# Assessment of Fatigue Life and Failure Criteria in Ultrasonic Testing Through Thermal Analyses
## 5 Conclusions  
This study aimed to investigate the impact of intermittent loading on material fatigue performance and evaluate existing models based on temperature during very high cycle fatigue (VHCF) tests. The key findings are summarized below:  
**Temperature Effects:**  
* In all T-N curves, temperature increases were observed as test time decreased. Figure 18: Fracture surfaces of the specimens (a) failed at 1.20 \(\times\) 10\({}^{6}\) and (b) 1.66 \(\times\) 10\({}^{5}\) cycles. * Specimens tested with intermittent loads of 300-200, 200-200, and 200-300 ms at stress levels of 40% and 45% \(\sigma_{\rm u}\) showed that the maximum temperature exceeded 300 \({}^{\circ}\)C.
* Fatigue life was shortened in these cases compared with tests with a 300-500 ms pulse-pause time due to high temperatures. * The steady-state temperature was not clearly defined for specimens that failed before reaching 10\({}^{7}\) cycles. * Specimens experienced an increase in surface temperature in relation to \(\rm f_{eff}\) for a constant \(\sigma_{\rm a}\). * Higher values of \(\rm f_{eff}\) and \(\sigma_{\rm a}\) led to a steeper temperature slope at the start of the test. * S-N Curve and Fatigue Life:
* An S-N curve was obtained under 300-500 ms pulse-pause conditions, exhibiting a higher level of scatter, as expected. * The maximum temperature observed during these loadings was below 40 \({}^{\circ}\)C.
* The Jiang and Fargione models were found to be unsuitable for the experimental conditions of this study. * The Q\({}_{\rm cyc}\)-N\({}_{\rm f}\) and R\({}_{\rm o}\)-N\({}_{\rm f}\) curves aligned with those found in the literature for low-frequency tests, indicating that N\({}_{\rm f}\) increases as Q\({}_{\rm cyc}\) and R\({}_{\rm o}\) decrease. **Model Accuracy and Prediction:**
* Fatigue functions for both models were derived, with predicted results showing good accuracy compared with experimental tests. * High scatter in ultrasonic fatigue tests may influence results. * The Meneghetti and Amiri-Khonsari models showed strong potential for predicting fatigue life based on temperature, even under VHCF conditions. **Crack Surface Observations:**
* Some specimens exhibited burning on the crack surfaces due to high temperatures, negatively impacting their fatigue performance. **Cooling System for VHCF Tests:**
* A cooling system is essential for VHCF tests to manage high temperatures. * Depending on the material studied, it may be necessary to increase pause times to maintain temperature control. * An additional cooling mechanism may be required to keep the test at room temperature. **Author Contributions:** Conceptualization and methodology, M.C.C.T. ; software, F.R.L. and T.G.d.S. ; validation, M.C.C.T. ; formal analysis, M.C.C.T. ; investigation, M.C.C.T. ; resources, M.V.S.P., F.R.L. and T.G.d.S. ; data curation, M.C.C.T. ; writing--original draft preparation, M.C.C.T. ; writing--review and editing, M.C.C.T. and R.F.M.S. ; visualization, R.F.M.S. ; supervision, M.V.S.P. ; project administration, M.V.S.P. ; funding acquisition, R.F.M.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This study was financed in part by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior--Brasil (CAPES)--Finance Code 001. This research was developed within the scope of the Research and Technological Development of the Brazilian Electric Energy Sector Program regulated by ANEEL, with the support of the Eneva. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author. **Acknowledgments:** This work was also supported by SENAI ISI Inspection and Integrity for conducting all infrared thermography tests.