单轴拉伸状态下橡胶隔振器的疲劳寿命预测研究

针对橡胶隔振器疲劳寿命预测问题,基于哑铃型橡胶材料试片的疲劳寿命试验数据,建立橡胶隔振器的疲劳寿命预测模型,进而提出橡胶隔振器疲劳寿命预测的研究方案。设计了一哑铃型橡胶试片进行拉伸疲劳试验,实测其疲劳寿命数据并以最小二乘法拟合了该试片疲劳寿命的幂函数预测模型。再通过有限元方法计算了橡胶隔振器的三种常见的疲劳寿命评价参数：对数主应变、柯西主应力和应变能密度,最后将橡胶隔振器的疲劳寿命预测值与实测值进行比对。结果发现：当疲劳失效标准静为刚度降级25%时,三种疲劳寿命评价参数均能较好地拟合成幂函数疲劳寿命预测模型,对数主应变的预测值较为接近实测值、应变能密度参数的预测值约为实测值2倍左右、柯西主应力预测寿命的可靠性验证系数均接近5,且三种疲劳损伤评价参数的疲劳寿命拟合曲线形状均较为相似。     

承受双向等拉应力平面贯穿型中心切口弹性应力集中系数的尺寸效应分析

采用线弹性有限元方法系统分析了350种不同尺寸条件下承受双向等拉应力平面贯穿型中心切口端部应力场,建立了垂直于切口长度方向的正应力分量Ktσy、应力第一不变量集中系数KtI1和Mises等效应力集中系数Ktσ等各项弹性应力集中系数的表达式。结果表明,各项弹性应力集中系数随切口长度与板宽之比2a/W的增大呈二次幂函数形式增大,随切口顶端半径与切口半宽之比r/a的降低呈指数函数形式增大。     

疲劳缺口系数Kf与理论应力集中系数Kt之间的关系

疲劳缺口系数Kf是表征缺口部位疲劳强度降低程度的参量,这一系数在结构抗疲劳设计中经常被使用.一般认为Kf与理论应力集中系数Kt呈线性关系,线性的斜率与不同材料类别(铝合金、钛合金和钢)有关.通过对大量不同材料、不同应力比和不同缺口试样形式的高周疲劳极限的系统分析,发现大多数情况下K f与Kt的呈线性关系,但有些情况下二者不服从线性关系.Kf与Kt的关系除与材料类别有关外还与实验的应力比有关.     

单轴拉伸局部屈服状态下的应力方程理论推导与分析

为了进一步研究屈服状态对小孔周围应力的影响,无限大平板在单轴拉伸下,小圆孔垂直方向上产生局部Tresca条件屈服状态下,对屈服圈外的应力方程进行理论推导,并就残余应力小孔应变法测量标准GB/T 31310—2014所推荐应变花尺寸的适用性进行了讨论分析。计算结果表明,GB/T 31310—2014推荐的应变花适用于残余应力水平低于0.971倍屈服强度的残余应力场测试。     

导弹横V型切口附近的应力场分析

本文利用弹性力学及复变函数理论，对异弹簧Ｖ型切口附近应力场进行了分析，给出了该切口附近应力分量的显式表达式，对该情况下的应力强度因子应如何定义进行了。     

拉伸正交各向异性有限宽板偏心圆孔的应力集中系数表达式

用近似解析方法推导出拉伸正交各向异性有限宽板偏心圆孔的应力集中系数的显式表达式,除了比较极端的情况外,该式的精度较好。误差估计基于与有限元分析结果的比较。当偏心度取为零,即变为有限宽板中心圆孔时,此简化情况下的应力集中系数表达式与文献值和有限元解吻合得很好。推导的方法虽然简单,但结果比较理想。利用所推导的表达式可以求解各向异性材料的应力集中系数问题。     

V 型切口在大理石切割爆破中的应用

大理石切割爆破技术是作者在岩石断裂力学理论和大理石矿实际切割要求的基础上提出的,而这个技术的关键是采用带 V 型切口的炮孔。本文从理论上论证了采用 V 型切口炮孔的合理性,同时给出了最佳 V 型切口角。文中附有某大理石矿采用 V 型切口炮孔爆破切割大理石的现场照片,切割效果是令人满意的。     

K节点应力集中系数的试验和数值研究方法

工程中常用的评价海洋平台中管节点疲劳寿命的方法是使用S-N曲线。当管节点承受疲劳载荷作用的时候,可以通过数值或者试验方法得到沿着焊缝处的热点应力幅的大小。然后通过S-N曲线,可以预测此节点在破坏前可以承受疲劳载荷的循环次数。应力幅的大小可以由应力集中系数这个参数来确定。对K型节点在承受基本载荷作用下的应力集中系数进行了数值和试验分析,得到了各种基本载荷作用下K节点沿着焊缝处应力分布情况和极值应力点的位置。     

