预应力混凝土结构抗爆性能试验研究

随着我国城市建设的飞速发展,人们迫切希望能将预应力混凝土结构用于平战结合的大跨度地下工程。但由于迄今为止国内外对预应力混凝土结构的抗爆性能知之甚少,长期以来预应力混凝土在抗爆结构中的应用一直是个禁区。本文给出了一例后张无粘结部分预应力混凝土梁板柱形结构模型在平面装药爆炸条件下的大比尺化爆相似模拟试验。模型的几何相似比例为1∶5,加载平面空气冲击波超压分别为70kPa、130kPa和320kPa。试验结果给出了在完全弹性状态、荷载略高于设计承载力状态和接近极限破坏状态下无粘结部分预应力混凝土结构的宏观试验现象和大量实测试验参数和实测波形曲线,并进行了简单的数据分析。试验结果表明:设计合理的预应力混凝土梁板柱形结构具有很好的延性,不会发生令人担心的脆性破坏,完全可以用于抗爆结构。     

Disk-shaped compact tension test for asphalt concrete fracture

A disk-shaped compact tension (DC(T)) test has been developed as a practical method for obtaining the fracture energy of asphalt concrete. The main purpose of the development of this specimen geometry is the ability to test cylindrical cores obtained from in-place asphalt concrete pavements or gyratory-compacted specimens fabricated during the mixture design process. A suitable specimen geometry was developed using the ASTM E399 standard for compact tension testing of metals as a starting point. After finalizing the specimen geometry, a typical asphalt concrete surface mixture was tested at various temperatures and loading rates to evaluate the proposed DC(T) configuration. The variability of the fracture energy obtained from the DC(T) geometry was found to be comparable with the variability associated with other fracture tests for asphalt concrete. The ability of the test to detect changes in the fracture energy with the various testing conditions (temperature and loading rate) was the benchmark for determining the potential of using the DC(T) geometry. The test has the capability to capture the transition of asphalt concrete from a brittle material at low temperatures to a more ductile material at higher temperatures. Because testing was conducted on ungrooved specimens, special care was taken to quantify deviations of the crack path from the pure mode I crack path. An analysis of variance of test data revealed that the prototype DC(T) can detect statistical differences in fracture energy resulting for tests conducted across a useful range of test temperatures and loading rates. This specific analysis also indicated that fracture energy is not correlated to crack deviation angle. This paper also provides an overview of ongoing work integrating experimental results and observations with numerical analysis by means of a cohesive zone model tailored for asphalt concrete fracture behavior.     

预应力混凝土水平加腋框架节点受力分析

本文对3个预应力混凝土框架节点进行了低周反复荷载作用下的抗震性能试验,研究了加腋节点的传力机理以及预应力对节点抗震性能的影响规律。研究表明,通过量测节点内的箍筋应变,穿过节点核心区的预应力筋对节点核心区双向交替斜向受压混凝土有重要约束作用,改善了节点的抗剪能力。梁端水平加腋节点中预应力筋不穿过节点核心区时,只要在节点核心区配置适量的箍筋,保持节点后期的强度和刚度,也能够满足抗震设防要求。     

预应力等效荷载的应用及非线性阶段计算方法

等效荷载概念在预应力结构的研究和设计中存在广泛应用．依据等效荷载的经典定义,对线性变换后预应力等效荷载是否变化进行论述,指出采用等效荷载法计算预应力混凝土受弯构件在使用阶段的预加力反拱值时,应取用净截面刚度．针对工程界提出的一些观点,用解析方法证明直线、抛物线组合线型力筋的预应力作用仍可用等效荷载法计算,以及等效荷载法并不存在与预应力筋线型相关的局限性．为回答无粘结、有粘结预应力筋应力增长后等效荷载是否变化的问题,提出实时等效荷载的概念,建立了预应力混凝土梁实时等效荷载的计算方法．推导基于实时等效荷载的承载力极限状态平衡方程,并证明其与将预应力筋完全作为抗力材料的平衡方程具有等价性．最后,介绍了实时等效荷载运用于预应力结构非线性阶段变形计算时所体现的优越性．     

