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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The mechanical properties have been estimated according to ASTM D638 standard. As it is possible to observe, the curves from the tested specimens are almost coincident with datasheet curve up to a strain level of 2%, then they deviate from the initial trend encountering the hardening region for a stress level of 40 MPa. The average yielding stress, estimated as the maximum of the stress-strain curve, is lower compared to the datasheet (Tab. 1), while the elongation at yielding is higher. The ultimate stress and the elongation at rupture are lower than the datasheet. On the other hand, the Poisson’s ratio and the tensile Young’s Modulus are in good agreement.

Tab. 1. Comparison of the mechanical properties of MJF PA12. PA12 MJF Experimental value

Datasheet HP MJF (ASTM D638)

Yielding Stress [MPa] Elongation at Yelding [%] Ultimate Stress [MPa] Elongation at rupture [%]

42.2±0.6 13.4±3.0 40.9±1.5 16.2±2.6

48 11 44 20

Poisson's ratio

0.467±0.033 1836.2±125.0

0.46 1800

Tensile Young Modulus [MPa]

4.2. Temperature evolution during static tensile tests During the static tensile tests, the superficial temperature has been measured with the IR camera, and its variation respect the initial value at the beginning of the test has been reported in Fig. 5 vs. the nominal applied stress level. As it is possible to observe, in the first phase there is a decrement of the temperature that follows a linear trend, according to Lord Kelvin’s thermoelastic law. At the testing time of t= 32.5 s, the temperature signal deviates from its initial linear trend, following a flatter profile up to a minimum of temperature, which correspond to the onset of irreversible plastic deformation within the material. According to the STM, it is possible to perform two linear regressions, the first for the Phase I, the latter for Phase II and make their intersection. The linear regressions presented in Fig. 5 are the ones that maximize the coefficient of determination (R 2 ) for the bilinear model that fits the temperature signal from the beginning of the tests up to the beginning of Phase III. Initial and final temperature points from both Phases I and II are excluded from the linear regression. The intersection point of the two linear regressions correspond to the limit stress, σ lim , i.e. the macroscopic nominal stress level that lead to the first micro damage within the material. The onset of this microdamage is the reason for which the temperature signal deviates from the initial linear trend. For the specimens reported in Fig. 5, the limit stress is equal to 31.4 MPa.