EFFECT OF THE POISSON RATIO ON THE PERFECTLY PLASTIC STRESS FIELD AT A STATIONARY PLANE-STRAIN CRACK TIP

Under the condition that all the perfectly plastic stress components at a crack tip are the functions of ? only, making use of equilibrium equations and Von-Mises yield condition containing Poisson ratio, in this paper, we derive the generally analytical expressions of perfectly plastic stress field at a stationary plane-strain crack tip. Applying these generally analytical expressions to the concrete cracks, the analytical expressions of perfectly plastic stress fields at the stationary tips of Mode I, Mode II and Mixed-Mode I-II plane-strain cracks are obtained. These analytical expressions contain Poisson ratio.

Stress and strain analysis on the anastomosis site sutured with either epineurial or perineurial sutures after simulation of sciatic nerve injury

The magnitude of tensile stress and tensile strain at an anastomosis site under physiological stress is an important factor for the success of anastomosis following suturing in peripheral nerve injury treatment. Sciatic nerves from fresh adult cadavers were used to create models of sciatic nerve injury. The denervated specimens underwent epineurial and perineurial suturing. The elastic modulus （40.96 ＋ 2.59 MPa） and Poisson ratio （0.37 ＋ 0.02） of the normal sciatic nerve were measured by strain electrical measurement. A resistance strain gauge was pasted on the front, back left, and right of the edge of the anastomosis site after suturing. Strain electrical measurement results showed that the stress and strain values of the sciatic nerve following perineurial suturing were lower than those following epineurial suturing. Scanning electron microscopy revealed that the sciatic nerve fibers were disordered following epineurial compared with perineurial suturing. These results indicate that the effect of perineurial suturing in sciatic nerve injury repair is better than that of epineurial suturing.

IMPROVEMENT ON THE CALCULATION OF PLASTIC STRAIN RATIO IN BCC MONOCRYSTALS

Two improvements have been made on the calculation of plastic strain ratio (r value) in BCC monocrystals by considering the differences in the critical shear stresses among the three slip systems and the rotation of crystal lattice. It is found that the results calculated by the improved method are more rational.

Effect of Strain Ratio on Fatigue Model of Ultra-fine Grained Pure Titanium

The ultra-fine grained(UFG)pure titanium was prepared by equal channel angular pressing(ECAP)and rotary swaging(RS).The strain controlled low cycle fatigue(LCF)test was carried out at room temperature.The fatigue life prediction model and mean stress relaxation model under asymmetrical stress load were discussed.The results show that the strain ratio has a significant effect on the low cycle fatigue performance of the UFG pure titanium,and the traditional Manson-coffin model can not accurately predict the fatigue life under asymmetric stress load.Therefore,the SWT mean stress correction model and three-parameter power curve model are proposed,and the test results are verified.The final research shows that the threeparameter power surface model has better representation.By studying the mean stress relaxation phenomenon under the condition of R≠-1,it is revealed that the stress ratio and the strain amplitude are the factors that significantly afiect the mean stress relaxation rate,and the mean stress relaxation model with the two variables is calculated to describe the mean stress relaxation phenomenon of the UFG pure titanium under different strain ratios.The fracture morphology of the samples was observed by SEM,and it was concluded that the final fracture zone of the fatigue fracture of the UFG pure titanium was a mixture of ductile fracture and quasi cleavage fracture.The toughness of the material increases with the increase of strain ratio at the same strain amplitude.

THE ANALYSES OF THE THREE-DIMENSIONAL STRESS STRUCTURE NEAR THE CRACK TIP OF MODE I CT SPECIMENS IN ELASTICPLASTIC STATE(Ⅰ)--THE ANALYSES OF CONSTRAINT PARAMETERS AND FRACTURE PARAMETERS

In the present paper,three dimensional analyses of some general constraint parameters and fracture parameters near the crack tip of Mode I CT specimens in two different thicknesses are carried out by employing ADINA program.The results reveal that the constraints along the thickness direction are obviously separated into two parts:the keeping similar high constraint field(Z_(1))and rapid reducing constraints one(Z_(2)).The two fields are experimentally confiremed to correspond to the smooth region and the shear lip on the fracture face respectively.So the three dimensional stress structure of Mode I specimens can be derived through discussing the two fields respectively.The distribution of the Crack Tip Opening Displacement(CTOD)along the thickness direction and the three dimensional distribution of the void growth ratio(V_(g))near the crack tip are also obtained.The two fracture parameters are in similar trends along the thickness direction,and both of them can reflect the effect of thickness and that of the loading level to a certain degree.

