PSI - Issue 68

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

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D’Andrea et al./ Structural Integrity Procedia 00 (2025) 000–000 at the yielding stress of the material ( y ). In the third phase there’s an exponential growth of temperature until the failure of the material. In Figure 2 stress and thermal trends over time measured during a static tensile are reported. The limit stress (σ lim ) can be estimated as the stress measured at the instant in which the change in slope occurs between the first and second phase at the temperature ΔT I-II . The deviation from linearity happens because of the presence of internal microdefects which plasticize when subject to increasing tensile force.

Figure 2. Temperature trend during a static tensile test

3. Materials and Methods Tensile and fatigue tests have been carried out by RFM (Rapid Fatigue Machine), a pneumatic benchtop universal testing machine patented by University of Messina’s academic spin-off KnoWow and designed in collaboration with Italsigma srl. Its main advantage is the simplicity of the system due to utilization of air as work fluid, which frees user from utilizing a complex system as the hydraulic one. It is equipped with 2.5 kN load cell and it can operate, during fatigue test, at frequency between 1-15 Hz. Its small dimensions force users to design specimens according to ASTM D638-type V standard, which is recommended for testing materials that are available only in very small quantities or restricted space for testing. It was chosen to use a thickness of 2 mm. Dimensions are reported in Figure 3.