Further development of digital shearography for dynamic deformation measurement and nondestructive testing

Abstract

Digital shearography is an interferometric technique for surface deformation measurement. It was invented to overcome several limitations of holography by eliminating the reference beam, thus leading to simplified setup, reduced coherence-length requirement of light source, and not requiring special vibration isolation. These distinct advantages have rendered shearography as a practical and employable measurement tool in industrial settings. In fact, it has already gained widely industrial acceptance, in particular for nondestructive testing applications. Shearography is particularly effective for detecting debonds in laminated composite materials. Hence, the rubber industry has been routinely employing shearography for evaluating tire quality, and the technique has been endorsed by the US Federal Aviation Administration (FAA) for inspecting aircraft tires. Shearography, however, is still relatively young and its full capability awaits further exploration. This research has developed several new techniques of shearography for dynamic deformation measurement and nondestructive testing. A new form of shearography referred to as Spatial-Frequency-Modulated (SFM) shearography has been developed. Traditionally, the output of shearography is in the form of a fringe pattern depicting the phase change due to deformation. To determine the phase change, it is necessary to capture at least three speckle images with different values of phase shifting at each deformed state. The requirement of phase shifting presents a problem in measuring dynamic deformation. The SFM shearography requires acquisition of only one single speckle image for each deformed state, thus enabling dynamic deformation measurement. In SFM shearography, a modulation fringe pattern in the form of a linear and parallel fringe lines is initially introduced before deformation. The production of the modulation fringe pattern is achieved by introducing a quadratic phase variation in the illumination laser light. Since shearography measures the first derivative of the phase change and the first derivative of a quadratic phase variation is a linear phase distribution, a fringe pattern of uniform spatial frequency (i.e. linear fringe lines of equal spacing) is produced. When the object is deformed, the SFM pattern will be distorted, thus resulting in a variation of spatial frequency change in the modulation fringe pattern, and the local frequency change carries the information about the deformation. In essence, a linear phase distribution is superimposed on the deformation phase of traditional shearography. To extract the deformation phase, an algorithm based on the Moiré effect is developed. A Moiré fringe pattern is generated by the multiplication of the deformed SFM pattern and a virtual linear grating digitally simulated in the computer memory. Since the phase of Moiré fringes can be changed by shifting the virtual grating, a conventional phase-shifted algorithm, such as the four-frame algorithm, can be used to determine the deformation phase. This process is carried out subsequent to a dynamic recording. The validity of the SFM shearography has been verified by measuring a plate structure of known deformation. In NDT applications, shearography reveals a material defect by identifying defect-induced deformation. Coupling with the SFM shearography, two new methods of nondestructive testing using dynamic stressing have been developed. Flash shockwave method irradiates an intense energy of very short duration to the test object surface thus inducing a transient thermal deformation. Induction method heats the electrically conducting test material by electromagnetic induction theory. Both methods do not produce intolerable rigid body motion nor change the reflective indexof surrounding air which can cause de-correlation in the speckle images. The dynamic deformation produced by both methods may be measured by SFM shearography. Another significant research contribution addresses a tall-building safety problem in Hong Kong. One of the main emphases in high-rise building maintenance is to locate areas where wall tiles are prone to detaching from external building walls, since fallen tiles endanger public safety. The research has developed sonic-shearography for testing the integrity of bonding between tiles and building walls. In this technique, the test area is excited using broadband frequencies generated by a computer-controlled powerful loudspeaker. The shearographic image of the test area is recorded continuously, and the recorded images are compared with one another. Since each pixel is corresponding to a point on the tiled wall surface, the algorithm developed to determine the peak deformation during the excitation allows the deboned area to be identified.

Date of Award	4 Oct 2010
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Michael HUNG (Supervisor)