Effect of strain rate on the forming behaviour of sheet metals

The strain rate dependence of plastic yield and failure properties displayed by most metals affects energies, forces and forming limits involved in high speed forming processes. This paper investigates the influence of the strain rate on the forming properties of one laboratory made and three commercial steel grades: a CMnAl TRIP steel, the ferritic structural steel S235JR, the drawing steel DC04 and the ferritic stainless steel AISI 409. First, split Hopkinson tensile bar (SHTB) experiments are carried out to assess the influence of the strain rate on the materials’ stress-strain curves. Subsequently, the obtained SHTB results, together with static tensile test results, are used to model the constitutive behaviour of the investigated steels using the phenomenological Johnson-Cook (JC) model and the Voce model, thus allowing dynamic modelling of forming processes. Finally, forming limit diagrams (FLDs) are calculated using the Marciniak-Kuczynski method. The results clearly show that the effect of the strain rate on forces and energies involved in a forming process, and the forming limits is non-negligible and strongly material dependent.     

Nanocrystalline refractory metals for extreme condition applications

For the last decade, there has been research aimed at engineering plastic instability into the deformation behavior of body centered cubic (b.c.c.) metals. At dynamic strain rates, the adiabatic shear band deformation mode has been shown to improve the performance of kinetic energy penetrator materials. However, for some b.c.c. metals the transition to localized plastic deformation dominates at all strain rates. This limits the traditional engineering properties (e.g., ductility and toughness) and feasibility of incorporation into a long rod penetrator system. Recently, we demonstrated that nanocrystalline tantalum shows significant promise as it deforms via adiabatic shear bands in dynamic compression but shows significant tensile elongation in quasi-static deformation.     

Indentation of lipid-based particle gels: An experimental,theoretical and numerical study

The rheological properties of lipid-based particle gels are critical in defining end-use properties. Although indentation is commonly utilized to investigate the end-use properties of these materials, a comprehensive methodology for acquiring and characterizing fundamental rheological properties via indentation is lacking. This study sought to develop the indentation method for obtaining the large-strain viscoplastic characteristics of three lipid systems that vary in composition and structure: shortening, margarine and butter. Uniaxial compression and conical indentation tests were performed at various rates. Constitutive behaviour was fitted with a viscoplastic model, with the rate-dependent plasticity defined by the overstress power law equation. The static stress–strain curve was defined by a linear elastic region, a strain hardening plastic deformation region, and a subsequent region of perfect plasticity. Forward predictions of the indentation load–displacement response were obtained via finite element analysis using compression test data. Good reverse predictions of the stress–strain properties from the indentation response were obtained through a novel method developed in this study.     

Gradient Elastic–Plastic Properties of Expanded Austenite Layer in 316L Stainless Steel

The gradient mechanical properties, variation of stress with strain and surface cracking behavior of expanded austenite developed on 316L austenitic stainless steel were investigated by nanoindentation tests, X-ray residual stress analysis and scanning electron microscope observation in four-point bending tests. The results show that the plastic properties of the carburizing layer including true initial yield strengths and strain hardening exponents increase significantly from substrate to surface, while the true elastic modulus just improves slightly. Due to the onset of plastic flow, the residual stresses are almost equivalent to the true initial yield strengths from surface to the depth of ～10 lm. The results of four-point bending tests show that surface stress increases linearly with the increase in strain until the strain reaches～1.0%, after that the plastic yield happens. The expanded austenite surface layer is brittle, and the cracks will be created at the strain of ～1.4%.The cracking stress is about～2.4 GPa.     

Microstructures,Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components

Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (σ-FFLD) and effective plastic strain-stress triaxiality space (ηEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using above-mentioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal.     

Critical evaluation of the indentation data analyses methods for the extraction of isotropic uniaxial mechanical properties using finite element models

Two different definitions of indentation strain and two different definitions of contact radius are being used in the current literature, leading to inconsistent estimates of mechanical properties (especially plastic properties). In this paper, we critically evaluate the validity of the different definitions of both the indentation strain and the contact radius by applying the protocols on datasets generated from a finite element simulation of spherical indentation. The finite element models allow assignment of a wide range of elastic–plastic properties to the sample while circumventing many of the uncertainties faced in experimental investigations, and thereby offer unique opportunities for critical validation of the different data analysis procedures. In particular, we are able to establish important connections between the indentation stress–strain curves and the conventional uniaxial stress–strain curves.     

