Formation Mechanism Of Shear Bands Research Articles

Dynamic behavior and formation mechanism of shear band during high-speed cutting Inconel 718

Dynamic behavior and formation mechanism of shear band during high-speed cutting Inconel 718

Numerical and experimental investigations on the laser assisted machining of the TC6 titanium alloy

In order to improve the machinability of difficult-to-machine materials in aviation, laser-assisted cutting technology has received extensive attention. This method of machining allows the workpiece to obtain a localized high temperature prior to cutting in order to benefit from thermal softening. A 3D transient laser heating model was developed for the finite element method to predict the preheating temperature of the cutting layer. The effect of cutting parameters (cutting speed, feed rates, depth of cut) on the magnitude, frequency and amplitude of the cyclic cutting forces was investigated both in conventional machining and in laser assisted machining. It is founded that the laser assistance decreases the flow stress of the material, magnitude of cutting forces and the amplitude of cutting forces. But the frequency of circulating cutting forces increase comparing with the CM (conventional machining). In addition, this paper considers the effect of the LAM (laser-assisted machining) technique on reducing the magnitude of cutting force fluctuations, which results in a flatter machined surface profile and reduces the number of surface defects. A chip morphology different from that of CM is created due to the influence of the cutting force by the temperature field in LAM. Furthermore, a “transitional chip” is discovered, which is a transition from a continuous chip to a segmented chip. The completely thermal-mechanical coupled orthogonal cutting model was established to reveal the evolution mechanism of shear band formation with laser assistance. The results show that shear bands can be formed faster and with less stress in LAM compared to CM. The model can effectively predict the shear angle and keep the prediction error within 10 %. The shear band formation processes in both machining methods were uncovered according to the models. The developed model is very useful for understanding the chip formation process and the cutting force variation in laser assisted machining.

Predictive model of chip segmentation in machining of high-strength metallic alloys

Chip segmentation is the dominant chip formation mechanism during the machining of high-strength metallic alloys which directly affects the machining productivity and the final product’s quality. This paper investigates the mechanism of shear band formation and its contact with the cutting tool for different titanium and nickel alloys. First, we reveal that the shear bands are in direct contact with the cutting tool due to rolling on its rake face, although the contact mechanism is different for titanium and nickel alloys. In addition, the effect of the stored energy at the tool-chip interface, which is believed to be the reason for chip segmentation in some studies, is investigated theoretically. We found that the stored energy at the tool-chip interface affects the strain rate temporarily and has minimal impact on the chip segmentation. Moreover, we provide a theoretical prediction of the workpiece material’s displacement, its distribution, and degree of segmentation for the first time without any post-mortem analysis of the produced shear bands. To this end, the rolling process must be taken into account for accurate prediction. We show that the segmented chip’s displacement can be accurately predicted without any fracture criterion since the shear band itself is a type of ductile material failure. It is shown that the shear bands can accommodate large strains (for example, around 45 when cutting Ti-6Al-4V at 60 m min−1 cutting speed) without fracture presumption.

Investigation of chip formation mechanism in ultra-precision diamond turning of silk fibroin film

The importance of silk fibroin particles (size from tens of nanometres to hundreds of microns) as effective drug carriers has been well identified due to their superior mechanical strength, biocompatibility and biodegradability. They are usually fabricated by methods that have difficulties in controlling the size and shape to meet the required functions. In addition, using chemical agents in the fabrication process may result in biological risks and degradation of the fibroin structures. In this study, ultra-precision diamond turning (UPDT) technology is researched as an alternative chemical-free manufacturing approach to generate silk particles in the form of cutting chips with controllable size and shape. The machinability in UPDT of silk fibroin film (hereinafter referred to as silk film) was discussed in terms of specific cutting force and chip morphology. The material removal in UPDT of silk film is taken place in the ductile regime. Experimental results reveal that a rise in the cutting speed could significantly reduce the specific cutting force, while decreasing the depth of cut (DOC) could increase the specific cutting force because of the size effect. Moreover, a sharp point tool using a less than 2.5 μm/rev feed rate is preferred to generate helical silk chips. The formation mechanism of shear bands and serrated chips in UPDT of silk film was investigated with the aid of a finite element and smoothed particle hydrodynamics (FE-SPH) hybrid numerical model with Cowper-Symonds (CS) material equation. The material parameters of silk film were determined for the first time at p = 7 and D = 1140 s−1. The simulated specific cutting force with this set of material parameters is 49.0% smaller than the measured one. The simulated chip morphology (e.g. shear band spacing) was also in good agreement with the measured values. The simulation study shows that the shear band formation is done via two joining parts: one part propagates from the cutting edge to the free surface, another part initiates on the free surface and evolves towards the cutting edge. The chip segment is formed by a microcrack propagating from the free surface to the cutting edge. The underlying mechanism of formation of the serrated chip is found to have its roots in the hierarchical structure of silk fibroin.

