A comparison of orthogonal cutting data from experiments with three different finite element models

The aim of this study is to compare various simulation models of orthogonal cutting process with each other as well as with the results of various experiments. Commercial implicit finite element codes MSC.Marc, Deform2D and the explicit code Thirdwave AdvantEdge have been used. In simulations, a rigid tool is advanced incrementally into the deformable workpiece which is remeshed whenever needed. In simulations with MSC.Marc and Thirdwave AdvantEdge, there is no separation criterion defined since chip formation is assumed to be due to plastic flow, therefore, the chip is formed by continuously remeshing the workpiece. However, in simulations with Deform2D, the Cockroft–Latham damage criterion is used and elements, which exceed the predefined damage value, are erased via remeshing. Besides this different modeling of separation, the three codes also apply different friction models and material data extrapolation schemes. Estimated cutting and thrust forces, shear angles, chip thicknesses and contact lengths on the rake face by three codes are compared with experiments performed in this study and with experimental results supplied in literature. In addition, effects of friction factor, different remeshing criteria, and threshold tool penetration value on the results are examined. As a result, it has been found that although individual parameters may match with experimental results, all models failed to achieve a satisfactory correlation with all measured process parameters. It is suggested that this is due to the poor modeling of separation.     

Revisiting the fundamentals of metal cutting by means of finite elements and ductile fracture mechanics

This paper investigates Atkins’ idea that the modelling of metal cutting must include the significant work involved in the formation of new surfaces as well as the traditional components of plastic flow and friction. New finite element and algebraic calculations are presented together with specially designed orthogonal metal cutting experiments performed on lead specimens under laboratory-controlled conditions. Independent determinations of the mechanical properties of lead were made and comparisons are given between theoretical predictions and experimental results. Calculations cover a wide range of topics such as material flow, chip-compression factor, primary shear plane angle, cutting force and specific cutting pressure. It is shown that the choice of lead as workpiece material reveals important facts that would be obscured were the usual sort of workpiece metals to be cut.The paper demonstrates quantitatively that while material flow, chip formation and the distribution of the major field variables can be modelled successfully by traditional ‘plasticity and friction only’ analyses, the contribution of ductile fracture mechanics is essential for obtaining good estimates of cutting forces and of the specific cutting pressure.     

Prediction of tool-chip contact length using a new slip-line solution for orthogonal cutting

Tool–chip contact length is an important parameter in machining. Several ways had been proposed in different works to find its value, which gave discordant results for the same set of cutting conditions. In this paper, a new slip-line solution for orthogonal cutting by a tool with unrestricted rake face is suggested. Based on the proposed solution, a new formula for tool–chip contact length has been obtained. Comparative analysis of different methods to predict tool–chip contact length has been done and experimental verification conducted. The suggested formula has shown to correspond well with experimental data and predicts tool–chip contact length better than other known solutions.     

Determination of workpiece flow stress and friction at the chip–tool contact for high-speed cutting

This paper presents a methodology to determine simultaneously (a) the flow stress at high deformation rates and temperatures that are encountered in the cutting zone, and (b) the friction at the chip–tool interface. This information is necessary to simulate high-speed machining using FEM based programs. A flow stress model based on process dependent parameters such as strain, strain-rate and temperature was used together with a friction model based on shear flow stress of the workpiece at the chip–tool interface. High-speed cutting experiments and process simulations were utilized to determine the unknown parameters in flow stress and friction models. This technique was applied to obtain flow stress for P20 mold steel at hardness of 30 HRC and friction data when using uncoated carbide tooling at high-speed cutting conditions. The average strain, strain-rates and temperatures were computed both in primary (shear plane) and secondary (chip–tool contact) deformation zones. The friction conditions in sticking and sliding regions at the chip–tool interface are estimated using Zorev's stress distribution model. The shear flow stress (k_chip) was also determined using computed average strain, strain-rate, and temperatures in secondary deformation zone, while the friction coefficient (μ) was estimated by minimizing the difference between predicted and measured thrust forces. By matching the measured values of the cutting forces with the predicted results from FEM simulations, an expression for workpiece flow stress and the unknown friction parameters at the chip–tool contact were determined.     

Study of the mechanism of nanoscale ductile mode cutting of silicon using molecular dynamics simulation

In cutting of brittle materials, it was observed that there is a brittle-ductile transition when two conditions are satisfied. One is that the undeformed chip thickness is smaller than the tool edge radius; the other is that the tool cutting edge radius should be small enough—on a nanoscale. However, the mechanism has not been clearly understood. In this study, the Molecular Dynamics method is employed to model and simulate the nanoscale ductile mode cutting of monocrystalline silicon wafer. From the simulated results, it is found that when the ductile cutting mode is achieved in the cutting process, the thrust force acting on the cutting tool is larger than the cutting force. As the undeformed chip thickness increases, the compressive stress in the cutting zone decreases, giving way to crack propagation in the chip formation zone. As the tool cutting edge radius increases, the shear stress in the workpiece material around the cutting edge decreases down to a lower level, at which the shear stress is insufficient to sustain dislocation emission in the chip formation zone, and crack propagation becomes dominating. Consequently, the chip formation mode changes from ductile to brittle.     