电磁平板自由胀形3D数值模拟(英文)

电磁成形是一种高速率成形方法,它能够有效提高金属板材的成形极限。但是电磁成形过程复杂,涉及到磁场?结构场之间的耦合分析。数值模拟提供一种手段去解决耦合问题。然而,大多数的数值模拟都限于2D。建立3D有限元模型去分析电磁平板胀形。成形过程中考虑了板料与底模的接触和板料变形对磁场的影响。板料中心节点和半径20mm处节点的位移随着时间的变化与实验结果一致。分析了塑性应变能和塑性应变。     

焊接塑性应变演变过程——低碳钢、不锈钢及钛合金塑性应变演变特点及规律

采用数值模拟方法对低碳钢、不锈钢及钛合金3种材料薄板电弧焊接塑性应变演变过程进行了分析.结果表明,在焊接过程中,焊缝及近缝区内无论是纵向塑性应变还是横向塑性应变均为压缩塑性应变,熔池凝固及高温区冷却产生的拉伸卸载不足以完全抵消加热过程产生的压缩塑性应变,最终残留在焊缝及近缝区内的塑性应变仍保持压缩状态,压缩塑性区的宽度明显宽于材料"力学熔化"区宽度.     

Mechanical Behavior of Reinforcement Stirrups BSt 500<Subscript>s</Subscript> at Corrosive Environment

To date, there is no widely accepted regulation on the minimum required diameter of the bending drums, which shape the stirrups of the main concrete reinforcing bars, varying between 4 and 12 times the diameter of the bent bar, and thus resulting in various plastic strains. In the present work, the influence of the degree of plastic strain on the mechanical properties of steel bar of Class BSt 500_s for concrete reinforcement stirrups is investigated, under the additional implication of laboratory corrosive conditions. The increase of the degree of the plastic strain of the stirrup bar had as a result a significant decrease in the ductility properties of the steel. Moreover, it is shown that the combination of plastic strain and corrosion causes additional damages, as strain fractures were recorded lower than that defined by the current specifications for concrete reinforcement, indicating the need for a new review of the relative specifications.     

Effect of the hydrobulging process on mechanical properties of spherical pressure vessels

This paper deals with the effect of large plastic deformation and angular deformation occurring during the hydrobulging process of polyhedral shells on mechanical properties. The results of the experiments show that strain ageing resulting from the plastic deformation causes increase of strength and decrease of plasticity and toughness of steel and its welded joints. The increasing or decreasing degree mainly depends on magnitude of the plastic deformation. The larger the plastic deformation, the more serious the unfavorable effect of the strain ageing. Appropriate heat treatment can eliminate the unfavorable effect and recover the mechanical properties to the original values.     

Integrated identification method of rheological model of sandstone in Sanmenxia bauxite

Based on the uniaxial compression creep experiments conducted on bauxite sandstone obtained from Sanmenxia, typical creep experiment curves were obtained. From the characteristics of strain component of creep curves, the creep strain is composed of instantaneous elastic strain, ?_me, instantaneous plastic strain, ?_mp, viscoelastic strain, ?_ce, and viscoplastic strain, ?_cp. Based on the characteristics of instantaneous plastic strain, a new element of instantaneous plastic rheology was introduced, instantaneous plastic modulus was defined, and the modified Burgers model was established. Then identification of direct screening method in this model was completed. According to the mechanical properties of rheological elements, one- and three-dimensional creep equations in different stress levels were obtained. One-dimensional model parameters were identified by the method of least squares, and in the process of computation, Gauss-Newton iteration method was applied. Finally, by fitting the experimental curves, the correctness of direct method model was verified, then the examination of posterior exclusive method of the model was accomplished. The results showed that in the improved Burgers models, the rheological characteristics of sandstone are embodied properly, microscopic analysis of creep curves is also achieved, and the correctness of comprehensive identification method of rheological model is verified.     

The impact of corrosion on the mechanical behavior of steel undergoing plastic deformation

In the present study the influence of the degree of plastic strain, due to various levels of plastic deformation during bending, on the mechanical properties of class BSt 500_s tempcore steel was investigated, under various levels of salt spray corrosion. The resulted mass loss was of realistic levels and comparable to the one created by natural environmental causes. The results showed that even though an increase in plastic deformation resulted in an expected marginal increase in strength properties, it had a negative effect in ductility. The international community has not reached a consensus yet concerning the minimum required bending roll diameter, for stirrup production, which ranges between 4–10 times the diameter of the steel bar to be bent. It was also shown that this combination is crucial since strain fractures were recorded under the minimum required values set by the most current design guide‐lines and design oriented research. The results of this investigation are intended to offer an in depth understanding of the impact of the underestimated factor of corrosion on the mechanical properties of steel undergoing plastic deformation in corrosive environments and to show the need of re‐examination of existing codes.     

