Acoustic emission analysis during fatigue crack growth in steel

Abstract

Emissions from fatigue crack growth have been examined for three steels — A533B steel, low carbon steel and H1 weld metal — using ACEMAN, a system for acoustic emission source location and analysis. The specimen size was sufficiently large for crack propagation under plane strain conditions and for time-difference analysis using multiple acoustic sensors. Signal recognition discriminators and location resolution allowed detection of signals below background noise. Emissions, detected during 80% of the upward-going part of the load cycles, most probably arose from the region of intense plastic strain near the crack tip and from unsticking behind the tip where crack closure had occurred. Expansion of the elastic-plastic interface with crack growth was ruled out as the dominant source mechanism on several grounds. With increasing crack severity, as determined by the cyclic stress intensity factor, ΔK, the rate of emission activity increased. The event count during the creation of unit crack area was, however, independent of crack growth rate. Thus, a specific emission parameter, γ, was derived, relating emission count to new crack area. For the three steels measured, γ lay in the range 2 – 44 events per square millimetre of crack growth. Analysis of signal amplitudes in A533B steel showed that amplitude spectra varied with ΔK. At the highest ΔK (67 MN m −3 2 ) a high amplitude emission source was activated.