Evaluation of Fatigue Crack Growth Characteristics on Stainless Steel SS 316 LN Using Acoustic Emission Technique

The type 316LN stainless steels used in the heat transfer circuit of a thermal power plant are subject to a combination of fluctuating loads and thermal transients. The simultaneous exposure to elevated temperatures combined with the cyclic load fluctuation can increase the susceptibility of stainless-steel structures to fail by fatigue. The acoustic emission (AE) technique can be used for the early defect detection and continuous fatigue damage assessment in the material under in-situ conditions.

 The present work focuses on acoustic emission monitoring during fatigue crack propagation in stainless steels. Physical Acoustics, USA supplied AE sensors and instrumentation was used for this study. The crack length increment was observed to be in agreement with the increase in AE cumulative count and cumulative energy over the test duration. Experiments were conducted on plain C(T) and side-grooved C(T) specimens to study the effect of stress state at the fracture process zone on the acoustic emission activity. It was noted that the side groove constrains the plastic zone.  The side grooved specimen indicated reduced acoustic activity which can be attributed to the predominant plane strain condition at the crack tip, which constrains the plastic zone in the vicinity of the crack tip. Further, during the fatigue crack growth testing, acoustic emission was not continuous and intermediate absence of acoustic activity was observed at room temperature, which may be due to the lack of crack advancement during every cycle. During the initial segment of crack growth, the cumulative AE counts when plotted with the applied stress intensity range(ΔK) indicated a sharp increase for a small increment in the ΔK. The crack growth rates corresponding to this ΔK range was negligible and the sudden rise in the acoustic activity may be attributed to the possible transition from ‘stage Ⅱ (a)’ to ‘stage Ⅱ (b)’ as reported in the literature. Similar transition in acoustic emission characteristics was observed in side grooved sample, however at a reduced magnitude. AE cumulative count and cumulative absolute energy after the transition indicated a relatively slower slope, indicating steady state crack growth. The cumulative change in the AE activity increased further with the rise in stress intensity range. However, the reduced AE activity was observed at the fast fracture zone. The fracture surface was evaluated using Scanning electron microscope to evaluate the underlying failure mechanism. The reduced AE activity at the fast fracture zone can be associated to the sudden change in failure mechanism from striation crack growth to void formation and dimple rupture associated with the fast fracture.

The work also attempts to correlate the AE parameters obtained during high temperature fatigue crack growth in stainless steel. A wave guide was used to transmit the acoustic waves to the acoustic sensor mounted outside the hot zone. Similar to the room temperature test, a qualitative correlation of AE parameters with the crack increment was established for the test conducted at elevated test conditions.