Punch Radius Research Articles

Comparison of thickness variation in multistage deep drawing of a stator motor housing by experimental and simulation methods

The objective of the present work is to study the thickness variation in multistage deep drawing of a stator motor housing by experimental and simulation methods. The final component of a stator motor housing is formed in five stages. The stator motor housing component in each stage is divided into different regions like punch flat region, punch radius region, wall surface and die flange region. The thickness variation is studied in all these regions to meet the required specifications. The thickness at each region is measured using an ultrasonic device. Finite element method (FEM) simulations were performed to predict the thickness of the stator motor housing at different regions. It was found that the simulated thickness at different regions is very close to experimental results and the experimental thickness variation in the stator motor housing was within the specification limits. A progressive tool is developed to reduce the process to form the stator motor housing so that production time can be saved and higher numbers of components can be produced. The stator motor housing is manufactured using this progressive tool and thickness is measured at the different regions.

Contact problem of a graded layer supported by two rigid punches

This paper considers the frictionless contact plane problem of an infinitely graded layer supported by two rigid cylindrical punches and subjected to a concentrated normal force by means of a rigid cylindrical punch. The layer is made of non-homogeneous material with an isotropic stress–strain law with exponentially varying properties. Rather than assuming vanishing displacements or stresses at the bottom of the geometry, this study examines the effect of supports on the graded layer. Using standard Fourier transform and the related boundary conditions, the plane elasticity equations are converted analytically to a system of singular integral equations in which the unknowns are the contact stresses and areas. Gauss–Chebyshev integration formulas are then employed to discretize and solve numerically the derived integral equations. Numerical results for the contact stresses and areas are provided for various dimensionless quantities including material inhomogeneity, distance between the punches, external load and upper and lower punch radii.

Simplified springback prediction of thick sandwich panel

Springback is one of the most important design behavior in air-bending processes. The sandwich panel exhibits more complicated bending and springback behavior due to substantial differences in mechanical properties between the foam core and the metallic skin sheet. In this paper, we will not only propose a semi-analytical model in order to easily predict springback in air-bending process of steel/polyurethane/steel sandwich panel, but also we will carry out experiments to measure springback amount. The semi-analytical model results and the experiment findings proved to be in a good agreement. In addition, numerical simulations and experiments were conducted to investigate the effects of punch radius, die opening, and the foam thickness on springback.

Development of optical-heating-assisted incremental forming method for carbon fiber reinforced thermoplastic sheet—Forming characteristics in simple spot-forming and two-dimensional sheet-fed forming

The aim of this study was to develop an incremental forming method for carbon fiber reinforced thermoplastics (CFRTPs) using an optical heating system. The developed forming machine was mainly composed of a vertically articulated robot to control the position of the blank sheet on the X-Y plane, a reciprocating mechanism to generate the reciprocating motion of the forming punch, an electrical cylinder to control the position of the reciprocating mechanism, and a halogen lamp for local heating. Discontinuous short-fiber CFRTP work sheets with thickness of 0.5, 1.0, and 1.5 mm were used for the experiment. Each work sheet was fixed to a blank holder and locally heated by a halogen lamp set under the work sheet. The heated region of the work sheet was formed using the reciprocating motion of a spherical forming punch with a punch radius of 0.5, 1.5, or 2.5 mm, which was located on the opposite side of the halogen lamp. In order to evaluate the fundamental forming characteristics of the developed system, simple spot-forming was performed without feeding a work sheet on the X-Y plane. The work sheet temperature was set to 200 °C at a distance of 3.0 mm from the optical axis of the halogen lamp for each work sheet thickness. This study examined the relationship between the pushing distance of the reciprocating forming punch to the work sheet and the forming height, which was the distance between the top of the formed concave portion and the flange region of the work sheet. The forming height was lower than the pushing distance of the forming punch, and this tendency was conspicuous when the work sheet was thin and the punch radius was small. Fractures in the formed part were caused by excessive thinning of the work sheet due to the tensile deformation at the sidewall of the formed shape. The symmetrical shape profile of the formed part following the forming punch shape could be obtained when the pushing distance did not exceed 7.0 mm with a work sheet thickness of 1.5 mm and punch radius of 2.5 mm. The shape profile of the formed product obtained by two-dimensional sheet-fed forming was also estimated to evaluate the advantage of the developed method. The forming characteristics in the two-dimensional sheet-fed forming were different from those in the spot-forming. The continuous formed shape according to the sheet-feeding path could be achieved by the two-dimensional sheet-fed forming, and the advantage of the developed method could be experimentally clarified.

