低温下船用钢材弹塑性曲线研究

【目的】分析不同低温下船用钢材单向拉伸应力-应变关系,对低温下船用钢材弹塑性曲线进行研究具有深远意义。【方法】选用EH36钢作为试验材料,对试件进行三种低温下的单向拉伸试验,在对试验数据进行分析后,采用弹性-幂次强化模型本构关系进行拟合,对比不同温度下单向拉伸应力-应变曲线。【结果】对EH36钢在不同低温下的单向拉伸应力-应变曲线进行分析,发现屈服强度和塑性应变随温度的降低而提高。【结论】EH36钢塑性强化参数采用有效塑性应变,计算得出预加应变时逆向加载的一维应力-应变关系,为此材料力学特性的研究提供参考。

考虑嵌固段桩前土体弹塑性变形的锚索微型桩群计算模型研究

锚索微型桩群在中小型边坡治理及边坡紧急抢险工程中的应用越来越广泛,但现有的计算模型对嵌固段桩前土体塑性变形及桩侧土体受力变形连续性考虑较少。针对这一问题,首先根据锚索微型桩群加固土质边坡的特点,以潜在滑裂面位置为分界线,将微型桩群分为受荷段与嵌固段进行分析。对于受荷段,将桩间土简化为水平土弹簧,有效考虑桩间土力的传递作用;对于嵌固段,采用理想弹塑性p-y曲线分析桩-土相互作用。其次,将冠梁处预应力锚索简化为弹性支座,将其作用力转移至后排桩桩顶,再由桩顶冠梁重分配至各排桩桩顶处,从而达到对桩顶位移的约束作用。最后,以帕斯捷尔纳克(Pasternak)双参数地基梁法为基础,建立考虑嵌固段桩前土体弹塑性变形及桩侧土体受力变形连续性的锚索微型桩群有限差分计算模型,并迭代求解得到该结构的水平位移与内力。利用该计算模型分别对微型桩群与锚索微型桩群加固土质边坡进行算例计算,与试验、数值模拟结果对比分析,验证该模型的合理性,并探讨土体剪切刚度、锚索束及桩径对该锚索微型桩群水平位移与内力的影响。研究表明:嵌固段桩前土体塑性变形及桩侧土体受力变形连续性对微型桩的水平位移与内力有较大影响;随着锚索束的增加,桩身最大正弯矩及水平位移均不断减小,可通过增设锚索束达到控制桩身变形与内力的目的。

两种不同需求曲线的能力谱评价方法比较

在计算能力谱方法需求曲线时,考虑了基于结构等效阻尼比和R--μT关系2种对弹性反应谱的折减方式,并对结构进行非线性时程分析作为比较依据.结果表明,采用能力谱评价方法对多层结构进行抗震能力分析是可行的,在弹性范围,2种折减方式所得结果基本一致,进入塑性后,有较大差别.传统能力谱法考虑等效阻尼比低估了结构的变形,有较大误差,主要原因是等效阻尼比的计算假设和实际阻尼表现不符,由于R-μ-T关系的改进能力谱法和时程分析结果基本吻合,在对结构进行抗震能力评价时,采用考虑R--μT关系的改进方法可有效地提高精度,满足工程需要.

SY740钢高温塑性试验研究

为支撑SY740非调钢连铸和轧制工艺制定及优化,预防和控制连铸坯表面裂纹的形成,提高和改善铸坯质量,助力产品研发及生产,利用Gleeble3800热模拟机试验研究SY740非调钢高温塑性。通过控制拉伸温度及应变速率,测定了该钢种不同温度的强度极限、断面收缩率,得出塑性变化特征,并绘制SY740非调钢高温塑性曲线。结果表明,SY740钢在650~900℃温度时,断面收缩率低于50,塑性较差,在950~1150℃温度时,断面收缩率大于60,塑性较好。试验研究成果对控制连铸拉坯过程矫直阶段温度预防连铸坯表面裂纹出现,提供了支撑。

