Testing Techniques for Mechanical Characterization of Nanostructured Materials
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Testing Techniques for Mechanical Characterization of Nanostructured Materials Carl C. Koch, Ronald O. Scattergood, K. Linga Murty, Ramesh K. Guduru, Gopinath Trichy, Koteswararao V. Rajulapati Materials Science and Engineering Department, North Carolina State University, Raleigh, NC 27695 ABSTRACT Testing methods are reviewed that can be applied to the small sample sizes which result from many of the processing routes for preparation of nanocrystalline materials. These include the measurement of elastic properties on small samples; hardness, with emphasis on nanoindentation methods; the miniaturized disk bend test (MDBT); the automated ball indentation test (ABI); the shear punch test; and the use of subsize compression and tensile samples. INTRODUCTION The mechanical behavior of nanostructured materials has become a topic of significant interest in the materials research community. The strength/hardness of nanostructured elemental metals has been found to be much higher – factors of from 2 to over 10 times – that of conventional, micron grain sized counterparts [1]. Ductility remains a concern but some recent results offer some optimism [2]. While some methods of preparation of nanostructured materials provide sufficient sample sizes for conventional mechanical test samples, most do not. For this reason, the bulk of mechanical property measurements over the years have concentrated on hardness. More recently, a variety of techniques have been developed/applied to reveal mechanical properties on samples of limited size that are not obtainable from simple hardness measurements. This paper will describe and discuss:the measurements of elastic properties on small samples; hardness with emphasis on nanoindentation methods; the miniaturized disk bend test (MDBT); the automated ball indentation test (ABI); the shear punch test; and the use of subsize compression and tensile samples. ELASTIC PROPERTY MEASUREMENT METHODS FOR SMALL SAMPLES In the early years of studies of nanocrystalline metals made by the inert gas condensation method, their elastic behavior exhibited lower modulus values than for conventional grain size materials of the same chemistry [3]. However, it is now believed that, except for the finest grain sizes (< 10 nm), these lower values were due to sample porosity or measurement inaccuracies and were not inherent properties of the nanocrystalline metals [4]. The variety of methods used to measure elastic moduli may be divided into “static” methods – essentially measuring the elastic slope of a stress-strain curve – and “dynamic” methods which involve resonance or pulse-echo techniques with elastic waves. The measurement of Young’s modulus from stress-strain curves on small samples can suffer from poor resolution of the measured strain. Early results had this problem and are presumably responsible for lower values of the modulus [4,5]. Careful measurements of strain by using, for example, a miniature strain gauge glued to the small tensile sample [6], or a laser interferometry method on micro-scale s
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