Understanding and suppressing shear band formation in strut-based lattice structures manufactured by laser powder bed fusion

The formation of shear band in metallic lattice structures under compression prompts an unstable stress–strain curve, resulting in a significant reduction in the energy absorption capacity. The development and formation of shear band need to be clarified, and versatile approaches to suppress the shear band are required. In this study, we explored the formation mechanism of shear band using a representative Al–12Si alloy lattice structure, consisting of a simple body-centered cubic (BCC) unit cell manufactured by laser powder bed fusion. The X-ray computed tomography observations of an interruptedly compressed lattice specimen and finite element analyses revealed that the crack initiations in the bent struts under the localized tensile stress trigger the formation of shear band. Accordingly, an attempt was made to fabricate various lattice structures consisting of BCC unit cells with different strut diameter ratios to suppress shear band formation. Finite element analyses revealed relaxed tensile stress inside the struts with the controlled diameter ratios, suggesting the suppressed formation of shear bands; this was demonstrated by compression tests. The present results provide new insights for achieving a stable compressive deformation and high energy absorption capacity of lattice structures.

Mechanism of shear band formation and dynamic softening in a two-phase (α2 + γ) titanium aluminide

Abstract

Characterization of Shear Band Formation and Microstructure Evolution during Orthogonal Cutting of Ti-5553: Part I—Shear Angle, Strain and Strain Rate

Ti-5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe) is a recently commercialized near-β titanium alloy employed in large-section forging of Boeing 787 landing gear. The machinability of Ti-5553 is low owing to its high strength at elevated temperature, low thermal conductivity and high chemical reactivity. Chip serration occurs during high-speed machining of Ti-5553, which is found to be attributed to the periodic formation of shear bands caused by thermoplastic instability within the primary shear zone. Microscopic observations indicate that the geometry and microstructure of shear bands are significantly related to the shear angle, shear strain and strain rate under the given cutting conditions. In this study, quick-stop turning tests were conducted to understand the chip formation mechanism in machining Ti-5553, and the machined chips were metallographically characterized to investigate their morphology and microstructure evolutions. In the Part I of this study, the shear angle, shear strain and strain rate were quantitatively determined, and the shear band microstructure was examined to reveal the effect of cutting conditions in dry turning of Ti-5553. This study advances the mechanism of shear band formation in machining Ti-5553 and develops a novel experimental approach to directly obtain the actual shear strain and strain rate based on the geometry of the machined chip.

Formation mechanism and characterization of shear band in high-speed cutting Inconel718

Nickel-based alloy Inconel718 is one kind of complex alloy with multiple components. There is complex cutting deformation (shear localization of material) in high-speed cutting process. The chip shows a serrated shape. Through analyzing the chip morphology, the change of microstructure and development of shear band is analyzed, and the formation mechanism of the shear band is revealed. Based on the formation mechanism of shear band, the coupled elastoplastic-damage constitutive model is proposed to describe the shear stress during the cutting process. The shear band width has been determined by micrographic observations and theoretical model calculation. The experimental and theoretical results show that the shear band width in chip formation decreases linearly with an increase in the cutting speed.

Ductile bulk metallic glass by controlling structural heterogeneities

A prerequisite to utilize the full potential of structural heterogeneities for improving the room-temperature plastic deformation of bulk metallic glasses (BMGs) is to understand their interaction with the mechanism of shear band formation and propagation. This task requires the ability to artificially create heterogeneous microstructures with controlled morphology and orientation. Here, we analyze the effect of the designed heterogeneities generated by imprinting on the tensile mechanical behavior of the Zr52.5Ti5Cu18Ni14.5Al10 BMG by using experimental and computational methods. The imprinted material is elastically heterogeneous and displays anisotropic mechanical properties: strength and ductility increase with increasing the loading angle between imprints and tensile direction. This behavior occurs through shear band branching and their progressive rotation. Molecular dynamics and finite element simulations indicate that shear band branching and rotation originates at the interface between the heterogeneities, where the characteristic atomistic mechanism responsible for shear banding in a homogeneous glass is perturbed.