Adhesion of a rigid punch to a confined elastic layer revisited

ABSTRACTThe adhesion of a punch to a linear elastic, confined layer is investigated. Numerical analysis is performed to determine the equivalent elastic modulus in terms of layer confinement. The size of the layer relative to the punch radius and its Poisson’s ratio are found to affect the layer stiffness. The results reveal that the equivalent modulus of a highly confined layer depends on its Poisson’s ratio, whereas, in contrast, an unconfined layer is only sensitive to the extent of the elastic film. The solutions of the equivalent modulus obtained from the simulations are fitted by an analytical function that, subsequently, is utilized to deduce the energy release rate for detachment of the punch via linear elastic fracture mechanics. The energy release rate strongly varies with layer confinement. Regimes for stable and unstable crack growth can be identified that, in turn, are correlated to interfacial stress distributions to distinguish between different detachment mechanisms.

Analysis of the bending effects and the biaxial pre-straining in sheet metal stretch forming processes for the determination of the forming limits

The most common failure criterion in sheet metal forming operations is the forming limit diagram (FLD). Typically, the FLD is evaluated with the Nakajima test, which is a stretch forming-based test that allows the analysis of stretching under different stress conditions. The results of the Nakajima tests are considered ideally achieved under linear strain path deformation. However, due to the hemispherical form of the punch, the induced strain path starts in biaxial condition and switches to a linear path after the first contact between punch and specimen. In addition, the strain distribution is evaluated on the outer surface of the specimen. Consequently, the results are affected by a non-linear strain path and a bending component due to the sheet thickness and punch diameter. These effects have a consequence on the FLD, and thus it is not directly comparable with the ideal FLD that is analytically evaluated since the analytical models in the literature do not consider the process effects like bending and the biaxial pre-straining. In this paper, a study of biaxial pre-straining and bending effects is observed for different stress conditions and materials. The investigation of the bending will be considered under the experimental and numerical aspect. Nakajima tests are conducted with different punch diameters to discover the effects of different material thickness to punch radius ratios. The results are also compared with the outcomes of a numerical model, used for the analysis of the strains on the outer, middle and inner layers. The biaxial pre-stretching is analysed by comparing the outcomes of the FLD achieved by the Nakajima punch with the results of the Marciniak test. In this paper, an efficient method based on the correlation coefficient for the evaluation of pre-straining is presented. The biaxial pre-stretching will be compared to the strain paths and the tendencies will be analyzed. Finally, considerations about the phenomenology of the effects in dependency of the material classes are given.

A Comparative Study on Formability of the Third-Generation Automotive Medium-Mn Steel and 22MnB5 Steel

Third-generation advanced automotive medium-Mn steel, which can replace 22MnB5 steel, was newly developed to improve the lightweight and crashworthiness of automobile. Studies on the formability and simulation method of medium-Mn steel have just been initiated. In this study, finite element simulation models of square-cup deep drawing were established based on various material property experiments and validated by experiments. The effects of blank holder force (BHF), fillet radii of tools (die and punch) on the maximum drawing depth (MDD), thickness distribution of the formed products, and the microstructure before and after forming were investigated and compared with those on 22MnB5 steel. Results show that the MDD of the two steels decreased with increased BHF but increased with the fillet radius of punch; however, the fillet radius of die showed no significant effect on the MDD for both steels. Compared with hot-formed 22MnB5 steel, the martensitic transformation of the hot-formed medium-Mn steel is rarely influenced by the process parameters; thus, it holds the complete, fine-grained, and uniform martensitic microstructure. Moreover, the medium-Mn has better formability, lower initial blank temperature, and smaller impact of BHF and fillet radius of tools on the hot-formed product. Thus, a theoretical basis for the replacement of 22MnB5 steel by medium-Mn steel in hot forming process is provided.

Numerical Analysis On The Effect Of Various Parameters On Fracture Limit For Deep Drawn Cups

