A Novel Method for In Situ Uniaxial Tests at the Micro/Nano Scale—Part I: Theory

A novel MEMS-based uniaxial testing apparatus and a specimen design are presented for measuring the mechanical response of material samples in situ in scanning electron microscope (SEM). The stage adopts an assembly approach, where specimens are fabricated independently, allowing testing of a variety of materials. The assembly approach, however, involves intrinsic challenges in achieving pure uniaxial loading at the micro/nanoscale due to off-axis loading (misalignment) errors. The effect of misalignment in stress evaluation increases with decreasing size scale of the sample-an issue that has received limited attention in the literature. Here, the source of intrinsic misalignment and its influence on the stress nonuniformity are explored analytically and numerically. This paper reveals substantial bending of the microspecimen due to small and often unavoidable off-axis loading. The proposed stage and the specimen ensure uniaxial loading by introducing hingelike self-aligning mechanisms both in the stage as well as in the specimen. The analysis offers the parameter space to design and optimize uniaxial tests at the micro/nanoscale.