Number of techniques are available to estimate the growth of fatigue cracks from standard fracture mechanics specimen geometries. However, many of them have restrictions of not being amenable to field studies. Acoustic emission technique (AET) seems to be being promising in this area, as long as the received signals are decoded to exclude the noise signals from other sources. This study has been carried out to characterise the growth of fatigue cracks using AET, with the objective of establishing a relationship between the acoustic emission counts as a function of crack growth rate or stress intensity factor range.

The Acoustic emission (AE) during fatigue crack growth tests on SEN specimen was acquired continuously using a 375 kHz resonant piezoelectric sensor. This sensor frequency was found to be optimum with minimum interference due to noise from the specimen with high vacuum grease. The acquired AE signals were amplified by a 40 dB fixed gain preamplifier and passed through a band pass filter having a range of 250 - 500 kHz. The preamplified and filtered AE signals were further amplified and analysed using a signal channel AE system. The overall gain selected was 65 dB. The threshold selected was 0.8 V which was well above the background noise level. The AE ringdown counts (RDC) and AE events were used for correlation of the AE activity with the FCG behaviour. The cumulative RDC (N) and the (da/dn) were measured at different number of cycles(n).

The initial slope change in AE plot indicate the transition from stage I to II. The presence of two substages IIa and IIb during state II FCG is also indicated by another slope change in the AE plot at 40 Mpam. This transition point in AE parameter is found to occur at a da/dn4x10^-7 m/cycle which is in agreement with the reported value. Similar transition behaviour in AE has been observed in AISI type 316 stainless steel also. The high AE activity observed during the substage IIa is attributed to extensive cyclic plasticity within the cyclic plastic zone (CPZ) and the increasing size of the CPZ with K under plane strain condition prevailing during stage IIa. The low AE activity observed during stage IIb is attributed to the reduction in the mean free path for dislocation movement and decrease in the size of the CPZ under plane stress condition prevailing during stage Iib. The variation in cumulative AE as a function of K during stage IIa shows a relationship as N= Kⁿ with n2.05 just similar to Paris law. This shows the possibility of evolving a quantitative relationship between AE and crack growth rate