Effect of Coarse Aggregate Size on and Crack Opening in Normal and Relationship between Stress High Strength Concretes

Fine and coarse aggregates play an important role in the fracture of concrete. However, quantitative information available on the effect of the coarse aggregate size on the fracture properties of concrete is still limited. In the present paper, the effect of coarse aggregate size （single grade of 5～10, 10～16, 16～20 and 20～25 mm） on stress-crack opening （σ-w） relation in normal and high strength concretes （compressive strength of 40 and 80 MPa, respectively） was studied. The investigation was based on three-point bending tests implemented by fictitious crack analysis. The result shows that coarse aggregate size and cement matrix strength significantly influence the shape of σ-w curve. For a given total aggregate content, in normal strength concrete, smaller size of aggregate leads to a high tensile strength and a sharp stress drop after the peak stress. The smaller the coarse aggregate, the steeper the σ-w curve. By contrast, in high strength concrete, the effect of aggregate size on σ-w relation almost vanishes. A similar σ-w relation is obtained for the concrete except for the case of 20～25 mm coarse aggregate size. The stress drop after the peak stress is more significant for high strength concrete than that for normal strength concrete. Meanwhile, the smaller the coarse aggregate size, the higher the flexural strength. Fracture energy and characteristic length increase with increasing coarse aggregate size in both normal and high strength concretes.

Mechanical properties of AISI 1045 ceramic coated materials by nano indentation and crack opening displacement method

An effective approach was conducted for estimating fracture toughness using the crack opening displacement (COD) method for plasma enhanced chemical vapor deposition (PECVD) coating materials.For this evaluation,an elastoplastic analysis was used to estimate critical COD values for single edge notched bending (SENB) specimens.The relationship between fracture toughness (KIC) and critical COD for SENB specimens was obtained.Microstructure of the interface between Al2O3-TiO2 composite ceramic coatings and AISI 1045 steel substrates was studied by using scanning electron microscope (SEM).Chemical compositions were clarified by energy-dispersive X-ray spectroscopy (EDS).The results show that the interface between of Al2O3-TiO2 and substrate has mechanical combining.The nanohardness of the coatings can reach 1 200 GPa examined by nanoindentation.The KIC was calculated according to this relationship from critical COD.The bending process produces a significant relationship of COD independent of the axial force applied.Fractographic analysis was conducted to determine the crack length.From the physical analysis of nanoindentation curves,the elastic modulus of 1045/Al2O3-TiO2 is 180 GPa for the 50 μm film.The highest value of fracture toughness for 1045/Al2O3-TiO2-250 μm is 348 MPa·ml/2.

Mathematical Model of a Hyperbolic Hydraulic Fracture with Tortuosity

The aim of the research is to study the propagation of a hydraulic fracture with tortuosity due to contact areas between touching asperities on opposite crack walls. The tortuous fracture is replaced by a model symmetric partially open fracture with a hyperbolic crack law and a modified Reynolds flow law. The normal stress at the crack walls is assumed to be proportional to the half-width of the model fracture. The Lie point symmetry of the nonlinear diffusion equation for the fracture half-width is derived and the general form of the group invariant solution is obtained. It was found that the fluid flux at the fracture entry cannot be prescribed arbitrarily, because it is determined by the group invariant solution and that the exponent n in the modified Reynolds flow power law must lie in the range 2 < n < 5. The boundary value problem is solved numerically using a backward shooting method from the fracture tip, offset by 0 < δ ≪ 1 to avoid singularities, to the fracture entry. The numerical results showed that the tortuosity and the pressure due to the contact regions both have the effect of increasing the fracture length. The spatial gradient of the half-width was found to be singular at the fracture tip for 3 < n < 5, to be finite for the Reynolds flow law n = 3 and to be zero for 2 < n < 3. The thin fluid film approximation breaks down at the fracture tip for 3 < n < 5 while it remains valid for increasingly tortuous fractures with 2 < n < 3. The effect of the touching asperities is to decrease the width averaged fluid velocity. An approximate analytical solution for the half-width, which was found to agree well with the numerical solution, is derived by making the approximation that the width averaged fluid velocity increases linearly with distance along the fracture.

CLOSED-FORM WEIGHT FUNCTION FOR EDGE CRACK PROBLEMS

An accurate crack surface opening displacement formula for edge cracks in finite bodies has been derived,and was subsequently used to develop wide-range closed-form weight functions for edge cracks. The analytical weight function is general and can be easily used to determine accurate stress intensity factors and crack surface opening displacements for arbitrarily loaded edge cracks with high efficiency.Examples have been given for problems of edge crack(s)with a number of typical load conditions for illustration.