Assessment of fracture characteristics from revised small punch test using pre-cracked specimen

Small punch (SP) test has been utilized to analyze the neutron irradiation damage of nuclear vessels. Since this technique is easy, simple, and nondestructive, it can be applied to evaluate the mechanical properties and material degradation of in-service components. Conventional SP test has evaluated the ductile-brittle transition temperature and the equivalent fracture strain by the interpretations of load-deflection curve and the change of specimen thickness, respectively. The assumption that fracture occurs at maximum load is, however, not reasonable because the crack initiates at smaller load. In this study, in order to evaluate quantitatively fracture characteristics based on fracture mechanics, the pre-crack is introduced to SP specimen and acoustic emission is used to determine the crack initiation point. Using the load at crack initiation point, the fracture toughness of thin plate is calculated through bending theory. Therefore, the fracture characteristics of thin plate can be evalualed more reliably by using revised SP test.     

台阶式凸模精冲工艺分析及数值模拟

采用平面压板的无齿圈精冲,可以有效降低模具制造成本,提高精冲材料利用率。但采用常规平面压板精冲很难获得较好的冲裁面质量。文章对常规平面压板精冲模具进行改进,通过采用台阶式凸模结构实现无齿圈精冲;采用DEFORM 2DTM软件对C20E-EN钢圆盘的台阶式凸模精冲过程建立轴对称模型进行有限元模拟,分析凸模台阶高度、台阶宽度和冲裁间隙等工艺参数对零件冲裁面光洁带高度和塌角高度的影响规律,获得了相对最佳工艺参数的配合方式。将模拟结果与台阶式凸模精冲实验结果进行比较表明,模拟结果和实验结果具有较好的一致性。     

Investigation into thermal characteristics of linear hotwire cutting system for variable lamination manufacturing (VLM) process by using expandable polystyrene foam

The dimensional accuracy and efficiency of VLM-S, which is a new rapid prototyping process using hotwire cutter and expandable polystyrene (EPS) foam sheet, depends significantly on the thermal fields of EPS foam sheet when the hotwire cuts the sheet. The objective of this study is to investigate thermal effects of the hotwire cutting on the sheets and to find relationships between process parameters in order to obtain optimal conditions for hotwire cutting and improve dimensional accuracy of the process. Several experiments were performed to find the relationships between maximum cutting speed and heat input, and between cutting offset and heat input. Numerical analyses were carried out to investigate the influence of the cutting parameters on temperature distribution around the hotwire and to estimate the amount of the sheet melted away. Moreover, the size of the thermal front as the hotwire is about to lose its stiffness was predicted to propose the optimal cutting conditions. Based on the results, the optimal cutting conditions of the hotwire cutting system for cutting of an EPS foam sheet were found. In addition, the outcomes of the present study were reflected on the fabrication of a spanner shape and a clover punch shape.     

On the fracture toughness of intermetallics estimated from the work of fracture

In the present paper the regression analysis of the fracture toughness data obtained from 3 and 4 pt bending of single-edge pre-cracked beam and chevron-notched beam specimens of various intermetallic alloys calculated from the work of fracture γ_wof and the maximum load on the load–displacement curve is carried out in order to establish a relationship between both quantities.     

基于裂纹尖端钝化能的断裂韧性估算方法

基于裂纹尖端塑性钝化能,从平面应力状态和平面应变状态分析KIc入手,建立了金属材料平面应变断裂韧性KIc值的能量估算方法,同时考虑了应力松弛和材料强化的影响,以及塑性钝化区形状和裂纹尖端应力状态的分布,并由此推导出了一个基于常规力学性能的断裂韧性KIc值的计算公式。利用LZ50车轴钢的试验数据进行了对比分析,标定出LZ50车轴钢的形状因子Z参数,并对比分析了KIc试验概率值分布与计算概率值分布规律,讨论了各影响因素对KIc值的影响规律。计算结果表明,所建立的KIc公式能够反映常规力学性能与平面应变断裂韧性间的关系。     

Optimising the AWJ cutting process of ductile materials using nozzle oscillation technique

Striations and roughness on workpiece surfaces produced by abrasive waterjet (AWJ) have been the most persistent problems that stand in the way of wider applications of the technology in industry. This paper presents the experimental investigation on the impact of using nozzle oscillation cutting technique in minimising or reducing these AWJ cut surface irregularities. The technique was used for cutting ductile materials, i.e. mild steel and aluminium, at various traverse speeds, oscillation angles and frequencies of oscillation. The results show that by oscillating the nozzle during cutting, the improvement in surface finish as measured by centre-line average R_a can be obtained by as much as 30%.     

