Use of double edge-cracked Brazilian disk geometry for compression-shear fracture investigation of rock

A new specimen geometry-the double edge-cracked Brazilian disk and a relevant fracture analysis by weight function method are proposed for the investigation of rock fracture caused by compression-shear loading. Not only can the mixed mode fracture with any ratio of KI /KII be achieved, but also the pure mode n crack extension can be obtained. The combined mode fracture analysis for this geometry shows that diametral compression in the far-field can induce a compression-shear stress state in the singular stress field ahead of crack tips. Experimental investigations conducted on marble specimens show that the pure mode [I crack extension can be obtained when the dimen-sionless crack length a>0. 7 and the inclined crack angle 5°≤ψ≤40°. Normalized mode I and mode II stress intensity factors decrease from -0. 45 and 2. 47 at ψ= 5° to - 1. 65 and 1. 52 at ψ=40°, respectively. The strains at three points of specimen are also measured in order to investigate the influence of stress singularity on initi     

硬岩脆性指标与弹性应变能关系初探

为探讨硬岩脆性指标与弹性应变能之间的关系,采用侧向应变差法确定起裂应力,以岩石起裂应力和峰值强度为桥梁,基于断裂力学理论和能量理论解释了脆性指标与弹性应变能内在的联系,并从两种岩石单轴压缩试验中构建了脆性指标与弹性应变能之间的函数关系.结果表明:脆性指标B_(13)、B_(14)与起裂弹性应变能(单变量)符合幂函数关系,脆性指标B_(15)、B_(16)与起裂弹性应变能(单变量)符合幂函数或线性函数关系,脆性指标B_(13)、B_(14)、B_(15)和B_(16)与峰值弹性应变能(单变量)之间无明显函数关系;新定义的脆性指标可由起裂和峰值弹性应变能(双变量)获得,或由起裂弹性应变能(单变量)获得,脆性指标与峰值弹性应变能关系式的构建,对于研究岩爆倾向性具有重要意义;根据新定义的脆性指标,利用两种岩石算例验证表明,采用起裂弹性应变能的岩石脆性指标计算方法具有一定的可靠性.通过这一研究,为岩石脆性指标与起裂弹性应变能关系的确定提供了一种新方法.     

复合型裂纹扩展的形状改变比能准则

为了预测金属材料裂纹在小范围屈服时的延性断裂，以复合型裂纹为研究对象，将裂纹前缘塑性屈服区内的总形状改变比能用于建立复合型裂纹的断裂准则。该准则表明了金属材料裂纹在扩展过程中，起决定作用的是形状改变比能，而不是整个应变能。能成功地预测了复合型裂纹的启裂角度以及临界荷载，将其结果与Sih的S准则和实验数据结果进行了比较。结果表明，复合型裂纹扩展的形状改变比能准则，在预测裂纹启裂角方面优于S准则，并且预测的临界荷载偏保守。在工程上，将其应用于复合型裂纹断裂判定是安全的。     

A MIXED METHOD OF DETERMINING THETHERMAL STRESS FOR COLD ROLLER

Coldrollerisanimportantcomponentofthemill.Thecoldrolleroftherollingprocessisusedathightemperature,whichwouldcracktherollerandshortentheservicelifeoftheroller.UPtonow,ageneralcalculationmethodofthethermalstressofthecoldrolleristoincreasethesafetycoefficient.Thiscalculationmethodlackstheoreticalanalysis.Inordertosolvethisproblem,thispaperappliesforthefirsttimethenewlydevelopedfiniteanalyticalmethodandtheboundaryfittingcoordinatetransformationmethodtomillpracticeandprovidesaanalyticalbasisoftemp…     

