Press hardening of alternative materials: conventional high- strength steels

The increase in strength of new high strength steels (HHS) and advanced high strength steels (AHHS) has led to forming issues, such as high springback, low formability, increase of forming forces and tool wear. These problems increase thecosts of manufacturing and maintaining stamping tools in the automotive industry. The aim of this research was to analyse the advantages of applying the press- hardening process toconventional HSS and AHSS steel to increase their formability and therefore reduce the number of forming steps and productioncosts. With this aim in mind, the press-hardening process was used to manufacture an industrialcomponent using four different automotive steel grades: dual phase (DP),complex phase (CP), transformation- induced plasticity (TRIP) and martensitic (MS) grade. Springback measurements werecarried out, together with an analysis of the obtained final mechanical properties and microstructures. The results showed that the formability of all the materials increased. The mechanical properties of theCP800 and TRIP700 materials were maintained or even improved, whereas those of the MS1200 and HCT980X materials were significantly reduced. Weconclude that press hardening is a suitable manufacturing process forCP800 and TRIP700components.     

Investigation of die radius arc profile on wear behaviour in sheet metal processing of advanced high strength steels

Advanced high strength steels (AHSS) are increasingly used in sheet metal stamping in the automotive industry. In comparison with conventional steels, AHSS stampings produce higher contact pressures at the interface between draw die and sheet metal blank, resulting in more severe wear conditions, particularly at the draw die radius. Developing the ability to accurately predict and reduce the potential tool wear during the tool design stage is vital for shortening lead times and reducing production cost. This paper investigates the effects of draw die geometry on the sheet metal tool wear distribution over the draw die radius using numerical and experimental methods. A numerical tool wear model is introduced and applied using the commercial software package Abaqus. Channel bend tests are carried out using an Erichsen sheet metal tester to verify the numerical model. Various geometries of radius arc profiles, including standard circular profiles, high elliptical profiles, and flat elliptical profiles, are numerically investigated, and the wear volume and contact pressure distribution along the radii are determined. The results show that the profile of the draw die radius has a significant effect on the wear distribution, and that a low contact pressure distribution can be achieved by using a combination of circular and high elliptical curved geometries.     

A Review on Fretting Wear Mechanisms,Models and Numerical Analyses

Fretting wear is a material damage in contact surfaces due to micro relative displacement between them. It causes some general problems in industrial applications, such as loosening of fasteners or sticking in components supposed to move relative to each other. Fretting wear is a complicated problem involving material properties of tribo-system and working conditions of them. Due to these various factors, researchers have studied the process of fretting wear by experiments and numerical modelling methods. This paper reviews recent literature on the numerical modelling method of fretting wear. After a briefly introduction on the mechanism of fretting wear, numerical models, which are critical issues for fretting wear modelling, are reviewed. The paper is concluded by highlighting possible research topics for future work.     

Finite element analysis of a polymer composite subjected to a sliding steel asperity Part II: Parallel and anti-parallel fibre orientations

Finite element (FE) micro-models have been developed in order to determine contact, stress and strain conditions produced by a steel asperity sliding on the surface of a fibre-reinforced polymer composite. Two cases were studied, i.e. a parallel and an anti-parallel fibre orientation relative to the sliding direction. In order to get more realistic simulation results relating to the failure conditions in the composite structure, FE contact macro/micro-models were used, contrary to the so far widely applied anisotropic analytical or numerical macro-models. To model a micro-environment as part of a macro-environment, the displacement coupling technique was introduced. The contact analysis operates on both the macro- and the micro-level, applying node-to-node contact elements. The contact results, especially the contact pressure distribution, can characterize the real fibre/matrix micro-system. Displacement and strain results lead to explanations of fibre related phenomena, matrix shear effects, and fibre/matrix debonding events. On the basis of the stress results, conclusions were drawn on the possible wear mechanisms of the fibre-reinforced polymer composite. For parallel fibre orientation, fibre/matrix debonding as a result of shear stresses at the interface, matrix shear type failure and fibre thinning are the dominant sliding wear mechanisms. If an anti-parallel fibre orientation is considered, matrix shear, tension/compression type fibre/matrix debonding and fibre thinning, associated with fibre cracking events, are the most dominant wear mechanisms. To study the wear mechanisms experimentally, diamond tip scratch tests were carried out, showing that the predicted failure events occur also in reality.     