Deep drawing is the process of converting a blank into cup shaped articles like kitchen sinks, cooking pans, automobile panels, gas tanks, fountain pen caps etc. Wrinkles and fractures are the major defects in deep drawn products. Fracture is the separation or fragmentation of a solid body into two or more parts under the action of stress. In deep drawn cups tearing is usually an open crack in the vertical wall which occurs near the base due to high tensile stress that causes thinning and fracture of the metals at this location. During this process the punch force acting on the bottom of the cup is transferred to the side of the cup. The narrow ring of metal just above the bottom of the cup is subjected to plane strain condition. As a result, failure of the cup easily happens in this zone due to necking induced by the tensile stress, leading to tearing. This type of failure can also occur on the flange as the metal is pulled over the sharp die corner. In addition to this sharp corner on the punch could also cause fracture of the cup along the corner. The objective of this work is to predict the fracture limit of deep drawn cups. This would help in preventing rejections in deep drawing industry. This can be achieved by setting the blank holder force appropriately. Also it would save material and reduce the total cost. In this work numerical simulations are conducted by considering five different parameters namely punch radius, die radius, clearance, coefficient of friction and punch diameter using finite element explicit solver LSDYNA. Modeling of the set up is done using hyper mesh. In this work simulations are carried out as per L-27 orthogonal array suggested by Taguchi. A combination of finite element method and Taguchi analysis is used to determine the influence of process parameters on fracture limit in deep drawing process. During analysis the value of optimum BHF is arrived by performing a number of trial runs. Also Column effect method and plotting methods are used for finding out the most influencing parameters and their interactions respectively for analysis. The studies reveal that punch diameter is most significant parameter for deciding fracture limit followed by die corner radius and clearance. In addition to this regression analysis is carried out for developing an empirical model using Minitab 17 for predicting fracture limit.

Deformation behavior of metal foil in micro pneumatic deep drawing process

The micro pneumatic deep drawing process which is expected to be extruded at elevated temperature was newly proposed. Before it is used in the micro pneumatic deep drawing process at elevated temperature, the stability of the micro pneumatic deep drawing process was examined by the micro pneumatic deep drawing process at RT in present study. The micro pneumatic deep drawing processes with different parameters were carried out to investigate the influence of deformation conditions on deformation behavior of pure titanium foil. The research results showed that the pressure of medium and punch radius have a strong influence on the deformation behavior and thickness distribution of pure titanium foils. The microcup with sound quality can be obtained by optimizing the deformation conditions.

Effect of die width on spring back of electrogalvanized CR4 steel during air bending

This study is an attempt for analyzing the effect on spring back due to changes in the die width during air bending of electro-galvanized CR4 steel. A universal testing machine, enabling recording of bending force and controlling of punch travel was used for conducting the experiments. A flexible die set with die width Wd = 40mm, Wd = 60mm and Wd = 80mm was used whose die width can be changed by simply changing a distance block. Die radius, punch radius and punch speed were kept constant. Galvanizing was done by electrodeposition method using alkaline non-cyanide baths. Different thickness of electro-galvanizing, Tg = 0 µm, 4 µm, 7 µm, and 10 µm were achieved by changing the timing of electrodeposition. An approach to vary one factor at a time was employed. Angles were measured by using optical profile projector and graphics. The Graphical method was used for analyzing the spring back. It has been witnessed that spring back increases with the increase in width of die for every punch travel and for non-galvanized as well as galvanized steels of different galvanizing thicknesses. An increase in spring back was observed when the galvanizing thickness was increased. The results are important for the tool engineers and process shop engineers for tool design and process control.

Applying 2<sup>k</sup> Factorial Design to Study on Parameters Affecting Springback of Forming of Advanced High Strength Steel Sheets (AHSS)

Advanced high strength steels (AHSS) are widely used in the automotive industry due to their appropriate strength to weight ratio. This alloy has unique hardening behavior and variable unloading elastic modulus; however, the unavoidable obstacle of AHSS sheet metal forming is springback. The springback is a result of elastic recovery and residual stress. The aim of this study is to determine the proper process parameters enabling the reduction of the springback defects in AHSS forming process. This work was divided into two parts, regarding to the effects of numerical parameters and process parameter on forming AHSS. In this paper, a U-shape forming was used to examine the springback behaviors, such as springback angle, sidewall curl, and thickness, through an experiment. To achieve this purpose, 2k factorial statistical experimental design has been employed to investigate the parameters affecting the springback of forming in AHSS to find out the main effect in the springback reduction focusing on using as a guideline for die design. It showed that the blank holder force is the most influential parameter. The second is the punch radius. However, the blank holder force and punch radius is not simple to adjust in die design, the die radius becomes the important parameter to be used to reduce the springback angle.

Mesh refinement study and experimental validation for stretch bending of sheet metals

For sheet metal parts with small radii and large bending angles, the sheet metal forming simulation reaches their application limits. Alternatives are complex shell formulations and volume elements. For volume elements, the necessary number of elements over the thickness is important. Valid values are not available depending on discrete radii. Therefore in this work, a convergence study is performed using the example of an angular stretch bend test with a radius to thickness ratio of 1. For various states of mesh refinement, simulations are performed, various results are presented, analysed and discussed with regard to convergence behaviour to the necessary number of elements in thickness direction. Recommendations for suitable validation variables are derived.Based on the refinement study, a simulation model for an experimental validation is developed. The experiments are carried out in a sheet metal forming machine. Experimental angular stretch bend test with a punch radius of 1 mm are performed until failure and the strain distribution on the top side of the sheet is measured. Finally, simulation and experiments are compared based on the surface strain.

