Flow stress model considering the transformation-induced plasticity effect and the inelastic strain recovery behavior

On the basis of continuum mechanics and the Mori-Tanaka mean field theory, a micro-mechanical flow stress model that considered both the transformation-induced plasticity （TRIP） effect and the inelastic strain recovery behavior of TRIP multiphase steels was presented. The relation between the volume fraction of constituent phases and plastic strain was introduced to characterize the transformation-induced plasticity effect of TRIP steels. Loading-unloading-reloading uniaxial tension tests of TRIP600 steel were carried out and the strain recovery behavior after unloading was analyzed. From the experimental data, an empirical elastic modulus expression is extracted to characterize the inelastic strain recovery. A comparison of the predicted flow stress with the experimental data shows a good agreement. The mechanism of the transformation-induced plasticity effect and the inelastic recovery effect acting on the flow stress is also discussed in detail.

Dilation angle variations in plastic zone around tunnels in rocks-constant or variable dilation parameter

Dilation angle is a significant parameter needed for numerical simulation of tunnels.Even though dilation parameter is physically variable and dependent on confinement and experienced shear plastic strain based on the existing dilation models,numerical simulations of tunnels and underground openings with constant dilation parameter usually lead to satisfactory results in practical use.This study aims to find out why constant dilation angle is enough under practical conditions to simulate numerically tunnels and underground excavations in spite of the fact that dilation angle is variable in laboratory and experimental scale.With this aim,this work studies how mobilized dilation angle varies in a plastic zone surrounding a tunnel.For the circular tunnel under uniform in situ stress field,the stepwise finite difference approximation analytical solution considering strain softening rock mass behavior with mobilized dilation angle was used to study how mobilized dilation angle varies in plastic zone around tunnel under very different conditions.In practical conditions determined in this study,dilative behavior of all over the plastic zone around the tunnel can be approximated to constant dilation angle in the middle region of the plastic zone.Moreover,the plastic zone displacements for mobilized and constant dilation angle models are compared with each other.Further investigation under more general non-uniform in situ stress conditions and non-circular tunnels is performed by using the commercial finite difference software to numerically simulate the Mine-by experimental tunnel of AECL(Atomic Energy of Canada Limited)and the arched tunnel.Although the Mine-by and arched tunnels were numerically simulated based on the mobilized dilation angle model,the variability associated with dilation angle around the simulated Mine-by and arched tunnels is insignificant,and dilation angle is approximately constant in the plastic zone.

EXACT SOLUTIONS OF NEAR CRACK LINE FIELDS FOR MODE I CRACK UNDER PLANE STRESS CONDITION IN AN ELASTIC－PERFECTLY PLASTIC SOLID

The near crack line analysis method has been used in the present paper,The classical small scale yielding conditions have been completely abandoned in the analyses and one inappropriate matching condition used to be used at the elasticplastic boundary has been corrected.The reasonable solution of the plastic stresses near the crack line region has been established.By matching the plastic stresses with the exact elastic stresses at the elastic-plastic boundary,the plastic stresses the length of the plastic zone and the unit normal vector of the elastic-plastic boundary near the crock line region have been obtained for a mode I crack under uniaxial tension,as well as a mode I crack under biaxial tension,which shows that for both conditions the plastic stress componentsσy, and σsy.he length of the plastic zone and the unit normal vector of the elastic-plastic boundary are quite the same while the plastic stress σs is different.

General forms of elastic-plastic matching equations for mode-Ⅲ cracks near crack line

Crack line analysis is an effective way to solve elastic-plastic crack problems. Application of the method does not need the traditional small-scale yielding conditions and can obtain sufficiently accurate solutions near the crack line. To address mode- Ⅲ crack problems under the perfect elastic-plastic condition, matching procedures of the crack line analysis method axe summarized and refined to give general forms and formulation steps of plastic field, elastic-plastic boundary, and elastic-plastic matching equations near the crack line. The research unifies mode-III crack problems under different conditions into a problem of determining four integral constants with four matching equations. An example is given to verify correctness, conciseness, and generality of the procedure.

Study on creep deformation and energy development of underground surrounding rock under four‐dimensional support

There is an urgent need to develop optimal solutions for deformation control of deep high‐stress roadways,one of the critical problems in underground engineering.The previously proposed four‐dimensional support(hereinafter 4D support),as a new support technology,can set the roadway surrounding rock under three‐dimensional pressure in the new balanced structure,and prevent instability of surrounding rock in underground engineering.However,the influence of roadway depth and creep deformation on the surrounding rock supported by 4D support is still unknown.This study investigated the influence of roadway depth and creep deformation time on the instability of surrounding rock by analyzing the energy development.The elastic strain energy was analyzed using the program redeveloped in FLAC3D.The numerical simulation results indicate that the combined support mode of 4D roof supports and conventional side supports is highly applicable to the stability control of surrounding rock with a roadway depth exceeding 520 m.With the increase of roadway depth,4D support can effectively restrain the area and depth of plastic deformation in the surrounding rock.Further,4D support limits the accumulation range and rate of elastic strain energy as the creep deformation time increases.4D support can effectively reduce the plastic deformation of roadway surrounding rock and maintain the stability for a long deformation period of 6 months.As confirmed by in situ monitoring results,4D support is more effective for the long‐term stability control of surrounding rock than conventional support.

