Thermo-mechanical homogenisation – application to HVOF thermal-sprayed WC-Co coatings

Hard material coatings are widely employed as wear protection for highly engrossed surfaces. For example, coatings consisting of tungsten carbide (WC) and cobalt (Co) are used for sheet metal forming tools. A relatively cost-efficient coating technique is the high velocity oxygen fuel (HVOF) thermal spraying process which, as a trade-off, induces a large amount of energy into the heterogeneous coating and the substrate. Hence, this leads to a complex transient, thermomechanically coupled problem. In order to predict the residual stresses during the quenching procedure, a two scale finite element framework is established wherein the scale bridging is performed by application of two different homogenisation approaches. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Meso-Macro Modelling of Coated Forming Tools under Thermal Shock Conditions

In hybrid-forming processes workpieces are heated up before forming in order to reduce the forming forces. They are innovative methods for the production of components with graded properties, particularly with regard to tailored material properties and geometrical shape. During service life the forming tools are subjected to cyclic thermal shock loading conditions which can result into damage and failure. For improvement of the tool durability in the hybrid-forming process coated forming tools with multilayered coating systems are considered to be applied in future. This contribution shows the actual state of work for the development of a twoscale FE model for the simulation of the multilayered coated forming tool. Within this model the three-dimensional model of the forming tool builds the macromodel. On the macrolevel the multilayered coating is discretized with one element over the coating thickness. The mesomodel of the coating considers the actual layer design with metallic and ceramic layers. The macro-meso transition is realized with a Taylor-assumption. As the microscale is not considered in our model, the constitutive equations are formulated on the mesoscale. The meso-macro transition is done using volume averaging procedures. Furthermore, a damage model is included for particular layers. The scalar damage variable is used in a thermo-mechanical coupled model for simulation of a reduced heat transfer through a partially damaged layer. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simulation of a Hybrid-Forming Process using Temperature- and Rate-Dependent Viscoplastic Material Models

Hybrid-forming processes for graded structures are quite innovative methods for the production of components with tailored properties, particularly tailored material properties and geometrical shape. In this contribution a hybrid-forming process based on the utilization of locally varying thermo-mechanical effects is investigated [1]. For process optimization and improvement of the resulting work piece the simulation of the entire forming process is necessary in modern engineering. The main topics of this contribution are the simulation of the cyclic thermal loaded forming tool and the simulation of the work piece treated at large deformations with phase transformations. For both materials temperature- and rate-dependent viscoplastic material models are applied and parameter identification using cyclic tensile-compression tests for the forming tool material and phase transformation tests for a low-alloy steel similar to the work piece material is presented. For validation of finite-element-calculations for the forming tool thermal shock experiments are performed with optical deformation measurements. For validation of finite-element-calculations for the work piece numerical results of geometry and structure after heating, forming and cooling are compared to experimental micro sections. Results concerning the forming tool will be used for future lifetime prediction and results concerning the work piece will be used for future specific setting of graded material properties. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Residual stress on the run out table accounting for multiphase transitions and transformation induced plasticity

The development of harder and thinner new steel grades requires computationally efficient numerical simulations of forming processes in order to optimize industrial conditions through parametric studies. Within this general framework, the present contribution deals with one particular process, namely the run out table. Thus, this paper focuses on the evolution of residual stresses of thin strips during cooling on the run out table. Due to the fact that the complete problem is a nonlinear multiphysics process, it is known that simulating such processes with fully coupled numerical procedures leads to high computational costs. Therefore, a simplified numerical strategy has been developed. This procedure consists of three steps: (i) solving the thermal problem coupled with multiphase transitions; (ii) computing thermal expansion, metallurgical deformation and transformation induced plasticity and (iii) solving the associated mechanical problem. Residual stress profiles through the strip thickness are also computed in order to evaluate classic flatness defects such as crossbow and longbow. A post-processing is also included in order to quantify out of plane displacements that would take place if the strip was cut off the production line. The post-processing consists in computing at finite strain the relaxation of residual stresses when the tension applied by the coiler is released. The proposed numerical strategy has been tested on common industrial conditions.     

