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This paper presents a comprehensive investigation on the incremental sheet metal forming assisted by ultrasonic vibration (UV). With the aid of a new acoustoplasticity constitutive model which can accurately capture the deformation response to ultra-high strain rates, extensive numerical analyses on incremental sheet metal forming under UV were conducted by using the finite element method. Corresponding experimental investigations were also carried out to examine the capacity of the constitutive and numerical models. The effects of critical parameters, such as tool size, tool revolution speed, feed rate, pitch and tool vibration amplitude, on the forming force, maximum sheet thickness reduction and springback, were investigated in detail. It was found that the models can realistically predict the incremental sheet metal forming under UV. It is expected that this research can facilitate the development of ultrasonic vibration–assisted forming techniques. [ABSTRACT FROM AUTHOR]

One of the most critical tasks within the scope of Design for Manufacturing (DfM) is to define the set of Geometrical Product Specifications (GPS) in the 3D model or in the engineering drawing that ensures the functionality and the interchangeability of parts, as well as the intended functional performance of an assembly. Several methodologies have been proposed for the optimal designation of such specifications; however, the majority of them do not effectively take into account the deformations that are inevitably induced during assembly and operation for the vast majority of mechanical components. Motivated by the widely accepted tolerancing practice for sheet metal parts in the automotive industry, where the distinction between free state and constrained state is considered, the paper investigates the influence of the deformations induced during assembly and operation on GPS. The effect of part stiffness in the resultant functional GPS of the assembly/component is explored, through CAD surfacing and non-linear numerical finite element analysis tools including the contact problem. The current stage of development of a novel, performance-based methodology for the GD&T design procedure is presented. The methodology is applied on a real-world mechanical assembly that is derived from tolerance stack up-related literature. This study illustrated is that for an unpredictably wide range of mechanical components the default, free-state GPS scheme should only be assigned after rigorous analysis of their compliance behaviour. The proposed approach will lead to deduce the correlation between production cost and performance through a further development in future study. [ABSTRACT FROM AUTHOR]

This paper focuses on numerical simulation of single point incremental forming (SPIF) process. Via a user-defined material (VUMAT) subroutine is employed to implement a non-associated mixed isotropic-kinematic hardening material model. The accuracy of the proposed non-associated flow rule model to predict the material behavior and the anisotropy in sheet metal forming simulations is examined by comparing numerical results with experimental measurements. The suggested model shows good agreement with experimental results compared to the associated Hill_R and Hill_S models. The proposed non-associated flow rule demonstrates a good efficiency to simulate the springback after sheet metal forming and to predict the thickness variation during incremental sheet metal forming process. The present non-associated model can improve the prediction of both anisotropic and hardening behaviors of material used in numerically controlled sheet metal forming technology. [ABSTRACT FROM AUTHOR]

In theoretical analyses, such as numerical simulations of the large plastic deformation of sheet metal forming, the material is usually assumed to be ideally uniform; thus, the actual material response during the process cannot be properly characterized. In this paper, a finite element method (FEM) model considering nonuniform material properties is proposed. In the model, the dispersion degree of mechanical parameters is characterized by the microhardness tested from sheet metal samples, and the discrete material properties conforming to the random distribution functions are automatically assigned to the elements in FEM by Python script. The digital image correlation (DIC) method is used to check the tensile instability deformation of sheet metal in the physical experiments of uniaxial tensile tests. The results show that the nonuniform finite element model can reveal the strain transfer and reproduce the diffuse instability in a wide range of plate samples. The deformation feature, crack and its direction are in good consistency with the experiment, and the predicted critical diffuse instability strain is of high accuracy compared with the DIC results. In addition, the influences of random distributions of mechanical properties and the mesh size effects on the simulation results of tensile instability are addressed. [ABSTRACT FROM AUTHOR]

