Digital shearography for NDT: Determination and demonstration of the size and the depth of the smallest detectable defect

Abstract

This paper presents a methodology of digital shearography for determining the size of the smallest detectable defect and the depth under different loading magnitudes for the purpose of nondestructive testing. Digital Shearography, an interferometric nondestructive testing (NDT) technique, has been proven to be a useful tool for material inspections and evaluations, especially for detecting delaminations/disbonds in composite materials and honeycomb structures. A commonly asked question in the field of NDT is about measuring sensitivity - specifically, what is the smallest detectable delamination/disbond and how deep can these be detected? Although various attempts to find the smallest detectable defect by shearography have been made, a numerical model for determining the size of the smallest detectable defect and the depth has not yet been developed. This paper did a study in this aspect, especially for NDT of delamination and disbond in polymers and honeycomb structures. First, a mechanical model based on the thin plate theory to calculate the expected bending of close-to-surface defects was proposed; the model built a relationship among the deformation caused by a defect, the size and the depth of the defect, as well as the load and the material properties. Second, the relationship between the relative deformation measured by shearography and the deformation induced by a defect was established based on the optimized shearing amount and the sensitivity of digital shearography. Based on these analyses, relationships between the size of the smallest detectable defect and the depth under different load amounts were established for different defect shapes. Finally, experimental validation based on different sizes of prefabricated defects were conducted to verify these relationships. The experimental results show that the model developed can provide useful estimation for NDT by digital shearography, especially with helping test engineers estimate the size of the smallest detectable defect and the depth with corresponding loading magnitudes.