Failure resistance of intermetallic matrix reinforced by wires

Composite materials with brittle matrices such as ceramics and intermetallic compounds have gained increased importance in application. Ceramics and intermetallic compounds possess unique heat-resistance at high temperatures. They are, however, vulnerable to brittle fracture. This problem can be overcome by reinforcing the intermetallic compounds with wires. NiAl-tungsten composite wire was manufactured by hot diffusion welding of alternate layers of the matrix and wires. These specimens were subjected to a three-point bending in the temperature range from 20° to 1000°C. Temperature dependence of the bending strength exhibited brittle to ductile transition behavior. At room temperature, unstable failure by bending is terminated in a stable fashion. Brittle fracture of the matrix and wire were observed. For text temperatures of 300°, 500° and 700°C, subcritical crack growth occurred where the matrix and wire showed brittle and ductile fracture, respectively. A pronounced necking of the specimen was observed as the temperature is increased. Substantial plastic deformation occurred when the test is performed at 1000°C. The critical stress intensity factor K_1c and specific work of fracture were measured and found to be two to three times larger than the intermetallic compounds without wire reinforcement.     

Multiscale modeling of crack initiation and propagation at the nanoscale

Fracture occurs on multiple interacting length scales; atoms separate on the atomic scale while plasticity develops on the microscale. A dynamic multiscale approach (CADD: coupled atomistics and discrete dislocations) is employed to investigate an edge-cracked specimen of single-crystal nickel, Ni, (brittle failure) and aluminum, Al, (ductile failure) subjected to mode-I loading. The dynamic model couples continuum finite elements to a fully atomistic region, with key advantages such as the ability to accommodate discrete dislocations in the continuum region and an algorithm for automatically detecting dislocations as they move from the atomistic region to the continuum region and then correctly “converting” the atomistic dislocations into discrete dislocations, or vice-versa. An ad hoc computational technique is also applied to dissipate localized waves formed during crack advance in the atomistic zone, whereby an embedded damping zone at the atomistic/continuum interface effectively eliminates the spurious reflection of high-frequency phonons, while allowing low-frequency phonons to pass into the continuum region.The simulations accurately capture the essential physics of the crack propagation in a Ni specimen at different temperatures, including the formation of nano-voids and the sudden acceleration of the crack tip to a velocity close to the material Rayleigh wave speed. The nanoscale brittle fracture happens through the crack growth in the form of nano-void nucleation, growth and coalescence ahead of the crack tip, and as such resembles fracture at the microscale. When the crack tip behaves in a ductile manner, the crack does not advance rapidly after the pre-opening process but is blunted by dislocation generation from its tip. The effect of temperature on crack speed is found to be perceptible in both ductile and brittle specimens.     

金属玻璃在不同温度下应力晶化行为的分子动力学模拟研究

金属玻璃的晶化对提高其力学性能尤其是延展性有重要意义．除了温度效应，大量文献证实应力是导致晶化的重要原因．本文通过分子动力学模拟，研究了金属玻璃铜在温度和应力两种因素联合作用下的晶化行为，分别研究了温度和应力两种因素的贡献．结果表明晶化比例随温度和应变的增大而增大，应变硬化与晶化比例之间存在正相关关系，证实了金属玻璃的应变硬化一定程度上由晶化引起．     

Time-temperature dependence in fracture behavior of high impact polystyrene

High Impact Polystyrene (HIPS) is one of the first toughened systems in which the brittle polystyrene becomes more ductile with the addition of an elastomer. However, it exhibits a ductile behavior only above a certain temperature and below a certain loading rate. Fracture in this material, like in most toughened systems, can become brittle when the temperature is lowered or the loading rate is increased. The correlation between temperature and loading rate seems to be controlled by the molecular relaxation according to the Arrhenius equation. The objective of this work is to foster the understanding of the effects of time and temperature on the fracture behavior of HIPS. The time and temperature dependence in fracture performance has been found to be governed by the strain energy density criterion. The theory allows prediction of fracture performance at various loading rates and temperatures. The brittle–ductile transition is controlled by an energy activation process. A peak in fracture energy always occurs at the transition region. This is attributed to the relaxation of the polymer macromolecules. The time and temperature dependence of this relaxation can be predicted by the Arrhenius equation. The rise in fracture energy at high loading speeds is not due to the higher frequency oscillations from dynamic effect but is controlled by the critical strain energy density.     