This paper presents a variant of the traditional 'mortise-and-tenon' joint, which has been used for thousands of years by carpenters and blacksmiths to connect wood or metal parts. The new proposed joint is utilized to fix longitudinally in position two metal sheets (or plates) perpendicular to one other by sheet-bulk metal forming, at room temperature. The development is performed by means of a combined finite element and experimental investigation focused on the identification of the major process parameters and on the understanding of their influence on the overall joining feasibility. Destructive testing is carried out to characterize the performance of the new proposed joint, and an analytical expression is provided to determine the maximum tensile force that the joint can safely withstand. [ABSTRACT FROM AUTHOR]

Numerical simulation is becoming one of the main methods to investigate various engineering problems with sophisticated conditions. A considerable amount of research is conducted on numerical analysis of sheet metal forming process to address different aspects of the problem. Among the numerical simulation methods for sheet metal forming process, finite difference method (FDM) and finite element method (FEM) have been the main methods. In this paper, the progresses in simulation techniques, advantage and disadvantage of numerical methods for simulating sheet metal forming process are discussed based on these numerical methods. Currently, FEM being the main simulation method for sheet metal forming, development in solution strategies and formulation, element selection are further brought into attention. Historical development of anisotropy and yield criteria, which are theoretical foundation for numerical simulation, and their application in simulation software are briefly classified. Formability of sheet metal is presented from the numerical simulation point of view. Numerical investigations on springback are reviewed in terms of simulation techniques and factors influencing springback. Simulation techniques for novel sheet metal forming techniques such as laser forming and incremental sheet forming (ISF) are presented. [ABSTRACT FROM AUTHOR]

In this paper, an iterative learning control (ILC) model is constructed with the state function, forming quality function, and learning updating law to optimize the sheet forming process. The state function, which is hardly calculated explicitly, is replaced by finite element simulation to predict the workpiece state with a specific drawbead geometry scheme. Then, the forming quality function is defined by accumulating the elements' thinning strain deviation with different weights. Afterwards, the learning updating law is designed to provide a new scheme of drawbead geometry based on the forming quality function multiplied by a learning gain. The ILC model guarantees a quick convergence and greatly improves the optimization efficiency for industrial application. To apply the ILC model to optimizing the drawbead geometry automatically, schemes of seamless integration of computer-aided engineering system into the CATIA platform and the drawbead geometry parameterization are adopted. Lastly, the rapidity and practicability of the optimization method for drawbead geometry are verified by the numerical experiment of the engine hood inner panel. [ABSTRACT FROM AUTHOR]

Currently, more and more tailor-welded blanks (TWBs) are used in the automobile industry. It is very important to locate the weld line and predict its movement during the forming process. The initial weld line can be predicted by one-step finite-element analysis according to the desired weld line in the final workpiece. Meanwhile, weld line movement during the deformation process can be evaluated in advance. In this paper, the procedures of finite-element analysis of one-step FEM with TWBs are established. The contact between tool and blank and the effect of restraining forces under the blankholder due to drawbead and blankholder pressure are considered as well. Forming limit diagram (FLD) is used to show not only the tendency of reduction in thickness and fracture but also of increase in thickness and wrinkle. Hydraulic controlled pads used to clamp the weld line during the deep drawing process are simplified as static external force to eliminate the movement of the weld line. In order to speed up and ensure the convergence of Newton-Raphson iterations, energy-based 3D mesh mapping algorithm is introduced to obtain the initial solution. The above-mentioned methods have been implemented in the authors’ in-house one-step FEM code InverStamp. A rectangular cup drawing case demonstrates that this approach can be easily implemented to evaluate weld line movement and develop initial blank in sheet metal stamping with tailor-welded blanks. [ABSTRACT FROM AUTHOR]

