PSI - Issue 76

Davide D’Andrea et al. / Procedia Structural Integrity 76 (2026) 151–158

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Figure 4. S-N curves obtained from constant amplitude fatigue tests on X, Y and Z direction’s specimen batch.

3.3. Risitano’s Thermographic Method (RTM) Fatigue strength and S-N curve of PA12 can be estimated in a rapid way, adopting the RTM, by monitoring the temperature evolution during a stress-controlled stepwise fatigue test. Three tests have been conducted on X and Y specimen sets, increasing of Δσ the maximum applied stress level starting from a minimum up to a maximum value every Δ N, with stress ratio R= 0.1 and frequency f= 1 Hz (Table 3). In Figure 5-a is reported the temperature evolution over cycles for the test Step_PA12_X_01. For each applied stress level, it is possible to observe a first increase of the temperature signal followed by a plateau, correspondent to the stabilization temperature Δ T st . By reporting the stabilization temperature vs. the applied stress level (Figure 5-b) it is possible to perform the linear regressions of the two temperature trends (blue and green markers) and evaluating at their intersection the fatigue limit according to RTM ( σ 0, RTM = 26.6 MPa).

(a)

(b)

Figure 5. RTM result for Step_PA12_SLS_X_01: a) temperature evolution vs. maximum applied stress vs. number of cycles; b) stabilization temperature vs. maximum applied stress.

From the temperature vs. number cycles curve (Figure 5-a), it is possible to calculate the Energy Parameter Φ as the integral of the curve. In Table 3 are reported the fatigue limits and the Energy Parameters estimated adopting the RTM. An average value of 27.4±2.4 MPa has been estimated adopting just six specimens and in less time compared to CA fatigue tests (41.3 h by RTM vs. 1899 h by CA fatigue tests).