A boundary elements formulation for 3D fretting-wear problems

Computational wear modeling is an extremely time-consuming problem, especially the 3D cases. In this work, a 3D boundary element method (BEM) formulation for wear modeling is proposed and applied to simulate 3D fretting-wear problems under gross sliding and partial slip conditions. The present formulation applies the BEM to approximate the elastic response of solids, and an augmented Lagrangian formulation to solve the contact problem. Contact restrictions fulfilment is established by a set of projection functions, and wear on contact surfaces is computed using the Archard wear law. The BEM proves to be a very suitable numerical method for this kind of mechanical interaction problems, considering only the boundary degrees of freedom involved in the problem and obtaining a very good approximation of contact tractions with a low number of elements. This is very interesting in terms of computational cost reductions of wear modeling, specially in 3D problems. In that regard, an acceleration strategy is applied to the proposed algorithm. It allows to obtain very important reductions on wear simulation times. The proposed methodology is therefore an efficient numerical tool for 3D fretting-wear problems modeling.     

Thermo-mechanically coupled FE analysis and sensitivity study of simultaneous hot/cold forging process with local inductive heating and cooling

The numerical investigation of the production of a stub shaft is presented, where the highly innovative metal forming technology of the simultaneous hot and cold forging is applied in combination with a hardening process performed directly in the closed forging dies after the forging step similar to press hardening of sheet metal. This complex forging process is completely analysed by means of a finite element simulation including the local inductive heating phase of the workpiece as well as the cooling process of the final stub shaft inside the forging dies. All relevant process parameters and the whole simulation model are documented in detail and the simulation results are discussed and validated by means of experimentally measured data, showing good agreement. Parameter studies for various properties of the model are carried out in order to investigate their influence on the geometry and the temperature field development, whereby a deeper understanding of the entire process is gained. Thus, a finite element benchmark analysis is provided for such a complex thermo-mechanically coupled structuring process.     

A discrete element method for the simulation of CFRP cutting

Orthogonal machining of unidirectional carbon fiber-reinforced polymer (UD-CFRP) composites is simulated using discrete element method (DEM). The objective of this work is to present a simple numerical model that allows the study the machining of unidirectional composites during orthogonal cutting. To control the physicochemical phenomena that occur during cutting, it is necessary to identify the parameters of contact, very difficult to measure experimentally. The DEM numerical simulation is presented then as an alternative to the problem. This tool has helped to recreate the physical mechanisms identified experimentally and to understand the origin of the abrasive wear of carbide tools. The observation of the chip formation using a high speed video camera made possible to validate qualitatively the results of numerical simulation by discrete elements. This tool can also determine the cutting forces quite close to reality.     

Failure of discontinuous railhead edges due to plastic strain accumulation

Railhead is perhaps the highest stressed civil infrastructure due to the passage of heavily loaded wheels through a very small contact patch. The stresses at the contact patch cause yielding of the railhead material and wear. Many theories exist for the prediction of these mechanisms of continuous rails; this process in the discontinuous rails is relatively sparingly researched. Discontinuous railhead edges fail due to accumulating excessive plastic strains. Significant safety concern is widely reported as these edges form part of Insulated Rail Joints (IRJs) in the signalling track circuitry. Since Hertzian contact is not valid at a discontinuous edge, 3D finite element (3DFE) models of wheel contact at a railhead edge have been used in this research. Elastic–plastic material properties of the head hardened rail steel have been experimentally determined through uniaxial monotonic tension tests and incorporated into a FE model of a cylindrical specimen subject to cyclic tension loading. The parameters required for the Chaboche kinematic hardening model have been determined from the stabilised hysteresis loops of the cyclic load simulation and implemented into the 3DFE model. The 3DFE predictions of the plastic strain accumulation in the vicinity of the wheel contact at discontinuous railhead edges are shown to be affected by the contact due to passage of wheels rather than the magnitude of the loads the wheels carry. Therefore to eliminate this failure mechanism, modification to the contact patch is essential; reduction in wheel load cannot solve this problem.     

Finite element hydrodynamic friction model for metal forming

The complex contact conditions on the three-dimensional (3-D) tooling-workpiece interface, such as non-penetrations, slip–stick phenomena and friction forces due to the relative motion of contacting surfaces, are of vital importance in metal forming operations. Usually, a lubricant is provided as an interface medium between the tool and the workpiece to avoid strain localization, wear and surface damage. Hence, a simple friction law such as Coulomb friction, involving only a constant friction coefficient, cannot model the contact phenomena accurately. In this research, a realistic friction model, which accounts for the tribological behaviour, and most importantly, the effect of surface roughness on the lubricated contact, is developed. This model has been implemented in a 3-D arbitrary Lagrangian Fulerian finite element code for metal forming analysis. The applicability of the proposed model is demonstrated by the simulation of fluid-lubricated thrust bearing and sheet metal stretch forming.     

