Search Results

A ductile failure criterion (DFC), which defines the stretching failure at localized necking (LN) and treats the critical damage as a function of strain path and initial sheet thickness, was proposed in a previous study. In this study, the DFC is revisited to extend the model to the low stress triaxiality domain and demonstrates on modeling forming limit curve (FLC) of TRIP 690. Then, the model is used to predict stretching failure in a finite element method (FEM) simulation on a TRIP 690 steel rectangular cup draw process at room temperature. Comparison shows that the results from this criterion match quite well with experimental observations.

Based on findings from micromechanical studies, a Ductile Failure Criterion (DFC) was proposed. The proposed DFC treats localized necking as failure and critical damage as a function of strain path and initial sheet thickness. Under linear strain path assumption, a method to predict Forming Limit Curve (FLC) is derived from this DFC. With the help of predetermined effect functions, the method only needs a calibration at uniaxial tension. The approach was validated by predicting FLCs for sixteen different aluminum and steel sheet metal materials. Comparison shows that the prediction matches quite well with experimental observations in most cases.

Hot stamping or so-called continuous press hardening is a process to make sheet metal parts with yield-tensile strength up to 1150Mpa-1550Mpa. Due to the high specific ratio of quenched Boron steels, which is higher than those of aluminum alloys and magnesium alloys, the components with low mass can be made from hot stamped Boron steels. In current industrial practice, direct hot stamping process, which forms a part directly from a flat sheet blank, is normally used to make geometries with relatively mild deformation, such as B-pillars, A-pillars etc. In this study, indirect hot stamping is introduced to develop geometries with a deep cavity and complex form features. Since the indirect hot stamping develops the part cavity depth in cold drawing and then forms detail features in hot stamping, part with complex geometry can thus be formed. A rocker component is chosen to demonstrate the technology.

Sheet metal forming at elevated temperatures, or so-called sheet metal warm/hot forming, is a relatively new forming process to make sheet metal parts with low mass. An accurate and convenient description of forming limit is critical for the success of forming process design and improvement. Strain-based Forming Limit Diagram has long been used to describe forming limit in cold sheet metal forming. However, at elevated temperatures, the formability of those sheet metals is strongly governed by both temperatures and strain rates. In order to extend the Forming Limit Diagram method into elevated temperature domain, a large number of forming limit curves are intuitively required to cover different temperatures and strain rates. It is not only costly to obtain but also inconvenient to apply those forming limit curves in industrial practice.

Due to its capability to make tubular components with high structural rigidity and low mass, tube hydroforming (THF) is an important manufacturing process to make lightweight automotive structural components, such as engine cradles, crank shafts, seat frames, roof bow and instrument panel beam etc. In order to integrate more functions, tube hydroformed components (THC) usually have complex geometry and are formed from a straight tube usually by three stages, which are programmable CNC bending, pre-crush and hydroforming. Since the complexity of component geometry, failures, such as fracture and buckling, could happen simultaneously at different spots. Tube bending and hydroforming process are designed to eliminate multiple failures and thus the THF development is tedious and time consuming. In this study, a multiobjective dynamic programming method is developed to optimize the THF process and demonstrated on an automotive component.

Multi-stage draw process is an important stamping method to make complex sheet metal parts for automotive industry. Due to complexity, its development is challenging and time consuming. In this study, the multi-stage draw development is modeled as a multi-stage decision making process. Based on sheet metal incremental plastic deformation theory, drawability and failure criteria, a dynamic programming type solution is proposed and demonstrated by FEM simulations on a rectangular geometry with high reduction ratio. Due to the recurrence nature of dynamic programming, the method provides a possibility of automatic process design of multi-stage draw.

Friction plays an important role in the deep drawing process. Previous research shows friction condition can be tailored by applying micro-textures on tooling surfaces. A friction model is proposed to reveal the mechanism of altering friction condition by configuring micro-texture. A discrete friction concept is proposed to improve drawability of sheet metal and demonstrates numerically on a non-symmetric geometry drawing process.

Progressive stamping is an important manufacturing process in making automotive parts. Springback and static loading performance are two critical concerns in the part development. An accurate FE-based prediction on springback and static loading performance could foresee and avoid costly pitfalls in future part development. A bracket, which has tight tolerance and loading performance requirement, is chosen for this study. Implicit and explicit solutions, two major algorithms for solving the FEM motion equations, are used to simulate progressive forming process, which is composed of rib embossing and multiple bending. The springback effect is predicted on the ‘part’ formed by both implicit and explicit solutions. Measurement on the physical part shows the implicit solution can provides a more accurate prediction on both the thickness and springback.

Bulb shields are important components in automobile headlamps and are primarily manufactured by progressive draw dies. Among progressive forming, multiple station progressive draw dies are the most formidable and thus among the most difficult of all progressive dies. Draw processes of a dome shape bulb shield are modeled and analyzed by FEM code DYNAFORM v5.2. Simulation prediction matches well with the measurement from the prototype test. A surface distortion problem is also identified and a solution is obtained through punch nose radius effect on stress distribution analysis. The tooling based on the suggested design has been manufactured and is running successfully in a Cleveland based production plant. Since the forming problems can be visually identified before physical tooling is manufactured, compared with the traditional development, the tooling tryout time has been shortened from several weeks to several days.