A Review on the Adiabatic Shear Banding Mechanism in Metals and Alloys Considering Microstructural Characteristics, Morphology and Fracture

Abstract

The current review work studies the adiabatic shear banding (ASB) mechanism in metals and alloys, focusing on its microstructural characteristics, dominant evolution mechanisms and final fracture. An ASB reflects a thermomechanical deformation instability developed under high strain and strain rates, finally leading to dynamic fracture. An ASB initially occurs under severe shear localization, followed by a significant rise in temperature due to high strain rate adiabatic conditions. That temperature increase activates thermal softening and mechanical degradation mechanisms, reacting to strain instability and facilitating micro-voiding, which, through its coalescence, results in cracking failure. This work aims to summarize and review the critical characteristics of an ASB’s microstructure and morphology, evolution mechanisms, the propensity of materials against an ASB and fracture mechanisms in order to highlight their stage-by-stage evolution and attribute them a more consecutive behavior rather than an uncontrollable one. In that way, this study focuses on underlining some ASB aspects that remain fuzzy, allowing for further research, such as research on the interaction between thermal and damage softening regarding their contribution to ASB evolution, the conversion of strain energy to internal heat, which proved to be material-dependent instead of constant, and the strain rate sensitivity effect, which also concerns whether the temperature rise reflects a precursor or a result of ASB. Except for conventional metals and alloys like steels (low carbon, stainless, maraging, armox, ultra-high-strength steels, etc.), titanium alloys, aluminum alloys, magnesium alloys, nickel superalloys, uranium alloys, zirconium alloys and pure copper, the ASB propensity of nanocrystalline and ultrafine-grained materials, metallic-laminated composites, bulk metallic glasses and high-entropy alloys is also evaluated. Finally, the need to develop a micro-/macroscopic coupling during the thermomechanical approach to the ASB phenomenon is pointed out, highlighting the interaction between microstructural softening mechanisms and macroscopic mechanical behavior during ASB evolution and fracture.

Numerical Investigation of the Damage Effect on the Evolution of Adiabatic Shear Banding and Its Transition to Fracture during High-Speed Blanking of 304 Stainless Steel Sheets

“…In this study, T DRX is estimated at 43% of T m , which remains in agreement with relevant work [29]. Finally, when the DRX softening effect is not considered on damage evolution, T cr is set equal to T m in order to capture local melting, which leads to micro-voiding and micro-cracking genesis at the following [1].…”

“…Therefore, the actual evaluation of the χ-value has proved crucial in predicting the peak force and ASB evolution, as its typical value of 0.9, which was globally considered during early studies, must be reassessed based on recent findings that highlight its influence from the material and the loading mode [27]. Finally, considering that the DRX temperature of 304SS is estimated at about 720 K and that the minimum examined χ-value of 0.5 for 304SS provides a maximum temperature marginally greater than 720 K, it is indicated that 304SS promotes DRX activation during ASB formation, which is not always guaranteed [1], underlining the importance of accounting for DRX softening effect during numerical computations.…”

“…In fact, the temperature field of 0.5 χ-value seems almost to reach the DRX temperature, as depicted in Figure 20b, indicating that the activation of DRX softening depends strongly on plastic work-to-internal heat conversion efficiency. Therefore, χ-value above 0.5 can activate DRX softening inside ASB, reacting to maximum temperature marginally greater than the critical one of 720 K, and considering that 304SS shows χ-values higher than 0.5 [30], it is concluded that 304SS promotes DRX during ASB formation, which is not always guaranteed for every material [1]. In addition, a higher χ-value results in an earlier temperature rise inside ASB, which allows it to reach a greater maximum temperature while it also activates the sharp damage increment earlier, thus facilitating fracture initiation, as confirmed in Figure 21a.…”

“…In that way, a smooth transition from very fine meshing to a coarser one is achieved, preventing numerical instabilities. The very fine FE mesh within ASB's expected region is necessary for capturing the occurrence of ASB and predicting its width, which has been estimated for about some decades in micrometers [1]. The ASB width is assessed through the visualization of shear strain distribution and obtaining the extent of critical strain transversely to the ASB direction.…”

“…In consequence, the significant temperature increase inside ASB provides a transformed banding type, with dynamic recrystallization (DRX) and phase transformation being the two dominant mechanisms. Thus, ASB has been described as thermomechanical strain instability, which is reached when thermal softening overcomes strain hardening, reacting to catastrophic and rapid failure through micro-voiding and cracking propagation [1].…”