Mathematical modeling of the processes occurring during high-temperature spray coating

We give a generalization of the mathematical model of the processes occurring in the preparation of gasthermal coatings in order to compute the radiation energy, thermal effects of turbulent flow of the gas jet of a plasmotron, spraying distance and required degree of preliminary heating of the base. The boundary conditions obtained describe the radiational/convective heat exchange of bodies with a medium through thin coatings taking account of the speed of flow of the gas jet.Translated fromMatematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 33, 1991, pp. 13–18.     

Deformation and Damage Analysis of a Hybrid-Forming Tool under Thermal Shock Conditions

In the hybrid–forming process for gradient structures [1] inhomogeneous cyclic thermo–mechanical stresses and strains lead to higher risks of failure of the forming tool. The main topic of this paper is the validation of finite element calculations for a tool–like specimen under complex thermo–mechanical loadings in order to predict the material behaviour [3]. To this end thermal shock experiments of tool–like specimens are performed. Optical measuring systems are used for three–dimensional digitalisation of the specimens to get a sufficient amount of data. Results of experimental optical measurings and results of finite element calculations are compared. Additionally, damage analysis using the eddy current method is performed to characterize the surface state of the cyclically thermal shocked specimens. This damage analysis provides data for lifetime prediction models under thermal shock conditions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Availability of inspected systems subject to shocks – A matrix algorithmic approach

We examine the limiting average availability of a maintained system that deteriorates due to random shock process and as a response to its usage (wear out). System’s failures are not self-announcing, hence, failures must be detected via inspection. We consider randomly occurring shocks that arrive according to a Poisson process and cumulatively damage the system. Two models are considered: in Model 1 the shock and wear out processes are independent of the external environment and in Model 2, the shocks arrival rate, the shock magnitudes and the wear out rate are governed by a random environment which evolves as a Markov process. We obtain the system’s availability for both models.     

Modeling of the Wear Particle Flow in Tribological Contacts

The dynamics of tribological systems with pronounced memory effects are characterized by the complex coupling of mechanical, thermal, and chemical processes. At the Institute of Dynamics and Vibrations in Braunschweig, Cellular Automata have been developed that facilitate the evaluation of dynamics in technically relevant systems within a reasonable time. The flow of wear material is decisive for the formation, localization, and reconstitution of load-bearing structures or films. In this paper, two opposed frictional contacts are chosen exemplarily for the investigation of the wear particle flow: a vehicle brake system and the sanding of wood. For this purpose, a modeling technique is proposed that is based on established force laws for micro particles and differentiates mechanisms of transport for different phases of the particle movement. The resulting transport of a single particle interacting with the topology of the frictional gap is analyzed for varying particle properties and process parameters. In a second step the insights are transferred to a set of rules for a Cellular Automaton which allows for quick evaluation and the incorporation of coupled effects. The validity and quality of the model transfer are discussed. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diagnosis of tool wear based on regression analysis and fuzzy logic

^** Email: Erkki.Jantunen{at}vtt.fi Tool wear monitoring is important for a number of reasons. Automaticdiagnosis of tool wear enables the unmanned use of flexiblemanufacturing systems and machine tools. Besides, a worn toolif unnoticed could cause a lot of damage, i.e. the machinedproducts could be damaged and unfit for their planned use. Assuch the machining process is very challenging to monitor dueto various reasons. Tool type and cutting parameters may varyresulting in variation of the monitored parameters. Also, therecan be a lot of noise in the measured signals. The paper dealswith the use of regression analysis techniques together withfuzzy logic in order to overcome the challenges in tool wearmonitoring. Regression analysis, based on a higher order polynomialfunction that emphasizes the most recent measured data and hasa limited number of terms, can very well follow and give prognosisof the development of the monitored parameters from such signalsas vibration, sound and acoustic emission. The use of fuzzylogic makes it possible to automatically define limits for themonitored parameters and to combine the information from a numberof signals. The proposed approach is tested with data from drillingtests.     

