Detection of sub-surface defects in vinyl tiles using double-exposure holographic interferometry

Abstract

Double-exposure holographic interferometry (DEHI) has been widely used in detecting surface and sub-surface defects in many application areas such as aircraft tires, musical instruments, artworks and many composite materials. Detection of sub-surface defects, like voids and cracks, is important in product quality assurance, long-term material stability and material property investigations. In DEHI the inspected sample is mechanically or thermally loaded and the resulting deformations are made visible as fringe patterns in a holographic interferogram. Local defects lead to typical local deformations, which deviate from the global deformation. These defects are then identified by their characteristic local interference patterns. DEHI is most successfully applied to problems where the defect exists near the surface of a material, which has either relatively low stiffness or is thin and the area of the defect is somewhat larger than the minimum detectable fringe spacing of about 0.1 cm. In the detection of sub-surface defects, the primary interest is the identification of the location and nature of the faults without performing any quantitative evaluation because of the many possible global fringe patterns and the variety of local patterns typical of the defects. In this paper, we have investigated the thermal response of a polymeric material and established the characteristic fringe patterns formed when there are sub-surface defects.