Numerical study on the effect of the thermo‐mechanical properties of alloys on the behaviour of super plastic forming tools

The paper describes a numerical study to investigate the effect of the thermo‐mechanical properties of heat resisting nickel and chromium alloys, used for super plastic forming (SPF) tools, on the tool service behaviour. The purpose of the paper is to rank the relative importance of each property studied for the heat resisting class of cast nickel and chromium alloys, subjected to repeated thermal cycles under typical industrial super plastic forming conditions. A finite element model of a tool block within an industrial press furnace was developed to simulate the typical thermal cycles of an super plastic forming tool, and predict the resulting mechanical performance. Important thermal and mechanical properties were identified for the cast nickel and chromium class of alloys studied in this paper and suitable ranges for the properties were determined for numerical simulations. The results include a quantitative analysis of the effect of the properties studied.     

Finite element simulations of ultrasonically assisted turning

Ultrasonically assisted turning is a promising machining technology, where high frequency vibration (f≈20 kHz) with an amplitude a≈10 μm is superimposed on the movement of the cutting tool. Ultrasonic turning yields a noticeable decrease in cutting forces, heat and noise radiation, as well as a superior surface finish, comparing to the conventional machining technology. The present study utilizes both experimental techniques and numerical (finite element) simulations to analyze the microstructural processes at the cutting tool–chip interface. High-speed filming of the chip–tool interaction zone during cutting and microstructural and nanoindentation analyses of the machined surfaces are used to compare process zones and deformation processes for both conventional and ultrasonically assisted technologies. The suggested finite-element (FE) model, which utilizes MSC Marc/Mentat general FE code, provides a transient analysis for an elasto-plastic material, accounting for the frictionless contact interaction between a cutter and workpiece as well as material separation in front of the cutting edge. A detailed analysis of cutting for a single cycle of ultrasonic vibration is carried out for isothermal conditions. Differences between conventional and ultrasonic turning in stress distribution in the process zone and contact conditions at the tool/chip interface are investigated.     

Experiences from experimental and numerical springback studies of a semi-industrial forming tool

The work reported in the present paper constitutes a part of a project on simulation of springback in sheet metal forming. Previous work in this project has been concentrated on material modeling and characterization with focus on springback applications. It has been demonstrated that, with proper considerations of all aspects of the material model and the material properties, excellent springback results can be obtained for simple problems. At the simulation of real, industrial parts, a number of additional problems are encountered. Many of these problems are associated with deviations from nominal geometries and other properties. These are examples of factors that influence the outcome of the forming process, but are unknown to the analyst, and can therefore not be considered in the simulation of the forming process. Other phenomena are known to exist, but due to their complexity, they are practically impossible to consider in industrial simulations. Examples of such phenomena are the true frictional behavior in contacts between the blank and the tools, and the flexibility of the press and the forming tool. The influence of these kinds of effects is discussed in the present paper. In the current study, a semi-industrial tool, specially designed to catch those springback problems that are encountered in the forming of real industrial, parts, is used. The tool includes several characteristics that can be found in typical forming tools, such as several draw radius steps and change-over in section geometries. Effects like flange/wall angle changes, sidewall curl and twist are obtained at springback. The sensitivity of the predicted springback is evaluated with respect to various numerical factors, such as the friction coefficient, the material model, and the mesh density. Finally, the quality of the predicted springback behavior for four different materials, commonly used in the automotive industry, is evaluated.     

Modelling the orthogonal machining process using coated carbide cutting tools

In this paper finite element methods were used to determine the influence of various coated and uncoated tungsten carbide cutting tools on the machining of a nickel-based super alloy Inconel 718. Disposable coated and uncoated carbide inserts were used both experimentally and as FEA models to study how the stress distribution within different coatings and carbide grades compared to each other, under a range of cutting conditions. Simulation of an orthogonal metal cutting process was performed using FORGE2, an elasto-visco plastic FEA code. All FE models were assumed to be plane strain. The results include the stress and temperature distributions through the primary shear zone, the chip/tool contact region and the coating/substrate boundaries. The tool wear and stress results from the FE modelling agree favourably with those obtained from experimental work.     

Mass conservation in numerical simulation of resin flow

The finite element/control volume (FE/CV) approach is commonly used for numerical simulation of resin flow in many composites manufacturing processes. The law of mass conservation is sometimes violated with the use of this approach. Especially, when the Galerkin formulation is used with isoparametric finite elements to obtain the pressure field, the balance of resin mass cannot be achieved.In this paper, reasons leading to such mass imbalance are investigated. A numerical model based on material incompressibility is developed to eliminate the problem. A few isothermal flow simulation examples are presented to demonstrate the application of the model. The results obtained are in excellent agreement with the analytical solutions.     

