尖锐V型切口混凝土梁的应力强度因子研究

对含尖锐V型切口构件的破坏评估通常是利用切口应力强度因子来确定,切口应力强度因子指的是切口周围渐进线弹性应力场强度.对于含尖锐V型切口构件来说,单位切口应力强度因子的大小是由V型切口角度决定.应变能量密度准则是根据一定体积内应变能的密度是否达到临界值来判断构件断裂破坏的准则,当这个体积足够小不影响Williams方程的高阶次解时,应变能量密度准则就能通过切口应力强度因子进行表示.考虑Ⅰ型荷载条件下,分别采用平均应变能量密度准则和Carpinteri有限断裂力学方法计算V型切口应力强度因子,两者的理论取值非常接近.同时通过试验,证明两种断裂准则给出的切口应力强度因子的理论值与实验数据吻合程度较好.     

基于有损伤体元件的广义Kelvin模型蠕变全过程探究

岩石的蠕变特性往往对隧道和地下工程的稳定性有着重要的控制作用.针对岩石蠕变的阶段性特征,可将岩石蠕变全过程分为四个阶段.广义Kelvin模型可较好地反映前三个阶段的岩石蠕变特性,不能理想地反映加速蠕变阶段特征.通过引入损伤体元件和Kachanov的损伤因子演化公式,构建了具有损伤体元件的广义Kelvin模型,从而建立了可以体现岩石蠕变全过程的蠕变模型,并提出较为简单的组合模型参数计算方法.该模型不仅能较好地反映岩石蠕变全过程,且模型参数易于确定.利用该模型对砂质泥岩单轴压缩蠕变实验曲线进行拟合分析,拟合效果良好,研究结果可为类似工程提供参考.     

极限应变法在圆形隧洞稳定分析中的应用

材料的屈服和破坏是不同的,屈服准则已有大量研究,但缺少严格的破坏准则.理想弹塑性模型用应力表述难以区别屈服与破坏,为此该文提出极限应变破坏判据,可用于判断材料的局部和整体破坏.给出了不同材料极限应变的确定方法,并作为破坏判据用于岩土类材料的稳定分析,称为极限应变法.将极限应变法应用于圆形隧洞,研究隧洞的破坏过程、围岩破坏深度及其安全系数,并与滑移线理论和实际模型试验的结果进行对比.研究表明:极限应变法能够判断圆形隧洞的破坏过程与极限状态,求得准确的安全系数,与滑移线场法和模型试验的结果一致,验证了极限应变法在隧洞中应用的可行性.极限应变判据具有明确的力学意义,能反映材料破坏的全过程,为岩土类材料极限分析提供了一种新的方法.     

周围应力与孔隙流体对突出煤力学性质的影响

本文在系统的实验与理论研究基础上,考查了周围应力与孔隙流体对突出煤变形与破坏的影响.提出了一个描述破坏后现象的模型,其判据是一个联系着有效围压与材料强度的无量纲参数.实验表明,吸附瓦斯对应力-应变曲线没有明显影响;有效应力中的孔隙压力项就是游离瓦斯的压力;含气体与液体的多孔介质会有不同有效应力关系.     

单轴拉伸条件下细观非均匀性岩石损伤局部化和应力应变关系分析

岩石在拉应力状态下的力学特性不同于压应力状态下的力学特性．利用细观力学理论研究了细观非均匀性岩石拉伸应力应变关系包括:线弹性阶段、非线性强化阶段、应力降阶段、应变软化阶段.模型考虑了微裂纹方位角为Weibull分布和微裂纹长度的分布密度函数为Rayleigh函数时对损伤局部化和应力应变关系的影响,分析了产生应力降和应变软化的主要原因是损伤和变形局部化.通过和实验成果对比分析验证了模型的正确性和有效性．     

基于分数阶损伤蠕变模型的圆形隧洞黏弹塑性解

深埋隧洞围岩变形是一个与时间相关的复杂力学过程.为了描述这一过程,首先基于分数阶理论,提出一个新的非线性蠕变损伤本构模型.然后基于该模型,并引入Hoke-Brown屈服准则,推导出深埋条件下圆形隧洞围岩位移的黏弹塑性解析解.最后,以锦屏二级水电站辅助洞为工程实例,对解析解的有效性进行验证,并分析了流变参数对流变位移的影响.研究结果表明:1)分数阶蠕变损伤本构可以较好的描述岩石蠕变全过程,即衰减蠕变、常速蠕变及加速蠕变过程.2)随着模型中分数阶阶次及损伤因子量值的增加,围岩的蠕变变形更为明显.3)解析曲线与现场实测位移平均值曲线在量值与形态上均吻合较好,验证了解析解的有效性.     

