纯钼薄板各向异性拉压不对称性本构建模研究

目的 对纯钼薄板的各向异性、拉压不对称性以及本构关系进行研究.方法 对纯钼薄板进行不同方向的单拉实验、中心带孔试样单拉实验,和纯钼薄板V形弯曲实验,同时结合有限元模拟反推材料力学性能以及对CPB06屈服准则及Swift强化模型进行参数标定,并进行模型可靠性验证.结果 纯钼薄板具有一定的面内各向异性、拉压不对称性以及显著...     

Effect of laser welding mode on the microstructure and mechanical performance of dissimilar laser spot welds between low carbon and austenitic stainless steels

This paper aims at investigating metallurgical and mechanical characterization of dissimilar laser spot welds between low carbon and austenitic stainless steel sheets. Microstructural examination, microhardness test and quasi-static tensile–shear test were performed. Mechanical properties of the welds were described in terms of peak load. The effects of laser mean power on the performance of dissimilar laser spot welds have been studied. It was found that increasing laser mean power leads to the transition of laser welding mode from conduction to keyhole. This transition causes a significant growth of the fusion zone size in the lower sheet, i.e. the low carbon steel sheet; since, the keyhole acts as an effective trap for the laser beam and will greatly increase the energy absorption from the incident laser beam.It is also shown that the fusion zone size in the weaker sheet, i.e. the low carbon steel sheet is the controlling factors in determination of the mechanical strength of dissimilar austenitic/ferritic laser spot welds.     

PS/改性PVA共混片材的性能研究

以丙三醇和乙二醇为复配增塑剂,对PVA进行改性,实现PVA的熔融加工,再采用熔融共混技术制备PS/改性PVA共混片材,改善了PS片材的力学性能,并使PS片材具有吸湿功能。实验结果表明:PS和PVA质量比为6:4时,共混片材力学性能达到最佳,吸湿效果良好,并具有保湿能力。     

板料硬度预报冲裁噪声的研究

考虑到板料硬度、强度与冲裁力的关系，对所提出的冲裁最大噪声即冲裁工艺噪声发射峰值的数学模型作了进一步的数学推导，获得了两个新的预报模型；取用几种钢、铝、铜板的维氏硬度及抗拉强度值，对这些模型进行试验研究，结果表明3个模型都是正确的，其中用硬度值预报是最为简便和精确的，为用板料的特征值来预报冲裁工艺噪声提供了一种简便可行的方法。     

Fatigue crack propagation characteristics of high‐tensile steel wires for bridge cables

The viability of single edge cracked sheet test method for rapidly determining the crack propagation characteristics of steel wires was investigated. First, fatigue tests under 3 different stress ratios were conducted on the sheet specimens which were manufactured from a kind of widely used cable wires. The test data were analysed, and the crack growth rates of sheet specimens were constructed by Walker model. Then, a series of fatigue tests were performed on notched round‐bar specimens to verify the predictability of Walker model parameters. Moreover, the experimental results obtained in different studies on crack propagation characteristics of steel wires were discussed. The results show that the crack propagation characteristics of sheet specimens behave a certain dependence on depth. The sheet crack growth laws can be well used to predict the fatigue life of notched bar specimens when the mechanical heterogeneity is considered. For bridge cable steels, the rational values for the exponent parameter of Paris law, m, should be close to 3.     

Predicting mechanical properties of single-walled carbon nanocones using a higher-order gradient continuum computational framework

The higher-order gradient continuum theory is employed to study the structural parameters and elastic properties of single-walled carbon nanocones (SWCNCs), where the higher-order Cauchy–Born rule is used to link the deformation of the carbon atomic structure to that of the continuum level. Unlike single-walled carbon nanotubes (SWCNTs), mechanical properties of SWCNCs vary along its side edge’s direction, owing to the monotonically increasing radius. In the constitutive model, a representative cell in an initial graphite sheet is selected to study the mechanical property of this domain of SWCNCs. By minimizing the potential energy of the representative cell in the undeformed SWCNC, structural parameters and elastic properties of the domain are obtained. The varying chiralities seem to have a large impact on mechanical properties of SWCNCs. Five kinds of SWCNCs are chosen in our study to test the influence of the apex angle on mechanical properties. The computational results demonstrate that mechanical properties of SWCNCs trend to the constant of graphite sheet when the radius is extremely large but this trend becomes mild as the apex angle increases.     

