混凝土弹塑性破坏计算模型

根据混凝土破坏面的几何特性,本文提出了改进的混凝土五参数椭圆抛物面破坏准则,并给出了混凝土线性混合强化弹塑性本构关系,有关计算参数可方便地采用试验指标确定。本文理论同试验资料吻合较好,采用的屈服与破坏准则消除了某些常用屈服面的奇异性,使计算处理简化,描述准确。     

Craze-initiation kinetics in polystyrene

In this study it is shown, for a commercial polystyrene grade, that the strain-rate dependence of craze initiation is equivalent to that of yielding. This implies that the kinetics of craze initiation are determined by the nonlinear flow behavior, and that the actual cavitation process is governed by an additional, apparently rate-independent, criterion. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2066–2073, 2004     

A non-destructive experimental investigation of elastic plastic interfaces of autofrettaged thick-walled cylindrical aluminium high pressure vessels

Positions of elastic plastic interfaces play a vital role in safe design and safe use of high pressure vessels. The ENGIN-X neutron diffractometer at the ISIS facility was used to measure the residual strain profiles in a series of aluminium vessels which had been subjected to different pressure levels. The positions of elastic plastic interfaces of the autofrettaged pressure vessels were identified. The results revealed that the residual strain magnitude and the depth of the plastic region will increase with increasing autofrettage pressure level. When autofrettage pressure produces an elastic-plastic boundary at a greater depth than the geometric mean position of the vessel wall, reverse yielding will occur, hence the loss of the vessels’ elastic ability to its subsequent loading. The neutron experimental results agreed well with both the suggestions from existing literatures and the results from FE simulations.     

Size effects due to secondary voids during ductile crack propagation

In the present study the size-effect due to a secondary void population during ductile fracture is investigated. Discrete primary voids are resolved in the process zone at the crack tip. A non-local GTN model is employed to describe the evolution of the secondary voids in the intervoid ligaments. The non-local GTN model contains an intrinsic length scale related to the size of the secondary voids. Hence, the ratio of the size of the primary and that of the secondary voids can be varied. The results show that small secondary voids can toughen the material. Such a behavior is in contrast to the prediction of cell model simulations. A theoretical reasoning of this effect and conclusions are given.     

圣维南原理的应用及屈服面形状大小的确定

在第1 部分,讨论弹性力学的圣维南原理在线弹性断裂力学中的应用,举例说明它的误用会引起很大的误差. 在第2 部分,讨论塑性力学中的Tresca 屈服面和Mises 屈服面的形状和大小,并推广到对Mohr-Coulomb 屈服面和Drucker-Prager 屈服面的描述,给出主应力空间中Mises 屈服面和Tresca 屈服面的形状和大小的三维图象,并以此更正和补充现有的弹塑性力学教材.     

On the temperature independence of statistical model parameters for cleavage fracture in ferritic steels

The work relates to the effect of temperature on the model parameters in local approaches (LAs) to cleavage fracture. According to a recently developed LA model, the physical consensus of plastic deformation being a prerequisite to cleavage fracture enforces any LA model of cleavage fracture to observe initial yielding of a volume element as its threshold stress state to incur cleavage fracture in addition to the conventional practice of confining the fracture process zone within the plastic deformation zone. The physical consistency of the new LA model to the basic LA methodology and the differences between the new LA model and other existing models are interpreted. Then this new LA model is adopted to investigate the temperature dependence of LA model parameters using circumferentially notched round tensile specimens. With the published strength data as input, finite element (FE) calculation is conducted for elastic-perfectly plastic deformation and the realistic elastic-plastic hardening, respectively, to provide stress distributions for model calibration. The calibration results in temperature independent model parameters. This leads to the establishment of a ‘master curve’ characteristic to synchronise the correlation between the nominal strength and the corresponding cleavage fracture probability at different temperatures. This ‘master curve’ behaviour is verified by strength data from three different steels, providing a new path to calculate cleavage fracture probability with significantly reduced FE efforts.     