AN INVESTIGATION ON ASYMPTOTIC NEAR-TIP FIELDS OF DYNAMIC CRACK IN AN ELASTIC-PERFECTLY PLASTIC COMPRESSIBLE MATERIAL

The stress and deformation fields near the tip of a mode-I dynamic crack steadily propagating in an elastic-perfectly plastic compressible material are considered under plane strain conditions. Within the framework of infinitesimal displacement gradient theory, the material is characterized by the Von Mises yield criterion and the associated J(2) flow theory of plasticity. Through rigorous mathematical analysis, this paper eliminates the possibilities of elastic unloading and continuous asymptotic fields with singular deformation, and then constructs a fully continuous and bounded asymptotic stress and strain field. It is found that in this solution there exists a parameter phi(0) which cannot be determined by asymptotic analysis but may characterize the effect of the far field. Lastly the variations of continuous stresses, velocities and strains around the crack tip are given numerically for different values of phi(0).

Elastic-plastic analysis of antiplane crack near crack surface region

The elastic-plastic stress distribution and the elastic-plastic boundary con- figuration near a crack surface region are significant but hard to obtain by means of the conventional analysis. A crack line analysis method is developed in this paper by consid- ering the crack surface as an extension of the crack line. The stresses in the plastic zone, the length, and the unit normal vector of the elastic-plastic boundary near a crack surface region are obtained for an antiplane crack in an elastic-perfectly plastic solid. The usual small scale yielding assumptions are not needed in the analysis.

海上风电单柱复合筒型基础复合加载条件下的承载特性

为研究新型海上风电单柱复合筒型基础在二维复合加载条件下的承载特性,利用有限元模型计算,首先得到模型在一维荷载空间下的极限承载力,随后通过施加不同的竖向荷载,得到在V-H、V-M、V-H-M荷载空间下的承载能力包络图,并分析不同二维荷载工况下的单柱复合筒型基础各位置处的土压力分布情况以及筒体底部等效塑性应变区的变化情况。结果表明:对于单柱复合筒型基础,在一定范围内,竖向荷载(0~0.15倍竖向极限承载力)的存在能够增大结构的水平及弯矩承载力;在V-H与V-M荷载空间中,基础各位置处的土压力变化、土体等效塑性应变变化具有一定的相似性,可互相对映参考;基础所承受的最大土压力位置与基础受力倾倒方向一致;随着竖向荷载的增加,结构达到极限承载力时,基础底部土体等效塑性应变区的范围与深度相应扩大。

热−力耦合作用下垃圾土体积变形特性和模型研究

城市生活垃圾填埋场因有机质降解产热导致其内部温度长期高于环境温度,引发工程运维问题。垃圾土温控三轴试验结果发现,温度变化引起的垃圾土体积应变与应力水平有关,且弹性体积应变占比较大,影响以塑性体积应变为硬化参数的弹塑性本构模型研究。在前期试验基础上,采用GDS温控应力路径三轴仪,研究了排水条件下温度对垃圾土中黏土体积变形特性和压缩特性的影响,并与相同条件下垃圾土体积变形特性进行比较,揭示了温度变化对垃圾土体积变形特性的影响机制。基于垃圾土温控三轴试验结果,推导了垃圾土热弹塑性压缩指数λT和热弹性压缩指数κT的确定方法;采用等效应力原则,假设温度变化引起的弹塑性体积应变等于某一等效应力增量引起的弹塑性体积应变,建立了垃圾土在热−力耦合作用下的体变计算模型,并对模型进行了验证分析。研究结果表明,孔隙比是影响垃圾土温度体积应变的主要因素;垃圾土的λT和κT随初始固结应力的增加而减小,比黏土高一个数量级;选取常温下相应固结应力水平的回弹指数κ和压缩指数λ比例值(κ/λ)能够较好地模拟热−力耦合作用下垃圾土的体积变形特性。

