PSI - Issue 33

Giacomo Risitano et al. / Procedia Structural Integrity 33 (2021) 748–756 Risitano et al./ Structural Integrity Procedia 00 (2019) 000–000

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materials. The adopted procedure includes the Static Thermographic Method, as well as the Thermographic Method (with the adoption of the temperature gradient over the number of cycles dT/dN and the stabilization temperature T stab ), the analysis of the thermoelastic signal (E- mode and D-mode) and the lost energy per cycle Q. In addition, traditional Staircase procedure for evaluate the fatigue limit has been performed. All the tests were carried out on the specimens of Fig. 3a, made of the same C45 steel. In Table 1 are reported the different values of the fatigue limit assessed with several techniques for a stress ratio of R= -1.

Table 1. Comparison of the different experimental techniques for the fatigue limit assessment (in MPa) for R= -1.

Barbagallo et al. 2021

Colombo et al. 2020

Test procedure

Present work

STM

222

TM (dT/dN) TM (T stab ) E-mode (ω) D-mode (2ω)

310 307

316±5 309±1 311±4 305±9

200÷220

Q

308 274

Staircase

It is possible to observe how the predicted value of the fatigue limit by means of the STM is well below the values estimated with other experimental techniques. Compared to the fatigue limit estimated by means of the traditional Staircase procedure, the limit stress is below the fatigue limit, hence the limit stress could be adopted as a more conservative design parameter compared to the other values. It is important also to point out that the different specimen preparations performed by other researchers, cleaning and/or machining, may severally affect the final value of the fatigue limit. In particular, the Staircase procedure was performed on different specimen geometries. Compared to the classical fatigue tests, but also to the other energetic approaches, the Static Thermographic Method was the most rapid procedure which required the lowest number of specimens. 5. Conclusion In this work, the energetic release during a tensile test of a C45 steel has been evaluated. Monitoring the superficial temperature with an infrared camera allowed the application of the Static Thermographic Method and of the Thermographic Method. The influence of the applied stress rate on the energetic release has been evaluated, showing how the choice of a proper stress rate assure adiabatic test conditions. For a stress rate of 400 MPa/min, the limit stress has been evaluated as the stress level at which the temperature deviates from its linear trend, obtaining a value of 222.2±4.0 MPa. Stepwise fatigue tests adopting the TM were performed showing how a temperature increment is present for stresses in the range of 200÷220 MPa. This indicates that fatigue damage begins for those stress levels. Comparisons with other energetic methodologies, with traditional Staircase tests and literature data have been performed, showing how the assessed limit stress value could be adopted as a more conservative fatigue design parameter. Energy methods allow a rapid estimation of the fatigue limit. In particular, 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.