Quantitative crack evaluation in slender reinforced concrete walls with rectangular section

Past earthquakes have shown that cracking affects post-earthquake functionality and accounted for huge repair costs for reinforced concrete(RC)wall buildings,even though the code-compliant seismic design prevents col-lapse.Engineers should know the maximum residual flexural crack width and volume of repair material needed for the flexural cracks to determine the damage degree and the repair cost.This paper presents the experimental campaign on four RC slender walls that investigated the effect of confining reinforcement and thickness of the wall on flexural crack parameters under quasi-static reversed cyclic loading.The width of all flexural cracks was measured when reaching each cycle peak drift and when unloading to zero lateral loads.Crack widths at peak and residual states increased with increasing peak drift.Based on the experimental observations,it was found that the maximum residual crack width is obtained as a simple function of the extreme tension fiber elongation of the wall tensile fiber within±30%error.In addition,this paper outlines methods to calculate the volume of repair material for flexural cracks from the extreme tension fiber elongation of the wall.With the funda-mental rules found from the experiment in this paper,it will become possible to obtain the maximum crack width and the volume of repair material from simple numerical analysis tools such as a multi-spring line element model.

Sensitive Evaluation on Early Cracking Tendency of Concrete with Inclusion of Light-burnt MgO

The advanced temperature and stress test machine was introduced to determine the early cracking tendency of concrete with inclusion of light-burnt MgO under full restraint by tracking the development of thermal,physical and deformation properties.Results showed that light-burnt MgO being incorporated ranging between 4 wt% and 6 wt% of cementitious materials was beneficial to increase the maximum compressive stress and cracking stress of concrete by 0.37 MPa and 0.2 MPa on average respectively.The second zero stress temperature was reduced by 11.4 ℃ and the maximum temperature was slightly reduced while cracking thermal impact was significantly enhanced from 59.8 ℃ to 66.2 ℃.Sensitive anti-cracking coefficient F was forwarded to assess the early cracking tendency of concrete and the inclusion of 4 wt% light-burnt MgO with activity of 109 s ranked the best in crack resistance.

Evaluation method of cracking resistance of lightweight aggregate concrete

The cracking behavior of lightweight aggregate concrete(LWAC) was investigated by mechanical analysis, SEM and cracking-resistant test where a shrinkage-restrained ring with a clapboard was used. The relationship between the ceramsite type and the cracking resistance of LWAC was built up and compared with that of normal-weight coarse aggregate concrete(NWAC). A new method was proposed to evaluate the cracking resistance of concrete, where the concepts of cracking coefficient ζt(t) and the evaluation index Acr(t) were proposed, and the development of micro-cracks and damage accumulation were recognized. For the concrete with an ascending cracking coefficient curve, the larger Acr(t) is, the lower cracking resistance of concrete is. For the concrete with a descending cracking coefficient curve, the larger Acr(t) is, the stronger the cracking resistance of concrete is. The evaluation results show that in the case of that all the three types of coarse aggregates in concrete are pre-soaked for 24 h, NWAC has the lowest cracking resistance, followed by the LWAC with lower water absorption capacity ceramsite and the LWAC with higher water absorption capacity ceramsite has the strongest cracking resistance. The proposed method has obvious advantages over the cracking age method, because it can evaluate the cracking behavior of concrete even if the concrete has not an observable crack.

Geochemical characteristics, forming conditions and resource evaluation of oil-cracking gas as ex-emplified by the platform area of the Tarim Basin

Based on the geochemical characteristics of oil-cracking gas and kerogen-cracking gas revealed by simu-lation experiments and the chemical composition of natural gases in actual gas reservoirs, two kinds of natural gases with different relationships between C2/C3 and C1/C2, C2/C3 and C1/C3, C2/C3 and 100×C1/(C1-C5) were identified in the Tarim Basin, and proposed further by the authors. The relationship charts of C2/C3 and C1/C2, C2/C3 and C1/C3, C2/C3 and 100×C1/(C1-C5) can be used to effectively distinguish oil-cracking gas from kerogen-cracking gas. Petro-leum geological analysis of the oil-cracking gas reservoirs showed that the distribution of oil-cracking gas is mostly related with deep-seated faults or faults with a large fault throw, and the burial depth of paleo-oil reservoir is rela-tively high; crude oil-cracking gas resources have been evaluated by using both forward and inversion methods. The plots of C2/C3 vs. C1/C2, C2/C3 vs. C1/C3, and C2/C3 vs. 100×C1/(C1-C5) were used to distinguish between oil-crack-ing gas and kerogen-cracking gas, and estimate the mixed ratios of the two kinds of natural gases in the main gas reservoirs of the platform area.

