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A unified and detailed evaluation of the coupling between specimen and test machine is presented. It is demonstrated that an accurate formulation of the specimen/machine coupling relation requires exact additivity of elastic and plastic strain contributions, and that additivity is only possible with a specific set of definitions for these strains. When these definitions are implemented to ensure 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 information about plastic-deformation models from raw data. Topics covered include (1) effect of machine stiffness on work-hardening behavior, (2) variation of total and plasticstrain 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.
tension test, stress relaxation, strain, strain rate, machine stiffness, crosshead speed
Principal Research Scientist, Battelle-Columbus Laboratories, Columbus, Ohio
Supervisor, Division 8316, Sandia National Laboratories, Livermore, Calif.
Supervisor, Division 5832, Sandia National Laboratoires, Albuquerque, N.M.