Acoustic-emission techniques are beginning to be frequently used as a means of detecting and locating deformation and fracture processes in both metallic and non-metallic engineering structures This has happened despite a relatively poor understanding of the basic physical processes involved in the generation, propagation, and detection of acoustic-emission signals, and of the influence in metals of factors such as composition and microstructure, upon the acoustic emission from different types of deformation and fracture processes The measurement of acoustic emission, potentially, could yield much information about dynamic aspects of these processes However, it is shown that the limitations of existing acoustic-emission recording instrumentation have enabled only qualitative information to be obtained to date Even this data, however, has demonstrated that metallurgical variables greatly affect the acousticemission response of metals, and simple models to account for this are discussed

Acoustic emission for physical examination of metals

Frequency spectrum analysis of acoustic emission signal obtained during tensile deformation and fracture of an AISI 316 type stainless steel

The authors describe a data acquisition system to determine the frequency spectrum between 500 KHz and 3 MHz of the initial acoustic emission. They measured the transfer function of the piezoelectric transducer and of the specimen for their results using a computer. They present an application of this device to the study of the emission of a low-carbon steel.

A spectrum analysis of acoustic emission

Based on the theory of F. Gilmore (Gilmore 1952 The growth or collapse of a spherical bubble in a viscous compressible liquid ) for radial oscillations of a bubble in a compressible medium, the sound emission of bubbles in water driven by high-amplitude ultrasound is calculated. The model is augmented to include expressions for a variable polytropic exponent, hardcore and water vapour. Radiated acoustic energies are calculated within a quasi-acoustic approximation and also a shock wave model. Isoenergy lines are shown for driving frequencies of 23.5 kHz and 1 MHz. Together with calculations of stability against surface wave oscillations leading to fragmentation, the physically relevant parameter space for the bubble radii is found. Its upper limit is around 6 μm for the lower frequency driving and 1–3 μm for the higher. The radiated acoustic energy of a single bubble driven in the kilohertz range is calculated to be of the order of 100 nJ per driving period; a bubble driven in the megahertz range reaches two orders of magnitude less. The results for the first have applications in sonoluminescence research. Megahertz frequencies are widely used in wafer cleaning, where radiated sound may be implicated as responsible for the damage of nanometre-sized structures.

Acoustic energy radiated by nonlinear spherical oscillations of strongly driven bubbles

The author presents the results of experiments on adhesively bonded aluminium-alloy lap joints where stress, far less than needed to break the joint, causes acoustic emission. The lap shear strength of those joints bonded with Redux 775 proved to be related to the cummulative acoustic emission energy per unit volume of the adhesive layer for different thicknesses of adhesive. What evidence there is for those joints bonded with the more ductile BSL 308 points to a similar relationship. The results demonstrate that acoustic-emission energy monitoring has potential as a non-destructive test of bond strength and merits further examination to establish the true relationship between bond strength and the total acoustic-emission energy released in the fracture of various structural adhesive systems.

Acoustic emission energy relates to bond strength

The author discusses the significance and measurement of the frequency and shape of acoustic emissions. He considers theoretical descriptions of acoustic emission and compares them with experimental evidence. He considers the implications of this understanding of the nature of acoustic emission and illustrates these implications by a description of a programme of investigation at AERE Harwell. This programme added to our knowledge of acoustic emission and the author considers the results in the light of other experiments. He concludes that spectral analysis could complement the use of amplitude distribution and ring-down counting but not enough is yet known of the general meaning of the spectrum of an acoustic emission.

G. Curtis

Papers

Acoustic emission —4: Spectral analysis of acoustic emission