Evaluation of residual stress relief of aluminum alloy 7050 by using crack compliance method

High strength aluminum alloys of 7xxx series have unacceptable levels of quenching residual stresses from solution heat treatment. The residual stress not only results in machining distortion and dimensional instability, but also increases the possibility of stress corrosion cracks. Therefore, it is necessary to reduce the residual stress to an acceptable level. The crack compliance method was adopted to study the influences of various stress relief methods on residual stress patterns in 7050 aluminum alloy. The results show that 90% residual stress can be eliminated by the cold stretching(Tx51) method. And a lower level of residual stress can be achieved by the uphill quenching(Tx53) method or the cold compression(Tx52). However, there is a very steep residual stress gradient normal to exterior surfaces.

Relationship among Parameters Evaluating Stress Corrosion Cracking

The threshold stress, σc, for sulfide stress corrosion cracking (SCC) of seven pipeline steels and five other steels, the critical stress, Sc, for seven pipeline steels and two drill rod steels with various strengths and the susceptibility to SCC, IRA or σf(SCC)/σf, for four pipeline steels, two drill rod steels and five other steels were measured. The results showed that there are no definite relationships among σc, Sc and IRA or σf(SCC)/σf.The threshold stress for hydrogen induced cracking (HlC) during charging with loading in the H2S04 solution, σc(H), decreased linearly with logarithm of the concentration of diffusible hydrogen c0, i.e., σc(H)=A-B Inco for four pipeline steels. σc(H) obtained with a special cathodic current ic, which was corresponding to the diffusible hydrogen concentration during immersing in the H2S solution, were consistent with /c for sulfide SCC for four pipeline steels. Therefore, σc for sulfide SCC can be measured using dynamically charging in the H2SO4 solution with the special cathodic current ic.

A NOVEL PARAMETER FOR EVALUATING THE FATIGUE CRACK GROWTH RATE IN CARBON STEELS

A novel parameter is suggested for evaluating the fatigue crack growth rate in carbon steels. Fatigue crack propagation tests of an annealed 0.42% carbon steel were carried out under different conditions to investigate the relationship between this dominating parameter and the crack opening displacement (COD). A new equation of fatigue crack growth rate is formulated in terms of the suggested parameter. The physical meanings of the material parameters in this equation are explored experimentally. Considering the relation of crack growth and deformation properties, a simple and applicable method is proposed to evaluate the fatigue crack growth rate. It is also observed that the material parameters in the fatigue crack growth rate equation of carbon steels are related linearly to the material strength. The results are in a good agreement with experimental results.

Low temperature cracking behavior of asphalt binders and mixtures: A review

Low temperature cracking(LTC)distress on pavement seriously affects road life.This paper finished a literature review of the research on the mechanism of LTC of asphalt composites(asphalt composites refers to asphalt binder and asphalt mixture in this article),test methods,factors contributing to LTC,measures to prevent and control the distress,and prediction of LTC in asphalt pavements.The following conclusions were obtained:the cracking mechanism of asphalt mixtures needs to be further revealed by means of simulation at the micro level,the BBR and 4 mm plate test(by DSR)methods are currently optimal,and a correlation between asphalt and asphalt mixture evaluation indexes needs to be established.Sensitivity analyses are needed for the factors affecting LTC of asphalt mixtures.It is necessary to calculate the contribution of each factor to the LTC of asphalt mixtures.The aim is to propose targeted improvement measures for the most unfavourable factors,as well as to carry out research and development of key materials for anti-cracking.Measures for the prevention and control of LTC of asphalt pavement are analyzed and discussed.Existing researches on the prediction of LTC of asphalt pavements is discussed.It is necessary to analyse the mechanical response of asphalt pavement,the damage process and the sensitivity of anti-cracking parameters on the basis of considering the complex geometrical characteristics and material properties of asphalt pavement materials.Finally,the mechanism of LTC,evaluation methods,factors influencing LTC,and remedial measures for asphalt composites were summarized,and future research prospects were suggested.This paper provides theoretical support for the further solution of LTC distress of asphalt pavement,which is effective on the improvement of pavement life.

