An improved strain-softening constitutive model of granite considering the effect of crack deformation

This paper presents an improved strain-softening constitutive model considering the effect of crack deformation based on the triaxial cyclic loading and unloading test results.The improved model assumes that total strain is a combination of plastic,elastic,and crack strains.The constitutive relationship between the crack strain and the stress was further derived.The evolutions of mechanical parameters,i.e.strength parameters,dilation angle,unloading elastic modulus,and deformation parameters of crack,with the plastic strain and confining pressure were studied.With the increase in plastic strain,the cohesion,friction angle,dilation angle,and crack Poisson's ratio initially increase and subsequently decrease,and the unloading elastic modulus and the crack elastic modulus nonlinearly decrease.The increasing confining pressure enhances the strength and unloading elastic modulus,and decreases the dilation angle and Poisson's ratio of the crack.The theoretical triaxial compressive stress-strain curves were compared with the experimental results,and they present a good agreement with each other.The improved constitutive model can well reflect the nonlinear mechanical behavior of granite.

Fatigue Life Evaluation Method for Foundry Crane Metal Structure Considering Load Dynamic Response and Crack Closure Effect

To compensate for the shortcomings of quasi-static law in anti-fatigue analysis of foundry crane metal structures,the fatigue life evaluation method of foundry crane metal structure considering load dynamic response and crack closure effect is proposed.In line with the theory of mechanical vibration,a dynamic model of crane structure during the working cycle is constructed,and dynamic coefficients under diverse actions are analysed.Calculation models of the internal force dynamic change process of dangerous cross-sections and a simulation model of first principal stress-time history are established by using the steel structure design criteria,which is utilised to extract the change of first principal stress of danger points over time.Then,the double-parameter stress spectrum is obtained by the rain flow counting method.The fatigue life calculation formula is corrected by introducing a crack closure parameter that can be calculated by the stress ratio and the effective stress ratio.Under the finite element model imported into Msc.Patran,crack propagation analysis is performed by the growth method in the fatigue integration module Msc.Fatigue.Taking the metal structure of a 100/40t-28.5m foundry crane with track offset as an example,the accuracy of calculation results and the feasibility and applicability of the proposed method are verified by theoretical calculation and finite element simulation,which provide a theoretical basis for improvement of the fatigue resistance design of foundry cranes.

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack

Shielding effect and emission criterion of a screw dislocation near an interracial blunt crack are dealt with in this paper. Utilizing the conformal mapping technique, the closed-form solutions are derived for complex potentials and stress fields due to a screw dislocation located near the interracial blunt crack. The stress intensity factor on the crack tips and the critical stress intensity factor for dislocation emission are also calculated. The influence of the orientation of the dislocation and the morphology of the blunt crack as well as the material elastic dissimilarity on the shielding effect and the emission criterion is discussed in detail. The results show that positive screw dislocations can reduce the stress intensity factor of the interfacial blunt crack tip （shielding effect）. The shielding effect increases with the increase of the shear modulus of the lower half-plane, but it decreases with the increase of the dislocation azimuth angle. The critical loads at infinity for dislocation emission increases with the increase of emission angle and curvature radius of blunt crack tip, and the most probable angle for screw dislocation emission is zero. The present solutions contain previous results as special cases.

Influence of Mineral Admixtures on the Electrodeposition Healing Effect of Concrete Cracks

Two types of solutions（Zn SO4, Mg SO4） were selected to study the influence of mineral admixtures on the electro-deposition healing effect of concrete cracks. Four parameters（i e, rates of weight gain, surface coating, crack closure and crack filling depth） were measured. The mineral composition of electro-deposits in the cracks was analyzed. The study shows that the healing effect of mortar specimens with 10% fly ash is the worst, while the healing effect of mortar specimens with 20% fly ash is better than that of the specimens without fly ash. The rates of weight gain, surface coating, crack closure and crack filling depth decrease with increasing content of the ground granulated blast-furnace slag（GGBS）. The mineral admixtures have no influence on the composition of deposits.

