A SELF-SIMILAR CRACK EXPANSION METHOD FOR THREE-DIMENSIONAL CRACKS IN AN INFINITE OR SEMI-INFINITE MEDIUM

The Self-Similar Crack Expansion (SSCE) method is used to calculate stress intensity factors for three-dimensional cracks in an infinite medium or semi-infinite medium by the boundary integral element technique, whereby, the stress intensity factors at crack tips are determined by calculating the crack-opening displacements over the crack surface. For elements on the crack surface, regular integrals and singular integrals are precisely evaluated based on closed form expressions, which improves the accuracy. Examples shaw that this method yields very accurate results for stress intensity factors of penny-shaped cracks and elliptical cracks in the full space, with errors of less than 1% as compared with analytical solutions. The stress intensity factors of subsurface cracks ate in good agreement with other analytical solutions.

SELF-SIMILAR CRACK EXPANSION METHOD FOR THE STRESS INTENSITY FACTOR ANALYSIS

The Self-Similar Crack Expansion (SSCE) method is proposed to evaluate stress intensity factors at crack tips, whereby stress intensity factors of a crack can be determined by the crack opening displacement over the crack, not just by the local displacement around the crack tip. The crack expansion rate is estimated by taking advantage of the crack self-similarity. Therefore, the accuracy of the calculation is improved. The singular integrals on crack tip elements are also analyzed and are precisely evaluated in terms of a special integral analysis. Combination of these two techniques greatly increases the accuracy in estimating the stress distribution around the crack tip. A variety of two-dimensional cracks, such as subsurface cracks, edge cracks, and their interactions are calculated in terms of the self-similar expansion rate. Solutions are satisfied with errors less than 0.5% as compared with the analytical solutions. Based on the calculations of the crack interactions, a theory for crack interactions is proposed such that for a group of aligned cracks the summation of the square of SIFs at the right tips of cracks is always equal to that at the left tips of cracks. This theory was proved by the mehtod of Self-Similar Crack Expansion in this paper.

DEVELOPMENT OF INCONEL~ ALLOY 783，A LOW THERMAL EXPANSION，CRACK GROWTH RESISTANT SUPERALLOY

Low thermal expansion superalloys have been used for a number of years in a variety of applications, including gas turbine engines. The low thermal expansion characteristics of the most widely used class of materials are derived from the ferromagnetic characteristics of Ni, Fe, and Co-based austenitic matrices containing little or no Cr.Alloy developments have been aimed at improving the oxidation resistance and stress accelerated grain boundary oxygen (SAGBO) attack.INCONEL alloy 783 is an oxidation resistant, low coefficient of thermal expansion superalloy developed for gas turbine applications. Alloy 783 represents a culmination in the development, of an alloy system with very high alumtnum content that, in addition to forming γ′,causes βaluminide phase precipitation in the austenitic matrix.This type of structure can be processed to resist both SAGBO and general oxidation,while providing low thermal expansion and useful mechanical properties up to 700℃.Key aspects of the alloy's development are presented.

Experimental study on expansion and cracking properties of static cracking agents in different assembly states

Rolled static cracking agent(RSCA)can solve the intractable problem of traditional bulk static cracking agent(BSCA)in engineering applications.This paper innovatively studies the rational water-cement ratio of BSCA and the immersion soaking time of RSCA under the condition of controlling temperature.Through the expansion and cracking performance experiments,the development characteristics of expansion pressure,the cracking effect of the single-hole specimen and the performance of hole spraying prevention under the action of BSCA and RSCA were compared and analyzed.The results show that:(1)The volume growth rate of static cracking agent decreases with the increase of water-cement ratio,and the fluidity increases with the increase of water-cement ratio.The rational water-cement ratio for BSCA application is 0.3,and the rational immersion time of RSCA is 2-2.5 min;(2)Under the bore diameters of 30,35,40 and 45 mm,the expansion pressure of BSCA with a water-cement ratio of 0.3 is 38.2,52.3,61.5 and 68 MPa,and the expansion pressure of RSCA immersed in water for 2.5 min is 43.5,58.8,69.5 and 75.1 MPa,respectively.Among them,the development speed of expansion pressure of BSCA is higher than that of RSCA,and the arrival time of the peak expansion pressure of RSCA is 1.7 times that of BSCA;(3)The crack initiation speed of single-hole specimen under the action of RSCA is 10.3%lower than that under the action of BSCA,but the cracking speed of the former is 72.6%higher than that of the latter;(4)The hole spraying occurs in BSCA under the bore diameter of 50,55 and 60 mm,while the hole spraying occurs in RSCA under the bore diameter of 60 mm.In terms of bore diameter,the hole spraying prevention of the RSCA is better than that of BSCA.The research results enrich the static blasting technology and provide data support and theoretical reference for field application.

