저자명 주근수 
년도 2014 
The tension/compression hardening behavior is important in sheet metal forming processes because of complicated lo-ading paths. Experimental methods to measure the tension/ compression behavior have not considered the effect of the strain rate although the strain rate is related to the hardening behavior of sheet metal. The tension/compression tests need to be conducted considering the strain rate to acquire accurate hardening behavior.

This paper deals with an experimental technique to measure the tension/compression behavior of sheet metal at strain rates from 0.001 s-1 to 50 s-1 for real sheet metal forming. The experimental method involves design of a specimen, a clamping device and devices to impose pre-strain and compression. In order to prevent buckling of a specimen, an opti-mum specimen design and a clamping device are required. Euler eqaution and secant formula proposed by Boger ea al [6] were applied to design a specimen for the tension/compression test at intermediate strain rates. The specimen having a new dimension was used for the tension/compression test at intermediate strain rates. A new clamping device was developed to prevent a sheet specimen from buckling under compression loading condition. Bae [2] proposed a   clamping device to impose sfficient clamping force. Compression type coil springs were used for the clamping device   proposed by bae to imopse clamping force. This clamping device, however, is not suitable for measuring the strain by  the DIC method. In this paper, a newly developed clamping device was used by modifying the device proposed by Bae. A newly developed device has an advantage for measuring the stain by the DIC method and imposing uniformly dis-tributed clamping force. The static material testing machine(Instron 5583) was utilized for tension/compression tests

from quasi-static states to low strain rates ranging from 0.001 s-1 to 0.1 s-1. High speed material testing machine(HSM-TM) was used for tension/compression tests at intermediate strain rates ranging from 1 s-1 to 50 s-1. Techincal functions, for imposing pre-strain and reverse loading, are required for HSMTM. Devices to impose pre-strain and reverse loading, therefore, were developed for HSMTM. Tension/compression tests ranging from 1 s-1 to 50 s-1 were available by using these devices.

Tension/compression tests have been conducted at various strain rates for SPCC and DP590 with pre-strain of 10%.   Hardening behaviors under the tension or compression loading condition were compared to each other at severe strain rates. Quantitative analysis was carried out by comparing the Bauschinger ratio with respect to the strain rate. Transient softening due to the Bauschinger effect was observed as well as the increase in flow stress with increasing the strain rate from the test results.

 

 

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