Institute of Fundamental Technological Research

The paper presents experimental analysis of relation between friction coefficient and contact pressure of MoS2 film deposited on Ti6Al4V substrate in contact with sapphire ball during reciprocating sliding motion. It is shown that the value of friction coefficient decreases with increasing contact pressure. A microscale modeling approach is next developed to mimic the experimental observations. Representative volume element is defined based on the actual topography of outer surface of MoS2 film. Assuming thermo-elastic material properties, the calculations on the asperity level are performed in two steps. Firstly, the mechanical contact between two surfaces is calculated. As a result, the relation between the global load and micro-stress distribution is obtained. Secondly, for a given stress load, thermal analysis is performed providing temperature fluctuation within simplified conical asperity. By assuming relation between friction coefficient and temperature on the microscale, it is possible to obtain macroscopic friction coefficient as a function of contact pressure. In the end, model results are compared with experimental data. The novel aspects of presented approach lie in the selection of three main factors on a micro-level defining macroscopic friction. They are actual surface topography, microscopic temperature and microscopic friction-temperature relation.

microscale modeling, friction coefficient, flash temperature, reciprocating motion test

The present work provides a formulation of a constitutive model for metals with the aim to simulate cyclic deformation under axial extension or compression assisted by cyclic torsional (or shearing) straining of specified amplitude and frequency. Such a mode of deformation was recently implemented in technological processes such as extrusion, forging and rolling, cf. Bochniak and Korbel (Eng Trans 47:351–367, 1999, J Mater Process Technol 134:120–134, 2003, Philos Mag 93:1883–1913, 2013, Mater Sci Technol 16:664–674, 2000). The constitutive model accounting for combined hardening (isotropic–kinematic) with both hardening and recovery effects is presented and calibrated for several materials: pure copper, aluminum alloy (2024), and austenitic steel. The experimental data are used to specify model parameters of materials tested, and next the cyclic response for different shear strain amplitudes is predicted and confronted with empirical data. The constitutive model is developed in order to simulate technological processes assisted by cyclic deformation

The effects commonly related with deformation caused by proportional and non-proportional loading types were identified experimentally. In the case of non-proportional cyclic loading along circular strain path the second order effects such as: phase shift between stress and strain signals was observed. An analysis of experimental data from tests under non-proportional cyclic loading along square strain path exhibited a significant reduction of stress independently on direction of deformation.

The paper also presents experimental results concerning evaluation of an influence of cyclic loading on stress variations during monotonic deformation carried out on the pure copper and X10CrMoVNb9-1 steel. All strain controlled tests were performed at room temperature using thin-walled tubular specimens. The experimental programme contained selected combinations of monotonic and cyclic loadings, i.e. the torsion-reverse-torsion cycles were superimposed on the monotonic tension. It is shown that such cycles associated with monotonic tension caused essential variations of tensile stress. For both materials, a significant decrease of the axial stress was visible. The effects observed during monotonic and cyclic loading combinations were theoretically described using the Mróz and Maciejewski model.

Proportional and non-proportional loadings, Hardening, Softening, Tensile stress, Strain energy, Yield surface, Modelling

Technological metal forming processes of extrusion, forging and rolling with imposed cyclic torsion or shear deformation are of engineering interest in view of their advantages with respect to monotonic loading processes. The present work is aimed at the analysis of an axisymmetric extrusion process assisted by cyclic torsion, which can be induced by a cyclically rotating die with a specified rotation amplitude and frequency. The material is assumed to be rigid-perfectly plastic and the upper-bound analysis for a kinematically admissible flow field is presented. The field parameters are specified from the condition of minimum plastic dissipation. The evolution of the extrusion force and torsional moment is studied and the corresponding plastic flow field is specified, with process control parameters such as the ratio of extrusion and rotation rates, and the amplitude of die rotation. For increasing rotation frequency, it is shown that localization of plastic flow occurs in the bottom zone close to the rotating die. The analysis allows for specification of process control parameters and their relation to extrusion force, torsional moment, and the associated plastic dissipation for varying reduction ratios.

