1. | Petryk H., Stupkiewicz S., Kucharski S., On direct estimation of hardening exponent in crystal plasticity from the spherical indentation test, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2016.09.025, pp.1-13, 2017Petryk H., Stupkiewicz S., Kucharski S., On direct estimation of hardening exponent in crystal plasticity from the spherical indentation test, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2016.09.025, pp.1-13, 2017Abstract: A novel methodology is proposed for estimating the strain hardening exponent of a metal single crystal directly from the spherical indentation test, without the need of solving the relevant inverse problem. The attention is focused on anisotropic piling-up and sinking-in that occur simultaneously in different directions, in contrast to the standard case of axial symmetry for isotropic materials. To correlate surface topography parameters with the value of material hardening exponent, a finite-element study of spherical indentation has been performed within a selected penetration depth range using a finite-strain crystal plasticity model. It is shown how the power-law hardening exponent can be estimated from the measured pile-up/sink-in pattern around the residual impression after indentation in a (001)-oriented fcc single crystal of a small initial yield stress. For this purpose, a new parameter of surface topography is defined as the normalized material volume displaced around the nominal contact zone, calculated by integration of the local residual height (positive or negative) over a centered circular ring. That indicator can be easily determined from an experimental topography map available in a digital form. Comparison is made with the estimates based on measurements of the contact area and the slope of the load–penetration depth curve in logarithmic coordinates. The proposed methodology is extended to estimation of the hardening exponent simultaneously with the initial yield stress when the latter is not negligible. Experimental verification for a Cu single crystal leads to promising conclusions. Keywords: Metal crystal, Elastoplasticity, Finite deformation, Strain hardening, Experimental identification | | 35p. |
2. | Tůma K., Stupkiewicz S., Phase-field study of size-dependent morphology of austenite–twinned martensite interface in CuAlNi, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2016.07.040, Vol.97-98, pp.89-100, 2016Tůma K., Stupkiewicz S., Phase-field study of size-dependent morphology of austenite–twinned martensite interface in CuAlNi, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2016.07.040, Vol.97-98, pp.89-100, 2016Abstract: Size-dependent microstructure of the interface layer between austenite and twinned martensite is studied using a recently developed finite-strain phase-field model. The microstructure is assumed periodic and two-dimensional, however, non-zero out-of-plane displacements are allowed so that the basic microstructural features, specifically the nominal orientation of the twinning and habit planes and the twin fraction, are consistent with the crystallographic theory of martensite. The phase-field computations are carried out for the CuAlNi shape memory alloy undergoing the cubic-to-orthorhombic transformation, and the corresponding four crystallographically distinct microstructures of the austenite–twinned martensite interface are studied in detail. The focus is on size-dependent morphology of the interface layer and on size-dependent interfacial and elastic micro-strain energy contributions. Two mechanisms of reducing the elastic micro-strain energy are revealed: formation of a non-planar zigzag-like interface and twin branching. Keywords: Microstructure, Phase transformation, Martensite, Phase-field method, Size effects | | 40p. |
3. | Tůma K., Stupkiewicz S., Petryk H., Size effects in martensitic microstructures: Finite-strain phase field model versus sharp-interface approach, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, ISSN: 0022-5096, DOI: 10.1016/j.jmps.2016.04.013, Vol.95, pp.284-307, 2016Tůma K., Stupkiewicz S., Petryk H., Size effects in martensitic microstructures: Finite-strain phase field model versus sharp-interface approach, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, ISSN: 0022-5096, DOI: 10.1016/j.jmps.2016.04.013, Vol.95, pp.284-307, 2016Abstract: A finite-strain phase field model for martensitic phase transformation and twinning in shape memory alloys is developed and confronted with the corresponding sharp-interface approach extended to interfacial energy effects. The model is set in the energy framework so that the kinetic equations and conditions of mechanical equilibrium are fully defined by specifying the free energy and dissipation potentials. The free energy density involves the bulk and interfacial energy contributions, the latter describing the energy of diffuse interfaces in a manner typical for phase-field approaches. To ensure volume preservation during martensite reorientation at finite deformation within a diffuse interface, it is proposed to apply linear mixing of the logarithmic transformation strains. The physically different nature of phase interfaces and twin boundaries in the martensitic phase is reflected by introducing two order-parameters in a hierarchical manner, one as the reference volume fraction of austenite, and thus of the whole martensite, and the second as the volume fraction of one variant of martensite in the martensitic phase only. The microstructure evolution problem is given a variational formulation in terms of incremental fields of displacement and order parameters, with unilateral constraints on volume fractions explicitly enforced by applying the augmented Lagrangian method. As an application, size-dependent microstructures with diffuse interfaces are calculated for the cubic-to-orthorhombic transformation in a CuAlNi shape memory alloy and compared with the sharp-interface microstructures with interfacial energy effects. Keywords: Phase-field method, Microstructure, Martensite, Size effects, Shape memory alloys | | 40p. |
4. | Temizer I., Stupkiewicz S., Formulation of the Reynolds equation on a time-dependent lubrication surface, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, ISSN: 1364-5021, DOI: 10.1098/rspa.2016.0032, Vol.472, No.20160032, pp.1-16, 2016Temizer I., Stupkiewicz S., Formulation of the Reynolds equation on a time-dependent lubrication surface, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, ISSN: 1364-5021, DOI: 10.1098/rspa.2016.0032, Vol.472, No.20160032, pp.1-16, 2016Abstract: The Reynolds equation, which describes the lubrication effect arising through the interaction of two physical surfaces that are separated by a thin fluid film, is formulated with respect to a continuously evolving third surface that is described by a time-dependent curvilinear coordinate system. The proposed formulation essentially addresses lubrication mechanics at interfaces undergoing large deformations and a priori satisfies all objectivity requirements, neither of which are features of the classical Reynolds equation. As such, this formulation may be particularly suitable for non-stationary elastohydrodynamic lubrication problems associated with soft interfaces. The ability of the formulation to capture finite-deformation effects and the influence of the choice of the third surface are illustrated through analytical examples. Keywords: Reynolds equation, elastohydrodynamic lubrication, soft interfaces, time-dependent curvilinear coordinates, finite deformations, objectivity | | 35p. |
5. | Stupkiewicz S., Lengiewicz J., Sadowski P., Kucharski S., Finite deformation effects in soft elastohydrodynamic lubrication problems, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2015.03.016, Vol.93, pp.511-522, 2016Stupkiewicz S., Lengiewicz J., Sadowski P., Kucharski S., Finite deformation effects in soft elastohydrodynamic lubrication problems, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2015.03.016, Vol.93, pp.511-522, 2016Abstract: Soft elastohydrodynamic lubrication regime is typical for many elastomeric and biological contacts. As one or both contacting bodies are then highly compliant, relatively low contact pressures may lead to large deformations which are neglected in the classical EHL theory. In the paper, the related finite-deformation effects are studied for two representative soft-EHL problems. To this end, a fully-coupled nonlinear formulation has been developed which combines finite-strain elasticity for the solid and the Reynolds equation for the fluid, both treated using the finite element method with full account of all elastohydrodynamic couplings. Results of friction measurements are also reported and compared to theoretical predictions for lubricated contact of a rubber ball sliding against a steel disc under high loads. Keywords: Soft-EHL problem, Finite deformation, Finite element method, Monolithic scheme | | 35p. |