Mineral Evaluations for Small Scale Miners:A Case for Zimbabwe and South Africa

Since the establishment of geological surveys in Southern Africa a lots of geological information has been systematically stored.Due to the vast amounts of data generated one of the main functions of

建筑工程混凝土裂缝分析与纤维混凝土应用评述

建筑工程的快速发展导致混凝土的抗裂控制成为复杂而持久的挑战。利用不同纤维的优势属性补偿素混凝土的性能缺陷,提高混凝土的抗裂性能,是目前一种经济有效的手段。文章首先对建筑工程混凝土裂缝的类型及成因进行了深入分析,在此基础上详细阐述了不同纤维增强混凝土的特点,对其增强效果进行了对比分析,最后给出了纤维混凝土的发展建议,希望能为纤维增强混凝土的发展与应用提供借鉴。

钢桥疲劳裂纹扩展行为研究及可靠度更新分析

疲劳断裂失效一直是导致钢结构桥梁发生破坏的重要原因之一。为了更好地利用断裂力学对此失效行为进行研究,开展桥梁钢板件标准疲劳裂纹扩展试验。利用金属磁记忆无损检测技术监测疲劳裂纹扩展行为,分析疲劳裂纹扩展过程中磁信号的变化情况。从理论及试验两方面探讨疲劳裂纹扩展长度磁信号量化方法,确定磁特征参数与裂纹扩展长度之间定量关系式。随后将所提关系应用于疲劳裂纹扩展剩余寿命预测确定性研究,并通过试验结果对其进行验证。最后考虑疲劳裂纹扩展的随机性及损伤信息更新的必要性,进一步开展疲劳可靠度评估研究。研究结果表明:磁信号梯度最大值可以较好地对裂纹扩展长度进行量化表征,剩余寿命预测结果与试验结果误差均在20%以内,基于磁信号的可靠性评估方法可以配合我们更好地制定桥梁维修及养护策略。

高温环境下花岗岩损伤演变及量化研究

为揭示高温环境下岩石的损伤演变,将花岗岩加热至600℃、800℃、1 000℃并分析其断口形貌特征,采用数字图像处理技术、损伤力学和“损伤微元”理论,结合各温度下花岗岩细观裂纹特性,建立了岩石损伤评价方法,并开展岩石损伤量化研究。结果表明:花岗岩的断口特征与裂纹的萌生和扩展相关,且裂纹数量、分布密度随温度上升而增加;各温度区间下,花岗岩细观裂纹长度服从对数正态分布,内部损伤表现形式以短裂纹为主,长裂纹为辅;不同温度下花岗岩细观裂纹由单方向延伸发育成多方向贯穿,最高损伤度和温度呈正相关。该研究揭示了高温下花岗岩内部裂纹损伤演化规律,该损伤评价方法对高温地层岩石稳定性评估和地下工程的维护具有重要的参考价值。

无导洞法连拱隧道先行洞仰拱开裂及受力分析

研究目的:为探明无导洞法连拱隧道先行洞仰拱衬砌开裂的原因,本文首先通过现场监测得到先行洞的受力规律,然后基于该受力特征提出一种考虑后行洞开挖的先行洞荷载-结构计算模型,进而结合现场监测和数值计算分析先行洞仰拱衬砌的开裂原因,最后评价先行洞二衬的安全性并给出相应的控制措施。研究结论:(1)当后行洞开挖前,先行洞二衬对称受力且各截面的安全系数大于2.4;当后行洞开挖后,先行洞二衬处于偏压受力状态,仰拱右侧的安全系数仅为1.25,其内表面恰是最大拉应力所在位置,与现场仰拱衬砌的开裂位置吻合;(2)先行洞仰拱衬砌开裂的原因是由于后行洞开挖导致的隧道偏压受力以及仰拱不均匀沉降;(3)提高支护抗力系数有助于改善衬砌结构的安全性,但支护抗力系数高于某值后安全系数变化不大;(4)本研究成果可为相关连拱隧道的设计和施工提供参考。

STUDY ON THE SMALL CRACKS OF COVER STRATA AFTER THE COMBINED MINING IN THE THIN SEAMS

By using the continuous exploration system with multi-pipeline in the borehole, the failure cracks of cover strata (specially, the small cracks developing) after the combined mining of 16, 17 thin seams of the 4th mining block in Zhaopo Mine is explored. After that, the researching result is considered as the output and the testing result of rock mechanics parameters is considered as the verifying data, the back analysis is made with the conception of the point safety degree. The relative height of the crack developing is obtaied. So, the evaluating of the safety of 16, 17 seams combined mining under the gob water with extreme distance is made and the mining has been safety. The scientific basis is provided for the evaluating of the mining safety with the similar conditions.

