按管环带区应力应变场的工程估算方法研究

本文实验测定了容器－接管联接处角焊缝区域（环带区）的应力集中系数，并与目前国内外几种主要的高应变区应力集中系数工程估算方法作了比较分析，还用实测值验证了ＣＶＤＡ－８４规范中采用的峰值应变近似计算方法。结果表明：曾广欣公式在屈服区有限情况下有效性良好，但在大范围屈服的情况下需作一定的修正。     

大开孔内压容器塑性极限载荷的有限元分析

具有大开孔（ｄ ／ Ｄ≥０．５）的圆柱壳是压力容器及管道连接中最常用的结构之一。采用非线性有限元法对这种结构进行了弹塑性分析,确定了孔边的应力集中系数,初始屈服的载荷及位置,塑性区扩展的规律,结构的变形等。最后通过结构上最大塑性变形点的载荷应变曲线,确定了其极限载荷值。同时对采用小变形和大变形理论的分析结果作了比较,结果表明,对于求解极限载荷而言,采用小变形理论亦可获得较精确的结果。     

压力容器多凹坑干涉效应分析

分析研究了多凹坑周围的应力分布以及应力集中的干涉效应。通过对单凹坑分析计算做出安全评价,建立了压力容器多凹坑模型。利用有限元软件ANSYS对该模型进行静力分析,得出沿环向排列的多凹坑最大主应力、应力集中系数和应变集中系数随凹坑数量变化的关系。通过对数据的分析和处理得到,在线弹性范围内薄壁圆筒形压力容器内沿环向排列的凹坑的最大主应力、应力集中系数和应变集中系数,随着凹坑个数的增加是线性递减的。     

聚氨酯复合泡沫塑料的动态压缩力学性能

针对几种不同密度、不同玻璃微珠填充比的聚氨酯复合泡沫塑料进行了动态压缩实验，研究了这类材料的宏观动态力学性能。结果表明，动态应力-应变曲线与准静态压缩加载下的应力-应变曲线具有相同的特征，也分为弹性区、平台区和致密区；在较大的应变率范围内，复合泡沫塑料的应变率效应是明显的，高密度复合泡沫塑料的屈服强度随应变率的增加而增加，而中、低密度材料的屈服强度则先随应变率的增加而提高，然后在某一高应变率下强度反而下降，材料表现出软化现象。     

中心带圆孔的正交异性FRP板孔边应力应变分析

本文首先针对正交异性FRP板,在二维弹性问题的有限元分析中,建立了三角形板元素刚度矩阵。之后,就中心带圆孔的正交异性FRP板,在单向拉伸载荷与材料主轴方向成不同角度情况下,进行了实际的孔边应力应变有限元分析与应变测量。对于孔边的应力集中系数,由有限元分析计算结果与无限正交异性平板解析解的比较,可知,无限平板的结果在一定程度上可用于有限宽板中。     

电极布置方式对CFRP电阻-应变特性的影响

用端部四电极布置方法和中间四电极布置方法对单向和正交CFRP材料在循环加载情况下的应变敏感性系数（GF）值进行了测试；结果表明：单向CFRP的GF随着电极布置方式的不同而反向，采用端部四电极布置方法其测试值为3．06、中间四电极布置方法为-4．76；研究了机理；为保证测得的电阻应变系数尽可能真实，应采用端部四电极布置方法。     

开孔间距对压力容器应力影响的有限元分析

采用ANSYS Workbench软件进行开孔间距对压力容器应力影响的有限元分析。结果表明:应力集中程度受开孔间距的影响,随着K值增大,应力集中系数减小,当K1.3时,应力集中系数基本不变。开孔接管区的最小应力反应了两孔间干涉作用的强弱,随着K值增大,开孔接管区的最小应力逐渐减小,说明两孔间的干涉作用逐渐减弱。     

聚酯短纤维/CR复合材料补强机理的研究

以不同直径、相同长径比的聚酯短纤维补强CR复合材料为研究对象，采用扫描电子显微镜观察、应力—应变曲线分析等手段，研究了复合材料的补强机理及破坏形式。结果发现，屈服强度由薄弱界面的剪切强度决定，而界面的剪切强度则取决于界面粘合、短纤维长度、直径以及含量等因素s拉伸过程中复合材料的界面按照剪切强度由小到大的顺序分段依次被破坏直至断裂；短纤维含量较低时，其拉伸强度主要由短纤维拨出后的基体承担，同时受应力集中影响很大，短纤维含量较高时断裂强度则更接近于屈服强度。     

