PSI - Issue 66

Davide D’Andrea et al. / Procedia Structural Integrity 66 (2024) 449–458 D’Andrea et al./ Structural Integrity Procedia 00 (2025) 000–000

451

3

ρ

density of the material [kg/m 3 ] stress level, uniaxial stress [MPa]

σ, σ 1 σ lim

fatigue limit estimated with the Static Thermographic Method [MPa]

σ U σ y

ultimate tensile strength [MPa]

Yielding Stress [MPa] σ 0 RTM Fatigue Limit estimated with RTM [MPa]

2. Theoretical background The Risitano’s Thermographic Method is a methodology that consist in determining the stabilization temperature associated to the stress level applied to the specimen. Surface temperature trend shows three different phases: the first phase in characterized by a temperature growth until a stabilization temperature is reached. Temperature remains constant throughout the second phase, since it reached its stabilization value (ΔT st ) and rises again in the third phase before the failure. It is possible 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 = N i 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 (Fig. 1).

Fig. 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 has been largely used and improved by many researchers [16,17]. 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)