DETERMINATION OF CLOSURE EFFECT IN FATIGUE CRACKING BY MEANS OF COMPLIANCE TECHNIQUE AND NUMERICAL METHOD

The crack tip strain gauge method in the compliance technique was used to determine the opening load of notched crack of axle steel,and the nonlinear finite element ADINA program, to which the cyclic stress-strain curve of axle steel was applied,was used to analyze the stress-strain field ahead of the crack tip and the opening load of notched crack.The results of both the compliance technique and the numerical method were in good agreement.In this pa- per,the concept of the sensitive point is proposed and the key to the determination of the crack opening load in the experiment is to place a strain gauge at sensitive point.It is certified by both experimental and numerical methods that the sensitive point has the best linear relation- ship character and the value of strain is much greater.

MODE Ⅰ AND MODE Ⅱ CRACK TIP ASYMPTOTIC FIELDS WITH STRAIN GRADIENT EFFECTS

The strain gradient effect becomes significant when the size of frac- ture process zone around a crack tip is comparable to the intrinsic material length l, typically of the order of microns. Using the new strain gradient deformation theory given by Chen and Wang, the asymptotic fields near a crack tip in an elastic-plastic material with strain gradient effects are investigated. It is established that the dom- inant strain field is irrotational. For mode Ⅰ plane stress crack tip asymptotic field, the stress asymptotic field and the couple stress asymptotic field can not exist si- multaneously. In the stress dominated asymptotic field, the angular distributions of stresses are consistent with the classical plane stress HRR field; In the couple stress dominated asymptotic field, the angular distributions of couple stresses are consistent with that obtained by Huang et al. For mode Ⅱ plane stress and plane strain crack tip asymptotic fields, only the stress-dominated asymptotic fields exist. The couple stress asymptotic field is less singular than the stress asymptotic fields. The stress asymptotic fields are the same as mode Ⅱ plane stress and plane strain HRR fields, respectively. The increase in stresses is not observed in strain gradient plasticity for mode Ⅰ and mode Ⅱ, because the present theory is based only on the rotational gradi- ent of deformation and the crack tip asymptotic fields are irrotational and dominated by the stretching gradient.

Self-catalyzed Effect and Cracking Risk in Mass Concrete Containing Micro-slag

The main results obtained from the experimental and engineering investigation on the heat evolution and cracking risk of a furnace concrete block were presented. The heat evolution of experimental mortars containing micro-slag under different environmental temperatures was instrumented in order to investigate the self-catalyzed effect, which was discovered in engineering. More-over,the thermal stress of the furnace concrete due to heat temperature rise was calculated to evaluate the cracking risk of mass concrete containing micro-slag due to self-catalyzed effect. The experimental results illustrate that with the development of hydration and initial temperature of mixture, the hydra-tion can be also accelerated and temperature of concrete will be continued to rise, which was the self-catalyzed effect. And the thermal stress due to self-catalyzed effect could not result in the cracking of furnace concrete.

EFFECTIVE ELASTIC MODULI OF AN INHOMOGENEOUS MEDIUM WITH CRACKS

In the present paper, the effective elastic moduli of an inhomogeneous medium with cracks are derived and obtained by taking into account its microstructural properties which involve the shape, size and distribution of cracks and the interaction between cracks. Numerical results for the periodic microstructure of different dimensions are presented. From the results obtained, it can be found that the distribution of cracks has a significant effect on the effective elastic moduli of the material.

The Effects of Fatigue Cracks on Fastener Loads during Cyclic Loading and on the Stresses Used for Crack Growth Analysis in Classical Linear Elastic Fracture Mechanics Approaches

