Waveform and Frequency Spectra of Acoustic Emissions

Abstract

Stress waves generated internally by deforming materials provide information about the operative deformation mechanisms. These stress waves are known as acoustic emissions. Acoustic emission stands alongside conventional ultrasonic methods as a useful technique of nondestructive testing. The frequency spectrum of the emissions varies according to the nature and location of the deformation, and it may also provide information about the integrity of the structure under test. On fundamental grounds we conclude that the emission waveform at source is a stress pulse (displacement step) whose frequency spectrum extends from zero up to a frequency which depends on the duration of the source event. Between generation and detection, the spectrum is modulated by structural resonances which tend to obscure the information contained in the source waveform. These effects should be borne in mind during the design of practical emission detection systems. Experimental results from adhesive bonds confirm the validity of our theoretical approach and show how information about the source event may be gained from the spectrum by looking beyond the structural resonances.