Discontinuous bifurcation analysis in fracture energy-based gradient plasticity for concrete

Conditions for discontinuous bifurcation in limit states of selective non-local thermodynamically consistent gradient theory for quasi-brittle materials like concrete are evaluated by means of both geometrical and analytical procedures. This constitutive formulation includes two internal lengths, one related to the strain gradient field that considers the degradation of the continuum in the vicinity of the considered material point. The other characteristic length takes into account the material degradation in the form of energy release in the cracks during failure process evolution.The variation from ductile to brittle failure in quasi-brittle materials is accomplished by means of the pressure dependent formulation of both characteristic lengths as described by Vrech and Etse (2009).In this paper the formulation of the localization ellipse for constitutive theories based on gradient plasticity and fracture energy plasticity is proposed as well as the explicit solutions for brittle failure conditions in the form of discontinuous bifurcation. The geometrical, analytical and numerical analysis of discontinuous bifurcation condition in this paper are comparatively evaluated in different stress states and loading conditions.The included results illustrate the capabilities of the thermodynamically consistent selective non-local gradient constitutive theory to reproduce the transition from ductile to brittle and localized failure modes in the low confinement regime of concrete and quasi-brittle materials.     

用试验室尺寸的试样预测混凝土及岩石结构的准脆性断裂

0Introduction Thefracturepropertiesofconcreteandrockmaterials,suchasfracturetoughnessKICand strengthfthavebeenofgreatinterestsformanyyearsbecauseoftheirimportantroleincontrolling thestructuralintegrityofvariousengineeringstructures[1-27].Varioustestingtec…     

混凝土梁三点弯曲断裂模式转变的近场动力学模拟

混凝土结构的宏观损伤开裂与其非均质微观结构紧密相关。底部带切口的混凝土梁在进行三点弯曲破坏时,随着切口的位置由梁中向梁边转移,裂纹由从切口处萌生并生长转变为从梁的中部萌生。本文采用半均质化近场动力学（IH-PD）模型和全均质化近场动力学（FH-PD）模型,分别对混凝土梁三点弯断裂问题进行模拟研究。IH-PD模型根据混凝土中骨料体积分数随机生成不同键的组合方式,将微观尺度的非均质性引入模型,无需详细描绘骨料形状和分布即可考虑混凝土非均质性。本文将IH-PD与FH-PD模型得到的断裂模式随切口位置的变化关系,与实验结果对比,分析微观结构对混凝土梁开裂的影响;基于非均质材料特征尺寸与IH-PD模型网格参数的相关性,模拟骨料大小对混凝土梁断裂模式的影响;另外,通过在IH-PD模型中设置预损伤的方式引入随机分布的孔隙,探讨孔隙率对混凝土断裂模式的影响。     

Laser speckle decorrelation as a noncontact method for brittle-ductile fracture differentiation

Quasi-static fracture in four-point-bend specimens of both brittle and ductile materials was examined using a method which required only photographic access to the specimen surface. Decorrelation of laser speckle patterns was used as a means to map out two-dimensional regions of high surface strain associated with crack propagation. ASTM A515 grade 70 steel was tested, at temperatures above and below its brittle-ductile transition temperature, by double-exposure speckle photography of the area ahead of the crack tip. The regions where the two speckle patterns were uncorrelated, determined by pointwise spatial filtering of the speckle interferograms, have been observed and are related to plastic deformation of the specimen surface near the crack tip. A subsequent comparison of the decorrelation zones resulting from brittle versus ductile states showed differences as expected in both zone size and shape.     

High temperature creep behaviour of an FeAl intermetallic strengthened by nanoscale oxide particles

The creep behaviour of an FeAl intermetallic strengthened by nanosized oxide particles has been examined at temperatures of 700–825 °C. For all temperatures the strain rate shows a power law dependence on the applied stress. At the lowest temperature and with the highest stresses there is evidence of a threshold stress produced by the difficulty of overcoming the particle barriers, while for higher temperatures as well as at low stresses there is no threshold stress and creep appears to be controlled by general climb. The fine oxide particles produce good strengthening at low temperatures but are more readily overcome at high temperatures due to their very small size and limited attractive relaxation force. Despite such fall in creep strength, this material remains one of the strongest iron aluminides to the temperature range evaluated.     