6. | Petryk H., Stupkiewicz S., A minimal gradient-enhancement of the classical continuum theory of crystal plasticity. Part I: The hardening law, ARCHIVES OF MECHANICS, ISSN: 0373-2029, Vol.68, No.6, pp.459-485, 2016Petryk H., Stupkiewicz S., A minimal gradient-enhancement of the classical continuum theory of crystal plasticity. Part I: The hardening law, ARCHIVES OF MECHANICS, ISSN: 0373-2029, Vol.68, No.6, pp.459-485, 2016Abstract: A simple gradient-enhancement of the classical continuum theory of plasticity of single crystals deformed by multislip is proposed for incorporating size effects in a manner consistent with phenomenological laws established in materials science. Despite considerable efforts in developing gradient theories, there is no consensus regarding the minimal set of physically based assumptions needed to capture the slip-gradient effects in metal single crystals and to provide a benchmark for more refined approaches. In order to make a step towards such a reference model, the concept of the tensorial density of geometrically necessary dislocations generated by slip-rate gradients is combined with a generalized form of the classical Taylor formula for the flow stress. In the governing equations in the rate form, the derived internal length scale is expressed through the current flow stress and standard parameters so that no further assumption is needed to define a characteristic length. It is shown that this internal length scale is directly related to the mean free path of dislocations and possesses physical interpretation which is frequently missing in other gradient-plasticity models. Keywords: gradient plasticity, geometrically necessary dislocations, single crystal, strain-hardening, internal length scale, size effect | | 20p. |
7. | Stupkiewicz S., Petryk H., A minimal gradient-enhancement of the classical continuum theory of crystal plasticity. Part II: Size effects, ARCHIVES OF MECHANICS, ISSN: 0373-2029, Vol.68, No.6, pp.487-513, 2016Stupkiewicz S., Petryk H., A minimal gradient-enhancement of the classical continuum theory of crystal plasticity. Part II: Size effects, ARCHIVES OF MECHANICS, ISSN: 0373-2029, Vol.68, No.6, pp.487-513, 2016Abstract: In our previous paper, a simple gradient-enhancement of the classical continuum theory of plasticity of single crystals deformed by multislip has been proposed for incorporating size effects. A single internal length scale has been derived as an explicit function of the flow stress defined as the isotropic part of critical resolved shear stresses. The present work is focused on verification whether the simplifications involved are not too severe and allow satisfactory predictions of size effects. The model has been implemented in a finite element code and applied to three-dimensional simulations of fcc single crystals. We have found that the experimentally observed indentation size effect in a Cu single crystal is captured correctly in spite of the absence of any adjustable length-scale parameter. The finite element treatment relies on introducing non-local slip rates that average and smoothen on an element scale the corresponding local quantities. Convergence of the finite element solution to the analytical one is also verified for the one-dimensional problem of a boundary layer formed at a constrained interface.
Keywords: gradient plasticity, geometrically necessary dislocations, boundary layer, size effects, indentation, finite element method | | 20p. |
8. | Stupkiewicz S., Piccolroaz A., Bigoni D., Finite-strain formulation and FE implementation of a constitutive model for powder compaction, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2014.09.027, Vol.283, pp.856-880, 2015Stupkiewicz S., Piccolroaz A., Bigoni D., Finite-strain formulation and FE implementation of a constitutive model for powder compaction, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2014.09.027, Vol.283, pp.856-880, 2015Abstract: A finite-strain formulation is developed, implemented and tested for a constitutive model capable of describing the transition from granular to fully dense state during cold forming of ceramic powder. This constitutive model (as well as many others employed for geomaterials) embodies a number of features, such as pressure-sensitive yielding, complex hardening rules and elastoplastic coupling, posing considerable problems in a finite-strain formulation and numerical implementation. A number of strategies are proposed to overcome the related problems, in particular, a neo-Hookean type of modification to the elastic potential and the adoption of the second Piola–Kirchhoff stress referred to the intermediate configuration to describe yielding. An incremental scheme compatible with the formulation for elastoplastic coupling at finite strain is also developed, and the corresponding constitutive update problem is solved by applying a return mapping algorithm. Keywords: Plasticity, Elastoplastic coupling, Finite element method, Automatic differentiation | | 45p. |
9. | Sadowski P., Kowalczyk-Gajewska K., Stupkiewicz S., Classical estimates of the effective thermoelastic properties of copper–graphene composites, COMPOSITES PART B-ENGINEERING, ISSN: 1359-8368, DOI: 10.1016/j.compositesb.2015.06.007, Vol.80, pp.278-290, 2015Sadowski P., Kowalczyk-Gajewska K., Stupkiewicz S., Classical estimates of the effective thermoelastic properties of copper–graphene composites, COMPOSITES PART B-ENGINEERING, ISSN: 1359-8368, DOI: 10.1016/j.compositesb.2015.06.007, Vol.80, pp.278-290, 2015Abstract: Significant research effort is concentrated worldwide on development of graphene-based metal-matrix composites with enhanced thermomechanical properties. In this work, we apply two classical micromechanical mean-field theories to estimate the effective thermoelastic properties that can be achieved in practice for a copper–graphene composite. In the modelling, graphene is treated as an anisotropic material, and the effect of its out-of-plane properties, which are less recognized than the in-plane properties, is studied in detail. To address the severe difficulties in processing of graphene-based metal-matrix composites, the copper–graphene composite is here assumed to additionally contain, due to imperfect processing, particles of graphite and voids. It is shown quantitatively that the related imperfections may significantly reduce the expected enhancement of the effective properties. The present predictions are also compared to the experimental data available in the literature. Keywords: Metal-matrix composites (MMCs), Mechanical properties, Thermal properties, Micro-mechanics, Graphene | | 40p. |
10. | Kowalczyk-Gajewska K., Sztwiertnia K., Kawałko J., Wierzbanowski K., Wroński M., Frydrych K., Stupkiewicz S., Petryk H., Texture evolution in titanium on complex deformation paths: Experiment and modelling, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ISSN: 0921-5093, DOI: 10.1016/j.msea.2015.04.040, Vol.637, pp.251-263, 2015Kowalczyk-Gajewska K., Sztwiertnia K., Kawałko J., Wierzbanowski K., Wroński M., Frydrych K., Stupkiewicz S., Petryk H., Texture evolution in titanium on complex deformation paths: Experiment and modelling, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ISSN: 0921-5093, DOI: 10.1016/j.msea.2015.04.040, Vol.637, pp.251-263, 2015Abstract: Texture evolution in commercially pure titanium deformed by equal-channel angular pressing (ECAP) and extrusion with forward–backward rotating die (KoBo) is studied both experimentally and numerically. New results are provided that demonstrate the effects of distinct and complex deformation paths on the texture in the ultra-fine grained (UFG) material obtained after severe plastic deformation (SPD). The numerical simulations are based on the self-consistent viscoplastic method of grain-to-polycrystal scale transition. A recently proposed modification of the probabilistic scheme for twinning is used that provides consistent values of the twin volume fraction in grains. The basic components of the experimentally observed texture are reasonably well reproduced in the modelling. The numerical simulations provide an insight into the internal mechanisms of plastic deformation, revealing substantial activity of mechanical twinning in addition to the basal and prismatic slip in titanium processed by ECAP. Keywords: Texture evolution, UFG materials, SPD processes, Crystal plasticity, Twinning | | 35p. |
