Nondestructive Testing And Fatigue Analysis Based On Lock-in Infrared Thermography

The grid stiffened composite structure is the next generation of advanced composite infrastructure with very promising future. Combining advantages of new material techniques and new structure design, it is widely used in industries due of its designable ability and various other advanced features. However, during design, production and implementation processes, unpredictable defects could be generated in grid stiffened composite structures, which seriously restrict their practical applications and the future of implementation. Fatigue failure is one of the most important failure modes for aircrafts, ships, motors and buildings. Fatigue failure often occurs in low stress level and it is unpredictable. Thus, it doesn't capture as much attention as it should. In this paper, based on lock-in infrared imaging theory, the non-destructive testing is applied to grid stiffened composite specimen with different kinds of defects. We also performed real time detection to fatigued specimen with defects and welded joint. The parameters of fatigue performance are also obtained. The study is supported by National Natural Science Foundation of China (No.10702012), a independent research project State Key Laboratory of Structural Analysis for Industrial Equipment (S08204) and National Natural Science Foundation of China (No.11072045). Major study of this thesis is as follows.1. Non-destructive testing on grid stiffened composite structure specimen based on Lock-in infrared Imaging theoryGrid stiffened composite structure is widely used because of its outstanding capabilities. However, regular testing method can not effectively detect defects inside of the material in a non-destructive way. Hence the application scope of grid stiffened composite structure is limited. In this thesis, based on Lock-in infrared Imaging theory, non-destructive testing is performed to detect potential defects, such as inclusion,crack and deboning. The result is verified by a Lock-in infrared Imaging system developed by Cedip, a French company. The impacts of different loading frequencies and loading strength have been studied. Combined with the tensile mechanical experiment, a testing on hat stiffened composite laminated plates has been performed. The results show that the non-destructive testing based on Lock-in infrared Imaging can successful detect unknown defects in grid stiffened composite structures.2. Non-destructive testing on specimen with defects and analysis of its parameters of fatigue performance Fatigue defects exist widely in industrialized manufacturing and human lives. Accidents caused by fatigue defects are extremely dangerous and destructive. However, it is challenging to predict the fatigue defects and it is difficult to conduct real-time monitoring. In this paper, non-destructive testing is applied to photo-stimulated loading on fatigue specimen with different defects. The relationships between testing accuracy and testing parameters such as testing frequency and defects size are studied. The study concludes that proper testing frequency is the key to success. TESCC is found out and the impact of loading stress amplitude on TESCC is analyzed. The fatigue life of the specimen is calculated using Lung method and the impact of defect depth on fatigue life is studied.3. Evaluation of fatigue performance for welded joints based on quantitative infrared thermographyEvaluation of fatigue performance of welded joints is very important and critical during the design of the welded joint structure, especially for the areas with stress concentration and residual stress. Currently the traditional methods are not able to accurately predict the life of welded joint structures, hence new evaluation system has to be developed. Based on the actual working environment of welded joint and considered the impact of constant mean stress, a model to rapidly predict parameters of fatigue performance and remaining life is developed. From the change of local hot spots detected by infrared camera, the evolution of defects is simulated and analyzed. This new methodology overcomes the restrictions of traditional fatigue testing methods and makes real-time testing possible.