Extrusion, Cyclic torsion, KOBO method, Upper-bound analysis, Process control parameters

Przedstawiono analizę procesu osiowosymetrycznego wyciskania metali przy nałożeniu cyklicznego skręcania wywoływa-nego matrycą ( metoda KOBO, [1÷4]). Założono uproszczony model sztywno-plastycznego materiału bez wzmocnienia i ki-nematycznie dopuszczalny mechanizm płynięcia. Z warunku równości mocy dyssypacji plastycznej i mocy sił zewnętrznych (siła osiowa i moment skręcający) określono wartość [...]

Two severe plastic deformation (SPD) processing techniques, namely equal-channel angular pressing (ECAP) and cyclic extrusion-compression (CEC), are investigated by using the finite element method. The major aspect examined is the non-uniformity of the accumulated, equivalent plastic strain after processing with the use of different shapes of the die. The quantitative effect of several parameters on the plastic flow is determined. It is found that the diameter ratio of the chambers and narrower channel in the CEC method, and also the inclination angle of connecting conical parts, can affect strongly the degree of strain non-uniformity. Comparison is made of distributions of equivalent strain after two passes of ECAP for two different routes and with two die profiles.

Technological metal forming processes of extrusion, forging and rolling with imposed cyclic torsion or shear deformation have been recently studied in view of their advantages with respect to monotonic loading processes, cf. Bochniak and Korbel [2–4]. The present work is aimed to analyze such process in the case of simple tension or compression of a cylindrical tube with imposed cyclic torsional deformation. The material element response is assumed to be rigid-perfectly plastic or elastic-perfectly plastic. For these models, the analytical solutions can be provided for the steady cyclic responses and the effect of two process parameters, namely the ratio of shear and axial strain rates η and the amplitude of shear strain γm, can be clearly demonstrated. Three different regimes of cyclic response can be visualized in the plane η, γm. The cyclic response of a cylinder under combined axial compression and cyclic torsion is predicted by considering a simplified model of a set of concentric tubes and neglecting their radial stress interaction. The axial force and torsional moment are then specified by averaging the responses of consecutive tubes. The cyclic response diagrams for the cylinder are then generated in terms of axial force and torsional moment related to axial deformation and angle of twist

cyclic torsion, plastic deformation of cylinders, elastic-perfectly plastic model, rigid-perfectly plastic model, analytical solutions of cyclic response of a cylinder

The damage state of a cracked material is assumed to be specified by crack density distribution on physical planes. The critical plane approach is used with account for damaged and intact area fractions on the plane. The maximum of failure function is specified for all potential failure planes and the critical plane orientation is determined. The resulting failure condition is applied to study strength evolution for triaxially compressed specimens with varying orientation of principal stress and damage tensor axes. Both Coulomb and non-linear failure conditions of Mohr type are applied to specify the representative critical plane. A general stress state is considered and the failure condition is specified for different relative orientations of orthotropy and principal stress axes.

The effects often taking place during proportional and non-proportional types of loading were identified experimentally and modelled numerically. In the case of non-proportional cyclic loading along circular strain path the second order effects such as: phase shift between stress and strain signals were discovered. An analysis of experimental data from tests under non-proportional cyclic loading along square strain path exhibited a significant reduction of stress independently on direction of deformation. The paper also presents experimental results concerning evaluation of an influence of cyclic loading on mechanical parameters during monotonic deformation carried out on the 2024 aluminium alloy and X10CrMoVNb9-1 steel. All strain-controlled tests were conducted at room temperature using thin-walled tubular specimens. The experimental programme contained selected combinations of monotonic and cyclic loadings, i.e. the torsion-reverse-torsion cycles were superimposed on the monotonic tension. It is shown that such cycles caused essential variations of the tension characteristics. A significant decrease of the tension stress was observed. The effects occured during monotonic and cyclic loading combinations were described using Mróz and Maciejewski model. Reasonable good predictions of: hysteresis loops and stress responses for the square and circular loading paths and tensile curves assisted by torsion cycles were obtained

cyclic loading, hardening, softening, complex stress state

The effects commonly taking place during proportional and non - proportional (circle and square) types of loading were identified experimentally. Experimental results of tests under cyclic loading assisted by monotonic deformation are compared to their predictions using the Mróz - Maciejewski model