插销式单闸板防喷器有限元分析

建立了插销式单闸板防喷器壳体、定位销轴和侧门的三维实体装配模型,并取出其关键部位做有效的简化得到有限元力学分析模型。考虑了材料的非线性以及接触问题的高度非线性问题,分析了在多种载荷情况下壳体、定位销轴和侧门上的应力、应变、位移情况和接触应力、接触状态,得出了应力集中区域,为防喷器设计的安全性和进一步优化设计提供了理论依据。     

浅析压力容器中的裂纹容限

应用Ｊ积分理论，讨论了屈服材料中小裂纹的Ｊ积分和ＣＯＤ与裂纹尺寸α及标称应变的函数关系，分析了压力容器高压力容器高应力区中裂纹容限，讨论了压力容器在设计压力及水压试验压力下，不同部位所允许的最大裂纹尺寸计算。     

The effect of time and temperature on the mechanical behavior of epoxy composites

A crosslinked epoxy resin consisting of a 60/40 weight ratio of Epon 815 and Versamid 140 and composites of this material with glass beads, unidirectional glass fibers and air (foams) were tested in tension, compression and flexure to determine the effect of time and temperature on the elastic properties, yield properties and modes of failure. Unidirectional continuous fiber-filled samples were tested at different fiber orientation angles with respect to the stress axis. Strain rates ranged from 10^?4 to 10 in./in.-min and the temperature from ?1 to 107°C. Isotherms of tangent modulus versus strain rate were shifted to form master modulus curves. The moduli of the filled composites and the foams were predictable over the entire strain rate range. It was concluded that the time-temperature shift factors for tangent moduli and the time-temperature shift factors for stress relaxation were identical and were independent of the type and concentration of filler as well as the mode of loading. The material was found to change from a brittle-to-ductile-to-rubbery failure mode with the transition temperatures being a function of strain rate, filler content, filler type and fiber orientation angle, indicating that the transition is perhaps dependent on the state of stress. In the ductile region, an approximately linear relationship between yield stress and log strain is evident in all cases. The isotherms of yield stress versus log strain rate were shifted to form a practically linear master plot that can be used to predict the yield stress of the composites at any temperature and strain rate in the ductile region. The time-temperature shift factors for yielding were found to be independent of the type, concentration and orientation of filler and the mode of loading. Thus, the composite shift factors seem to be a property of the matrix and not dependent on the state of stress. The compressive-to-tensile yield stress ratio was practically invariant with strain rate for the unfilled matrix, while fillers and voids raised this ratio and caused it to increase with a decrease in strain rate. The yield strain of the composites is less than the unfilled matrix and is a function of fiber orientation and strain rate.     

The fracture mechanism of polylactic acid resin and the improving mechanism of its toughness by addition of acrylic modifier

The fracture mechanism of polylactic acid (PLA) resin and the improving mechanism of its toughness by addition of an acrylic modifier were examined. Plane strain compression testing of PLA clearly showed strong softening after yielding. Because the stress for craze nucleation of PLA was close to the yield stress, brittle fractures resulted. The addition of an acrylic modifier to the PLA significantly lowered the yield stress and formed many voids. The release of the strain constraint because of the formation of many voids and the decrease of yield stress resulted in the relaxation of stress concentration, and the toughness was improved. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Estimating time and temperature dependent yield stress of cement paste using oscillatory rheology and genetic algorithms

A controlled shear stress–shear rate rheometer was used to determine the viscoelastic behavior of cement paste incorporating various superplasticizers and subjected to prolonged mixing at high temperature. At a low applied shear stress range, the oscillatory shear strain/stress curve of cement paste was characteristic of a linear elastic solid; while the higher stress range was characteristic of a viscous liquid exhibiting a linear strain increase with increasing applied shear stress. The transition from solid-like to liquid-like behavior occurred over a very narrow stress increment. This transition stress corresponded to the yield stress parameter estimated from conventional flow curves using the Bingham model. The yield stress from oscillatory shear stress tests was estimated using the intersection between the viscous part of the oscillatory shear strain/stress curve and the oscillatory shear stress axis. In this study, equations describing the variation of shear strain versus shear stress beyond the solid–fluid transition for cement pastes incorporating various superplasticizers at different ambient temperatures and mixing times were developed using genetic algorithms (GA). The yield stress of cement pastes was subsequently predicted using the developed equations by calculating the stress corresponding to zero strain. A sensitivity analysis was performed to evaluate the effects of the mixing time, ambient temperature, and superplasticizer dosage on the calculated yield stress. It is shown that the computed yield stress values compare well with corresponding experimental data measured using oscillatory rheology.     

