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

Non-destructive evaluation (NDE) of materials for characterising various key microstructural features, mechanical properties (tension, creep, fatigue crack growth, hardness and fracture toughness), deformation and damage mechanisms has attracted considerable attention in the past 20 years as a primary step towards ensuring structural integrity of components. However, until recently, the correlations between the various NDE parameters and material properties have been only empirical and based on physical principles. The interaction between the NDE probing medium and the mechanical behaviour is not yet fully understood. The purpose of this review is to discuss the progress made in the application of non-destructive testing (NDT) techniques in evaluating various microstructural features and mechanical properties with emphasis on recent studies. Reinterpretation of older data, in the light of present understanding of the interaction of the NDE probing medium with material parameters, is carried out selectively. The NDT techniques evaluated include acoustic emission, ultrasonic attenuation and velocity, magnetic hysteresis parameters, magnetic Barkhausen emission, acoustic Barkhausen emission, laser interferometry, positron annihilation, X-ray diffraction and small angle neutron scattering. Critical assessments of the applicability of the various NDE techniques for the material parameters are provided.

Assessment of microstructures and mechanical behaviour of metallic materials through non-destructive characterisation

The acoustic emission (AE) generated during tensile tests performed on Q345 steel specimens was investigated in this study. Two typical types of AE signals were obtained during the test: Type A signals with burst type waveforms in the micro-yielding and work-hardening stages and Type B signals with continuous-type waveforms during the discontinuous yielding (Luders band propagation) stage. The effects of the strain rate and notch on these two types of signals were studied. The AE energy of Type B signals increased with increasing strain rate and was much higher in notched specimens compared to unnotched ones at high strain rates. However, for Type A signals, less AE energy was generated in notched specimens than in unnotched specimens, and it showed little sensitivity to changes in the strain rate. These results were fully analyzed by relating the AE behavior to the deformation properties and transmission electron microscope (TEM) observations of the materials. Thus, the AE source mechanisms and the effects of the strain rate and notch during plastic deformation were determined for discontinuous yielding materials.

Effects of strain rate and notch on acoustic emission during the tensile deformation of a discontinuous yielding material

Acoustic emission (AE) has been measured during subcritical crack growth of Fe, Ni, 304 SS, a tool steel, and an Fe−Si single crystal. Tests were conducted using compact tension type samples in a variety of environments. Iron with two levels of S and P grain boundary concentrations was tested in Ca(NO3)2 at +750mV; Ni with P segregation was tested in 1N H2SO4 at +900 mV; two heats of 304 SS were tested in Na2S2O3 at open circuit potentials; Fe and Ni with S-enriched grain boundaries were tested in 1N H2SO4 at cathodic potentials; and RDS tool steel and an [001] oriented Fe−Si single crystal were tested in gaseous hydrogen at a pressure of 1 atm. The number of AE events/mm2 of crack extension increased with an increase in transgranular fracture for all of these test conditions with the exception of the Fe−Si single crystal. In this case, crack extension was 100 pct transgranular, but the number of events/mm2 was negligible. It was concluded that the short, discontinuous crack jumps were not detectable with the AE monitoring equipment. Likewise, for the intergranular subcritical crack growth, it was concluded that crack extension was either anodic dissolution with no accompanying AE or short, discontinuous crack jumps with nondetectable AE. It also was postulated that the transgranular fracture surfaces accompanying the intergranular subcritical crack growth resulted from ligaments which fractured behind the advancing crack front.

K.N. Tandon

Papers

Acoustic emissions during deformation of polycrystalline silicon—iron

An acoustic emission study on deformation behaviour of strain-aged polycrystalline samples of Fe-3% Si