Microscopic characteristics of different fracture modes of brittle rock

Ａｓｕｒｖｅｙｏｆｆｒａｃｔｕｒｅｍｅｃｈａｎｉｃｓｌｉｔｅｒａｔｕｒｅｒｅ ｖｅａｌｓｔｈａｔｒｅｓｅａｒｃｈｗｏｒｋｏｎｒｏｃｋｆｒａｃｔｕｒｅｍｅｃｈａｎ ｉｃｓｈａｓｍａｉｎｌｙｆｏｃｕｓｅｄｏｎＭｏｄｅⅠｆｒａｃｔｕｒｅ (ｏｐｅｎ ｉｎｇｍｏｄｅ) ,ｉｎｐａｒｔｉｃｕｌａｒ,ｏｎＭｏｄｅⅠｆｒａｃｔｕｒｅｔｏｕｇｈｎｅｓｓｔｅｓｔｉｎｇ[1 2 ] .Ｈｏｗｅｖｅｒ,ｐｒｅ ｅｘｉｓｔｉｎｇｃｒａｃｋｓｉｎｒｏｃｋｍａｔｅｒｉａｌｓｏｒｄｉｓｃｏｎｔｉｎｕｉｔｉｅｓｉｎｒｏｃｋｍａｓｓｅｓａｒｅｓｅ…     

Press cutting of thin-walled round pipe for producing curvilinear end

In order to obtain a basic understanding of the unwanted distortions in the pipe wall during the press cutting process, the deformation of a thin-walled round pipe to form a curvilinear end was numerically and experimentally studied. Vector analysis was used to study the relationship between the punch shape and the collapse of the cut-end. Stamping experiments on AISI 1020 steel pipe were conducted using different angles α and β defining the shape of the punch. The elasto-plastic finite element method that allows consideration of a ductile fracture was also employed to study the process. The results show that the deformation of the pipe end after press cutting is influenced mostly by the shape of the punch. A satisfactory quality of the curvilinear end of the pipe can be obtained if the appropriate geometric parameters of the punch are chosen. The pipe-wall collapse in the upper part of the section is decreased when α and β increase. The recommended values for α and β lie within 30°–50°. The hole on the underside of the punch has less influence on the quality of the cut-end, and the wall distortion and the generation of burr on the cut-end can be satisfactorily simulated using the fracture criterion of Brozzo or the normalized criterion of Cockcroft and Latham.     

Dynamic recrystallization behavior of 35CrMo structural steel

1　ＩＮＴＲＯＤＵＣＴＩＯＮＤｕｒｉｎｇｈｏｔｗｏｒｋｉｎｇ ,ｓｅｖｅｒａｌｍｅｔａｌｌｕｒｇｉｃａｌｐｈｅｎｏｍｅｎａｓｕｃｈａｓｗｏｒｋ ｈａｒｄｅｎｉｎｇ (ＷＨ ) ,ｄｙｎａｍｉｃｒｅｃｏｖｅｒｙ (ＤＲＶ) ,ａｎｄｄｙｎａｍｉｃｒｅ ｃｒｙｓｔａｌｌｉｚａｉｔｏｎ (ＤＲＸ )ｏｃｃｕｒｓｉｍｕｌｔａｎｅｏｕｓ ｌｙ[1 5 ] .Ｅｓｐｅｃｉａｌｌｙ ,ｔｈｅｏｃｃｕｒｒｅｎｃｅｏｆＤＲＸ ,ｃａｎｒｅｆｉｎｅｇｒａｉｎａｎｄｒｅｄｕｃｅｄｅｆｏｒｍａｔｉｏｎｒｅｓｉｓｔａｎｃｅｉｎｐｒａｃｔｉｃａｌｈｏｔ ｗ…     

Rock fracture under anti-plane shear (Mode III) loading

Anti-plane punch-through shear test and anti-plane four-point bending test are used to study the crack initiation and propagation under anti-plane shear (Mode III) loading. The tensile and shear stresses at the crack tip are calcualted by finite element method. The results show that under Mode III loading the maximum principal stress σ₁ at crack tip is smaller or a little larger than the maximum shear stress τ_max. Since the tensile strength of brittle rock is much lower than its shear strength, σ₁ is easy to reach its critical value before τ_max reaches its critical value and thus results in Mode I fracture. The fracture trajectory is helicoid and the normal direction of tangential plane with the fractured helicoid is along the predicted direction of the maximum principal stress at the notch tip. It is further proved that Mode I instead of Mode III fracture occurs in brittle rock under Mode III loading. The fracture mode depending on the fracture mechanism must be distinguished from the loading form. Foundation item: Project (50374073) supported by the National Natural Science Foundation of China; project (2002032256) supported by the Postdoctor Science Foundation of China     

