Development of a novel stressing method by means of thermal shock for non-destructive testing of large structures using shearography

Abstract

This applied research aimed to further develop shearography for nondestructive testing of large structures. Shearography is a coherent-optical method originally developed for full-field measurement of deformation. It was invented to overcome several limitations of holography by eliminating the requirement for a separate reference beam. Consequently, the method is more robust and can be employed in industrial environments. Shearography has received considerable industrial acceptance for nondestructive testing, in particular, the US Federal Aviation Administration (FAA) had specified it for nondestructive inspection of aircraft tires. In nondestructive testing, shearography revealing a material defect by detecting the defect-induced deformation anomaly. Since the underlying principle of this technique is based upon the response of a defect to the applied stress, it is necessary to apply suitable stress to deform the test object during inspection. The development of shearographic NDT applications has therefore become the research of a practical means of stressing the object that would readily reveal flaws. Ideally, the stress mode should be similar to the service stresses so that flaws that are critical and detrimental to the service life of the object would be revealed, and cosmetic flaws that do not undermine the structural integrity of the test object can be ignored. For large structures, such as a bridge or a pressure vessel, the application of shearography is very difficult, as it is not practical to stress the entire structure. Further, shearography has very limited tolerance to rigid body motion likely to occur in large structures during stressing, as excessive rigid-body motion would cause speckles de-correlation, resulting in degradation of fringe quality. A novel pulsed thermal stressing technique has been developed in this research for NDT of large structures. The technique is based on applying a pulse of heat which produces a local thermal deformation to a large structure. This stressing technique does not produce intolerable rigid body motion. In this research study, the technique has been successfully applied to (1) crack detection in panels and pipes for the pressure vessel industry; (2) assessment of adhesive debond in composites for automotive fabrication; (3) inspection of bonding integrity of laminated fiber reinforcement for concrete structures; (4) examination of spalling mosaic tiles in high-rise building façade, a serious public safety concern in Hong Kong. In addition, a new single-frame phase determination algorithm has been developed to obtain the shearographic phase data. This algorithm is essential for capturing the highly transient deformation induced by the pulsed thermal stressing whereas traditional temporal phase shift technique is not applicable.

Date of Award	2 Oct 2007
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Michael HUNG (Supervisor)