耐候钢JNS高温力学性能研究

采用Gleeble热／力模拟试验机，对耐候钢JNS连铸坯进行了高温力学性能测试，得到了在600～1300℃范围内的应力应变曲线，塑性、强度和硬化系数。测试结果表明，JNS钢断面收缩率均在60％以上，塑性比较好。在塑性曲线上存在两个低塑性区，一个在700℃以下，另一个在二次脆性区800～950℃内。抗拉强度和屈服强度随着温度升高是降低的，在1200℃以上强度很低，受力易产生裂纹。高温段的平均硬化系数较低，变形能力较差，出现裂纹的倾向较高。为避开二次脆性区，矫直点前铸坯的温度应控制在1000℃以上，以防止裂纹的出现。

建筑结构基于性能的抗震评估的等ζ_y延性谱法

利用水平地震作用下单自由度体系的运动方程,以屈服承载力系数ζy为参量,推导出适时延性系数动力方程,从而建立了基于屈服承载力系数的延性需求谱。结合结构的非线性静力pushover分析,提出了基于性能的抗震评估方法——等ζy延性谱法。用等yζ延性谱法对一个6层钢筋混凝土框架结构进行了抗震评估,并与非线性时程分析进行了比较。结果表明,用等yζ延性谱法对结构进行抗震性能评估是可行的,并且具有较高的精度。

耐硫酸露点腐蚀Q315NS钢的高温力学性能研究

采用Gleeble-3500热模拟实验机,对耐硫酸露点腐蚀钢Q315NS连铸板坯进行了高温力学性能实验,获得了600~1400℃范围内应力-应变曲线、热塑性曲线和强度曲线。结果表明:在600~870℃内,断面收缩率随着温度先升高后降低,但其数值均低于60%,最低值仅为20%;在920~1200℃内,断面收缩率均在80%以上,塑性好;在1250℃到熔点温度范围内,塑性又迅速下降。在连铸坯矫直和弯曲时应避免脆性区。将矫直弯曲温度控制在920~1200℃,可减少裂纹产生。

隧道锚的抗拔安全系数确定方法

为了探求合理的隧道锚抗拔安全系数评估方法,依托西南某跨越金沙江的大桥隧道锚工程,分别采用应力积分法、塑性屈服区体积-荷载曲线和位移-荷载曲线求解隧道锚的抗拔安全系数,并将3种方法的计算结果与规范法的结果加以对比.结果表明:规范法忽略了岩体对锚碇的夹持作用,故其计算所得抗拔安全系数相对最小,仅为2.60;应力积分法取用设计荷载工况对应的应力场,夹持效果不明显,因而计算结果接近于规范法,采用应力积分法所得锚碇-围岩界面及围岩内破坏面的抗拔安全系数分别为2.64和2.88;采用塑性屈服区体积-荷载曲线及位移-荷载曲线时,容许荷载作用下的锚碇-岩体能够充分接触,夹持效果显著,故所得抗拔安全系数最大,其值均为6.50;通过极限摩阻力来反推锚碇-岩体界面夹持力,可得考虑夹持效应的隧道锚的抗拔安全系数.

DETERMINATION OF PRESSURIZING CURVE AND COMPUTER CONTROL OF THICKNESS THINNING FOR SUPERPLASTIC BULGING

This paper proposes the assumption that the flow with viscous friction is the stretch of part of the sheet that lies along the walls of a die during the process of superplastic bulging according to superplastic flow equation and geometrical model of bulging of a sheet into a long trapezoid groove or truncated cone, by introducing the friction-factor P which describes the friction effect on the process. Also, the paper proposes the method of controlling thickness nonuniformity and develops the equipment which for uniform thickness of bulging, is automatically controlled with a computerl it also analyzes the important innuence of lubrication on thickness distribution of bulging materials. By the assumption, the relationship between bulging pressure and time is obtained in bulging of a sheet into the groove and cone, and p-t curve of multi-mould-cavity complicated bulging is discussed based on the analysis of single-mould-cavity bulging characteristics.

