Acoustic Emission Source Characterization Research Articles

The response of a rectangular parallelepiped to a simulated acoustic emission burst

The forced vibrational response of the rectangular parallelepiped is of particular interest in the study of wave propagation in three-dimensional solids and especially in the characterization of acoustic emission sources. There has been considerable interest in studying source mechanisms in order to predict, and perhaps eventually control, flaw growth in structural materials. The acoustic emission source waves generated by flaw growth are thought to be pulselike functions of stress (force) which are produced by the step displacements associated with material yielding. Much of this type of emission in solids is produced internally and can therefore be modeled as a body force phenomenon; as such, this paper presents a normal mode solution for the response of a rectangular parallelepiped to an impulsive body force. The numerical results prior to any reflections from the boundaries show good agreement with the infinite space solution.

Acoustic emission source characterization

The source function for glissile dislocation loops and microcracks have been modeled in terms of time dependent force dipoles and the acoustic emission waveforms, at the epicentre, evaluated for an isotropic elastic half-space. A specimen geometry has been designed that allows microcracking to occur under the influence of an applied stress but which also approximates to a half-space for a short period of time after operation of an event. The epicentre waveforms of such a specimen have been measured, quantitatively, with a broad band capacitance transducer and the waveforms recorded by Biomation transient recorders and stored on computer disk. The inverse transfer function for the experimental configuration has been calculated and used to determine the source function of carefully characterized brittle cleavage and intergranular microcracking in mild steel and electrolytic iron at 77K. The microcrack lengths deduced from acoustic emission measurements were consistent with fractographic observations. The velocity of microcrack growth, also deduced from acoustic emission measurements, indicated the existence of a limiting microcrack speed of about half the shear wave speed in both materials. [This work has been jointly funded by A.M.T.E., Holton Heath and RAE Farnborough through MOD (Procurement Executive) and by AERE, Harwell.]

The forced vibrational response of a rectangular parallelepiped with stress‐free boundaries

Most of what is known about the nature of acoustic emission sources has been learned through the use of piezoelectric transducers coupled to rectangular parallelepiped or plate‐type specimens. Unfortunately, the output of the piezoelectric transducer is not a true picture of the source signal; rather, it is the acoustic emission signal modified by the responses of the specimen, the specimen‐transducer interface, and the transducer. As a first step in determining what acoustic emission looks like, this paper presents a closed‐form normal mode solution for the forced vibrational response of a rectangular parallelepiped with stress‐free boundaries. Knowing the specimen response will allow the calibration of piezoelectric transducers for all three wave types—dilational, transverse, and surface waves—and this, in turn, will allow the characterization of acoustic emission sources.

Characterization of acoustic emission sources, sensors, and propagation structures

Acoustic emission signals carry potentially useful information about the criticality of deformation source mechanisms in materials, but signal processing techniques such as threshold counting, RMS voltage recording, peak detection, and spectral analysis often failed to extract such information unambiguously. The difficulty lies in the determination of the transfer characteristics of the structure and the sensor. We report experimental results of a computer‐based test system consisting of a large plate and a capacitive transducer [N. N. Hsu, J. A. Simmons, and S. C. Hardy, Materials Evaluations 35(10), 100–106 (1977)]. The transient wave propagation behavior of the plate and the transfer function of the capacitive transducer are known. Consequently, unknown sources can be characterized by solving the Toplitz matrix directly. Explicit force‐time functions of simulated signals such as breaking glass capillaries, breaking pencil leads, dropping steel balls, electrical arcs, and pulsed piezoelectric transducer...