Effect of specimen thickness on Mode II fracture toughness of rock

Anti-symmetric four-point bending specimens with different thickness, without and with guiding grooves, were used to conduct Mode II fracture test and study the effect of specimen thickness on Mode II fracture toughness of rock. Numerical calculations show that the occurrence of Mode II fracture in the specimens without guiding grooves (when the inner and outer loading points are moved close to the notch plane) and with guiding grooves is attributed to a favorable stress condition created for Mode II fracture, i.e. tensile stress at the notch tip is depressed to be lower than the tensile strength or to be compressive stress, and the ratio of shear stress to tensile stress at notch tip is very high. The measured value of Mode II fracture toughness K _{II C} decreases with the increase of the specimen thickness or the net thickness of specimen. This is because a thick specimen promotes a plane strain state and thus results in a relatively small fracture toughness. Foundation item: The National Natural Science Foundation of China (No: 49672164) Biography of the first author: RAO Qiu-hua, doctor of engineering, born in 1965, majoring in rock fracture mechanics.     

Creep properties and permeability evolution in triaxial rheological tests of hard rock in dam foundation

Triaxial creep tests were carried out under seepage pressure by using rock servo-controlled triaxial rheology testing equipment. Based on experimental results, rock rheological properties influenced by seepage-stress coupling were studied, and variations of seepage rate with time in complete creep processes of rock were analyzed. It is shown that, when the applied stress is less than failure stress level, the creep deformation is not obvious, and its main form is steady-state creep. When applied stress level is greater than or less than but close to fracture stress, it is easier to see the increase of creep deformation and the more obvious accelerative creep characteristics. The circumferential creep deformation is obviously higher than the axial creep deformation. At the stage of steady-state creep, the average of seepage flow rate is about 4.7×10⁻⁹ m/s at confining pressure (σ ₃) of 2 MPa, and is about 3.9×10⁻⁹ m/s at σ ₃ of 6 MPa. It is seen that the seepage flow rate at σ ₃ of 2 MPa in this case is obviously larger than that at σ ₃ of 6 MPa. At the stage of creep acceleration, the seepage flow rate is markedly increased with the increase of time. The variation of rock permeability is directly connected to the growth and evolution of creep crack. It is suggested that the permeability coefficient in complete creep processes of rock is not a constant, but is a function of rock creep strain, confining pressure, damage variable and pore water pressure. The results can be considered to provide a reliable reference for the establishment of rock rheological model and parameter identification.     

Failure mode and strength anisotropic characteristic of stratified rock mass under uniaxial compressive situation

A stratified rock mass model was founded by FLAC^3D. The failure mode and anisotropic characteristic of strength for stratified rock mass were analyzed. The analysis results show that the numerical simulation can visually reflect the failure modes of rock samples under different inclination angles β of structural plane. The stiffness of rock sample before peak strength changes in the compressive procedure. With the increase of β, the compressive strength σ _c of rock sample decreases firstly and then increases; when β is in the range of 20°–30° and 80°–90°, σ _c has the largest sensitivity to β; while β falls in the range of 30°–70°, σ _c varies little. When ϕ _j <β<90° (β _j is friction angle of structure plane), the results obtained from numerical simulation and theoretical analysis are in almost the same values; while β⩽ ϕ _j or β=90°, they are in great different values. The results obtained from theoretical analysis are obvious larger than those from numerical simulation; and the results from numerical simulation can reflect the difference of compressive strength of rock samples for the two situations of β⩾ ϕ _j and β=90°, which is in more accordance with the real situation. Foundation item: Project (50099620) supported by the National Natural Science Foundation of China     

