一种马铃薯临界损伤跌落高度的测定方法

针对马铃薯这类不易观察跌落损伤的果蔬产品,提出了一种测定其临界损伤跌落高度的方法。首先对马铃薯进行准静态压缩,得到其产生损伤时的压缩变形量。同时,通过测量马铃薯在不同跌落高度下的跌落信号,分析得到反映不同跌落高度与跌落变形量之间关系的数学模型。对应压缩变形量在此数学模型中的值,即可得到马铃薯的临界损伤跌落高度,为38cm。     

硅橡胶泡沫材料的应力松弛性能研究

在不同温度、湿度、压缩变形量下,对硅泡沫材料的应力松弛性能进行了实验研究,得到了温度、湿度、压缩变形量、时间对硅泡沫材料应力松弛影响的初步规律.     

Nb-Ti微合金高强钢动态再结晶动力学及临界条件

在Gleeble-3500 热模拟试验机上对Nb-Ti微合金高强钢进行了热模拟压缩试验,研究了其在变形温度为900~1 100 ℃、应变速率为0. 01~5 s-1 、最大变形量为70%条件下的动态再结晶行为。对流变曲线的分析及微观组织观察结果表明,低温高应变速率下流变曲线未显现出典型动态再结晶特征,但此条件下已发生动态再结晶。使用双曲正弦形Arrhenius关系计算的Nb-Ti微合金钢变形激活能为404 kJ/mol。利用加工硬化原理和Cingara-McQueen模型确定了动态再结晶初始临界应力和应变,分析了由Cingara-McQueen模型计算临界应力值偏高的原因,建立了临界应力、应变和Z参数之间的定量关系,得到了动态再结晶临界应力和应变方程:σ_c=0.335Z0.144,εc=0.005 9Z0.079。通过对θ-ε曲线进行分析,建立了最大软化速率处应变(εm)和变形条件的关系。在此基础上使用Avrami型动态再结晶动力学模型计算了不同变形条件下的再结晶体积分数,结果表明此模型可准确预测Nb-Ti微合金高强钢动力学。     

23CrNi3Mo钢动态再结晶行为和组织演变研究

利用Gleeble-1500热模拟试验机进行单道次热压缩变形,通过控制变形温度(900,950,1 000,1 050℃)和应变速率(0.01,0.1和1 s-1)研究了23CrNi3Mo钢的动态再结晶行为,分析了变形温度、应变速率对动态再结晶行为和组织演变的影响.结果表明:23CrNi3Mo钢动态再结晶的再结晶激活能为293.23 kJ/mol;采用θ-σ模型较精确地获得了发生动态再结晶的临界应变与峰值应变,并确定了其平均比值εc/εp=0.63;确定了材料常数和热变形本构方程.通过对热模拟实验数据的分析和显微组织观察,建立了不同变形条件下动态再结晶模型和晶粒尺寸模型.     

基于热压缩的Cu-15Ni-8Sn合金动态再结晶行为研究

目的 获得Cu-15Ni-8Sn合金动态再结晶临界模型,描述热变形参数对动态再结晶晶粒尺寸的影响规律.方法 基于前期通过Gleeble-3500热模拟机得到的热压缩实验数据,分析Cu-15Ni-8Sn合金在不同热变形参数下的再结晶晶粒尺寸及流变应力数据,采用线性回归拟合等方式建立动态再结晶模型,并利用数值模拟与实验相结合的方法 验证模型精确度.结果 采用YADA模型描述Cu-15Ni-8Sn合金的动态再结晶,通过线性拟合求得模型参数C1=11895.554,C2=0.1503,C3=0.1553,C4=3.933×10?4,C5=2995.6409,数值模拟与实验所得的平均晶粒度分别为16.7μm和15.5μm.结论 变形温度和变形速率对Cu-15Ni-8Sn合金热变形中的再结晶过程有重要影响.变形温度越高,临界应变越小,越容易发生动态再结晶,动态再结晶晶粒尺寸越大;应变速率越小,动态再结晶晶粒尺寸越大.研究所构建的Cu-15Ni-8Sn合金动态再结晶临界模型具有较高精度,将为后续该合金热塑性变形工艺设定提供理论基础.     