Thermal Residual Stresses and Triaxiality Measures

High performance ceramics have found their way into many highly challenging engineering tasks. For example silicon nitride is one of the best choices, if a material for demanding applications like metal forming and cutting is required. Due to the brittle nature of these hard and strong materials it is useful to know about thermal residual stresses, which can arise during the sintering process. In order to gain insight into the material behaviour, a single grain inclusion is exposed to thermal loads. Due to thermal mismatch, it undergoes a residual stress and strain field. The geometry of the model and the material data are motivated by the properties of silicon nitride. The stress fields are analyzed by three different measures for stress triaxiality. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of optimal metal flow in incremental sheet forming

Sheet metal forming processes are manufacturing processes in which a piece of sheet metal is shaped to a specified geometry, e.g. a car door. A promising new forming process is incremental sheet metal forming, in which the deformation is imposed by a progressive, localised plastic deformation induced by a pin-like forming tool that moves under numerical control along a pre-defined trajectory. This process offers the possibility to control the metal flow by adjusting the trajectory of the forming tool. Mathematically, sheet metal forming processes can be considered as a mapping between the initial, undeformed sheet metal and the final, deformed state. In most applications the surface area of the sheet metal is enlarged during the deformation. In this case, an ideal mapping would produce a homogeneous stretching of the sheet metal such that the final sheet thickness is the same everywhere. In this work, we analyze the following question: for each point in the initial configuration, what must be its location on the final geometry such that the thickness is the same everywhere? We construct a special type of surface evolution that combines flow along the surface normal with appropriate tangential velocity corrections, and show that the flow yields a constant sheet thinning on a sheet metal. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Virtual process design on combined quasi-static and high-speed forming

Sheet metal forming processes play a key role in a vast number of manufacturing cycles. The pursuit of novel approaches and enhancements of process chains is limited by the tremendous cost of experimental investigations. Hence, virtual process design is a suitable tool to overcome this limitation and to investigate new aspects of the processes via computer aided design. This work focuses on the optimization of a class of triggering pulses utilized in combined quasi-static and electromagnetic high-speed forming that avoid reverse currents. As typical example double exponential pulses are treated. Non-linear, constrained optimization exploiting a LSDYNA simulation of the forming process is used to determine parameters defining a triggering current pulse, yielding an enhanced forming result, in terms of sharper drawing radii. A more detailed discussion of the method is presented in [1]. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wear resistance of bodies protected by a thin composite coating

The mechanical contact interaction of bodies with a thin composite coating is investigated with account of wear. The thermal effects are not considered. The coating is modeled by a thin plate. Between the body and the coating is an interlayer, which is modeled by a Winkler body with one modulus of subgrade reaction. Under the action of a rigid stamp on the coating, the process of abrasive wear proceeds. The contact interaction of the coating with the base is described by using the model of an intermediate layer. To determine the stress-strain state of the coating, equations of the generalized theory of plates including the shear strains and the compression of normal are utilized. For the contact wear problem formulated, the basic integral equation with a Fredholm-type kernel is derived, and its solution algorithm is proposed. Numerical results are presented. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 3, pp. 319–330, May–June, 2006.     

Optimal tool replacement for processes with low fraction defective

Tool wear is a frequent and natural part in many machining processes and is a systematic assignable cause. The fraction of defectives would rise as the tool deteriorates. When the fraction defective reaches a certain level, the tool must be replaced. To minimize the defective parts and the overall tool costs, the optimal tool replacement time needs to be determined. Process capability indices (PCIs) have been effectively used in the manufacturing industry to measure the fraction of defectives. Conventional methods of capability measurement become inaccurate since the process data is contaminated by the assignable cause variation. In order to determine the optimal tool replacement time to maintain maximum product quality, conventional capability calculation must be modified. Considering process capability changes dynamically, an estimator of C_pmk is investigated. We obtain an exact form of the sampling distribution in the presence of a systematic assignable cause. This study provides an effective management policy for optimal tool replacement under low fraction of defectives. To illustrate the application of this procedure, a case study involving the tool wear problem is presented.     

