A modified damage model for advanced high strength steel sheets

More often than not, better formability in the simple tension test implies better formability performance in other stretching modes, especially in hole expansion performance since deformation in the hole expansion test is perceived to be in the same simple tension deformation mode. However, when the hole expansion formability is evaluated particularly for the twinning induced plasticity (TWIP) steel, its performance is so poor compared to other automotive steels, even though the TWIP steel has significantly superior formability in the simple tension test. Therefore, hole expansion formability was experimentally and numerically studied for advanced high-strength grade steel sheets, TWIP940 and a transformation induced plasticity (TRIP) 590 steel sheet, as well as a high-strength grade 340R steel sheet, particularly in conjunction with formability in the simple tension test and its surface condition sensitivity. In order to characterize mechanical properties, simple tension tests were performed to determine anisotropic properties and strain rate sensitivities. To account for macro-crack formation, an inverse calibration method based on a damage model utilizing a triaxiality-dependent fracture criterion and hardening behavior with stiffness deterioration was developed. In this approach, the damage model was inversely calibrated by performing numerical simulations and experiments for the simple tension test (with specimens prepared by milling and punching). Then, the damage model was applied to formability study in the hole expansion test. The damage model along with the anisotropic yield function Hill (1948) incorporated into the ABAQUS/Explicit FEM code performed well to predict hole expansion ratios (HER) and their surface condition sensitivity, elucidating the cause of the lukewarm hole expansion performance and strong surface condition sensitivity of the TWIP steel compared to the others.     

Fracture strength for a high strength steel bridge cable wire with a surface crack

In this paper, the fracture strength of a cracked suspension bridge wire is determined based on linear elastic fracture mechanics (LEFM). The wire is 5 mm in diameter, with an original ultimate strength of 1725 MPa and ultimate elongation that ranges between 5.5% and 6%. The average value of for the wire fracture toughness, K_C, was recently evaluated by the author. The state of practice is to use the ultimate strength of the cracked wire as obtained from tensile tests. This approach may overestimate the strength of the wire due to possible delamination and crack tip plasticity. A case study for a group of in situ wire breaks retrieved from a suspension bridge cable is presented. The failure analysis is performed based on both the fracture toughness criterion and the net section theory. The fracture toughness criterion produced more realistic results for the fracture strength of the wire. The decline in the fracture toughness and the corresponding reduction in the fracture strength of cracked degraded wire are predicted making use of the strain energy density criterion.     

A new method of analyzing surface cracks

The authors have developed a new line-spring boundary element method in thepresent paper,which combines the advantage of the line-spring model with that of theboundary element method.This method reduces the three-dimension problem of thesurface cracks into a quasi-one-dimension problem and can be used to analyze thesurface cracked plate under various loading conditions.In this paper theoreticalanalyses and numerical verifications are carried out.The calculated results arereported,which indicate that the present method is efficient and can be used to analyzethe surface crack problem on a personal computer.     

Dynamic crushing characteristics of high strength steel cylinders with elliptical geometric discontinuities

Thin-walled members are commonly used as energy absorbers in engineering structures and often contain cutouts. This study performed numerical simulations of high strength steel cylindrical shells with elliptical cutouts subjected to dynamic axial impact. The LS-DYNA code was the primary analytical tool used to analyze the influence of cutout locations, cutout shapes and symmetry of cutout on the energy absorption capabilities and the crush characteristics of tubes with a cutout. For high strength steel tubes made from a rate sensitive material, the stress-strain curves of different strain rates were used to elucidate the effect of dynamic impact on the strain rate. Our results show that collapse crushing behavior is strongly influenced by the location and symmetry of cutouts and the variation of major axis influences the peak crush load.     

A study on deformation characteristic of ultra high strength steel and its fracture

A new shape on the load and elongation curve of ultra high strength structural steel in tension is measured by means of the advanced physical methods. A method of calculating true stresstrue strain is found in accordance with the whole load-elongation curve. The calculation results show that during the period of deformation, whetner prior to or after the maximum load, all are staged power hardening. The method of calculating hardening in dex is finally studied for discussion.The Iron and Steel Research Institute of the Anshan Iron and Steel Comapny     

Experimental investigation of the behavior of extra high strength steel

This paper comprises a study concerning the mechanical behavior of extra high strength steel. This is investigated by means of biaxial testing of flat cross-shaped specimens in the full σ₁-σ₂ plane, a concept developed earlier at Steel Structures, Luleå University of Technology. Furthermore, new specimen designs had to be developed to enable testing of a material with high yield strength and low ultimate over yield strength ratio, such as the extra high strength steel Weldox 1100. The tests are performed in two steps: one initial loading followed by unloading and a subsequent loading in a new direction. The test results, containing data from 15 biaxial tests, are characterized by a slightly anisotropic initial yield criterion where the proof stress in compression is consequently somewhat higher compared to the results in tension. In the subsequent loading the observed phenomena are a Bauschinger effect in the direction opposite the initial loading direction and that the transition from elastic to plastic state in subsequent loadings is gradual and direction-dependent.     

钢纤维高强混凝土冲击压缩的试验研究

介绍了利用100 mm SHPB装置获得钢纤维高强混凝土冲击压缩应力-应变曲线的试验研究。同一类试样在静态和动态共4个不同应变率下的试验结果揭示混凝土是应变率敏感材料,其破坏应变、峰值应变和弹性模量表现出显著的应变率强化效应。从静态和动态压缩下混凝土损伤演化的不同形式对这种应变率强化效应进行了详细讨论。从相近应变率下不同钢纤维含量试样的试验结果中,发现冲击压缩下钢纤维对混凝土的增强效应随应变率的增大而减弱。从钢纤维对混凝土静态和动态压缩下损伤演化形式的影响,讨论了钢纤维对混凝土的这种增强效应。     

高斯过程响应面法研究

新型的高斯过程响应面法具有灵活性好、精度高和能量化不确定性等优点,但某些关键技术细节还存在争议。本文简要介绍了高斯过程响应面法的基本理论,结合算例详细讨论了关键技术细节的处理方法;提出了先验期望函数选用低阶多项式、相关函数选用高斯形式、粗糙度参数用边缘后验众数法固定、模型有效性必须经过验算点检验等应用建议。     

A combined phenomenological model for the representation of anisotropic hardening behavior in high strength steel line pipes

Line pipes have anisotropic mechanical properties, such as tensile strength, ductility and toughness. These properties depend on both prestrain during the cold forming process and on the anisotropy of the mother plates. In this study, a phenomenological model combining isotropic and kinematic hardening is developed to represent anisotropic hardening behavior of high strength steel line pipes. The model is adjusted on experiments carried out on smooth and notched axisymmetric bars and plane strain specimens. The model is used to simulate bending tests carried out on large pipes containing a geometric imperfection. Numerical results suggest that prestraining in pipe forming process significantly affects the bending capacity of pipes.     

高强度钢板热成形数值模拟-静力显式

在已建立的高强度钢热成形热、力、相变耦合本构方程的基础上,基于虚功率方程及持续平衡方程建立了热成形静力显式多场耦合有限元列武。将热成形过程中的相变潜热引入温度场,并进行了有限元分析;在自主开发的商业化金属成形CAE软件KMAS(King-Mesh Analysis System)基础上,开发了考虑多场耦合的非线性、大变...