The light-weight and safe design of auto-body structures becomes an important challenging issue in the automotive industry in order to increase the fuel efficiency satisfying the emission-gas regulation of vehicles and to ensure the safety of passengers against the car crash. In order to achieve the two contradictory purposes, the crash analysis of the high speed deformation has to be accurately carried out with the accurate stress-strain curves at the high strain rate. The flow stress of a material generally increases as the strain rate increases, and that is regarded as the inherent characteristics of a material. An accurate car crash simulation needs accurate information of material data at the strain rate up to several hundred per second since the strain rate of the local deformation in the car crash reaches up to the range of 300 /sec - 500 /sec. Particularly, material properties of steel sheets is very important to the car crash since 70 % of an auto-body is composed of steel sheets and deformation of steel sheets is more sensitive to the strain rate than that of other materials.
Experimental techniques to obtain the relation of the stress and the strain vary according to the strain rate. At the low strain rate under about 1/sec, 'general mechanical or hydraulic tensile testing machines are used to acquire the stress-strain curve since the process of deformation is quasi-static and isothermal. When a steel sheet deforms faster than tens per second, the inertia effect and the stress wave propagation become so important that the tensile properties are changed remarkably with respect to the amount of the strain rate. The split Hopkinson pressure bar apparatus is a popular experimental technique for identification of dynamic material characteristics at the high strain rate ranged from 1000 to 10000/s. However, at the intermediate strain rate up to 500 /sec, the split Hopkinson bar is no longer applicable since the quasi-static and dynamic aspects of deformation take place simultaneously. In the intermediate strain rate, some special testing equipment needs to be developed to obtain the stress-strain curve.
In this paper, the tensile testing method with variation of the strain rate at the intermediate strain rate is established and tensile tests of 22 different steel sheets for an auto-body have been performed to obtaine dynamic material properties. In order to apply material properties from tensile tests to the crash analysis, a new material constitutive equation is suggested to describe dynamic behavior of steel sheets for an auto-body accurately.
A high speed tensile testing machine was developed for tensile tests at the intermediate strain rate and has the maximum velocity of 7 m/sec to obtain the tensile material properties at the strain rate of up to 500/sec. A simple jig fixing a specimen was designed to diminish the load ringing phenomenon induced by unstable stress propagation at the high strain rate and to enhance the accuracy to acquire the stress. The dimensions of specimens that can provide the reasonable results were determined by the finite element analysis.
The tensile testing of steel sheets for an auto-body have been carried out to obtain stress-strain curves at the strain rate of 1 /sec to 200 /sec. The test results provide new tendency of stress-strain curves at the intermediate strain rates and demonstrate that the strain hardening and the strain rate hardening are strongly coupled together. The low strength steel is more sensitive to the strain rate than the high strength steel and the strain rate sensitivity generally decreases as the strain increases and the strength of a steel sheet increases. In contrast to the conventional concept that the tensile elongation is generally decreased as the strain rate is increased, experimental results of steel sheets reveal a different viewpoint. The elongation is decreased as the strain rate is increased at the low strain rate up to 0.5 Is. However the elongation increases at the range from 0.5 /s to about 10-20/s and decreases again above 10-20 Is as the strain rate increases. It is noted from the experiment that as the strain rate increases, the development of necking is restrained, so the necking spreads to the neighboring region.
In the past, the material properties at the intermediate strain rate have been obtained from interpolation of results obtained from the quasi-static test and the high strain rate test such as a split Hopkinson bar, but this interpolation cannot describe the behavior of steel sheets at the intermediate strain rate. So a new material constitutive equation is needed to represent the variation of the stress accurately at the intermediate strain rate. In new suggested material constitutive equation, the term related to the strain rate sensitivity of is separated from a quasi-static stress-strain curve and is composed of the strain and the strain rate. Stress curves of arbitrary type can be applied to the quasi-static stress-strain curve and the number of coefficients in the term for the strain rate sensitivity is 4. New suggested material constitutive equation is verified for 22 steel sheets of an auto-body. It is conclude from verification that n ew material constitutive equation accurately describes the behavior of steel sheets for an auto-body.
본 논문의 목적은 중변형률 속도에서의 변형률 속도에 따른 동적 물성시험 방법을 확립하여 차체용 강판의 동적 물성 데이터베이스를 구축하고, 동적 물성치를 정확히 표현하는 물성 구성방정식을 제안하는 것이다. 이를 위하여 500 /sec 이하까지 시험 가능한 고속 인장재료시험기를 개발하였으며, 고속 인장시험에서 발생하는 하중 떨림 현상을 개선하고 중변형률 속도에서의 인장시편을 결정하였다. 이를 바탕으로 22종의 차체용강판의 동적 인장시험을 수행하고 물성 데이터베이스를 구축하였다. 충돌해석에 적용하기 위하여 변형률 속도 경화를 표현할 수 있는 물성 구성방정식을 제안하였으며, 이를 차체용강판의 시험결과와 비교 검증하였다.