PERFECTLY PLASTIC FIELDS AT A RAPIDLY PROPAGATING PLANE-STRESS CRACK TIP

Under the condition that all the perfectly plastic stress components at a crack tip are the functions of only, making use of the Mises yield condition , steady-state moving equations and elastic perfectly-plastic constitutive equations, we derive the generally analytical expressions of perfectly plastic fields at a rapidly propagating plane-stress crack tip. Applying these generally analytical expressions to the concrete crack, we obtain the analytical expressions of perfectly plastic fields at the rapidly propagating tips of modes I and II plane-stress cracks.

ANISOTROPIC PLASTIC FIELDS AT A RAPIDLY PROPAGATING PLANE-STRESS CRACK-TIP

Under the condition that all the stress components at a crack-tip are the functions of 0 only, making use of the equations of steady-state motion. Hill anisotropic yield condition and stress-strain relations, we obtain the general solution of anisotropic plastic field at a rapidly propagating plane-stress crack-tip. Applying this general solution to four particular cases of anisotropy, the general solutions of these four particular cases are derived. Finally, we give the anisotropic plastic field at the rapidly propagating plane-stress mode I crack-tip in the case of X=Y=Z

ANISOTROPIC PLASTIC FIELDS AT A RAPIDLY PROPAGATING CRACK-TIP

Under the condition that all the stress components at a crack-tip are the functions of only, making use of the equations of steady-slate motion, stress-strain relations and Hill anisotropic yield conditions, we obtain the general solutions at a crack-tip in both the cases of anti-plane and in-plane strains. Applying these general solutions to the concrete cracks, the anisotropic plastic fields at the rapidly propagating tips of mode III and mode I cracks are derived.

Finite deformation analysis of crack tip fields in plastically compressible hardening–softening–hardening solids

Crack tip fields are calculated under plane strain small scale yielding conditions. The material is characterized by a finite strain elastic-viscoplastic constitutive relation with various hardening-softening-hardening hardness functions. Both plastically compressible and plastically incompressible solids are considered. Displacements corresponding to the isotropic linear elastic mode I crack field are prescribed on a remote boundary. The initial crack is taken to be a semi-circular notch and symmetry about the crack plane is imposed. Plastic compressibility is found to give an increased crack opening displacement for a given value of the applied loading. The plastic zone size and shape are found to depend on the plastic compressibility, but not much on whether material softening occurs near the crack tip. On the other hand, the near crack tip stress and deformation fields depend sensitively on whether or not material softening occurs. The combination of plastic compressibility and softening (or softening-hardening) has a particularly strong effect on the near crack tip stress and deformation fields.

SUPPLEMENTARY STUDY ON ANISOTROPIC PLASTIC STRESS FIELDS AT A STATIONARY PLANE-STRESS CRACK-TIP

Under the hypothesis that all the perfectly plastic stress components at a orach tip are the functions of θ only, making use of yield conditions and equilibrium equations. we derive the generally analytical expressions of the perfectly plastic stress field at a crack tip. Applying these generally analytical expressions to the concrete cracks, the analytical expressions of perfectly plastic stress fields at the tips of Mode Ⅰ Mode Ⅱ, Mode Ⅲ and Mixed Mode Ⅰ-Ⅱ cracks are obtained.

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.

SOME POSSIBLE ALTERNATIVE SOLUTIONS OF NEAR TIP FIELDS FOR STEADY CRACK GROWTH IN ELASTIC PERFECTLY-PLASTIC MEDIA

Some possible alternative solutions of near-tip fields are studied for plane-strain Mode-I qua- si-static steady crack growth in incompressible(v=1/2)elastic perfectly-plastic media.A group of four-sector so- lutions and a three-sector solution in which the elastic-unloading region vanishes are given.Stress functions,plas- tic flow factors and plastic strains in each region are also given.

ANISOTROPIC PLASTIC STRESS FIELDS AT A SLOWLY PROPAGATING CRACK TIP

Under the condition that any perfeetly plastic stress components at a crack tip are nothing but the Junctions of 0 only, making use of equilibriumequations,Hill ani.sutropic yield condition and unloading stress-strain relations, in this paper, we derive the general analytical expressions of anisotropic plastiestress Jields at the slowly steadyhe slowly steady propagatin tips of plane and anti-phane strain,Applying these general analytical expressions to the concrete cracks the attchvtical expressions of anisotropie plastic stress fields at the slowly steady propagating tips of Motle I and Motle III cracks are obtained. For the isolropic plastic material, the anisotropic plastic stress fields at a slowly propagating crack tip become the perfeeby plastic mress fields

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.