11. | Stupkiewicz S., Piccolroaz A., Bigoni D., Elastoplastic coupling to model cold ceramic powder compaction, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, ISSN: 0955-2219, DOI: 10.1016/j.jeurceramsoc.2013.11.017, Vol.34, pp.2839-2848, 2014Stupkiewicz S., Piccolroaz A., Bigoni D., Elastoplastic coupling to model cold ceramic powder compaction, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, ISSN: 0955-2219, DOI: 10.1016/j.jeurceramsoc.2013.11.017, Vol.34, pp.2839-2848, 2014Abstract: The simulation of industrial processes involving cold compaction of powders allows for the optimization of the production of both traditional and advanced ceramics. The capabilities of a constitutive model previously proposed by the authors are explored to simulate simple forming processes, both in the small and in the large strain formulation. The model is based on the concept of elastoplastic coupling – providing a relation between density changes and variation of elastic properties – and has been tailored to describe the transition between a granular ceramic powder and a dense green body. Finite element simulations have been compared with experiments on an alumina ready-to-press powder and an aluminum silicate spray-dried granulate. The simulations show that it is possible to take into account friction at the die wall and to predict the state of residual stress, density distribution and elastic properties in the green body at the end of the forming process. Keywords: Ceramic forming, Granular material, Elastoplasticity, Constitutive model, Material modelling | | 50p. |
12. | Stupkiewicz S., Denzer R., Piccolroaz A., Bigoni D., Implicit yield function formulation for granular and rock-like materials, COMPUTATIONAL MECHANICS, ISSN: 0178-7675, DOI: 10.1007/s00466-014-1047-8, Vol.54, pp.1163-1173, 2014Stupkiewicz S., Denzer R., Piccolroaz A., Bigoni D., Implicit yield function formulation for granular and rock-like materials, COMPUTATIONAL MECHANICS, ISSN: 0178-7675, DOI: 10.1007/s00466-014-1047-8, Vol.54, pp.1163-1173, 2014Abstract: The constitutive modelling of granular, porous and quasi-brittle materials is based on yield (or damage) functions, which may exhibit features (for instance, lack of convexity, or branches where the values go to infinity, or ‘false elastic domains’) preventing the use of efficient return-mapping integration schemes. This problem is solved by proposing a general construction strategy to define an implicitly defined convex yield function starting from any convex yield surface. Based on this implicit definition of the yield function, a return-mapping integration scheme is implemented and tested for elastic–plastic (or -damaging) rate equations. The scheme is general and, although it introduces a numerical cost when compared to situations where the scheme is not needed, is demonstrated to perform correctly and accurately. Keywords: Plasticity, Return mapping algorithm, Automatic differentiation | | 40p. |
13. | Korelc J., Stupkiewicz S., Closed-form matrix exponential and its application in finite-strain plasticity, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.4653, Vol.98, pp.960-987, 2014Korelc J., Stupkiewicz S., Closed-form matrix exponential and its application in finite-strain plasticity, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.4653, Vol.98, pp.960-987, 2014Abstract: A new method to compute numerically efficient closed-form representation of matrix exponential and its derivative is developed for 3 × 3 matrices with real eigenvalues. The matrix exponential is obtained by automatic differentiation of an appropriate scalar generating function in a general case, and highly accurate asymptotic expansions are derived for special cases in which the general formulation exhibits ill-conditioning, for instance, for almost equal eigenvalues. Accuracy and numerical efficiency of the closed-form matrix exponential as compared with the truncated series approximation are studied. The application of the closed-form matrix exponential in the finite-strain elastoplasticity is also presented. To this end, several time-discrete evolution laws employing the exponential map are discussed for J2 plasticity with isotropic hardening and nonlinear kinematic hardening of Armstrong–Frederick type. The discussion is restricted to the case of elastic isotropy and implicit time integration schemes. In this part, the focus is on a general automatic differentiation-based formulation of finite-strain plasticity models. Numerical efficiency of the corresponding incremental schemes is studied in the context of the FEM. Keywords: automatic differentiation, symbolic methods, ADB form, exponential map, kinematic hardening | | 40p. |
14. | Kucharski S., Stupkiewicz S., Petryk H., Surface Pile-Up Patterns in Indentation Testing of Cu Single Crystals, EXPERIMENTAL MECHANICS, ISSN: 0014-4851, DOI: 10.1007/s11340-014-9883-1, Vol.54, pp.957-969, 2014Kucharski S., Stupkiewicz S., Petryk H., Surface Pile-Up Patterns in Indentation Testing of Cu Single Crystals, EXPERIMENTAL MECHANICS, ISSN: 0014-4851, DOI: 10.1007/s11340-014-9883-1, Vol.54, pp.957-969, 2014Abstract: Nano- and micro-indentation of Cu single crystals is performed in directions not aligned with crystallographic axes. Such tests correspond to mechanical characterization of incidentally oriented grains in a polycrystalline or composite material. Orientation and size dependence of complex patterns of surface piling-up and sinking-in around the imprint are investigated. Experimental observations are compared with finite element simulations based on the large deformation crystal plasticity theory. Keywords: Copper, Nanoindentation, AFM, EBSD, Crystal plasticity, Finite element method | | 35p. |
15. | Gourgiotis P.A., Stupkiewicz S., Macroscopic stress and strain in a doubly periodic array of dislocation dipoles, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, ISSN: 1364-5021, DOI: 10.1098/rspa.2014.0309, Vol.470, pp.20140309-1-16, 2014Gourgiotis P.A., Stupkiewicz S., Macroscopic stress and strain in a doubly periodic array of dislocation dipoles, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, ISSN: 1364-5021, DOI: 10.1098/rspa.2014.0309, Vol.470, pp.20140309-1-16, 2014Abstract: It is known that in two-dimensional periodic arrays of dislocations the summation of the periodic image fields is conditionally convergent. This is due to the long-range character of the elastic fields of dislocations. As a result, the stress field obtained for a doubly periodic array of dislocation dipoles may contain a spurious constant stress that depends on the adopted summation scheme. In the present work, we provide, based on micromechanical considerations, a simple physical explanation of the origin of the conditional convergence of lattice sums of image interactions. In this context, the spurious stresses are found in a closed form for an arbitrary elastic anisotropy, and this is achieved without using the stress field of an individual dislocation. An alternative procedure is also developed where the macroscopic spurious stresses are determined using the solution of the Eshelby's inclusion problem. Keywords: dislocation dynamics, conditional convergence, micromechanics, Eshelby’s inclusion problem | | 35p. |
16. | Stupkiewicz S., Lewandowski M.J., Lengiewicz J., Micromechanical analysis of friction anisotropy in rough elastic contacts, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2014.07.013, Vol.51, No.23-24, pp.3931-3943, 2014Stupkiewicz S., Lewandowski M.J., Lengiewicz J., Micromechanical analysis of friction anisotropy in rough elastic contacts, INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES, ISSN: 0020-7683, DOI: 10.1016/j.ijsolstr.2014.07.013, Vol.51, No.23-24, pp.3931-3943, 2014Abstract: Computational contact homogenization approach is applied to study friction anisotropy resulting from asperity interaction in elastic contacts. Contact of rough surfaces with anisotropic roughness is considered with asperity contact at the micro scale being governed by the isotropic Coulomb friction model. Application of a micro-to-macro scale transition scheme yields a macroscopic friction model with orientation- and pressure-dependent macroscopic friction coefficient. The macroscopic slip rule is found to exhibit a weak non-associativity in the tangential plane, although the slip rule at the microscale is associated in the tangential plane. Counterintuitive effects are observed for compressible materials, in particular, for auxetic materials. Keywords: Contact, Friction, Anisotropy, Roughness, Micromechanics | | 35p. |
