爆扩桩在基础工程中应用的研究

本文以工程实例叙述爆扩桩施工工艺,试压结果和经济技术比较,证明爆扩桩具有施工方便、工期短、机械化程度低、降低工程成本等优点,适用于土质基础工程的短桩.     

消除负摩阻力扩体挤密桩的研制及力学特性分析

针对湿陷性黄土地区等不良土质地区的桩基负摩阻力问题,提出一种既可以消除负摩阻力,又能提高承载力的扩体挤密新型桩,对其结构构成和工作机理进行详细阐述。根据扩体挤密桩的结构特性,建立扩体装置扩张的菱形孔扩张模型,采用复变函数方法,求解了扩体装置扩张的弹塑性解及极限扩张角,推导出该新型桩的单桩极限承载力计算公式。结合算例采用有限元法和提出的理论计算方法对该新型桩的挤土效应和承载特性进行了分析。结果表明:扩体装置对土体应力的影响范围约为扩体装置扩开宽度的2倍~3倍;扩体装置扩张形成的塑性区范围大小在竖直方向上和水平方向上基本相等;各扩体装置承力时具有一定的顺序效应和时间效应,扩体装置的承力能力与周围土体的性质有关;数值模拟结果与理论计算结果基本一致,验证了理论分析方法的正确性;与普通直桩及套管桩相比,扩体挤密桩消除负摩阻力效果良好,提高承载力作用显著,受力机制科学合理,为湿陷性黄土地区桩基工程的设计提供一定的理论依据和参考。     

复合载体夯扩桩承载力性状有限元分析

本文对复合载体夯扩桩进行了有限元分析，在分析中通过现场取样和模型试验确定夯扩挤密区范围，通过分段线性插值确定夯扩影响区土单元的材料参数，从而能够较为准确地分析这种桩承载力的特殊性状。本文根据计算结果讨论了桩身的荷载传递规律、承载力的计算及最优夯扩体直径的确定，同时也比较了这种桩与扩底灌注桩在承载力特性方面的区别，从而揭示了复合载体夯扩桩承载力性状的基本规律，说明了复合载体夯扩桩是一种承载力性能优越的桩型。     

复合载体夯扩桩施工技术

随着工程建设的发展,复合载体夯扩桩地基处理技术在土木工程建设中得到越来越多的推广应用,并产生了良好的社会效益和经济效益。复合载体夯扩桩是一种新的施工技术,实践证明,该桩可大幅提高桩基的承载力,是一种技术可行、经济适用的地基加固新技术。     

边坡工程抗滑桩合理桩间距初探

对边坡工程防滑桩的合理间距进行探讨,要考虑抗滑桩间存在的土拱效应因素桩间距的影响,在对土拱效应分析之后就可以根据桩间静力平衡条件、跨中截面强度状况以及拱脚处所在截面强度状况合理推断出桩间距。在掌握了合理桩间距的计算公式之后,就可以通过计算公式看出桩间距与决定条件的关系,通常而言,如果保持其他因素不变,桩间距与桩后土体粘聚力成正比,与内摩擦角成正比,而与桩后坡体推力成反比。实际工程应用中我们也发现,这个桩间距公式的推导比较合理,很好地考虑了各方面的影响因素,根据计算出来的结果设置抗滑桩的桩间距,防滑效果甚好,所以说该计算公式实用价值比较高。     

某载体桩工程质量事故分析及处理方法

<正>0序言桩基础广泛应用于工业建筑、民用建筑、道路桥梁、港口航道、水利堤坝、边坡治理、基坑维护等工程。复合载体夯扩桩施工技术作为一种近年来较新的施工方法,它是在锤击沉管灌注桩与扩底桩的基础上发展起来的一种将桩端夯扩成扩大头的新桩型。它继承了沉管桩能挤密桩周土层,直接浇注混凝土等优点,又能充分利用桩端的扩大头提高承载力。具有承     

