Coming Full Circle: The Application of Microtechnology Techniques to Evaluate Bulk Materials
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Coming Full Circle: The Application of Microtechnology Techniques to Evaluate Bulk Materials* David Read and Nicholas Barbosa Materials Reliability Division, National Institute of Standards and Technology, Boulder, CO 80305, U.S.A. ABSTRACT A tensile test procedure that accommodates specimens with gage section 25 m thick, 70 m wide and 360 m long was developed and demonstrated. The instrumentation and technique were adapted from those previously developed and used to test thin films, by increasing both the force capacity of the load cell and the stiffness of the pull rod. Specimens with bow-tie geometry were fabricated by photolithography from nominally 25 m thick full hard stainless steel 302 foil. A silicon test frame fabricated by bulk micromachining techniques included tapered grips in the form of recesses in its top surface that accepted and retained the specimen grip sections. One grip was on the fixed outer portion of the frame. The other grip was on a plate suspended in the center of the frame by long slender silicon beams. Force was imposed on this plate by pin loading. The force was measured by use of a custom load cell. The displacement was measured by sub-pixel digital image correlation to surface features on the two ends of the gage section, applied to images with a resolution of approximately 0.8 m per pixel. Yield and ultimate strengths and elongation values consistent with vendor-provided information were obtained. The values of Young’s modulus were scattered but within the range of expected behavior for the specimen material. INTRODUCTION The mechanical performance of materials exposed to harsh environments is important in a variety of contexts, for example, in nuclear power systems and chemical reactors. The search for greater efficiency in measuring the mechanical properties of such materials has advanced in recent years through the gradual downsizing of specimens. Techniques developed recently for mechanical testing of metal films with thicknesses of one micrometer or less illustrate new approaches that may be useful for testing small specimens of traditional bulk materials. The downsizing is advantageous for the development of new materials for harsh environments for the following reasons: 1. Supplies of special specimen material, e.g. nanostructured developmental alloy, may be limited. 2. Smaller specimen size means that a smaller “harsh environment” is needed, which leads to safer tests and less waste material. 3. Certain harsh treatments, e.g. irradiation methods, may be limited in penetration depth into the material surface. With a sufficiently small specimen, full thickness treatment may be convenient. 4. Smaller specimens may facilitate testing in situ, within the harsh environment. *Contribution of the National Institute of Standards and Technology, an agency of the U. S. Department of Commerce. Not subject to copyright in the U.S.A.
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EXPERIMENT Specimen and Test Frame Stainless steel 302 was selected as the specimen material for its similarity to widely used structural stainless st
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