Acoustic emission monitoring of fatigue in 7010 aluminium alloys

Abstract

Acoustic emission (AE) has been monitored in test pieces of 7010 aluminium alloy containing growing fatigue cracks. Both high-purity and commercial casts were studied in underaged, peak-caged, and overaged conditions, using tension–tension fatigue with R = 0·1 and 0·5, at crack growth rates in the range 0·1–3 μm/cycle. The reduction of grip noise and discrimination of AE events with sources distant from the plane of the crack were achieved by careful design of test piece geometry and clamping, and by the use of two broadband point transducers with calibrated response. Analysis of the load distribution at which AE events occurred at the crack plane showed that most events occurred near peak loading. However, the frequency of detected AE events was sometimes greatly reduced by adding oil to the crack, implying that crack-face rubbing was one source of emission. This was confirmed by fractographic examination and suggests that the practice used by some workers of gating–out all but those signals generated close to peak load in order to eliminate rubbing signals is not necessarily effective. The frequency of events remaining after the removal of rubbing signals was shown to be consistent with the size of the plastic zone at the crack tip, the rate of crack growth, and the inclusion size/density distribution. It is estimated that the fracture of inclusions of area greater than ~40 μm2 could be detected by the testing system. The commercial cast contained more inclusions of this size than the pure cast, in proportion to the increased AE event frequency.MST/184