对于正交异性材料屈服与流动的探讨

假定正交异性材料的屈服准则与各向同性材料的Huber-Mises准则同构,提出了无量纲应力屈服准则,进而推导了与之相关的塑性流动规则.用不同的简单应力状态下的应力-应变试验曲线,可以得到不同的广义等效应力-应变关系.     

皮肤层蠕变分析的混合解法

本文利用积分变换方法求解皮肤层在表面压力作用下的蠕变响应问题。使用数值方法分别求解了双积分逆变换和由表面应力边界条件所得的第一类Ｖｏｌｔｅｒｒａ积分方程。计算所得的加载瞬时和平衡状态时的位移结果分别与不可压缩和可压缩单相弹性体的应移值相同，证明了本文方法的正确性。文中还给出了皮肤层在压力作用下的蠕变响应曲线。     

混凝土轴拉加卸载随机损伤模型的建立与试验验证

为了分析及深入探讨混凝土在受拉加载及卸载情况下的力学特性,基于随机损伤本构关系提出了一种混凝土轴拉加卸载模型,推导出了混凝土加卸载的应力 应变关系表达式.为了印证理论表达式,进行了混凝土轴向拉伸及加卸载的试验研究,测得了混凝土的材料参数及其相应的轴拉加卸载应力-应变曲线.结合模型的计算结果,对混凝土的轴拉加卸载试验结果进行了对比分析,结果表明:混凝土轴拉加卸载模型能够预测混凝土的极限强度,同时能描述混凝土的强度软化、加载过程中的弹模折减及卸载后的塑性变形.     

冲击压缩下陶瓷材料中的破坏波模型

从多晶陶瓷材料细观结构非均匀性及其导致的应力奇异性分析出发,建立了陶瓷材料在冲击压缩下的本构关系,以及以表征材料损伤和破坏的非弹性体积应变为传播特征的破坏波控制方程,破坏层的非弹性体积应变包括由微裂纹成核、扩展引起的膨胀体积应变和由气孔塌陷引起的压缩体积应变两部分．结合92.93%氧化铝陶瓷板碰撞试验,数值模拟了冲击压缩下陶瓷材料中破坏波的传播过程,并对跨越破坏波阵面应力历程和剪切强度的变化规律进行了分析．     

单轴作用下岩土材料的双重介质本构模型

基于弹塑性力学和损伤力学理论,将岩土材料视为孔隙-裂隙双重介质,假设孔隙介质不发生损伤,而裂隙介质随应变的增加发生损伤,建立了单轴作用下岩土类材料的双重介质本构模型隐式表达式,并利用Newton迭代法得出了材料的全程应力-应变曲线．分析结果表明,岩土材料中裂隙空间展布的多态性（均匀展布、集中展布和随机展布）是岩土材料本构关系千变万化的根本原因．由于双重介质本构模型将岩土材料的弹性主体（孔隙介质部分）和损伤主体（裂隙介质部分）分化开来,对于研究岩土或含损伤材料的破坏具有实用价值和理论意义．     

Cyclic yield strength in definition of design limits for fatigue and creep

This study proposes a cyclic yield strength (CYS, σ^c_y) as a key characteristic for the definition of safe design for engineering structures operating under fatigue and creep conditions. CYS is defined on a cyclic stress-strain curve, while monotonic yield strength (MYS, σ^m_y) is defined on a monotonic stress-strain curve. Both values of σ^c_y and σ^m_y are identified using a 2-steps fitting procedure of the experimental stress-strain curves using Ramberg-Osgood and Chaboche material models. Comparison of σ^c_y and fatigue endurance limit σ^f_lim on the S-N fatigue curve reveals that they are approximately equal. Hence, basically safe fatigue design is guaranteed in purely elastic domain defined by the σ^c_y. A typical creep rupture curve in time-to-failure approach for creep analysis has 2 inflections corresponding to the σ^c_y and σ^m_y. These stresses separate 3 sections on the creep rupture curve, which are characterised by 3 different creep fracture modes and 3 creep deformation mechanisms. Thus, basically safe creep design is guaranteed in linear creep domain with brittle failure mode defined by the σ^c_y. These assumptions are confirmed for several structural low- and high-alloy steels for normal and high-temperature applications. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stress-strain state of two-way glass-reinforced plastics with allowance for resin yield point