PERIODICAL INTERFACIAL CRACKS IN ANISOTROPIC ELASTOPLASTIC MEDIA

By using Fourier transformation the boundary problem of periodical interfacial cracks in anisotropic elastoplastic bimaterial was transformed into a set of dual integral equations and then it was further reduced by means of definite integral transformation into a group of singular equations. Closed form of its solution was obtained and three corresponding problems of isotropic bimaterial, of a single anisotropic material and of a bimaterial of isotropy- anisotropy were treated as the specific cases. The plastic zone length of the crack tip and crack openning displacement ( COD) decline as the smaller yield limit of the two bonded materials rises, and they were also determined by crack length and the space between two neighboring cracks . In addition , COD also relates it with moduli of the materials .

Simulating hydraulic fracturing processes in laboratory-scale geological media using three-dimensional TOUGH-RBSN

In this context, recent developments in the coupled three-dimensional(3 D) hydro-mechanical(HM)simulation tool TOUGH-RBSN are presented. This tool is used to model hydraulic fracture in geological media, as observed in laboratory-scale tests. The TOUGH-RBSN simulator is based on the effective linking of two numerical methods: TOUGH2, a finite volume method for simulating mass transport within a permeable medium; and a lattice model based on the rigid-body-spring network(RBSN) concept. The method relies on a Voronoi-based discretization technique that can represent fracture development within a permeable rock matrix. The simulator provides two-way coupling of HM processes, including fluid pressure-induced fracture and fracture-assisted flow. We first present the basic capabilities of the modeling approach using two example applications, i.e. permeability evolution under compression deformation, and analyses of a static fracturing simulation. Thereafter, the model is used to simulate laboratory tests of hydraulic fracturing in granite. In most respects, the simulation results meet expectations with respect to permeability evolution and fracturing patterns. It can be seen that the evolution of injection pressure associated with the simulated fracture developments is strongly affected by fluid viscosity.

Engineering critical assessment on welding flaws of the X65 UOE girth weld based on CRACKWISE

Focused on the girth weld of a φ762 x 28.6 mm （X65） offshore UOE pipe, the fracture toughness properties at different weld joint positions were tested. Meanwhile,the largest size of different cracks under service condition was calculated by using CRACKWISE. The service life of the UOE pipe with postulated crack-like flaws was calculated by considering various fatigue factors, such as vibrations caused by ocean current and fluctuations of inner pressure. The assessment process and results can be used to direct the repair of weld flaws laid in pipes or to assess the reliability of in-serving pipes.

A Straightforward Direct Traction Boundary Integral Method for Two-Dimensional Crack Problems Simulation of Linear Elastic Materials

This paper presents a direct traction boundary integral equation method(DTBIEM)for two-dimensional crack problems of materials.The traction boundary integral equation was collocated on both the external boundary and either side of the crack surfaces.The displacements and tractions were used as unknowns on the external boundary,while the relative crack opening displacement(RCOD)was chosen as unknowns on either side of crack surfaces to keep the single-domain merit.Only one side of the crack surfaces was concerned and needed to be discretized,thus the proposed method resulted in a smaller system of algebraic equations compared with the dual boundary element method(DBEM).A new set of crack-tip shape functions was constructed to represent the strain field singularity exactly,and the SIFs were evaluated by the extrapolation of the RCOD.Numerical examples for both straight and curved cracks are given to validate the accuracy and efficiency of the presented method.

聚乙烯醇纤维混凝土动态断裂过程试验研究

为探究纤维体积掺量对聚乙烯醇纤维增强水泥基复合材料(PVA-ECC)断裂过程的影响,基于50 mm的分离式霍普金森压杆(SHPB)装置对不同纤维体积掺量(0%、0.75%、1.50%、2.25%、3.00%)的PVA-ECC中心切槽半圆盘弯曲(NSCB)试件进行冲击试验,同时结合超高速数字图像(DIC)相关试验系统对PVA-ECC材料的动态断裂过程进行试验研究,得到了预制裂纹尖端张开位移的变化规律以及各组试件的临界裂缝尖端张开位移(CTODC)。结果表明,当不添加PVA纤维或添加较少(小于1.50%)时,裂尖宏观裂纹基本出现在裂尖荷载的峰值时刻处,而随着PVA纤维掺量的增加,裂尖宏观裂纹的出现显著早于裂尖荷载的峰值时刻,并且纤维体积掺量越大,裂尖宏观裂纹出现得越早,裂纹扩展至完全断裂的时间也显著增加。添加聚乙烯醇纤维可以显著提高混凝土试件的CTODC值,提高试件的阻裂能力,相同冲击荷载下,体积掺量为2.25%的聚乙烯醇纤维试件具有较大的CTODC值。

