Vacuum excitation in shearographic NDT

Materials of composite nature, such as for example carbon fibre skin, sandwiching Nomex core and other honeycomb structures, are increasingly being utilised 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 turnkey solution for only composites vacuum testing.