节点域对多层钢结构动力损伤的影响

为明确节点域对多层钢框架整体结构性能的影响,针对4组研究模型,运用动力弹塑性时程分析,从整体结构的层间位移、耗能、建筑塑性变形能量速度换算值、滞回曲线、最大延性比和最大累积塑性延性比研究了节点域对整体钢框架的影响。研究结果表明:考虑节点域影响之后,首层层间位移显著降低,且各层位移趋于均匀;节点域耗能显著且各层均匀;塑性变形能量速度换算值、最大延性比和最大累积塑性延性比均增大;以上分析均表明节点域的影响在整体结构中起着积极的作用,结构设计不可忽略。

基于三剪能量屈服准则的岩石弹塑性耦合模型

为建立能够准确描述岩石变形过程中能量演化规律的力学模型,提出利用三剪能量屈服准则和弹性剪应变能获取岩石黏聚力与内摩擦角,以及基于三剪能量塑性势函数获取剪胀角的方法,依据所提方法和岩石循环加卸载试验数据,得到了各级围压下不同塑性累积阶段岩石的弹性模量、黏聚力、内摩擦角及剪胀角等参数,发现随着塑性变形的增大,弹性模量呈负指数关系减小,黏聚力峰前线性增大而峰后呈负指数减小,内摩擦角满足比例参数为1、形状参数为1.5的Weibull函数,剪胀角线性减小;随着围压的增大,弹性模量和黏聚力均呈线性关系增大而剪胀角呈线性减小。建立岩石三剪能量弹塑性耦合力学模型,实现了模型在有限差分软件中的二次开发并编写了能量演化监测程序。模拟岩石室内试验,结果表明所建模型能较准确地描述岩石后继屈服过程中的硬化软化、剪胀、围压效应以及能量演化等力学行为。研究成果可为从能量角度研究岩体失稳提供理论支撑和现实手段。

Research on the mechanism of the two-dimensional ultrasonic surface burnishing process to enhance the wear resistance for aluminum alloy

The gradient nanostructure is machined on the aluminum(Al)alloy by the two-dimensional ultrasonic surface burnishing process(2D-USBP).The mechanism of why the gradient nanostructure enhances wear resistance is investigated.The mechanical properties and microstructure characterization for the gradient nanostructure are performed by operating a nanoindenter,transmission electron microscopy(TEM),and electron backscattered diffraction(EBSD).Dry wear tests are performed on the samples before and after machining to evaluate the wear resistance and mechanisms.The effect of the gradient nanostructure on the wear resistance is explored by developing the crystal plasticity(CP)finite element and molecular dynamics(MD)models.The characterization results show that the 2D-USBP sample prepared a gradient structure of~600μm thick on the aluminum surface,increasing the surface hardness from 1.13 to 1.71 GPa and reducing the elastic modulus from 78.84 to 70.14 GPa.The optimization of the surface microstructure and the increase of the mechanical properties effectively enhance the wear resistance of the sample,with 41.20%,39.07%,and 54.58% of the wear scar areas for the 2D-USBP treated samples to the original samples under 5,10,and 15 N loads,respectively.The gradient nanostructure hinders the slip of dislocations inside the sample during the wear process and reduces the size and scope of plastic deformation;meanwhile,the resistance to deformation,adhesion,and crack initiation and propagation of the sample surface is improved,resulting in enhanced wear resistance.

MBE In_xGa_(1-x)As/GaAs(001)系统晶格失配的研究

本文利用X射线双晶衍射和电子探针显微分析方法,研究了MBE In_xGa_(1-x)As/GaAs(001)系统的晶格失配与In含量x值的关系.在外延层厚度t_(?)～2μm的情况下,获得范性应变临界组份x_c=0.114.当x<x_c时,外延层生长方向应变(?)_1⊥与x成线性关系;当x>x_c时,外延层出现范性形变.

厚顶煤巷道煤柱尺寸留设及“控顶限帮”支护技术研究

为研究存在地质构造的情况下,刀把型工作面缩面期间煤柱留设尺寸以及支护难题,结合东滩煤矿1310工作面地质条件,通过理论分析及数值模拟等研究方法,确定缩面后刀把型工作面煤柱尺寸,提出了“控顶限帮”支护原理,实现对厚顶煤巷道深浅围岩的协同控制。研究结果表明,留设40 m保护煤柱,配合支护方案,可充分确保1310辅助轨顺安全稳定。结合缩面后现场监测情况,认为缩面后40 m煤柱宽度与支护方案合理可行。

弹塑性梁板结构冲击相似性方法研究

针对传统的冲击相似性方法在处理弹塑性梁和板结构时无法同时考虑弹性效应、应变率效应和应变硬化效应的问题,提出了一种基于弹塑性材料的相似性修正方法。在现有理想弹塑性材料相似性方法基础上,通过建立应变率区间和应变区间上比例模型预测的屈服应力和原型屈服应力的最佳逼近关系,直接获得不同弹塑性材料之间的应力比例因子。通过矩形梁质量冲击模型和圆板质量冲击模型,分析了该方法的准确性和适用性。分析结果表明,该修正方法可以同时考虑真实金属材料的弹性效应、应变率效应和应变硬化效应,比现有的相似性方法适用性更广,准确性更高。

