Quantitative non-destructive evaluation of advanced composite panels usingoptical interferometric techniques

Composite panels have been assessed for structural integrity using optical interferometric methods. Both holographic interferometry and shear speckle pattern interferometry, also known as shearography, have been used for asse ssment. It is found that measurement of displacement derivatives using shea rography may be more appropriate for the purpose. To develop the electronic shearography technique, a digital image processing system which incorporat es an image grabber board and a self-developed computer program has been de veloped. A Wollaston prism, together with a polariser was selected as an im age shearing device. A coherent laser beam after being spatially filtered a nd expanded, illuminates the object surface. The scattered light, before be ing recorded by the CCD camera is split into two slightly shifted images by a Wollaston prism. The object is doubly exposed with minute displacements in between and a shearogram was formed. The fringes in the shearogram depic t the displacement derivative in the image shear direction. This derivative is proportional to the strain of the specimen. The method has the advantag e of being real-time and vibration insensitive. It also measures the effect of defects on strain distributions rather than merely detecting the presen ce of defects. Thus it can be used to screen the cosmetic defects from the detrimental defects in the test-piece.