Finite Strain and the 0.01 Percent Offset Yield Strength

Abstract

Abstract The validity of the stress at 0.01 percent offset as an approximation to the elastic limit was evaluated for a high-strength material, textured copper-beryllium CA-172. Specimens were loaded in uniaxial tension and precise determinations were made of the load-extension diagram. In loading to a strain of 0.85 percent, offsets of ∼0.02 percent were observed with insignificant permanent strain (<5 × 10−5) remaining on release of the load. Permanent strain of 0.01 percent occurred only after an offset of 0.05 percent and a strain of 1 percent. The nonlinearities observed correspond to previously reported higher-order elastic effects in copper. A priori corrections to the offset strength for elastic nonlinearities are impractical since the nonlinearities depend on both the higher-order elastic constants of the material and the preferred orientation. The limitations of the offset yield strengths are discussed and the suggestion made that dynamic energy absorption techniques be considered to measure the onset of plastic flow; that is, plastic strains of about 0.01 percent.