residual stresses and strains in orthogonal metal cutting

1、International Journal of Machine Tools received in revised form 29 October 2002; accepted 3 January 2003AbstractThe finite element method is used to simulate and analyze the orthogonal metal cutting process under plane

2、strain conditions, with focus on the residual stress and strain fields in the finished workpiece. Various modeling options have been employed. The frictional interaction along the tool-chip interface is modeled with a mo

3、dified Coulomb friction law. Chip separation is modeled by the nodal release technique based on a critical stress criterion. Temperature-dependent material properties and a range of tool rake angle and friction coefficie

4、nt values are considered. It is found that while thermal cooling increases the residual stress level, the effects of the rake angle and the friction coefficient are nonlinear and depend on the range of these parameters.

5、The predicted residual stress results compare well with experimental observations available in the literature. ? 2003 Elsevier Science Ltd. All rights reserved.Keywords: Finite element simulation; Orthogonal metal cuttin

6、g; Residual stress1. IntroductionMachining operations such as orthogonal metal cut- ting are complex nonlinear and coupled thermomechan- ical processes. The complexities are due to large strain and high strain-rate in th

7、e primary shear zone and due to the contact and friction between the chip and tool along the secondary shear zone. In addition to the above, complexities are also caused by local heat generation through the conversion of

8、 plastic work in the chip during chip formation and the frictional work between the tool and chip. An undesired byproduct of the metal cutting process is the creation of residual stresses and strains in the freshly cut w

9、orkpiece, which is known to affect the integrity of the newly finished surface, including short- ened creep and fatigue lives of the machined component under service loads. Hence a careful assessment of the residual stre

10、ss and strain fields in the workpiece is neces- sary for optimizing the cutting process and for safe- guarding against the premature failure of machined parts under creep and fatigue loading conditions. A significant amo

11、unt of metal-cutting research work? Corresponding author. Tel.: +1-803-777-7144; fax: +1-803-777- 0106. E-mail address: deng@engr.sc.edu (X. Deng).0890-6955/03/$ - see front matter ? 2003 Elsevier Science Ltd. All rights

12、 reserved. doi:10.1016/S0890-6955(03)00018-Xhas been carried out in the past 60 years. Amongst the earliest work were analytical models developed by Mer- chant [1,2] and Piispanen [3] on the mechanics of metal cutting. T

13、hese models are known as the shear-angle models in that they provide empirical relations between the shear angle, the rake angle and the coefficient of friction. These models can also be used to estimate forces, stresses

14、, strains, and energy consumption in the metal cutting process under plane strain conditions. More sophisticated shear-angle models were later developed to include the effect of various design para- meters. Lee and Shaff

15、er [4] proposed a shear-angle model based on the slip-line field theory, which assumes a rigid-perfectly plastic material behavior and a straight shear plane. Kudo [5] modified the slip-line model by introducing a curved

16、 shear plane to account for the con- trolled contact between curved chip and straight tool face. Palmer and Oxley [6] and Oxley et al. [7] con- sidered viscoplastic conditions and included work hard- ening and strain-rat

17、e effects. Doyle et al. [8] studied the effect of interfacial friction between the chip and the tool. Trigger and Chao [9] analyzed the effect of local heating in metal cutting. Among the various numerical techniques for

18、 studying metal cutting, the finite element method has been widely applied. The versatility of the finite element method allows it to take into account large deformation, strain575 C. Shet, X. Deng / International Journa

19、l of Machine Tools & Manufacture 43 (2003) 573–587have provided a good understanding of the metal cutting process. In particular, these studies have covered issues such as large strains and strain rates, the steady-s

20、tate response, the effect of friction and local heating and the chip separation criteria. It appears, however, that not much computational work has been carried out to under- stand issues relevant to the surface integrit

21、y of machined parts. Residual stresses are known to cause poor surface integrity. Henriksen [26] conducted a series of tests to understand residual stresses in the machined surface of steel and cast iron parts under vari

22、ous cutting conditions. He reported that residual stresses could be as high as 689.48 MPa (100 ksi). He also found that residual stresses were usually tensile in ductile materials (e.g. carbon steel) and compressive in b

23、rittle materials (e.g. cast iron). Various reasons have been attributed to the cause of residual stresses in the workpiece. Liu and Bar- ash [27] observed that the mechanical deformation of the workpiece surface induced

24、residual stresses. Kono et al. [28] and Tonsoff et al. [29] revealed that residual stresses are dependent on the cutting speed. Matsumoto et al. [30] and Wu and Matsumoto [31] observed that the hardness of the workpiece

25、material has a significant influence on the residual stress field. Konig et al. [32] showed that friction in metal cutting also contributes to the formation of residual stresses. Field et al. [33] reviewed various method

26、s for determining the surface integrity of machined parts, such as micro-hardness evaluation, X-ray diffraction, and layer removal-deflec- tion techniques. An early analytical model for predicting residual stresses was p

27、roposed by Okushima and Kakino [34], in which residual stresses were related to the cutting force and temperature distribution during machining. In another analytical model (Wu and Matsumoto [31]) a connection was made b

28、etween residual stresses and the hardness of the workpiece. Shih and Yang [18] conduc- ted a combined experimental/computational study of the distribution of residual stresses in a machined workpiece. More recently, Liu

29、and Guo [35] used the finite element method to evaluate residual stresses in a workpiece. They also observed that the magnitude of residual stress reduces when a second cut is made on the cut surface. While existing stud

30、ies on residual stresses in machined parts have provided important insights, issues such as residual strain distributions, the effect of tool rake angle, the level of contribution from each stage of the cutting-cooling p

31、rocess, are still not well understood. To this end, the objective of this investigation is to understand how the tool-chip interfacial friction and the tool rake angle affect the formation and distribution of residual st

32、resses and strains in machined parts, and div- ide the cutting-cooling process into four stages and investigate the contribution of each stage. The finite element method is used to simulate the orthogonal metalcutting pr

33、ocess. A simulation procedure has been developed through the use of several advanced modeling options in the general-purpose code ABAQUS [36]. An updated Lagrangian formulation suitable for large strain deformations is e

34、mployed. Plane strain conditions are assumed. Strain-rate effects are included with an over- stress viscoplastic constitutive model. Frictional contact along the tool-chip interface is made to obey a Modified Coulomb Fri

35、ction Law. Adiabatic heating conditions are used to account for temperature rise due to local heating induced by plasticity and friction. Chip separation from the workpiece is modeled using a stress-based chip sep- arati

36、on criterion. Temperature-dependent material properties are considered. This study provides a detailed exposition of stress and strain field evolution at different stages after cutting, and of the formation of residual s

37、tresses and strains near the finished surface of the work- piece.2. Finite element model descriptionFig. 1 shows a schematic diagram of the orthogonal metal cutting process, in which a continuous chip is being taken off

38、from the workpiece by a cutting tool that is moving relative to the workpiece with a constant velo- city. In order to model chip separation and treat frictional interactions in the tool-chip-workpiece system, three conta

39、ct pairs are defined, as shown in Fig. 1. Contact Pair 1 defines the cutting path, where the two contact surfaces are represented by two sets of nodes (one on each surface) that are paired and bonded together. When the c

40、hip separation criterion is satisfied, the contact node pair immediately ahead of the tool tip is separated, enabling the tool to advance incrementally. As the tool breaks the contact node pairs, materials forming the ch