低碳Q690qENH高强桥梁钢的动态再结晶行为

对低碳Q690qENH高强桥梁钢进行压缩实验,研究了动态再结晶行为。结果表明,在低碳Q690qENH高强桥梁钢的轧制热变形过程中,其软化以动态回复为主,只在0.1 s^-1和0.2 s^-1低应变速率下才发生明显的动态再结晶.通过计算将应力因子α修正为0.0099 MPa^-1,得到了实验钢的动态再结晶激活能,建立了动态再结晶动力学模型。采用P-M-K法确定了εc/εp约为0.72,且峰值应变与Z/A满足幂函数关系,建立了动态再结晶临界应变模型,其计算值与热变形中的显微组织演变规律一致。研究了温度对动态再结晶过程中界面迁移速率的影响规律。     

挤压态FGH4096合金室温变形及临界晶粒长大研究

基于挤压态FGH4096合金圆柱和双圆锥台试样室温压缩变形及热处理实验,结合DEFORM有限元软件数值模拟,确定了挤压态FGH4096合金室温压缩临界晶粒长大的临界等效应变窗口条件,分析了挤压态FGH4096合金室温压缩临界晶粒长大机制.结果表明:挤压态FGH4096合金圆柱试样在压缩速度0.01 mm/s下的室温压缩临界等效应变窗口为0.03～0.07,双圆锥台试样在压缩速度0.05 mm/s下的室温压缩临界等效应变窗口为0.005～0.06.挤压态FGH4096合金室温压缩临界晶粒长大与合金变形量和γ'相溶解后晶界的颗粒钉扎作用消失有关,呈现为在小变形区域粗大晶粒吞噬细小晶粒的异常晶粒特征.     

冲击荷载下纳米碳纤维混凝土的动态受压力学特性

为探究纳米碳纤维混凝土的动态受压力学特性,本研究利用Φ100 mm分离式霍普金森压杆装置,分别对素混凝土及体积掺量为0.1％、0.2％、0.3％、0.5％的纳米碳纤维混凝土试件开展冲击压缩试验,对比分析了混凝土在5组不同应变率水平下混凝土的动态抗压强度、动态压缩变形和冲击韧性的变化特征.结果表明:各掺量下纳米碳纤维对混凝土动态抗压强度、动态压缩变形和冲击韧性均具有不同程度的增强作用.纳米碳纤维掺量为0.3％时对动态抗压强度的增强效果最佳;纳米碳纤维仅在0.2％掺量时对混凝土动态弹性模量表现为提升作用,其他掺量下反而使其动态弹性模量有所下降.素混凝土及纳米碳纤维混凝土的动态抗压强度、冲击韧性和动态弹性模量均随应变率水平的提高而逐渐增大,表现出显著的线性关系.动态峰值应变、极限应变虽整体上随应变率的上升呈递增趋势,但二者并不存在明显的线性相关性.适量纳米碳纤维的掺入能够发挥小尺寸效应及增强、阻裂作用,改善基体的微观结构,提升混凝土在冲击荷载作用下的动态受压力学特性.     

混凝土三轴蠕变统计损伤模型研究

为探讨混凝土三轴蠕变特性,采用SAM-2000微机控制电液伺服材料三轴试验机并配备压力自平衡式三轴压力室对混凝土进行了三轴压缩试验和三轴蠕变试验,结果表明:围压的存在可以显著提高混凝土的承载能力,围压越大,混凝土的轴向承载能力越高,蠕变变形量和变形速率越小。假设混凝土破坏为其内部微元破坏累积所致,微元破坏概率与混凝土应变存在一定关系P[#x003b5;],且服从Weibull分布,结合三轴压缩试验结果,计算得到了描述混凝土损伤特性的统计损伤变量;基于Burgers流变模型,通过引入损伤变量建立了混凝土三轴蠕变统计损伤模型,利用MATLAB软件进行了模型参数拟合,经对比分析,模型曲线与试验曲线拟合结果较理想,主偏应力越大,拟合结果越精确。此模型可供今后研究参考。     

