A dislocation density material model based on model-based-phenomenology has been used to predict orthogonal cutting of stainless steel Sanmac 316L. The chip morphology and the cutting forces are used to validate the model. The simulated cutting forces and the chip morphology showed good conformity with practical measurements. Furthermore, simulation of cutting process utilizing the dislocation density based material model improved understanding regarding material behaviour such as strain hardening and shear localization at the process zone.
The problem of calibrating material models with tests in a limited range of conditions and then applying outside this range is discussed. This is the case when machining simulations are performed where very high strain rates (>50000s-1) can be obtained. The paper discusses the Johnson-Cook model, an empirical model that is common for high strain rate applications and a physical based dislocation density model. Test data for AISI 316L ranging from 0.001 to 10s-1 and room temperature up to 1300°C are used for calibration of the models and thereafter additional tests up to 9000s-1 at varying initial temperatures are compared with the model predictions
Metal cutting is one of the most common metal shaping processes. Specified geometrical and surface properties are obtained by break-up of the material removed by the cutting edge into a chip. The chip formation is associated with a large strain, high strain rate and a locally high temperature due to adiabatic heating which make the modelling of cutting processes difficult. This study compares a physically based plasticity model and the Johnson-Cook model. The latter is commonly used for high strain rate applications. Both material models are implemented into the finite element software MSC.Marc and compared with cutting experiments. The deformation behaviour of SANMAC 316L stainless steel during an orthogonal cutting process is studied.
Simulation based design enables rapid development of products with increased customer value in terms of accessibility, quality, productivity and profitability. However simulation of metal cutting is complex both in terms of numeric and physics. The work piece material undergoes severe deformations. The material model must therefore be able to accurately predict the deformation behavior for a large range of strain, strain rates (>50000 s-1) and temperatures. There exist a large number of different material models. They can be divided into empirical and physically based models. The far most common model used in simulation of metal cutting is the empirical Johnson-Cook plasticity model, JC model. Physically based models are based on the knowledge of the underlying physical phenomena and are expected to have larger domain of validity. Experimental measurements have been carried out in order to calibrate and validate a physical based material model utilizing dislocation density (DD) as internal variable. Split-Hopkinson tests have been performed in order to characterize the material behavior of SANMAC 316L at high strain rates. The DD model has been calibrated in earlier work by Lindgren et al. based on strain rate up to 10 s-1 and temperatures up to 1300 °C with good agreement over the range of calibration. Same good correspondence was not obtained when the model was extrapolated to high strain rate response curves from the dynamic Split-Hopkinson tests. These results indicate that new deformation mechanisms are entering. Repeating the calibration procedure for the empirical JC model shows that it can only describe the material behavior over a much more limited range. A recalibrated DD model, using varying obstacle strength at different temperatures, was used in simulation of machining. It was implemented in an implicit and an explicit finite element code.Simulation of orthogonal cutting has been performed with JC model and DD model using an updated Lagrangian formulation and an implicit time stepping logic. An isotropic hardening formulation was used in this case. The results showed that the cutting forces were slightly better predicted by the DD model. Largest error was 16 % compared to 20 % by the JC model. The predicted chip morphology was also better with the DD model but far from acceptable. Orthogonal cutting was simulated using an updated Lagrangian formulation with an explicit time integration scheme. In this case were two hardening rules tested, isotropic hardening and a mixed isotropic-kinematic hardening. The later showed an improvement regarding the feed force prediction. A deviation of less than 8% could be noticed except for the feed force at a cutting speed of 100 m/min. The time stepping procedure in combination with the mesh refinement seems to be able to capture the chip segmentation quite well without including damage evolution in the material model.Further works will mainly focus on improving the DD-model by introducing relevant physics for high strain rates.