17. | Lengiewicz J., Wichrowski M., Stupkiewicz S., Mixed formulation and finite element treatment of the mass-conserving cavitation model, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2013.12.012, Vol.72, pp.143-155, 2014Lengiewicz J., Wichrowski M., Stupkiewicz S., Mixed formulation and finite element treatment of the mass-conserving cavitation model, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2013.12.012, Vol.72, pp.143-155, 2014Abstract: A mixed formulation of the mass-conserving cavitation model is developed. The cavitation problem is formulated in terms of the hydrodynamic pressure and a complementary variable representing the void fraction in the cavitation zone. Weak form of the mass-balance equation is consistently derived, and it exhibits subtle differences with respect to the available formulations. Finite element treatment preserves the two-field formulation, and a semi-smooth Newton method is applied to solve the resulting discretized equations. A monolithic Newton-based scheme is also applied to solve the fully coupled elastohydrodynamic lubrication problem in the soft-EHL regime. Numerical examples illustrate the performance of the computational scheme. Keywords: Lubrication, Cavitation, Reynolds equation, Soft-EHL problem | | 35p. |
18. | Zhao Y., Chen X., Park C., Fay C.C., Stupkiewicz S., Ke C., Mechanical deformations of boron nitride nanotubes in crossed junctions, JOURNAL OF APPLIED PHYSICS, ISSN: 0021-8979, DOI: 10.1063/1.4872238, Vol.115, pp.164305-1-9, 2014Zhao Y., Chen X., Park C., Fay C.C., Stupkiewicz S., Ke C., Mechanical deformations of boron nitride nanotubes in crossed junctions, JOURNAL OF APPLIED PHYSICS, ISSN: 0021-8979, DOI: 10.1063/1.4872238, Vol.115, pp.164305-1-9, 2014Abstract: We present a study of the mechanical deformations of boron nitride nanotubes (BNNTs) in crossed junctions. The structure and deformation of the crossed tubes in the junction are characterized by using atomic force microscopy. Our results show that the total tube heights are reduced by 20%–33% at the crossed junctions formed by double-walled BNNTs with outer diameters in the range of 2.21–4.67 nm. The measured tube height reduction is found to be in a nearly linear relationship with the summation of the outer diameters of the two tubes forming the junction. The contact force between the two tubes in the junction is estimated based on contact mechanics theories and found to be within the range of 4.2–7.6 nN. The Young's modulus of BNNTs and their binding strengths with the substrate are quantified, based on the deformation profile of the upper tube in the junction, and are found to be 1.07 ± 0.11 TPa and 0.18–0.29 nJ/m, respectively. Finally, we perform finite element simulations on the mechanical deformations of the crossed BNNT junctions. The numerical simulation results are consistent with both the experimental measurements and the analytical analysis. The results reported in this paper contribute to a better understanding of the structural and mechanical properties of BNNTs and to the pursuit of their applications. Keywords: Atomic force microscopy, Elastic moduli, Nanotubes, Carbon nanotubes, Finite element methods | | 30p. |
19. | Stupkiewicz S., An ALE formulation for implicit time integration of quasi-steady-state wear problems, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2013.03.023, Vol.260, pp.130-142, 2013Stupkiewicz S., An ALE formulation for implicit time integration of quasi-steady-state wear problems, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2013.03.023, Vol.260, pp.130-142, 2013Abstract: A fully coupled implicit scheme is developed for quasi-steady-state wear problems. The formulation admits finite configuration changes due to both deformation and wear. The unconditionally stable implicit backward-Euler scheme is used for time integration of the shape evolution problem. Thus, the solution may proceed with large time increments, contrary to the commonly used explicit forward-Euler scheme, in which the time increment is restricted by the stability condition. This comes at the cost that the shape transformation mapping constitutes an additional unknown. As a result, a kind of an arbitrary Lagrangian–Eulerian (ALE) formulation is obtained in which the problem is solved simultaneously for the nodal positions and displacements. The incremental coupled problem is solved using the Newton method which leads to a highly efficient computational scheme, as illustrated by two- and three-dimensional numerical examples. Keywords: Contact, Wear, Shape evolution, Arbitrary Lagrangian–Eulerian formulation, Automatic differentiation | | 45p. |
20. | Stupkiewicz S., Petryk H., A robust model of pseudoelasticity in shape memory alloys, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.4405, Vol.93, No.7, pp.747-769, 2013Stupkiewicz S., Petryk H., A robust model of pseudoelasticity in shape memory alloys, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.4405, Vol.93, No.7, pp.747-769, 2013Abstract: A model of pseudoelasticity in shape memory alloys is developed within the incremental energy minimization framework. Three constitutive functions are involved: the Helmholtz free energy and rate-independent dissipation that enter incrementally the minimized energy function, and the constraint function that defines the limit transformation strains. The proposed implementation is based on a unified augmented Lagrangian treatment of both the constitutive constraints and nonsmooth dissipation function. A methodology for easy reformulation of the model from the small-strain to finite-deformation regime is presented. Finite element computations demonstrate robustness of the finite-strain version of the model and illustrate the effects of tension–compression asymmetry and transversal isotropy of the surface of limit transformation strains. Keywords: shape memory alloys (SMA), phase transformation, energy methods, finite element method, augmented Lagrangian method | | 40p. |
21. | Lengiewicz J., Stupkiewicz S., Efficient model of evolution of wear in quasi-steady-state sliding contacts, WEAR, ISSN: 0043-1648, DOI: 10.1016/j.wear.2013.03.051, Vol.303, pp.611-621, 2013Lengiewicz J., Stupkiewicz S., Efficient model of evolution of wear in quasi-steady-state sliding contacts, WEAR, ISSN: 0043-1648, DOI: 10.1016/j.wear.2013.03.051, Vol.303, pp.611-621, 2013Abstract: A computationally efficient model of evolution of contact and wear is developed for a general periodic pin-on-flat problem with the focus on the pin-on-disc configuration and Archard wear model. The evolving contact state is assumed to be fully controlled by the wear process except during a short initial transient period controlled by both wear and elasticity. The contact pressure distribution is thus obtained by considering only the local wear model and the geometry of the conforming contact, without referring to the underlying elasticity problem. Evolution of the contact state is then obtained by time integration of the resulting rate-problem, and two computational schemes are developed for that purpose employing either the forward- or the backward-Euler method. The model is successfully verified against a three-dimensional finite element model. A dimensionless wear-mode index specifying the relative magnitude of wear coefficients of the contact pair is introduced, and model predictions are presented as a function of this parameter. Keywords: Contact mechanics, Wear, Simulation, Quasi-steady-state process, Rigid-wear model, Pin-on-disc | | 35p. |
22. | Kowalczyk-Gajewska K., Stupkiewicz S., Modelling of Texture Evolution in Kobo Extrusion Process, ARCHIVES OF METALLURGY AND MATERIALS, ISSN: 1733-3490, DOI: 10.2478/v10172-012-0160-y, Vol.58, No.1, pp.113-118, 2013Kowalczyk-Gajewska K., Stupkiewicz S., Modelling of Texture Evolution in Kobo Extrusion Process, ARCHIVES OF METALLURGY AND MATERIALS, ISSN: 1733-3490, DOI: 10.2478/v10172-012-0160-y, Vol.58, No.1, pp.113-118, 2013Abstract: The paper is aimed at modelling of evolution of crystallographic texture in KOBO extrusion which is an unconventional process of extrusion assisted by cyclic torsion. The analysis comprises two steps. In the first step, the kinematics of the KOBO extrusion process is determined using the finite element method. A simplifying assumption is adopted that the material flow is not significantly affected by plastic hardening, and thus a rigid-viscoplastic material model with no hardening is used. In the second step, evolution of crystallographic texture is modelled along the trajectories obtained in the first step. A micromechanical model of texture evolution is used that combines the crystal plasticity model with a self-consistent grain-to-polycrystal scale transition scheme, and the VPSC code is used for that purpose. Since each trajectory corresponds to a different deformation path, the resulting pole figures depend on the position along the radius of the extruded rod. Keywords: plasticity, microstructure, crystallographic texture, KOBO extrusion | | 20p. |