高糖浓度对软骨断裂力学性能的影响

高糖环境会对骨节关炎的发病机制、病理进展及治疗结果均造成不可逆的影响。同时,软骨产生裂纹及缺损破坏是骨关节炎的主要病因。通过单轴拉伸试验,研究了不同糖浓度对裂纹软骨断裂性能的影响。结果表明:随着细胞外糖浓度的增加,对关节的软骨细胞及胶原纤维产生相应的影响。糖化会使软骨产生明显的基质硬化现象,含裂纹软骨的弹性模量随糖化程度的增大而增大,同时,其力学性能明显降低。

国内外标准中裂纹尖端张开位移的计算结果差异

对12Cr1MoV钢板进行了不同试验温度下的断裂韧度测试,再按照3个标准分别计算了裂纹尖端张开位移,并分析其差异。结果表明:按照ASTM E1820—2020b和GB/T 21143—2014标准计算得到的裂纹尖端张开位移较小,而按照ISO 12135:2021标准计算得到的裂纹尖端张开位移较大。当材料的断裂韧性计算结果在裂纹尖端张开位移的最小允许值附近时,计算依据的标准不同,会导致判定结论的不一致。

裂缝参数对储层渗透率和孔隙度影响实验

中国致密油藏和特低渗透气藏的油气产量近年增加很快,多裂缝或缝网等大型压裂成为开采此类油气藏的重要手段。为了深入了解裂缝对油气渗流的影响,优化裂缝长度和开度等参数,以油藏工程理论为指导,以物理模拟为技术手段,开展了裂缝对致密油藏和特低渗透气藏储层渗透率和孔隙度影响实验研究。结果表明:随裂缝贯穿程度和开度增加,岩心孔隙度增大,但增幅较小;与孔隙度相比较,裂缝可明显提高岩心渗透性;当裂缝贯穿程度超过60%时,渗透率开始明显增大,当贯穿程度达到100%时,裂缝岩心渗透率要比基质岩心渗透率高10~1 000倍。

裂纹尖端张开角及在输气管线止裂预测中的应用

介绍了一种近年来新发展的输气管线断裂控制参量———裂纹尖端张开角 (CTOA)的计算模型及方法。国际上计算输气管线动态断裂与止裂的PFRAC软件中视裂纹扩展速度为常数。针对这一问题 ,利用Alliance管线的实物爆破试验结果 ,通过分析和模拟提出了一种新的输气管线裂纹扩展速度的衰减模式 ,并引入到PFRAC计算程序中。采用改进的计算方法和程序 ,结合西气东输管线 ,研究了CTOA与止裂长度的关系 ,分析了输送压力、管材壁厚等因素对西气东输管线动态断裂及止裂的影响。

基于弥散与分离裂缝模型的混凝土开裂比较研究

裂缝扩展是影响混凝土结构非线性响应的重要因素,其扩展深度与张开位移是评价结构安全性的重要指标。基于弥散裂缝框架,该文建立了等效裂缝张开位移与损伤因子、断裂带宽度之间的函数关系。分别采用弥散、分离两类裂缝模型模拟I型断裂和I-II混合型断裂试验,二者在结构承载力、裂缝扩展和裂缝张开位移方面均获得了基本一致的结果。数值算例表明弥散裂缝模型与分离裂缝模型计算精度接近但其效率更高。

三点弯曲梁法研究砼双K断裂参数及其尺寸效应

本文采用三点弯曲梁试件利用在试验中测得的最大荷载 Pm ax及对应的裂缝口张开位移CMODc等参数 ,根据虚拟裂缝模型求得了砼裂缝亚临界扩展量 Δac、起裂断裂韧度 KiniIC、失稳断裂韧度 KunIC(即双 K断裂参数 )及临界裂缝尖端张开位移 CTODc。结果表明 ,砼双 K断裂参数及 CTODc均与试件尺寸无关 ,可以作为描述砼结构裂缝的起裂、稳定扩展及失稳断裂全过程的材料常数 ,而且试验方法简单 ,无需加卸载过程且在普通实验室即可实现。