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。

增材制造金属材料的疲劳性能研究进展

金属增材制造作为前沿热点制造技术之一,近年来在各种重要工业领域的研究和应用日益广泛。利用增材制造技术制备金属材料的过程中,不可避免会造成材料表面粗糙、气孔、未熔合等缺陷,虽然工艺技术的改进可以在一定程度上减小缺陷程度,但至今仍无法完全消除这些缺陷。增材制造金属材料的过程中,缺陷部位通常会成为应力集中源诱发疲劳裂纹的形核,造成金属材料的疲劳寿命下降。首先从表面质量、内部缺陷及微观结构等方面阐述了增材制造金属材料疲劳性能的影响因素;其次从宏观与微观角度概括了疲劳裂纹萌生/扩展机理的研究现状与进展;总结了热处理、表面优化、电磁辅助以及超声辅助等疲劳延寿技术的研究进展;最后讨论了基于机器学习技术的疲劳寿命评估模型,同时展望了机器学习和人工智能技术在增材制造金属材料领域的应用前景,为推动增材制造金属材料的发展和应用提供了借鉴与参考价值。

飞机结构X射线裂纹图像智能评定

飞机结构X射线图像评定过程存在复杂背景下裂纹分割不准确、检出难等问题。基于高效层聚合网络提出一种飞机结构X射线裂纹图像智能评定模型(ELAN-Seg),将ELAN-Seg模型和DeepLabv3+模型的射线图像裂纹分割能力进行对比,结合图像处理技术对模型分割的裂纹长度进行评估,利用飞机强度试验及外场维护过程采集的X射线图像对模型进行验证。结果表明:分割的最小裂纹长度约为3 mm,ELAN-Seg模型对复杂背景射线图像裂纹分割更加准确,裂纹漏检率小于3.8%,该模型具有工程适用性。

825镍基高温合金凝固裂纹敏感性评测

【目的】凝固裂纹敏感性是目前国产镍基825合金工程化应用关注的重点技术问题。【方法】采用横向位移凝固裂纹试验(TMW裂纹试验)以及相图计算方法对2种不同成分的镍基825合金的凝固裂纹倾向进行研究。并利用光学显微镜及扫描电镜对焊接接头凝固裂纹的断口形貌及显微组织进行分析,使用热力学相图软件Pandat,采用Scheil凝固模型计算两种合金的T-(fs)^(1/2)凝固曲线及凝固裂纹敏感指数。【结果】研究结果表明,镍基825 B合金表现出比镍基825 A合金更高的凝固裂纹敏感性,镍基825 A合金的凝固裂纹敏感指数为3452℃,而镍基825 B合金的凝固裂纹敏感指数为4074℃。【结论】825 B合金的凝固裂纹敏感性计算结果高于825 A合金,与TMW测试结果吻合,表现出更高的凝固裂纹敏感性。

岩石爆破基础理论研究进展

随着仪器科学、测试技术和爆破领域相关交叉学科的跨越式发展,岩石爆破基础理论研究取得了丰硕的研究成果,为工程爆破技术开发和应用提供了科学指导。提出了岩石爆破基础理论研究的2个关键科学问题,即:炸药爆炸做功与岩体破碎耗能的耦合作用机理、炸药能量释放与爆炸裂纹扩展的精细控制原理。指出以岩石本构关系、动静关系、波纹关系和损伤评价为主线的系列研究是解决关键科学问题的具体途径,系统梳理了现阶段岩石爆破基础理论的研究进展。在此基础上,提出了3点研究展望,即:岩石爆破内部三维裂隙场的实时监测、考虑爆破物理过程的数值仿真算法、围岩爆破损伤的精准评价。

基于不同温度SCB试验的沥青混合料低温抗裂性能评价

为更方便、准确地评价沥青混合料低温抗裂性能,通过对半圆弯曲(SCB)试验对断裂能成分进行分析,探究了断裂能随温度升高而增加的原因,提出了基于SCB试验计算沥青混合料黏性耗散能方法,并用于评价沥青混合料低温抗裂性能。通过设计SCB试验,测试并评价了6种沥青混合料低温抗裂性能。结果表明:沥青混合料断裂能随测试温度升高而增加主要归因于黏性耗散能的增加,通过SCB试验预估沥青混合料黏性耗散能,用以评价沥青混合料低温抗裂性能;以黏性耗散能作为低温抗裂性能评价指标,对比不同再生沥青混合料的黏性耗散能的变化幅度,可以发现最大变化幅度(AC20R40与AC20R40RA试验组的差异)达到72.2%,远大于断裂能的变化幅度,可有效区分不同再生沥青混合料的低温抗裂性能,同时也有利于降低试验误差对评价结果的影响;随着试验温度升高,沥青混合料从脆性破坏逐渐转为延性破坏,断裂面中集料断裂面积逐渐减少,且材料原子或分子振动更剧烈,造成沥青混合料表面能逐渐减小,所以出现断裂能随温度先增大后减小的变化规律,其由黏性耗散能引起,断裂能峰值点温度即为黏性耗散能峰值点温度;建议黏性耗散能以―12℃与0℃温度下SCB试验的断裂能之差进行计算,用以评价沥青混合料低温抗裂性能。