Energy consumption in rock fragmentation at intermediate strain rate

In order to determine the relationship among energy consumption of rock and its fragmentation, dynamic strength and strain rate, granite, sandstone and limestone specimens were chosen and tested on large-diameter split Hopkinson pressure bar (SHPB) equipment with half-sine waveform loading at the strain rates ranging from 40 to 150 s⁻¹. With recorded signals, the energy consumption, strain rate and dynamic strength were analyzed. And the fragmentation behaviors of specimens were investigated. The experimental results show that the energy consumption density of rock increases linearly with the total incident energy. The energy consumption density is of an exponent relationship with the average size of rock fragments. The higher the energy consumption density, the more serious the fragmentation, and the better the gradation of fragments. The energy consumption density takes a good logarithm relationship with the dynamic strength of rock. The dynamic strength of rock increases with the increase of strain rate, indicating higher strain rate sensitivity. Foundation item: Projects(50674107, 10472134, 50490274) supported by the National Natural Science Foundation of China     

A simple method of determining stress intensity factor KI from isochromatic fringe loops

A new photoelastic method of obtaining mode I stress intensity factor (SIF) is presented. The method considers the influence of far field stress, σ _ox , on the value of SIF. The only information needed for K _I calculation is the area between isochromatic fringe loops. The method is examined by two kinds of specimen in different load cases. Experimental results show that it is quite simple and of high precision. Project supported by the National Natural Science Foundation of China Synopsis of the first author Chen Feng, associate professor, born in October 1949, worked on experimental fracture mechanics in Sweden from 1990 to 1995 as a visiting scholar. Major research fields include perturbation and weight function method and its application, experimental mechanics, rock fracture mechanmics.     

Asymptotic analysis of weakly nonlinear and forced oscillations

The restriction ofK-B averaging method is discussed and asymptotic solution of the weakly nonlinear and forced oscillationu″+ω ₀ ² u=εkcosωt−εu ³ is obtained by Struble technique. The conclusion about this oscillation derived with other method is discussed. The results show thatK-B method will break down whena andθ in the zeroth solution of above eqation are not slowly varying functions of timet. The stationary solution of weakly nonlinear oscillation,u″+ω ₀ ² u=εkcosω(ε)t-εu ³ is also analysed. Synopsis of the author Chen Feng, associated professor, born in October 1948, worked on experimental fracture mechanics in Sweden from 1990 to 1995 as a visiting scholar. Major research fields include perturbation and weight function method and its application, experimental mechanics. currently interested in rock fracture mechanics.     

Deformation behavior and microstructural evolution of Al-Zn-Cu-Mg-Sc-Zr alloy during high temperature compression

The deformation behavior of a new Al-Zn-Cu-Mg-Sc-Zr alloy was investigated with compression tests in temperature range of 380–470 °C and strain rate range of 0.001–10 s⁻¹ using Gleeble 1500 system, and the associated microstructural evolutions were studied by metallographic microscopy and transmission electron microscopy. The results show that true stress—strain curves exhibit a peak stress, followed by a dynamic flow softening at low strains (ɛ<0.05). The stress decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener-Hollomon exponential equation with the activation energy for deformation of 157.9 kJ/mol. The substructure in the deformed specimens consists of few fine precipitates with equaixed polygonized subgrains in the elongated grains and developed serrations at the grain boundaries. The dynamic flow softening is attributed mainly to dynamic recovery and dynamic recrystallization. Foundation item: Project(2006AA03Z523) supported by the National High-Tech Research and Development Program of China     

Semi-analytical weight function method for center-cracked circular disk subjected to diametral compression

Ｓｉｎｃｅｔｈｅｃｅｎｔｅｒ ｃｒａｃｋｅｄｃｉｒｃｕｌａｒｄｉｓｋｓｐｅｃｉ ｍｅｎｃａｎｂｅａｖａｉｌａｂｌｅｄｉｒｅｃｔｌｙｆｒｏｍｒｏｃｋｃｏｒｅ ,ａｎｄｍｉｘｅｄ ｍｏｄｅｆｒａｃｔｕｒｅｒａｎｇｉｎｇｆｒｏｍｐｕｒｅｍｏｄｅⅠｔｏａｎｙＫⅡ/ＫⅠ ｒａｔｉｏｃａｎｂｅｅａｓｉｌｙａｃｈｉｅｖｅｄｕｓｉｎｇｔｈｉｓｇｅｏｍｅｔｒｙ ,ｉｔｉｓｗｉｄｅｌｙｕｓｅｄｉｎｔｈｅｍｉｘｅｄ ｍｏｄｅｆｒａｃｔｕｒｅｉｎｖｅｓｔｉｇａｔｉｏｎｏｆｂｒｉｔｔｌｅｍａｔｅｒｉａｌｓ .Ｔｈｅｐｒｏｂｌｅｍｃｏ…     

