Micro-structure mechanical failure characterization using rotating Couette flow in a small gap

Abstract

A new method for testing the failure rates of micro-mechanical structures is presented. The technique uses Couette flow in a narrow-gap channel to induce different forces and loading on an array of structures. The Couette flow is induced by a rotating disc, which allows multiple devices to be tested at different designed stress levels depending on rotation speed and radial position. As an example, SU-8 micro-structures are used to present a general testing procedure that can be applied to other components. The forces acting on the micro-structures are due to fluid shear stress, centrifugal forces from rotation and form drag, all of which are characterized as they vary with radial position for one rotation rate. One series of tests with a single circumferential row of identical micro-structures is performed to determine the relative importance of these forces on revolutions to failure and failure rate of the structures in the row. Two additional series of tests are conducted to determine the effects of also adding unsteady loading from wakes to the structures. This unsteady loading from wakes is induced by time-varying velocity and pressure variations, which are imposed when additional rows of micro-structures are placed at smaller radial positions compared to the row being tested. Weibull failure rate approaches are used to provide information on failure rate as dependent upon cumulative revolutions. As such, the testing approach is useful for failure characterization of thin-film adhesion, self assembled nano-layers and micro-mechanical structures.