地冲击下新型脆断构件防护性能实验研究

为对地冲击作用下地下结构进行有效防护,提出一种泡沫混凝土材质的新型防护构件。与使用实心泡沫混凝土层的防护机理不同,本文中提出的构件在地冲击作用下,首先发生脆断破坏,然后破碎块体间搭接折断、挤压密实。通过构造设计,截断地冲击荷载,减弱荷载传递,改变被保护结构上的荷载形式。通过场地爆炸实验对比了不同防护措施下（无防护、实心泡沫混凝土层防护及新型构件防护）被保护结构的动力响应。实验结果表明:新型构件防护通过脆断、破碎块体的搭接、挤压密实表现出较实心泡沫混凝土层防护更好的防护效果;新型构件防护由于脆断特性,在较小荷载下即可显著削弱荷载传递,避免了实心泡沫混凝土层防护中负效果的出现;地冲击荷载较强时,构件防护层趋于压实,其防护效果逐渐接近实心泡沫混凝土层。     

On Publications on the Brittle Fracture Mechanics of Prestressed Materials

The paper analyzes new publications on the brittle fracture mechanics of prestressed materials. It is found out that new scientific results published in the International Journal of Solids and Structures in 2002 are a particular case of the results obtained at the Institute of Mechanics 20 years ago     

Dynamic fracture surface energy values and branching instabilities during rapid crack propagation in rubber toughened PMMA

An experimental device based on strip band geometry has been designed to explore the brittle behaviour of polymers during rapid crack propagation. The macroscopic crack speed is found to be quasi-constant along an entire RT-PMMA specimen, even in the case of crack branching and until arrest, if any. At the macroscopic branching velocity, the experimental fracture surface energy and the fracture surface roughness have no single values in RT-PMMA. In fact, the macroscopic fracture surface energy increases with the number of instabilities or frustrated micro-branches.     

The effect of crack orientation on fracture behavior of tantalum by multiscale simulation

The quasicontinuum (QC) multiscale method is used to investigate anisotropic fracture behaviors of body-centered cubic (BCC) rare metal tantalum (Ta) loaded in Mode I and different fracture mechanisms are discussed from nanoscopic to continuum perspectives to have a deep understanding of brittle and ductile fracture. Initial crack deflection, brittle fracture by cleaving along low surface energy plane, ductile fracture as a result of dislocation emission and fracture accompanied by deformation twinning are all observed near crack tips of different crystal orientations. Particularly, some of these fracture mechanisms are found to be consistent with the latest experimental results. By examining different fracture behaviors, we find the surface energy and the available slip planes play a combined role in determining the fracture mechanisms near a crack tip. Both isotropic and anisotropic critical stress intensity factors are derived and compared for different crack orientations. A straightforward criterion that is proved to be applicable is used to distinguish brittle fracture from ductile fracture.     

高温空气下C/SiC复合材料断裂韧性实时测试和微观结构表征分析

为了研究高温空气下C/SiC复合材料断裂韧性和微观结构,采用单边切口梁三点弯曲法实时测试了C/SiC复合材料在高温空气下的断裂韧性,并采用电子扫描显微镜 (scanning electron microscope,SEM)和X 射线衍射分析仪 (X-ray diffraction, XRD)分析了复合材料在不同温度下的破坏断口和失效机制。研究结果表明随测试温度升高,C/SiC复合材料断裂韧性降低,材料的断裂形式由脆性断裂逐渐演变成塑性断裂。从室温升温到1 000 ℃测试温度条件下,C/SiC复合材料的断裂韧性由12.5 MPa·m1/2降低为10.96 MPa·m1/2,降幅仅为12%,C/SiC复合材料高温断裂韧性良好。不同温度下,材料呈现出不同形式的断裂形貌。常温下断口形貌主要可以看到纤维拔出的现象,随着温度的升高,该现象基本消失,断裂截面变得更平整,材料的强度主要取决于基体的强度。     

脆性材料破坏过程分析的数值试验方法

文中运用作者新近开发的材料破坏过程分析MFPA2D系统,以岩石、混凝土等非均匀脆性材料的破坏过程分析为例,说明了数值模拟方法给脆性材料破坏理论发展所带来的契机,并简述了MFPA2D在煤层移动、地下工程稳定性、地震孕育机制,以及复合材料破坏问题研究中的应用前景．