This paper presents a more accurate predictor-corrector scheme that combines the stream line method (SLM) and feasible sequential quadratic programming (FSQP) using the explicit dynamic finite element method (FEM) to design the optimum blank in the deep drawing process of square cup with flange. It is clear that faster convergence and better results of calculating optimum blank shape are guaranteed when FSQP uses a better initial guess. But it is not easy to guess the initial blank shape due to the variation of blank thickness, material anisotropy, and friction on the flange area at the beginning in the deep drawing process. SLM can obtain a preliminary prediction of the optimum blank shape with a little computational effort, so with SLM it is feasible to predict the initial guess of optimum blank with the assumption of fixed height of square cup with flange. FSQP can continue to adopt the predictor obtained by SLM to correct the optimum blank efficiently and accurately. Then the optimum blank is used in the final simulation and experiment. From comparison of the target shape between the simulated and experimental results, a good correspondence is confirmed. Other comparisons of the punch load, punch stroke, and wall thickness of the target square cup also show good agreement. [ABSTRACT FROM AUTHOR]

Blank holder force (BHF) is one of the key parameters affecting the process of sheet metal stamping. Numerous researchers have demonstrated two primary failures (wrinkling and tearing) could be avoided if appropriate variable BHF profile was implemented during punch stroke. Several approaches are proposed to determinate optimal BHF by experiment or FEM simulation. In this study, new control objective was adopted based on the definition and analysis of BHF formability window. The PID controller was integrated into FEM code to conduct closed-loop forming simulation of step rectangle box, through which the optimal BHF trajectories were determined for each separated binder. The trajectories were verified on a multipoint variable BHF hydraulic press and showed good effect to avoid wrinkling and tearing. The proposed BHF determination approach in this paper can be applied to stamping parts of different geometry due to the application of new objective, which makes it a robust strategy. [ABSTRACT FROM AUTHOR]

Prediction of the influence of tool wear influence in sheet metal blanking/punching processes is investigated in this paper using the finite element method. In order to simulate accurately sheet metal-cutting processes by a material shearing mechanism, a finite element model valid for the numerical description of such processes has been developed. Damage and crack propagation has been taken into account by means of an elastoplastic constitutive law to study the effects of the variation of process parameters on the geometry of the sheared edges and the evolution of the force-punch penetration. Furthermore, a numerical investigation has been carried out to study the effect of tool wear on burr formation. During analysis, the initiation of a crack is assumed to occur, at any point in the sheet metal where the damage reaches a critical value. Crack propagation is simulated by the propagation of a completely damaged area. This is taken into account in the finite element model by a decrease of the stiffness of the broken elements. [ABSTRACT FROM AUTHOR]

Plastic deformation of the sheet metal occurs synchronously with elastic deformation of the elastomer in flexible die forming process using elastic die. Thus, adjusting elastic deformation state of the elastomer to adapt to stress state of the sheet metal may be a potential way to improve sheet-forming capability. In this work, various deformation modes of the flexible dies were established using elastomers with different structures, which aims at developing a new way to improve sheet-forming capability through controlling in-plane stress state of the sheet metal. Digital image correlation (DIC) technique was used to capture the displacement and strain field of the sheet metal in bulge tests. Finite element analysis (FEA) was conducted to obtain pressure field of the flexible die and in-plane stresses of the sheet metal. The results showed that forming load, bulged height, and bulge strain could be adjusted by using flexible dies with different structures. The sharp increase of normal pressure in local region at the flexible die/sheet interface leads to non-uniform distribution of radial and circumferential stresses of the sheet metal. A design strategy of flexible die with a hollow structure (i.e., type 6) was proposed to reduce localization of interfacial normal pressure. Due to evenly-distributed normal pressure and beneficial friction at the flexible die/sheet interface, radial and circumferential stresses subjected by the sheet metal were relieved effectively. And the consequent increase of bulge height and bulge strain indicates the improvement of sheet-forming capability. [ABSTRACT FROM AUTHOR]