On the modelling of complex 3D bulk metal forming processes via the pseudo‐concentrations technique. Application to the simulation of the Mannesmann piercing process

The modelling of complex 3D metal forming processes using the flow formulation, implemented via the pseudo‐concentrations technique, requires the development of robust computational strategies for dealing with the velocity and pseudo‐concentration boundary conditions in the zone where the blank–tools contact is developed. A new algorithm, designed to fulfil those requirements, is presented in this paper. The Mannesmann piercing process is a metal forming operation used in industry for manufacturing metal seamless pipes. The results of the Mannesmann process finite element simulation are particularly dependent on the accuracy and stability of the algorithm used to describe the contact boundary conditions between the forming tools and the blank. The validation of the finite element model is performed by comparing the numerical predictions obtained using the new algorithm with the results of industrial tests. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical simulations of low-velocity impact on an aircraft sandwich panel

The potential hazards resulting from a low-velocity impact (bird-strike, tool drop, runway debris, etc.) on aircraft structures, such as engine nacelle or a leading edge, has been a long-term concern to the aircraft industry. Certification authorities require that exposed aircraft components must be tested to prove their capability to withstand low-velocity impact without suffering critical damage.

This paper describes the results from experimental and numerical simulation studies on the impact and penetration damage of a sandwich panel by a solid, round-shaped impactor. The main aim was to prove that a correct mathematical model can yield significant information for the designer to understand the mechanism involved in the low-velocity impact event, prior to conducting tests, and therefore to design an impact-resistant aircraft structure.

Part of this work presented is focused on the recent progress on the materials modelling and numerical simulation of low-velocity impact response onto a composite aircraft sandwich panel. It is based on the application of explicit finite element (FE) analysis codes to study aircraft sandwich structures behaviour under low-velocity impact conditions. Good agreement was obtained between numerical and experimental results, in particular, the numerical simulation was able to predict impact damage and impact energy absorbed by the structure.     

Effects of friction on roller expanded tube–tubesheet joint strength

Roller expansion of new tubes in enlarged heat exchanger tubesheet holes requires higher rolling torques which may result in over-thinning of the tube wall. This affects adversely the tube–tubesheet joint strength, which is measured in terms of residual contact pressure between the expanded tube and tubesheet. The axial force required to cause the mechanical failure of the tube–tubesheet joint has therefore been considered as an indication of the integrity of the joint. This force is influenced by several parameters such as the type of tube and tubesheet materials, the initial clearance and coefficient of friction. In the present work, an axisymmetric finite element (FE) model is used to evaluate the combined effects of friction between tube and tubesheet, initial clearance and tube material strain hardening on the strength of the tube–tubesheet joint. The FE results show that the increase in friction between tube and tubesheet results in higher residual contact stress and lower cutoff clearances. The residual contact stress also increased linearly with increasing tube material strain hardening level for all friction coefficients.     

Damage Model and Progressive Failure Analyses for Filament Wound Composite Laminates

Recent improvements in manufacturing processes and materials properties associated with excellent mechanical characteristics and low weight have made composite materials very attractive for application on civil aircraft structures. However, even new designs are still very conservative, because the composite failure phenomenon is very complex. Several failure criteria and theories have been developed to describe the damage process and how it evolves, but the solution of the problem is still open. Moreover, modern filament winding techniques have been used to produce a wide variety of structural shapes not only cylindrical parts, but also “flat” laminates. Therefore, this work presents the development of a damage model and its application to simulate the progressive failure of flat composite laminates made using a filament winding process. The damage model was implemented as a UMAT (User Material Subroutine), in ABAQUS^TM Finite Element (FE) framework. Progressive failure analyses were carried out using FE simulation in order to simulate the failure of flat filament wound composite structures under different loading conditions. In addition, experimental tests were performed in order to identify parameters related to the material model, as well as to evaluate both the potential and the limitations of the model. The difference between numerical and the average experimental results in a four point bending set-up is only 1.6 % at maximum load amplitude. Another important issue is that the model parameters are not so complicated to be identified. This characteristic makes this model very attractive to be applied in an industrial environment.     

Finite element analysis on nanoindentation with friction contact at the film/substrate interface

Finite elements (FE) provide a numerical method to calculate complex nanoindentation problems. Correlation of FE analysis with experimental data on nanoindentation may lead to improved characterization of the mechanical properties of thin films and coating systems. In this study, a model of the friction contact at the interface of thin films and substrates is established using FE analysis about a cone indenter, which imitates a Berkovich nanoindenter. Finite element nanoindentation simulations were performed at three different interface friction contact conditions. The following conclusions were depicted through the study of the simulation data. First, for increasing values of the friction coefficient, the indenter’s force versus displacement response of the film/substrate-friction-contact (F/SFC) model coincides with the response of the film/substrate-perfectly-bonded (F/SPB) model. Second, when the indenter’s maximum displacement is less than 10% of the film thickness the deformed nanoindentation area is concentrated under the indenter tip for both F/SFC and F/SPB models. Third, a mechanical response is generated along the F/SFC interface while the mechanical response along the F/SPB interface is negligible. Finally, the nanoindentation simulation data indicate that the calculated mechanical properties intrinsically depend on the interfacial contact conditions of the film/substrate even when the maximum displacement of the indenter is controlled within the 10% of the thin-film thickness.