Entire deformational characteristics and strain localization of jointed rock specimen in plane strain compression

Shear band (SB), axial, lateral and volumetric strains as well as Poisson’s ratio of anisotropic jointed rock specimen (JRS) were modeled by Fast Lagrangian Analysis of Continua (FLAC). Failure criterion of rock was a composited Mohr-Coulomb criterion with tension cut-off. An inclined joint was treated as square elements of ideal plastic material beyond the peak strength. Several FISH functions were written to automatically find the addresses of elements in the joint and to calculate the entire deformational characteristics of plane strain JRS. The results show that for moderate joint inclination (JI), strain is only concentrated into the joint governing the behavior of JRS, leading to ideal plastic responses in axial and lateral directions. For higher JI, the post-peak stress-axial and lateral strain curves become steeper as JI increases owing to the increase of new SB’s length. Lateral expansion and precursor to the unstable failure are the most apparent, resulting in the highest Poisson’s ratio and even negative volumetric strain. For lower JI, the entire post-peak deformational characteristics are independent of JI. The lowest lateral expansion occurs, leading to the lowest Poisson’s ratio and positive volumetric strain all along. The present prediction on anisotropic strength in plane strain compression qualitatively agrees with the results in triaxial tests of rocks. The JI calculated by Jaeger’s formula overestimates that related to the minimum strength. Advantages of the present numerical model over the Jaeger’s model are pointed out.

Influence of the Impeller/Guide Vane Clearance Ratio on the Performances of a Nuclear Reactor Coolant Pump

An AP1000 nuclear reactor coolant pump is considered to assess the influence of the Impeller/Guide vane clearance on the performances of this type of pumps.Experiments and numerical simulations relying on an unidirectional fluid-solid coupling approach are used to investigate the problem(stress,strain and mode of the rotor).The results reveal the relationship existing between the hydraulic performance of the nuclear reactor coolant pump and the clearance ratio.The effect of clearance ratio on the maximum equivalent stress on the back surface of the impeller blade is greater than that on the working surface(the maximum equivalent stress on the back surface of impeller blade is about three times that on the working surface).The clearance ratio has a scarce effect on the first six natural frequencies of the rotor of the nuclear reactor coolant pump.The related vibrational modes have different waveforms.

Cyclic behavior of reinforced sand under principal stress rotation

Although the cyclic rotation of the principal stress direction is important,its effect on the deformation behavior and dynamic properties of the reinforced soil has not been reported to date.Tests carried out on large-scale hollow cylinder samples reveal that the cyclic rotation of the principal stress direction results in significant variations of strain components(ε,ε,εand γ) with periodic characteristics despite the deviatoric stress being constant during tests.This oscillation can be related to the corresponding variations in the stress components and the anisotropic fabric that rotate continuously along the principal stress direction.Sand under rotation appears to develop a plastic strain.Similar trends are observed for reinforced sand,but the shear interaction,the interlocking between particles and reinforcement layer,and the confinement result in significant reductions in the induced strains and associated irrecoverable plastic strains.Most of the strains occur in the first cycle,and as the number of cycles increases,the presence of strains becomes very small,which is almost insignificant.This indicates that the soil has reached anisotropic critical state(ACS),where a stable structure is formed after continuous orientation,realignment and rearrangement of the particles accompanied with increasing cyclic rotation.Rotation in the range of 60°-135° produces more induced strains even in the presence of the reinforcement,when compared with other ranges.This relates to the extension mode of the test in this range in which σ>σand to the relative approach between the mobilized plane and the weakest horizontal plane.Reinforcement results in an increase in shear modulus while it appears to have no effect on the damping ratio.Continuous cycles of rotation result in an increase in shear modulus and lower damping ratio due to the densification that causes a decrease in shear strain and less dissipation of energy.