Re-evaluation of the basic mechanics of orthogonal metal cutting: velocity diagram, virtual work equation and upper-bound theorem

This paper re-evaluates the known velocity relationships expressed in the form of a velocity diagram in orthogonal metal cutting, arguing that the metal cutting process be considered as cyclic and consisting of three distinctive stages. The velocity diagrams for the second and third stages of a chip-formation cycle are discussed. The fundamentals of the mechanics of orthogonal cutting, which are the upper-bound theorem applied to orthogonal cutting and the real virtual work equation, are re-evaluated using the proposed velocity diagram and corrected relationships are proposed. To prove the theoretical results, the equation for displacements in the deformation zone is derived using the proposed velocity relationships. To prove that the displacements in the deformation zone follow the derived equation and that this zone consists of two unequal parts, a metallographical study of chip structures has been carried out. To estimate the variation of stress and strain in the deformation zone quantitatively, a microhardness scanning test was conducted.Because it is proved that the chip formation process is cyclic, its frequency is studied. It is shown that when the noise due to various inaccuracies in the machining system is eliminated from the system response and thus from the measuring signal, and when this signal is then properly processed, the amplitude of the peak at the frequency of chip formation is the largest in the corresponding autospectra.     

Rapid determination of the fracture toughness of metallic materials using circumferentially notched bars

Three types of material whose fracture toughness tests were previously performed by using circumferentially notched bars, namely (1) a dual-phase steel with three different morphologies; (2) an Al-Zn-Mg-Cu-wrought alloy; and (3) Al-Si-cast alloys with three different Si contents, were investigated in terms of accuracy and reliability of the testing method. Also, the advantages of using circumferentially notched bars for fracture toughness determination of metallic materials were discussed. With the help of stress concentration factors, which are associated with the bluntness of the notch, correction factors for the fracture toughness calculations are derived. The corrected fracture toughness values are found to be close to the uncorrected ones, implying that the testing procedure is reliable.     

An FEM investigation into the behavior of metal matrix composites: Tool–particle interaction during orthogonal cutting

An analytical or experimental method is often unable to explore the behavior of a metal matrix composite (MMC) during machining due to the complex deformation and interactions among particles, tool and matrix. This paper investigates the matrix deformation and tool–particle interactions during machining using the finite element method. Based on the geometrical orientations, the interaction between tool and particle reinforcements was categorized into three scenarios: particles along, above and below the cutting path. The development of stress and strain fields in the MMC was analyzed and physical phenomena such as tool wear, particle debonding, displacements and inhomogeneous deformation of matrix material were explored. It was found that tool–particle interaction and stress/strain distributions in the particles/matrix are responsible for particle debonding, surface damage and tool wear during machining of MMC.     

An investigation into the forging of Bi-metal gears

This paper introduces a method for the production of bi-metal gears using the forging technique. To study the process, model materials of copper (tooth ring material) and lead (core material), were used for both experimentation and simulation. Firstly, experimental setup and test procedures are introduced and the bi-metal gears are forged with different thicknesses of the outer ring material. A simplified FE model is established based on the symmetry of a gear forging process, which enables the 3D FE analysis to be carried out in an efficient manner. The material flow and thickness distribution of the experimentally forged bi-metal gears are analysed and compared with FE predictions. The effect of friction on the axial lock caused by the material flow of the forged gears is also studied. Finally, simulations of different combinations of the inner core and outer ring materials, specifically steel (ring material), copper (ring and core material) and lead (core material) are performed. The numerical and experimental data showed that: thin rings can deform excessively, affecting the structure of the gear; and that both tooling friction and flow stress can significantly affect the relative material flow between the core and the ring.     

Application of fracture mechanics to the interpretation of bond strength data from ASTM standard C633-79

The debonding specimen used in ASTM Standard C633- 79 has a nonuniform stress distribution at the interface between the coating and the substrate.^[1] This means that bond strengths determined according to the standard could be significantly lower than actual strengths. A new specimen, 50% longer than the standard specimen, was developed to alleviate this problem. The elongated specimen has a uniform stress distribution that is equal to the uniform stress assumed by ASTM Standard C633- 79. Thus, bond strengths obtained using the elongated specimen are higher and more representative of the actual bond strength than estimates obtained from the standard specimen. In this work, a procedure is developed to transform the existing bond strength values obtained using the C633- 79 Standard specimens to the more representative bond strength values that would be obtained if the tests were repeated using the elongated specimens. A combination of finite- element analyses and laboratory test data is used to identify the relation between the bond strength values of standard specimens and those of elongated specimens. Examples are presented and the procedure is verified by comparisons with bond strength data for Colmonoy No. 6 and aluminum oxide coatings.     