Hot V-bending behavior of pre-deformed pure titanium sheet assisted by electrical heating

The whole hot V-bending process assisted by electrical heating (EH) of pre-deformed pure titanium sheet, from initial cyclic bending-unbending deformation to EH of sheet, followed by hot V bending and further rebound after unloading, were systematically studied by experiments and FEM numerical simulations. Besides, the differences and corresponding mechanisms in heating efficiency and bending behavior between EH and furnace heating (FH) were compared and analyzed. Specifically, the 1-mm-thick sheets were subjected to three passes of cyclic bending-unbending with 90o bending angle and 5 mm punch radius. Subsequently, the sheets were electrically heated with a series of current intensities, 50, 100, 150, 200, 250, 300, and 350 A, and bent after heating. The microstructures in different characteristic layers after cyclic deformation were observed; the temperatures varying with heating time under various current were measured and the heating efficiency was referred to FH in corresponding ambience temperatures. Additionally, the heat transfer situation of samples to the die in bending and its effect on springback after unloading were discussed. The variation of bending force to punching stroke of the samples heated by both modes, combined with their underlying mechanisms of difference, was tested and analyzed.

Experimental and Finite Element Results for Optimization of Punch Force and Thickness Distribution in Deep Drawing Process

Carrying out various experiments organized based on the design of experiments method, the deep drawing process is investigated entirely in this paper. The influences of eight main process parameters including punch and die radii, blank thickness, punch velocity, lubrication conditions at the interfaces of blank-die, blank-punch, and blank–blank holder as well as the blank holder force on the process outputs comprising punch force and sheet thickness variation are investigated. The analysis of variance method is considered to define the procedure by which these eight parameters be adjusted for deep drawing process to minimize the punch force and maximize the uniform thickness distribution in the products. It was found that for punch force, in addition to blank thickness, die and punch radii as well as the BHF are, respectively, the main essential parameters. Besides, blank thickness, die radius, and lubrication condition at the holder-blank interface are the most effective parameters on thickness distribution. Accordingly, initial blank thickness and die shoulder radius could be taken into account as two important parameters in deep drawing process should be set up appropriately to accomplish the minimum punch force and uniform thickness distribution together.

Effect of coating thickness on the stress profile at the epoxy/silicone interface subjected to pull-off loading: A finite element analysis

Previous experimental studies of silicone coatings have shown three distinct types of release behavior in the tensile flat punch test, depending on coating thickness. The mechanical response in the punch test is highly dependent upon the Poisson's ratio of the coating and its confinement ratio (punch radius divided by coating thickness). This study developed a high accuracy finite-element model of the punch test using the adaptive p-method with extensive mesh refinement to produce smooth stress profiles up to the punch edge. Stress distributions were found for a wide range of confinement parameters and Poisson's ratios. At a typical Poisson's ratio of 0.49, the highest center stress occurred for the intermediate thickness coatings—not thin or thick. Also, the thickest coatings demonstrated steadily increasing high stress towards the edge, while other thicknesses showed the steep singularity at the edge with a protective stress depression bordering inside it. The results further help explain why the critical pull-off force continues to increase as the thickness decreases, even with different release mechanisms. The stress profiles for thick coatings have almost no sensitivity to Poisson's ratio, unlike other thicknesses which show high sensitivity. Edge peeling is most likely to occur for all thick coatings, while other debonding modes are most likely for thin and intermediate thickness coatings. Together, results show the stress mechanics of the flat punch test follow three distinct types of confinement.

Failure of Physical Vapour Deposition Coating Zirconium Nitride on the Punch of Clinching Tool

Abstract A tool with a punch of ø5 mm and a die with a specially formed circular cavity and an annular gap was used for mechanical joining of thin hot-dip galvanized steel sheets. The active parts of punch and die were covered by PVD coating of ZrN type with LARC technology. The punches and the dies were tested in a complex tool by joining thin hot-dip galvanized steel sheets with the pressing force of 7,000 N. Decohesion of coating with width of 100 – 200 μm was observed in the perimeter of cylindrical part of ø5×4 mm in the in edge of punch radius R = 0.5 mm with deposited ZrN coating after the creation of 150 mechanical joints. The decohesion of PVD coating occurred mainly in the surroundings of the radius R = 0.5 mm on the front plane of ø15 mm part.

