PSI - Issue 76

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

157

Table 3. Stepwise fatigue tests characteristics and RTM results for both specimen’s batch .

σ max [MPa] 22÷44 24÷45 24÷45 22÷44 22÷44 22÷44

ΔN [cycles]

Φ [cycles K]

σ 0, RTM [MPa]

Test ID

R

Δσ [MPa]

N f [cycles]

Step_PA12_X_01 Step_PA12_X_02 Step_PA12_X_03 Step_PA12_Y_01 Step_PA12_Y_02 Step_PA12_Y_03

2 1 1 2 2 2

19549 34538 33207 21704 24232 15525

162708 238529 275943 122669 143028 146885 181627 61097

26.6 26.5 27.1 27.6 31.9 24.9 27.4

0.1

3000

Mean value Standard Deviation

2.4

Polymeric materials subjected to cyclic stress exhibit damping effect; indeed, to avoid its influence on the estimation of the number of cycles to failure according to RTM, the increase in temperature due to material dumping (grey triangles, Figure 5-b) has been subtracted from the recorded temperature (green squares, Figure 5-b) according to the procedure proposed in (Davide D’Andrea et al., 2025) . From the CA fatigue test campaign, the fatigue curve with a scatter band with a probability of survival 90-10% can be estimated. By dividing the Energy Parameter with the stabilization temperatures, the number of cycles to failure can be predicted according to RTM (Figure 6-a). The prediction performed adopting raw temperature data underestimates material’s fatigue life ; indeed, even if most of points fall inside CA fatigue tests S- N curve’s scatter band, they do not follow a similar trend, since the inverse slope is higher. In Figure 6-b are reported fatigue life predictions obtained from the equivalent stabilization temperatures, which are corrected from the damping contribution due to the self-heating effect. Data derived from each stepwise fatigue test fall inside CA dispersion band and S-N curve calculated by Basquin law shows a similar trend.

(a)

(b)

Figure 6. S-N curves calculated from: a) raw stabilization temperatures; b) equivalent stabilization temperatures compared to CA tests