Abstract
Selective laser sintered parts in polyamide are increasingly being used as functional parts in a variety of applications. The SLS technique offers distinct advantages over conventional production techniques, such as low cost, low production time and high flexibility, particularly when a small batch of lightweight components with a complex geometry is needed. However, knowledge of the fatigue properties of these components is still very limited and fatigue life predictions are in practice often not possible. Therefore tensile tests and uni-axial constant amplitude fatigue tests were performed on toroidal test specimens in a force controlled servohydraulic test rig. During these experiments force and displacement were monitored. Also the temperature of the critical cross section was measured using a thermocouple and an infrared temperature sensor.
The test specimens were produced with state of the art rapid manufacturing equipment from Materialise. Using the EOS sinter station P730, PA12 powder with a particle size of 60 to 90 μm was sintered with a CO2 laser. To minimize statistical scatter in the fatigue results, the authors have selected an ideal combination of production parameters. Furthermore, all the specimens were manufactured at the same time. This was done in a horizontal plane in the middle of the build platform with the scanning path in the longitudinal direction of the test pieces.
The results of the tensile tests show little macroplastic deformation, a small elongation at break and a surface fracture perpendicular to the loading direction, which is typical for a brittle material. The specimens subjected to repeated tensile and compression stresses show larger necking regions and a fracture surface under an angle of 45° with the loading direction. This is due to the rise in temperature which causes a decrease in stiffness and an increase in macroplastic deformation. This phenomenon is also clearly visible when studying the elastic hysteresis curves. The symmetrical hysteresis curves indicate an identical elastical behaviour in tension or compression. Furthermore, the results also point to cyclic dependable creep (ratchetting) and cyclic softening. Finally, it is found that the densities of the components play a major role in the fatigue life. The presented results are the first step for accurate fatigue life predictions of selective laser sintered components in polyamide.
Keywords: Fatigue tests, Selective laser sintering, Polyamide, RP&M, S-N approach