PSI - Issue 28

Giacomo Risitano et al. / Procedia Structural Integrity 28 (2020) 1449–1457 G. Risitano et al./ Structural Integrity Procedia 00 (2019) 000–000

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in 2000 (La Rosa and Risitano, 2000). Since that year, thermography has been adopted widely from researchers all around the world. It has been applied on notched and plain steel specimens (Corigliano et al., 2020; Foti et al., 2020; Ricotta et al., 2019; Rigon et al., 2019, 2017) as well as on composites materials under static and dynamic loads (Crupi et al., 2015; Huang et al., 2020; Vergani et al., 2014). In the recent years it has been applied also on materials such as polyethylene (Risitano et al., 2020, 2018) and 3D-printed polyamide (Santonocito, 2020). In 2013, Risitano and Risitano proposed the Static Thermographic Method (STM) (Risitano and Risitano, 2013) as a very rapid procedure to estimate the fatigue limit of the material assessing the end of the linearity of thermoelastic effect during a static traction test performed under adiabatic conditions (Guglielmino et al., 2020). Several authors have investigated the temperature evolution of the material during fatigue loading conditions (Jiang et al., 2004; LY et al., 2011) but, as we are aware, no studies regarding the modelling of temperature evolution during a static tensile test has been conducted. In the present work, experimental tests and numerical simulations are performed on plain specimens made of a medium carbon steel C45 during a static tensile test. Differences between the experimental and numerical temperature trend indicates how some irreversible damage phenomena arises within the material, leading to the end of the thermoelastic effect assessed by means of an infrared camera. Nomenclature c specific heat capacity of the material [J/kg.K] I 1σ first invariant of the stress tensor [MPa] K m thermoelastic coefficient [MPa -1 ] Q heat generated by plastic work [J] R load ratio t test time [s] T, T i instantaneous value of temperature [K] T 0 initial value of temperature estimated at time zero [K] W p plastic work per volume unit [J/m 3 ] α thermal diffusivity of the material [m 2 /s] β Taylor-Quinney coefficient ΔT s absolute surface temperature variation during a static tensile 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] ρ density of the material [kg/m 3 ] σ stress level [MPa] σ D critical macro stress that produces irreversible micro-plasticity [MPa] σ lim fatigue limit estimated with the Static Thermographic Method [MPa] σ 1 uniaxial stress [MPa]

2. Theoretical background 2.1. Temperature trend in static tensile tests

In this section, a simplified temperature model for engineering materials under static tensile condition is exposed. It is based on the fundamental assumption that fatigue failures occur within the material where the local stress distribution, amplified by structural or superficial micro defects, is capable of producing local micro plastic deformation (Risitano and Risitano, 2013). The local stress state can be linked to a macroscopic nominal stress value (load/area) that introduce in the material the first micro plasticization. The relationship between the applied stress, or strain, and the corresponding temperature change in solid material consist of two contributions due to a thermoelastic and a thermoplastic effect (Equation (1)) (LY et al., 2011).