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.     

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.     

由压痕蠕变试验确定材料的蠕变性能参数

研究由平头压痕蠕变试验来确定受压材料蠕变性能参数的可行性。利用有限元蠕变分析确定在定压痕应力下的压痕蠕变率,重点放在稳定压痕蠕变率和受压材料蠕变性能参数的关系上。详细地研究了压头形状、大小和宏观约束对压痕蠕变响应的影响:当压头的尺寸和受压材料为同一数量级时,宏观约束将有十分明显的影响。提出两种方法来由压痕蠕变试验来确定受压材料蠕变性能参数,并给出了算例,结果有利于准确认识平头压痕蠕变试验,从而拓宽其应用范围。     

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.     

变应力寿命试验的统计分析

本文利用 S-N-P 曲线和累积损伤原理,给出了变应力下产品的寿命分布,此分布不仅与应力水平有关,而且与各应力水平的次序有直接关系.在此分布的基础上,给出了变应力寿命试验的统计分析方法.这种方法不仅适用于疲劳试验,而且还可用于单项可变动应力寿命试验和加速寿命试验,以及各种应力组合的可变应力的寿命试验和加速寿命试验.为利用可变动组合应力的加速寿命试验,估计正常可变应力下的寿命特征提供了一种解决的方法.     

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。     

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.     

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.     

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.