An elasto-plastic constitutive model for soft rock considering mobilization of strength

A new elasto-plastic constitutive model is presented in the framework of plasticity theory. The strength characteristics of a diatomaceous soft rock is investigated. The friction angle and cohesion of soft rock are mobilized as a function of plastic strain. A hyperbolic hardening function for the mobilized friction and a mixed parabolic and exponential equation for the mobilized cohesion are proposed. In view of the unified strength theory and the mobilizations of strength components, a yield function is given. A plastic potential function is determined by using the non-associated plastic flow rule. An elasto-plastic constitutive model is developed and verified. The results indicate that the proposed model can predict the behavior of soft rock accurately. The advantages of the proposed constitutive model are analyzed. The evidences support that the proposed constitutive model is a mixed hardening/softening model. A hump hardening/softening function for mobilized friction is extended to a more generalized condition.

Constitutive modeling and springback prediction of stress relaxation age forming of pre-deformed 2219 aluminum alloy

Stress relaxation ageing behavior of pre-deformed AA2219 is studied through stress relaxation age experiments and finite element(FE) simulation. The results show that the stress can promote the process of ageing precipitation, and shorten the time to reach the peak strength. Meanwhile,the residual stress and yield strength increase along with the increase in the initial stress. Based on microstructure evolution and ageing strengthening theory,a unified constitutive model is established and incorporated into the FE simulation model through a user subroutine. It is found that the relative error of the radius is 3.6% compared with the experimental result and the springback is 16.8%. This indicates that the proposed stress relaxation ageing constitutive model provides a good prediction on the springback of such stiffened panel during its ageing process.

Fractional-order visco-plastic constitutive model for uniaxial ratcheting behaviors

This paper proposes a novel unified visco-plastic constitutive model for uniaxial ratcheting behaviors. The cyclic deformation of the material presents remarkable time-dependence and history memory phenomena. The fractional(fractional-order)derivative is an efficient tool for modeling these phenomena. Therefore, we develop a cyclic fractional-order unified visco-plastic(FVP) constitutive model. Specifically, within the framework of the cyclic elasto-plastic theory, the fractional derivative is used to describe the accumulated plastic strain rate and nonlinear kinematic hardening rule based on the Ohno-Abdel-Karim model. Moreover, a new radial return method for the back stress is developed to describe the unclosed hysteresis loops of the stress-strain properly.The capacity of the FVP model used to predict the cyclic deformation of the SS304 stainless steel is verified through a comparison with the corresponding experimental data found in the literature(KANG, G. Z., KAN, Q. H., ZHANG, J., and SUN, Y. F. Timedependent ratcheting experiments of SS304 stainless steel. International Journal of Plasticity, 22(5), 858–894(2006)). The FVP model is shown to be successful in predicting the rate-dependent ratcheting behaviors of the SS304 stainless steel.

Development of a unified viscoplasticity constitutive model based on classical plasticity theory

The traditional unified viscoplasticity constitutive model can be only applied to metal materials.The study of the unified constitutive theory for metal materials has discovered the correlation between the classical plasticity theory and the unified viscoplasticity constitutive model,thus leading to the con-cepts of the classic plastic potential and yield surface in the unified constitutive model.Moreover,this research has given the continuous expression of the classical plastic multiplier and presented the corresponding constructive method,which extends its physical significance and lays down a good foundation for the application of the unified constitutive theory to the material analysis in more fields.This paper also introduces the unified constitutive model for metal materials and geo-materials.The numerical simulation indicates that the construction should be both reasonable and practical.

Unified elastoplastic finite difference and its application

Two elastoplastic constitutive models based on the unified strength the- ory （UST） are established and implemented in an explicit finite difference code, fast Lagrangian analysis of continua （FLAC/FLAC3D）, which includes an associated/non- associated flow rule, strain-hardening/softening, and solutions of singularities. Those two constitutive models are appropriate for metallic and strength-different （SD） materials, respectively. Two verification examples are used to compare the computation results and test data using the two-dimensional finite difference code FLAC and the finite element code ANSYS, and the two constitutive models proposed in this paper are verified. Two application examples, the large deformation of a prismatic bar and the strain-softening be- havior of soft rock under a complex stress state, are analyzed using the three-dimensional code FLAC3D. The two new elastoplastic constitutive models proposed in this paper can be used in bearing capacity evaluation or stability analysis of structures built of metallic or SD materials. The effect of the intermediate principal stress on metallic or SD mate- rial structures under complex stress states, including large deformation, three-dimensional and non-association problems, can be analyzed easily using the two constitutive models proposed in this paper.

