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2012 Contents

Determining Residual Stresses with the Aid of Optical Interference Techniques (2012)


Most manufactured components contain residual stresses, introduced during the manufacturing process, which have the potential to affect the performance and life expectancy of the final product. .

This paper presents further findings of a research project which investigates the suitability of using optical interference techniques, namely Digital Shearography and Electronic Speckle Pattern Interferometry, to non-destructively determine inherent material properties including residual stresses. The method relies on comparing the deformation characteristics of samples containing residual stresses to the deformation characteristics of fully annealed samples. Initial test results using Digital Shearography applied to cantilever samples were encouraging and have been extended to steel and aluminium flat bars subjected to 3 point bending tests using Digital Shearography and Electronic Speckle Pattern Interferometry. In this paper the two inspection techniques are described and applied to the prepared samples in order to determine the deformation curves in response to the applied stresses. From the test results the magnitude of the induced residual stresses for the different samples are calculated and presented. Comparisons between the information obtained from both techniques are drawn and discussed and pertinent aspects highlighted.

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Using UT to characterize defects in composites detected with Digital Shearography


This paper reports on a nondestructive application where the use of traditional ultrasonic testing may aid in characterizing the defects that are detected using the laser based optical interference technique known as Digital Shearography. The composite materials tested are both of the laminate and sandwich type construction. The portable Digital Shearography system employed with regard to evaluating the integrity of composite materials has proven to be particularly user friendly, revealing the whole field under test in almost real time. Barely visible or invisible impact damage, delaminations and cracks in composite specimens were localized using mild surface heating between the two captured images required by the Digital Shearography technique. Subsequent testing, using a portable ultrasonic tester, aimed at characterizing the defects as they were detected in terms of their extent and depth position relative to the near or test surface.

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