Line field analysis and complex variable method for solving elastic-plastic fields around an anti-plane elliptic hole

A new approach is proposed to solve the elastic-plastic fields near the major-axis line of an elliptical hole. The complex variable method is used to determine the elastic fields near the major-axis line of the elliptical hole. Then, by using the line field analysis method, the exact and new solutions of the stresses, strains in the plastic zone, the size of the plastic region and the unit normal vector of the elastic-plastic boundary near the major-axis line of the elliptical hole are obtained for an anti-plane elliptical hole in a perfectly elastic-plastic solid. The usual small scale yielding assumptions are not adopted in the analysis. The present method is simple, easy and efficient. The influences of applied mechanical loading on the size of plastic zone are discussed.

Self-sustained firing activities of the cortical network with plastic rules in weak AC electrical fields

Both external and endogenous electrical fields widely exist in the environment of cortical neurons. The effects of a weak alternating current （AC） field on a neural network model with synaptic plasticity are studied. It is found that self-sustained rhythmic firing patterns, which are closely correlated with the cognitive functions, are significantly modified due to the self-organizing of the network in the weak AC field. The activities of the neural networks are affected by the synaptic connection strength, the exterrtal stimuli, and so on. In the presence of learning rules, the synaptic connections can be modulated by the external stimuli, which will further enhance the sensitivity of the network to the external signal. The properties of the external AC stimuli can serve as control parameters in modulating the evolution of the neural network.

Effect of alternate magnetic field on LY12 structure properties after thermal plastic forming

The powerful alternate magnetic field treatment is an effective not-heat treatment, which improves the coriaceous performance of the material. In order to reveal the effect rule of the powerful alternate magnetic field on the structure capability after thermal plastic forming, the experimental methods were adopted to compare the microcosmic structure of the LY12 aluminium alloy test pieces before and after the powerful alternate magnetic field treatment. The mechanism of the structure refining was analyzed theoretically. According to the effect rule of the alternate magnetic field on critical grain growth work and the magnetic vibration-constriction mechanism, the structure dynamics factors were analyzed. The results show that, after a certain powerful alternate magnetic field treatment, the mechanical capability of the LY12 aluminium alloy after thermal plastic forming can be reinforced, the structure intertwist deriving from the thermal plastic forming becomes even and the branch crystal is also smashed, consequently refines the structure. The powerful alternate magnetic field treatment can be regarded as an effective method to improve metal structure performance after heat plastic forming.

CONSTRUCTION OF QUANTUM FIELD THEORY OF DISLOCATIONS BASED ON THE NON-RIEMANNIANPLASTICITY

A new microscopic approach was proposed, which bridges the order gap between the dislocation theory and the crystalline plasticity based on the quantum field theory of dislocations. The Ginzburg-Landau equation was derived rigorously from the quantized Hamiltonian for a crystal body containing a large number of dislocations, which gives the reaction-diffusion (RD) type differential equations. The RD equation describes periodic patterning shown in PSBs, etc.. relationship between the proposed theory and the concepts appeared in the non-Riemannian plasticity was extensively discussed by introducing the gauge field of dislocations. (Edited author abstract) 15 Refs.

基于弹塑性井周的井筒-射孔地层破裂压力预测模型

深部“高地应力、高温度、高地层压力”的赋存条件,使储层呈现脆性-延性-应变硬化的塑性特征。针对深部塑性储层的水力压裂改造,存在人工裂缝起裂、延伸压力高的挑战。本文通过开展实时高温高围压的储层岩石三轴压缩实验,明确了储层岩石的塑性破坏特征。建立了深部储层岩石的塑性硬化本构模型,推导了井周的弹塑性应力场分布。结合断裂力学理论,建立了考虑井周塑性区的射孔尖端应力强度因子计算模型,并提出相应的迭代求解方法。采用模型预测室内实验与新疆某油田实际条件下的水力压裂裂缝起裂压力,模型预测结果与实测结果对比表明,考虑弹塑性井周应力场的破裂压力预测值比实际压裂预测值高,储层塑性特征不利于水力裂缝的起裂。模型预测值与油田实测结果的误差为6.6%,验证了模型的可靠性。考虑弹塑性井周的破裂压力预测模型可保证压裂设计的安全性,为深部储层的压裂方案设计提供有效指导。

Design of New Farm Machine for Chopping Field Straw,Pressing it into Soil and Cleaning up Plastic Film

In this paper,we design a farm machine integrating the functions of chopping field straw,pressing field straw into soil,breaking stubble,and cleaning up plastic film,in order to improve the efficiency of plastic film cleaning,enrich the function of straw chopping equipment,reduce the noise generated at work,and further realize the intelligent harmony. The remote control button is used to control the work of the entire system,and the bluetooth remote control module and matrix keyboard are used to control the vehicle. With STM32 microcontroller as the main control chip,the machine controls the motion of the electric motor by relay,and employs LCD screen to realize real-time display of system work. It is convenient and easy to operate,with good human-computer interaction.

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).

