| 1. |
- Larsson, Ragnar, 1960, et al.
(author)
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Ductile dynamic fracture modeling using embedded discontinuities in CGI machining simulations
- 2012
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In: proc. 83rd Annual Meeting of the International Association of Applied Mathematics and Mechanics.
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Conference paper (peer-reviewed)abstract
- A major driving force for the industry to simulate various manufacturing processes is the incorporation of new design materials e.g. in order to promote lightweight design often leading to significant changes in manufacturing conditions, which can be assessed in an efficient way by simulation rather than more expensive testing. In the current contribution we are concerned with the constitutive modeling of Compacted Graphite Iron (CGI) with respect to orthogonal machining simulations. Although CGI consists in general of pearlite, graphite and ferrite, focus is placed on the constitutive modeling of the pearlitic phase since this is the dominating constituent with respect to the machinability issues. In this study, the continuum hardening response is modeled using theJohnson-Cook (JC) plasticity model; a model that has been extensively used in the literature for the modeling effects of large strains, high strain rates and hightemperatures related to machining. In earlier works, the JC plasticity model has been used along with ductile fracture response that has been described with theelement deletion based on Johnson-Cook dynamic failure criterion. In the current development we it is proposed to use a continuum damage approach for the continuous behavior up to the critical stress-strain states where discontinuous bifurcation occurs. Whenever a critical state has been diagnosed, a Cohesive Zone (CZ) is established so that the actual critical stress state is located right at the onset of stress degradation in the CZ. Both pre-peak continuum damage and post-peak CZ damage, representing distributed and localized damageevolution, respectively, are considered in the formulation. Both the pre- and post-peak damage evolutions are defined as a post-processing of the effectivestress response. The localized cohesive zone damage is kinematically realized as an element embedded discontinuity which is introduced elementwise, therebyfacilitating the model developments in standard FE-packages. The orthogonal machining simulations show that the new continuum damage model appears to be sensitive to element locking as well as significant element size dependence without the cohesive zone enhancement of the model. In order to investigate the extent of locking behavior in simulations and also the possible pathologicalmesh dependency the series of 2D shear test simulations with both triangular and rectangular elements with different sizes have been conducted and results are compared.
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| 2. |
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| 3. |
- Larsson, Ragnar, 1960, et al.
(author)
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Rate Sensitive Continuum Damage Models and Mesh Dependence in Finite Element Analyses
- 2013
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In: 3rd International Conference on Material Modeling. - 9788389687838 ; , s. 253-
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Conference paper (peer-reviewed)abstract
- The experiences from the orthogonal machining simulations show that the Johnson Cook dynamic failure model appears to exhibit a significant element size dependence.Such pathological mesh dependence is a direct consequence of the use of damage models unless some type of egularization is introduced. The current contribution investigates the extent of possible pathological mesh dependence,and a comparison of the resulting behavior in the case of the Johnson Cook (JC) plasticity model combined with two types of damage evolutions. Both the plasticity and the damage models considered in the formulation are rate dependent and the damage evolutions for both models are defined as a post-processing of the effective stress response. The results show that both damage models, with a realistic representation of the pearlite material properties,exhibit a similar extent of mesh dependence and that viscous regularization effects do not suffice in the current investigation. As a remedy for the observed mesh sensitivity, it is proposed to use a continuum-damage approach for the modeling of continuous deformation behavior up to the critical point along the stress-strain curve where discontinuous bifurcation occurs. Whenever a critical stress-strain state has been diagnosed, a Cohesive Zone (CZ) is established to allow for objective fracture energy release as the stress is degraded in the CZ. The formulation is made in a completely general kinematical context thus allowing for large deformations, central in machining applications. To characterize the homogenized continuous/discontinuous macro-behavior,a discontinuous enhancement of XFEM--type is proposed at a sub-scale based on homogenization theory. It appears that the associated local momentum balance is manifested by the condition for traction continuity across the discontinuity surface. Thereby, the formulation generically contains e.g. the classical condition for diagnosing discontinuous bifurcation. In the corresponding FE-application, localized CZ damage is kinematically realized as an element embedded discontinuity, which is introduced elementwise, thereby facilitating the model implementation in standard FE-packages. Both pre-peak continuum behavior and post-peak CZ behavior are modeled using the concept of (visco)-plasticity coupled to damage, representing distributed and localized damage evolutions by separate constitutive modeling considerations, respectively. To arrive at a computationally attractive formulation, both the pre- and post--peak damage evolutions are defined as a post-processing of the effective stress response. In the considered numerical examples of typical shear and tensile deformation the new continuous/discontinuous ductile fracture modeling exhibits no significant element size dependence.
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| 4. |
- Ljustina, Goran, 1971, et al.
