Real Time Detection of Microcracks in Brittle Materials Using Stress Wave Emission (SWE)

Stress wave emission (SWE) is defined as the elastic waves released within a material as a result of crack propagation due to the application of stress. A SWE in a brittle material is a relatively low-amplitude stress wave that is caused by the release of energy by microstructural events. This type of a phenomenon is best described by the "Griffith Theory of Solids." Since the SWE's are generally thought of as step or delta functions due to the nature of the generating source, their frequency spectrum is very broad. A series of experiments in SWE were conducted to determine the characteristics of the emissions, the instrumentation requirements, and to better understand the phenomenon for its application to brittle materials (glass and ceramic (Al
2
0
3
) thin film substrates). It was found that the detected SWE's released have a frequency spectrum very much like acoustic white noise over the frequency range of 0.1 kHz to approximately 100 MHz. The detection of SWEs in ceramic substrates was accomplished using a broadband 700 kHz piezoelectric transducer whose output amplified and filtered signal indicated the presence of a substrate microcrack. In order to distinguish true signals from faulty ones, a simple likelihood ratio detector was devised, where its threshold was previously experimentally determined.