基于截面配筋率的钢筋混凝土统一本构关系研究

为避免数值化分析中直接处理钢筋与混凝土之间的复杂关系,推导出钢筋混凝土构件轴心受压统一理论公式,做了不同配筋率与不同混凝土强度等级下的钢混棱柱体轴心受压试验,利用试验量测的荷载位移数据,结合Matlab曲线拟合工具的线性回归与拟合功能,建立了一种基于截面配筋率的能够描述钢筋混凝土材料力学性能的统一本构关系。对试验梁进行数值化分析表明,所建立的数值化模型应力应变特征与试验结果吻合较好,证明了该模型在钢筋混凝土实际运用过程中的可行性与适用性。

砌体受压本构关系统一模型的研究

依据国内外对砌体受压本构关系的研究成果,总结了影响砌体受压本构关系的主要因素;提出了反映上述主要因素的砌体受压本构关系统一模式.该模型适用于各类砌体、不同受力状态、加载方式、加载方向以及加载速率、是否存在横向水平约束等不同情形,具有广泛适用性.

粘弹塑性理论在混凝土变形中的应用

首先文章介绍了粘弹塑性统一本构模型的基本方程,根据混凝土的特点对统一本构模型进行了简化,简化后的模型对混凝土的无损伤段的变形模拟,取得了满意的结果。之后应用粘弹塑性统一本构模型对混凝土进行跳跃实验预测,同样获得了预期效果,证明粘弹塑性统一本构模型在混凝土的变形预测中的应用是成功的。

双剪统一弹塑性本构模型及其工程应用

本文提出了基于俞茂宏统一强度理论的双剪统一相关和非相关流动的弹塑性本构模型，并给出了该统一弹塑性本构模型的有限元实施方法。重点讨论了所谓“奇异屈服面”奇异性的处理方法，定义了两类不同类型的奇异形式，给出了它们的不同处理方法。该方法既直观、简单又便于有限元的实施，它对于其它类型“奇异屈服面”角点奇异性的处理同样适用。应用基于该统一弹塑性本构模型的有限元程序ＵＥＰＰ（ＵｎｉｆｉｅｄＥｌａｓｔｏ－ＰｌａｓｔｉｃｆｉｎｉｔｅｅｌｅｍｅｎｔＰｒｏｇｒａｍ），验证了作者提出的双剪统一弹塑性本构模型及其实施方法的正确性。统一强度理论和双剪统一弹塑性本构模型可以广泛应用于各种土木、机械、航空和岩土工程的结构分析。

岩石三维弹塑性损伤本构模型研究

基于应变等效假设和真实应力概念的弹塑性损伤理论,在真实应力空间内,结合非线性统一强度模型,以广义塑性剪应变为硬化参数,并同时考虑应力水平对硬化速率的影响,建立了无损状态下岩石材料的弹塑性表达式;在名义应力空间内建立考虑围压对损伤速率影响的损伤演化方程,从而建立了岩石材料的三维弹塑性损伤本构模型。本构模型中各物理参数意义明确,与材料试验结果的对比表明,所建立的三维弹塑性损伤本构模型可较好地描述岩石材料在多轴受力情况下的变形与强度特性,为岩体工程的复杂非线性受力分析提供理论依据。

基于统一强度理论的软岩损伤统计本构模型研究

基于统一强度理论,在考虑洛德参数的基础上,得到随洛德参数和中间主应力系数变化的材料统一强度参数,建立可以考虑中间主应力的统一强度理论平面形式的强度准则。假定软岩微元强度分布统计概率,定义软岩的统计损伤变量,依据统一强度理论建立三轴应力状态下软岩的损伤统计本构模型。通过软岩的常规三轴试验结果对其进行验证,对偏应力–应变的理论结果与试验结果进行比较。研究结果表明,该本构模型能够较好地反映软岩的偏应力–应变关系,尤其是应变软化特性。而且依据统一强度理论和统一强度内摩擦角的发挥,进一步分析表明洛德参数以及中间主应力系数b对偏应力–应变关系有影响,软岩的偏应力–应变曲线先随洛德参数的增大而上升到一定值,而后随洛德参数的增大而降低;随中间主应力系数b的增大而不断增加。

粘弹塑性统一本构模型理论

文章在已有的统一本构模型的基础上 ,将粘弹性变形引入到统一本构模型之中 ,成功地改善了材料过渡段的变形模拟情况。通过 Hastelloy- X的变形模拟及与其它统一本构模型的变形模拟比较 。

考虑颗粒破碎的粗粒土剪胀性统一本构模型

粗粒土作为无黏性散粒状材料具有状态依赖特性,土体的剪切特性受密度和应力水平影响。易破碎是粗粒土的另一个特点,颗粒破碎影响粗粒土的剪胀、内摩擦角、峰值强度和渗透系数。为了能够准确地描述粗粒土的应力-应变关系,采用初始状态参量描述粗粒土的内部状态,根据三轴试验数据建立考虑颗粒破碎耗能的应力-应变关系,采用相关联流动法则推导考虑颗粒破碎的粗粒土剪胀性"统一本构模型",并建立初始状态参量与模型参数之间的关系。所建立的统一本构模型既考虑了颗粒破碎对剪胀、内摩擦角的影响,又考虑了剪切特性对土体初始状态的依赖。采用变异粒子群算法拟合试验曲线,确定模型参数。模型计算结果能够很好地拟合试验曲线。采用同一组参数对假定的初始状态进行模拟计算,计算结果表明,模型能够模拟不同初始密度和应力水平下粗粒土变形的一般规律。