(author)
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A FE based machining simulation methodology accounting for cast iron microstructure
- 2014
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In: Finite Elements in Analysis and Design. - : Elsevier BV. - 0168-874X. ; 80, s. 1-10
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Journal article (peer-reviewed)abstract
- In the present paper the effect of graphite nodularity in the microstructure of cast iron is assessed with respect to its machinability at orthogonal cutting. A micro-structure based model of the cast iron material has been developed based on analyses of micro-graph images. The image analysis combines pearlitic grains with graphite nodules to produce the micro-structures. Continuous deformation behavior of pearlite and graphite phases is described using the Johnson-Cook (JC) viscoplasticty model including temperature dependence. Of particular concern is the modeling of the chip formation, induced by the micro-structure morphology and the mechanical behavior of the work piece material in the vicinity of the insert. This is described by the element deletion methodology along with the Johnson-Cook failure criterion to realize the fracture kinematics. A parametric study is carried out by varying the nodularity of the graphite. To validate the model, the finite element results are compared to machining test results carried out on a Compacted Graphite Iron (CGl) material in a related paper It turns out that the FE-simulated and experimentally observed cutting force and chip formation compare fairly well.
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| 5. |
- Ljustina, Goran, 1971, et al.
(author)
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Constitutive modeling and validation of CGI machining
- 2010
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In: 23rd Nordic Seminar on Computational Mechanics, 21-22 October, Stockholm.
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Conference paper (other academic/artistic)abstract
- A driving force for the industry to simulate various manufacturing processes is the incorporation of new design materials – e.g. in order to promote lightweight design –often leading to significant changes in manufacturing conditions, which can be assessed in an efficient way by simulation rather than more expensive testing. The current contribution is concerned with the constitutive modeling of Compacted Graphite Iron (CGI) and its application to simulate orthogonal machining thereof. Although CGI consists in general of pearlite, graphite and ferrite, focus is put on the constitutive modeling of the pearlitic phase since it is the dominating constituent with respect to machinability. The continuum hardening response is modeled with the Johnson-Cook (JC) plasticity model and the ductile fracture response with the Johnson-Cook dynamic failure criterion, both involving effects of largestrains, high strain rates and high temperatures. Furthermore, the model has been calibrated against experimental data found in the literature for a pearlitic rail steel considered as a good representative for the pearlitic phase in the CGI-material. This assumption is strengthened by the good correlation obtained between simulated chip formation and cutting forces in thecurrent work and the experimentally obtained chip formation and measured cutting forces in a related project. The proper representation of the finite deformation inelasticity problem is also discussed. The traditional way to represent the material in metal cutting applications via hypoelastic-inelastic material models, incorporating the objective corotational Jaumann stressrate can be shown to give an inaccurate response for particular loading situations, e.g. in simple shear where a spurious softening response is obtained. Therefore, we instead propose to use the alternative Green-Naghdi corotational stress rate to formulate the hypoelasticinelastic response which does not show this type of unphysical behavior. 2D orthogonalcutting simulations have been conducted in Abaqus/Explicit using both types of models (based on the Jaumann and the Green-Naghdi stress rate respectively) and the results thereof are compared and discussed. The results of these models are also compared with the results of a thermodynamically consistent hyperelastic-inelastic material model using the same type of JC-hardening.
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| 6. |
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| 7. |
- Ljustina, Goran, 1971, et al.
(author)
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Ductile dynamic fracture modeling using embedded strong discontinuities in CGI machining simulations
- 2012
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In: Proceedings of the 25th Nordic Seminar on Computational Mechanics.
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Conference paper (peer-reviewed)abstract
- In order to model the material behavior in machining simulations it is proposed to use a continuum damage approach on the basis of the Johnson-Cook (JC) plasticity model for the ductile continuous behavior up to the critical stress–strain states where discontinuous bifurcation may occur. Whenever a critical state has beendiagnosed, a Cohesive Zone (CZ) is established so that the actual critical stress state is located right at the onset of stress degradation in the CZ. Both pre–peak continuum damage and post–peak CZ damage are considered in the formulation. The localized cohesive zone damage is kinematically realized as an element embedded discontinuity, cf. [2], which is introduced elementwise, thereby facilitating the model developments in standard FE-packages.
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| 8. |
- Ljustina, Goran, 1971, et al.
