Mechanical response of mesoscopic aluminum rings under uniaxial compression
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Research Letter
Mechanical response of mesoscopic aluminum rings under uniaxial compression Bin Zhang, Shahrior Ahmed, Shuai Shao, and W.J. Meng, Mechanical & Industrial Engineering Department, Louisiana State University, Baton Rouge, LA 70803, USA Address all correspondence to W.J. Meng at [email protected] (Received 25 January 2018; accepted 16 April 2018)
Abstract The strength of materials exhibits size effects at sample dimensions 100 µm in characteristic dimension to the ∼10 and ∼1 µm scales. The mechanical behavior of metals at the sub-mm to μm scales is an important consideration for forming process design and implementation. Over the past two decades, various measurements of the flow stress of metals have been performed on specimens with characteristic external sizes ranging from ∼1000 to ∼1 µm and below, and strong size effects on the strengths of materials have been observed. For moderately large sample sizes, i.e., in the ∼100 to ∼10 µm range, size effects have been observed and associated with strain gradients imposed by deformation geometry, such as in torsion of thin wires[5] and bending of thin foils.[6] In wire torsion and foil bending, it was shown that materials increasingly strengthen as the characteristic specimen dimension decreases,[5,6] due to increase in imposed strain gradients. In the absence of strain gradients, size effects have also been observed in samples
with smaller sizes. At diameters ranging from ∼40 µm down to ∼0.2 µm, axial compression of single crystal cylindrical pillar specimens showed a pronounced increase in their flow stress as the diameter decreases.[7,8] Such increase in strength has been attributed to a dislocation starvation mechanism, where the nucleation, subsequent propagation, and escape of dislocations carry a dominant influence on plastic deformation. In contrast to the above two examples of “smaller is stronger”, when poly-crystalline cylinders with diameters ranging from ∼3000 µm down to ∼500 µm were tested under uniaxial compression, the flow stress was reported to decrease with decreasing cylinder diameter with the grain size held constant.[1,2,9] In the mesoscopic range of characteristic dimensions between ∼400 and ∼20 µm, data sets on the mechanical response of materials are scarce in the current literature. Further, mechanical response of materials obtained from simple testing geometries with free external specimen surfaces, such as uniaxial compression of free-standing cylinders, may be significantly different from their behavior during actual microforming operations, in which almost all external surfaces of the material being formed are in contact with the forming tool and are subjected to significant external constraints.[10] Obtaining reliable uniaxial compression response of materials in this meso-range of characteristic specimen dimensions is thus of interest because: first, data obtained may elucidate mechanisms responsible for the transition from a “smaller is weaker” to a “smaller is stronger” behavior; second, knowledge gained from such simple tests wit
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