Brake Block Internal Air Bubble Detection By Lock-in Thermography

Brake block existing detection method mainly relies on artificial judgement, and its detection efficiency is low. There is few research about brake block non-destructive detection automatically. Infrared thermography technology has advantages of non-contact, large detection area, which is applied widely in many fields. This dissertation explores a lock- in thermography detection method based on induction heating for brake block internal air bubble detection. The theoretical research, simulation analysis and experimental verification are discussed in this dissertation.According to the structural characteristics of brake block, the lock- in thermography detection system based on induction heating is designed. The building process of detection system is introduced. The design principle of excitation coil is discussed. Under the condition of different excitation coil, the response characteristic of the induction heater is measured, and to realize the accurate control of heating time. Data acquisition software is developed. In order to realized the data post-processing immediately, the measures of reducing the amount of data collected by reducing the sampling frequency and choosing reasonable observation area are adopted. The experiment conditions of lock- in thermography are analyzed, mainly involving the influence of modulation frequency, air convection and heating uniformity. The simulated defect sample is tested, to verify the validity of the lock- in thermography.A three dimensional model is established and the brake block heat transfer process is simulated. Through the simulation analysis, the phase variation law with structure and internal air bubble are proved. Through phase subtraction can effectively reduce phase change caused by structure, improve the signal- to-noise ratio, and the phase change caused by internal air bubble is more obvious. The brake block samples are tested, and validate the results. Experimental results show that the lock- in thermography detection method can effective ly detect the brake block internal air bubble.