A new criterion to determine wrinkling in deep drawing and stretch forming processes

Abstract

At present, in order to improve the global warming effect and the deterioration of the environment, series of solutions have been proposed for the automotive industry that uses fossil fuels as the driving energy source. The lightweight of cars is one of the most important solutions. The light weight of a car implies that the car's body weight should be reduced as much as possible while ensuring the strength and safety performance of the car, thereby improving the power of the car, reducing fuel consumption, and reducing exhaust pollution. However, thinner and lighter sheet formed parts, although successfully reducing the weight of the body, they have presented unprecedented challenges and requirements for the metal forming technology.The major problems faced by the forming technology applied in automotive industry are, for example, how to use lightweight materials to maintain the structural properties, or to get even better properties than traditional formed parts, and how to avoid and reduce the defects often appearing in forming processes. For this reason, the accuracy of finite element analysis (FEA) in metal forming technology is more important than ever, since the used new lightweight materials and the thin sheet materials usually have reduced bending resistance which can easily lead to surface defects like wrinkles during manufacturing process like deep drawing and stretch forming. If such surface defects can be predicted in production design stage and SE-stage (Simultaneous Engineering), the manufacture period and cost of a new car model will be reduced by changing the part and structure design in advance. This work focuses on the wrinkling criteria used in FEA to predict surface defects in forming process. For that reason, two kinds of conventional wrinkling criteria were first reviewed in Chapter 4. The first conventional wrinkling criteria mentioned in this work was based on force-displacement curve determined from experiments, for example “turning point” approach and “gradient of wrinkling height development” approach was reviewed in Chapter 4.1. However, these approaches cannot be used in FEA to predict surface defects caused by wrinkling. Furthermore, the second conventional wrinkling criteria discussed in this work was based on the wrinkling limit curve (WLD) in forming limit diagram (FLD). The feasibility of this kind of strain-based wrinkling criterion is also discussed in Chapter 4.1. Experiments at the Institute for Metal Forming Technology (IFU) have shown that this strain-based wrinkling criterion does not accurately reflect the wrinkles in the complex deep drawing situations. Only simple geometry like conical-cup test (CCT) and car-fender-based geometry (KVG) can be validated with strain-based wrinkling criterion under limited conditions. As shown in this work, firstly, conventional wrinkling criteria do not accurately predict wrinkles during the forming process. Secondly, traditional wrinkling criteria cannot predict changes in surface quality during the forming process. Therefore, a new criterion that can predict wrinkles and changes in surface quality during manufacturing process was developed in this work. For that reason, this new criterion aims to establish a new approach for predicting the surface quality of sheet metal parts, taking into consideration tool contact. This newly developed criterion is called “Double-Surface Compressive-Stress-Approach” (DSCS-approach). By using this approach, not only wrinkles, but also wrinkling initiation and slight surface defects, like surface lows and buckling effects, can be predicted using FEA. Two essential time points, namely “wrinkling initiation” (WI) and “developed wrinkle” (DW), can be defined using DSCS-approach by evaluating the surface stress difference between outer side and inner side of component in FEA. By using these two time points, the development of surface defects caused by wrinkling phenomenon can be observed according to reference experiments at IFU. The introduced and performed reference experiments are, buckling tests with modified Yoshida specimens and mini-conical-cup tests. In the time point “wrinkling initiation”, the investigated part region, which reveals wrinkling tendency, shows the maximum buckling height and tends to change surface defect type from “buckling” to “wrinkling”. When the local bending resistance is high, the surface profile remains in a buckling state. The phenomenon was observed during buckling tests with G-Series modified Yoshida specimen (M-YBT-G, “G” means specimen with large shoulder radius) and by performing mini conical cup tests with thick materials. On the other hand, when the local bending resistance is low, a typical wrinkled surface profile occurs. This phenomenon was observed during buckling tests with K-Series modified Yoshida specimen (M-YBT-K, “K” indicates specimens with small shoulder radius), and by performing mini conical cup tests with thin sheet metal materials. The concept of DSCS-approach was continuously developed as regression model considering the effecting parameters “sheet metal thickness” and “local curvature” in this work. Finally, regression model was prepared as User-Defined-Variable-Files (UDV-Files) which can be implemented in FEM software like AutoForm R6 and R7. The DSCS-approach was validated. The validation results show that the DSCS-approach can accurately predict the location of surface defects. In addition, by using the DSCS-approach, the first wrinkles can be predicted at the correct drawing depth. In addition, by setting the critical strain value in the UDV-file, it is possible to detect the wrinkles in the elastic deformation stage (small effective strain value). Since the DSCS-approach is a stress-based criterion, there is no need to consider the effects of non-linear strain paths in UDV-files. In future, if enough sheet metal materials are validated, an individual model should be developed to calculate the critical value of stress between outer side and inner side at defined “wrinkling initiation” and “developed wrinkles”. By using this stress-based wrinkling criterion, the wrinkling position actually cannot be stored in AutoForm software. In this work, the wrinkling positions were manually marked at corresponding drawing depth.