Rockburst mechanism in soft coal seam within deep coal mines

A number of rockburst accidents occurring in soft coal seams have shown that the rockburst mechanism involved in soft coal seams is significantly different from that involved in hard coal seams. Therefore, the method used to evaluate rockburst in hard coal seams is not applicable to soft coal seams. This paper established an energy integral model for the rockburst-inducing area and a friction work calculation model for the plastic area. If the remaining energy after the coal seam is broken in the rockburstinducing area is greater than the friction work required for the coal to burst out, then a rockburst accident will occur. Mechanisms of ‘‘quaking without bursting" and ‘‘quaking and bursting" are clarified for soft coal seams and corresponding control measures are proposed as the optimization of roadway layouts and use of ‘‘three strong systems"(strong de-stressing, strong supporting, and strong monitoring).

Effects of axial cyclic loading at constant confining pressures on deformational characteristics of anisotropic argillite

Triaxial cyclic loading tests have been performed to assess the influence of plastic deformation on inelastic deformational properties of anisotropic argillite with bedding planes which is regarded as a kind of transversely isotropic media.Considering argillite's anisotropy and inelastic deformational properties,theoretical formulae for calculating oriented elastic parameters were deduced by the unloading curves,which can be better fitted for the description of its elasticity than loading curves.Test results indicate that with the growth of accumulated plastic,strain,the apparent elastic modulus of argillite decreases in a form of exponential decay function,whereas the apparent Poisson ratio increase in a form of power equation.A ratio of unloading recoverable strain to the total strain increment occurred during a loading cycle is defined to illustrate the characteristic relations between anisotropic coupled elasto-plastic deformation and plastic strain.It is significant to observe that high stress level and plastic history have an inhibiting effect on argillite anisotropy.

Theoretical model and testing method for ball indentation based on the proportional superposition of energy in pure elasticity and pure plasticity

For a homogeneous,continuous,and isotropic material whose constitutive relationships meets with the Ramberg-Osgood law(R-O law),the energy in the elastoplastic indentation with a ball indenter was theoretically analyzed,and the proportional superposition of energy in pure elasticity and pure plasticity during indentation was considered based on the equivalence of energy density.Subsequently,a Proportional Superposition-based Elasto Plastic Model(PS-EPM)was developed to describe the relationships between the displacement and the load during the ball indentation.Furthermore,a new test method of Ball Indentation based on Elastoplastic Proportional Superposition(BI-EPS)was developed to obtain the constitutive relationships of R-O law materials.The load–displacement curves predicted using the PS-EPM model were found to agree closely with the Finite Element Analysis(FEA)results.Moreover,the stress vs.strain curves predicted using the BI-EPS method were in better agreement with those obtained by FEA.Additionally,ball indentation was performed on eleven types of metal materials including five types of aluminum alloys and six types of steel.The test results showed that the stress vs.strain relationships and the tensile strength values predicted using the proposed BI-EPS method agreed well with the results obtained using conventional uniaxial tensile tests.

基于弹塑性分析的局部过度应变评定方法及其应用

在弹塑性分析法中,防止局部过度应变失效的评定是压力容器分析设计中不可缺少的一环。首先阐述了采用弹塑性分析法评定局部过度应变的理论基础;然后以筒体正交接管为例,详细阐释了应用弹塑性分析法评定局部过度应变的两种流程。结果表明,在进行局部过度应变评定时,应根据容器或元件加载时是一次加载或多次加载的情况,分别选择相对应的方法进行评定。相比于传统的弹性应力分析设计方法,弹塑性分析法能够得到压力容器结构中更精确的应力应变分布状况。

考虑弹性模量劣化的洞室围岩弹塑性分析

洞室开挖伴随岩体参数的劣化,描述参数的劣化对指导工程实践具有重要意义。为描述变形参数的劣化,基于霍克-布朗准则,利用损伤门槛值表征弹性模量劣化程度,通过泰勒级数简化求解含损伤门槛值的平衡方程得到塑性区半径解析解并给出损伤门槛值的取值方式,结合有限差分软件FLAC^(3D)对比验证了公式合理性。基于常弹性应变和变弹性应变2种假设,联合非关联流动准则获得了含损伤门槛值的塑性区位移表达式。最后,通过算例分析了不同参数下塑性区半径及洞壁位移变化规律。结果表明:劣化越严重,洞室越容易失稳;随着损伤门槛值增大,2种假设下的位移差值和塑性区半径逐渐增加,常弹性应变假设得到的位移偏小。为避免洞室失稳破坏,要重视支护的施作。