PSI - Issue 34

Dario Santonocito et al. / Procedia Structural Integrity 34 (2021) 211–220 D. Santonocito et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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2. Theoretical Background 2.1. Thermographic Method

As observed by La Rosa and Risitano (La Rosa and Risitano (2000)), during a fatigue test, performed at a stress level above the fatigue limit σ 0 of the material and at a given stress ratio R and test frequency f, the temperature evolution exhibits three phases (Fig. 1a). In the first phase (Phase I), there is an increment until the temperature stabilize at a value equal to ΔT st (Phase II). As the material approaches to fail, temperature experiences a very rapid increment (Phase III), compared to the previous one. Fatigue limit can be identified in a rapid way as the first stress level at which the stabilization temperature is noticeably higher compared to the previous value. For each constant amplitude (CA) fatigue test, it is possible to evaluate the energy parameter Φ as the subtended area of the temperature v ersus number of cycle curve (ΔT -N). Generally, the higher the applied stress, the higher the stabilization temperature, but the energy parameter could be assumed as material property, at a given stress ratio and test frequency. It is also possible to perform a stepwise fatigue test (Fig. 1b), increasing the applied stress level and registering the relative stabilization temperature. As the specimen fail, it is possible to evaluate the energy parameter Φ and assess the number of cycles to failure for each stress level, as the specimen would be stressed at that stress level with CA tests, simply dividing the energy parameter for the different stabilization temperatures and neglecting Phase I and III, usually smaller compared to Phase II. In this way, knowing the N- σ values, it is possible to obtain the complete SN curve of the material with a very limited number of tests.

(a)

(b)

Fig. 1. Fatigue assessment by Thermographic Method: a) temperature trend during a fatigue test; b) rapid stepwise fatigue test.

2.2. Static Thermographic Method In 2013, Risitano and Risitano (A. Risitano and Risitano (2013)) proposed a very rapid procedure to assess the first damage within the material monitoring its temperature evolution during a uniaxial tensile test. During a static tensile test of common engineering materials, the temperature evolution, detected by means of an infrared camera, is characterized by three phases (Fig. 2): an initial approximately linear decrease due to the thermoelastic effect ( obeying to Lord Kelvin’s law, Phase I), then the temperature deviates from linearity until a minimum temperature value (Phase II), therefore it experiences a very high further increment until material failure (Phase III). Under uniaxial stress state and in adiabatic test conditions, Equation 1 can be simplified as:

1  T K T m s  = −

(1)