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2006 Contents



This paper deals with the methodology of determining residual stresses using Digital Shearography (DS). DS is a non-contacting, whole field non-destructive testing and evaluation technique which uses the monochromatic characteristics of a laser to produce speckle interference images. Once processed into images, the resultant fringe patterns reveal surface displacement gradients with sensitivities greater than the wavelength of the laser light used for the experiment.

In a simple experiment, a rectangular aluminium plate was subjected to uniform loading and the out-of-plane surface displacement gradient field was obtained through DS. Subsequently a comparison of these measurements was made with measurements obtained after the plate had been subjected to shot peening. The above method aimed to identify the extent to which residual stresses had been imposed on the rectangular plate due to the shot peening process, which is known to induce such stresses. The results of this investigation are presented and discussed.

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Materials of composite nature such as for example carbon fibre skin, sandwiching Nomex core and other honeycomb structures are increasingly being utilized in the Aerospace industry due to the benefits of reduced weight, the ease of formability and good mechanical properties that they offer. Composite materials are difficult to inspect for flaws such as delaminations and disbonding using traditional NDT techniques, however Digital Shearography is demonstrably well suited for the purpose and particularly when applying vacuum stressing.

The paper reports on developments regarding the challenge of transferring Shearographic inspection from the confines of the laboratory to the industrial environment, the development of a portable vacuum chamber which would be placed on the surface to be inspected, and guidelines on the technique's sensitivity or resolution in attempting to detect hidden flaws of differing reduced magnitude. The results indicate that vacuum stressing is particularly desirable when inspecting laminates for disbonding or delaminations because its nature dramatically highlights the flaw, with the added benefit of allowing the detection of relatively small magnitude defects.

Notwithstanding the fact that the Shearographic method is an optical interferometric technique, an additional optical element, such as the window of a vacuum chamber, interposed between the shearography optics and the surface of the component under test, did not affect the procedure in any manner. This fact provided the confidence to proceed in developing the portable vacuum chamber, in that the current Digital Shearography system's versatility would be conserved by not transforming it to a turn key solution for only composites vacuum testing.

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It is a well known and documented fact that engineering components develop defects and flaws during service which eventually may lead to malfunction or total failure. Typically this would be the result of excessive distortion leading up to the formation and propagation of cracks. The work presented here describes the successful development of a displacement and crack sensing system or sensor, utilizing optical fibres. These were laid equally spaced across a host specimen specially chosen to detect with ease, displacement and the development of a crack.

As the crack propagated it opened, which coincided with the particular elongation suffered by each individual optical fibre. As a result of the fibre's elongation, the light passing through it, suffered a loss of intensity which was sensed by specially designed photo detector. An attempt to develop a governing sensor equation is presented. Experiments validate some of the parameters and their relationship however the sensor equation requires further attention. The practical work on this subject accomplished the development of a complete system comprising the sensor, the light emitting and detecting modules. It is a promising method for monitoring structural health.

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