저자명 배기현 
년도 2011 
The sheet metal forming is a popular manufacturing technique to obtain the desired shape of a product by imposing the plastic deformation. Recently, many parts of an auto-body structure are manufactured by using the sheet metal forming process. In order to reduce the manufacturing time and cost of sheet metal parts, the numerical method using the finite element analysis has been actively adopted for the process design in the tryout stage. However, the accuracy of the numerical method is still not satisfactory for predicting the spring-back phenomenon. Spring-back usually occurs the severe assembly problem of automotive parts by welding. To improve the spring-back predictability of the numerical method, many researchers have been conducted to obtain the accurate hardening behavior during the tension/compression loading condition. However, researchers have been only concerned with the pre-strain in tension/compression experiments. The strain rate is well known as an important parameter to influence the hardening behavior of sheet metals. The sheet metal experiences the sensitive strain rate change during the forming process since the punch progresses with the velocity of several m/sec to form the product. Therefore, the strain rate effect should be considered in tension/compression experiments to obtain accurate hardening behavior during the actual forming process.
This paper deals with the tension/compression hardening behavior of auto-body steel sheets considering the pre-strain and the strain rate. To conduct tension/compression experiments with respect to the pre-strain and the strain rate, an experimental method was established by using a newly developed clamping jig of the specimen to prevent buckling. The measured hardening behavior was fitted by the two-surface model which is a representative constitutive model for describing the tension/compression hardening curve.
To prevent buckling of a specimen is the most important point to conduct successful tension/compression experiments of sheet metals. In order to prevent buckling, a specimen should be designed carefully and the sufficient clamping force should be applied to the specimen during the tension/compression experiment. Recently, Boger et al performed the optimum specimen design using Secant formula and Euler equation. Based on their research result, a new specimen shape was designed for the tension/compression test by modifying the standard specimen for the static and dynamic tensile test. The clamping force was determined by using the analytical model to calculate the clamping force to suppress the buckling or the wrinkling at the flange in the forming process proposed by Wang and Cao.
A new clamping jig which has the small and simple structure was manufactured for the tension/compression experiment considering the pre-strain and the strain rate. To impose the desired clamping force, compression-type coil springs were used for the new clamping jig. After that, specific dimensions of the clamping jig were determined by considering the specimen and spring shape. Gap controllers were used to impose the accurate clamping force to the specimen by controlling the deflection of coil springs. Before the tension/compression experiment, the compression test of coil springs was conducted to check the linearity and repeatability of springs since the performance of coil springs is directly related to the reliability of the clamping jig. After the verification of the performance of coil springs, the tension/compression experiment of auto-body steel sheets was performed with respect to the pre-strain and the strain rate.
INSTRON8801 testing machine, which is generally used for the dynamic material fatigue test, was used to impose the tension/compression loading condition. The velocity range to provide reliable experimental data is limited with respect to the actuator displacement since the testing machine uses the hydraulic system. The limit strain rate of with respect to the actuator displacement was determined based on the normalized time-displacement curve during the tension/compression loading. Based on the range of the limit strain rate with respect to the actuator displacement, experimental table was established for the tension/compression test of auto-body steel sheets considering the pre-strain and the strain rate.
Before the tension/compression test, the cyclic loading test was performed with respect to the strain rate. For the cyclic loading test, the amplitude of the cycle is fixed to 1 mm. The maximum and the minimum load were measured in the tension and compression state of each cycle, respectively.  From the measured data, the cyclic hardening or softening phenomenon with the strain rate sensitivity were observed according to auto-body steel sheets.
The tension/compression hardening behavior of auto-body steel sheets is measured with respect to the pre-strain and the strain rate. During experiments, the Teflon film is attached between the specimen and the clamping plate in order to improve the lubrication condition. The measured data was transformed to true stress-strain curve by correcting the strain error and the frictional and biaxial effect of the test jig. After the correction procedure, reliable stress-strain curves were obtained with respect to the pre-strain and the strain rate. From stress- strain curves, the hardening behavior of auto-body steel sheets was compared carefully with respect to the pre-strain and the strain rate. Bauschinger and transient behaviors in the compression state were also compared to explain effectiveness of the pre-strain and the strain rate quantitatively.
The two-surface model proposed by Lee et al. was employed to describe the tension/compression hardening behavior. The gap between the loading and bounding curves was fitted by Swift model. The conventional two-surface model is modified in order to consider the pre-strain and the strain rate effect simultaneously. The sensitivity of the pre-strain and the strain rate is approximated by the exponential-type function. An accuracy of the modified two-surface model was evaluated by comparing the experimental data and the fitted line by the modified two-surface model. The spring-back predictability of the numerical simulation can be improved by implementing the tension/compression hardening behavior using the two-surface model.
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