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2011 Contents

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Modal frequency vs. Shearography in detecting and locating voids/delaminations in sandwich composites (2011)

ABSTRACT

This paper reports on the attempt by the authors to establish a novel non-destructive testing protocol in detecting the presence and location of subsurface defects, such as voids or de-laminations, in sandwich composites.

The work performed was based on the premise that the presence of a defect in a composite structure would affect its modal frequencies, when subjected to vibratory excitation. Experiments were carried out using various types of sandwich composites fashioned as built in cantilever beams which were perturbed by airborne vibrations. As predicted by a parametric finite element analysis the modal frequencies of the cantilever were modestly affected by the presence of a defect, however its position along the length of the beam could not easily be determined. For comparison purposes the same cantilever specimens were subjected to thermal excitation and inspected using a portable digital shearography system. As expected the presence and location of the defects were readily and vividly identified

Complete Paper (480kB)


A Comparison of Results Obtained from Alternative Digital Shearography and ESPI Inspection Methods(2011)

ABSTRACT

Digital Shearography (DS) and Electronic Speckle Pattern Interferometry (ESPI) are two optical interference techniques used, amongst other, in the field of non destructive testing. DS captures the rate of surface displacement and ESPI the displacement of an object in response to an applied stress. The addition of phase stepping techniques allows for the surface displacement of the object to be reconstructed. By applying a suitably large shear to the DS process, pseudo ESPI results can be obtained without all the inherent environmental vibration considerations associated with ESPI, thus potentially extending the use of conventional DS equipment to ESPI applications

This paper describes the above mentioned inspection techniques and examines the fringe pattern results and unwrapped displacement map results obtained from DS, pseudo ESPI and conventional ESPI techniques applied to 2 samples containing defects. In particular the results from the DS and pseudo ESPI techniques are compared with the reference ESPI results to determine how well they correlate, which ultimately attempts to determine the suitability of the extended DS technique as an alternative ESPI inspection technique.

Complete Paper (480kB)


Determining Residual Stress And Young’s Modulus – Can Digital Shearography Assist(2011)

ABSTRACT

Residual Stresses are inherent in most materials and structures which have been exposed to a machining or manufacturing process. They are known to have both a beneficial as well as detrimental influence on the performance of manufactured components and yet often go by undetected.

This paper presents the results of a research project aimed at determining the use of Digital Shearography as a suitable method to identify inherent material and structural properties. In order to apply the technique, 3 samples are prepared, one in its fully annealed state and the other 2 with different levels of residual stresses introduced into one of the sample surfaces. This is repeated for three different materials. Investigations are then conducted to determine the samples deflection curvatures in response to an applied load. From these deflections the investigation attempt to determine the Young’s Modulus and magnitude of residual stresses present. The results are presented and compared with tensile specimen results for accuracy. From the results obtained it is apparent that Digital Shearography can be used to detect the presence of residual stresses and also determine the material’s Young’s modulus.

Complete Paper (480kB)