Existence of Localizing Solutions in Plasticity via Geometric Singular Perturbation Theory

Shear bands are narrow zones of intense shear observed during plastic deformations of metals at high strain rates. Because they often precede rupture, their study attracted attention as a mechanism of material failure. Here, we aim to reveal the onset of localization into shear bands using a simple model from viscoplasticity. We exploit the properties of scale invariance of the model to construct a family of self-similar focusing solutions that capture the nonlinear mechanism of shear band formation. The key step is to desingularize a reduced system of singular ordinary differential equations and reduce the problem into the construction of a heteroclinic orbit for an autonomous system of three first-order equations. The associated dynamical system has fast and slow time scales, forming a singularly perturbed problem. Geometric singular perturbation theory is applied to this problem to achieve an invariant surface. The flow on the invariant surface is analyzed via the Poincaré--Bendixson theorem to construct a heteroclinic orbit.

Formation and microstructure of shear bands during hot rolling of a Mg–6Zn–0.5Zr alloy plate with a basal texture

A Mg–6Zn–0.5Zr alloy (ZK60) plate with a strong basal texture was rolled at 400°C to reductions of 9–48%. Microstructure and texture of the hot-rolled plates were systematically studied with the aim to disclose the formation mechanism of shear bands during hot rolling. Our results show that dynamic recrystallization (DRX) prefers to occur at grain boundaries in a large amount at low rolling reductions. With increasing reductions, the DRXed regions cluster and widen by consumption of the parent grains. Finally, large shear bands appear in the form of symmetric pairs, inclining at an angle of±40° from the rolling direction (RD). Both equiaxed and elongated DRXed grains in submicron scale exist in the shear bands. Deformation twins mainly appear in the regions outside of the shear bands. Both micron-scaled and submicron-scaled {101¯1} contraction twins or {101¯1}-{101¯2} double twins are detected. It seems that those deformation twins do not play an important role in the formation of shear bands during hot-rolling at 400°C. The DRXed grains in the vicinity of grain boundaries have similar orientations to that of their parent grains, while basal poles of the DRXed grains in double twins greatly incline from that of their parent twins. The hot rolling results in scattering of basal poles toward the transverse direction (TD). The maximum intensity of basal poles varies with rolling reductions.

Finite Element Analysis of Clay Shear Band under Plane Strain Condition

The clay shear band analysis under plane strain condition were based on modified cam clay model in finite element software ABAQUS. Normally Consolidated Clay and lightly overconsolidated clay were analyzed in the article. Loading speed, size of soil specimen, different consolidation state (Normally Consolidated Clay and lightly overconsolidated clay) are considered. These factors have effect on the distribution of Pore water pressure, the effective stress path of nodes in shear band and out of shear band and the angle between the shear band and horizontal line. All the differentia and effects were used to analyze the production conditions and formation mechanism of shear band. The results show that size of clay specimen can influence the number of shear band. When the height-width ratio is large, the number of shear band will increase.

Process Simulation and Formation Mechanism of Shear Band in Soil

Shape multiformity and formation mechanism of shear band in normal consolidated soil were analyzed based on Modified Cam-Clay model under plane strain condition. The results proved that shear band forms because of impact action caused by strain energy releasing. End restraint and initial defect can induce and control the formation process and shape of shear band. The interaction of different boundary and initial defect settings was analyzed in order to obtain the mechanism and deformation process of shear band.

Testing and Numerical Simulation Research on Shear Band of Soil

The triaxial equipment of GDS with digital image processing technique is used in drained test of sand. The result shows that the end restraint and initial defect have significant effect on mechanical properties of sand sample. End restraint and initial defect of sample both can cause inhomogeneous deformation, then form macroscopic shear band in soil testing and numerical simulation. Shape multiformity and formation mechanism of shear band in normal consolidated soil is analyzed based on Modified Cam-Clay Model under plane strain condition. The result shows that end restraint and initial defect can induce and control the formation and shape of shear band. The impaction of different boundaries and initial defect setting are analyzed.

