Henri Champliaud

Modeling and simulation of asymmetrical three-roll bending process

Abstract Asymmetrical three-roll bending is one of the three-roll bending processes widely used in metal forming due to its simple configuration. Modeling and simulation of the process based on finite element analyses has great interests to industrial practices. This paper deals with the prediction of the position of the lateral roll during cylindrical roll bending. In the numerical model, the rolls are assumed to be rigid bodies and the plate is assumed to be made of elasto-plastic material, with a bilinear material model corresponding to the results from tensile tests. Shell elements are applied to the plate and automatic node-to-surface contacts are selected for the interfaces between the plate and the rolls. The nonlinear equations are resolved by fully integrated Belytschko–Tsay shell formulation under the well-known ANSYS/LS-DYNA environment with explicit time integration. The positions of the lateral roll predicted by the numerical simulations agree well with experiments.

Numerical study of non-kinematical conical bending with cylindrical rolls

Abstract Although desired cones can be manufactured by kinematical conical roll bending, less manufacturing cost is still largely required. This paper presents the modelling and simulation of non-kinematical conical roll bending process with cylindrical rolls. Such process is achieved by using attachments to reduce the velocity on the area close to the top edge of the plate. In contrast with a kinematical conical roll bending machine, the driving outer rolls of a non-kinematical conical roll bending machine are allowed to slide on the plate close to the top edge. The modelling is based on finite element method under ANSYS/LS-DYNA environment. The bent cones obtained from numerical simulations compare well with the desired cones. Numerical simulation investigations show that the adaptive capacity of the model is available within a range of the top–bottom radius ratio of the desired cones between 0.44 and 0.8.

Three-stage process for improving roll bending quality

Abstract Continuous pyramidal three-roll bending process has the simple configuration. However, two planar zones exist near the leading and trailing edges after a conical roll bending. This paper proposes a three-stage process to improve the geometrical quality of a bent cone for providing an alternative process for the manufacture of the crowns of Francis turbines for hydro plants or the towers of wind power plants. The process was simulated under the commercial software LS-DYNA with an explicit scheme and ANSYS with an implicit scheme. The geometrical quality of the bent cone was improved after the final stage of the process.

Understanding and modelling the torsional stiffness of harmonic drives through finite-element method

Abstract Torsional stiffness or rigidity is a crucial characteristic in the design of transmission devices, including harmonic drives (HDs). Among the various design aspects constituting a reduction mechanism in robotic systems, torsional stiffness is an important factor for positioning accuracy and control issues. One of the major advantages of HDs is their capacity to present a high reduction ratio while maintaining a small hardware size. However, manufacturing these drives remains a complex and costly process due to the high precision of its machined components; as a result, the use of such drives is still limited only to high-end mechanical products and technologies. Given these costs, numerical analysis becomes an effective alternative for obtaining valuable data through simulations, without the need for prototypes. This article presents a finite-element model to reproduce the behaviour of the torsional stiffness of an HD. The numerical model allows an evaluation of the effects of various geometrical parameters on the torsional stiffness of the HD. The numerical model of the HD can be used for optimization purposes, i.e. to develop an HD with a high torque capacity combined with a high-rated lifespan.

Investigation of non-kinematic conical roll bending process with conical rolls

Abstract Although high quality cones can be produced with kinematic conical roll bending due to the non-slide condition between the conical rolls and the plate, the process flexibility is limited because the conical rolls cannot be reused to produce cones of different cone angles. In this paper, a non-kinematic three-roll bending process is proposed to reduce manufacture costs by reusing existing conical rolls. In this process, an attachment is added to the top edge of the plate to obtain required circumferential velocity as the top/bottom radius ratio of the desired cone is smaller than the rolls. The top sides of the conical rolls slide on the plate to reduce the local velocity near the top edge and an appropriate velocity near the top edge of the plate can be obtained by adjusting the friction coefficients between the rolls and the plate. This flexible process can provide performance similar to the kinematic conical roll bending process. The process modeling is based on the finite element method under ANSYS/LS-DYNA environment. A relation between the gap size of the bent cone and the friction at the plate/attachment contact interface has been investigated by using numerical simulations. The simulation results give a well bent cone compared with an ideal cone.