Metal cutting is one of the most frequently used forming processes in the manufacturing industry. Extensive effort is made to improve its process and simulation has become an integrated part, not only in the product development process but also in the customer relations. However, simulation of metal cutting is complex both from numerical as well as physical point of view. Furthermore, modelling the material behaviour has shown to be crucial. Errors in the material model cannot be reduced by the numerical procedures. The magnitudes of strain and strain rate involved in metal cutting may reach values of 1-10 and 103-106 s-1. The dissipative plastic work together with the chip tool friction also leads to locally high temperatures. These extreme ranges of conditions imply that a diversity of physical phenomena is involved and it is a challenge to develop a material model with adequate accuracy over the whole loading range. Furthermore, this intense and severe deformation represents thermo-mechanical behaviour far from what is generated from conventional material compression and tension testing. A highly desirable feature is also a material model that can be extrapolated outside the calibration range. This is not trivial since materials exhibit different strain hardening and softening characteristics at differentstrains, strain rates and temperatures. Models based on modelling some aspects of the underlying physical process, e.g. the generation of dislocations, are expected to have a larger range of validity than engineering models. Though, engineering models are the far most common models used in metal cutting simulations.The scope of this work includes development of validated models for metal cutting simulations of AISI 316L stainless steel. Particular emphasis is placed on the material modeling and high strain rate plasticity phenomena. The focus has been on a physically based material model. The approach has been to review the literature about flow stress models and phenomena and particularly at high strain rates. A previous variant of a dislocation density model has thereafter been extended into high strain rate regimes by applying different mechanisms. Some of the models have been implemented in commercial finite element software for orthogonal cutting simulations. Experimental measurements and evaluations that include SHPB-measurements, cutting force measurements, quick-stop measurements and some microstructural examinations has been conducted for calibration and validation. The compression tests, within a temperature and strain rate range of 20-950 °C and 0.01-9000 s-1 respectively, showed that the flow stress increased much more rapidly within the dynamic loading range and hence depends on the strain rate. The dynamic strain aging (DSA) that has been observed at lower strain rates is non-existent at higher strain rates. The temperature and strain-rate evolution is such that the DSA is not necessary to include when modelling this process. Furthermore the magnetic balance measurements indicate that the martensite transformation-strengthening effect is insignificant within the dynamic loading range.In the present work the concept of motion of dislocations, their resistance to motion and substructure evolution are used as underlying motivation for description of the flow stress. A coupled set of evolution equations for dislocation density and mono vacancy concentration is used rendering a formulation of a rate-dependent yield limit in context of rate-independent plasticity. Dislocation drag due to phonon and electron drag, a strain rate dependent model of the subcell formation, a strain rate and temperature dependent recovery function and a structural dependent thermally activated stress component have among others been considered. Best predictability was obtained with a strain rate dependent subcell formulation. Dislocation drag did not improve the predictability within the measured testing range. Although showed to has a greater influence outside the range of calibration when extrapolated. It has been shown that extrapolation is uncertain. Results from experiments and modelling of material behaviour and metal cutting together with the literature indicate that the predictability of the material behaviour within and outside the measured testing range can be further tuned by implementing models of the phenomenon mechanical twining and recrystallization.
Machining one of the most common manufacturing processes within the industry but it is also a process with extreme conditions in the vicinity of the cutting insert. Due to diversity of physical phenomena involved machining has proven to be complex and difficult to simulate. The chip formation process is in the vicinity of the cutting insert associated with highly localized severe deformations accompanied by high local temperatures rise. Furthermore, the strain rate can in the primary zone be very high (>50000 s -1), far beyond what can be reached with conventional mechanical material tests. Therefore, the possibility to extrapolate the material model outside the calibration range with respect to strain rate is a wanted feature. It is recognized that the mechanical behavior at high strain rate differs considerably from that observed at low strain rates and that the flow stress increase rapidly with the strain rates above ∼1000 s -1. The predictive abilities outside as well as inside the calibration range of the empirical Johnson-Cook plasticity model and a dislocation density based model are compared and discussed with reference to AISI 316L stainless steel. The results clearly show the difficulty of obtaining a comprehensive material model that predicts the material behavior across the loading conditions that can occur in machining with good accuracy and that the accuracy of extrapolation is uncertain
Chip formation in metal cutting is associated with large strains and high strain rates, concentrated locally to deformation zones in front of the tool and beneath the cutting edge. Furthermore, dissipative plastic work and friction work generate high local temperatures. These phenomena together with numerical complications make modelling of metal cutting difficult. Material models, which are crucial in metal cutting simulations, are usually calibrated based on data from material testing. Nevertheless, the magnitude of strains and strain rates involved in metal cutting are several orders higher than those generated from conventional material testing. A highly desirable feature is therefore a material model that can be extrapolated outside the calibration range. In this study, two variants of a flow stress model based on dislocation density and vacancy concentration are used to simulate orthogonal metal cutting of AISI 316L stainless steel. It is found that the addition of phonon drag improves the results somewhat but the addition of this phenomenon still does not make it possible to extrapolate the constitutive model reliably outside its calibration range.