PSI - Issue 77

Tomasz Rogala et al. / Procedia Structural Integrity 77 (2026) 11–17

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Tomasz Rogala et al. / Structural Integrity Procedia 00 (2026) 000–000

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originates from the viscoelastic properties of the polymers, which absorbs energy during cyclic loading and coverts it into thermal energy. The relatively low thermal conductivity of PMCs amplifies this e ff ect, leading to localized temperature growth, especially in regions of elevated stress concentration Amraei et al. (2024a). Because the temperature rise from self-heating is correlated with the level of cyclic loading, this e ff ect becomes more pronounced at higher stress levels. At elevated stress amplitudes, excessive heating can accelerate material degradation, leading to a reduction in fatigue life. In contrast, at lower cyclic stress levels, self-heating occurs alongside typical mechanical damage mechanisms but results in only a modest temperature increase. This limited thermal e ff ect generally does not significantly influence the material’s life during fatigue testing Amraei and Katunin (2022). Under such conditions, the temperature typically stabilizes after an initial transient phase of loading during the fatigue testing. These moderate values of stabilized relative temperatures ∆ T s , when associated with their corresponding stress levels σ , are critical inputs in thermographic analysis e.g. for ∆ T s − σ approach. The outcomes of the analysis in the form of fatigue strength predictor σ TT of the material’s thermomechanical response are commonly used to predict fatigue strength σ SN .

3. Thermographic approaches and methods of estimation

3.1. Thermographic approaches

Thermographic approaches used for rapid evaluation of the fatigue strength σ TT of PMCs are based on the observa tion of noticeable changes in the thermomechanical fatigue responses which are derived as a result of an appropriately designed increasing amplitude step test Luong (1998), Huang et al. (2020). This test is usually conducted under the condition of the constant mean stress or constant stress ratio, at di ff erent levels of the cyclic loadings [ σ 1 ,σ 2 , ...,σ k ], where the lowest value of cyclic stress level σ 1 allows to observe thermomechanical response due to the self-heating phenomenon, and the highest cyclic stress level σ k is the stress level that still provide stabilized temperature response. An example thermal response for the increasing amplitude test is shown in Figure 1(a).

Fig. 1. (a) Thermomechanical response for the increasing amplitude test; (b) exemplary ∆ T s − σ chart; (c) exemplary ˙ q − σ chart.