黄土地基夯扩挤密桩单桩静载荷试验研究

分析了夯扩灰土挤密桩单桩承载力机理及不同条件下的单桩静载荷试验Q-S曲线特征规律,进行了三种不同夯击能的单桩静载荷试验,提出了夯击能对单桩承载力特征值的影响。     

基桩承载力动态测试的最小击振能量研究

经长期对桩的承载力的现场测试研究发现，动力打桩公式在测试贯入度时无法考虑土体对桩的影响，而“低应变”法的击振能量太小，同时要计算“参振系统”的参振土体的质量，特别是，该计算方法又没有足够的理论依据。另一方面，“高应变”法对一般单桩的击振能量：赶大导致桩士系统进入较大的塑性破坏。因此，采用一维单自由度模式分析系统最小击振力，利用一维杆件振动模式导出桩土系统的0阶与1阶振动频率（准静态），得出桩顶振动位移与桩土系统最小击振力的函数关系，能够克服“低应变”法和“高应变”法存在的缺陷。实践检验表明，所提出的测试方法能较好地解决桩基础承载力测试的工程应用问题。     

饱和软粘土中沉桩后桩周土体固结分析

假定沉桩过程是一个平面应变圆孔扩张问题, 采用了修正剑桥模型, 给出了软粘土中沉桩过程后初始时刻超孔隙水压力沿桩径分布的解析函数, 并与Cao 等人的数值解以及Gibson 提出的公式进行了比较。根据土骨架的弹性位移特性以及水流的连续性条件, 得到了桩周土体固结的控制方程。运用分离变量法并结合边界条件以及初始条件得到了桩周土中超孔隙水压力消散的级数解答, 该解答可以作为孔压静力触探反求固结系数的一个理论依据。通过2 个算例分析了土体的应力历史以及刚度对桩侧超静孔压消散的影响;算例分析表明, 随着超固结比的增大, 归一化后的塑性区半径以及桩侧超孔隙水压力均在减小;桩侧的超静孔压消散前期较快, 后期较慢。     

金属棒材拉丝应力统一解

材料拉压强度不相等时棒材拉丝问题的应力解答是研究材料成形工艺人员所关心的问题。根据统一强度理论,分析了棒材拉丝问题,得到考虑材料拉压比影响的棒材拉丝应力统一解和最大截面缩减率统一解。分析过程中引入一个反映棒材拉制和压制工艺的系数,将棒材拉丝问题的拉制解和压制解统一起来。当材料拉压强度相同时,棒材拉丝应力的Mises解答是棒材拉丝应力统一解的特例。在拉丝过程中,棒材拉丝问题的最大截面缩减率随材料拉压比变化而变化。当材料的拉压强度相等时,棒材拉丝最大截面缩减率的Mises解答是最大截面缩减率统一解的特例,也是它的最大值。     

Dynamic plastic response of a circular plate based on unified strength theory

The study and design of structures under dynamic loads require a knowledge of the plastic response and deformation behavior under impact loading. The calculations of dynamic plastic response of structures are useful for the design and investigation of colliding vehicle, engine and various impacting structures. The unified solutions of dynamic plastic load-carrying capacities, moment fields and velocity fields of a simply supported circular plate are introduced. The strength is different in tension and compression and the effect on the yield criteria is taken into account by using the unified strength theory. Upper bound and lower bound plastic responses of the plate, under moderate partial uniformly distributed impulsive loading, are obtained. The static and kinematical admissibility of the dynamic plastic solutions are discussed. The unified solutions of the static plastic load-carrying capacities, moment fields and velocity fields of a simply supported circular plate are also obtained according to the dynamic solutions in this paper. The solutions are suitable for many materials with or without different strengths in tension and compression and the effect of intermediate principal stress.The solutions based on the Tresca, the von Mises, the Mohr–Coulomb theory, and the twin-shear strength theory, as well as the unified yield criterion, are all the special cases of the unified solutions. The influences of the coefficient of failure criteria, b, and tension-compression strength ratio, α, on the dynamic and static solutions, are investigated. It is shown that the effects of different strengths in tension and compression and yield criteria on the dynamic load-carrying capacity are greater than in the static plastic limit state.     

