PSI - Issue 33

Dario Santonocito et al. / Procedia Structural Integrity 33 (2021) 724–733 Santonocito et al./ Structural Integrity Procedia 00 (2019) 000–000

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4.3. Traditional fatigue tests

In order to compare the value of the limit stress, assessed by STM, and of the fatigue limit, assessed by TM, a series of CA fatigue tests has been performed on 9 specimens, with the same stress ratio and test frequency of the stepwise fatigue test. In Fig. 8 are reported the real applied stress level vs. the number of cycles to failure. The run out limit (i.e. specimen do not fails) has been set at 10 6 cycles. The fatigue limit, with a probability of survival PS= 50%, is equal to 13.5 MPa; which is in good agreement with the values of the limit stress by STM and fatigue limit by TM.

Fig. 8. SN curve of welded HDPE and comparison with other fatigue limits.

Compared to the traditional fatigue test, energy methods and, in particular, the Static Thermographic Method, allow the fatigue assessment with a very limit number of specimens and testing time. In fact, it is possible to obtain information regarding damage of materials by performing a static tensile test and monitoring the temperature trend. 5. Conclusion In this work, the energetic release of welded specimens made of HDPE has been evaluated. The superficial temperature of the specimens has been monitored with an infrared camera during static tensile and fatigue test allowing, respectively, the application of the Static Thermographic Method and of the Thermographic Method. The limit stress has been evaluated as the stress value that introduce in the material the first irreversible plastic damage. It has been evaluated as the point where temperature trend deviates from linearity, obtaining a value of 13.9±0.1 MPa. Stepwise fatigue tests adopting the TM were performed showing how a significative temperature increment is present for stress level above 14.0 MPa. This indicates that fatigue damage begins for those stress levels. Comparison with traditional fatigue test has been performed showing how the limit stress and the TM’s fatigue limit fall within the scatter band of the material and have value near the fatigue limit for a PS= 50% (13.5 MPa) Within the Energy methods, the Static Thermographic Method is a rapid test methodology able to predict the fatigue properties of the materials from a static tensile test, even with a limited number of specimens and in a short amount of time.

References

Amiri, M., Khonsari, M.M., 2010. Rapid determination of fatigue failure based on temperature evolution: Fully reversed bending load. Int. J.