TiC(30vol%)/Cu-Al_2O_3复合材料热变形及动态再结晶

采用真空热压-内氧化烧结法制备了TiC(30vol%)/Cu-Al2O3复合材料,测试其基本性能,对其微观组织进行了观察分析。利用Gleeble-1500D热力模拟试验机,在变形温度450~850℃、应变速率0.001~1s-1、变形量50%的条件下,对TiC(30vol%)/Cu-Al2O3进行了热压缩变形试验。通过对流变应力进行分析和计算,构建了该复合材料的本构方程及动态再结晶临界应变模型。利用加工硬化率-应变曲线的拐点和对应偏导曲线最小值的判据,建立了动态再结晶临界应变与Zener-Hollomon参数之间的函数关系。结果表明:TiC(30vol%)/Cu-Al2O3复合材料的真应力-真应变曲线以动态再结晶软化机制为特征,峰值应力随变形温度的降低或应变速率的升高而增加;计算得出该复合材料的热变形激活能为211.384kJ/mol。     

数字图像相关方法辅助的IM7/8552碳纤维/环氧树脂复合材料单向带层合板沿厚度方向非线性本构参数识别

提出了采用数字图像相关(DIC)方法和有限元模型修正(FEMU)技术相结合,通过短梁剪切(SBS)试验获得碳纤维增强环氧树脂(IM7/8552)正交各向异性复合材料单向带层合板沿厚度方向压缩本构关系参数的试验方法。该方法根据假设材料初始本构,采用3D有限元模型(FEM)计算获得主平面压头下方沿厚度方向的应力和应变分布,以DIC实测应变和有限元计算应变之间的方差建立目标函数,并在FEM中进行迭代更新,收敛后获得材料本构参数。由于选择的试样加载形式近似静定结构,试样表面的应力分布对材料本构关系及参数弱相关,上述迭代过程进一步转化为通过全场有限元计算应力和DIC实测应变之间的最小二乘回归识别假设本构关系及参数。因此,该方法具有以下优点:迭代过程中不需要建立针对识别参数的显式敏感度矩阵,识别效率高;识别过程对初始材料本构参数不敏感。     

超弹性形状记忆合金的神经网络连续本构模型

对超弹性形状记忆合金(SMA)丝在不同应变幅值和荷载速率下进行加卸载单轴拉伸试验,分析其滞回特性随环境因素的变化规律。将径向基函数神经网络(RBFNN)和Graesser模型结合起来,Graesser模型参数取自试验曲线,能由数学式确定的模型参数和应变幅值、荷载速率一起作为网络的输入信息,不能由数学式确定的模型参数作为输出神经元。数值计算表明,RBFNN可以精确地预测Graesser模型参数,且计算的SMA应力-应变曲线与Graesser模型结果吻合很好。     

循环荷载下奥氏体型和双相型不锈钢材料本构关系研究

为研究循环荷载下不锈钢材料的本构关系,对奥氏体型S30408不锈钢和双相型S220503不锈钢材料进行了单调拉伸和大应变超低周循环加载试验。采用三种常用的单调拉伸本构模型对所得应力-应变曲线进行拟合,得到相应单调荷载下材料本构参数;采用Ramberg-Osgood本构模型对循环骨架曲线进行拟合,得到材料循环强化参数;利用Chaboche塑性本构模型,标定了两种材料的循环本构参数。结果表明:在单调拉伸荷载下,G-R-O本构模型更适用于拟合不锈钢材料的单调拉伸本构;在循环荷载下,不锈钢材料滞回曲线饱满,且随着应变增大,两种材料在加载后期均表现出了明显的循环强化现象;Ramberg-Osgood本构模型对骨架曲线拟合较好,有限元计算结果和试验滞回曲线吻合度高;表明该文标定出的强化参数、循环本构参数可用于结构体系地震响应分析之中,为准确分析不锈钢结构在地震作用下的受力性能提供参考。     

The application of a three-surface kinematic hardening model to repeated loading of thinly surfaced pavements

This paper examines the application of kinematic hardening to modelling the behaviour of thinly surfaced pavements dominated by the clay subgrade. An existing three-surface kinematic hardening model has been found to predict too much shear strain and therefore too much settlement under repeated loading for certain stress conditions. Under some stress conditions, the model also predicts an accumulation of negative shear strain with increasing number of cycles of load, leading to a pavement rut depth which decreases with increasing numbers of cycles. Consequently, a new model has been developed by modifying the flow rule and hardening modulus. The new model requires 10 parameters, most of which can be determined directly from simple triaxial tests. The new model is validated against drained cyclic triaxial results in order to determine model parameters, and it is shown that the new model predicts better the accumulation of shear strain and the problem of accumulation of negative shear strain is eliminated. This new model is applied to the repeated loading of a thinly surfaced pavement and is seen to predict realistic resilient and permanent deformations.This revised version was published online in September 2005 with a corrected sequence of authors.     