Effects of high hydrostatic pressure on the mechanical behavior of a crystalline polymer–polyoxymethylene

The true stress-true strain behavior of polyoxymethylene, n(-CH₂O), as an example of a bulk semi-crystalline polymer, has been investigated for constant hydrostatic environmental pressures from 1 atmosphere to 8 kilobars with the principal objectives of elucidating the factors controlling flow and fracture. Experiments were conducted in uniaxial tension at room temperature and constant strain rate. The tensile observations were supplemented by measurements of bulk compressibility and stress relaxation behavior at pressure. In contrast with metals and inorganic compounds, the modulus, yield stress and fracture stress of POM increase strongly with pressure by a factor of approximately three at 8 kilobars. The modulus increase is shown from the stress relaxation measurements to be associated with a pressure-induced increase in the β-transition temperature which points to the potential usefulness of the concept of pressure-temperature super-position of mechanical behavior. The characteristics of the pressure dependence of the yield stress demonstrate that yield criteria based on continum mechanics considerations, including the Mohr or Coulomb-Navier criterion, are not valid for general deformation (non-plane strain) conditions in this polymer. The concept of a critical volume change determining the initiation of yielding is suggested to be applicable to semi-crystalline polymers. Comparison with analogous changes in yield stress with temperature points to an increasing contribution to the control of yielding by the initially disordered regions with increasing pressure or decreasing temperature. The fracture behavior observed at pressure eliminates the concepts of a critical stress as a fracture criterion for POM and of a simple reduction in normal stress at points of stress concentration as the principal effect of the applied pressure on fracture.     

A correlation of Young's modulus with yield stress in oriented poly(vinyl chloride)

The variations of tensile and compressive yield stresses and of Young's modulus of oriented poly(vinyl chloride) sheet with direction and with degree of orientation, represented by birefringence, are shown. Young's modulus was calculated from elastic stiffness constants measured by an ultrasonic pulse method at 5MHz with estimated strain and strain rate amplitudes of 2 × 10^?5 and 100s^?1. Yield strains were about 5 × 10^?2 measured at strain rates of about 2 × 10^?2s^?1. Although the measuring conditions were so different there was found to be a close correlation between tensile yield stress and Young's modulus, the two quantities being connected by a simple linear relationship, as direction of measurement and degree of orientation were varied. Compressive yield stress did not correlate with Young's modulus, and changed little with direction or degree of orientation by comparison with tensile yield stress. The empirical linear relationship between tensile yield stress and Young's modulus, difficult to account for theoretically, might form the basis of a method for determining tensile yield stress ultrasonically.     

Tensile properties of microbial β‐glucan films

β‐glucan films with different plasticizer concentrations (glycerol and water) were deformed in tension and various mechanical characteristics (modulus of elasticity, yield stress, strength, and strain at rupture point) were evaluated. The influence of both plasticizers was of additive nature describable by the total plasticizer content. The influence is reflected by an exponential decrease of either modulus of elasticity or yield stress with increasing total plasticizer content. The film strength decreased linearly with plasticizer concentration, whereas the decline in strain at the rupture point followed a power function. It was also found that the stress parameters can be expressed via modulus of elasticity: the strength as linear function of the square root of the elasticity modulus and the yield stress as a linear function of the latter parameter. On the basis of such relations a quadratic function between strength and yield stress was proposed. This relation well describes the character of the deformation curves that were obtained at different deformation conditions. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

The effect of time and temperature on the mechanical behavior of a “plasticized” epoxy resin under different loading modes