Effect of temperature on the creep behavior of a Ni–Cr–Fe–Al alloy: a comparison of the experimental data and a model

In order to characterise the mechanical behaviour of sandwich structures, which combine an interlayer of a woven wire mesh between two thin walled sheet metals, creep tests at 650, 680 and 750 °C were carried out on sheet metals made of the nickel based alloy Nicrofer 6025 HT (2.4633). In addition to the tests the creep behaviour was simulated by a model, which considers the creep rate as a function of the applied stress σ and the internal deformation resistance including an internal back stress σ_i and a particle resistance σ_P. The damage is included by a damage parameter D, which converges to “one” with increasing damage. A concluding comparison with the creep test results shows that the model is able to describe the creep behaviour of the investigated sheet metals.     

初始晶粒尺寸对高温交叉轧制镁合金板材组织和力学性能的影响

目的 揭示晶粒尺寸对多道次高温交叉轧制AZ31镁合金板材组织和力学性能的影响规律及机制.方法 通过对不同初始晶粒尺寸的镁合金板材进行高温交叉轧制变形及热处理,获得不同状态的镁合金板材,采用金相显微分析、X射线衍射(XRD)分析及室温拉伸实验等手段研究镁合金板材的晶粒组织(形态、尺寸、取向)及力学性能.结果 经过多道次交叉轧制后,不同初始晶粒尺寸的板材均发生了孪生现象,但粗晶板材的晶粒尺寸仍明显大于细晶板材.退火处理后,细晶退火态板材组织均匀性较好,而粗晶退火态板材的组织虽有细化,但并不均匀.随着交叉轧制次数的增加,两种板材内非基面取向晶粒都有所增加.退火后粗晶板材中非基面取向晶粒更多.结论 晶粒细化和晶粒取向强化导致退火后的交叉轧制细晶镁合金板材具有更高的强度.粗晶板材伸长率较高主要与其具有更多的非基面取向晶粒有关.     

Effects of defects in hybrid sheet moulding compound – Evaluation of defects and the impact on mechanical properties

Sheet moulding compound is a widely used fibre‐reinforced material. Generally, it consists of discontinuous glass fibres in a thermoset matrix system. Due to the finite fibre length, mechanical properties of structural components are limited. To overcome this drawback, sheet moulding compound is locally reinforced with a unidirectional carbon fibre sheet moulding compound material in the approach presented in this contribution. The manufacturing of this hybrid material consisting of discontinuous glass fibre sheet moulding compound and continuous carbon fibre sheet moulding compound can result in different defects, such as folds or fibre misalignments. These defects may affect mechanical properties of the hybrid material. Consequently, this article deals with the investigation and analysis of defective hybrid sheet moulding compound components, which were examined by means of tensile tests. Results point out that investigated defects have different effects on mechanical properties. However, independent from the type of defect, mechanical properties were reduced. With a reduction of 68.86 %, folds have one of the greatest influences on tensile strength. In addition, depending on the angle deviation, even greater reductions can occur. Furthermore, the reduction of the mechanical properties can be identified clearly with increasing angle deviation.     

磷酸三钙/海藻酸钙复合平板膜的制备及其力学性能

本文提供了一种制备磷酸三钙/海藻酸钙（TCP/CA）复合平板膜的方法,以提高CA水凝胶的强度,并使其力学性能容易测试。在水溶液中将TCP与海藻酸钠复合,制备了TCP/CA复合平板膜,采用红外光谱和扫描电子显微镜对其进行表征,研究了不同TCP含量的TCP/CA复合平板膜的溶胀性能与复合平板膜在湿态和干态下的力学性能。结果表明：随着TCP含量的增加,TCP/CA复合平板膜在生理盐水中的溶胀率逐渐降低;TCP/CA复合平板膜在干态和湿态下的极限应力值均随着TCP含量的增加呈现先增加后减小的趋势;干态下的极限应力、模量和断裂能要明显大于湿态下的。该TCP/CA复合平板膜制备方法简单、厚度可控、易批量生产,在控制释放和组织工程领域有潜在的应用前景。     