Relationship between attachment probability and surface energy in adhesion process of gold particles to oil-carbon agglomerates

1　INTRODUCTIONInflotationfield ,someattentionshavebeenpaidtomodelingtheprocessofflotationinordertoobtainadeepunderstandingofhowamineralparticleisat tachedtoabubble .Schulze[1] analyzedtheelementaryprocessesofflotationintermsofphysico chemistry .Anfrunsetal[2 ] studiedthebubble particleattachmentexperimentallyinthecaseofpotentialflow .Solarietal[3] investigatedtheeffectofbubblesizesonadhesionprobability .Polatetal[4 ] estimatedthetruedistribu tionofflotationrateconstantsforafirst orderkin…     

A new method of characterizing equivalent strain for equal channel angular processing

In order to establish the quantitative relationship between equivalent strain and the performance index of the deformed material within the range of certain passes for equal channel angular processing (ECAP), a new approach to characterize the equivalent strain was proposed. The results show that there exists better accordance between mechanical property (such as hardness or strength) and equivalent strain after rolling and ECAP in a certain range of deformation amount, and Gauss equation can be satisfied among the equivalent strain and the mechanical properties for ECAP. Through regression analysis on the data of hardness and strength after the deformation, a more generalized expression of equivalent strain for ECAP is proposed as: ɛ=k ₀exp[−(k ₁ M−k ₂)²], where M is the strength or hardness of the material, k ₁ is the modified coefficient (k ₁∈(0, 1)), k ₀ and k ₂ are two parameters dependent on the critical strain and mechanical property that reaches saturation state for the material, respectively. In this expression the equivalent strain for ECAP is characterized novelly through the mechanical parameter relating to material property rather than the classical geometry equation. Foundation item: Projects(50471102, 50671089) supported by the National Natural Science Foundation of China     

Fracture evolution characteristics of sandstone containing double fissures and a single circular hole under uniaxial compression

The uniaxial compression experiments on the sandstone samples containing double fissures and a single circular hole were carried out by using electro-hydraulic servo universal testing machine to investigate the effect of rock bridge angle β and fissure angle α on mechanical properties and evolution characteristics of cracks.The results show that the peak strength,peak strain and elastic modulus of defected specimens decrease comparing with those for intact sample,and show a decreased trend firstly and then increase with β changing from 0° to 90°.The peak strength and elastic modulus achieve the minimum value as the rock bridge angle is 60°,while the peak strain reaches the minimum value with the rock bridge angle of 45°.The crack initiation of tested rock samples occurs firstly in stress concentration areas at tips of prefabricated fissures under uniaxial compression,and then propagates constantly and coalescences with the prefabricated hole.Some secondary cracks initiate and propagate as well until buckling failure happens.The rock bridge angle has a great influence on crack initiation,coalescence,final failure mode,crack initiation stress and transfixion stress.The peak strength varies significantly,while the elastic modulus and peak strain change slightly,and the failure modes are also different due to the influence of fissure angle.     

Analysis of fracture process zone in brittle rock subjected to shear-compressive loading

1　INTRODUCTIONFracture process zone (FPZ) is very importantto study some physical phenomena, such as nonlin ear behavior of brittle rock and initial angle ofcrack extension which has to be taken into accountin order to predict the initial angle of crack exten sion and the fracture locus. Besides, the structureand size of FPZ ahead of crack tip are crucial forthe application of conventional fracture criteria,since the stress or energy in these criteria shouldbe ca…