Al/Ti异质合金回填式搅拌摩擦点焊数值模拟

采用耦合的欧拉-拉格朗日(CEL)算法对2 mm厚的5A02铝合金/TC4钛合金板异质金属回填式搅拌摩擦点焊过程展开模拟研究,分析了旋转速度和停留时间对焊点成型过程中温度场、等效塑性应变场以及材料流动的影响。结果表明,在扎入初期,热累积作用明显,升温速率相对较快,随焊接过程进行,产热会由摩擦产热转变为摩擦和塑性变形共同产热,升温速率放缓。当搅拌头转速为2000 r/min、扎入速度为1 mm/s、扎入深度为1.9 mm、回填速度为1mm/s、停留时间为0 s时,铝合金焊核区温度峰值达到558℃,等效塑性应变峰值达到120。在模拟的工艺区间内,焊核区峰值温度与旋转速度呈正相关,停留时间对温度的影响并不明显。

不同种植方式对高寒旱区地膜小麦耗水特征和产量的影响

于2019—2022年在陇中高寒旱区以裸地条播为对照(CK),设置全膜覆土穴播(FM)和膜侧沟播(FS)两种覆盖方式,研究不同覆盖种植方式对冬小麦耗水特性、生长发育及产量的影响。结果表明:与CK相比,FM和FS处理播种期~拔节期0~20 cm土层土壤温度分别平均提高3.1℃和2.1℃,灌浆期分别降低0.6℃和1.0℃。覆盖能不同程度提高冬小麦各生育期0~200 cm土层土壤含水量,其中出苗期、返青期、拔节期和灌浆期提高幅度均高于20%。与CK相比,FM处理返青后冬小麦耗水量平均显著提高29.2%,返青前显著降低42.4%;FS处理返青期~灌浆期耗水量提高12.6%,返青前降低25.7%。各处理冬小麦基本苗、分蘖数、公顷穗数、穗粒数、千粒重和产量均表现为FM>FS>CK,处理间差异显著,其中FM和FS处理产量分别较CK平均提高74.7%和45.4%;处理间耗水量差异不显著;FM处理水分利用效率最大,较CK平均提高67.3%,FS次之,较CK平均提高46.1%。综上,地膜覆盖可调节土壤水分状况,改善冬小麦生长发育和成穗情况,显著提高产量和水分利用效率,其中全膜覆土穴播调节效应优于膜侧沟播,是适宜在高寒旱区地膜小麦生产中推广应用的种植方式。

全生物降解地膜对湖南不同地区果蔬生长的影响

针对塑料地膜使用造成土壤环境污染的问题,研究以白色/黑色全生物降解地膜和普通塑料地膜为参试材料,在湖南省长沙市、沅江市、汉寿县等地区开展了西瓜、莴苣、秋葵等作物的覆膜试验,探究全生物降解地膜对不同作物的生长、品质、产量及土壤性质等指标的影响,观察全生物降解地膜的降解情况。结果发现,两种全生物降解地膜对湖南各地区果蔬生产均起到提质增效的作用。沅江地区黑色降解地膜覆盖栽培西瓜,西瓜单瓜重增加25.8%,叶片数增加45.5%,显著高于未覆膜处理;覆盖白色降解地膜的秋葵叶片长度比对照高42.1%,叶片宽度比对照高55.9%,叶片数增加43.2%,产量增加55.5%。普通塑料地膜在作物收获后未能降解,白色和黑色全生物降解地膜的降解率分别为80.3%~90.7%和63.0%~80.3%。综合不同地区的试验结果,认为两种降解地膜均可作为普通塑料地膜的潜在替代品,具有较大的推广前景。

三向非等压应力场下围岩主应力差与塑性区分布关系研究

首先,以弹性理论为基础推导出考虑巷道轴向应力影响的主应力差解析式,通过变化水平侧压比、轴向侧压比研究完全弹性条件时不同应力场下主应力差分布形态演化规律。其次,推导出考虑轴向应力作用的围岩塑性区近似解,研究不同应力环境下围岩的塑性区形态演化规律。最后,对比分析主应力差及塑性区形态特征,研究二者分布形态的对应关系,并通过数值模拟验证理论分析的正确性。研究结果表明:在弹性理论下,三向应力场条件的主应力差值分布会呈现圆形、椭圆形、蝶形3种形态的演化规律;在考虑巷道塑性破坏的情况下,主应力差值在σx、σz较小一侧形成峰值壳,在较大一侧形成卸压壳。在不同水平侧压比及轴向侧压比情况下,围岩塑性区形态会呈现圆形—椭圆形、椭圆形—蝶形、蝶形恶性扩展3个阶段的演化过程。巷道主应力差的分布形态在一定程度上可以反映塑性区形态,主应力差和塑性区在相同应力条件下的形态特征具有逐一对应关系,且二者形态在很大程度上由水平侧压比决定,轴向侧压比对塑性区形态影响较小。160206回风巷在偏转式主应力差集中区形成非对称异化特征。本文基于分析及现场破坏情况提出的联合协同支护技术在现场应用效果较好。