螺栓杆应力集中系数的研究

螺栓球节点栓杆上的应力集中是导致栓杆发生疲劳破坏的主要原因。该文通过理论推导及数值分析的方法研究了螺栓杆的应力集中系数。在忽略螺纹升角的情况下,根据变形协调条件研究了螺栓杆的轴力梯度变化,采用数值计算确定常用螺栓球的螺纹位移系数及螺纹力计算公式。采用局部建模,通过迭代计算的方法确定螺栓杆应力集中系数。与完整模型的计算结果比较表明该方法的计算结果具有较高的精度,且计算效率高。     

焊接钢管节点热点应力集中系数参数方程的适用性研究

由于各几何特征参数对焊接钢管节点应力集中系数影响复杂,参数方程给出的计算结果都存在不同程度的不确定性。本文在收集大量实验数据的基础上,对一些常用参数方程计算结果的不确定性进行了分析讨论,给出了各方程在不同荷载条件下计算结果的变异系数值,并对其适用性进行了评价,可为海洋平台钢管结构的疲劳可靠性分析和管节点应力计算提供参考。     

Development of a uniaxial ice tensile specimen for low temperature testing

The objective of this research was to develop uniaxial ice tensile specimens that could be tested at temperatures ranging from 255 K down to 150 K. Standard tensile specimens were produced using a mold constructed from a room temperature vulcanizing rubber which was equipped with aluminium end mounts. Tensile tests showed that the casting procedure specimens that failed consistently in the small cross-sectional region and the maximum tensile strength data were reproducible to within experimental error.     

Stress intensity factor for a cracked specimen under compression

For a cracked specimen under compression, a set of complex stress functions is proposed and by using the boundary collocation method, the unknown coefficients of these complex stress functions are determined. Based on the calculation results of the boundary collocation method, the formulas of the stress intensity factor for a cracked specimen under compression are obtained, and by using these formulas, the influence of confining stress on stress intensity factor is analyzed.     

Experiments on concrete under uniaxial impact tensile loading

A problem of practical importance for designing of structural elements is discussed in this paper—the behaviour of concrete subjected to uniaxial impact tensile loading. The “Split Hopkinson Bar” technique was adopted for testing concrete in uniaxial tension at stress rates between 2 and 60 N/mm²/ms. A remarkable increase in tensile strength was observed due to high stress rate. The ratio of impact and static tensile strength varied between 1.33 and 2.34 for various concrete mixes. The influence of maximum aggregate size, water-cement ratio, cement content, cement type and quality, specimen humidity, static compressive strength and loading/casting direction upon the uniaxial impact tensile strength was studied. The high stress rate resulted in an increase of the modulus of elasticity of concrete in uniaxial tension. An explanation for the observed phenomena is suggested.     

Experiments on concrete under repeated uniaxial impact tensile loading

The fatigue behaviour of concrete was a new aspect in the experimental program focused on the behaviour of concrete subjected to uniaxial impact tensile loading. The results obtained in repeated impact tensile loading tests (at stress rates of 2–6 N/mm². ms) indicated that the impact fatigue was associated with progressive crack propagation resulting in accumulative damage of concrete. The upper stress limits varied between 1.7 and 0.6 times the static tensile splitting strength and specimen withstood 1 to 6,055 loading cycles respectively. The influence of water-cement ratio, cement content, specimen humidity, loading/casting direction and compressive strength upon the impact fatigue tensile strength of concrete was studied. The phenomena observed are discussed with the aid of fracture mechanics concepts.

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Résumé Un aspect nouveau dans la recherche du comportement du béton soumis en traction uniaxiale à des charges de choc est le comportement à la fatigue. Ce problème de fatigue a une grande importance pour des pieux de fondation. On a obtenu 89 résultats en faisant des essais répétitifs de choc en traction où la vitesse de mise en tension variait entre 2 et 6 N/mm². ms. Les limites des contraintes ont varié entre des valeurs égales à la résistance du choc en traction uniaxiale (1,7 f_b,r) et 0,6 fois la résistance statique à la traction par fendage. Les éprouvettes ont résisté à des variations de charge de 1 à 6 055. Les résultats montrent que la fatigue au choc allait de pair avec une augmentation progressive de la fissuration provoquée par la dégradation accumulative du béton. Il a été constaté que la résistance du béton à un effort de chocs repétés en traction augmente quand la valeur du facteur eau-ciment et la teneur en ciment augmentent. L'humidité des éprouvettes n'influence pas la résistance à la traction par chocs répétés. La résistance déterminée sur des éprouvettes chargées perpendiculairement à la direction du bétonnage était plus élevée que la résistance mesurée dans un essai où la direction de chargement et du bétonnage étaient parallèles. En général un béton de qualité inférieure a une meilleure résistance aux chocs répétés. On propose des explications des phénomènes observés par les théories de la mécanique de la rupture.

