Fracture to shear-banding failure mode transition (FMT) is an important physical phenomenon in solid materials when a fracture is subjected to dynamic loads. Since its observation, FMT has long been considered as an abrupt change, which is triggered by a critical impact velocity. However, with a novel experimental scheme and a newly-designed dynamic shear fracture specimen, we present a different viewpoint based on experimental observations that FMT is actually not an abrupt change, but a continuous evolving process of distinct microstructures dominated by a thermo-plastic mechanism. The mode II dynamic fracture toughness K_IId was determined over a large range of loading rates. It's observed that with the increase of loading rates, K_IId rose continuously while the failure modes changed gradually from coalescence of stretched dimples to a combination of dimples and ASBs, and then to extensive ASBs. The initiation and propagation of the fracture was also observed by a high-speed camera, and it has a good consistence with the development of the microstructures in the FMT. This discovery will provide a brand-new understanding to FMT, and lay the foundation for developing of new criteria and theories for selection of failure modes in solid materials.

当断裂承受动态载荷时，断裂至剪切带破坏模式转变（FMT）是固体材料中的重要物理现象。自从观测以来，FMT一直被认为是由临界冲击速度触发的突然变化。但是，采用新颖的实验方案和新设计的动态剪切断裂标本，我们基于实验观察结果提出了不同的观点，即FMT实际上不是突变，而是由热塑性机制主导的不同微结构的连续演化过程。 。在较大的加载速率范围内确定了模式II动态断裂韧性K _IId。据观察，随着装载率的增加，K _IId连续上升，而破坏模式从拉伸的凹痕的聚结逐渐变为凹痕和ASB的组合，然后变为广泛的ASB。高速照相机还观察到了裂缝的萌生和扩展，它与FMT中微观结构的发展具有良好的一致性。这一发现将为FMT提供全新的理解，并为开发新标准和理论以选择固体材料的失效模式奠定基础。