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Design of sheet metal stamping simulation system

Abstract This paper discusses the design and implementation of computer simulation system for sheet metal stamping process, including structured and object-oriented methods

keywords sheet metal stamping computer simulation finite element method system design

sheet metal stamping computer simulation involves mathematics, mechanics, material science, stamping technology, computer science, computer graphics... It needs to integrate multidisciplinary knowledge for research; It is highly theoretical and widely applied. In order to develop a computer simulation software of sheet metal forming process with high calculation efficiency, strong adaptability, stability, reliability and complete functions, it is necessary to integrate stamping process CAE and die CAD, that is, on the one hand, CAD system is used to establish geometric model for numerical analysis, on the other hand, numerical analysis is used to evaluate CAD design results, optimize process parameters and optimize die structure. The application of sheet metal stamping computer simulation technology in engineering practice is a powerful means to fundamentally improve the current mold design mode, is one of the key factors to promote the technological progress of mold industry, and is of great significance to realize the modernization of industrial production

1 simulation principle

sheet metal stamping is to use the mold to make the metal sheet plastic (1) it is not allowed to bend over and place the sample after the pendulum is raised; In the experiment, it is a forming method that allows people to deform in the swing plane of the pendulum to produce shell parts. In sheet metal stamping, due to the complexity of workpiece deformation law, forming defects such as cracking and wrinkling are easy to occur in stamping. In addition, the workpiece produces internal stress in balance with the die load in stamping forming. After the die is removed after stamping, the workpiece will rebound due to the release of internal stress. After cutting the process waste, it will rebound again, making the final shape of the workpiece difficult to be accurately controlled. The distribution of displacement, stress and strain of workpiece in the forming process can be obtained by forming process simulation; The possible wrinkling can be predicted by observing the deformation shape of the workpiece after displacement; According to the position of the principal strain at each discrete point on the sheet metal forming limit curve or by using the damage mechanics model, the possible fracture in the forming process can be predicted; The springback process can be simulated by releasing the external force on the workpiece or the binding force on the cut part, and the shape of the workpiece after springback and the distribution of residual stress and residual strain can be obtained. This can provide a scientific basis for optimizing stamping process and die design

The core of computer simulation of sheet metal stamping is the application of numerical methods to analyze and study the plastic forming of sheet metal. As the most widely used and vital method in numerical analysis, finite element method has become the most effective method for numerical analysis of sheet metal forming [1,2]. Implicit integration method and explicit integration method are two main algorithms for geometric and material nonlinear problems in finite deformation of continuous media. Implicit algorithm is to establish a high-order nonlinear equation group based on the principle of virtual work, and use Newton Raphson iterative calculation to solve the equation group. The calculation is accurate and reliable, but in each incremental step, it is necessary to form a large sparse stiffness matrix for repeated iterative calculation, which requires a large amount of calculation and takes up 2 Comparing the performance of nylon material with that of traditional oil pan material, if you want to keep ahead, you can't relax. There is a lot of storage space, and there is a very serious convergence problem, especially in the analysis of highly nonlinear processes such as sheet metal forming. Therefore, the development of computer simulation software for sheet metal stamping forming process uses less implicit algorithm and more explicit algorithm. The quasi-static sheet metal forming problem is virtually regarded as a dynamic process, and the dynamic explicit algorithm is used to analyze it. Based on the time center difference scheme, the calculation of the finite element equation is explicit, avoiding the iterative calculation and the convergence problem caused by nonlinearity. Using lumped mass matrix to decouple the simultaneous equations into independent equations can greatly simplify the calculation. Since the 1980s, the explicit algorithm has been applied in sheet metal forming analysis and gradually achieved satisfactory results. For the system dynamic problem, the basic idea of the explicit algorithm is that the virtual work equationcan be obtained from the virtual work principle. In the formula,

fi is the physical density; ρ Is the mass density; ν Is the damping coefficient; Ti is the force boundary Γσ External force acting on; TCI is the external force acting on the contact boundary. The finite element equation

can be obtained by finite element discretization, in which m is the consistent mass matrix; C is the uniform damping matrix; P is the external force vector; F is the internal force vector

the mass matrix is diagonalized by using the method of concentrated mass. On this basis, the damping matrix is also diagonalized. The central difference method is used to discretize the time, and multiple independent equations can be solved without iteration. The explicit algorithm also has its own problems, such as the influence of dynamic effect, that is, the virtual inertia force. In addition, due to the limitation of the calculation stability of the central difference method on the time step, if the springback of the stamping part is analyzed as the transition process of the dynamic system with the dynamic explicit algorithm, it will require a lot of calculation steps and cost a lot to make the system reach a stable equilibrium state. The implicit algorithm is unconditionally stable, so a large enough time step can be used, and the number of calculation steps is very small. Using explicit algorithm and implicit algorithm to analyze the forming process and springback can give full play to their respective strengths, which is a more popular explicit implicit synthesis algorithm at present

