Characterization of Layer-Delaminations by Ultrasonic Speckles
Sprache des Titels:
Combined electrical mechanical and thermal loads influence highly structured specimen over time and might cause a variety of failure modes. Some of these failures can be such as to only gradually diminish the nominal behaviour of the components so one cannot conclude there is no evolving problem if nominal function is observed. Standard non destructive testing methods are only partly able to detect the developing delamination or progressive generation of voids due to stress. These delaminations can cause voids and cracks in the still operative specimen that might never be of any problem in the nominal operating performance but on the other hand they might further the degradation of the component to ultimately result in a catastrophic failure. We investigate the gradual emergence of so called ultrasonic (US) speckles as a hint to degradation processes deep in the volume of the specimen caused by scatterers (small voids or small cracks) that are too small to be directly imaged by ultrasound techniques. However, due to their density evolving over time they expose t.hemselvs by forming US speckles that increase in contrast over time. In this paper we are applying the well known theory on optical laser speckles caused by micro roughness on a specimens surface to the formation of ultrasound speckles caused by randomly distributed sub wavelength scatterers within the volume of a specimen. Both effects are due to random phase variations in the former case operating on laser light caused by height variations of the scattering profile and in the latter case by sound velocity variations along the propagation path within a specimens volume. We can show that the speckle contrast is a good measure of the average total volume of scattering voids and thus of the onset and evolution of delaminations as defined by Goodman of partly developed speckles.