Interaction among fractures and stress field computation of fracture systems

The interaction problem among fractures under the action of compressional stress is studied in this paper by using the finite element method and boundary element method respectively.The mechanical criteria which differentiate between the independent fractures and fracture systems and their computation methods are presented in this paper.The proportional conditions between length and spacing of fractures that exist interaction for several kinds of fracture groups of different geometric arrangement are given.The effect of interaction among fractures on the displacement field,stress field and strain energy distribution are computed.The relations between the fracture system of conjugate array and conjugate earthquakes are also discussed in this paper.

Experimental investigation on strength and deformation of plain high-strength concrete under multiaxial stress state

To investigate the strength and deformation behavior of plain high-strength concrete （HSC） under muhiaxial stress states, a large static-dynamic true triaxial machine was employed, and muhiaxial tests were performed on 100 mm × 100 mm × 100 mm cubes concrete specimens. Friction-reducing pads were three-layer plastic membranes with glycerine in-between for the compressive loading plane. The tensile loading plane of concrete samples was processed by attrition machine, and then the samples were glued up with the loading plate with structural glue. Failure modes of specimens were described. The principal static compressive strengths, strains at the peak stress and stress-strain curves were measured, and the influence of stress ratios on them was analyzed as well. Experimental results show that the ratio of the compressive strength σ3f over the uniaxial compressive strengthfo depends on brittleness-stiffness of concrete besides stress state and stress ratios. The formula of Kupfer-Gerstle＇ s and Ottosen＇ s failure criterion for plain HSC under biaxial compression and muhiaxial stress state is proposed respectively.

Non-complete relief method for measuring surface stresses in surrounding rocks

The measurement of surface stresses in surrounding rocks with the use of a relief method of annular hole-drilling was studied by numerical analysis. The stress relief process by hole-drilling was then simulated with the use of finite element method. The influences of the borehole diameter(d), the initial stresses and the ratio of the initial principle stresses on the variations of the remained stress and the released stress in function of the relief depth(h) were discussed. The relation between the non-dimensional ratio of the released principle strains and that of the initial principle stresses, and the effect of the elastic modulus and the Poisson ratio of the rock mass on the stress relief curves were studied. The results show that the stress relief behavior formulated with the non-dimensional ratio of the released stress and the ratio of h/d is only sensitive to the ratio of the initial principle stresses and the Poisson ratio. The stresses are completely released when h equals 1.6d, and the tensile stresses take place on the bore core surface in the relief measurement process. Finally, a non-complete relief method of annular hole-drilling for measuring surface stress in surrounding rocks is proposed and the procedure is presented.

Effect of fiber-reinforcement on the mechanical behavior of sand approaching the critical state

Several types of ground improvement methods that employ fiber-reinforcement have been developed in recent years.A series of consolidated drained triaxial compression tests has been conducted here to examine the effect of short fibers on the mechanical properties of Toyoura sand.Sand with 0%,0.2%,0.4%,and 1%fiber contents,prepared to yield random distribution,was sheared under several confining pressures and controlled via their initial relative densities.The test results showed that the maximum and residual deviatoric stresses increased,whereas the volumetric expansion decreased with an increase in fiber content.Although the stress ratio h(=q/p′)and specific volume changed depending on the fiber content and confining pressure with shear progression,they each reached the same values for a definite fiber content at the end of shearing,independent of initial relative density.In other words,the unique critical state line can be found for a definite fiber content.Moreover,the greater the fiber content,the larger the slope of the critical state line at the end of shearing.Additionally,as the length of fibers shortened with the same percentage of fiber inclusions in sand,the deviatoric stress and the stress ratio decreased,approaching the shear-strain-volumetric response of unreinforced sand.