An in-depth analysis of the synchronization between the measured and predicted cutting forces for developing instantaneous milling force model

This paper proposes an analytical approach to synchronize the measured and predicted cutting forces for calibrating instantaneous cutting force coefficients that vary with the instantaneous uncut chip thickness in general end milling. Essential issues such as the synchronization criterion, phase determination of measured cutting forces, specification of calibration experiments and related cutting parameters are highlighted both theoretically and numerically to ensure the calibration accuracy. A closed-form criterion is established to select cutting parameters ensuring the single tooth engagement. Numerical cutting simulations and experimental test results are compared to validate the proposed approach.     

An investigation into the corrosion of Ag coins from the Greek colonies of Southern Italy. Part I: An in situ FT-IR and ERS investigation of the behaviour of Ag in contact with aqueous solutions containing 4-cyanopyridine

The electrochemical behaviour of polycrystalline Ag and Ag(1 1 1) electrodes in contact with neutral chloride and perchlorate solutions containing 4-cyanopyridine (4CP) was studied. The understanding of the interaction of organic adsorbates with precious metals and alloys in given environments can provide guidelines for the design of suitable corrosion inhibition systems meeting the peculiar requirements of archaeological standards. 4CP adsorbs onto Ag both in the presence and in the absence of chlorides and acts as a reversible film former, suitable for use with historical items. 4CP has been shown to alter markedly the anodic kinetics of Ag: active corrosion in chloride environment is changed to a pseudo-passivating behaviour and potential oscillations, typical for the formation of thick AgCl layers, are suppressed.In situ spectroelectrochemical FT-IR measurements were carried out using both p- and s-polarised radiation in order to clarify the potential-dependent behaviour of 4CP. FT-IR data were complemented by in situ electroreflectance experiments, which highlighted changes in the electronic structure of the Ag surface in the presence of 4CP. Cyclic voltammetry and differential capacitance measurements were employed to asses the electroactivity and adsorption ability of 4CP. The results obtained disclosed that 4CP is adsorbed intact in a neighbourhood of the corrosion potential in perchlorate and chloride solutions, it desorbs at high anodic potentials and it reacts cathodically.     

Effects of deviations from stoichiometry on the strength anomaly and fracture behavior of B-doped FeAl

Tensile tests were conducted at temperatures ranging from 145–1123 K on four different FeAl alloys, containing 40, 43, 45, and 48 at.% Al, each doped with 0.12 at.% B. The alloys were initially heat treated to obtain a relatively large grain size (˜200 μm), after which they were given a long, low-temperature anneal (673 K for 5 d), to minimize, respectively, the effects of grain boundary strengthening and thermal vacancies on the measured yield strengths. Each alloy displayed bcc-type behavior at low temperatures (yield strength decreasing with increasing temperature), followed by a strength anomaly at intermediate temperatures (yield strength increasing with increasing temperature), and a sharp drop in yield strength at elevated temperatures (beyond the anomalous strength peak). Thermal vacancies that are generated during the hold time at the test temperature may contribute to the production of the strength anomaly. In specimens not given the vacancy-minimizing anneal, quenched-in vacancies were found to substantially increase low-temperature strength, thereby masking the yield strength anomaly. As the Al concentration of FeAl increased, the prominence of the yield strength anomaly decreased. Ductility also exhibited a peak at elevated temperatures, first increasing with temperature until it reached a maximum value and then decreasing with further increases in temperature. The peak in ductility occured at lower temperatures as the Al content increased. The fracture mode in all four alloys was mixed (intergranular + transgranular) at cryogenic temperatures, predominantly intergranular at around room temperature, dimpled rupture at peak ductility, and intergranular cavitation at elevated temperatures where the ductility dropped.     

An investigation of the deformation mechanisms in sn5% sb alloy using tensile, creep and abi tests from ambient to 473k

Tensile, creep, and automated ball indentation (ABI) tests have been conducted to study deformation mechanisms in Sn5%Sb alloy between ambient and 473 K. A power law relationship was obtained between minimum creep rate and applied stress, with stress exponent,n=5 and activation energy,Q=12.6±1.1 kCal/ mole. At 473 K, a transition fromn=5 ton=3 was observed at low stresses. ABI tests showed a power law relationship between strain rate and ultimate tensile stress with values ofn=5 andQ=13.0±1.8 kCal/mole. Tensile results were in broad agreement with the creep and ABI data. A new deformation mechanism is proposed for then=5 region involving viscous glide of dislocations assisted by dislocation core diffusion.