基于性能抗震评估的等ζ_y延性谱法探究

运用水平地震作用下单自由度体系的运动方程，以屈服承载力系数为参量，推导出适时延性系数动力方程，从而建立了基于屈服承载力系数的延性需求谱。结合结构的非线性静力pushover分析，提出了基于性能的抗震评估方法-等￡γ延性谱法。用等￡γ延性谱法对-6层钢筋混凝土框架结构进行了抗震评估，并和非线性时程分析进行了比较。结果表明，用等￡γ延性谱法对结构进行抗震性能评估是可行的，并且具有较好的精度。

润滑剂类型对ABS树脂润滑性能的影响

本文采用HAAKE—9000流变仪,在一定转速和温度条件下,测定了不同润滑剂对ABS体系的塑性形变曲线,并对各种曲线进行了分析,为今后在制作轿车门内基板的ABS体系中确定润滑剂的种类提供了有益的参考。

热轧H型钢厚度自动控制系统模型研究与实现

随着H型钢应用范围的不断扩大,人们对相应的产品尺寸要求也越来越高。本文以莱钢H型钢生产线精轧控制系统为原型,介绍了厚度自动控制系统的原理、AGC自动控制系统数学模型的建立,以及影响H型钢产品尺寸的主要因素。着重分析了精轧控制系统中静态AGC厚度控制系统与动态AGC厚度控制系统的区别和实现的效果。

插层改性粘土的可加工性能研究

本文利用聚丙烯酸对粘土进行插层处理,通过绘制泥料的塑性流变曲线,并对曲线进行线性回归拟合和分析,研究插层改性粘土的可加工性能。

Effect of surface nanocrystallization and PPEC time on complex nanocrystalline hard layer fabricated by plasma electrolysis

Size distribution of nano-carbides produced by duplex treatments of surface nanocrystallization(by surface severe plastic deformation) and plasma electrolytic carburizing on CP-Ti was investigated.Skewness and kurtosis of Gussian shape distribution curves were studied and the effect of time was determined.The usage of longer time is more suitable for achieving less size of complex nano-carbides.Surface roughness of treated samples was measured.It is observed that there is an optimum level for time on surface roughness increasing(difference between two measured data).

基于Pushover分析的预压装配式框架结构整体抗震性能系数研究

对8个按中国规范设计的不同层数、不同设防烈度的预压装配式框架结构中的梁柱节点进行有限元分析,根据有限元分析得到的各节点截面弯矩-转角曲线计算其初始转动刚度和定义了预压装配式梁柱节点的塑性铰骨架曲线。在此基础上分别对8个预压装配式框架结构在倒三角分布和均匀分布两种水平侧向力分布模式下进行了Pushover分析,研究了不同分布方式对预压装配式框架结构Pushover分析结果的影响。通过能力谱法得到了各结构在小震和大震下的最大层间位移角,结果表明预压装配式结构能够满足抗震规范中关于小震和大震下的位移限值要求。根据Pushover能力曲线得到整体抗震性能系数,给出了结构超强系数、反应修正系数、位移放大系数的建议值,可为预压装配式框架结构的设计提供依据。

Influences of strain softening and seepage on elastic and plastic solutions of circular openings in nonlinear rock masses

Considering the influence of strain softening, the solutions of stress, displacement, plastic softening region radius and plastic residual region radius were derived for circular openings in nonlinear rock masses subjected to seepage. The radial stress distribution curve, ground reaction curve, and relation curve between plastic softening region radius and supporting force in three different conditions were drawn respectively. From the comparisons among these results for different conditions, it is found that when the supporting force is the same, the displacement of tunnel wall considering both seepage and strain softening is 85.71% greater than that only considering seepage. The increase values of radial displacement at 0.95 m and plastic softening region radius at 6.6 m show that the seepage and strain softening have the most unfavorable effects on circular opening stability in strain softening rock masses.