Finite Element Method Investigation of the Effect of Cold Expansion Process on Fatigue Crack Growth in 6082 Aluminum Alloy

Cold expansion is an efficient way to improve the fatigue life of an open hole. In this paper, three finite element models have been established to crack growth from an expanded hole is simulated. Expansion and its degree influence are studied using a numerical analysis. Stress intensity factors are determined and used to evaluate the fatigue life. The residual stress field is evaluated using a nonlinear analysis and superposed with the applied stress field in order to estimate fatigue crack growth. Experimental test is conducted under constant loading. The results of this investigation indicate expansion and its degree are a benefit of fatigue life and a good agreement was observed between FEM simulations and experimental results.

Simulation of Corrosion-Induced Cracking of Reinforced Concrete Based on Fracture Phase Field Method

Accurate simulation of the cracking process caused by rust expansion of reinforced concrete(RC)structures plays an intuitive role in revealing the corrosion-induced failure mechanism.Considering the quasi-brittle fracture of concrete,the fracture phase field driven by the compressive-shear term is constructed and added to the traditional brittle fracture phase field model.The rationality of the proposed model is verified by a mixed fracture example under a shear displacement load.Then,the extended fracture phase model is applied to simulate the corrosion-induced cracking process of RC.The cracking patterns caused by non-uniform corrosion expansion are discussed for RC specimens with homogeneous macroscopically or heterogeneous with different polygonal aggregate distributions at the mesoscopic scale.Then,the effects of the protective layer on the crack propagation trajectory and cracking resistance are investigated,illustrating that the cracking angle and cracking resistance increase with the increase of the protective layer thickness,consistent with the experimental observation.Finally,the corrosion-induced cracking process of concrete specimens with large and small spacing rebars is simulated,and the interaction of multiple corrosion cracking is easily influenced by the reinforcement spacing,which increases with the decrease of the steel bar interval.These conclusions play an important role in the design of engineering anti-corrosion measures.The fracture phase field model can provide strong support for the life assessment of RC structures.

Study on Expansion Cracking of Hydration in Concrete Aggregates

In accordance with a fresh accident by severe expansion cracks of structural elements , based on systematic detection and analyses such as X-ray diffraction, differential thermal analysis, scanning electron mi-croscory, chemical analysis, petrographic analysis, electronic probe analysis, and atomic absorption spectroscopy analysis, it is pointed out that the dominant reasons lie in the hydration reaction of magnesia in concrete aggregates , resulting in a volume expansion in structure members. A wholly new corresponding .strengthening method is applied to the, cracked elements and turned out to be effective.

Repairing of Macroscopic Ceramic Cracks by Tiny Expansion Filling Composites

Using SiC, Al, Al_2O_3, ZrO_2 and Y_2O_3 as raw materials, based on chemical reactions those can cause expansion effect to compensate for the sintering shrinkage at elevated temperature among them, such as Al oxidizes to Al_2O_3, SiC converts to SiO_2, and their reaction productions react further to form mullite (3Al_2O_3·2SiO_2), a sort of tiny expansion composites were prepared and designed which can be used to repair macroscopic cracks of sintered ceramics. The total expanding extent can be adjusted by proportion of raw materials or control of sintering temperature. Filling this expandable ceramic ingredient in cracks of real ceramic part in a designed paste state, cracks could be repaired by a re-sintering process.

Analysis of Crack Expansion and Morphology of Cross-Laminated Timber Planar Shear Test

To describe the dynamic cracking process of the CLT vertical layer,the correlation between a load-displacement curve,specimen cracking,and planar shear failure mechanism of the CLT were explored.A three-point bending test and an improved planar shear test are used to evaluate the shear performance of the CLT.In this study,the load-displacement curve is recorded,the experimental part is synchronized with the video,the dynamic process of cracking of the vertical layer is observed and analyzed throughout the test.From the load-displacement curve,the image characteristics of the initial cracking and the sudden increase of the cracking of the specimen are summar-ized.The description results of the whole dynamic process of the CLT vertical layer cracking are analyzed by pla-nar shear strength value,cracking phenomenon,and azimuth angle of cracking surface.The main conclusions show that the three-point bending test and the improved plain shear test can be used to test the plain shear strength of the CLT,with a difference of only 5.7%.The original crack and the new crack expansion account for 18.9%and 81.1%of the main cracking surface,respectively.And the vertical layer of the CLT specimen under three-point bending has three cracking morphologies,such as radial shake,ring shake,neither along with the radial shake nor along with the ring shake.The azimuth angle of the cracking surface of the CLT vertical layer under planar shear is quite consistent with the first main plane azimuth of the vertical layer of the CLT specimens under the three-point bending test and the shearing test.The shape in the cracking direction of the left half-span or the right half-span of the vertical layer of the specimen is similar to the Chinese character eight.

