It has been established that hydrogen assisted subcritical crack growth in Fe-3wt%Si single crystals is discontinuous while accompanied by substantial plasticity. Prop osed micromechanisms of this process are addressed via observed fine-scale {100} cleavage features, acoustic emission tracking and computer simulation analysis. The near crack tip stress distribution in an elastic-plastic analysis enabled insight into how dislocation shielding led to mirocrack nucleation. Such discretized computational models include: the stress due to the dislocation self field, the stress from crack-dislocation interactions, the stress from the crack-external stress interaction and the external applied stress. Stress distributions in both macroscopic and microscopic scales of interest were also examined, appropriate to different slip systems. It was found that the stress at the crack tip became slightly compressive while the position of the maximum stress, approaching the theoretical strength shifts to about 2–30 nm from the crack tip. The high stress region may be correlated with the observed 1 μm discontinuous crack instabilities as detected by acoustic emission. The mutual feedback from experimental findings related to acoustic emission and crystallographical habits vis-a-vis theoretical aspects are analyzed. This approach results in a micromechanical model which has implications to both hydrogen embrittlement thresholds and the ductile-brittle transition.

Dislocation modeling and acoustic emission observation of alternating ductile/brittle events in Fe-3wt%Si crystals

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

Acoustic emission analysis during fatigue crack growth in steel

The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6Al-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 10 μm. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.

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Carl E. Hartbower

Papers

Acoustic emission from low-cycle high-stress-intensity fatigue