Determination of mining-induced stresses using diametral rock core deformations

Knowledge of ground stresses is crucial for ground control activities such as the design of underground openings,selec-tion of support systems,and analysis for stability.However,it is a known fact that far field stresses experience changes in orientation and magnitude due to the presence of geological structures and due to the excavations created by mining activi-ties.As a result,in-situ stresses around drifts,ramps,and stopes in underground mines are quite different from far field or pre-mining stresses.The purpose of this research is to develop a simple and practical methodology for determining in-situ stresses.Stress relief occurs once the rock core is drilled off.Such relief is a function of the surrounding stress field.This study uses exploration rock cores that are drilled off for the purpose of orebody definition in the underground mine.The method measures and analyzes the diametral core deformations in laboratory.Two case studies from operating underground mines are presented for demonstration.In these case studies,rock core deformations are measured with a customized test apparatus and rock samples were prepared and tested for Young's modulus and Poisson's ratio.The differential stress,namely the difference between the local principal stresses in the plane perpendicular to the core rock axis is calculated.It is shown that this methodology is useful for determining the brittle shear ratio in the rock mass,which is of primary interest to ground control studies.

基于能量转化机制的岩石强度的演化机理研究

针对岩石强度的演化机理是提高强度准则计算精度与适用性的前提,基于能量转化是物质物理过程的本质属性,弹性应变能是材料破坏的内在机理,通过试验与理论研究,探究了围压、中间主应力及泊松比对岩石强度演化规律的影响.结果表明:岩石破坏与应力状态及岩石变形特性有关,忽略泊松比的影响是中间主应力效应提出的理论依据,并指出围压效应与中间主应力效应忽略变形影响,既与试验结果不一致,又是据此建立的强度理论精度较差的内在原因.岩石强度的演化是围压、中间主应力及泊松比共同作用的结果,并据此研究了静水应力状态下材料破坏特性、围压区间性及三轴拉伸强度恒大于三轴压缩强度条件.理论及试验表明,体现了围压、中间主应力及泊松比影响的强度理论具有较高计算精度与适用性.研究成果对于精准地描述岩石破坏特性,建立普遍适用的强度理论具有重要意义.

重塑黄土的往返加卸载真三轴试验研究

针对不同应力路径往返加卸载条件下黄土累积塑性应变发展特性,以重塑黄土为研究对象,利用真三轴仪开展不同应力路径的往返加卸载试验研究,探究不同应力路径、应力幅值下黄土的循环应力时程曲线、滞回曲线、骨干曲线及累积塑性应变曲线的影响规律。揭示了应力路径对重塑黄土力学特性的影响,描述了各主应力与中主应力比b和应力幅值的相关性,提出了重塑黄土的滞回曲线近似呈椭圆形,长轴斜率随中主应力比b值的增大而增大,随应力幅值的增大而减小;随着中主应力比b值的增大,循环应力-应变骨干曲线随之硬化,累积塑性应变曲线依次降低,且累积塑性应变发展更早的进入平缓阶段,为解决相关黄土工程问题提供了参考。

纤维水泥土动力性能研究进展

纤维水泥土是在水泥土中均匀掺入纤维而形成的复合材料,其拥有良好的强度、韧性和抗形变能力。目前纤维水泥土静力性能的研究较为完善,但针对其动力性能的研究仍有不足,因此从动应力-动应变关系、动模量、阻尼比和动强度4个动力性能指标对现有纤维水泥土动力研究成果进行分析,并指出纤维水泥土动力性能研究存在的问题和不足之处,以期为未来高性能水泥土的发展提供借鉴。