Study on Expansion of Steam Cracking Unit to 660 kt/a at Yanshan Petrochemical Company

Yanshan Petrochemical Company after having expanded its 300 kt/a steam cracking unit to 450 kt/a in 1994 is still experiencing such problems as low feedstock flexibility, high energy consumption and smaller scale of ethylene unit. In order to fully improve technical capability of steam crackers, reduce energy consumption, improve feedstock flexibility and increase production capacity, a lot of technical revamp cases on steam cracking were studied and compared. Revamp of relevant facilities has expanded the ethylene capacity to the target of 660 kt/a with the actual capacity reaching 710 kt/a. This revamp project has remarkably reduced the energy consumption, which is capable of using naphtha, light diesel fuel, heavy diesel fuel and the hydrocracked tail oil as the steam cracking feedstock. This project is the first to apply refrigeration by means of a mixed cooling agent and has succeeded in using C3 catalytic rectification/hydrogenation technology, which has given an impetus to the progress of steam cracking industry in the world.

Study and Strategy on Expansion of Steam Cracking Unit to 650 kt/a at Yanshan Petrochemical Company

After having expanded its capacity from 300 kt/a to 450 kt/a in 1994, the steam cracking unit of Yanshan Petrochemical Company was still experiencing such problems as low feedstock flexibility, high energy consumption and smaller production capacity as compared to other ethylene units constructed in the same time. In order to fully improve technical capability, reduce energy consumption, improve feedstock flexibility and increase production capacity of steam crackers, a lot of technical transform schemes on steam cracking were studied and compared with attention focused on the feasibility, safety, operability of the related revamp schemes, as well as the advanced nature and economical features. The transform scheme decided includes following projects. For the pyrolysis quench compression system, the pyrolysis gasoline fractionation tower is retrofitted on the site, quench water tower, pyrolysis gas compressor and steam turbine are replaced by new ones respectively. For the separation system a low pressure methane removal technology is adopted with the demethanizer tower replaced, coupled with the C 3 catalytic rectification/hydrogenation technology, while the high pressure depropanizer is replaced with addition of a propylene rectification tower. The fractionation flow sheet and equipment are adjusted with addition of a propylene compressor and compressor for binary mixture of methane ethylene cooling agents. The revamped complex has expanded the ethylene capacity to the target of 660 kt/a with the actual capacity reaching 710 kt/a, remarkably reduced the energy consumption, and is capable of using naphtha, light diesel fuel, heavy diesel fuel and the hydrocracked tail oil as the steam cracking feedstock. This project is the first to apply refrigeration by means of a mixed cooling agent and has succeeded in using C\-3 catalytic rectification/hydrogenation technology, which has given an impetus to the progress of steam cracking industry in the world.

Cracking in an expansive soil under freeze–thaw cycles

Expansive soils located in cold regions can easily endure the action of frost heaving and cyclic freezing–thawing. Cracking can also occur in expansive clayey soils under freeze–thaw cycles, of which little attention has been paid on this issue.In this study, laboratory experiment and cracking analysis were performed on an expansive soil. Crack patterns were quantitatively analyzed using the fractal concept. The relationships among crack pattern, water loss, number of freeze–thaw cycles, and fractal dimension were discussed. It was found that crack patterns on the surface exhibit a hierarchical network structure that is fractal at a statistical level. Cracks induced by freeze–thaw cycles are shorter, more irregularly oriented,and slowly evolves from an irregularly rectilinear pattern towards a polygonal or quasi–hexagonal one; water loss, closely related to specimen thickness, plays a significant role in the process of soil cracking; crack development under freeze-thaw cycles are not only attributed to capillary effect, but also to expansion and absorption effects.