2 system implementation

the development of sheet metal stamping simulation system can adopt structured method or object-oriented method

2.1 structured method

in the mid-1970s, the design idea of structured system developed. Using structured method to design complex sheet metal stamping simulation system is widely used in actual software development. Through system analysis, in the design stage of the system, determine the overall structure of the stamping simulation software system and the relationship between modules, define the interface between modules, design the global data structure, determine the interface between the system and other software and users, and design the specific algorithm and data structure of functional modules and other internal details

structured system design emphasizes top-down functional decomposition, and decomposes the system level by level into modules and sub modules. When the simulation system of sheet metal stamping is divided into modules, the coupling degree between modules should be reduced as much as possible to increase the cohesion of each module. Make full use of these two complementary design principles, and try to improve the independence of the module. When modifying and maintaining a module, the scope of modification can be controlled to a minimum, and the impact on other modules will be minimized

2.2 object-oriented method

although the traditional structured software engineering method improves the development efficiency of sheet metal stamping simulation software and the maintainability of the system to a certain extent, it still does not improve the reusability, scalability and the ability to embed other systems of the software. The reason is that the structured method adopts the task-oriented view, that is, it is designed for a certain task, This methodology has created a gap between the analysis stage and the design stage, resulting in the inconsistency between the problem domain in the analysis stage and the solution domain in the design stage. The object-oriented method is a new software engineering method established to get rid of this inconsistency. Its guiding ideology density is about 1.04~1.06 g/cm3. It is to establish the problem domain model according to people's usual way of thinking, and design the software that can naturally express the solution method as much as possible. Therefore, it is necessary to establish the concept of directly expressing the things that make up the problem domain and the interrelationships between things, and establish a description paradigm suitable for people's way of thinking. In object-oriented method, object and message passing represent things and the interrelationship between things respectively; Class and inheritance are descriptive paradigms that adapt to people's way of thinking; Methods are various operations that act on objects: the above concepts constitute the basic content of object-oriented methodology. The basic characteristics of objects and classes lie in the encapsulation and inheritance of objects. Through encapsulation, the independence and information concealment of objects are improved; Through inheritance, the interconnection between classes is realized, resulting in dynamic bunching and entity polymorphism. The object-oriented method improves the consistency of analysis, design and implementation, and makes the system reusable and extensible

the rapid development of finite element method and its application field requires that the software developed with it as the core has a high degree of reusability and scalability. At the same time, finite element software has increasingly become an integral part of cad/cam software. It requires that it has a higher ability to be embedded in other systems. Using traditional methods to develop finite element software is difficult to meet these requirements. It is imperative to develop object-oriented finite element methods and technologies, Therefore, object-oriented method is gradually considered to develop sheet metal stamping simulation software system, which is a new attempt and needs time to explore. Object oriented design uses the formal analysis model to expand its construction part, improve the description of classes and object entities, and then form a complete software model. This software model can be realized by the solution space object provided by a programming language, so as to get the required software. The process of designing a sheet metal stamping simulation system using object-oriented method can be summarized as follows: ① define the attributes of each object class; ② Determine the relationship between classes and objects, and between classes; ③ Define the communication mechanism between objects, mainly to determine the message mode of objects and the message transmission between objects, so as to constitute the control flow and information flow of the system; ④ Determine the state of each object and the method to realize each state

2.3 system implementation

in essence, stamping simulation is to describe the stamping process of sheet metal in punch, die Constrained motion under the action of tools such as blank holder moving in a given way (deformation movement)], therefore, when establishing the object model, the structure of the stamping system can be described by the aggregation tree shown in Figure 1. It shows that the stamping system is composed of tools and workpieces. The tools are usually punches, dies, blank holder rings, reverse punches and other molds, and the workpieces represent sheet metal deformation parts. In the stamping process, the properties of each tool are different, and the role is different, and the sheet metal is formed under the constraint of the tool. From here, we can see that the object model By combining the concept of object-oriented with the concept of information modeling commonly used in traditional methods, the common information model is improved and broadened, and the comprehensibility and expression ability of the model are enhanced

the module composition of the sheet metal stamping simulation system and the call relationship between modules are shown in Figure 2. The front and rear processing module is the information exchange and scheduling center. In addition to completing the work of forming the analysis model of discrete geometric model and visualizing the calculation results, it is also responsible for the task of information intersection and transmission, and commanding the work of several other modules; The geometric modeling module is used to carry out the initial design and optimize the design according to the analysis results of the simulation by the expert system; The two modules of forming and springback simulation make numerical analysis of the deformation process of the discrete workpiece, and the expert system module evaluates the quality of the design according to the calculation results, determines the design or puts forward correction opinions

the process of sheet metal stamping forming simulation system is shown in Figure 3, and the functions of each part are as follows: ① geometric modeling. Establish punch and blank holder

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