High strength threaded fasteners are widely used in the aircraft industry, and service experience shows that for structures where shear loading of the joints is significant, like skin splices, fuselage joints or spar caps-web attachments, more cracks are initiated and grow from the edges of the fastener holes than from features like fillets radii and corners or from large access holes. The main causes of this cracking are the stress concentrations introduced by the fastener holes and by the threaded fasteners themselves, with the most common damage site being at the edge of the fastener holes. Intuitively, it is easy to visualize that after the crack initiation, during the growth stages, some of the load transferred initially by the fastener at the cracked hole will decrease, and it will be shed to the adjacent fasteners that will carry higher loads than in uncracked condition. Using currently available computer software, the method presented in this paper provides a relatively quick and quantitatively defined solution to account for the effects of crack length on the fastener loads transfer, and on the far field and bypass loads at each fastener adjacent to the crack. At each location, these variations are determined from the 3-dimensional distribution of stresses in the joint, and accounting for secondary bending effects and fastener tilt. Two cases of a typical skins lap splice with eight fasteners in a two rows configuration loaded in tension are presented and discussed, one representative for wing or fuselage skins configurations, and the second case representative for cost effective laboratory testing. Each case presents five cracking scenarios, with the cracks growing from approx. 0.03 inch to either the free edge, next hole or both simultaneously.

Shielding effect of a nano-circular inclusion acting on semi-infinite wedge cracks

The model of a screw dislocation near a semi-infinite wedge crack tip inside a nano-circular inclusion is proposed to investigate the shielding effect of nano inclusions acting on cracks. Utilizing the complex function method, the closed-form solutions of the stress fields in the matrix and the inclusion region are derived. The stress intensity factor, the image force, as well as the critical loads for dislocation emission are discussed in detail. The results show that the nano inclusion not only enhances the shielding effect exerted by the dislocation, but also provides a shielding effect itself. Moreover, dislocations may be trapped in the nano inclusion even if the matrix is softer than the inclusion. This helps the dislocation shield crack, and reduces the dislocation density within the matrix.

The Performance of SUF High Effective Concrete Crack-patching Material

The physical properties and microstructure of SUF are investigated to develop a highly effective cement matrix crack-patching material for concrete cracks. The SEM and XRD determination of hardened SUF shows that the microstructure of SUF is dense and compact with a lot of C-S-H gels and ettringite. Also, the mechanism of shrinkage compensating is discussed.

Effect of Pulse Parameters on Deposition in Concrete Crack using Pulse Current Electro-deposition

To evaluate the effect of pulse parameters on the formation of electrodeposits in concrete cracks, five different types of pulse current were set up, and ZnSO_4 and MgSO_4 solutions were used as the electrolytes. The rate of weight gain, rate of surface coating, rate of crack closure and crack filling depth were measured. Scanning electron microscopy was used to assess the morphology of the electrodeposits, and energy dispersive spectroscopy was used to analyze the mineral composition of the electrodeposits in the cracks. The experimental results demonstrate that, among five different pulse parameters, when T_（on）/T_（off）=0.8 ms/0.8 ms, the healing effect of electro-deposition is the best. The pulse mode hardly affects the mineral composition of the electrodeposits but changes the micromorphology. In addition, for both ZnSO_4 and MgSO_4 solutions, when T_（on）/T_（off）=0.8 ms/0.8 ms, the crystal structure of the electrodeposits is the most uniform and the densest.

MODE Ⅰ CRACK TIP FIELD WITH STRAIN GRADIENT EFFECTS

A plane strain mode 1 crack tip field with strain gradient effects is investigated.A new strain gradient theory is used.An elastic-power law hardening strain gradient material is considered and two hardening laws,i.e.a separation law and an integration law are used respectively.As for the material with the separation law hardening,the angular distributions of stresses are consistent with the HRR field,which differs from the stress results;the angular distributions of couple stresses are the same as the couple stress results.For the material with the integration law hardening,the stress field and the couple stress field can not exist simultaneously,which is the same as the conclusion,but for the stress dominated field,the an- gular distributions of stresses are consistent with the HRR field;for the couple stress dominated field,the an- gular distributions of couple stresses are consistent with those in Ref.However,the increase in stresses is not observed in strain gradient plasticity because the present theory is based on the rotation gradient of the deformation only,while the crack tip field of mode 1 is dominated by the tension gradient,which will be shown in another paper.