Origami-based sheet metal (OSM) bending is a promising new die-free folding technique for sheet metal. OSM bending principle is based on deforming the material along a pre-defined fold line, which is determined using material discontinuity (MD) produced by laser or waterjet cutting. The objective of this work is to study and evaluate the fracture in OSM bending under the influence of various MD types, kerf-to-thickness (k/t) ratios, and sheet thicknesses. The research goal is to provide information on selecting an optimized k/t ratio and type of MD that allows for fracture-free bending. Four different ductile fracture criteria (DFC) are used and calibrated from experimental data to forecast fracture. The DFC calibration is used to produce a set of critical damage values (CDV) for assessing the possibility of fracture in the OSM bending. In addition, the study provides fracture evaluation using finite element analysis (FEA) integrated with experimental cases for a broader range of OSM bending parameters and MDs. The results demonstrated that an MD with a higher k/t ratio is less likely to fracture during the OSM bending, whereas a higher sheet thickness increases the possibility of fracture. Furthermore, the study identifies the k/t ratio limit that ensures successful bending without fracture and categorizes MD types into two groups based on fracture likelihood. The fracture in the first group is dependent on the limiting k/t ratio, whereas the possibility of fracture in the second group is independent of the k/t ratio due to its topology. [ABSTRACT FROM AUTHOR]

The clinched process of heterogeneous materials is increasingly used in automobile, aerospace, and household appliances manufacturing. To investigate the forming quality of clinched joint for dissimilar sheet metal, the clinching experiments are carried out, and the cross-section morphology of clinched joints is observed. The finite element model for forming steel-aluminum clinched joint is established. The influence of process parameters (forming process parameters, geometry parameters, and concave die structural parameters) on the forming quality of steel-aluminum clinched joint is analyzed. The evaluation of the joint after forming includes the critical dimension, deformation, and neck-lock ratio. Then, the strength of the joint is measured by the tensile shear test. The changing law of strength and the neck-lock ratio is analyzed. The design strategy of different process parameters is proposed. The results show that the forming process of the joint is well predicted by numerical simulation, and the joint quality is good. The neck lock ratio of the joint with the highest tensile and shear strength is close to 1 when the forming force is 40 kN. The tensile strength and shear strength of the joint are increased by 125% and 62.35%. The design strategy is verified by experiments. The shear strength of the joint is increased by 16.49%. [ABSTRACT FROM AUTHOR]

This study aims to develop a universal, robust, and linear method to obtain an initial guess configuration for the multi-step finite element method (FEM) solver of sheet metal forming. Using the decoupling theory, the deformation at each step in the multi-step FEM solver of the sheet metal forming is decoupled into two independent deformation modes: bending-dominated deformation and stretching-dominated deformation. The configuration of the bending-dominated deformation constrained by the sliding constraint surface is considered as the initial guess configuration for the current step in multi-step FEM solver. To get an accurate initial configuration at each step, the method of Laplace-Beltrami operator (LBO) on a simplicial surface is employed to obtain the initial guess configuration effectively. Several numerical examples are provided for validation and verification of the proposed method through its applications for complicated sheet metal workpieces of finite element simulations. The results show that the proposed method on the simplicial surface for the initial guess configuration within a few iterations to be significantly effective. [ABSTRACT FROM AUTHOR]

There are some problems in the CNC incremental forming with the sheet metal part containing vertical wall, such as excessive thinning and easy fracture. A new CNC incremental forming method was proposed based on the feature recognition for the multidirectional adjustment of the sheet metal posture. According to the feature recognition, the multi-feature of the sheet metal part was decomposed into a number of the forming feature units according to the size and distribution of the forming angles, so that the forming angles of the surface are less than the forming limit angle. The feature recognition and forming feature unit generation algorithm, sheet posture control method, and sheet posture surface generation algorithm were studied, and the finite element analysis and the forming experiments based on the proposed algorithms were also presented. The research results show that the proposed method can change the sheet metal posture in real time during the CNC incremental forming process and realize the non-rupture forming with the large forming angle parts such as vertical wall parts. [ABSTRACT FROM AUTHOR]