玻璃纤维/聚丙烯复合材料层合板拉深成型性

为研究纤维增强树脂复合材料零部件快速成型,加速复合材料零部件大规模产业化量产,以玻璃纤维/聚丙烯复合材料层合板为实验对象,首先利用设计加工的拉深成型模具,进行了玻璃纤维增强热塑性树脂复合材料（Glass fiber reinforced thermoplastic resin composite,GFRTP）板材外表面纤维方向和模具长轴方向为0°和90°的试件在不同温度和不同拉深深度条件下的深拉深成型实验,将成型件制备金相试件在光学显微镜下进行微观组织观察,并对试件的成型情况和不同拉深力-行程曲线进行分析。其后进行了GFRTP板材外表面纤维方向和模具长轴方向为0°、45°和90°的试件的不同温度下的浅拉深成型实验,并对成型后的试验件进行了室温条件下的拉伸性能测试,对其拉伸失效情况及具体力学性能进行了对比分析。试验结果表明,在室温25℃到基体树脂的熔融温度165℃之间,随着温度的升高,板材的极限拉深深度增大,最大拉深力呈下降趋势。在选取的试验温度范围内,85℃时试件成型性能较好且0°试件优于90°试件,温度对拉深成型试件的皱曲改善不明显。浅拉深成型试件拉伸力学特性受试件铺层纤维方向的影响较大,防止皱曲等缺陷的发生对GFRTP板材拉深成型十分重要。     

Formability studies on plasma arc welded duplex stainless steel 2205 sheet

Sheet metal forming is cost-effective manufacturing process and hold a significant key position in fabrication works. Welding being a popular technique the industries primarily prefer for joining of sheet metal formed parts. The inherent material properties changes at the weld metal and heat affected zone post welding process, hence the impact and changes on mechanical strength aspects need to be studied. The current study focuses on influence of plasma arc welding on formability of 1.6 mm thick duplex stainless steel 2205 sheet using Erichsen cupping test by gauging the height of the cup formed. The performed tests such as uniaxial tensile, microhardness showed better mechanical properties and decrease in formability noted from Erichsen cupping test for weld metal compared to base metal. Finite element analysis of Erichsen cupping test is conducted using ABAQUS software and results are matched with experimental outcomes for validation. The comparison shows that a deviation of less than 5 % is noticed between actual and predicted formability index values. Microstructure examination reveals that, equiaxed grains at the weld center and columnar grains at sides typically formed in the weld metal region. The decrease in formability of weld blank compared to base blank is attributed to an increase in ferrite content. This is supported by amount of ferrite content measured in weld metal is 55 % and base metal is 52 %. The scanning electron micrographs (base and weld blank) reveal the mode of failure is ductile.     

Dynamic mechanical properties of paper: effect of density and drying restraints

The tensile dynamic mechanical properties of paper sheets with densities between 300 and 1000 kg m^–3 have been measured at room temperature in the frequency range 0.1 to 10 Hz. Two series of sheets have been investigated; one which had been restrained during drying and the other which had been allowed to shrink freely. In general, paper was found to be a non-linear viscoelastic material. The dynamic modulus decreased and the mechanical loss factor increased as the strain amplitude of the applied sinusoidal deformation was increased. The non-linear character of paper was more pronounced at lower sheet densities. The modulus increased strongly with increasing sheet density and was always higher for sheets that had been restrained during drying. The sheets were anisotropic and the ratio of the modulus in the machine direction (MD) to that in the cross-direction (CD) was of the order of 2 to 3.5 depending on the density and the drying conditions. The mechanical loss factor (extrapolated to zero strain amplitude) decreased with increasing sheet density and was always higher in the CD than in the MD. The freely dried sheets were characterized by a higher value of the mechanical loss factor, tan , than sheets that had been restrained during drying. It is suggested that the influence of the sheet density and the drying restraints on the dynamica mechanical properties of paper is associated with interfibre friction and/or bending and shearing of the cellulose fibres.     