23. | Lengiewicz J., Stupkiewicz S., Continuum framework for finite element modelling of finite wear, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2010.12.020, Vol.205-208, pp.178-188, 2012Lengiewicz J., Stupkiewicz S., Continuum framework for finite element modelling of finite wear, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2010.12.020, Vol.205-208, pp.178-188, 2012Abstract: A finite deformation contact problem with friction and wear is studied in which the shape changes due to wear are finite. Accordingly, in addition to the initial configuration and the current configuration, an intermediate time-dependent configuration is introduced that corresponds to the undeformed body of the shape changed due to wear. Two time scales are also introduced in order to distinguish the fast time of the actual deformation (contact) problem from the slow time of the wear process (shape evolution problem). Separation of these time scales allows us to partially decouple the deformation problem and the shape evolution problem. Shape parameterization is introduced and the corresponding shape update scheme is formulated as a minimization problem. In particular, a second-order scheme is developed which exploits shape sensitivities of the deformation problem. Numerical examples are provided to illustrate the performance and accuracy of the proposed numerical schemes. Keywords: Frictional contact, Wear, Sensitivity analysis | | 40p. |
24. | Petryk H., Stupkiewicz S., Instability of equilibrium of evolving laminates in pseudo-elastic solids, INTERNATIONAL JOURNAL OF NON-LINEAR MECHANICS, ISSN: 0020-7462, DOI: 10.1016/j.ijnonlinmec.2011.07.005, Vol.47, pp.317-330, 2012Petryk H., Stupkiewicz S., Instability of equilibrium of evolving laminates in pseudo-elastic solids, INTERNATIONAL JOURNAL OF NON-LINEAR MECHANICS, ISSN: 0020-7462, DOI: 10.1016/j.ijnonlinmec.2011.07.005, Vol.47, pp.317-330, 2012Abstract: This study is concerned with isothermal stability of equilibrium of evolving laminated microstructures in pseudo-elastic solids with a multi-well free energy function. Several possible modes of instability associated with phase transition between energy wells are analysed. The related rate-independent dissipation is included by imposing a threshold value on the thermodynamic driving force. For a homogenized phase-transforming laminate with no length scale it is shown that localization instability is a rule in case of a non-zero interfacial jump of a directional nominal stress, irrespectively of actual boundary conditions. A stabilizing effect of elastic micro-strain energy at the boundary of the localization zone is demonstrated for laminates of finite spacing. Illustrative numerical examples are given for an evolving austenite–martensite laminate in a crystal of CuZnAl shape memory alloy. Keywords: Microstructures, Phase transformation, Laminates, Energy methods, Stability | | 35p. |
25. | Petryk H., Stupkiewicz S., Modelling of microstructural evolution on complex paths of large plastic deformation, INTERNATIONAL JOURNAL OF MATERIALS RESEARCH, ISSN: 1862-5282, DOI: 10.3139/146.110683, Vol.103, No.3, pp.271-277, 2012Petryk H., Stupkiewicz S., Modelling of microstructural evolution on complex paths of large plastic deformation, INTERNATIONAL JOURNAL OF MATERIALS RESEARCH, ISSN: 1862-5282, DOI: 10.3139/146.110683, Vol.103, No.3, pp.271-277, 2012Abstract: A procedure for the modelling of microstructural changes induced by non-uniform large plastic deformation of metals is developed. For a given plastic working process, a set of deformation paths for different initial locations of a material element is generated first for a non-hardening material by using the finite element method. Next, changes of a cellular microstructure and related hardening effects along each path are calculated by using a recently proposed model. The procedure is applied to a non-conventional process of cold extrusion assisted by cyclic rotation of the die. The evolution of microstructural parameters, their effect on strain hardening and a distribution map over the specimen cross-section are calculated. Keywords: non-uniform deformation, dislocation cells, grain refinement, hardening, quantitative prediction | | 30p. |
26. | Stupkiewicz S., Maciejewski G., Petryk H., Elastic micro-strain energy of austenite–martensite interface in NiTi, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, ISSN: 0965-0393, DOI: 10.1088/0965-0393/20/3/035001, Vol.20, pp.035001-9, 2012Stupkiewicz S., Maciejewski G., Petryk H., Elastic micro-strain energy of austenite–martensite interface in NiTi, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, ISSN: 0965-0393, DOI: 10.1088/0965-0393/20/3/035001, Vol.20, pp.035001-9, 2012Abstract: The interfacial energy due to elastic micro-strains at the austenite–twinned martensite interface is calculated for the NiTi shape memory alloy undergoing cubic-to-monoclinic B2 ↔ B19' transformation. For each crystallographically distinct microstructure, an energetically favourable local shape of the interface is determined. The approach employs finite element computations and energy minimization with respect to shape parameters, taking into account elastic anisotropy of the phases and finite-strain kinematics. The effect of atomic-scale interfacial energy is studied. Keywords: microstructure, martensitic phase transformation, shape memory alloys (SMA), interface structure, micromechanical modelling | | 30p. |
27. | Stupkiewicz S., Górzyńska-Lengiewicz A., Almost compatible X-microstructures in CuAlNi shape memory alloy, CONTINUUM MECHANICS AND THERMODYNAMICS, ISSN: 0935-1175, DOI: 10.1007/s00161-011-0222-9, Vol.24, pp.149-164, 2012Stupkiewicz S., Górzyńska-Lengiewicz A., Almost compatible X-microstructures in CuAlNi shape memory alloy, CONTINUUM MECHANICS AND THERMODYNAMICS, ISSN: 0935-1175, DOI: 10.1007/s00161-011-0222-9, Vol.24, pp.149-164, 2012Abstract: A systematic study of a specific martensitic microstructure, called the X-microstructure, is carried out with the focus on the CuAlNi shape memory alloy undergoing the cubic-to-orthorhombic transformation. The set of all crystallographically distinct candidate X-microstructures is determined, and it is shown that, according to the crystallographic theory of martensite, none of them is compatible. Almost compatible X-microstructures, which involve elastic strains, are thus examined. These microstructures are searched in the neighborhood of all candidate X-microstructures by minimizing the total elastic strain energy with respect to the microstructure parameters. Several low-energy X-microstructures are found, and it is shown that the total elastic strain energy correlates reasonably well with one of the indicators which characterize incompatibility of the corresponding candidate X-microstructure. Keywords: Microstructure, Martensitic phase transformation, Shape memory alloys (SMA), Energy minimization | | 20p. |
28. | Lengiewicz J., Korelc J., Stupkiewicz S., Automation of finite element formulations for large deformation contact problems, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.3009, Vol.85, pp.1252-1279, 2011Lengiewicz J., Korelc J., Stupkiewicz S., Automation of finite element formulations for large deformation contact problems, INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, ISSN: 0029-5981, DOI: 10.1002/nme.3009, Vol.85, pp.1252-1279, 2011Abstract: The aim of this paper is to present a general method for automation of finite element formulations of large deformation contact problems. A new automatic-differentiation-based notation is introduced that represents a bridge between the classical mathematical notation of contact mechanics and the actual computer implementation of contact finite elements. Automation of derivation of the required formulas (e.g. element residual and tangent matrix) combined with automatic code generation makes the finite element implementation possible at a moderate effort. Accordingly, several 3D contact formulations have been implemented in this work, including penalty and augmented Lagrangian treatments of contact constraints, and several contact smoothing techniques. A typical benchmark problem could thus be executed in an objective way leading to a comprehensive study of the efficiency and the accuracy of various formulations of 3D contact finite elements. Keywords: automatic differentiation, symbolic methods, automation, frictional contact, contact smoothing, augmented Lagrangian method | | 40p. |