Application and evaluation of hyper- and hypoelastic-based plasticity models in the FE simulation of metal forming processes

Metal forming processes are usually accompanied by large plastic strains and rotations of the material elements which emphasizes the need for reliable finite strain elastoplasticity models in corresponding FE simulations. In this work, two specific finite strain hyper- and hypoelastic-based plasticity models with combined nonlinear isotropic and kinematic hardening are presented and compared in numerical FE simulations. Although both models led to remarkably different results in a shear-dominated single element deformation test, the structural simulation of a standard deep drawing process delivered nearly congruent results which suggests that both models are equally well-suited for modeling metals in common forming processes. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wear and fluid-solid-thermal transient coupled model for journal bearings

In this study, the transient interactions between the sliding wear behaviour and fluid–solid–thermal (FST) characteristics of journal bearings are revealed using an established mathematical model. The calculated temperature distribution is validated by a comparison with experimental results from the literature. Furthermore, a wear test for lubricated journal bearings is conducted to verify the predicted wear rate. The time-varying wear and FST performances of the journal bearing, including the wear rate, wear depth, fluid pressure, contact pressure, and maximum temperature are calculated numerically. Through numerical simulations, the effects of the boundary friction coefficient and surface roughness on the wear and FST performances are evaluated. To demonstrate the importance of considering the three-dimensional (3D) thermal effect during the wear analysis of lubricated journal bearings, the numerical results predicted by the isothermal model and the thermal model are compared systematically within a wide range of operating conditions. The numerical results reveal that the worn surface profile slightly decreases the maximum temperature. Additionally, the worn region is primarily located at both edges of the bearing, and the time-varying worn surface profile may be beneficial for improving the hydrodynamic effect. Furthermore, the effect of the 3D thermal characteristics on the wear prediction of journal bearings cannot be ignored when the external load, boundary friction coefficient, surface roughness are relatively large.     

An Efficient Thermo-Kinematic (ETK) Model for Long-Time Simulation of High Speed Railway Brake Dynamics

High speed railway brakes transfer a large amount of kinetic energy into heat during the brake operation. Due to its design one major problem is tapered wear, which could significantly reduce brake performance and safety as well as increase aftermarket costs. Modeling and simulation of these brake systems is rather complicated with respect to the coupled multiphysics, multiscaled phenomena, friction and wear, especially on long time scale. Here, we present the so-called Efficient Thermo-Kinematic (ETK) model, which include the complexities of the multiphysics of the brake process in the efficient way. Using this model, we are able to proceed the simulation of the brake dynamics in long-time scale. The partial differential equations are derived based on the coupled manner. The simulation with ETK model shows reasonable results with very fast computational time. The ratio of computing time to real time can reach 1:2000 (with a standard personal computer). Thus, investigations on tapered wear can be performed not only in one brake operation but in many times of brake operations. The simulation results will support the development process of railway brakes in order to mitigate tapered wear of the brake pads. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Computational Homogenisation of Polycrystalline Elastoplastic Microstructures at Finite Deformation

During sheet bulk metal forming processes both, flat geometries and three-dimensional structures change their shape significantly while undergoing large plastic deformations. As for forming processes, FE-simulations are often done before in situ experiments, a very accurate material model is required, performing well for a huge variety of different geometrical characteristics. Because of the crystalline nature of metals, anisotropies have to be taken into account. Macroscopically observable plastic deformation is traced back to dislocations within considered slip systems in the crystals causing plastic anisotropy on the microscopic and the macroscopic level. A finite crystal plasticity model is used to model the behaviour of polycrystalline materials in representative volume elements (RVEs) of the microstructure. A multiplicative decomposition of the deformation gradient into elastic and plastic parts is performed, as well as a volumetric-deviatoric split of the elastic contribution. In order to circumvent singularities stemming from the linear dependency of the slip system vectors, a viscoplastic power-law is introduced providing the evolution of the plastic slips and slip resistances. The model is validated with experimental microstructural data under deformation. Through homogenisation and optimisation techniques, effective stress-strain curves are determined and can be compared to results from real forming processes leading to a suitable effective material model. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Computation of the thermal field in tools during machining processes

This paper deals with the modelling and computation of the thermal field inside turning tools during machining processes. The analysis is developed for the parabolic heat equation both in the transient and steady case as well as for inverse-type problems where some information on the solution is obtained from experimental data. Some numerical simulations are performed and a discussion on some research perspectives is proposed.     

Contributions to the theory of first‐exit times of some compound processes in queueing theory

We consider compound processes that are linear with constant slope between i.i.d. jumps at time points forming a renewal process. These processes are basic in queueing, dam and risk theory. For positive and for negative slope we derive the distribution of the first crossing time of a prespecified level. The related problem of busy periods of single‐server queueing systems is also studied. This revised version was published online in June 2006 with corrections to the Cover Date.