PSI - Issue 75

John Hock Lye Pang et al. / Procedia Structural Integrity 75 (2025) 29–34 Author name / Structural Integrity Procedia (2025)

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3. Results and Discussion 3.1. Fatigue Test Results

Figure 3 shows the stress-life (S-N) behavior of as-built and post-treated 316L stainless steel specimens, plotted on a double logarithmic scale. Fatigue test data for the as-built specimens include results published by Kedziora et al. [9], along with two additional data points from this study. As shown in Figure 3, the two data points align closely with the literature S-N curve, demonstrating the validity of using the literature data as a baseline for evaluating the fatigue performance of the post-treated specimens. The fatigue test data were analyzed using the Basquin equation. The empirical Basquin models for the as-built condition, as well as post-treatment Sequence 1 and post-treatment Sequence 2, are listed in Eqs. (1), (2), and (3). − = 1762.179 ∙ −0.121 (1) 1 = 1503.210 ∙ −0.104 (2) 2 = 1426.790 ∙ −0.098 (3) The as-built specimens demonstrated the lowest fatigue performance among the three processing conditions and showed the most rapid degradation due to fatigue across the applied stress range, with the steepest fitted fatigue strength exponent of -0.121. The specimens that underwent post-treatment performed milder slopes and superior fatigue lives, demonstrating the effectiveness of the two proposed post-treatment sequences in improving fatigue performance. Sequence 2 achieved further fatigue performance improvement by incorporating shot peening to Sequence 1, which shifted the S-N curve upward without significantly changing the exponent. For instance, at the stress range of 390 MPa, the specimen post-treated by Sequence 1 failed after 402,261 cycles, whereas the specimen processed by Sequence 2 lasted for 661,911 cycles before failure, performing a 65% increase in fatigue life. The findings suggest that while HIP and surface machining effectively enhance fatigue performance, incorporating shot peening provides additional benefits by altering surface conditions.

Figure 3 S-N curves of post-treated and as-built 316L stainless steel specimens.

3.2. Crack Initiation Mechanism

Following fatigue tests, a fractographic analysis was conducted to investigate the crack initiation mechanism responsible for the fatigue failure of the post-treated specimens. Figure 4 illustrates the fracture surfaces of specimens post-treated using Sequence 1 and Sequence 2 under a stress range of 390 MPa. The specimen processed by Sequence