(author)
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Hypo-- and hyperinelasticity applied to modeling of compacted graphite iron machining simulations
- 2011
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In: Proceedings of the 24th nordic seminar on computational mechanics. - 1799-4896. - 9789526043470 ; , s. 59-62
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Conference paper (peer-reviewed)abstract
- In the present paper we are concerned with constitutive modeling and validation of the thermomechanically coupled Compacted Graphite Iron (CGI) machining problem.Particular emphasis is placed on the significance of the choice of different hypoelastic-inelastic formulations in terms of four different objective stress rate formulations.We also relate to a thermodynamically consistent hyperelastic--inelastic formulation based on multiplicative decomposition of the deformation gradient. The consequently induced tangent material behavior is then derived in the spatial setting in terms of the Oldroyd stress rate,and it is compared to the hypo--formulations.The Johnson--Cook (JC) model is taken as the main prototype for the modeling of isotropic hardening, strain rate and temperature dependencies.The different models are compared in simple shear, uniaxial tensile--compressive tests, and finally in a representative CGI--machining simulation, and the resulting mechanical isothermal behavior obtained from the different ways of establishing the objective stress rate are surprisingly similar.
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| 9. |
- Ljustina, Goran, 1971, et al.
(author)
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Hypo– and hyperinelasticity applied to modeling of compacted graphite iron machining simulations
- 2013
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In: European Journal of Mechanics, A/Solids. - : Elsevier BV. - 0997-7538. ; 37, s. 57-68
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Journal article (peer-reviewed)abstract
- In the present paper we are concerned with constitutive modeling and validation of the thermomechanically coupled Compacted Graphite Iron (CGI) machining problem. Particular emphasis is placed on the significance of the choice of different hypo–inelastic models induced by different objective stress rate formulations. We also relate to a thermodynamically consistent hyperelastic–inelastic formulation based on multiplicative decomposition of the deformation gradient. The consequently induced tangent material behavior is then derived in the spatial setting in terms of the Oldroyd stress rate, and it is compared to the hypo–formulations. The Johnson–Cook (JC) model is taken as the main prototype for the modeling of isotropic hardening, strain rate and temperature dependencies, which is considered reframed within the Perzyna visco–plasticity framework, thereby highlighting the quasistatic and rate dependent properties of the model. The different models are compared both on the material point level (simple shear and uniaxial tensile–compressive tests) and on the structural level (FE analysis of a 2D shear test and in representative CGI–machining simulations) and the resulting mechanical isothermal behavior obtained from the different ways of establishing the objective stress rate are surprisingly similar. Based on the results obtained a hypo–inelastic formulation based on a modified Oldroyd stress rate is proposed due to its link to thermo–mechanical consistency and relative computational efficiency.
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| 10. |
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| 11. |
- Ljustina, Goran, 1971
(author)
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Modeling and Simulation of CGI Machining on Microstructure Level
- 2010
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Licentiate thesis (other academic/artistic)abstract
- incorporation of new design materials – e.g. in order to promote lightweight design – oftenleading to significant changes in manufacturing conditions, which can be assessed in anefficient way by simulation rather than more expensive testing. The current contribution isconcerned with the constitutive modeling of Compacted Graphite Iron (CGI) and itsapplication to simulate orthogonal machining thereof. Although CGI consists in general ofpearlite, graphite and ferrite, focus is put on the constitutive modeling of the pearlitic phasesince it is the dominating constituent with respect to machinability. The continuum hardeningresponse is modeled with the Johnson-Cook (JC) plasticity model and the ductile fractureresponse with the Johnson-Cook dynamic failure criterion, both involving effects of largestrains, high strain rates and high temperatures. Furthermore, the model has been calibratedagainst experimental data found in the literature for a pearlitic rail steel considered as a goodrepresentative for the pearlitic phase in the CGI-material. This assumption is strengthened bythe good correlation obtained between simulated chip formation and cutting forces in thecurrent work and the experimentally obtained chip formation and measured cutting forces in arelated project. The proper representation of the finite deformation inelasticity problem is alsodiscussed. The traditional way to represent the material in metal cutting applications viahypoelastic-inelastic material models, incorporating the objective corotational Jaumann stressrate can be shown to give an inaccurate response for particular loading situations, e.g. insimple shear where a spurious softening response is obtained. Therefore, we instead proposeto use the alternative Green-Naghdi corotational stress rate to formulate the hypoelasticinelasticresponse which does not show this type of unphysical behavior. 2D orthogonalcutting simulations have been conducted in Abaqus/Explicit using both types of models(based on the Jaumann and the Green-Naghdi stress rate respectively) and the results thereofare compared and discussed. The results of these models are also compared with the results ofa thermodynamically consistent hyperelastic-inelastic material model using the same type ofJC-hardening.