The results of a theoretical study of the stress-strain state of a two-way glass-reinforced plastic under uniaxial load are presented. It is assumed that the reinforcing fibers are elastic up to failure, whereas the resin is elastic only at stresses below the yield point. It is found that with these assumptions the stress-strain curve is composed of three line segments.Mekhanika Polimerov, Vol. 1, No. 2, pp. 55–58, 1965     

Long-Term Strength Criteria for Some Polymer Materials under a Plane Stress State

The problem of the choice and experimental justification of long-term failure criteria for isotropic polymer materials in creep under a plane stress state is considered. The criteria are defined by a linear two-parameter interpolation relating two stress-tensor invariants which limit the interpolation range with respect to the conditions of viscous and brittle failure and take into account the signs of principal stresses. The base experiment for determining material constants includes standard tests on long-term strength in uniaxial tension and a test on long-term strength under a plane stress state. The failure criteria have been approved in constructing unified long-term strength diagrams for thin-walled tubular specimens made of rigid polyvinylchloride and high-density polyethylene under the action of internal pressure, pressure with axial tension, torsion, and axial tension with torsion.     

Relationship between polymer creep curves and stress-strain diagrams

For polymers whose () diagram has the form of a monotonically increasing curve the creep rate is monotonically damped with time. If, however, the stress-strain diagram has a maximum, the creep curve becomes stepped: after first slowing, the creep sharply accelerates, and then slows down again. The strains corresponding to characteristic points on the stress-strain curves coincide with those corresponding to singular points on the creep curves.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, Vol. 4, No. 5, pp. 787–792, September–October, 1968.     

Mechanical properties of blends of liquid crystalline copolyesters with polyprophylene

Conclusions A significant effect of the addition of LCP on the mechanical properties and their anisotropy has been established. Already, if one considers the shape of curves of the stress-strain relationship it can be seen that curves typical for semicrystalline polymers (pure polypropylene) with clearly visible yield point and significant cold drawing leading to an anisotropic stiffening are changing into curves without yielding and with a brittle failure (LC-rich blends). Generally, the tensile elasticity modulus increases with increasing LCP content for both MD and TD. The maximum value of anisotropy of elastic properties was noted for a rather low content of LCP (c = 5%). On the contrary, the stress at yield decreases with increasing LCP content. The same was observed for the strain at yield but in both cases an important increase of anisotropy has taken place. Consequently, the total elongation during drawing (strain at break) showed a drastic decrease for blends with higher LCP content (about 60–80 times). The addition of the LCP to polypropylene has led to a stiffness increase (higher elasticity modulus) but simultaneously to a considerable plasticity decrease. As a confirmation of these observations, there served also the creep test where a decrease of the creep compliance (by two times) for LC-rich blends as compared with pure PP was noted.It also should be emphasized that, generally, a smaller effect of LCP content on the elastic deformation was noted than that on the time dependent effects (nonelastic creep deformation).Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 4, pp. 442–450, July–August, 1994.     

Mechanical Properties of High-Density Polyethylene/Chlorinated Polyethylene Blends

Results of experimental investigation of mechanical properties of high-density polyethylene (HDPE)/chlorinated polyethylene (CPE) blends in tension are reported. The specimens of pure HDPE, CPE, and nine types of HDPE/CPE blends, with different component ratios at 10 wt.% intervals, are examined. The features of the stress-strain curves obtained are discussed. Data on the influence of blend composition on the elastic modulus, yield stress, breaking stress, and ultimate elongation are obtained. The results of investigations into the creep behavior are also presented. It is found that the creep compliance obeys the power law of creep with coefficients depending on blend composition.     

Deformability of thermoplastics in physiological salt solution

Conclusions 1. The creep of thermoplastics in physiological salt solution is characterized by change in the rate of creep deformation relative to the creep in air. The bending creep deformations of HMWPE and phenylone and the compressive creep deformation of HMWPE are described by a binomial equation [Eq. (5)] and the compressive creep deformation of phenylone is described by Eq. (6).2. The lifetime of the thermoplastics studied under static compression and bending determined relative to limiting deformations is described by Eq. (7). Under the same stresses and limiting deformations, the lifetime of phenylone in physiological salt solution is greater than the lifetime of HMWPE.3. After the simultaneous action of physiological salt solution and static stress over one month, most of the characteristics of short-term strength in phenylone are significantly reduced due to swelling in the model medium.Paper presented at the Third All-Union Conference on Polymer Mechanics, Riga, 1976.N. N. Priorov Central Scientific-Research Institute of Traumatology and Orthopedics, Moscow. Kazan Construction Engineering Institute. Translated from Mekhanika Polimerov, No. 2, pp. 325–331, March–April, 1978.