# Strain Rate of Metal Deformation in the Machining Process from a Fluid Flow Perspective
## 1 Introduction  
Correctly describing the mechanical behavior of metal materials during the deformation process under different strain rate conditions is an important subject of solid mechanics. [PERSON] [1] divided the strain rates into three categories: low strain rate, \(10^{-4}\)-\(10^{-1}\) s\({}^{-1}\); middle strain rate, \(10^{-1}\)-\(10^{2}\) s\({}^{-1}\); and high strain rate, \(10^{2}\)-\(10^{4}\) s\({}^{-1}\). Metal deformation will show different behaviors under different strain rates, such as elasticity, plasticity or viscosity [2]. The material response at a low or medium strain rate is usually described by one behavior, elasticity or plasticity, or a combination of the two. With increasing strain rate, the metal deformation form will change from static to dynamic behavior. The effects of material viscosity should be considered because it describes the viscous behavior of the materials in terms of the relevant strain rate (strain rate includes time factors). Regardless of ductile fracture or brittle fracture, it is a process that develops at a limited rate. Therefore, the mechanical properties of the materials are essentially strain-rate dependent. However, the relevant rate cannot accurately describe the viscous behavior of materials at high strain rates. Machining involves large plastic deformation; in this process, the strain rate increases up to \(10^{5}\) s\({}^{-1}\) orders of magnitude with increasing cutting speed [3, 4]. Studies of the cutting process usually focus on elastic-plastic deformation; as a result, the theoretical calculation results evidently differ from theexperimental data. Therefore, the viscous behavior of material deformation during cutting is worthy of further study and exploration. The influence of material viscosity on the deformation of materials under high strain rates has been increasingly investigated. [PERSON] (2016) studied the deformation behavior of aluminum under different strain rates and reported that viscous stress should be considered in evaluating the flow stress of materials. [PERSON] (2017) pointed out that the flow stress of a material is composed of three parts: thermal activation, athermal resistance and viscous damping parts. [PERSON] (2017) studied the dynamic behavior and deformation mechanism of C5191 bronze under high strain rates. The results indicate that the dominant mechanism of plastic deformation is due to the increase and change in dislocation motion at different strain rates. The microscopic mechanism of plastic deformation is dislocation movement. In studies of dislocation motion, the effect of material viscosity is commonly considered. In computer simulations, the rate-dependent constitutive model is typically used to describe the viscosity of materials. [PERSON] is one of the first scholars to study the viscosity of solid materials and establish a simple mechanical model. He developed a uniaxial material constitutive model under stress by using a combination of plastic and viscous components to describe the viscoplasticity properties. For viscoplastic materials, the yield conditions are not only related to stress, plastic strain and strengthening properties, but also to the parameters that reflect the viscosity of the material. The viscous characteristics of the materials are important in the case of explosive impact loading, where the strain rate of the material increases dramatically in a very short time and the dynamic mechanical response of the material is similar to that of the fluid. Therefore, many scholars have studied plastic deformation at high strain rates from the point of view of fluid flow. Solids and fluids are two natural forms of matter in nature and are differentiated in terms of response to shear stress; in contrast to fluid, a solid can withstand certain shear forces. Under certain extreme conditions, solids may behave like fluids.