     

Doped silicon under uniaxial tensile strain investigated by PAC

The recent application of strained silicon into transistor design has led to significant progress in increasing the performance of devices. However, up to now little is known about the mechanical behaviour of the strained Si layers and the elastic properties of the deformed semiconductor lattice. The perturbed angular correlation method is ideally suited to study strain related local phenomena in silicon using the acceptor ¹¹¹In as probe. In the past, the influence of external uniaxial strain on In acceptors in pure Si and on donor-acceptor pairs in silicon has been investigated intensively (G. Marx, R. Vianden, Phys. Lett. A210, 364 1996; G. Tessema, PhD thesis, University of Bonn, 2003). In the course of these studies it was found that the unpaired indium probes on regular lattice sites showed an unexpected reaction to uniaxial strain, which depended on the dopant species. Our current experiments reveal that the tension induced local deformation around the In probe atoms also depends on the concentration of the co-implanted donors suggesting a local change of the elastic properties of silicon. In addition, the reaction of the silicon lattice to tensile strain applied along different crystal axes showed strong differences.     

Pore size characteristics of nonwoven structures under uniaxial tensile loading

Nonwovens are highly porous structures consisting pores of complex shapes and sizes which are responsible for desired functional characteristics. In general, a nonwoven is often subjected to uniaxial tensile loading in various applications and it is of paramount importance to account for changes in structural characteristics including pore sizes during the loading conditions. In this research work, the pore size of thermally bonded nonwoven structures under uniaxial tensile loading at various levels of strains has been investigated. A theoretical model has been proposed that accounted for fibre reorientation and changes in the fibre volume fraction during the application of tensile strain. A comparison has been made between theoretical and experimental pore size distributions of thermally bonded nonwoven structures at defined levels of strains. Moreover, an attempt has been made to rationalise some of the contradictory literature results of pore size distributions of nonwoven structures under uniaxial tensile loading.     

Strain-driven phase transition of molybdenum nanowire under uniaxial tensile strain

The phase transition processes of a single Mo nanowire under uniaxial tensile strains have been studied with molecular dynamics simulation. Two phase transitions were observed during the uniaxial tensile processes. The first one is the configuration of Mo nanowire transformed from BCC structure to FCC structure and the second one is that transformed from FCC-to-BCC. The phase structures have also been demonstrated with radial distribution function (RDF) analyses. A pseudoelasticity behavior was observed under large uniaxial tensile strain. Two average atomic energy curves of BCC and FCC structures as the function of the layer-space along [0 0 1] direction were obtained with embedded-atom method potential calculations, with which the strain-driven BCC-to-FCC and FCC-to-BCC phase transition mechanisms have been clarified clearly for Mo nanowire under uniaxial tensile strain.     

Damage to flax fibre slivers under monotonic uniaxial tensile loading

Usual testing methods developed for synthetic fibre bundles or slivers do not suit for plant fibres which exhibit specific characteristics. This paper reports damage investigation in flax fibre slivers (collection of disentangled and aligned fibre bundles more or less bonded by bark residues or other tissues) especially prepared for uniaxial tension testing using a new methodology and a dedicated device.The analysis of the acoustic emission data by using clustering methodology allowed categorizing three populations of damage mechanisms. Identification of the damage mechanisms was made by correlation of the tensile loading curves with acoustic emission data and by optical observations. Different damage parameters defined from the mechanical data (released elastic energy) were compared with those from acoustic data (energy of the hits). The results showed that the extension of damage vs. deformation can be expressed by using either of these two data sets.     

Stress concentration formulae useful for any shape of notch in a round test specimen under tension and under bending

In this work stress concentration factors, K_t , for a round bar with a circular-arc or V-shaped notch are considered on the basis of exact solutions for special cases and accurate numerical results. Then, a set of K_t formulae useful for any shape of notch is proposed. The conclusions can be summarized as follows. (i) For the limiting cases of deep (d) and shallow (s) notches, the body force method is used to calculate the K_t values. Then, the formulae are obtained as K_td and K_ts . (ii) On the one hand, upon comparison of K_t and K_td it is found that K_t is nearly equal to K_td if the notch is deep or blunt. (iii) On the other hand, if the notch is sharp or shallow, K_t is mainly controlled by K_ts and the notch depth. (iv) The notch shape is classified into several groups according to the notch radius and notch depth. Then, the least-squares method is applied for the calculation of K_t /K_td and K_t /K_ts . (v) Finally, a set of convenient formulae is proposed that are useful for any shape of notch in a round test specimen. The formulae give SCFs with <1% error for any shape of notch.