Fatigue testing of bulk materials using a microsystems based approach

Abstract

Fatigue, the degradation of a material's mechanical properties due to cyclic loading, is a critical issue limiting the reliability of structural materials[1]. Fatigue testing of materials is typically carried out in controlled laboratory conditions on specially prepared specimens, and the results are extrapolated to real world conditions. In the past two decades, conventional fatigue testing machines and specimens have undergone miniaturization for the purpose of evaluating the fatigue properties of miniaturized mechanical components such as sensors and biomedical implants, with the smallest test specimens having dimensions on the order of 1 mm length [2] or consisting of foils and wires [3]. Challenges with miniaturization include difficulty in specimen handling, gripping, and alignment. At the same time, MEMS technology has been used to fabricate the actuators and sensors for fatigue testing of thin films [4]. In this approach, the specimen is typically part of the MEMS actuator and is fabricated in-situ. While this eliminates the problems with specimen gripping and alignment, it limits the specimen materials to those from which MEMS actuators and sensors can be readily fabricated, is destructive to the MEMs device, and furthermore is typically limited to thin films. We seek to use the advantages of MEMS to study the fatigue properties of bulk materials rather than thin films, but at the micrometer scale. This allows for greater accuracy and spatial resolution, compared to the state of the art, of property measurements of structural materials such as aluminum and stainless steel alloys as well as other materials used in civil infrastructure, aerospace, transportation and energy industries. Our approach is to use MEMS as chip-scale re-useable test instruments into which small specimens cut from bulk materials can be inserted and tested [5]. We describe the design of the MEMS test instrument and the metal foil specimen, whose gage section was 135 um wide and 25 um thick. The test instrument was fabricated from silicon and glass wafers, and the specimens were etched from commercially available Al 1145 H19 foil. Our S-N curve agrees within expectation with published values for similar aluminum alloys tested using conventional methods at much larger specimen size scales, and the fracture surface shows distinct regions corresponding to slow and fast crack growth. We envision this test technique as a tool to further the study of the fatigue properties of structural materials.