Scale effects in microindentation of ductile crystals

Indentation testing has long been a standard test used to classify all types of materials. In the past several decades the scale of indentation testing has moved into the micron and even sub-micron range. For many types of materials, at these small length scales, the hardness of the material measured by the indentation test depends on the depth of the indentation. This indentation size effect was not observed at the larger length scales. Because indentation testing with conical or pyramidal indenter tips is geometrically similar, the existence of a size effect was surprising. Since the size effect associated with microindentation was discovered, many theories about its cause have been proposed. Several of the theories suggest that the source of the indentation size effect is experimental error. Such factors as inaccurate measurement of the contact area, indenter tip deformities, improper surface preparation, lateral movement of the indenter tip, inhomogeneity of the material, compliance of the test fixture, anisotropic deformations, thermal drift, and noise have been cited as areas where experimental error may play a role in the size effect. Another group of theories suggests that there are actual physical causes for the size effect. Some of the proposed physical causes of the size effect are friction between the specimen and the indenter tip, elastic recovery of the indent, material pile-up and sink- in, work-hardened surface material, oxidized surface layers, and variation of material parameters due to the stress state of the material. One area that has received some attention recently as a possible cause of the indentation size effect is hardening resulting from geometrically necessary dislocations (GNDs). GNDs arise due to curvature in the crystalline lattice from gradients of plastic shear strain. As the indentation depth decreases, the relative strain gradients within the test specimen increase.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Mechanical Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2000

Author dc:creator

• Nielson, Gregory Nolan, 1974-

Advisor dc:contributor.advisor

• David M. Parks.

• Mechanical Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/89289

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/89289