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| 12. |
- Ljustina, Goran, 1971
(author)
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Modeling of cast iron materials related to machining
- 2013
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Doctoral thesis (other academic/artistic)abstract
- A microstructure level simulation model of cast iron machining based on micrograph image analysis has been developed. The simulation tool has been developed for theorthogonal machining process involving 2D representations of a range of cast iron microstructures. The benefits achieved from this approach are: a better understanding of the machinability of the work-piece material related to its composition and microstructures, and qualitative predictions of e.g. the cutting force connected to a variation of the microstructural properties. Compacted graphite iron was taken as a prototype material due to unsolved issues regarding the machining of this type of cast iron in industry.As to the modeling, focus is placed on the pearlitic phase since it is the dominating constituent with respect to strength, and the continuous deformation behavior is described using the Johnson-Cook (JC) visco-plasticity model. Various formats of this model, using both hyperelastic-inelastic and hypoelastic-inelastic formulations, have been used to investigate possible differences in response and computational efficiency. In orderto further describe the material degradation during machining, a continuum damage evolution is proposed as an enhancement of the JC model. Finite element results obtained from many simulations have been compared to machining tests with promising results.However, a severe mesh dependence has been observed in the simulations caused by the local damage modeling. A special investigation of this mesh dependence has beenundertaken based on the resulting behavior of the Johnson Cook (JC) plasticity model combined with two different types of damage formulations. The results show a similarextent of the mesh dependence for both damage models, and that the viscous regularization efects, due to rate dependence of the model, are absent. As a remedy to the observed mesh dependence, the final contribution is concerned with ductile dynamic fracture modeling using FE-element embedded discontinuities. To characterize the homogenized continuous/discontinuous macro-behavior, a discontinuous enhancement is proposed at a sub-scale based on homogenization theory. In the corresponding FE-application, localized cohesive zone damage is kinematically realized as an element embedded discontinuity, which is introduced elementwise, thereby facilitating the model implementation in standard FE packages. In the considered numerical examples the proposed continuous/discontinuous ductile fracture modeling exhibits no significant element size dependence.
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| 13. |
- Ljustina, Goran, 1971, et al.
(author)
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Rate Sensitive Continuum Damage Models and Mesh Dependence in Finite Element Analyses
- 2013
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In: MekIT'13 Seventh National Conference on Computational Mechanics. - 9788232102662 ; , s. 21-32
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Conference paper (peer-reviewed)abstract
- The experiences from the orthogonal machining simulations show that the Johnson Cook dynamic failure model appears to exhibit a significant element size dependence.Such pathological mesh dependence is a direct consequence of the utilization of local damage models unless some type of regularization is introduced. The current contribution is an investigation of the extent of the possible pathological mesh dependence and a comparison of the resulting behavior in the case of the Johnson Cook (JC) plasticity model being combined with two types of damage evolution. The first damage model is the Johnson Cook dynamic failure model where the development of the "damage" does not affect the response until the critical state is reached. The second is a continuum damage model based on work done by Cocks and Ashby, where, on the other hand, the damage variable is affecting the material response (and result in a softer development of stress vs. strain). Both the plasticity and the damage models considered in the formulation are rate dependent and the damage evolutions for both models are defined as a post-processing of the effective stress response. The investigation is conducted based on simulation of a series of 2D shear tests utilizing FE-representations, including elements with different sizes, of the plane strain plate with pearlite material properties. Furthermore, the crack propagation is described by the element deletion method. A one way of reducing the mesh dependence is to include a viscosity in the material law. Since the both damage models used in the investigation are rate dependent, different deformation rates have been used in order to reveal possible effects of the viscous regularization. The results show that, for both damage models, and with a realistic representation of the pearlite material properties, a similar extent of mesh dependence is obtained and that the viscous regularization effects are absent in the current investigation.
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| 14. |
- Ljustina, Goran, 1971, et al.
(author)
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Rate Sensitive Continuum Damage Models and Mesh Dependence in Finite Element Analyses
- 2014
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In: The Scientific World Journal. - : Hindawi Limited. - 1537-744X .- 2356-6140. ; 2014
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Journal article (peer-reviewed)abstract
- The experiences from orthogonal machining simulations show that the Johnson-Cook (JC) dynamic failure model exhibitssignificant element size dependence. Such mesh dependence is a direct consequence of the utilization of local damage models. Thecurrent contribution is an investigation of the extent of the possible pathological mesh dependence. A comparison of the resultingJC model behavior combined with two types of damage evolution is considered. The first damage model is the JC dynamic failuremodel, where the development of the “damage” does not affect the response until the critical state is reached. The second one is acontinuumdamagemodel, where the damage variable is affecting thematerial response continuously during the deformation. Boththe plasticity and the damagemodels are rate dependent, and the damage evolutions for bothmodels are defined as a postprocessingof the effective stress response. The investigation is conducted for a series of 2D shear tests utilizing different FE representations ofthe plane strain plate with pearlite material properties. The results show for both damage models, using realistic pearlite materialparameters, that similar extent of the mesh dependence is obtained and that the possible viscous regularization effects are absentin the current investigation.
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