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

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In Eq. 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 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 Fig. 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.

Fig. 2. Temperature trend during a static tensile test

3. Materials and methods Specimens of PA12 were obtained by Multijet Fusion (MJF) printing technique, with an HP Jet Fusion 4200 printer. The MJF system consist of deposition of PA12 powder in the printing chamber, then a fusing and detailing agent is injected to define the syntherization zone and, finally, by means of infrared energy, the powder is fused. This sequence of operation is repeated layer by layer allowing a very fast production of devices. The specimens have a dog-bone shape (Fig. 3a), with a cross section of 5x3 mm 2 and built along the Z direction adopting a “balanced” profile”. Mechanical tests were performed adopting the Rapid Fatigue Machine (RFM) developed by the University of Messina’s spin-off KnoWow S.r.l. in collaboration with Italsigma S.r.l. (Fig. 3b). The machine, covered by patent, is electro-pneumatically actuated with a maximum load capacity of 2.5 kN and can perform static and fatigue tensile tests with frequency up to 15 Hz. It is equipped with a Digital Image Correlation (DIC) system and an IRtech Model XT IR camera. DIC system consists of a Basler video-camera acA2440-35uc which can acquire 35 frame per second with a full resolution of 2464 x 2056 px. DIC software, which has been developed in Python language using µDIC open-source library [18], allow to extract displacement and strain field in longitudinal and transversal direction. IR camera can acquire temperature’s field with a thermal sensitivity of 80 mK and it is controlled by Timage Connect software. Data acquired are processed in Python environment