Experiments on Strain Rate History and Temperature Effects During the Plastic Deformation of Close-Packed Metals

Abstract

A study is presented on the influence of strain rate and strain rate history on the flow stress of several metals. Experiments were performed on specimens of four polycrystalline metals: 1100-0 aluminum, OFHC copper, AZ31 B magnesium, and commercially pure zinc. The experiments involve, the use of a modified Kolsky bar to increase abruptly the imposed strain rate, initially 2 × 10−4 s−1, by a factor of more than 106. Tests were performed at selected temperatures in the range T ≤ (1/2)Tm. The results include complete stress-strain curves for deformation at constant strain rates as well as for deformation involving a sharp increment in strain rate. The difference in flow stress at a given value of strain for stress-strain curves obtained at constant but different strain rates provides one measure of the influence of strain rate on the flow stress. However, the results of the incremental strain rate experiments show that both strain rate and strain rate history contribute to this difference in flow stress. Hence, interpretation of both the incremental and the constant strain rate tests offers a means for distinguishing between the effects of strain rate as opposed to those of differences in strain rate history.

Strain Rate and Strain Rate History Effects in Two Mild Steels.

“…In nearly al l cases , the stress-strain curve following the strain rate increment quickly joins the all-d ynamic curve, implying no "memory" effect in sharp contrast to the results of Senseny et al [13] for fcc and hcp metals , who found that the stress after the increment in strain rate was significantly lower than the all-d ynamic flow stress and approached it only graduall y with further straining. Thus , it appears that strain rate history has significantly less effect on flow stress in low carbon HRS , a bcc me ta l , than in aluminum or copper , fc c me ta l s , or magnesium or zinc , which ar e hcp metals [13]. One should note also the contrasting bahvior , as for upper and lower yield points , with the results of Frant : and Duffy [161 .…”

A constitutive description of the deformation of copper based on the use of the mechanical threshold stress as an internal state variable

“…We noted that the strain rate dependence of the deviation from the C-S law was consistent with the concept of a "recovery strain" contribution due to dynamic recovery, but that the addition of a second, shorter-range obstacle could also lead to the observed behavior. Fortunately, to a first approximation, these deviations can be ignored; but the detailed g, (8) variation should be incorporated into the model, either with tabular or empirical formats, for the best predictions.…”

Rise time in shock consolidation of materials

“…Existing experimental evidence indicates that the flow strength remains relatively high and strongly rate dependent under these conditions. 10,11,15,16 Figure 2 summarizes the variation of flow stress with shear strain rate for copper over a wide range of strain rates. The prediction of Eq.…”