Experimental investigation of large radius air bending

Usage of high-strength steels with limited bendability in conventional air bending is not obvious due to the deformation limits of these materials. An attractive solution for deformation reduction is the utilization of large radius punches. The bending process for large radius tooling is dissimilar to conventional air bending, due to the presence of the multi-breakage phenomenon. This phenomenon has been observed by many researchers, but understanding of the influencing parameters is limited. Multi-breakage poses a serious obstacle, since it changes the loading scheme from the typical three-point to four-point bending. In this contribution, an extensive experimental exploration of large radius bending leads to conclusions on the influence of tool dimensions and material characteristics. Air bending experiments have been performed for five materials, four thicknesses, four punch radii, and four die openings. Based on the observed force measurements, the process can be defined as large radius bending when the ratio punch radius/die opening is larger than 1:4. Trends for the influencing parameters of the measured data can be used for further investigation of large radius air bending.

Numerical and experimental investigation on forming stacer using compositing stretch and press bending process

A continuous cold forming method, named compositing stretch and press bending (CSPB), is an effective process to manufacture a stacer, which is a typical cylinder element with a thin strip for deployable booms in various spacecraft missions. The forming tools in CSPB are a pair of punch and die; a stacer is formed continuously by pulling the strip from the gap between the tools. The process of compositing stretch and press bending is introduced, and the forward bending and the reverse bending occur during this process. The stability of cold forming workpieces has been increased due to those processes above. In this paper, the forming process is analyzed by the numerical simulation and experimental method. The key process parameters such as post tension, die gap, punch radius, and strip thickness are studied. The macromorphology of the stacer formed by CSPB is measured and analyzed. And, the deploying force and the bending stiffness of the stacer under different treatments are investigated. The results show that the simulation results and the experimental results are in good conformity, and a stacer with good forming precision and performance can be obtained by the CSPB process without any other stabilization treatments.

Meta-model Based Approach to Minimize the Springback in Sheet Metal Forming

Springback occurs at the end of sheet metal forming process where the geometric changes takes place in the part when the load is removed. It is an important factor to determine the quality of sheet metal forming. In this paper, the factors which affects the springback is determined and its influence over springback is studied analytically. The sheet metal forming process is performed using finite-element tool ABAQUS, so as to obtain the springback value for different process parameter. Punch radius and clearance gap along the horizontal axis between punch and die are the two process parameters considered for this study. For this study, 2D metal forming process has been performed and the springback angle is calculated at the corner area close to the holder. Kriging interpolation technique is used to build the surrogate model for the optimization process. Finite element method provides the data to build the metamodel which also helps in reducing the number of the real tests. Kriging or Gaussian process regression is an interpolation method which interpolates the values, modeled by a Gaussian process governed by prior covariance to optimize the smoothness of the fitted values. Kriging was applied to simulate the complex springback process. The sample data required to build the surrogate model was generated using MATLAB. Latin hypercube sampling technique has been utilized to generate the input sample points. The meta- model built with help of kriging interpolation for the sheet metal forming process is validated using RMSE method. With help of this approach we are able to predict the best process parameter which results in lower springback for the sheet metal forming is obtained, thereby reducing the spring back considerably. This paper gives a new approach that involves in reducing the springback phenomenon in sheet metal forming process with help of meta-model technique.

Two-stage forming approach for manufacturing ferritic stainless steel bipolar plates in PEM fuel cell: Experiments and numerical simulations

Multi-stage micro-channel forming by stamping, as a method for cost effective and efficient for mass production, was performed for ultra-thin ferritic stainless steel sheets with thicknesses of 0.1 and 0.075 mm, as a good substitute for traditional graphite bipolar plates of proton exchange membrane fuel cell. Attention was directed to enhance the final forming depth and minimize localized thinning, extremely important aspects of the micro-channel on bipolar plate, by the proposed forming process. A forming depth at the first forming stage was chosen as a process variable, and its effect on the formability of the micro-channel at the second forming stage was experimentally investigated. Finite element simulations for the two-stage forming process were conducted to optimize the punch radius and forming depth at the first stage for improving the formability. The comparative study between the simulations and the experimental results could validate improvements in the formability by the proposed approach. In particular, this study could support the existence of an optimum forming depth at the first forming stage. Based on the simulation results, a mathematical model was established to identify the dominant factor needed for formability improvement and to propose a methodology for the process optimization of the multi-stage forming.