Measuring flow curve and failure conditions for a MEMS-scale electrodeposited nickel alloy^,†

Implementation of advanced electroplated Nialloy materials in MEMS-scale mechanisms requires performance predictions that are based on actual, rather than idealized, material properties. Accurate characterization of material properties is necessary to the formulation of material models used in finite element analysis (FEA) simulations that can be relied upon for critical design applications. Quantitative material models, and specifically those implemented in FEA codes, require the use of the true stress-true strain material flow curve as input. Simulations of failure conditions require that a failure criterion be known, such as the failure strain. The subject of this paper is a method for obtaining such characterization of MEMS-scale electroplated materials; it addresses the challenges associated with measurement of stress and, particularly, strain that arise from geometric constraints on MEMS-scale specimens and instrumentation. These include thicknesses a few tenths of a millimeter and below, component dimensions 1 mm and less and planar geometries. A method for refining the material quasi-static flow properties, by inverse-modeling of data from quasi-static tests, is reported. Tests were performed on an electrodeposited nickel alloy by use of a small-scale conventional loading apparatus, with digital imaging to measure specimen displacements. Simulations were carried out using two general purpose, commercially available FEA codes. Inverse modeling was applied to the data to obtain true stress-true strain flow curves. The FEA simulations of the local displacements converged to the experimental results for both commercial codes. Forward-modeling was then used to determine the maximum equivalent true plastic strain in the specimen neck; the calculated values converged to significantly different values in the two codes. This result suggests a need for caution in the use of reported values for maximum effective plastic strain as a failure criterion in FEA simulations.