PSI - Issue 77

Tomasz Rogala et al. / Procedia Structural Integrity 77 (2026) 11–17 Tomasz Rogala et al. / Structural Integrity Procedia 00 (2026) 000–000

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Horst (2017), Shabani et al. (2021), Premanand et al. (2023), Premanand and Balle (2025) for very-high cycle fatigue (VHCF) and thermographic techniques (TT) Luong (1998), Huang et al. (2017), Katunin and Wachla (2019), Jia et al. (2023), Dolbachian et al. (2025) which allow for the approximate estimation of the fatigue strength for high-cycle fatigue (HCF).

Nomenclature

σ SN fatigue strength determined using stress-life curve σ TT fatigue strength determined using thermographic technique ∆ T s stabilized relative temperature

˙ q diss heat dissipation rate ˙ q gen heat generation rate σ maximum stress S - N stress-life curve TT thermographic technique BL bilinear method

AC normalized angle change method MCR minimum curvature radius method MPD maximum perpendicular distance method PMC polymer-matrix composite

Accurate determination of the fatigue strength across di ff erent lifetimes requires conventional fatigue testing. While reliable, this process is labourious. However, when an exact fatigue prediction is not required it can be replaced by the faster TT, which o ff er a rapid estimation of fatigue strength. This technique enables the prediction of the fatigue strength of PMCs in the high-cycle fatigue regime (HCF) using various approaches. The most commonly used approaches are the ∆ T s − σ chart and the ˙ q − σ chart Luong (1998), Yang et al. (2020). The TT relies on observing the thermomechanical behaviour of composites, allowing the extraction of a predictive feature that correlates strongly with the fatigue strength σ SN obtained from the traditional S - N curve. This predictive feature is typically identified as a noticeable change that occurs in the thermomechanical response Guo et al. (2015), Mehdizadeh and Khonsari (2018). For PMCs, it marks the transition from a thermal response driven primarily by internal friction and self heating, to a response increasingly influenced by microdamage mechanisms such as matrix cracking and small-scale delaminations at higher fatigue loadings Huang et al. (2020). This characteristic change is often referred to as the value of the fatigue strength determined by the thermographic technique σ TT . A wide range of estimation methods have been used to estimate σ TT Huang et al. (2017), Jia et al. (2023), Dolbachian et al. (2025), Amraei et al. (2024b), Amraei et al. (2025). Unfortunately, the TT is applied very selectively. In most studies, the reported σ TT values are related to specific PMCs Bagheri et al. (2014), Charca et al. (2024), Colombo et al. (2011), Gornet et al. (2013), Gornet et al. (2018), Harizi et al. (2019), Najd et al. (2022), Pathak et al. (2025), Peyrac et al. (2015),Amraei et al. (2024b), and are vali dated using particular estimation methods and approaches, often without broader generalization. To address this issue, the present study evaluates various fatigue strength estimation methods for di ff erent PMCs previously investigated individually in selected literature by the authors Amraei and Katunin (2025), Amraei et al. (2025). Specifically, we access the reliability of di ff erent approaches and estimation methods in determining σ TT by comparing them with corresponding σ SN . This analysis provides a quantitative assessment of selected approaches and estimation methods, supporting informed decision-making in the selection of appropriate thermographic techniques.

2. The self-heating phenomenon

When PMCs are subjected to fatigue loading, mechanical energy is generated ˙ q gen and dissipated ˙ q diss mainly as heat due to self-heating e ff ect and microdamage accumulation Huang et al. (2020). The self-heating phenomenon