QUANTITATIVE PERCUSSION DIAGNOSTICS FOR DETECTING ULTRAFINE CRACKS

摘要

Conventional nondestructive testing (NDT) techniques are often incapable of detecting ultrafine cracks commonly produced by fatigue and other damage processes. When unloaded, these cracks have internal gap spaces that can be as small as a few nanometers. For example, radiographic images do not have the resolution to reveal these cracks. A novel NDT approach has emerged that overcomes this limitation by inducing and measuring the response to low-stress percussion events. This technology, quantitative percussion diagnostics (QPD), administers a reproducible level of kinetic energy by accelerating a probe to a predetermined velocity just prior to impact with the specimen. As such, the loading rate and amplitude resulting from the percussion are completely governed by the mechanical properties and integrity of the specimen. The stresses produced by the percussion event are typically on the order of a few megapascals while the displacements can be less that 100 nm. For a defect free sample, the percussion force as a function of time follows a nearly symmetric uniform peak that can be predicted by Hertzian impact theory. When a crack is present, experimental results as well as finite element simulations have shown that the shape of the force-time response changes significantly. Our results also indicate that this change in percussion response increases as the size of the crack increases.