Specimen-test machine coupling and its implications for plastic-deformation models

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The equation relating uniaxial strain rates to the crosshead speed of a test machine of arbitrary stiffness is carefully examined. It is demonstrated that an accurate formulation of the specimen/machine coupling relation requires exact additivity of elastic and plastic strain contributions and that this is only possible with a specific set of definitions for these strains. When these definitions are implemented to insure additivity, two strain-dependent terms in the coupling relation are identified that have been treated as constants in previous studies. One term is shown to account for the decreasing true total strain rate with increasing plastic strain at ... continued below

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Pages: 36

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Holbrook, J.H.; Swearengen, J.C. & Rohde, R.W. January 1, 1981.

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Description

The equation relating uniaxial strain rates to the crosshead speed of a test machine of arbitrary stiffness is carefully examined. It is demonstrated that an accurate formulation of the specimen/machine coupling relation requires exact additivity of elastic and plastic strain contributions and that this is only possible with a specific set of definitions for these strains. When these definitions are implemented to insure additivity, two strain-dependent terms in the coupling relation are identified that have been treated as constants in previous studies. One term is shown to account for the decreasing true total strain rate with increasing plastic strain at a fixed crosshead speed. The second term accounts for strain dependence of the combined specimen/machine modulus due to change in length and area of the specimen. Several applications of the coupling relation are given that illustrate the importance of treating the specimen and test machine as a well-defined system when inferring infomation about plastic-deformation models from raw data. Topics covered include (1) effect of machine stiffness on workhardening behavior, (2) variation of total and plastic strain rate during tensile deformation, (3) effect of machine stiffness on stress relaxation behavior, (4) relationship between stress rates before and after a change in crosshead speed, and (5) deductions about mobile dislocation density from observed stress-time behavior.

Physical Description

Pages: 36

Notes

NTIS, PC A03/MF A01.

Source

  • ASTM conference on mechanical testing for deformation model development, Bal Harbour, FL, USA, 12 Nov 1980

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  • Report No.: SAND-80-1840C
  • Report No.: CONF-801148-5
  • Grant Number: AC04-76DP00789
  • Office of Scientific & Technical Information Report Number: 6501705
  • Archival Resource Key: ark:/67531/metadc1209404

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Office of Scientific & Technical Information Technical Reports

Reports, articles and other documents harvested from the Office of Scientific and Technical Information.

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  • January 1, 1981

Added to The UNT Digital Library

  • July 5, 2018, 11:11 p.m.

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  • Oct. 26, 2018, 1:41 p.m.

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Holbrook, J.H.; Swearengen, J.C. & Rohde, R.W. Specimen-test machine coupling and its implications for plastic-deformation models, article, January 1, 1981; United States. (digital.library.unt.edu/ark:/67531/metadc1209404/: accessed January 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.