Distortion and residual stresses in electron beam-welded hydroelectric turbine materials

Heavy-section assembly of hydroelectric turbine runner materials using single-pass, autogenous EBW was demonstrated to penetrate a 90-mm-thick butt joint. The welding-induced distortions and residual stresses were characterised to understand the impact of the materials and process conditions (e.g. preheating and/or PWHT). Using 3D optical measurements, the angular distortions of EB-welded UNS S41500 and CA6NM steels were determined to be 0.13° and 0.38°, respectively. The longitudinal residual stresses, measured through the contour method, had a M-shaped distribution throughout the thickness with minimum (∼−500 MPa) compressive stresses in the FZ and maximum (∼600 MPa) tensile stresses in the HAZ. After PWHT, the tensile and compressive stresses reduced to ∼100 MPa.

On the Use of Dual Kriging Interpolation for the Evaluation of the Gasket Stress Distribution in Bolted Joints

The leakage behavior of bolted joint is very much dictated by the gasket contact stress. In particular, the non-uniform distribution of this stress in the radial direction caused by the flange rotational flexibility has a major influence on the leak tightness of some gasket types. The current ASME flange design rules and the new ASME proposed design rules addresses this effect by introducing the concept of gasket effective width for which the validity of the suggested values has not been verified. This paper presents a simple comprehensive analytical approach based on the dual kriging interpolation technique to predict the gasket contact stress distribution in floating type bolted joints. The kriging methodology is shown to be very efficient when nonlinear modeling such as gasket material mechanical behavior is involved. Together with the flange rotational flexibility, this technique implemented in the “SuperFlange” program is supported and validated by numerical FEA conducted on different flange sizes and gasket materials combinations.Copyright

Heat-Assisted Roll-Bending Process Dynamic Simulation

Abstract The forming forces during roll-bending process can be reduced by heating the workpieces. Heat-assisted roll-bending is a promising alternative to the costly and time-consuming casting process that is usually selected for manufacturing large and thick axisymmetric parts made of high-strength steel. In this paper, a computer-aided simulation program has been built in the Ansys/LS-Dyna environment to study the relationships between temperature, applied forces and plate thickness. The finite element modelling of the formed geometry is sequential with first a thermal simulation followed by a structural one. The numerical results are then compared to analytical ones. The analyses of the process with numerical simulations yield a better understanding of the mechanism of the process and provide an opportunity for the design of an efficient heating system to control the heat energy to be input in the workpiece during the roll-bending process.

Study of True Stress-Strain Curve after Necking for Application in Ductile Fracture Criteria in Tube Hydroforming of Aerospace Material

Tube hydroforming (THF) is an advanced metal forming process that is used widely in automotive industry, but the application of the THF process in aerospace field is comparatively new with many challenges due to high strength and limited formability of aerospace materials. The success of THF process largely depends on many factors, such as mechanical properties of the material, loading path during the process, tool geometry and friction condition. Due to complexity of this process, finite element modeling (FEM) can largely reduce the production cost. One of the important input in FEM is the material behavior during hydroforming process. The true stress-strain curve before necking can be easily determined, using either tensile testing or bulge testing, but for an accurate failure prediction in a large deformation, such as hydroforming, the study of true stress-strain curve after necking is important because it improves the quality of the analysis due to utilizing a real extended stress-strain curve. Hence, the objective of this research was to establish a methodology to determine the true stress-strain curve after necking in order to predict burst pressure in the THF of aerospace materials. Uniaxial tensile tests were performed on standard tensile samples (ASME E8M-04) to determine the true stress-strain before and after necking, using an analytical method presented in this study. To validate the approach, burst pressure in the THF process was predicted using the extended stress-strain curve in conjunction with Brozzos decoupled fracture model. The approach was evaluated using data obtained from the free expansion (tube bulging) tests performed on stainless steel 321 tubes with 2 inches diameter and two different thicknesses, 0.9 mm and 1.2 mm. The comparison of the predicted and measured burst pressures was promising, indicating that the approach has the potential to be extended to predict formability limits in THF of complex shapes.

Modeling circular inductors coupled to a semi-infinite magnetic medium considering the proximity effect

Abstract This paper presents a simple 2D electromagnetic model that solves the high-frequency current distribution, considering the proximity effect, in a planar spiral coil above a linear conductive or non-conductive semi-infinite magnetic medium. The conductive regions are divided into axisymmetric elements in which the current is assumed constant. The current flowing in each element depends on its complex impedance and is computed by Kirchhoff’s circuit law. To take the effect of the magnetic medium into account, a new set of mutual inductance formulas are presented. Those formulas are expressed in terms of elliptic integrals and the fast-converging arithmetic–geometric mean iteration of Gauss. The geometric mean distance method is used to deal with elements of arbitrarily shaped cross-section. Elliptic integrals are also used to express the magnetic flux density. The current distribution, the magnetic field and the equivalent impedance computed with the multifilament model agree well with the results obtained using commercial finite element software.