积石山M6.2地震诱发秦阴村黄土震陷灾害研究

2023年12月18日在甘肃省临夏州积石山县发生的M6.2地震诱发了多处黄土震陷灾害。通过对积石山县石塬镇秦阴村黄土震陷灾害进行现场调查,得出灾害的主要特征;利用原状黄土开展室内试验,研究秦阴村黄土的物性参数特征、微结构特征和震陷性,结合现场调查结果分析震陷灾害产生的主要原因。研究结果表明:秦阴村震陷区内土体酥裂,裂缝发育,边缘土体坍落或产生滑坡;土层呈反倾台阶状震裂破坏,最大沉降量57 cm,裂缝最大长度约300 m,宽度约83 cm。震陷区黄土土质疏松、含水率较高、架空孔隙发育、颗粒胶结较弱,具备产生震陷的土性条件。震陷区黄土层厚度较大,地震的等效动应力大于土体产生震陷的临界动应力,且地震动持续时间较长、卓越周期较大,使得场地产生了严重的震陷破坏。

不同钙源对微生物自修复混凝土裂缝的修复效果及其机理分析

本研究在模拟裂缝区溶液中,研究了不同钙源下溶液中的Ca^(2+)浓度和钙沉淀能力的变化。制备了微生物自修复砂浆试件,研究含不同钙源的微生物修复剂对砂浆试件强度的影响;以面积修复率和吸水率来评价0.25~0.40 mm裂缝的自修复效果,并研究了裂缝的修复方向。此外,通过扫描电镜和X射线衍射仪分析了砂浆试件裂缝区的修复产物,并详细探讨了不同钙源对砂浆裂缝的自修复机理。研究结果表明,在不同钙源下,枯草芽孢杆菌的矿化机理是不同的,造成了溶液中Ca^(2+)浓度和钙沉淀能力的差异,并且钙源也会影响微生物自修复砂浆的抗折抗压强度。当钙源为乳酸钙时,砂浆裂缝的修复效果最好,其次是醋酸钙和硝酸钙。在裂缝修复过程中,修复产物从裂缝表面往裂缝内部生成,因而随着裂缝深度的增加,修复产物的生成量逐渐降低,并且砂浆试件裂缝处的白色物质主要是方解石碳酸钙。本研究证明了微生物诱导碳酸钙沉积(MICP)技术可以较好地应用于水泥基材料裂缝的自修复,有利于推动微生物自修复混凝土的发展与进步。

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to Stress Corrosion Cracking (SCC) and General Corrosion

This work investigates the relative aggressiveness of nitrate solutions at different pH values on mild steel towards stress corrosion cracking (SCC) and general corrosion. Electrochemical behavior and stress corrosion cracking sus-ceptibility measurements were carried out in 52 Wt% ammonium nitrate solutions at 368° K and various pH values ranging from 0.77 to 9.64. Constant load stress corrosion test at 90% yield stress was conducted. Tested specimens were prepared and examined using the scanning electron microscope (SEM). The potentiodynamic polarization curves for different pH values again emphasized the validity of the gravimetric measurements and hence the mechanism of cracking was attributed to the stress that assisted the dissolution process.

地震放大作用下裂缝边坡稳定性极限分析及影响规律研究

大量试验数据和震后调查表明,地震加速度沿坡高具有显著放大效应并导致产生坡顶裂缝。为探究放大效应对边坡稳定性和坡顶裂缝的影响,基于极限分析上限法和拟静力法,建立考虑放大效应的裂缝边坡稳定性分析方法,得到地震放大作用对边坡稳定性和裂缝开展情况的影响规律。计算结果表明:裂缝边坡安全系数随地震放大系数增加而降低,陡坡和缓坡在考虑地震放大效应后安全系数均下降约10%,坡顶裂缝随地震放大系数增大逐渐远离坡肩,但裂缝深度会减小,其中当坡底地震加速度较大时放大效应对裂缝位置和深度影响显著。根据地震裂缝边坡安全系数的上限解c/γHtanφ与边坡参数F/tanφ值呈现幂函数的特征,通过拟合获得了安全系数计算表达式,可在实际工程中快速评估地震放大效应对裂缝边坡安全系数的影响。