基于数字散斑应变测量法的薄板各向异性力学性能研究

系统地说明了采用数字散斑相关法研究薄板各向异性的实验方法和数据处理方法,进而对SPCC钢板和AA6061铝板的各向异性及其演化规律进行了研究。结果表明:散斑应变测量法是一种获取薄板力学性能的有效手段,其最大优点在于能够获得变形过程中的整体应变场,这是研究复杂加载条件下材料力学性能的关键;对于SPCC钢板,其流动应力的各向异性并不严重,但全量和增量形式的Lankford系数(r值和r′值)均表现出了明显的各向异性,且其值随着变形的增加而逐渐降低,这与传统的采用引伸计进行应变测量时只能获得恒定的Lankford系数不同;对于AA6061铝板,其流动应力和r值的各向异性均不明显,但与轧制方向成不同角度试样的伸长率表现出了明显的差异,并且流动应力的加工硬化速率和r′值在拉伸真实应变处于0.15～0.20时出现了剧烈的波动;随着变形的增加,两种薄板应变的各向异性都逐渐增强,SPCC钢板增强得更为明显。     

Finite element modelling and experimental investigation of single point incremental forming process of aluminum sheets: influence of process parameters on punch force monitoring and on mechanical and geometrical quality of parts

New trends in sheet metal forming are rapidly developing and several new forming processes have been proposed to accomplish the goals of flexibility and cost reduction. Among them, Incremental CNC sheet forming operations (ISF) are a relatively new sheet metal forming processes for small batch production and prototyping. In single point incremental forming (SPIF), the final shape of the component is obtained by the CNC relative movements of a simple and small punch which deform a clamped blank into the desired shape and which appear quite promising. No other dies are required than the ones used in any conventional sheet metal forming processes. As it is well known, the design of a mechanical component requires some decisions about the mechanical resistance and geometrical quality of the parts and the product has to be manufactured with a careful definition of the process set up. The use of computers in manufacturing has enabled the development of several new sheet metal forming processes, which are based upon older technologies. Although standard sheet metal forming processes are strongly controlled, new processes like single point incremental sheet forming can be improved. The SPIF concept allows to increase flexibility and to reduce set up costs. Such a process has a negative effect on the shape accuracy by initiating undesired rigid movement and sheet thinning. In the paper, the applicability of the numerical technique and the experimental test program to incremental forming of sheet metal is examined. Concerning the numerical simulation, a static implicit finite element code ABAQUS/Standard is used. These two techniques emphasize the necessity to control some process parameters to improve the final product quality. The reported approaches were mainly focused on the influence of four process parameters on the punch force trends generated in this forming process, the thickness and the equivalent plastic deformation distribution within the whole volume of the workpiece: the initial sheet thickness, the wall angle, the workpiece geometry and the nature of tool path contours controlled through CNC programming. The tool forces required to deform plastically the sheet around the contact area are discussed. The effect of the blank thickness and the tool path on the punch load and the deformation behaviour is also examined with respect to several tool paths. Furthermore, the force acting on the traveling tool is also evaluated. Similar to the sheet thickness, the effect of wall angle and part geometry on the load evolution, the distribution of calculated equivalent plastic strain and the variation of sheet thickness strain are also discussed. Experimental and numerical results obtained allow having a better knowledge of mechanical and geometrical responses from different parts manufactured by SPIF with the aim to improve their accuracy. It is also concluded that the numerical simulation might be exploited for optimization of the incremental forming process of sheet metal.     

