硅纳米线的分子动力学模拟

本文采用分子动力学软件包LAMMPS研究了无缺陷和有缺陷的硅纳米线的拉伸力学性能,得到其应力-应变曲线,曲线表明拉伸过程中无明显的塑性流动,但会出现颈缩现象;当硅纳米线存在缺陷时,屈服强度减小为无缺陷时的79%;且容易形成单个晶胞被拉伸现象.研究表明,温度对硅纳米线的屈服强度有显著影响,温度升高会大大降低硅纳米线的屈服强度.     

超高强度钢AF1410塑性流动特性及其本构关系

在本文中,为揭示超高强度钢AF1410的塑性流动性,并研究其塑性流动本构关系,利用CSS4410电子万能试验机和改进的Hopkinson拉压杆技术,对AF1410钢在温度从100K到600K,应变率从0.001/s到2000/s,塑性应变超过20%的塑性流动特性进行了试验研究。结果表明,拉伸加载下AF1410钢屈服强度低于压缩屈服强度,且随应变率增加,拉压屈服强度差值越来越大;该材料塑性流动应力对应变率敏感性低,而对温度较为敏感;随应变率的提高,该材料拉伸失效应变减小,但温度对失效应变无明显影响。最后基于位错的运动学关系,借助试验数据,获得了AF1410钢的塑性流动物理概念本构模型,并通过与经典J-C模型的结果对比对该物理概念本构模型进行了评估分析,表明该物理概念本构模型在较宽温度和应变率范围能较好的预测AF1410钢的塑性流动应力。     

预应变对中碳钢扭转疲劳特性的影响

应用扭转疲劳试验机对退火后的非预应变45#钢及其3种预应变材料进行 实验. 实验结果表明：预应变量小于5%时,与非预应变材料相比疲劳极限降低10\%; 预应变量大于5{%}时,随着预应变量的增加,其疲劳极限有恢复的趋势,当预应变量达到 8%时,预应变材料与非预应变材料的疲劳极限几乎相等;当循环应力达到1*10^7$时,各种试件表面都有明显的停留裂纹存在.     

H13钢的高温拉伸性能研究

利用高温传感器在 MTS880试验机上系统地研究了 H13钢由室温到700℃温度范围内的应力应变特性,并给出其应力应变关系随温度的变化规律,同时对拉伸试件断口进行了微观分析。研究结果表明：由室温到600℃的温度范围内,H13钢的屈服强度、抗拉强度、弹性模量随温度的增加而线性地缓慢降低;室温拉伸试验的断口表现为多点起裂的撕裂断口,表明 H13钢的强度较高,但韧性较低;400℃时断口为典型的韧性断口形貌,强韧性很好;550℃和600℃时断口为韧窝状断口,具有一定的强度和韧性;当温度高于600℃时 H13钢的强度和韧性快速下降,失去了承载能力。     

预应变对中子辐照高纯铝拉伸性能的影响

金属材料的中子辐照硬化和脆化一直都是核能安全领域十分关注的重要问题之一.为了进一步认识预应变对中子辐照金属材料塑性形变和最终断裂特性的影响规律,及其微观机理,本文研究了10%拉伸预应变高纯铝的拉伸应力-应变曲线、失稳应力和失稳应变等随辐照剂量的变化规律.结果表明,辐照剂量越高,预应变高纯铝内部孔洞的尺寸和数密度越高,导致屈服强度和极限拉伸强度越高,均匀延伸率和失稳应变越小,表现出典型的辐照硬化和脆化效应,但失稳应力与辐照剂量几乎无关.相同辐照剂量条件下,预应变引入的高密度位错能够显著降低辐照孔洞的尺寸和数密度,加之辐照退火效应的综合影响,导致预应变能够降低高纯铝屈服强度的增长率和失稳应变的下降率,从而表现出一定的抑制辐照硬化和脆化的能力,预应变还能够提高高纯铝的失稳应力,但整体而言预应变并不能提高高纯铝的延性.最后,基于J-C本构模型的中子辐照退火态金属材料的脆化模型能够直接应用于预应变金属材料,且模型预测结果与实验结果吻合较好.     

