<title>Nondestructive evaluation of aircraft structure using lock-in thermography</title>

Abstract

This paper describes experiments conducted with the lock-in thermographic procedure. A carbon fiber reinforced composite specimen with defects of various sizes and depths below the test surface was analyzed. The detectivity of AGEMA 900 lock- in system was investigated. The experimental results show that the detectivity of lock-in thermography depends on inspection frequency, intensity of heat source, resolution of lock-in system, distance between the IR camera and the object. It was found that inspection frequency has significant effect on the phase difference produced by a certain defect. At blind frequency, the defect produces no difference or very small difference. There are 2 optimum frequencies at which the defect produces maximum positive and negative phase differences respectively. At a depth that both high frequency and low frequency thermal waves can reach, lock-in thermography is more sensitive at high frequency. Lock-in phase sensitive thermography was found to be more sensitive than conventional reflection and transmission thermography. Sight line angle, which is the angle between the surface plane of the object and the sight line of the camera, has no significant effect on detectivity of lock-in thermography.