非晶复合材料制备与性能

总结了近年来本课题组在外加强化相非晶复合材料制备方面取得的主要研究结果。通过制备过程凝固控制获得了性能优异的非晶复合材料,其中金属W／Zr基非晶合金双连续相复合材料压缩强度达到3450MPa,压缩应变为48％;金属Ti／Mg基非晶合金双连续相复合材料压缩强度达到1750MPa,塑性应变为30％;8％Nb颗粒／Mg基非晶复合材料压缩强度达到900MPa,塑性应变为12．1％;6％SiC颗粒／Zr基非晶复合材料压缩强度达到2230MPa,塑性应变为3％。     

Phase transformation behavior of pseudoelastic NiTi shape memory alloys under large strain

Although it is known that the plastic deformation after transformation could stabilize martensite and make the transformation irreversible, there lacks a systematic research on the effect of plasticity on phase transformation behavior of NiTi shape memory alloys (SMAs). Therefore, the present study focuses on this aspect of NiTi SMAs. A series of tensile cycling experiments are performed on a NiTi SMA at room temperature. Attention has been paid to the characteristics of the phase transformation stresses, the residual and recoverable strain and the dissipated and recoverable energy density as functions of deformation cycles and maximum strain amplitude. With the increasing of plastic strain amplitude at the first loading cycle, the stress–strain curves reach a stable state sooner during cycling. It is concluded that a small amount of plastic strain at the first loading cycle is helpful to get good stable mechanical properties.     

Modelling of plastic flow, heat transfer and microstructural evolution during rolling of eutectoid steel rods

A study was undertaken in an attempt to quantitatively describe the rolling process of eutectoid steel rods. Finite element method was employed to model plastic flow and heat transfer in the deformed material, and heat transfer during cooling to ambient temperature. The numerical calculation gave an assessment of the strain, strain rate and temperature distributions in the work piece. This allowed the prediction of the austenite microstructure evolution during the process. Finally, the relationships describing the microstructure-mechanical properties were used to quantitatively characterize the influence of processing parameters on strength of rods after rolling.     

球化钢损伤和断裂的实验与理论研究

对４５，Ｔ８和ＧＣｒ１５三种球化钢的力学性能，断裂及微观损伤机理进行了研究，结果表明，碳化物粒子的存在引起的孔洞形核及长大是导致这类材料断裂破坏的主要原因；材料的孔洞体积分数随应变而增加，且有较多碳化物粒子的材料孔洞体积分数增长较快；孔洞损伤可导致材料的软化，推导出碳化物粒子和孔洞损伤影响的等效应硬化指数以及断裂应变的解析表达式，断裂应变的解析表达式为ε＝ｎｍ（１－ｆｖ）（１－ｆｐ）／βｆｖ（１＋     

Influence of uniaxial plastic deformation on magnetic Barkhausen noise in steel

Magnetic Barkhausen noise (MBN) measurements were made on hot-rolled mild steel samples uniaxially deformed to differing magnitudes of plastic strain, to study the dependence of MBN activity on plastic strain. The results indicated an initial increase in MBN energy with increasing plastic strain followed by a decrease at higher plastic deformations. At still higher plastic deformations, the MBN energy was found to be almost independent of plastic strain. The results are explained in terms of different mechanisms of interaction of domain walls with dislocations, with increasing plastic strain. The behavior of MBN energy with plastic strain was found to be anisotropic and the angular MBN measurements indicated that the deformation-induced easy axis of magnetization changed direction with increasing plastic strain. At higher deformations, the MBN activity was largely controlled by the deformation-induced anisotropy, due to residual stress.     

温度和应变率对低银无铅焊料力学行为的影响

在不同工况下,采用电子万能材料试验机和分离式霍普金森压杆装置(SHPB)对低银无铅焊料Sn0.3Ag0.7Cu分别进行准静态和动态实验,分析了应变率和温度对Sn0.3Ag0.7Cu动态力学性能的影响。结果表明:低银焊料Sn0.3Ag0.7Cu的应力-应变曲线具有温度软化效应与应变率硬化效应。在不同的温度范围内,应变率硬化效应与温度软化效应对低银焊料Sn0.3Ag0.7Cu的塑性变形的影响是不同的。基于Johnson-Cook模型对实验数据进行拟合、修正得到低温和中高温下Sn0.3Ag0.7Cu的动态本构关系,并且与实验数据进行比较,两者在材料的塑性平台区表现出高度的一致性。     

高强度双相钢的成形性能

采用光学显微镜、单向/拉压、万能成形、三点弯曲等试验机检测了780~1180 MPa级双相钢的显微组织、力学性能、FLC(成形极限曲线)和冷弯性能。结果表明,随着双相钢的屈服强度增加,其伸长率、n值和r值降低,晶粒细化,马氏体含量增加,平面应变成形极限值从22.7%下降至12.6%,拉伸极限从15.8%下降至8.6%,三点弯曲最小相对弯厚半径从1.13增加到2.86;980 MPa级双相钢随着屈服强度增加,组织均匀性提高,胀形极限从22.8%增加到34.5%,最小相对弯厚半径从2.50减小到1.82;CR420/780DP、CR700/980DP、CR820/1180DP钢的包申格效应常数分别为0.82、0.78和0.79。CR420/780DP和CR820/1180DP钢在压缩过程中塑性变形初始阶段的加工硬化速率高于反向拉伸和单向拉伸塑性变形初始阶段的加工硬化速率。