纳米金属丝拉伸破坏及其应变率效应

建立了适于研究纳米金属快速变形破坏过程的分子动力学模型,并对不同应变率工况下不同截面尺寸单晶镍纳米丝的零温单向拉伸破坏过程进行了分子动力学模拟.模拟得到各种纳米镍丝的应力-应变曲线、屈服应变、屈服强度、断裂强度和初始弹性模量,提出了纳米金属丝快速变形力学性能的应变率效应预测公式并加以验证.计算表明金属纳米丝的屈服应变与尺寸和应变率无关,屈服强度、断裂强度和弹性模量与应变率呈对数关系.     

不同晶向金属纳米线拉伸力学性能分子动力学研究

在经典等温分子动力学框架下,采用位移加载方式,准静态条件下数值模拟常温条件下金属纳米线的单向拉伸,研究了面心立方晶格（FCC）单晶金属纳米线的弹塑性力学性能。研究发现 <100>,<110>,<111>三个不同晶向纳米线拉伸时呈现不同的拉伸变形力学性能。其中<111>晶向拉伸有最高的屈服强度,<110>晶向屈服屈服最小,特别的是<100>晶向拉伸时屈服应变最大。由于不同的晶向对应纳米线的不同表面,三个晶向的纳米线拉伸呈现不同的应力应变关系曲线,变形过程中的局部结构具有不同的演化方式。分析了纳米单晶铜线的三个晶向拉伸表现不同的等效弹性刚度和屈服强度,讨论了相关的局部位错结构演化过程和与位错发射分解剪应力相关的纳米线塑性变形机理。     

对称拉压循环对HRB400E钢弹塑性行为影响的试验研究

对HRB400E钢在单向拉伸、拉压循环和循环后拉伸载荷作用下的力学行为进行了试验研究,采用0.3%、0.4%、0.5%、0.6%、0.7%、0.8%、0.9%和1%八种对称应变幅对HRB400E薄壁圆管试件施加100周次拉压循环载荷,循环后施加单轴拉伸载荷直至试件断裂。试验数据分析表明,循环加载过程中HRB400E钢前10周应力应变曲线曲率明显变小,峰值应力明显下降,呈现循环软化特性,随着应变循环幅值的增大,循环滞回曲线稳定所需循环周次逐渐减小。HRB400E钢在不同应变幅循环后的拉伸弹性模量均出现下降,下降幅度约为3%～8%,屈服应力会随着预循环应变幅的增大而增大,随着预循环应变幅增大材料的硬化效应有所减弱。与单轴拉伸相比,循环后拉伸的强度极限和断裂应变无明显变化,塑性变形能力没有明显下降,破坏为典型的塑性剪切断裂模式,薄壁圆管试件断口中的断裂纤维区和剪切唇区位于接近管壁处,断面径向中部的断裂扩展区中,在大韧窝周围存在多个较小韧窝。     

双向受压状态下的混凝土率效应试验研究

利用三峡大学自行研制的大型多功能三轴材料试验机,完成了4种侧应力等级和3个数量级加载速率的C30混凝土立方体试件双向受压试验,系统地探讨了不同侧应力和不同加载速率下混凝土的强度特性及变形特性。研究表明,侧应力和应变速率都能提高混凝土的应变峰值,但侧应力对混凝土的应变峰值的影响较应变速率的影响大;随着侧应力的增加,混凝土的率敏感性逐渐降低;在有侧应力的情况下,随应变速率的增加,混凝土弹性模量变化逐渐减小;随应变速率的增加,应力应变曲线上升段更加陡峭,曲线更加饱满,混凝土剩余强度越来越高;双轴受压情况下,混凝土的应力应变曲线下降段趋势比无侧应力状态下较平缓,随侧应力的增加下降段曲线越来越平缓,混凝土的应力应变曲线更加饱满,峰值点无明显尖峰出现,混凝土剩余强度越来越高。     

汽车薄钢板应力应变曲线及屈服轨迹的研究

采用十字形双向拉伸的实验方法对两种汽车用薄钢板BH220和SPEN进行了不同 加载路径下的双向拉伸试验，得到了不同应力状态下的应力应变关系曲线，同时，根据单位 体积塑性功相等的原则，确定了两种钢板等效塑性应变从0.2\%$\sim$2\%的实验屈服轨迹. 结果分析表明：不同加载路径下板料的应力应变关系不同，随着加载比例由单拉到等双拉状 态，板料的硬化指数逐步增大；实验屈服轨迹呈外凸性，且以等双拉为界的上下部分屈服轨 迹不对称，随着变形程度的增加，屈服轨迹向外扩大，但单拉时强化程度最小，而等双拉 时最大. 对BH220和SPEN钢板的实验屈服轨迹与几种常用理论屈服轨迹的比较发现，Hosford各向 异性屈服准则的理论轨迹与实验结果最为接近，Hill48准则与实验结果相差最大，此外一 向被视为只适用于各向同性材料的Mises准则与实验结果也较为接近，其他几个屈服准则的 理论屈服轨迹与实验点相差较大.     