A continuum damage mechanics model with the strain-based approach to biaxial low cycle fatigue failure

A continuum damage mechanics model for low cycle fatigue failure of initially isotropic materials under biaxial loading conditions is presented. The expression for the equivalent strain in the fatigue damage evolution equation contains the three material parameters, and the strain intensity as well as the maximum principal strain and the volume strain for amplitudes. It is shown how these material parameters can be determined from a series of basic experiments using a cruciform specimen. Particular expressions for the equivalent strain with a smaller number of material parameters and invariants are obtained. Model predictions are found to be in satisfactory agreement with the experimental low cycle fatigue data under full ranged biaxial loadings obtained in the test using a cruciform specimen.     

碎石料应力路径大型三轴试验的离散元模拟研究

碎石料在高填方工程中得到广泛应用。由于其粒径较大,对碎石料力学特性的研究通常需要大型的试验设备。该文探索通过离散元数值模拟方法研究碎石料的应力路径相关力学特性。离散元程序可以将若干球形单元粘结成一个捆绑单元,从而模拟可破碎的碎石颗粒。该文首先简要介绍了离散元的计算原理、特点和模拟过程,并建立碎石料试件的离散元模型。通过对比一组恒定围压下的大型三轴试验,确定了各参数,对所建模型进行了标定。进而通过该模型独立预测碎石料试件在三种不同应力路径加载时的应力-应变关系及体变特性,并与相应的应力路径大型三轴试验结果进行对比,验证了离散元方法可以较为准确地计算碎石料沿不同应力路径加载时的力学特性,同时还讨论了碎石料的小应变刚度特性。     

Identification of Damage and Plasticity Parameters for Continuum Damage Mechanics Modelling of Carbon and Glass Fibre‐Reinforced Composite Materials

Abstract: An experimental test series was carried out to determine input parameters for a well‐known continuum damage mechanics elementary ply plasticity model. A full suite of data was obtained for a carbon fibre and an S2‐glass fibre‐reinforced composite material, both currently used in the aerospace industry. Models were implemented using the experimentally determined input parameters and predictions for in‐plane behaviour found good agreement with experiments for both material systems. In addition, model predictions for cyclic loading accurately captured reload moduli and plastic strain magnitude.     

Low‐cycle fatigue life prediction of various metallic materials under multiaxial loading

This paper proposed a simple life prediction model for assessing fatigue lives of metallic materials subjected to multiaxial low‐cycle fatigue (LCF) loading. This proposed model consists of the maximum shear strain range, the normal strain range and the maximum normal stress on the maximum shear strain range plane. Additional cyclic hardening developed during non‐proportional loading is included in the normal stress and strain terms. A computer‐based procedure for multiaxial fatigue life prediction incorporating critical plane damage parameters is presented as well. The accuracy and reliability of the proposed model are systematically checked by using about 300 test data through testing nine kinds of material under both zero and non‐zero mean stress multiaxial loading paths.     

Effect of strain rate on stepwise fatigue and creep slow crack growth in high density polyethylene

The effects of frequency and R-ratio (the ratio of minimum to maximum stress in the fatigue loading cycle) on the kinetics of step-wise crack propagation in fatigue and creep of high density polyethylene (HDPE) was characterized. Stepwise crack growth was observed over the entire range of frequency and R-ratio examined. A model relating crack growth rate to stress intensity factor parameters and applied strain rate was proposed by considering the total crack growth rate to consist of contributions from creep and fatigue loading components. The creep contribution in a fatigue test was calculated from the sinusoidal loading curve and the known dependence of creep crack growth on stress intensity factor in polyethylene. At a very low frequency of 0.01 Hz, fatigue crack growth rate was found to be completely controlled by creep processes. Comparison of the frequency and R-ratio tests revealed that the fatigue loading component depended on strain rate. Therefore, crack growth rate could be modeled with a creep contribution that depended only on the stress intensity factor parameters and a fatigue contribution that depended on strain rate.