Acoustic emission from cleavage microcracking in alloy steels

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Abstract
Acoustic emission (AE) monitoring, in liaison with fracture mechanics methods, is used to study cleavage crack growth in laboratory specimens of several alloy steels. Experimental results indicate that, during the onset of rapid unstable crack growth, spontaneous acoustic events are emitted. Fractographic studies on fracture surfaces have demonstrated that a large number of energetic signals resulted from the local brittle fracture at the crack tip. It is shown that the stress intensity factor corresponding to the first transor intergranular cleavage crack extension provides predictive information regarding the final unstable failure. A model of cleavage crack extension based on the local critical fracture stress criterion is used to interpret the emission behaviour from the materials tested. The difference in AE response during toughness testing is attributed to the microstructural features. These results indicate that incorporating AE techniques in detecting brittle fracture will enhance the failsafe design of the structural components.