Unloading of thick-walled cylinders that have been plastically deformed

Two analytical solutions are used to predict load-strain relations for unloading of thick-walled cylinders. The solutions assume that the material is an isotropic-hardening material that obeys the von Mises yield condition. The loading function for the material for the unloading of the cylinders was obtained from tension-compression specimens that were unloaded and reverse loaded from several points along the tension stress-strain diagram. Good agreement is indicated between the unloading load-strain curves obtained from two thick-walled cylinders made of SAE 1045 steel and the curves predicted by the analytical solutions. The analytical solutions predict that the beneficial circumferential compressive residual stresses at the inside of the cylinders decrease by about 50 percent during the unloading.     

Strain-history effect on isotropic and anisotropic plastic behavior

An experimental investigation was performed to evaluate the effect of strain history on an initially isotropic material. A hot-rolled 2.5-in.-diam bar of SAE 1045 steel provided all the test specimens. Axial and circumferential compression data indicated that the steel was isotropic. Additional tension and torsion data indicated that the steel was an isotropic-hardening von Mises material; this was also confirmed by proportionate loading of thin-walled cylinders such that the ratio of axial to circumferential stresses was either 0, 1/2, 1, 2 or ∞. Two additional sets of cylinders were preloaded either in simple axial tension or as closed-ended cylinders to an effective plastic strain of 0.006 before they were proportionately loaded. The preloading had a pronounced effect on yield surfaces for reloading if the effective plastic strain on reloading was only slightly greater than that for the preloading. The effect of preloading on the yield surfaces was small when the effective plastic strain was three to four times that for the preloading. Hill's anisotropic theory was used to predict stress-strain relations for several of the reloaded cylinders. Good agreement was obtained between theory and experiment.     

Mechanical behavior of metals under tension- compression loading at high strain rate

By using a new technique based on a split Hopkinson pressure bar method, a sequenced reverse test (quasi-static tensile prestress, followed by dynamic compression and then followed by dynamic tension) at high strain rate was performed and tension-compression stress-strain relations were derived by using one-dimensional stress wave analysis. Three materials, 2017 aluminium alloy, 0.45% carbon steel, and pure aluminium, were investigated at low and high strain rates, and the strain rate effect on the reverse loading stress-strain curves was compared to that on the loading stress-strain curves. It was found that reduction of yield stress is always associated with load reversals, and the strain rate effect on the reverse loading (tension) is almost the same as that during loading (compression) at higher values of reverse deformation.     

Plastic deformation of aluminium alloy under abruptly-changing loading or strain paths

The effect of deformation history on the plastic behaviour of thin-walled tubular specimens of aluminium alloy 5056 was examined for three types of abruptly-changing loading or strain paths: namely, reverse loading after pre-loading, orthogonal straining after pre-strain, and orthogonal reloading after pre-loading and then perfect unloading, by applying combined loadings of axial force and torque. The experimental results revealed the following trends. The relation between the magnitudes of stress and strain after the comer is expressed for every pre-strain by a single curve parallel to the extension of the pre-loading curve, when the strain after the corner exceeds 1.2 per cent. Moreover, the relations between the stress reduced by pre-stress and strain after the corner for various values of the same type of pre-strain agree with each other; however, the relation for the tensile pre-strain differs clearly from that for the torsional pre-strain. Thus, A.A. Il'yushin's postulate of isotropy does not hold accurately for the above loading histories for the aluminium alloy even if the effect of the third invariant of stress deviator is eliminated.     

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.     