Microstructure and mechanical behavior of ECAP processed AZ31B over a wide range of loading rates under compression and tension

In this work, a commercial magnesium alloy, AZ31B in hot-rolled condition, has been subjected to severe plastic deformation via four passes of equal channel angular pressing (ECAP) to modify its microstructure. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of the as-received, ECAPed and mechanically loaded specimens. Mechanical properties of the specimens were evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates. The yield strength, ultimate strength and failure strain/elongation under compression and tension were compared in detail to sort out the effects of factors in terms of microstructure and loading conditions. The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression, and the stress–strain curves exhibit clear sigmoidal shape, pointing to dominance of mechanical twinning responsible for the plastic deformation under compression. All compressive samples fail prematurely via adiabatic shear banding followed by cracking. Significant grain size refinement is identified in the vicinity of the shear crack. Under tension, the yield strength is much higher, with strong rate dependence and much improved tensile ductility in the ECAPed specimens. Tensile ductility is even much larger than the malleability under compression. This supports the operation of 〈c + a〉 dislocations. However, ECAP lowers the yield and flow strengths of the alloy under tension. We attempted to employ a mechanistic model to provide an explanation for the experimental results of plastic deformation and failure, which is in accordance with the physical processes under tension and compression.     

在复杂应力状态下厚壁圆筒的极限分析

应用双剪统一强度理论,考虑材料的拉压异性和同性,推导了在内压力和轴力联合作用下的厚壁圆筒的塑性极限载荷计算公式,并且绘制了其极限载荷线图。在这些计算公式中,当其系数取不同的值时,就能得到按Tresca屈服准则、线性逼近的Mises屈服准则和双剪应力屈服准则的计算结果。应用其极限载荷线图,根据其承受的载荷大小,就能判断厚壁圆筒是否达到了屈服极限状态。绘制了在不同屈服准则下的极限载荷线图,以便对其差异进行比较。     

管桩承载性状的数学描述

基于5根桩现场试验资料的分析,对表征管桩承载性状的桩顶荷载-桩顶沉降曲线、桩身压缩-桩顶荷载曲线进行了数学描述。结果表明:1)用Boltzmann数学模型拟合5根桩的桩顶荷载-桩顶沉降曲线,效果很好,相关系数都达到了0.996以上。2)从综合系数-桩顶荷载曲线图上可发现5根桩的曲线形状有3种类型。3根短桩的综合系数大,2根超长桩的综合系数小。这些反映出综合系数是一个变量而非常数,要由它来确定桩身压缩难度较大。3)用Boltzmann数学模型对桩身压缩-桩顶荷载关系曲线进行回归拟合,效果也很好,各单桩的相关系数都达到了0.996以上,5根桩的综合相关系数为0.9871。这样就可根据已有试桩资料的回归拟合曲线及其方程式来预测和计算该场地及其类似场地中钢管桩的桩身压缩。4)Boltzmann数学模型是适合描述钢管桩桩顶荷载-桩顶沉降、桩身压缩-桩顶荷载这两类曲线的,它为桩身压缩及其极限值的预测、为桩顶极限承载力的判定提供了新方法。5)极限荷载与长径比成大致的负线性关系。     

Long-term creep-rupture failure envelope of epoxy

An accelerated testing methodology based on the time-temperature superposition principle has been proposed in the literature for the long-term creep strength of polymer matrices and polymer composites. Also, it has been suggested that a standard master curve may be a feasible assumption to describe the creep behavior in both tension and compression modes. In the present research, strength master curves for an aerospace epoxy (8552) were generated for tension and compression, by shifting strength data measured at various temperatures. The shift function is obtained from superposition of creep-compliance curves obtained at different temperatures. A standard master curve was presented to describe the creep-rupture of the polymer under tension and compression. Moreover, long-term creep-rupture failure envelopes of the polymer were presented based on a two-part failure criterion for homogeneous and isotropic materials. Ultimately, the approach presented allows the prediction of creep-rupture failure envelopes for a time-dependent material based on tensile strengths measured at various temperatures, considering that the ratio between tensile and compressive strengths is known.     

