Forming
- Mechanical Properties
- Simulative Formability Experiment
- Surface Friction Characteristic
- Coating Adhesion
- Dent resistance of sheet and parts
- Engineering application of CGA (Circle Grid Analysis) technology
- Finite Element Simulation
Mechanical Properties
The first thing for the automobile customer is to form the auto sheet to the desired shape and dimensions. The forming of the auto sheet is a very complex process related to the mechanical property, friction coefficient, die and stamping conditions. The mechanical properties are principally concerned by customer, which includes ¦Òs, ¦Òb, ¦Ä, r value, n value. Generally, the higher ¦Ä, n value and r value is useful for forming process.
Simulative Formability Experiment
Besides the fundamental mechanical properties, it is also important to evaluate the simulative formability corresponding to the typical forming processes. The relationships between the material properties, die and stamping parameters can be studied by the digital video, image processing and numerical analysis.
The main items of simulative formability experiment include: springback performance of "U-bending test" test, hydraulic bulge test, cupping test for evaluating bulging performance, forming limit diagram test (FLD), drawing experiment for evaluating drawing ability, conical cup experiment for evaluating both drawing and bulging ability, hole-expansion experiment for flanging ability and so on.
FLD experiment is extremely important among the previous experiments. FLD is composed of ultimate strain in different strain path, which describes the forming limit under various strain conditions and is very helpful for the analysis to the forming process of complex parts. The typical FLD can be shown as follows:
Surface Friction Characteristic
Besides the forming property of steel, the friction condition between steel surface and die is also an important factor for the formation result, but it is difficult to obtain friction condition accurately. The drawbead tester is a special equipment to investigate the surface condition, which can simulate forming process and measure friction coefficient for researching the influence of lubrication, roughness and temperature on formation. It can also measure the degree of coating adhesion to determine the surface quality and corrosion resistant. The purpose of friction experiment is to estimate the friction character of auto sheet or lubricating oil. The operating temperature is from normal temperature to 120oC.
Coating Adhesion
More and more Zn-coated sheets are used in automotive, and in result the research of adhesion is becoming more and more important and necessary. Experiments can evaluate the adhesion characteristics of Zn-coated steel sheet in the condition of draw bead. The testing temperature is from normal temperature to 120oC.
Dent resistance of sheet and parts
More and more high strength and ultra-thin steel sheets are used in exposure parts in automotive industry in order to reduce weight for saving energy. Dent resistance is used to evaluate the alternation of stiffness caused by thinner sheet under the cross direction concentrated load. It contains local dent resisting force and dent reisistance rigidity.
Dent resistance experiment can also evaluate the dent resistance of simulative parts. It can give a repetitive comparable outcome in quasistatic (low velocity) dynamic state (high velocity). Consequently it is usually used to choose material.
Local dent resisting force will increase when thickness is increased. It can also be seen from the above diagram: Local dent resisting force will increase with the increasing of yield strength.
Because the dent resistance experiment can give a repetitive comparable outcome in quasistatic (low velocity) dynamic state (high velocity), it also can estimate the service performance of finished product.
Engineering application of CGA (Circle Grid Analysis) technology
The safety of forming can be evaluated with the use of forming limit diagram (FLD), CGA and thickness-strain analysis (TSA). The CGA technology can be illustrated by the following process:
From the curve, it is very clear that the part is in the safe state through CGA.
Finite Element Simulation
Computer simulation technology of automobile, a kind of computer aided engineering (CAE), is used during of the whole course of auto designing stage. Stiffness analysis, crash-simulation and modal analysis are used to check if the auto structure meets the safe requests. VPG is used to check if the auto structure, the parts and the welded joints have good ability to withstand fracture. Stamping simulation is used to check if the parts have good formability. So the computer simulation technology of automobile effectively shorten the cycle of auto design, decrease the cost and increase the competitive ability of car-manufacturers.
The formability of the part is improved by optimizing the dispose and intensity of drawbeads and adding cutting.