大理岩试样循环加载强化作用的试验研究

利用不同晶粒大理岩块加工成完整和含孔道的圆柱试样,在伺服实验机上分别进行单轴和常规三轴循环加载试验,其中三轴循环加载包括维持围压恒定和卸载围压两种情况.结果表明,循环加载确实可使试样的强度增加,强度大致提高5%-10%.岩石的材料强度和承载能力是两个不同的概念.大理岩晶粒之间多为裂隙,轴向压缩加载时晶粒局部接触应力远高于名义应力,在岩样未达到峰值应力之前,接触处已经产生很大的变形乃至出现局部破坏,形成的碎屑在卸载时可以脱落充填到附近的空隙,提高岩样承载能力.这种强化特征是与摩擦相关的承载能力,并非材料强度;与多次循环加载造成材料强度逐步劣化不同.从卸载点附近卸载与再加载应力应变曲线的关系,可以确认卸载是否引起岩石承载能力的增加.     

Thermo-mechanical measurement of elasto-plastic transitions during cyclic loading

The surface temperature of stainless steel SS304 low cycle fatigue specimens subjected to cyclic loading was studied using infrared thermography technique. The thermal data mapped onto the various stages of cyclic stress-strain curve shows the ability of these measurements to identify the yield points in both the compression and tension loading. Based on the results of this study, it is possible to identify the state of stress for materials such as elastic tension, plastic tension, elastic compression, plastic compression during cyclic loading using infrared thermographic data. The thermo-elastic slope and thermo-plastic slope was observed to be dependent on the prior loading cycles.     

预应变对中子辐照高纯铝拉伸性能的影响

金属材料的中子辐照硬化和脆化一直都是核能安全领域十分关注的重要问题之一. 为了进一步认识预应变对中子辐照金属材料塑性形变和最终断裂特性的影响规律, 及其微观机理, 本文研究了10%拉伸预应变高纯铝的拉伸应力-应变曲线、失稳应力和失稳应变等随辐照剂量的变化规律. 结果表明, 辐照剂量越高, 预应变高纯铝内部孔洞的尺寸和数密度越高, 导致屈服强度和极限拉伸强度越高, 均匀延伸率和失稳应变越小, 表现出典型的辐照硬化和脆化效应, 但失稳应力与辐照剂量几乎无关. 相同辐照剂量条件下, 预应变引入的高密度位错能够显著降低辐照孔洞的尺寸和数密度, 加之辐照退火效应的综合影响, 导致预应变能够降低高纯铝屈服强度的增长率和失稳应变的下降率, 从而表现出一定的抑制辐照硬化和脆化的能力, 预应变还能够提高高纯铝的失稳应力, 但整体而言预应变并不能提高高纯铝的延性. 最后, 基于J-C本构模型的中子辐照退火态金属材料的脆化模型能够直接应用于预应变金属材料, 且模型预测结果与实验结果吻合较好.      

Tensile Test Machine for Unsymmetrical Materials

A novel solution to overcome the shortcoming of conventional tensile test machines in dealing with unsymmetrical materials and off-axis testing of composites is presented. Conventional testing machines are designed on the basis of subjecting a specimen to axial load to determine the stiffness and strength of the material. For specimens with unsymmetrical cross-section this method is no longer valid due to induced additional bending stresses. To overcome this problem a novel tensile test machine was designed, which allows bending deformation, thus subjecting the specimen to pure tension instead of axial loading. To validate the design, the machine was fabricated and employed for tensile testing of an aluminum specimen with unsymmetrical cross-section. The comparison of test results from a conventional machine and from analytically calculations, based on pure tension, reveals that conventional machine generates significant errors, while the results from new machine are in good agreement. The machine was then used to test a functionally graded beam.     

A New Experimental Technique for the Multi-axial Testing of Advanced High Strength Steel Sheets

This paper deals with the development of a new experimental technique for the multi-axial testing of flat sheets and its application to advanced high strength steels. In close analogy with the traditional tension-torsion test for bulk materials, the sheet material is subject to combined tension and shear loading. Using a custom-made dual actuator hydraulic testing machine, combinations of normal and tangential loading are applied to the boundaries of a flat sheet metal specimen. The specimen shape is optimized to provide uniform stress and strain fields within its gage section. Finite element simulations are carried out to verify the approximate formulas for the shear and normal stress components at the specimen center. The corresponding strain fields are determined from digital image correlation. Two test series are performed on a TRIP-assisted steel sheet. The experimental results demonstrate that this new experimental technique can be used to investigate the large deformation behavior of advanced high strength steel sheets. The evolution of the yield surface of the TRIP700 steel is determined for both radial and non-proportional loading paths.