drawbead modeling technique is presented to improve the accuracy of finite element simulations in terms of part draw-in and thickness predictions and validated with channel drawing experiments of a high-strength low-alloy steel. The drawing characteristics of 1.5-mm thickness blanks are obtained with strip drawing tests with a round drawbead, and drawbead model parameters are computed for three bead settings. The consequences of bending deformation cycles are determined experimentally on strip draw-in and thickness values, and model limitations of equivalent drawbead elements are also assessed for test conditions in which the drawbead restraint force is lower than the sectional yield limit. The influence of omitted drawbead geometry and overestimated drawbead-exit thickness are described using an analytical model, and a closed form expression is obtained to correct draw-in model error under sectional deformation conditions. Blank thickness and equivalent strain at the drawbead exit are additional drawbead model parameters of the proposed technique. Then, drawing simulations of a variable section, open-ended channel part are performed. The drawbead design, bead settings and tool-blank interface conditions are identical to those in strip drawing tests. Computed draw-in and thickness distributions were compared with measurements on produced channels using an experimental channel draw die. It is concluded that simulation models, based on drawbead force parameters only, overestimate blank thickness at the die entry and bring about relatively high draw-in values along part border lines. The thickness distribution predicted with proposed technique shows an enhanced correlation with on-part thickness measurements, and bead penetration effects on channel border lines are also simulated acceptably. [ABSTRACT FROM AUTHOR]

The present study aims to determine the optimum blank shape design for deep drawing of arbitrary shaped cups with a uniform trim allowance at the flange, i.e., cups without ears. The earing, or non-uniform flange, is caused by non-uniform material flow and planar anisotropy in the sheet. In this research, a new method for optimum blank shape design using finite element analysis is proposed. The deformation process is first divided into multiple steps. A shape error metric is defined to measure the amount of earing and to compare the deformed shape and target shape set for each stage of the analysis. This error metric is then used to decide whether the blank needs to be modified. The blank geometry change is based on material flow. The cycle is repeated until the converged results are achieved. This iterative design process leads to optimal blank shape. To test the proposed method, three examples of cup drawing are presented. In every case converged results are achieved after a few iterations. The proposed systematic method for optimal blank design is found to be very effective in the deep drawing process and can be further applied to other sheet metal forming applications such as stamping processes. [ABSTRACT FROM AUTHOR]

The deep drawing process, one of the sheet metal forming methods, is very useful in the industrial field because of its efficiency. The limiting drawing ratio (LDR) is affected by many material and process parameters, such as the strain-hardening exponent, the plastic strain ratio, friction and lubrication, the blank holder force, the presence of drawbeads, the profile radius of the die and punch, etc. In order to verify the finite element method (FEM) simulation results of the LDR, the experimental data are compared with the results of the current simulation. The influences of the process parameters such as the blank holder force, the profile radius of the die, the clearance between the punch and the die, and the friction coefficient on the LDR are also examined. The abductive network was then applied to synthesize the data sets obtained from the numerical simulation. The predicted results of the LDR from the prediction model are in good agreement with the results obtained from the FEM simulation. By employing the predictive model, it can provide valuable references to the prediction of the LDR under a suitable range of process parameters. [ABSTRACT FROM AUTHOR]

A comprehensive finite element method employing a subroutine to link up submodules of commercial code ANSYS is proposed to perform analysis of projection welding in quantitative detail. In order to solve the complexity due to dynamic changes in heat and electrical current flow paths, as well as temperature-dependent material properties, information about contact interfaces and the geometry of the projection areas have been taken into account. By updating parameter information among these submodules in an incremental manner, a truly thermal-electrical-mechanical coupled numerical analysis is realized for projection welding simulation. A case study of an automotive door assembly welding process is carried out and a series of experiments is conducted to confirm the validity of the newly developed method. The agreement between the experimental and numerical results is satisfactory, indicating that the incrementally coupled finite element method may be suitable for projection welding research. Finally, future work to extend this method in optimizing projection welding process design is also presented. [ABSTRACT FROM AUTHOR]