29. | Stupkiewicz S., Lengiewicz J., Korelc J., Sensitivity analysis for frictional contact problems in the augmented Lagrangian formulation, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2010.03.021, Vol.199, No.33 - 36, pp.2165-2176, 2010Stupkiewicz S., Lengiewicz J., Korelc J., Sensitivity analysis for frictional contact problems in the augmented Lagrangian formulation, COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING, ISSN: 0045-7825, DOI: 10.1016/j.cma.2010.03.021, Vol.199, No.33 - 36, pp.2165-2176, 2010Abstract: Direct differentiation method of sensitivity analysis is developed for frictional contact problems. As a result of the augmented Lagrangian treatment of contact constraints, the direct problem is solved simultaneously for the displacements and Lagrange multipliers using the Newton method. The main purpose of the paper is to show that this formulation of the augmented Lagrangian method is particularly suitable for sensitivity analysis because the direct differentiation method leads to a non-iterative exact sensitivity problem to be solved at each time increment. The approach is applied to a general class of three-dimensional frictional contact problems, and numerical examples are provided involving large deformations, multibody contact interactions, and contact smoothing techniques. Keywords: Sensitivity analysis, Direct differentiation method, Frictional contact, Augmented Lagrangian method | | 32p. |
30. | Stupkiewicz S., Petryk H., A bi-crystal aggregate model of pseudoelastic behaviour of shape-memory alloy polycrystals, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, ISSN: 0020-7403, DOI: 10.1016/j.ijmecsci.2009.09.012, Vol.52, pp.219-228, 2010Stupkiewicz S., Petryk H., A bi-crystal aggregate model of pseudoelastic behaviour of shape-memory alloy polycrystals, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, ISSN: 0020-7403, DOI: 10.1016/j.ijmecsci.2009.09.012, Vol.52, pp.219-228, 2010Abstract: A multi-scale model of stress-induced phase transformation and martensite variant reorientation in shape memory alloy (SMA) polycrystals is developed. It is proposed to include neighbouring-grain interaction in a simple manner by introducing an intermediate bi-crystal level into the sequential averaging scheme for SMA. The constitutive relationships are defined by specifying the free energy and dissipation functions. At the level of a single grain, the rate-independent dissipation function is used that incorporates the dissipation due to forward and reverse austenite-to-martensite transformation as well as reorientation of martensite variants. The global response of the model is simulated numerically by minimizing the total incremental energy supply. Specific examples are calculated for a NiTi polycrystal for proportional and non-proportional loading paths. Keywords: Phase transformation, Dissipation, Microstructure, Multi-scale model, Incremental energy minimization | | 32p. |
31. | Petryk H., Stupkiewicz S., Interfacial energy and dissipation in martensitic phase transformations. Part I: Theory, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, ISSN: 0022-5096, DOI: 10.1016/j.jmps.2009.11.004, Vol.58, pp.390-408, 2010Petryk H., Stupkiewicz S., Interfacial energy and dissipation in martensitic phase transformations. Part I: Theory, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, ISSN: 0022-5096, DOI: 10.1016/j.jmps.2009.11.004, Vol.58, pp.390-408, 2010Abstract: This paper is a continuation of the Part I (H. Petryk, S. Stupkiewicz, Interfacial energy and dissipation in martensitic phase transformations. Part I: Theory. J. Mech. Phys. Solids, 2010, doi:10.1016/j.jmps.2009.11.003). A fully three-dimensional model of an evolving martensitic microstructure is examined, taking into account size effects due to the interfacial energy and also dissipation related to annihilation of interfaces. The elastic micro-strain energy at microstructured interfaces is determined with the help of finite element computations and is approximated analytically. Three interface levels are examined: of grain boundaries attained by parallel martensite plates, of interfaces between austenite and twinned martensite, and of twin interfaces within the martensite phase. Minimization of the incremental energy supply, being the sum of the increments in the free energy and dissipation of the bulk and interfacial type at all levels, is used as the evolution rule, based on the theory presented in Part I. An example of the formation and evolution of a rank-three laminated microstructure of finite characteristic dimensions in a pseudoelastic CuAlNi shape memory alloy is examined quantitatively. Keywords: Microstructures, Phase transformation, Grain boundaries, Energy methods, Shape memory alloys (SMA) | | 32p. |
32. | Petryk H., Stupkiewicz S., Maciejewski G., Interfacial energy and dissipation in martensitic phase transformations. Part II: Size effects in pseudoelasticity, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, ISSN: 0022-5096, DOI: 10.1016/j.jmps.2009.11.003, Vol.58, pp.373-389, 2010Petryk H., Stupkiewicz S., Maciejewski G., Interfacial energy and dissipation in martensitic phase transformations. Part II: Size effects in pseudoelasticity, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, ISSN: 0022-5096, DOI: 10.1016/j.jmps.2009.11.003, Vol.58, pp.373-389, 2010Abstract: A model of evolving martensitic microstructures is formulated that incorporates the interfacial energy and dissipation on three different scales corresponding to the grain boundaries attained by martensite plates, the interfaces between austenite and martensite plates, and the twin interfaces within martensite plates. Three different time scales are also considered in order to clarify the meaning of rate-independent dissipation related to instabilities at more refined temporal and spatial scales. On the slowest time scale, the process of deformation and martensitic phase transformation is investigated as being composed of segments of smooth quasi-static evolution separated by sudden jumps associated with creation or annihilation of interfaces. A general evolution rule is used in the form of minimization of the incremental energy supply to the whole compound thermodynamic system, including the rate-independent dissipation. Close relationship is shown between the evolution rule and the thermodynamic condition for stability of equilibrium, with no a priori assumption on convexity of the dissipation function. It is demonstrated that the extension of the minimum principle from the first-order rates to small but finite increments requires a separate symmetry restriction imposed on the state derivative of the dissipation function. Formulae for the dissipation associated with annihilation of interfaces are proposed that exhibit limited path-independence and satisfy that symmetry requirement. By exploiting the incremental energy minimization rule with the help of the transport theorems, local propagation conditions are derived for both planar and curved phase transformation fronts. The theory serves as a basis for the algorithm for calculation of the stress-induced evolution of martensitic microstructures along with their characteristic dimensions and related hysteresis loops in shape memory alloys; the examples are given in Part II of the paper. Keywords: Microstructures, Phase transformation, Grain boundaries, Energy methods, Stability | | 32p. |
33. | Sadowski P., Stupkiewicz S., Combined effect of friction and macroscopic deformation on asperity flattening, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2010.04.011, Vol.43, pp.1735-1741, 2010Sadowski P., Stupkiewicz S., Combined effect of friction and macroscopic deformation on asperity flattening, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2010.04.011, Vol.43, pp.1735-1741, 2010Abstract: The combined effect of friction and macroscopic plastic deformation on asperity flattening is studied. Crushing of a periodic array of wedge-like asperities is formulated as a rigid-viscoplastic periodic indentation problem with superimposed macroscopic deformation. A micromechanical framework is developed and the corresponding boundary value problem is solved using the finite element method. An anomalous regime of asperity flattening is predicted at low flattening rates, in which the effect of friction on asperity flattening is opposite to that observed in the absence of macroscopic deformation and also at high flattening rates. An incremental elastoplastic analysis confirms this finding. Keywords: Asperity flattening, Real contact area, Surface layer, Metal forming | | 32p. |
