PSI - Issue 68

Danilo D’Andrea et al. / Procedia Structural Integrity 68 (2025) 746–755 D’Andrea et al./ Structural Integrity Procedia 00 (2025) 000–000

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to determine the Energy Parameter Φ calculated as the area under the curve describing temperature’s trend over number of cycles N (measured as cycles∙K). It was observed that the higher the applied stress, the higher the stabilization temperature value will be, and that the energy parameter remains constant regardless of the applied stress following the relation: Φ = Ni ΔT st (1) Additionally, it has been observed that stabilization temperatures could be measured applying increasing stress level on a single specimen until fatigue failure (Figure 1).

Figure 1. Temperature evolution during stepwise fatigue tests. The fatigue limit is finally calculated as the stress level which correspond to a negligible increase in temperature (ΔT st ∼ 0 K). RTM was largely used and improved by many researchers (Williams et al. 2013; Fargione et al. 2002; Dario Santonocito, Stanislava Fintová, Vittorio Di Cocco, Francesco Iacoviello, Giacomo Risitano, n.d.). Even in the case of specimens subjected to static tensile tests, the thermal behavior is characterized by three phases. In the first phase, a linear trend of temperature decrease is observed following the thermoelastic effect enunciated by Lord Kelvin:

T 0 ⋅ σ 1 =-K m T 0 ⋅ σ 1

α ρ ⋅ c

ΔT s =-

(2)

In equation 2, ΔT s stand for surface’s temperature, α is the coefficient of thermal expansion, ρ is the material density, σ 1 is the principal stress applied, T 0 is the absolute temperature of material and c is its specific heat capacity at constant pressure; K m is the thermoelastic constant. In the second phase it can be observed a deviation from the initial linear trend, caused by appearance of the first microdamage, until a minimum value of temperature is reached