钢纤维高强混凝土断裂能及裂缝张开位移

通过对钢纤维高强混凝土和高强混凝土试件的楔劈拉伸试验,探讨了钢纤维体积率(φf)和切口相对深度对钢纤维高强混凝土断裂能(GfF)和临界裂缝张开位移(cδ)的影响。结果表明:随着φf的增加,GfF、临界裂缝嘴张开位移(fδmc)、临界裂缝尖端张开位移fδtc均线性增加,钢纤维高强混凝土断裂能增益比、临界裂缝嘴张开位移和临界裂缝尖端张开位移的增益比亦基本呈线性增加。随着切口深度的增加,GfF和高强混凝土断裂能GF都有不同程度的减小。相对于高强混凝土,切口深度变化对GfF的影响较小。在试验的基础上,提出了GfF和Qfmc的计算公式。

混凝土三点弯曲梁裂缝断裂全过程数值模拟研究

考虑裂缝黏聚力的作用,基于Paris位移公式推导出混凝土三点弯曲梁裂缝扩展过程中断裂过程区上的裂缝张开位移的解析表达式。采用起裂韧度作为裂缝起裂及扩展的判断标准,提出了荷载作用下混凝土裂缝起裂、扩展及失稳破坏全过程的数值模拟方法,并分别与国内外断裂试验实测值及有限元计算值进行了比较。结果表明,本文提出的数值模拟方法形式简单且精度较好。

渗透水压作用下类岩石材料张开型裂纹启裂特性研究

在探讨渗透水压和远场应力共同作用下张开型裂纹的启裂规律及裂纹尖端应力强度因子的演化规律的基础上,建立压剪应力场和渗流场共同作用下含预置裂纹类岩石材料的损伤断裂力学模型和裂纹尖端应力强度因子演化方程,提出运用裂纹尖端应力强度因子作为判断压剪岩石裂纹的启裂准则。研究结果表明:张开型裂纹尖端应力强度因子受围压、渗透水压力、裂纹尖端曲率半径以及裂纹倾角等因素的影响;裂纹启裂角随预制裂纹角度的变化不大,其值约为70.5°;裂纹启裂强度与渗透水压力、裂纹长度、裂纹尖端曲率半径成反比,与围压的大小成正比,此外还与裂纹倾角有关。算例验证表明,运用不同的断裂判断准则均可得出岩石裂纹初裂强度随渗透水压力的增大而呈减小的趋势。且进一步的试验也验证了启裂强度与渗透水压成反比而与围压成正比;当裂隙角度为30°时裂纹启裂强度最大,60°次之,45°最小。提高渗透水压可显著降低张开型裂纹的启裂强度,这一结果可为深部高应力岩体诱导破裂提供新的思路。

Stanstead花岗岩动态断裂性能

采用一种新的方法研究Stanstead花岗岩的动态断裂性能,包括起裂韧度、断裂能、传播韧度和裂纹传播速度。该方法采用分离式霍普金森压杆加载的带预制裂纹的半圆盘三点弯试样,同时采用激光位移计监测试样的裂纹面张开位移。在动态力平衡的条件下,起裂韧度由准静态公式计算得到。通过裂纹面张开位移数据推算出2个碎片的残余动能,从而计算出平均传播断裂能和传播韧度。裂纹传播平均速度由黏接在试样上的一系列裂缝计测量得到。试验结果表明,该花岗岩的起裂韧度和传播韧度都与加载速率有关,传播韧度大于起裂韧度,传播韧度随着裂纹传播速度的提高而提高。通过裂纹传播速度和传播韧度的关系拟合得到材料的止裂韧度及裂纹传播极限速度。得到的Stanstead花岗岩与Laurentian花岗岩结果对比表明,Stanstead花岗岩颗粒较大,起裂、止裂韧度较小;Laurentian花岗岩颗粒较小,传播韧度较小,裂纹传播极限速度较大,裂纹容易传播。

基于虚拟裂缝模型的混凝土双K断裂参数

本文将虚拟裂缝模型与线弹性断裂力学相结合，利用楔入劈拉试件在实验中测得的最大荷载Ｐｍａｘ及对应的裂缝口张开位移ＣＭＯＤｃ，求得了混凝土裂缝亚临界扩展量Δａｃ的解析解，据此计算了最大尺寸为７００×７００×２００ｍｍ的楔入劈拉混凝土试件的起裂断裂韧度ＫｉｎｉＩｃ、失稳断裂韧度ＫｕｎＩｃ及临界裂缝尖端张开位移ＣＴＯＤｃ，并与试验结果进行了比较．研究结果表明，当试件高度ｈ＞４００ｍｍ时，ＫｉｎｉＩｃ、ＫｕｎＩｃ、ＣＴＯＤｃ是一个与试件高度无关的断裂参数．