Theoretical and experimental study of initial cracking mechanism of an expansive soil due to moisture-change

Swelling and shrinkage due to moisture-change is one of the characteristics of the expansive soil,which is similar to the behavior of most materials under thermal effect,If the deformation is restricted,stress in expansive soil is caused by the swell-shrinking.The stress is defined as "moisture-change stress" and is adopted to analyze swell-shrinkage deformation based on the elasticity mechanics theory.The state when the total stress becomes equal to the soil tensile strength is considered as the cracking criterion as moisture-change increases.Then,the initial cracking mechanism due to evaporation is revealed as follows:Different rates of moisture loss at different depths result in greater shrinkage deformation on the surface while there is smaller shrinkage deformation at the underlayer in expansive soil;cracks will grow when the nonuniform shrinkage deformation increases to a certain degree.A theoretical model is established,which may be used to calculate the stress caused by moisture-change.The depth of initial cracks growing is predicted by the proposed model in expansive soil,A series of laboratory tests are carried out by exposing expansive soil samples with different moisture-changes.The process of crack propagation is investigated by resistivity method.The test results show good consistency with the predicted results by the proposed theoretical model.

An improved outer pipe method for expansive pressure measurement of static cracking agents

Static cracking agent(SCA)is actively investigated as an alternative to explosive blasting for rock breakage due to its immense expansion property.SCA can eliminate the negative effects of shock,noise and harmful gases encountered in explosive blasting processes.Accurate measurement and deep understanding of the expansive properties of SCAs are important in their industrial application.An improved outer pipe method(OPM),termed the upper end surface method(UESM),is proposed in this paper to overcome the shortcomings of the OPM in the expansive pressure measurement of SCAs.Numerical simulation is used to proof the concept and a mathematical model established to relate the internal pressure and the radial strains at different positions in the upper end surface method test equipment.The new equipment is calibrated using oil pressure and strain measurements.The calibrated equipment is then used to measure the expansion pressure of SCA at three different water contents to proof its potential.The differences in the measurements with OPM and UESM at three different moisture contents are less than 4%.The experimental results confirm the accuracy and applicability of the more user friendly and less expensive UESM in the measurement of the expansive pressures of SCAs.

Cracking and Mechanical Properties of Airport Concrete Pavement with Fiber and Expansive Agent

Polypropylene fiber and expansive agent are used in airport concrete to improve its shrinkage cracking resistance and mechanical properties.The concrete specimens with amount content of polypropylene fiber or expansive agent or both of them are prepared.The morphology of specimens is observed by scanning electron microscope,the time when the first crack occurred is recorded through slap test,and the mechanical properties such as compressive strength and impact energies of concrete are measured.The results show that polypropylene fiber in concrete can reduce the shrinkage and delay the first crack,improve the impact resistance obviously,and improve the compressive strength slightly.Expansive agent can compensate the shrinkage and reduce cracks of concrete pavement markedly,and improve the mechanical properties of concrete pavement slightly.The study provides recommendations for cracking control of airport concrete pavement in the future.

Static solution on four-region spherial cavity expansion model in a pressure sensitive medium

Spherical cavity expansion model is often used to study the mechanic characteristics of pressure sensitive mediums. The most important one we do in the paper is that we construct a four-region model with σθ≠0 in damage region,which is different from what Satapathy did before and is more reasonable. By adopting this model,different constitutive equations were constructed by different method-elastic mechanics in elastic region,damage mechanics and fracture mechanics in damage region,and macro-micro mechanics theory in plastic region. Then using Durban's self-similarity assumption,the control differential equations with boundary conditions were established,and the static numerical solution of stress field and displacement field in the three different regions of elastic,damage and plastic area were discussed respectively. Results showed that this four-region model can describe precisely the mechanic characteristics of pressure sensitive mediums under initial pressure.

干湿循环作用下剑麻纤维加筋膨胀土的抗裂作用及影响因素

为研究干湿循环作用下纤维加筋膨胀土的抗裂效果,分别开展了素土和剑麻纤维加筋膨胀土的干湿循环试验,并采用图像处理技术提取试样表面裂隙参数,分析了剑麻纤维含量及长度、干湿循环次数、试样含水率对裂隙发育的影响.结果表明:1)剑麻纤维加筋膨胀土具有较好的抗裂性能,且剑麻纤维的掺入对膨胀土的裂隙率和裂隙平均宽度影响较大,相较于素土试样,最优加筋土试样的裂隙率和裂隙平均宽度均比素土试样减小了约1/2.2)纤维长度相同时,随着纤维含量的增大,裂隙率、裂隙总长度、裂隙平均宽度和分形维数均呈先减小后增大的趋势,且纤维含量为0.4%时各参数值最小;纤维含量相同时,纤维长度对各裂隙参数影响不大.3)随着干湿循环次数的增加,加筋土和素土试样裂隙参数均呈逐渐增大趋势,但纤维加筋土裂隙率和裂隙平均宽度的增大幅度均比素土小;从第5次干湿循环开始,各裂隙参数增长趋缓.4)单次脱湿过程中,试样含水率由20%降至10%时,裂隙急剧发育,且素土裂隙的发育对含水率的变化更加敏感,含水率低于10%时,随着含水率的减小,试样裂隙率变小并趋于稳定;相同含水率条件下,剑麻纤维加筋土具有更好的抗裂性能.5)剑麻纤维加筋膨胀土的抗裂机理主要表现在两方面,一方面是剑麻纤维的掺入增大了膨胀土的渗透系数,促进了试样内水分的均匀分布,减小了试样各处的胀缩差异;另一方面纵横交错的剑麻纤维约束了聚集体之间大孔隙的收缩.

