Speckle technique for fracture measurement in metallic structures

When a rough object is illuminated by coherent light, the scattered light p roduces a random speckle effect or luminous twinkling. This effect can be u sed as an optical non-destructive test for detecting and measuring surface deformations. This technique is based on holographic interferometry, where CCD cameras are used as recording mean. For this reason the technique becom es an opto-electronic one. Thus, a fringe pattern that appears by overlappi ng two wavefronts coming from the object under study, before and after ifs deformation, is digitally recorded. This image is computer processed to int erpret the deformations suffered by the object. As a result one can study, in full field, an object under different load conditions, with no necessity of being in contact. The present work shows the potential of the speckle t echniques, extending its application towards the detection and measurement of fractures in different metallic objects. Speckle techniques are theoreti cally described and experimental results of a fracture measurements in a me tallic plate under mechanical load are shown.