Bend testing of micro-scale bulk metal specimens using a chip-scale test instrument

We describe bend testing on micro-scale specimens of 302 stainless steel, using a MEMS test instrument. Bend testing is a common way of measuring the flexural stiffness of structural materials across many size scales, from thin laminate sheets to large weldments. Whereas the stiffness of a material under tensile loading is given by the Young's Modulus, the flexural stiffness, or the stiffness in bending, is much lower. In the past two decades, conventional materials testing machines and the specimens themselves have undergone miniaturization for the purpose of evaluating the mechanical properties of miniaturized mechanical components such as sensors and biomedical implants, for which the smallest specimen dimension is typically around 1 mm [2]. Another driver for miniaturizing the testing apparatuses is to test materials with inherently small form factors such as wires and thin films [3]. Now the emerging 3D printing technology is creating another need for material property measurement at micrometer size scales, to accurately capture the property gradients resulting from the layered manufacturing. However, with ever increasing miniaturization comes increasing difficulty in specimen handling, gripping, and alignment. Concurrently, MEMS technology has been used over the past 2 decades to fabricate small actuators and sensors for mechanical testing of materials of thin films [4] or nanoscale materials such as nanowires. We seek to use the advantages of MEMS to study the mechanical properties of bulk materials rather than thin films, but at the micrometer scale. We believe this will result in greater accuracy and spatial resolution of property measurements of structural materials used in civil infrastructure, aerospace, transportation and energy industries, as well as characterizing manufacturing processes that lead to steep property gradients such as 3D printed components. Our approach is to use MEMS actuators as chip-scale re-useable test instruments into which small specimens sectioned from bulk materials can be inserted and tested [5], to reduce the cost and time to obtain large data sets and to allow the measurements to be done in-situ in harsh environments. We will describe the design of a micro-size 302 stainless steel specimen, and the use of a MEMS test instrument for performing the bend testing on the specimens. The specimen's gage section was 350 um long, 65 um wide and 25 um thick, and was made by lithographic etching of a foil. The MEMS test instrument was fabricated from silicon and glass wafers. The specimens were inserted into the MEMS test chip and the silicon actuator applied static bending loads to the specimen. Displacements were measured from optical microscope images, and the force was calculated from the applied voltage and the known (measured) stiffness of the silicon actuator. The applied force from the MEMS actuator was measured directly, without any specimen, using a custom table-top force probe and load cell apparatus, and was in agreement with the force calculated from the applied voltage. The flexural stiffness of the micro specimens were measured, using the MEMS test device, at 90 – 130 N/m. These values were validated by independently testing the specimens with the much larger table-top force probe. We thus show that our MEMS test chip can be used to perform bending tests on micro scale specimens of bulk materials, but with a 1000-fold reduction in size compared to table-top force-measuring apparatuses.