Two-scale finite element analyses for bendability and springback evaluation based on crystallographic homogenization method

Abstract In this study, a relationship between the sheet metal formability, such as the bendability and springback property, and the crystal texture was investigated by using our two-scale finite element (FE) analysis code based on the crystallographic homogenization method (Nakamachi et al., 2010 [1] ). Our code employed a two-scale finite element model, such as the microscopic polycrystal structure and the macroscopic elastic plastic continuum, which can predict the anisotropic plastic deformation of sheet metal in the macro-scale, and the crystal texture and hardening evolutions in the micro-scale. The macro-FE model consisted of the die, punch and sheet metal. The die and punch were modeled as the rigid bodies for “V-bending” process analyses. The measured crystal orientation distribution was adopted as the initial texture in the microscopic polycrystal FE model, which corresponded to the three-dimensional representative volume element (RVE). RVE model was featured as 3×3×3 equi-divided solid finite elements, totally 27 FEs with 216 crystal orientations assigned at the integration points of micro-finite elements. The bendability was evaluated by the surface wrinkle growth index and the strain localization – the shear band formation – by using two-scale FE results. On the other hand, the springback property was defined by the angular difference between before and after the punch and die removing process – the post-process. At first, we investigated the bendability of single crystal sheets, which have typical preferred orientations of the copper alloy sheet, to elucidate the fundamental mechanism of shear band formation and wrinkle growth in the V-bending process. Next, we analyzed V-bending processes of four copper alloy polycrystal sheets to evaluate the bendability and springback property. The Cube-dominant texture sheet of the Corson series copper alloy and the {001}〈110〉-dominant texture sheet of the phosphor-bronze alloy show a high-bendability. On the other hand, the S-dominant texture sheet of the Corson series copper alloy and the Brass-dominant texture sheet of the phosphor-bronze alloy show a low-bendability. By contraries, the Cube-dominant and {001}〈110〉-dominant texture sheets have a low-springback property, and the S-dominant and Brass-dominant textures sheets have a high-springback property. Our two-scale FE results of four copper alloy sheets in the V-bending and the post-process were compared with experimental results, and finally the validity of our two-scale FE code was confirmed.

Shear band formation during hot compression of AZ31 Mg alloy sheets

Shear band formation has been reported to depend strongly on micro-structural factors, such as grain size, texture, alloying elements, and also on forming processes. The formation mechanism of shear bands has thus been investigated in the present work in relation to improve the forming limit of AZ31 Mg alloy. A series of compression tests have thus been carried out using strip-cast (SC) and hot-rolled (AR) AZ31 Mg alloys to compare their texture effects. The tests were performed under the strain rate of 10−1/s at room temperature (RT) and 300°C to clarify temperature dependence of shear band formation. Shear bands in SC specimens were observed to develop mainly at the boundaries of cast structures by generating compression twins and dynamic recrystallization (DRX) induced by the activation of the <a> and <c+a> slips as well as twinning at elevated temperatures. AR specimens exhibited, on the other hand, well-developed basal-fiber texture to generate shear bands through fine grains developed by DRX along the geometrically maximum shear stress directions.

Microscopic Analysis of Sand Shear Band in Direct Shear Test Based on Particle Flow Theory

In order to analyze the microscopic shear mechanics behavior of sands, a numerical simulation is carried out in sand direct shear test with commercial DEM software PFC2D. By compiling FISH function, PFC2D is able to visualize principal stresses and their inclination inside the sample. In the simulation, particle rotation gradient and particle displacement vector are observed to analyze the microscopic formation mechanism of shear band. The strain and deformation is concentrated in a band whose thickness is about 10 times of average particle diameter and the band is close to the shear plane. The porosity ratio in shear band is higher than other parts of the sample with shear process.

Formation mechanism of shear bands in ultrafine lamellar Ti–Fe eutectics

Shear bands with alternating lamellar and serrated structures were observed on the fracture surface of an Ti–Fe ultrafine eutectic. The shear bands are composed of coalesced subgrains. Hence, a subgrain rotational mechanism is proposed to explain their formation. The work is a direct experimental evidence of the hypothetical rotational mechanism.

Room temperature plastic deformation behavior of ZrCuNiAl bulk metallic glasses

The Zr62.55Cu17.55Ni9.9Al10 bulk metallic glass (BMG) was prepared by using copper-mold suction-casting. X-ray diffraction and differential scanning calorimetry were utilized to determine its structure and thermal stability. Uniaxial compression and Rockwell indentation tests were adopted to study the plastic deformation behavior at room temperature. The results show that the glass transition temperature and the onset temperature of exothermic reaction of the BMG are 651.5 and 748 K, respectively. During the compression test, the BMGs undergo an engineering strain of about 2.5%, i.e., true strain of 2.8%, and then fracture. The BMGs deform via the formation and propagation of shear bands. Under indentation loading, the BMGs deform through the formation of radiation-like and circular shear bands. The circular shear bands form earlier than the radiation-like ones. The formation mechanism of shear bands in the BMGs was analyzed and discussed.

Hyperbolic Conservation Laws

Hyperbolic Conservation Laws