The effect of stress concentration on the upper yield stress was investigated for an annealed mild steel wire by performing uniaxial tension tests. In order to avoid such a situation that initial yielding occurs at the specimen-clamping ends due to stress concentration, a non-uniform annealing method (the annealing temperature was the highest at the center of the specimen) was employed. For a non-uniformly annealed specimen the upper yield stress was almost twofold of the lower yield stress. In contrast, for a uniformly annealed specimen, the difference between the upper and lower yield stresses was not so considerably high, because the site of clamping-induced plastic pre-strain played a role of starting point of yielding. To examine the effect of stress concentration on the upper-yield point phenomena, FE simulation of the uniaxial tension tests was conducted using a viscoplastic constitutive model proposed by one of the authors (F. Yoshida, Int. J. Plasticity 16 (2000) 359). The results of numerical simulation were in good agreement with the experimental observations. From the results of the experiments and the numerical simulation, it is concluded that, although a material element itself possesses considerably high upper-yield stress, the observed one in ordinal uniaxial tension experiment is generally not so high because of the stress concentration.
(Received 2004/12/27; Accepted 2005/10/20; Published 2006/1/15)
Keywords: stress concentration, upper yield point, Lüders band, viscoplastic constitutive model, FE model, plastic strain, lower yield strength
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