34. | Sadowski P., Stupkiewicz S., A model of thermal contact conductance at high real contact area fractions, WEAR, ISSN: 0043-1648, Vol.268, pp.77-85, 2010Sadowski P., Stupkiewicz S., A model of thermal contact conductance at high real contact area fractions, WEAR, ISSN: 0043-1648, Vol.268, pp.77-85, 2010Abstract: Thermal contact conductance (TCC) is studied in the whole range of real contact area fractions between zero and unity. For this purpose, a two-scale model is developed in which the effective (macroscopic) TCC coefficient is obtained from the solution of the heat conduction problem at the scale of asperities. Additional thermal resistance at the real contact spots is included in the model. The model is applied for several real 3D roughness topographies for which the effective TCC coefficient is determined as a function of the real contact area fraction and the local TCC coefficient at real contact spots. An analytical function is found which approximates this relationship in the whole range of parameters, and a characteristic length-scale parameter is introduced which characterizes the effective TCC properties of a rough surface. Keywords: Roughness topography, Real contact area, Thermal contact conductance, Characteristic length | | 32p. |
35. | Stupkiewicz S., Petryk H., Grain-size effect in micromechanical modelling of hysteresis in shape memory alloys, ZAMM-ZEITSCHRIFT FUR ANGEWANDTE MATHEMATIK UND MECHANIK, ISSN: 0044-2267, DOI: 10.1002/zamm.201000008, Vol.90, pp.783-795, 2010Stupkiewicz S., Petryk H., Grain-size effect in micromechanical modelling of hysteresis in shape memory alloys, ZAMM-ZEITSCHRIFT FUR ANGEWANDTE MATHEMATIK UND MECHANIK, ISSN: 0044-2267, DOI: 10.1002/zamm.201000008, Vol.90, pp.783-795, 2010Abstract: Size effects in pseudoelastic polycrystalline shape memory alloys are studied by considering a representative spherical laminated domain (subgrain) and its interfacial energy at three scales: at the subgrain boundaries, at the austenite–martensite interfaces, and at the twin boundaries. Two sources of interfacial energy are accounted for, namely the atomic-scale energy of twin and phase boundaries and the elastic strain energy at microstructured interfaces, the latter being predicted theoretically. The evolution of microstructure of the representative domain is determined using the incremental energy minimization rule applied to the sum of the increments in the Helmholtz free energy and rate-independent dissipation. The size-dependent part of dissipation is estimated by assuming that negative increments in interfacial energy, associated with annihilation of interfaces, cannot be reverted back into the bulk free energy and are thus dissipated. Simple analytic formula for the interfacial energy dissipated in a complete forward-reverse transformation cycle is derived and combined with a micromechanical model of a polycrystalline NiTi shape memory alloy. A numerical example illustrating size-dependent hysteresis in the stress-induced martensitic transformation is presented. Keywords: interfacial energy, incremental energy minimization, dissipation, martensitic transformation | | 20p. |
36. | Petryk H., Stupkiewicz S., Energia powierzchniowa, dyssypacja i efekty skali w modelowaniu mikrostruktur martenzytycznych, Czasopismo Techniczne. Mechanika, ISSN: 0011-4561, Vol.107, No.20, pp.99-108, 2010Petryk H., Stupkiewicz S., Energia powierzchniowa, dyssypacja i efekty skali w modelowaniu mikrostruktur martenzytycznych, Czasopismo Techniczne. Mechanika, ISSN: 0011-4561, Vol.107, No.20, pp.99-108, 2010Abstract: W niniejszym artykule przedstawiono energetyczne podejście do wieloskalowego modelowania ewolucji mikrostruktur martenzytycznych w stopach z pamięcią kształtu. Energia swobodna Helmholtza i energia dyssypowana w układzie reprezentowane są przez sumy członów odpowiadających energii objętościowej oraz energii powierzchniowej na granicach mikro-strukturalnych pomiędzy poszczególnymi wariantami martenzytu, fazami lub ziarnami. Ewolucja mikrostruktury jest wyznaczana drogą przyrostowej minimalizacji całkowitej energii dostarczanej do rozpatrywanego układu w procesie makroskopowo quasi-statycznym i izotermicznym. Ogólną procedurę zastosowano do numerycznych symulacji powstawania i ewolucji warstwowych struktur martenzytycznych indukowanych naprężeniowo w stopach z pamięcią kształtu. W energii powierzchniowej uwzględniono energię mikroodkształceń sprężystych w otoczeniu granic mikrostrukturalnych, wyznaczoną przy użyciu metody elementów skończonych. Policzone przykłady opisują ewolucję mikrostruktury martenzytycznej w formie laminatu trzeciego rzędu w stopie CuAlNi dla przemiany β1→γ1′ oraz jej zależność od sposobu uwzględnienia dyssypacji energii powierzchniowej. Keywords: energia powierzchniowa, minimalizacja energii, przemiana martenzytyczna | | 6p. |
37. | Stupkiewicz S., Finite element treatment of soft elastohydrodynamic lubrication problems in the finite deformation regime, COMPUTATIONAL MECHANICS, ISSN: 0178-7675, DOI: 10.1007/s00466-009-0394-3, Vol.44, pp.605-619, 2009Stupkiewicz S., Finite element treatment of soft elastohydrodynamic lubrication problems in the finite deformation regime, COMPUTATIONAL MECHANICS, ISSN: 0178-7675, DOI: 10.1007/s00466-009-0394-3, Vol.44, pp.605-619, 2009Abstract: Soft elastohydrodynamic lubrication (EHL) problem is studied for a reciprocating elastomeric seal with full account of finite configuration changes. The fluid part is described by the Reynolds equation which is formulated on the deformed boundary of the seal treated as a hyperelastic body. The paper is concerned with the finite element (FE) treatment of this soft EHL problem. Displacement-based FE discretization is applied for the solid part. The Reynolds equation is discretized using the FE method or, alternatively, the discontinuous Galerkin method, both employing higher-order interpolation of pressure. The performance of both methods is assessed by studying convergence and stability of the solution for a benchmark problem of an O-ring seal. It is shown that the solution may exhibit spurious oscillations which occur in severe lubrication conditions. Mesh refinement results in reduction of these oscillations, while increasing the pressure interpolation order or application of the discontinuous Galerkin method does not help significantly. Keywords: Contact, Elastohydrodynamic lubrication, Finite element method, Discontinuous Galerkin method, Elastomeric seal | | 32p. |
38. | Stupkiewicz S., Marciniszyn A., Elastohydrodynamic lubrication and finite configuration changes in reciprocating elastomeric seals, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2008.08.008, Vol.42, pp.615-627, 2009Stupkiewicz S., Marciniszyn A., Elastohydrodynamic lubrication and finite configuration changes in reciprocating elastomeric seals, TRIBOLOGY INTERNATIONAL, ISSN: 0301-679X, DOI: 10.1016/j.triboint.2008.08.008, Vol.42, pp.615-627, 2009Abstract: A computational framework has been developed for a fully coupled analysis of elastohydrodynamic lubrication and finite deformations of elastomeric reciprocating seals in hydraulic actuators. The relevant formulation is provided, which consistently treats finite configuration changes of the seal modelled as a hyperelastic (Mooney-Rivlin) solid. The steady-state hydrodynamic lubrication is modelled using the classical Reynolds equation. Coupling of the solid and fluid parts is fully accounted for, including friction due to shear stresses in the lubricant film. Detailed results of finite element simulations are provided for two benchmark problems of O-ring and rectangular rod seals in a wide range of process parameters. Keywords: Elastohydrodynamic lubrication, Hyperelastic model, Dynamic sealing | | 32p. |
