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A fundamental study on the ductility of high strength steels in crash deformation is carried out to investigate the effect of the local ductility of various materials on automobile crashworthiness, considering the prestrain induced by press forming in the manufacturing process. In this study, a newly developed 980 MPa-grade steels [1], ‘jetQTM’, is investigated to clarify its advantage in term of crashworthiness in comparison with the conventional DP (Dual Phase) and TRIP steels. Quasi-static axial crushing tests are performed to evaluate the crashworthiness of the different types of steel. Based on the experimental results, the effect of the local ductility of high-strength steel on the risk of material fracture is discussed. In this paper, a new bending test method, orthogonally reverse bending, (ORB), is proposed to simulate the fracture that occurs during crash deformation considering press-forming strain.

New highly ductile AHSS steel grades with tensile strength greater than 980 MPa have been developed with the aim of combining high strength and excellent formability. The new jetQ-Family offers high local and global ductility while still fulfilling standards for resistance towards hydrogen embrittlement and weldability. These improved properties are based on their specifically engineered microstructure, which utilize the TRIP-mechanism in a strengthened matrix. This work shows how the microstructure plays a significant role for the tensile testing as well as hole-expansion. Based on the increased yield strength a better crash performance compared to conventional DP steel grades can be attained. The local ductility is demonstrated with excellent hole expansion ratios and high resistance to sheared edge failure. In combination with improved bending angles and thickness strain at fracture a robust process for manufacturing of components can be achieved.

A fundamental study on the ductility of high strength steels under impact deformation is carried out to investigate the effect of the local ductility of various materials on crash performance. In this study, newly developed 980 and 1180 MPa grade steels are investigated to clarify their advantages in term of crash performance compared to conventional DP (Dual Phase) steels. The features of the developed steel, named as jetQ are higher yield strength and higher local ductility due to an optimized microstructure by the quenching and partitioning process (QP) [1, 2]. The bending test according to VDA 238-100 is performed while observing the fracture propagation during the bending test. Fracture strain in the tensile tests is evaluated by a three-dimensional shape measurement technique for the fracture surface. Both three-point bending tests and axial impact tests are performed to evaluate the crashworthiness of different types of steel.

Initiation and propagation of ductile fractures in crashed automotive components made from high strength steels are investigated in order to understand the mechanism of fracture propagation. Fracture of these components is often prone to occur at the sheet edge in a strain concentration zone under crash deformation. The fracture then extends intricately to the inside of the structure under the influence of the local stress and strain field. In this study, a simple tensile test and a 3-point bending test of high strength steels with tensile strengths of 590 MPa and 1180 MPa are carried out. In the tensile test, a coupon having a hole and a notch is deformed in a uniaxial condition. The effect of the notch type on the strain concentration and fracture behavior are investigated by using a digital imaging strain measurement system.

A simple testing method is proposed in order to investigate ductile fracture in crashed automotive components made from advanced high strength steels. This type of fracture is prone to occur at spot-welded joints and flange edges. It is well known that the heat affected zone (HAZ) is a weak point in high strength steel due to the formation of annealed material around the spot-welded nugget, and the flange edge also has low ductility due to the damage caused by shearing. The proposed method is designed to simulate a ductile fracture which initiates from a spot-welded portion or a sheared edge in automotive components which are deformed in a crash event. Automotive steel sheets with a wide range of tensile strengths from 590MPa to 1470MPa are examined in order to investigate the effect of material strength on fracture behavior. The effects of material cutting methods, namely, machining and shearing, are also investigated.

In order to reduce automobile body weight and improve crashworthiness, the use of high-strength steels has increased greatly in recent years. An optimal combination of both crash safety performance and lightweight structure has been a major challenge in automobile body engineering. In this study, the Cockcroft-Latham fracture criterion was applied to predict the fracture of high-strength steels. Marciniak-type biaxial stretching tests for high-strength steels were performed to measure the material constant of the Cockcroft-Latham fracture criterion. Furthermore, in order to improve the simulation accuracy, local anisotropic parameters based on the plastic strain (strain dependent model of anisotropy) were measured using the digital image grid method and were incorporated into Hill's anisotropic yield condition by the authors. In order to confirm the validity of the Cockcroft-Latham fracture criterion, uniaxial tensile tests were performed.

In order to evaluate the crashworthiness of impact energy absorbing parts for automobiles, it is very important to obtain accurate stress-strain (s-s) relationships for steel sheets under various strain rates. However, at high strain rates over 10/s, stress wave interference in the load cell obstructs precise measurement of the s-s relationship with conventional tensile testing equipment. Various new tensile testing machines have been developed to obtain accurate s-s relationships at high strain rates. Six of these machines (coaxial Split Hopkinson Pressure Bar method, Non-coaxial Split Hopkinson Pressure Bar method, One Bar method, Sensing Block Testing System, and two types using the Hydraulic Servo method) were used to obtain s-s relationships for high strength steel sheets. In this paper, the s-s relationships are compared and the characteristic of the curves was discussed.

In this study, an FEM simulation method was investigated in order to predict the dent resistance and stiffness performance of automotive exterior body panels. The method was based on the combination of a forming simulation and a static dent simulation. The dent simulation was carried out with material models taking forming effects, such as work hardening and thickness variation into account. The modeling method of material properties with the forming effects was discussed to improve the prediction accuracy. For the validation of the accuracy, forming and denting tests were carried out for a door model panel using mild steels or high strength steels. The results of the dent simulations showed good agreement with the experimental results.

Finite element modeling based on both 3-D shape measurement and experimental stress-strain relationship was applied to the FEM simulation for estimating the crashworthiness of automobile parts. Compared with the result dynamic crash test using hat-square-column type specimen made of 300 - 590MPa grade steels, the accuracy of the FEM simulation was evaluated for various modeling methods. It was revealed that the modeling of corner radius, bulging of specimen wall and strain rate sensitivity of materials played important roles in predicting the actual dynamic deformation process and the force-stroke relationship.

In order to estimate and improve the crashworthiness of practical automotive parts, dynamic crash test was conducted by using double hat specimen composed of the materials with different strength and thickness. Estimation method of the average force of the specimen was discussed. From the test results, it was clear that absorbed energy of the specimen composed of the materials with different strength and thickness of plate can be evaluated from the linear mixture law for the relations of the absorbed energy and the thickness of the materials used. And the “effective width theory” is useful way to estimate the average force of the parts.

In order to improve the capability of frontal or rear automotive members to absorb impact energy, a basic study was performed to evaluate the effect of strength, micro-structure, and plate thickness on dynamic deformation phenomena using tensile specimens and hat square columns. Tensile strengths of the materials were selected from 340 to 1180 MPa class steels. Five kinds of micro-structure, Ferrite, Ferrite + Bainite, Ferrite + Martensite, Ferrite + Pearlite, Ferrite + Bainite + γ, and Martensite, were investigated. Tensile tests were carried out under strain rates from 10-1 to 103 /sec. and crash tests of hat square columns were conducted under test speeds from 0.1 to 14.0 m/sec.