Superplastic deformation behavior and mechanism of 1420 Al-Li alloy sheets with elongated grains

True stress-true strain curve,microstructure and texture information were obtained to investigate the superplastic deformation behavior of 1420 Al-Li alloy sheets with initial elongated grains.From the true stress-true curve,the stress increases with the increase of strain to 0.15,then dramatically decreases with the increase of strain to 0.80,and finally keeps almost a horizontal line.Meanwhile,initial elongated grains are gradually changed into equiaxed grains and the initial strong Brass {0 1 1} <2 1 1> and S {1 2 3} <6 3 4> orientations are turned into nearly random orientation with increasing strain.All these results suggest that dislocation activity is the dominant mechanism during the first stage,then dynamic recrystallization occurs,and grain rotation is expected as an accommodation for grain boundary sliding(GBS).At larger strains,grain boundary migration(GBM) becomes necessary to accommodate GBS.

Press cutting of thin-walled round pipe for producing curvilinear end

In order to obtain a basic understanding of the unwanted distortions in the pipe wall during the press cutting process, the deformation of a thin-walled round pipe to form a curvilinear end was numerically and experimentally studied. Vector analysis was used to study the relationship between the punch shape and the collapse of the cut-end. Stamping experiments on AISI 1020 steel pipe were conducted using different angles a and β defining the shape of the punch. The elasto-plastic finite element method that allows consideration of a ductile fracture was also employed to study the process. The results show that the deformation of the pipe end after press cutting is inβuenced mostly by the shape of the punch. A satisfactory quality of the curvilinear end of the pipe can be obtained if the appropriate geometric parameters of the punch are chosen. The pipe-wall collapse in the upper part of the section is decreased when a and β increase. The recommended values for a and β lie within 30°-50°. The hole on the underside of the punch has less inβuence on the quality of the cut-end, and the wall distortion and the generation of burr on the cut-end can be satisfactorily simulated using the fracture criterion of Brozzo or the normalized criterion of Cockcroft and Latham.

Plastic characterization of metals by combining nanoindentation test and finite element simulation

Materials with the same elastic modulus E and representative stress and strain （σr,εr） present similar indentation-loading curves, whatever the value of strain hardening exponent n. Based on this definition, a good approach was proposed to extract the plastic properties or constitutive equations of metals from nanoindentation test combining finite element simulation. Firstly, without consideration of strain hardening, the representative stress was determined by varying assumed representative stress over a wide range until a good agreement was reached between the computed and experimental loading curves. Similarly, the corresponding representative strain was determined with different hypothetical values of strain hardening exponent in the range of 0-0.6. Through modulating assumed strain hardening exponent values to make the computed unloading curve coincide with that of the experiment, the real strain hardening exponent was acquired. Once the strain hardening exponent was determined, the initial yield stress ay of metals could be obtained by the power law constitution. The validity of the proposed methodology was verified by three real metals： AISI 304 steel, Fe andA1 alloy.

Generatrix shape optimization of stiffened shells for low imperfection sensitivity

According to previous studies,stiffened shells with convex hyperbolic generatrix shape are less sensitive to imperfections.In this study,the effects of generatrix shape on the performances of elastic and plastic buckling in stiffened shells are investigated.Then,a more general description of generatrix shape is proposed,which can simply be expressed as a convex B-spline curve(controlled by four key points).An optimization framework of stiffened shells with a convex B-spline generatrix is established,with optimization objective being measured in terms of nominal collapse load,which can be expressed as a weighted sum of geometrically imperfect shells.The effectiveness of the proposed framework is demonstrated by a detailed comparison of the optimum designs for the B-spline and hyperbolic generatrix shapes.The decrease of imperfection sensitivity allows for a significant weight saving,which is particularly important in the development of future heavy-lift launch vehicles.