Mechanics of energy transfer and failure of ductile microscale beams subjected to dynamic loading

The mechanical response of microelectromechanical systems (MEMS) under impulse loading conditions has not been thoroughly studied to date, partially because of the lack of means to provide such extreme loading rates to miniature devices. However, the increasing use of MEMS-based sensors and actuators in adverse environments, which include extreme strain rate loading, has motivated the investigation of the response of MEMS components under these conditions. In this work, basic and mostly commonly employed Au MEMS components were subjected to impulse loads of 40 ns in duration, which were generated by a high power pulsed laser in order to achieve acceleration levels on the order of 10⁹g. This allowed for the microdevice mechanical/structural response to be investigated at time scales that were of the order of wave transit times in the substrate and the devices. Basic microscale structures, such as cantilevers and fixed-fixed beams of uniform cross-section, were employed to facilitate comparisons with companion finite element simulations in order to gain insight into the mechanisms responsible for impulsive deformation at the microscale. The simulations investigated the effect of loading rate, boundary conditions, beam length, material constitutive response, and damping on the final deformed shapes of the beams. It was found that contact and momentum transfer mechanisms were responsible for the large permanent beam deflections which were measured postmortem. Additionally, the effects of both damping and material property rate dependence were found to be dominant in determining the final deformed shape of the beams. In fact, our observations suggest that the contributions of material rate dependence and damping are not simply additive, but rather involve a coupling between them that affects the final structure response.