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임지호(Jiho Lim),김석봉(Seokbong Kim),허훈(Hoon Huh),권순용(Soonyong Kwon),박성호(Sungho Park) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.10
This paper introduces a newly developed high speed material testing apparatus for tensile tests at the strain rate up to 500 /sec. The tensile material properties of sheet metals are indispensible for the accurate crashworthiness analysis of auto-bodies since the local strain rate reaches to 500 /sec in the car crash. An appropriate experimental method has to be developed to acquire the tensile material properties at the intermediate strain rate ranged from 0.003/sec to 200/sec. Tensile tests of various different steel sheets for an auto-body were performed to obtain the dynamic properties with respect to the strain rate. The dimensions of specimens that can provide the reasonable results were determined by the finite element analysis. A special jig fixture of a load cell is designed to reduce the load ringing phenomenon induced by unstable stress propagation at the high strain rate. Stress - strain curves were acquired for each steel sheet from the dynamic tensile test and utilized to obtain the relationship of the stress to the strain rate.
중변형률속도에서의 재료의 동적거동에 대한 물성구성방정식
임지호(Jiho Lim),허훈(Hoon Huh) 한국자동차공학회 2007 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-
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 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/s ~ 500/s. 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. In this paper, a new material constitutive equation is suggested to describe dynamic behavior of material at the intermediate strain rate ranged from 1/s to 1000/s. In new suggested material constitutive equation, the term related to the strain rate sensitivity is separated from a quasi-static stress-strain curve and is composed of functions of the strain and the strain rate. Any stress-strain curves of arbitrary type can be applied to the quasi-static stress-strain curve.
임지호(Jiho Lim) 한국자동차공학회 2010 한국자동차공학회 학술대회 및 전시회 Vol.2010 No.11
Spot welding is the dominating process for joining of an automotive body due to the reliability and low cost. The crashworthiness which is one of the most important requirements in a car is largely affected by the strength of individual spot welds. Therefore, many car designers have tried to apply behavior of spot welds to crash analysis in order to increase the accuracy of the simulation. Although the strength of spot welds is required, it is very difficult to obtain those properties with respect to the thickness, materials and welding conditions. In this paper, the strength of spot welds is predicted from many experimental results. Welding parameters are analyzed to determine major effective factors, and prediction equations of tensile shear and cross tension strengths are developed by the regression model with the second polynomial function of thickness, tensile strength and nugget ratio. And, the application limit of predicted models is established from investigation of data with high error.
임지호(Jiho Lim),김석봉(Seokbong Kim),김진성(jinsung Kim),허훈(Hoon Huh),이강욱(Kangwook Lee) 한국자동차공학회 2004 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-
In the past, the static load-displacement curve was sufficient to predict the deformation of plastics. As plastics have increasingly been used in more applications such as automotive pasts, the dynamic material properties considering the strain rate and temperature become important. In plastics, the stress is typically very sensitive to the strain rate, but results for dynamic material properties at the intermediate strain rate ranged from 1/sec to 500/sec do not have been published sufficiently. This paper introduces a newly developed high speed material testing apparatus for tensile tests at the strain rate up to 500/sec and dynamic material properties of plastics from experiments. Stress-strain curves were acquired for the polycarbonate and the polypropylene from the dynamic tensile test and utilized to obtain the relationship of the stress and the fracture elongation to the strain rate.
임지호(Jiho Lim),김석봉(Seokbong Kim),김진성(Jinsung Kim),허훈(Hoon Huh),권순용(Sunyong Kwon),윤치상(Chisang Yoon),박성호(Sungho Park) 한국자동차공학회 2004 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-
While the formability is important and indispensable for success in very complicated sheet metal forming, it seems that few studies has been carried out about the formability of sheet metal at the high strain rate. The present experimental results report that the elongation is dependent on the crosshead speed in tensile tests. In this paper, the tensile elongation has been obtained from various steel sheets for an auto-body at the intermediate strain rate. The strain rate in the experiment is ranged from 0.003/sec to 200/sec. The experimental result demonstrates that the tensile elongation does not decrease as the strain rate increases. This tendency has varieties depending on the microstructure and forming history of sheet metal. Some high strength steels have the tendency that the tensile elongation increases as the strain rate increases, while others not. This phenomenon is very important not only in sheet metal forming but also in the crash worthiness evaluation to predict the fracture and tearing of sheet metal members.
Hat형 압축 충돌시편을 이용한 자동차강판의 충돌성능 평가
임지호(Jiho Lim) 한국자동차공학회 2012 한국자동차공학회 부문종합 학술대회 Vol.2012 No.5
It is very difficult to design a car considering the crash performance for each part due to tight development schedule. So, crash tests of simplified specimens such as a hat type specimen are used to estimate crash behavior of real parts indirectly. In this paper, compressed crash analysis and test of a top hat specimen are performed to assess crash characteristics of automotive steel sheets, especially AHSS. Fracture of material and spot welds are not considered in the analysis. Crash tests are carried out by a high speed crash tester which is servo-hydraulic type and is able to fire a carriage to a specimen horizontally. Thickness, section length and material strength are varied to investigate effects on the mean load. The mean load is proportional to thickness exponentially and increases in proportion to material strength linearly. The mean load according to the section length shows very big scattering because the collapse mode differs very according to dimension. From these results, an equation to predict the mean load is proposed with respect to thickness, section length and material strength. Fracture of spot welds is occasionally observed on some specimens of material of which tensile strength is lower than 800 MPa, but that does not almost affect crash performance. In case of 980DP, fracture of spot welds always happens and deteriorates energy absorption and enlarges scattering.
임지호(Jiho Lim),정연일(Younil Chung) 한국자동차공학회 2009 한국자동차공학회 학술대회 및 전시회 Vol.2009 No.11
A crash box of an automotive body should has characteristics of low impact force and high energy absorption in order to reduce the repairing cost and to achieve light-weight design. But, conventional crash box with collapse type have high impact force by buckling and low energy absorption due to local deformation by bending, so it is necessary to develop a crash box with new concept. Although an energy absorber with tube expansion type used in a train meets both requirements, this is not proper to adopt an automobile due to high weight. In this paper, novel crash box is developed to absorb the energy by double walled collapse of an expanded and a fixed tubes after the tube expansion of an expanded tube. This crash box should be satisfied not to have the buckling of both tubes before completion of the expansion. In order to adopt new developed crash absorber with tube expansion type to a crash box of an automotive body, the interrelation between of design parameters of a crash absorber and the absorbed energy is studied. And, tube expansion type is compared with conventional collapse type in a viewpoint of the impact force and the crash energy absorption. Finally, values of design parameters which are the thickness, the radius, the length, the expansion angle, the expansion ratio and the friction coefficient are determined from the parameter study.