多频段循环荷载下海洋黏土动力特性试验研究

为探究海洋土的动力特性及动力影响因素,采取原状黄海海洋黏土,利用GCTS动三轴试验系统开展室内试验研究,探讨偏应力水平、动应力幅值和动荷载频率对海洋土动应力应变关系和动孔压的影响规律,分析动剪切模量和阻尼比的变化特征。试验结果表明:海洋黏土的动应变随偏应力水平提高和动应力幅值增大而增大,且具有显著的频率效应,低频段动荷载(0.02~0.1 Hz)引起的动应变累积量高于高频段动荷载(1~10 Hz)的应变累积量;在低频段动荷载作用下,土样内超静动孔压显著上升,而在高频段下,动孔压的累积变化量很小;土样的动剪切模量对动荷载的频率反应不十分明显,仅在低频段,动剪切模量会随动荷载频率的提高略有下降,但是会随动应力幅值增加而显著提高,随偏应力水平提高而显著下降;土样的阻尼比对动荷载频率也很敏感,在低频段,阻尼比随频率的增大而降低,在0.1 Hz时的阻尼比最低;在高频段,土样的阻尼比随动荷载频率增加明显提高,提高幅度可达到1倍以上;阻尼比会随动应力幅值增长而略有减小,随偏应力水平提高而略有增加。

大尺寸岩石在低频周期扰动载荷下的力学特性试验研究

岩石长期处于高静应力状态且频繁受到周期扰动载荷的影响是引发各种地质灾害的关键因素。利用自制的CX-8568冲击扰动围岩试验系统对大尺寸红砂岩开展了低频周期扰动载荷下的力学特性试验,分析了高应力状态下不同扰动平均应力、不同扰动载荷幅值下岩石的应变、损伤变量的演化规律。结果表明:岩石在周期扰动载荷下的破坏存在门槛值,其载荷低于门槛值时岩石不会发生破坏,岩石弹性模量存在强化现象;高扰动平均应力、高扰动载荷幅值均为岩石破坏的不利因素;当扰动上限应力超过岩石单轴抗压强度后,扰动平均应力和扰动载荷幅值的增大对岩石破坏时的循环次数影响愈发显著;当扰动上限应力相同,但扰动平均应力和扰动载荷幅值不同时,岩石破坏对扰动平均应力的敏感度高于扰动载荷幅值;周期扰动载荷下岩石破坏较静态加载更剧烈;随着扰动平均应力或扰动载荷幅值的提高,岩石破坏形态由以拉剪破坏为主导过渡至以拉伸劈裂破坏为主导。

冻融循环作用下黄土累积塑性应变演变规律

为探究黄土路基在冻融循环和交通荷载耦合作用下的累积塑性应变变化规律,选取西宁地区重塑黄土为研究对象,采用GDS双向动三轴测试系统对其进行一系列动三轴试验,研究不同冻融循环次数、围压、动应力幅值以及频率对累积塑性应变的影响规律,并通过引入拟合参数建立考虑多因素的累积塑性应变预测模型。结果表明:累积塑性应变随冻融循环次数的增大而增大,在6次冻融循环后增长速率减缓且趋于稳定;减小动应力幅值和增大围压能显著抑制累积塑性应变的发展;加载初期累积塑性应变随频率变化不明显,随着振次的增加,频率作用凸显,累积塑性应变随着频率的增大而减小;基于试样的累积塑性应变演变规律,分别采用幂指数模型和对数模型进行拟合,发现后者拟合效果好;综合考虑4种因素对累积塑性应变的影响,建立累积塑性应变预测模型,并对试验的实测值与预测值进行对比,验证模型的可行性。研究成果可为季冻区黄土路基永久变形的计算提供理论参考依据。

A fast numerical method of introducing the strengthening effect of residual stress and strain to tensile behavior of metal matrix composites

Thermal residual stress and strain(TRSS)in particle reinforced metal matrix composites(PRMMCs)are believed to cause strengthening effects,according to previous studies.Here,the representative volume element(RVE)based computational homogenization technique was used to study the tensile deformation of PRMMCs with different particle aspect ratios(AR).The influence of TRSS was assessed quantitatively via comparing simulations with or without the cooling process.It was found that the strengthening effect of TRSS was affected by the particle AR.With the average strengthening effect of TRSS,a fast method of introducing the strengthening effect of TRSS to the tensile behavior of PRMMCs was developed.The new method has reduced the computational cost by a factor 2.The effect of TRSS on continuous fiber-reinforced metal matrix composite was found to have a softening-effect during the entire tensile deformation process because of the pre-yield effect caused by the cooling process.