统一弹塑性本构模型在FLAC^(3D)中的计算格式

统一强度理论是一个有着坚实理论基础、可表达各种岩土材料强度特性且被广泛应用的新的强度理论。FLAC3D是一个具有强大的计算分析功能且专门针对岩土工程问题开发并被广泛应用的数值分析软件。若能将二者结合起来,无疑会大大促进岩土工程领域相关问题的解决。针对这一问题,研究了统一弹塑性本构模型在有限差分方法中应用的基本理论格式。根据FLAC3D软件中UDM接口计算格式的要求,将统一强度理论引入其中,详细推导了统一弹塑性本构模型在FLAC3D软件中应用的计算公式。由于统一屈服面在应力空间内由12个面组成,在应力角为[0,π/3]的范围内,统一屈服面由两个相交面组成,为计算塑性因子及处理计算中应力超出屈服面的应力调整问题,利用应力角分析了应力空间的分区,推导了区域分界面的公式。最后,编制了相应的UDM接口程序对一圆形隧洞进行了弹塑性分析,对比了计算结果与解析解,结果很好地验证了此计算格式及相应接口程序的正确性。统一强度理论和FLAC3D软件的结合,将使二者的优点得以充分发挥,以解决更多的岩土工程问题。

土体热力学本构模型的改进、验证及有限元分析

首先给出了建立土体弹塑性本构模型的热力学方法的基本思路,它不仅具有紧凑的数学结构,而且能自动满足热力学定律,具有较严密的数学物理基础。随后在简要介绍与分析Collins的热力学土体模型的基础上,针对其硬化定律的不足,引入统一硬化参量对其进行改进,从而将该模型扩展为既能反映砂土剪胀等变形特性,又适用于黏土的统一热力学模型。编制了改进前、后的土体热力学模型的试验验证程序与有限元程序,通过对计算结果的对比分析,初步验证了模型的适应性与有效性。

金属材料本构模型的研究进展

介绍了金属材料加工硬化、动态回复和动态再结晶流变行为的传统本构模型;概述了统一本构理论模型的产生、通用形式和共同特点,并对Miller、Walker、B-P和Chaboche等几种典型的统一本构模型重点进行了介绍。

西安东门城墙的弹塑性有限元分析

西安东门城墙是我国的重点保护文物，对其研究意义重大．采用俞茂宏统一强度理论建立的弹塑性本构模型对其研究，得到了不同强度准则下的荷载系数位移曲线、塑性区分布、主应力迹线、弹塑性极限等结构特性．研究表明，不同的强度理论（如ＭｏｈｒＣｏｕｌｏｍｂ理论、广义双剪强度理论和ＤｒｕｃｋｅｒＰｒａｇｅｒ准则）对城墙结构分析有很大的影响．

一种黏弹塑性统一本构模型

经过对大量有关统一本构模型的文献资料分析，指出了现有统一本构模型存在的问题，并通过对材料实验数据的分析，指出了黏弹性现象在实验中的表现，并据此将黏弹性引入到弹性黏塑性统一本构模型［１］之中，建立了黏弹塑性统一本构模型；通过模型的数值模拟证明：模型计算结果无论在变形趋势上，还是在数值精度上都与实验数据符合得很好，克服了此前统一本构模型存在的问题．黏弹塑性统一本构模型的产生将统一本构理论的内涵扩大到黏弹性范围，进而构造了一个黏弹塑性理论的新框架．

聚丙烯纤维再生混凝土损伤本构模型的研究

基于Weibull统计分布理论和Lemaitre等效应变假定原理,推导出聚丙烯纤维再生混凝土单轴受压损伤本构模型,对聚丙烯纤维再生混凝土进行单轴受压试验,根据试验数据,确定了该模型参数,通过试验曲线和模型曲线的对比分析发现两者拟合较好。基于考虑中间主应力、拉压性能影响的适用于任何材料的统一强度理论,推导出聚丙烯纤维再生混凝土的双剪损伤本构模型,建立了纤维混凝土损伤本构模型从单轴到双轴的转化方法。

连续冷却过程中的相变本构方程

如何有效地预测在不同工艺条件下材料的微观结构和力学性能,是许多钢铁公司非常关注的重要课题之一。以连续冷却过程中的自由能理论为基础,成功地开发出一套在连续冷却过程中统一的相变本构方程。该本构方程可用来预测钢材在不同的冷却速度下从奥氏体到铁素体、珠光体或贝氏体的开始转变点和结束转变点,同时还可用来预测各相转变的体积分数。利用优化算法通过缩小试验数据和预测数据之间的误差来确定本构方程中的材料常数,得出DINSAE5140钢种相变本构方程的模型参数,并且利用该相变本构方程对DINSAE5140钢种的相变过程进行详细地研究。研究结果表明,利用所开发的模型参数得出的预测连续冷却转变(Continuous cooling transformation,CCT)曲线和相变过程中各相的转变体积分数与试验数据非常一致,该统一的本构方程的建立为预测相变过程提供了一种有效的方法。