and strain fields much ahead of upper yield point. Following band formation, band front propagates and at 63 s formation of second band takes place from the upper end of gauge length after which both the band fronts propagate and cover the complete gauge length at 110 s as depicted by the thermal and strain fields in Fig. 25.3. At 110 s, center of the gauge length in the strain field is observed to be yielded relatively less in comparison with other zones. Following this, uniform strain hardening takes place and therefore thermal and strain evolutions are expected to occur over the entire gauge length. But, during the initial stage of strain hardening, till 114.5 s, thermal evolutions are found to be localized in the center of the gauge length as shown in Fig. 25.4a. This information is not conveyed by the corresponding strain field in Fig. 25.4b due to the record of prior plastic deformation by band fronts propagation. However, strain rate field shown in Fig. 25.4c clearly depicts such strain localization in the center of the gauge length at this instant. Such continuation of strain localization during the initial stage of strain hardening is called as delayed yielding which has also been observed earlier using strain gauges [10]. Figure 25.5c depicts the axial profiles of strain and strain rate along the line marked in the corresponding strain field and strain rate field in Fig. 25.5a, b at 60 s where first band front from the lower end of the gauge length has propagated to some distance along the gauge length. The region with linear variation of strain from the band front to within the band is called as the Fig. 25.4 Thermal field (a), strain field (b) and strain rate field (c) at 114.5 s Fig. 25.5 Strain field (a), strain rate field (b) and their axial profiles (c) at 60 s 190 S. Nagarajan et al.
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