渗透树脂联合美白技术修复微裂氟斑牙

背景:微研磨、家庭美白联合渗透树脂治疗氟斑牙具有良好的效果,但该方法对氟斑牙微裂纹的影响尚不清楚。目的:探究微研磨、家庭美白联合渗透树脂修复微裂氟斑牙的效果。方法:①临床研究:选择2020年7月至2021年3月就诊于天津医科大学口腔医院修复科的23例微裂氟斑牙患者,包括255颗微裂氟斑牙,均接受牙齿微研磨术、家庭美白与渗透树脂联合治疗,对比治疗前及治疗结束后1周、1个月的牙齿颜色、牙齿敏感程度和牙齿疼痛阈值。②体外实验:收集牙齿表面至少存在一条裂纹的氟斑牙60颗,随机分3组处理:对照组不进行任何处理,美白组进行微研磨与家庭美白处理,联合组进行微研磨、家庭美白和渗透树脂联合处理,每组20颗,测量处理后3组牙齿样本的显微硬度。结果与结论:①临床研究:治疗结束后6个月,255颗微裂氟斑牙中牙齿美白治疗显效207颗、有效48颗,总体治疗有效率为100%。随着治疗时间的延长,中、重度牙齿度敏感程度占比呈下降趋势,治疗结束后6个月,255颗氟斑牙中无重度敏感、15颗为中度敏感、125颗为轻度敏感、115颗无敏感,与治疗前的氟斑牙敏感程度相比差异有显著性意义(P<0.05)。治疗前与治疗结束后1周、6个月的牙齿疼痛阈值比较差异无显著性意义(P>0.05)。②体外实验:美白组牙齿显微硬度值低于对照组、联合组(P<0.05),对照组与联合组牙齿显微硬度值比较差异无显著性意义(P>0.05)。③结果表明,微研磨、家庭美白联合渗透树脂治疗微裂氟斑牙具有较好的疗效。

基于混凝土损伤塑性模型的弯曲损伤RC/PC桥梁动力学特性研究

基于混凝土损伤塑性模型,通过数值模拟研究了带有弯曲损伤的RC/PC桥梁动力学特性的退化情况.首先,提出了适用于RC/PC桥梁非线性振动分析的数值模型构建方法.其次,通过RC简支梁受弯破坏试验,将试验结果与数值模拟结果进行比较,验证了数值模型的准确性.最后,基于厦门快速公交系统中某一联三跨PC连续梁桥建立了三维有限元模型,研究其在不同弯曲损伤程度下前三阶频率和振型的变化情况.结果表明,带有弯曲损伤的RC/PC桥梁的动力学特性会随着损伤程度的增加而呈现不同程度的改变.带有混凝土裂缝的桥梁的微幅振动仍然是线性的,然而随着振幅的增大频率开始降低,使振动呈现出非线性.此外,当裂缝闭合时,前三阶自振频率与振型变化不明显,难以用于识别弯曲损伤;当裂缝张开时,前三阶自振频率与振型出现明显变化,可作为损伤检测指标来识别损伤.

自修复混凝土修复性能评估中的若干关键技术与方法研究综述

混凝土的自修复可有效抑制混凝土开裂并降低其开裂对材料力学和耐久性能的不利影响,从而提高混凝土结构的安全性和韧性、延长混凝土结构的使用寿命、降低混凝土结构的维护成本,是推动混凝土材料与结构可持续发展的一种极具前景的技术方法。然而,目前关于混凝土自修复性能评估过程中的裂缝制备和自修复环境选择、效果评价指标、修复机制表征方法等关键技术手段尚未形成统一认识与标准。基于此,本文分析了采用加载和非加载方式制备自修复混凝土裂缝的特点和自修复环境的选择原则,比较了基于裂缝特征、力学性能、耐久性能和微观结构的自修复效果评价方法,总结了自修复混凝土修复机理的研究方法,最后,提出了自修复混凝土研究中存在的问题和潜在的改进措施,并展望了自修复混凝土的发展趋势和应用前景。