39. | Petryk H., Stupkiewicz S., Kuziak R., Grain refinement and strain hardening in IF steel during multi-axis compression: Experiment and modelling, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, ISSN: 0924-0136, DOI: 10.1016/j.jmatprotec.2007.11.068, Vol.204, pp.255-263, 2008Petryk H., Stupkiewicz S., Kuziak R., Grain refinement and strain hardening in IF steel during multi-axis compression: Experiment and modelling, JOURNAL OF MATERIALS PROCESSING TECHNOLOGY, ISSN: 0924-0136, DOI: 10.1016/j.jmatprotec.2007.11.068, Vol.204, pp.255-263, 2008Abstract: The effect of severe plastic deformation (SPD) during cyclic multi-axis compression on grain refinement and strain hardening in interstitial free (IF) steel is studied quantitatively. In the experimental part, the material samples were cold deformed in the MAXStrain(R) system by successive compression in two mutually orthogonal directions. The electron backscatter diffraction (EBSD) technique was used to measure the average spacing of the dislocation cell (low angle) and cell-block (high angle) boundaries. In the modelling part, the decrease in size of dislocation cells and cell blocks was expressed in terms of the effective plastic strain defined such that strain-rate reversals slow down its accumulation. The strengthening effect of microstructural evolution was included in the continuum mechanics framework of finite strain plasticity. Examples of simulation of the behaviour of IF steel severely deformed by multi-axis compression are calculated and compared to experimental data. Keywords: Modelling, Microstructure, Hardening, Severe plastic deformation | | 27p. |
40. | Stupkiewicz S., Maciejewski G., Petryk H., Low-energy morphology of the interface layer between austenite and twinned martensite, ACTA MATERIALIA, ISSN: 1359-6454, DOI: 10.1016/j.actamat.2007.07.034, Vol.55, No.18, pp.6292-6306, 2007Stupkiewicz S., Maciejewski G., Petryk H., Low-energy morphology of the interface layer between austenite and twinned martensite, ACTA MATERIALIA, ISSN: 1359-6454, DOI: 10.1016/j.actamat.2007.07.034, Vol.55, No.18, pp.6292-6306, 2007Abstract: A micromechanical scheme is developed for predicting the morphology and interfacial energy of the interface layer between the parent phase and internally twinned martensite. Low-energy morphologies are determined by minimizing, with respect to shape parameters, the elastic microstrain energy associated with local incompatibility of transformation strains. The computational scheme involves a finite element solution to a problem of non-linear elasticity with eigenstrains, shape sensitivity analysis with respect to general shape parametrization and minimization employing a gradient-based algorithm. As an application, low-energy morphologies are studied for the austenite–martensite interface in the cubic-to-orthorhombic transformation in a CuAlNi shape memory alloy. Discussion of the results of the analysis includes comparison to alternative simplified methods in terms of the predicted morphologies and the corresponding interfacial energies. Keywords: Microstructure, Martensitic phase transformation, Shape memory alloys (SMA), Interface structure, Micromechanical modelling | | 32p. |
41. | Petryk H., Stupkiewicz S., A quantitative model of grain refinement and strain hardening during severe plastic deformation, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ISSN: 0921-5093, DOI: 10.1016/j.msea.2006.08.076, Vol.444, pp.214-219, 2007Petryk H., Stupkiewicz S., A quantitative model of grain refinement and strain hardening during severe plastic deformation, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ISSN: 0921-5093, DOI: 10.1016/j.msea.2006.08.076, Vol.444, pp.214-219, 2007Abstract: The effect of severe plastic deformation (SPD) on grain refinement and strain hardening in polycrystalline metals is studied quantitatively. The decrease in size of dislocation cells and cell-blocks is expressed as a function of the effective plastic strain influenced by strain-rate reversals. The estimated growth of the high-angle boundary area fraction depends on the complexity of the three-dimensional deformation path. The strain hardening due to both dislocation and boundary strengthening is described in terms of microstructural parameters and incorporated in the continuum mechanics framework of finite strain plasticity. The proposed model provides a tool for quantitative comparison of different SPD processes. Examples of simulation of the behaviour of pure aluminium deformed by equal channel angular pressing (ECAP) and cyclic extrusion–compression are calculated. Keywords: Modelling, Microstructure, Severe plastic deformation, Ultra-fine grained materials | | 27p. |
42. | Richert M., Petryk H., Stupkiewicz S., Grain refinement in AlMgSi alloy during cyclic extrusion-compression: Experiment and modelling, ARCHIVES OF METALLURGY AND MATERIALS, ISSN: 1733-3490, Vol.52, pp.49-54, 2007Richert M., Petryk H., Stupkiewicz S., Grain refinement in AlMgSi alloy during cyclic extrusion-compression: Experiment and modelling, ARCHIVES OF METALLURGY AND MATERIALS, ISSN: 1733-3490, Vol.52, pp.49-54, 2007Abstract: The effect of severe plastic deformation (SPD) during cyclic extrusion-compression (CEC) on grain refinement and strain hardening in AlMgSi alloy is studied quantitatively. New experimental results are presented showing that the average microband thickness and grain size decrease below 100 nm, i.e. a nanocrystalline material is obtained. In the modelling part, the decrease in size of dislocation cells and microbands is expressed in terms of the effective plastic strain defined such that strain rate reversals slow down its accumulation. Examples of simulation of the behaviour of AlMgSi alloy severely deformed by cyclic extrusion-compression are calculated and compared to experimental data. Keywords: Modelling, Microstructure, Hardening, Severe plastic deformation | | 20p. |
43. | Stupkiewicz S., Petryk H., Finite-strain micromechanical model of stress-induced martensitic transformations in shape memory alloys, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ISSN: 0921-5093, DOI: 10.1016/j.msea.2006.01.112, Vol.438-440, pp.126-130, 2006Stupkiewicz S., Petryk H., Finite-strain micromechanical model of stress-induced martensitic transformations in shape memory alloys, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, ISSN: 0921-5093, DOI: 10.1016/j.msea.2006.01.112, Vol.438-440, pp.126-130, 2006Abstract: A micromechanical model of stress-induced martensitic transformation in single crystals of shape memory alloys is developed. This model is a finite-strain counterpart to the approach presented recently in the small-strain setting [S. Stupkiewicz, H. Petryk, J. Mech. Phys. Solids 50 (2002) 2303–2331]. The stress-induced transformation is assumed to proceed by the formation and growth of parallel martensite plates within the austenite matrix. Propagation of phase transformation fronts is governed by a rate-independent thermodynamic criterion with a threshold value for the thermodynamic driving force, including in this way the intrinsic dissipation due to phase transition. This criterion selects the initial microstructure at the onset of transformation and governs the evolution of the laminated microstructure at the macroscopic level. A multiplicative decomposition of the deformation gradient into elastic and transformation parts is assumed, with full account for the elastic anisotropy of the phases. The pseudoelastic behavior of Cu–Zn–Al single crystal in tension and compression is studied as an application of the model. Keywords: Microstructures, Laminates, Finite deformations, Micromechanics, Shape memory alloys | | 27p. |
44. | Maciejewski G., Stupkiewicz S., Petryk H., Elastic micro-strain energy at the austenite-twinned martensite interface, ARCHIVES OF MECHANICS, ISSN: 0373-2029, Vol.57, No.4, pp.277-297, 2005Maciejewski G., Stupkiewicz S., Petryk H., Elastic micro-strain energy at the austenite-twinned martensite interface, ARCHIVES OF MECHANICS, ISSN: 0373-2029, Vol.57, No.4, pp.277-297, 2005Abstract: A micromechanical scheme is developed for the analysis of elastic micro-strains induced by local incompatibilities at the austenite-twinned martensite interface. The aim of the paper is to estimate the elastic micro-strain energy which is an important factor in the formation of microstructures during the martensitic transformation. The finite deformation framework is applied, consistent with the crystallographic theory of martensite, and full account is taken for elastic anisotropy of the phases. As an example, the microstructures in the cubic-to-orthorhombic transformation in CuAlNi shape memory alloy are analyzed by the finite element method for the assumed class of zigzag shapes of the austenite-martensite interface at the micro-level. Finally, the effect of the interphase boundary energy on the microstructure of the transition layer is studied. Keywords: microstructure, martensitic phase transformation, shape memory alloys (SMA), interface structure, micromechanical modelling | | 27p. |