Strain/stress measurements using electronic speckle pattern interferometry

In its classical application Electronic Speckle-Pattern Interferometry (ESPI) is used to measure deformations with high resolution(1). Additionally, this method is also able of measuring the 3D topography of technical surfaces even with discontinuities(2,3,4,5). If adequate set-ups are used, combined measurements of shape and deformation can also be carried out(6). Especially in production of machine sand other metal parts, the determination of the stress and strain e.g. of welding points is a very important issue for evaluating about the quality of the specimen under test(7). Here, the use of wire resistance strain gauges is state-of-the-art for measuring length variations of parts under mechanical load(8). It is very time-consuming to prepare the corresponding measurement environment for strain gauges. Moreover, only very limited information about the strain can be measured by this means because all information is integrated over the whole area covered by the strain gauges without lateral resolution. In order to extend this kind of metrology to a matrix of some thousands points even including sensitivity in the out-of-plane-direction, ESPI-methods can be used, As described in this paper, it is therefore necessary to perform both, the shape and the deformation measurement to obtain the necessary information. Of course, the directions of sensitivity depend on the contour of the test specimen and can be determined due to the previously measured topography. In this paper current work on the field of stress and strain measurements with ESPI is described. The experimental result of several technical applications is shown and it is compared to measurements with conventional strain gauges. Further possible technical applications are discussed and the prototype of an ESPI stress sensor is presented.