Reflective fringe patterns technique for non-destructive subsurface flaw detection

This paper presents a novel optical technique, Reflective Fringe Pattern (RFP), for non-destructive testing of objects having specularly or semi-specularly reflective surfaces. The principles are based on measuring the surface deformation of a test object and identifying deformation anomalies caused by subsurface flaws. In the set-up, a computer-generated fringe pattern displayed on a computer monitor is placed in front of the test object surface (which acts as a mirror) and forms a mirror image of the fringe pattern. The fringe pattern is perturbed according to the surface slope distribution. In the measurement, the fringe phase distributions before and after deformation are separately determined, and the difference of the phase distributions depicts the change of surface slope due to the deformation. In this paper, experimental demonstrations of the present technique to subsurface crack detection and adhesive bonding evaluation are presented. Two methods are also designed to enhance the flaw visibility on the output of RFP.