The effect of different loading scenarios on deformation shape and fatigue life of plastics

Compliant mechanisms are a relatively new technology which often utilize elastic body deformation to transfer an input force and displacement to an output force and displacement at a different location. The inherent function of these devices makes them susceptible to field failures. Consider the Britax B-Safe 35 infant car seat series, which was recalled in June of 2017. The products were recalled because of the potential for a compliant member in the unit's chest clip to fail in field, presenting a choking hazard for the children in the seat. This single component failure led to full-system failure as the infant car seat's primary function was no longer fulfilled. Field failures like this underscore the importance that testing be properly performed to ensure compliant mechanisms in the marketplace do not affect product reliability. One such form of applicable testing is for fatigue; however, further research is needed to ensure the deformation shape tested during fatigue tests resembles that of the compliant member in the system in the field. Three-point bending tests and dual-directional cyclic loading tests were performed using three different testing apparatuses with various deformation shapes which may reflect various loading scenarios in reality. Fatigue data was generated using ABS plastic sheets with multiple deformation shapes. This paper includes the results from the three-point bending and fatigue tests on ABS plastic sheets (5.0” × 0.5” × 0.125”) which provides insight on the influence that different deformation shapes have on the fatigue life of ABS plastic.