Temperature Rise Associated with Adiabatic Shear Band: Causality Clarified

One of the most important issues related to adiabatic shear failure is the correlation among temperature elevation, adiabatic shear band (ASB) formation and the loss of load capacity of the material. Our experimental results show direct evidence that ASB forms several microseconds after stress collapse and temperature rise reaches its maximum about $30\mu \mathrm{s}$ after ASB formation. This observation indicates that temperature rise cannot be the cause of ASB. Rather, it might be the result of adiabatic shear localization. As such, the traditional well-accepted thermal-softening mechanism of ASB needs to be reconsidered.

Figure 1

Deformation history of a shear-compression specimen, showing the time, shear stress, and temperature. Numbers in the boxed legend indicate the eight detectors (or locations) where temperatures are measured. The three stages (I, II, and III) are marked by vertical dotted lines. The arrows $a-l$ correspond to the times at which the high-speed photographing system captures snapshots of the specimen. Four representative snapshots of the specimen are shown in Fig. 2 that correspond to the times of $a$, $f$, $h$, and $l$ in Fig. 1. T-Cal, the dashed line, is the calculated temperature based on the conversion of mechanical work to heat (taking 1.0 as the Taylor-Quinney factor). The dashed green and red lines denote the appearances of ASB and crack, respectively.

Figure 2

Four representative high-speed photographic snapshots to show the deformation of a shear-compression specimen under dynamic loading. The letter at the upper left corner of each snapshot correpsonds to the time of Fig. 1. (a) Before any plastic deformation; (f) Uniform deformation at the peak stress or immediately before softening; (h) Adiabatic shear band—deformation is now highly localized, contrasting (f); (l) presents a crack as a consequence of the ASB.

Figure 3

Summary of the measured and calculated ($\beta =1.0$) temperature rise at the maximum shear stress for the shear-compression tests.

Figure 4

Time sequence of the occurrence of typical events in the dynamic shear failure process. It shows that peak shear stress comes first, followed by ASB initiation, and the maximum temperature rise comes last. These results greatly clarified the causality of these important events during dynamic loading of the shear-compression specimen.