Modeling of resistance spot welding of multiple stacks of steel sheets

Weld quality is a major challenge for resistance spot welding of multiple stacks of steel sheets. Because of the differences in mechanical and physical properties of steel sheets and the sheet gage variation, the contact state between sheets and welding current flow throughout the stack joint is complicated. As a result, discrepant weld sizes at the faying interfaces become an issue. In this study, a coupled thermal–mechanical/thermal–electrical incremental model has been developed to reasonably predict the weld nugget formation process of resistance spot welding of a sheet stack made of 0.6 mm thick galvanized SAE1004+1.8 mm thick galvanized SAE1004+1.4 mm thick galvanized dual-phase (DP600) steel using published thermal, electrical, and mechanical properties. It was found that the weld nugget on the faying interface of DP600 forms earlier than that on the other interface, which agrees well with the experimental results. Based on the coupled model, the effects of the sheet gage combination and steel grade combination were examined. The results show that, for a multiple stacks of steel sheets SAE1004 + SAE1004 + DP600, the critical ratio of sheet thickness between the top and bottom sheets is approximately 1:3. The model could provide an important guidance in the selection of the welding variables, sheet gage and steel grade to meet the weld quality of steel component.     

Semi‐empirische Berechnungsmethode zur Bestimmung des Grenzformänderungsdiagramms auf Basis mechanischer Kennwerte

Semi‐empirical calculation method for prediction of Forming Limit Curves based on mechanical properties For characterisation purposes of sheet metal forming processes concerning feasibility of material mainly the Forming Limit Curve (FLC) is commonly used. This failure model can either be measured experimentally or can be predicted by using semi‐empirical approaches. These mathematical approaches mainly are validated for Mild Steels. However, prediction of FLCs for modern High Strength Steels or Aluminium sheet alloys is not possible with acceptable accuracy today. This is why this contribution deals with a new semi‐empirical approach for FLC prediction, which is valid for all sheet metal materials used in car body production. This approach uses a correlation of mechanical properties of uniaxial tensile test an experimentally determined limit strains.     

Design optimisation of biaxial tensile test specimen using finite element analysis

One of the most restricting aspects of the biaxial tensile test for sheet metal is the design of the cruciform specimen. Although specimens of the cruciform type have been investigated quite extensively previously, no standard geometry for the cruciform specimen exists. Using a specifically designed pantograph apparatus for operation in a standard tensile testing machine, various cruciform specimens machined from low-carbon cold rolled steel sheet were analysed experimentally. Finite element modelling of the specimens was conducted in parallel to the experimental test programme to establish optimum specimen geometry. Through a process of optimisation, a standard cruciform specimen was designed which can be used to accurately predict the mechanical behaviour of the mild steel when formed in multiple directions simultaneously. This paper describes the optimisation process and the results obtained from both the experimental testing and numerical modelling.     

An experimental study on the comparative assessment of hydraulic bulge test analysis methods

An in-depth understanding and full characterization of mechanical behavior for sheet materials are required since it is critical to establish the highly reliable material models over a broad range of strain levels for accurate modeling and analysis of sheet material deformation processes such as stamping, hydroforming, deep drawing, etc. Hydraulic bulge testing of sheet materials has been known to provide flow stress properties at higher strain levels compared to commonly used tensile tests mainly due to the fact the tested specimens are strained under biaxial loading conditions. However, analysis of the hydraulic bulge test data has not been standardized yet as there have been numerous approaches developed and adopted throughout the years. In this study, different approaches for the analysis of hydraulic bulge were compared with experimental results to determine the best combination in obtaining accurate flow curves models at room and elevated temperature conditions for different lightweight materials of interest for several industrial applications (AA5754 and AISI 201). It was determined that Panknin’s bulge radius and Kruglov’s thickness calculation approaches are the best combination to accurately obtain the flow curves at both cold and elevated temperature conditions.     

Anisotropy of mechanical properties in quaternary Al-Li-Cu-Mg alloys

Considerable anisotropy in the mechanical properties of quaternary Al-Li-Cu-Mg alloys was observed in both plate and sheet products. These alloys showed more than 100% increase in tensile ductility in the test direction oriented at 45–60° to the rolling direction as compared to that in the rolling direction (longitudinal, L). A concomitant decrease in strength was also found. These alloys exhibit superior low cycle fatigue resistance in the long-transverse (LT) direction as compared to the longitudinal (L) direction. Another observation is the occurrence of strength differential (S-D), which is seen to be directional. The trends in S-D are similar under monotonic as well as cyclic loading conditions. The fracture resistance is also highly anisotropic. An attempt is made here to correlate the observed anisotropy in the mechanical behaviour of these alloys with the microstructure and crystallographic texture.