Abstract
This paper presents a method by which not only the shape but also the depth of defects in a plate is inspected nondestructively. A sandwich structure sample plate which consisted of three layers was made in order to demonstrate the usefulness of the method ; the middle layer is 1 mm in thickness and has six different round and square holes in it, the upper layer is 0.125 mm in thickness and the lower one is 0.254 mm in thickness. The plate is vibration stressed and inspected from both the upper layer side and the lower layer side by ESPI (Electronic Speckle Pattern Interferometry). A defect has two different resonant frequencies f and f' for the both sides. The shape of a defect is decided by ESPI fringe patterns and the depth of it is calculated only by knowing f, f' and the thickness of the plate. The experimental results showed the accuracy of the depth of a defect is 2 × 10-2 and 1 cm diameter sized defects can be observed in the 15 cm × 15 cm area.
