A continuum surface layer effect in polycrystalline aggregates

Uniaxial tension tests on uniform cross section specimens of Ti-6Al-4V and aluminum 7475-T651 show that the stress in the direction of load application determined by x-ray diffraction and the nominal applied stress display the expected linear correspondence up to a maximum stress somewhat below the bulk yield paint. Above this stress, the xray stress is noticeably less than expected. Upon unloading, there is a substantial acquired compressive residual stress. Because the x-ray diffraction measurements provided average stress values in a surface layer of only a few grain diameters, these results suggest that the surface layer of the metal is yielding at a lower stress than the bulk. This anomalous behavior is duplicated qualitatively with a continuum model of an aggregate of soft and hard square grains obeying the Von Mises yield criterion. The effect is purely mechanical. No material effects, such as lower yield point or dislocation density for the surface grains, are invoked. The continuum effect decreases rapidly with depth, becoming negligible for depths exceeding 2-3 grain or domain diameters. Further work is required to refine the model for real systems and to assess the importance of material effects acting in conjunction with the continuum effect.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Engineering Mechanics

Department dc:contributor.department

Engineering Mechanics

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

1994

Author dc:creator

• Laurent, Michel P.

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

etd-06082009-170819

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/42890