多频率重复荷载作用下布敦岩沥青混凝土宏细观开裂特性分析

针对布敦岩沥青混凝土路面在不同工况下的使用寿命问题,对多种路用环境及不同频率重复荷载作用下布敦岩沥青混凝土小梁试件进行三点弯曲重复加载试验,采用数字图像相关(digital image correlation,DIC)法分别对布敦岩沥青混凝土、基质沥青混凝土和SBS改性沥青混凝土在5种路用环境下的疲劳损伤进行分析。结果表明:在不同路用环境和多频率的作用下,疲劳寿命排序为SBS改性沥青混凝土>布敦岩沥青混凝土>基质沥青混凝土;布敦岩的掺加对沥青混凝土疲劳性能起到了提升作用,但对车速变化的敏感程度较强;布敦岩沥青能有效提升沥青混凝土的抗疲劳开裂性能;同种材料在相同重复荷载作用次数时的损伤程度随着加载频率的增加而减小;布敦岩的掺加可以提高混合料的抗损伤能力,延缓沥青混凝土出现加速开裂的时间点;布敦岩沥青混凝土宏观裂纹出现的时间点为重复荷载次数占比为70%时。研究成果有利于工程提前制定相应的防治措施。

氯盐与硫酸盐复合侵蚀下钢筋混凝土锈裂行为

为评估海洋、盐湖等环境中钢筋混凝土结构耐久性,通过试验与理论相结合探究氯盐与硫酸盐复合侵蚀下钢筋混凝土锈裂特性。通电腐蚀5%(质量分数)NaCl、5%NaCl+5%Na_(2)SO_(4)溶液中钢筋混凝土试件,对比分析混凝土表观形貌、钢筋锈蚀特征。设计伴随的混凝土腐蚀试验,类比保护层腐蚀劣化,分析混凝土力学性能。结果表明:硫酸盐的存在改变胀裂前混凝土形貌,使得单一氯盐侵蚀下的“白须”消失,表面粉化并出现盐结晶,延长胀裂时间;复合侵蚀下钢筋锈蚀率低于单一氯盐侵蚀,二者均显著低于法拉第定律理论值;锈胀裂缝宽度与钢筋锈蚀率线性相关,硫酸盐的存在增大裂缝随钢筋锈蚀发展的速率;通电环境中,受腐蚀混凝土的抗压强度先升高后降低,劈裂抗拉强度不断降低。提出受腐蚀混凝土的抗拉强度演化经验公式。在经典锈胀模型的基础上考虑锈蚀产物对裂缝的填充作用,并将硫酸盐的影响考虑至混凝土抗拉强度、钢筋腐蚀电流密度中,建立复合侵蚀下钢筋混凝土胀裂时间预测模型,并验证了模型的有效性。

考虑干湿循环累积损伤的膨胀土土水特征研究

为探明干湿循环中膨胀土裂隙发育对其土水特征的影响,制备经历不同次数干湿循环的柱状土样,并对其土水特征曲线进行测定。结合CT扫描结果,基于裂隙的分形特征构建土柱截面和整体损伤变量,通过其变化规律阐明裂隙发育对土水特征的影响。结果表明:裂隙的分形维数沿深度方向逐渐减小,大体呈线性变化规律,不同循环次数下裂隙的开展过程具有相似性;基于裂隙分形维数构造的损伤变量能够较好描述干湿循环对膨胀土的累积损伤效应,土柱的整体损伤变量随干湿循环次数逐渐增加,且增速逐步提高;提出的基于土柱整体损伤变量进行修正的Fredlund-Xing模型能够较好拟合裂隙膨胀土的土水特征曲线。研究成果能够为后续裂隙膨胀土的渗流和工程应用研究提供理论支持。