PSI - Issue 34

Dario Santonocito et al. / Procedia Structural Integrity 34 (2021) 211–220 D. Santonocito et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 2. Temperature trend during a static tensile test

The use of high precision IR sensors allows to define experimental temperature vs. time diagram during static tensile test in order to define the stress at which the linearity is lost. In 2010, Clienti et al. (Clienti et al. (2010)) for the first time correlated the damage stress σ lim related to the first deviation from linearity of ∆T temperature increment during static test (end of phase I) to the fatigue limit of plastic materials. If it is possible during a static test to estimate the stress at which the temperature trend deviates from linearity, that stress could be related to a critical macro stress σ lim which is able to produce in the material irreversible micro-plasticity. This critical stress is the same stress that, if cyclically applied to the material, will increase the microplastic area up to produce microcracks, hence fatigue failure. 3. Materials and Methods The material under study is a polyamide 12 (PA12), which has high mechanical properties, good chemical resistance and biocompatibility. A set of specimens was produced by Selective Laser Sintering (SLS), an additive manufacturing technique that adopts a laser beam to sintering plastic or composite powder. The specimens were produced according to the geometry 1A prescribed by ISO 527-2 standard, with a nominal cross section of 10 mm x 4 mm, adopting the EOS commercial powder PA2200. The specimens building orientation angle was of 0° respect the X machine direction, allowing a volume internal grid-like structure due to the orthogonal laser beam trajectories (Stoia et al. (2019)) (Fig. 3a). Static tensile tests were performed on a set of three specimens, adopting a servo-hydraulic load machine ITALSIGMA 25 kN with a crosshead speed of 5 mm/min, with constant temperature and relative humidity (23 °C and 50% RH). During the tests, axial strain on the specimens were measured by an extensometer, with an initial gauge length of 25 mm, and the specimen surface temperature was monitored with an infrared camera FLIR A40, adopting a sample rate of 1 image per second (Fig. 3b). A series of fatigue tests, with a stepwise increase of the stress level of Δσ = 1 and 2 MPa, were performed adopting the same loading machine, with a stress ratio R= 0.1, a number of cycles per block ΔN= 3000 and test frequency f= 1 Hz to prevent higher temperature increment. Also during the fatigue tests the temperature evolution o f the specimens was monitored with the infrared camera. The specimen’s failure surfaces were analysed adopting a high-definition optical microscope LEICA.