A microbeam bending method for studying stress-strain relations for metal thin films on silicon substrates

We have developed a microbeam bending technique for determining elastic-plastic, stress-strain relations for thin metal films on silicon substrates. The method is similar to previous microbeam bending techniques, except that triangular silicon microbeams are used in place of rectangular beams. The triangular beam has the advantage that the entire film on the top surface of the beam is subjected to a uniform state of plane strain as the beam is deflected, unlike the standard rectangular geometry where the bending is concentrated at the support. To extract the average stress-strain relations for the film, we present a method of analysis that requires computation of the neutral plane for bending, which changes as the film deforms plastically. This method can be used to determine the elastic-plastic properties of thin metal films on silicon substrates up to strains of about 1%.Utilizing this technique, both yielding and strain hardening of Cu thin films on silicon substrates have been investigated. Copper films with dual crystallographic textures and different grain sizes, as well as others with strong 〈1 1 1〉 textures have been studied. Three strongly textured 〈1 1 1〉 films were studied to examine the effect of film thickness on the deformation properties of the film. These films show very high rates of work hardening, and an increase in the yield stress and work hardening rate with decreasing film thickness, consistent with current dislocation models.     

Deformation Behavior Modeling of SMAs Under Cyclic Loading Based on Rational Interpolation北大核心CSCD

提出了一个有限弹塑性模型,用来模拟形状记忆合金(shape memory alloys,SMAs)在循环荷载下的变形行为.首先,通过分析上下屈服阶段形函数的特点,利用有理插值方法给出循环荷载下的应力-应变形函数显式表达,可以精确匹配任意形状的实验数据;其次,基于对数客观率,构建了有限弹塑性J2流模型,耦合了屈服中心的移动和屈服面的增大;再次,从单轴情况出发,推导得到了单个循环下的三个硬化函数显式表达,再引入局部因子和多轴扩展不变量,构造了光滑统一且多轴有效的硬化函数;最后,将模型得到的结果与经典实验结果比较,证明了新方法的有效性.该文创新点如下:第一,通过改进传统的背应力演化方程,使得新模型产生强烈的Bauschinger效应,从而使新方法具备模拟SMAs特殊变形行为的能力;第二,新的光滑统一硬化函数在单个循环下会自动退化,得到精确符合实验数据的结果;第三,利用本构方程推导得到有效塑性功演化规律,而有理插值得到的形函数中包含依赖有效塑性功的参数,给出这些参数方程以后使得模型具备了预测变形的能力.     

新型让压锚杆作用机理研究

软岩大变形工程存在因开挖卸荷导致较大的应力释放,在工程施工过程中将会产生较大变形的特点,如沿用传统的锚杆进行支护,岩体的大变形所产生的过大拉力将会造成锚杆的拉断破坏.针对软岩大变形工程存在的该类问题,介绍了一种新型的端部具有让压装置的新型压力分散型让压锚杆,并对该让压锚杆的作用机理和力学行为进行分析,得出锚固体的作用机理与传统压力分散型锚杆相比分为3个阶段,并通过试验的方式确定了该让压锚杆的让压力.这种新型让压锚杆在将锚固体压力分散的同时,实现大变形的让压,适宜在大变形工程中进行应用.     

Inelastic axial-flexure–shear coupling in a mixed formulation beam finite element

In this paper a beam element that accounts for inelastic axial-flexure–shear coupling is presented. The mathematical model is derived from a three-field variational form. The finite element approximation for the beam uses shape functions for section forces that satisfy equilibrium and discontinuous section deformations along the beam. No approximation for the beam displacement field is necessary in the formulation. The coupling of the section forces is achieved through the numerical integration of an inelastic multi-axial material model over the cross-section. The proposed element is free from shear-locking. Examples confirm the accuracy and numerical robustness of the proposed element and showcase the interaction between axial force, shear, and bending moment.