Epoxy–Versamid specimens were loaded in tension, compression, and flexure at different strain rates and temperatures to determine mode of failure, yield stress and strain, and tangent and relaxation moduli. Stress-strain curves were used to define brittle, ductile, ductile-rubbery, and rubbery modes of behavior which prevailed in different temperature-strain rate regions. The time-temperature superposition principle was applied to yield stress, initial tangent moduli, and relaxation moduli data for all three types of loading. The transition regions, tangent and relaxation moduli, and shift factors were the same in tension, compression, and flexure. Thus the most convenient mode of loading can be used to determine the general time-temperature dependence. The ratio of compressive-to-tensile yield stress was almost constant over the entire ductile region. Flexural yielding data were used to predict yield stress in tension and compression, and stress relaxation master curves were shown to be related to elastic modulus vs. strain rate curves. The yielding phenomenon was interpreted using Eyring's theory of non-Newtonian viscoplastic flow. The apparent activation energy and activation volume were larger for tension than compression. A theory is offered to explain why yielding can occur in a cross-linked system.     

Yield Stress Measurement of Silicon Nitride Suspensions

Yield stress values of silicon nitride suspensions were measured via a novel slotted plate device and a constant stress rheometer and the results were compared. Test platform velocity, associated with the slotted plate method, was found to have a substantial effect on dynamic yield stress but not on static yield stress. The effects of suspension concentration and temperature on yield stress values were investigated. Yield stress measurements on mixtures of silicon nitride and alumina particles, as well as creep tests were performed.     

The notch sensitivity of polymeric materials

Stress–strain tests were made on about five dozen polymeric materials using unnotched and notched specimens containing six different types of notches. Notches decrease the strength, but they decrease the elongation to break even more drastically in general. Notch sensitivity factors are defined for strength and for energy to fracture in such a manner that the greater the notch sensitivity factor, the greater is the effect of a notch relative to the unnotched material. The notch sensitivity factor for breaking (or yield) strength is not the same as the notch sensitivity factor for energy to fracture as measured by the area under the stress–strain curve. Brittle polymers and composites tend to have greater notch sensitivity factors for strength than ductile polymers. For brittle polymers, the notch sensitivity factor for energy to fracture tends to increase with the elongation to break of the unnotched polymer. Notches generally are more detrimental to ductile polymers than to brittle ones as far as the energy to fracture is concerned. For ductile polymers, the shape of the stress–strain curve is important in determining the sensitivity to notches. The ratio of the upper to lower yield strengths should be small for low notch sensitivity. It is desirable to have the breaking strength greater than the yield strength. Glass fibers and filler in ductile matrices increase the notch sensitivity for strength but decrease the sensitivity for energy to fracture relative to the unfilled polymer. Rubber–filled polymers have a reduced notch sensitivity for strength relative to the unfilled polymer, but the notch sensitivity for energy to fracture may be either increased or decreased, depending upon the system. The energy to fracture for notched specimens correlates better with Izod impact strength than does the energy to fracture for unnotched specimens. It is recommended that notched stress–strain specimens be routinely measured along with unnotched specimens.     

Miscibility and mechanical properties of sulfonated polystyrene–polyurethane blends

A sulfonated polystyrene (SPS) and a polyurethane containing a tertiary amine group (NPU) were blended in solution. The effect of blend composition was studied in the blend of SPS with 9.83 mol % of sulfonation (SPS-9.83) and NPU with 33 mol % of MDEA (NPU-33). As the SPS concentration increases, a significant improvement of miscibility is observed. The tensile strength of the blends is greater than either pure NPU or SPS. A maximum strength and a maximum density occur at 50 wt % SPS. The stress–strain curve shows a well-defined yield when the SPS concentration in the blend is 30 or 50 wt %. The yield is more dramatic in the blend with 50 wt % SPS than that of 30 wt % SPS. At a lower SPS concentration, the blend behaves like a rubber, while a higher SPS concentration in the blend results in a brittle failure before yield. An increase in the sulfonation level of SPS in the SPS–NPU-33 (30/70) blends leads to an improved miscibility. A significant enhancement of tensile strength is observed as the sulfonation increases. A clear yield point on the stress–strain curves occurs when the sulfonation of SPS in the blend is 4.79 mol % or greater. Increasing the MDEA content of NPU up to 8.3 mol % can lead to an enhancement of tensile strength. A further increase in the MDEA content has little influence on the tensile strength, but a clear yield on the stress–strain curve occurs. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2035–2045, 1998