On Laser Ultrasonic Guided Waves

In aircraft non-destructive testing technologies,the corrosion detection of aging aircraft and the damage detection of new materials have always been the focus of research.Metal and its composite structures are often used in the manufacture of main structures and pairs.Carbon fiber reinforced composite materials are widely used in the fuselage,wings,tail and other parts.The strength and integrity of structural materials are important guarantees for safe flight,and frequent inspections and maintenance are required to eliminate potential hazards.The research on nondestructive testing of aviation materials has huge social and economic benefits.Laser ultrasonic guided wave testing technology is a new type for nondestructive testing that uses pulsed laser to excite and continuous laser to detect guided waves to evaluate the status of the workpiece.This technology has the advantages of laser ultrasonic testing and guided wave.And it can achieve complete noncontact detection and be used for the extreme environments such as high temperature and high pressure,have high sensitivity to small defects and so on.The laser ultrasonic technique has the potential for damage detection and research value.Aimed at solving the problem of non-destructive testing for aerospace structures,laser ultrasonic guided wave technology has attracted more and more attention.Based on all-optical laser ultrasonic detection system(pulsed laser is excited at a fixed position,continuous laser scanning is received),corrosion detection for metal structure and delamination detection for composite material are focused.Four laser ultrasonic guided wave damage imaging methods are proposed and developed.Combined with the laser ultrasonic numerical model constructed by finite element method,the wavefield and wavenumber characteristics in aerospace structures are analyzed.The applicable conditions of different damage imaging methods are also refined,preparing for the actual detection.The main research contents and innovative work of this paper are summarized as follows:(1)Multi-frequency local wavenumber based on space windows is developed for quantitative assessment of hidden corrosion in thin metal plates,achieving threedimension accurate damage imaging.This local wavenumber is obtained from a shortwindowed Fourier transform in space domain of the wavefield.In the researches on corrosion detection,the local wavenumber based on spatial window is rarely studied.In addition,the selection of the mode and frequency for corrosion damage detection of thin metal plate lacks the corresponding theory.Sensitive mode and frequencies can be obtained from the analysis of wavenumber to thickness changes.The frequency selection is critical for the algorithm and it can be concluded by discussing the influence of frequency on imaging results.The better results of corrosion detection can be achieved by multi-frequency method.The algorithm has been verified in simple and complex corrosion situations.(2)Multi-frequency local wavenumber based on wavenumber window is proposed for quantitative assessment of hidden corrosion in thin metal plates,achieving threedimension accurate damage imaging.This local wavenumber estimation is obtained from a sliding wavenumber window that maximizes the wave energy at each grid point.For a full non-contact laser ultrasound system,the damage imaging results obtained by single-frequency local wavenumber have background noise and low fidelity,which can be improved by using multi-frequency methods.The application of multi-frequency local wavenumber based on wavenumber window needs specific instructions on wavenumber window and frequency.The size of wavenumber window and frequency selection for this algorithm are also discussed using numerical simulation methods.The out-of-plane displacement is also studied for corrosion detection.The algorithm has also been verified in simple and complex corrosion.(3)The algorithm of localized wave energy is proposed for quantitative assessment of delamination damage in metal composite structure,achieving accurate damage imaging.Localized wave energy is acoustic energy in space under specific wavenumber.New wavenumber components occur in damaged composite plates and its localized wave energy can be used for delamination identification.Guided waves have different sensitivities to delamination damage of different sizes and positions,so it is necessary to study multi-frequency methods for detection.The analysis of localized wave energy in metal composite structure with delamination can help get its guided wave propagation characteristics.The influence of scattering waves on localized wave energy is studied in detail.The choice of wavenumber threshold and frequency is elaborated in detail,and multi-frequency experimental results show lower noises and more discernible profile of delamination region in two specimens.(4)The algorithm of localized wave energy with different time windows is proposed for quantitative assessment of delamination damage in carbon fiber matrix composites,achieving three-dimension accurate damage imaging.The application of laser techniques to detect delamination faces difficulties with ultrasonic wave excitation because of its low thermal conductivity.In this study,aluminum foil was pasted onto the surface of the composite so that it was vulnerable to ablation and the acquired signals were usable.The effects of different aluminum foil sizes and shapes on the wavefield were studied for the composites.Most of the literature only focuses on the detection of a single delamination damage,and the detection of multiple delamination damages should be paied more attention.The localized wave energy with different time windows can realize the imaging of each damage in carbon fiber reinforced composites with multiple delamination damages.