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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Risitano proposed an innovative approach for the rapid prediction of the fatigue limit and the S-N curve, using a very limited number of tests: the Thermographic Method (TM) (La Rosa and Risitano, 2000). In 2013, Risitano and Risitano (A. Risitano and Risitano, 2013) proposed the Static Thermographic Method (STM) as a rapid test procedure to derive the fatigue limit of the material evaluating the temperature evolution during a static tensile test. Infrared Thermography (IR) has been applied for the analysis of different materials subjected to different loading conditions: notched and plain steel specimens under static and fatigue tests (Corigliano et al., 2020, 2019; Foti et al., 2020; Ricotta et al., 2019; Rigon et al., 2019; Antonino Risitano and Risitano, 2013), laminated composites under tensile static loading (Vergani et al., 2014), polyethylene under static and fatigue loading (Risitano et al., 2020, 2018), PA66GF30 composites under static and fatigue loading (Crupi et al., 2015b), steels under high cycle (Amiri and Khonsari, 2010; Curà et al., 2005; Meneghetti et al., 2013) and very high cycle fatigue regimes (Crupi et al., 2015a; Plekhov et al., 2015), high strength concrete (Cucinotta et al., 2021), and recently also on 3D printed plastic and metals (Santonocito, 2020; Santonocito et al., 2021). The aim of this research activity is the application of the Static Thermographic Method (STM) and of the Thermographic Method (TM) during static tensile and fatigue tests for the assessment of the fatigue life of a medium carbon steel of the class C45. Tensile tests were carried out and infrared camera has been adopted during all static tests in order to assess the influence of the applied stress rate on the energetic release of the material. In addition, stepwise fatigue tests are carried out and the obtained value of the fatigue limit is compared with the one obtained by STM. A comparison of the different found fatigue limits is performed in order to highlights the differences between several energy based methods for fatigue assessment. This research activity is part of the collaboration between the University of Messina and several others Italian universities within the AIAS group on Energetic Methods, made on the same material. Nomenclature c specific heat capacity of the material [J/kg.K] f test frequency [Hz] K m thermoelastic coefficient [MPa -1 ] R stress ratio t test time [s] T, T i instantaneous value of temperature [K] T 0 initial value of temperature estimated at time zero [K] α thermal diffusivity of the material [m 2 /s] ΔT s absolute surface temperature variation during a static tensile test [K] ΔT st stabilization temperature reached during fatigue test [K] ΔT 1 estimated value of temperature for the first set of temperature data [K] ΔT 2 estimated value of temperature for the second set of temperature data [K] Φ Energy Parameter [Cycles K] ρ density of the material [kg/m 3 ] σ stress level [MPa] σ lim fatigue limit estimated with the Static Thermographic Method [MPa] σ 0 fatigue limit [MPa] σ 1 uniaxial stress [MPa]

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