环形均布荷载作用下简支圆板的塑性极限分析

本文采用双剪统一屈服准则首次对受环形均布荷载作用下的简支圆板进行了塑性极限分析,得出了相应的统一解形式。已有的Tresca准则、Mises准则、双剪应力准则的解答是文中解答的特例或逼近,它可以适用于不同性状的拉压同性材料。用本文的解还可以推出多种荷载作用下简支圆板的塑性极限荷载。     

基于统一强度理论考虑拉压模量不同散体材料桩承载力计算

考虑岩土类材料拉压模量不同和应变软化特性,运用空间轴对称的统一强度理论分析了柱形孔扩张问题,推导出了圆孔扩张问题的应力场、位移场及最终扩张压力的统一解表达式,并在此基础上推导出了散体材料桩极限承载力计算公式。将该公式应用于某高速公路碎石桩复合地基中碎石桩极限承载力的计算,计算值与试验值吻合良好。最后,分析了不同拉压模量比、软化特性参数及其他计算参数对计算结果的影响。分析结果表明:采用传统弹性理论,不考虑拉压模量不同及应变软化的计算方法,会带来较大的误差。     

Off-axis strength differential effects in unidirectional carbon/epoxy laminates at different strain rates and predictions of associated failure envelopes

A difference between off-axis tensile and compressive strengths in a unidirectional carbon/epoxy laminate is examined at 100 °C for different fiber orientations and strain rates. By comparing their predictions with experimental results, the Tsai–Wu, Hoffman, Hashin–Rotem failure criteria that can distinguish between the off-axis strengths in tension and compression are evaluated for the accuracy of prediction of the off-axis strength differential (SD) effect and of the failure envelopes associated with off-axis loading at different strain rates. It is shown that the failure envelope associated with off-axis compression is unsuccessfully predicted by these failure criteria. The comparison suggests that the SD effects in the longitudinal, transverse and shear strengths should be taken into account for accurate prediction of the off-axis failure envelope. On the basis of this experimental implication, simple modifications to the representative failure criteria are attempted in which both the normal and shear SD effects are taken into account.     

Room‐Temperature Mechanical Properties of V20Nb20Mo20Ta20W20 High‐Entropy Alloy

Refractory high‐entropy alloys (HEAs) have shown promising high temperature strengths, while their mechanical behaviors at room temperature are rarely reported. In this work, the room‐temperature mechanical properties of V₂₀Nb₂₀Mo₂₀Ta₂₀W₂₀ refractory HEA under various different loading modes including tension, compression, bending, shear loading, and microhardness are investigated. The results show that this alloy exhibits very high compressive strength but quite low strengths under tension, bending, and shear loading, similar to the conventional brittle materials. However, pronounced plasticity and slip bands are observed in compression samples, and no indentation cracking is observed in low‐load microhardness tests, which indicate the potential ability of plastic deformation in this refractory HEA. The present work suggests that the microstructure or composition of this HEA should be carefully tailored before its practical usage to suppress its large tendency for cracking and eventually improve its ductility and strength under tension.

     

Microscopic failure mechanisms of fiber-reinforced polymer composites under transverse tension and compression

The mechanical behavior of unidirectional fiber-reinforced polymer composites subjected to tension and compression perpendicular to the fibers is studied using computational micromechanics. The representative volume element of the composite microstructure with random fiber distribution is generated, and the two dominant damage mechanisms experimentally observed – matrix plastic deformation and interfacial debonding – are included in the simulation by the extended Drucker–Prager model and cohesive zone model respectively. Progressive failure procedure for both the matrix and interface is incorporated in the simulation, and ductile criterion is used to predict the damage initiation of the matrix taking into account its sensitivity to triaxial stress state. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. And then the effects of interfacial properties on the damage behavior of the composites are assessed. It is found that the interfacial stiffness and fracture energy have relatively smaller influence on the mechanical behavior of composites, while the influence of interfacial strength is significant.