基于应力三轴度的TA18钛合金损伤失效研究

对TA18钛合金材料设计不同缺口尺寸的拉伸试样,进行不同应力状态下的室温拉伸试验及断口形貌观察,通过试验和数值计算结合的方法研究TA18钛合金的韧性断裂特性,分析了应力状态对微观断裂机制的影响规律.利用Bridgman正向计算法修正颈缩失稳后的应力数据,建立了TA18钛合金的Johnson-Cook(J-C)本构模型,计算了拉伸试样的平均应力三轴度和断裂应变,回归确定了TA18钛合金损伤失效模型.结果 表明:不同应力状态下的TA18钛合金断裂应变各不相同,断裂应变随着应力三轴度的增大而减小,断口韧窝尺寸与应力三轴度呈正相关关系,所建立的损伤失效模型能够描述该材料的断裂特性.     

断裂仿真中的网格尺寸效应与断裂曲线修正

有限元仿真模拟方法可实现对结构材料从变形到破坏全过程的有效预测，而仿真模型建立的精度对于预测的准确性至关重要。应用有限元对标优化方法建立DP780材料的光滑拉伸、缺口拉伸及剪切拉伸试样的准静态仿真模型。划分不同网格尺寸，逆推得到材料的大变形本构曲线，以及各试样的应力三轴度、罗德角参数，系统研究了网格尺寸对材料拉伸本构曲线及韧性断裂参数逆推结果的影响。基于Lou-Huh(2012)断裂准则模型建立了不同网格尺寸下的断裂曲线。采用GISSMO理论建立断裂应变的网格尺寸效应修正函数，并对平面应力条件下的断裂曲线进行修正。研究结果表明：随着网格的细化，塑性应变-真应力曲线的对标结果向大应变方向延伸并趋于收敛，相同试样所标定的应力三轴度值与临界断裂应变值增大。随着网格尺寸的增大，临界断裂应变呈现指数形式递减，网格尺寸大于2 mm后趋于平稳。基于网格尺寸效应修正的韧性断裂模型可有效降低材料断裂仿真分析对网格尺寸的敏感性。     

基于裂纹扩展模型的脆性岩石破裂特征及力学性能研究

为了探究初始微裂纹参数分布对岩石破裂特征及力学性能的影响,进一步系统地了解脆性岩石破裂演化过程,依据线弹性断裂力学理论,建立了非均质性二维细观弹性损伤模型,并运用FLAC^2D数值分析软件,数值模拟研究了单轴压缩条件下不同形态岩石试样的破裂过程。研究结果表明,当初始微裂纹长度和角度服从不同的随机分布时,岩石材料表现出不同的破裂特征,其中初始微裂纹长度和角度均服从正态分布时,岩石破裂区域较完整;初始微裂纹长度或角度服从均匀分布和指数分布时,岩石破裂区域较分散;初始微裂纹角度对于解释脆性岩石单轴抗压试验时岩石试样出现剪切破坏和劈裂破坏的原因具有一定的指导意义,且当初始微裂纹角度均值ɑ=45°时,模型具有最小的峰值强度和轴向最大应变。模型还模拟了脆性岩石单轴抗压试验、巴西劈裂试验和断裂韧度试验的演化过程,模拟结果与试验结果具有较高的一致性。     

基于RFPA-3D含裂隙矿体破裂扩展数值模拟研究

为了深入探究不同参数条件下含裂隙矿体的力学特性及破裂扩展路径、破坏模式及破裂扩展规律,借助RFPA-3D真实数值模拟仿真软件开展了3种不同长度预制裂隙及5种不同角度下的单轴压缩数值模拟试验。结果表明:(1)随着裂隙长度增大,模型试样峰值应力逐渐降低;随着裂隙倾角增大,模型试样峰值应力出现“V”字型变化规律;当裂隙倾角为60°时,模型试样峰值应力最低。(2)张拉应力是引起模型试样裂隙尖端产生起裂的主要因素,剪切应力是造成裂隙横向扩展并形成破碎区的主要原因。(3) 0°～30°模型试样以张拉劈裂破坏为主,45°～60°模型以剪切破坏为主,90°模型试样呈现出明显的剪切-张拉组合破坏。     

恒湿环境下不同含水饱和度砂岩制备方法及压缩试验

针对目前制备不同含水饱和度岩石试样存在的精度低和试样内部含水分布不均匀等问题,基于渗透技术和化学热力学原理,设计了一种恒湿环境下不同含水饱和度岩石试样制备装置,并通过核磁共振成像实验验证了该装置精确制备水分均匀分布岩石试样的合理性。同时,利用该装置开展了不同含水饱和度条件下砂岩常应变率压缩试验。结果表明：砂岩的单轴抗压强度和弹性模量均随着含水饱和度的增加而降低,随着围压的增大,试样砂岩压密阶段减弱,试样的变形经历弹脆性—弹塑性—应变硬化的过程。研究结果为不同含水饱和度岩石试样的精确制备提供了新的方法和思路。     

基于NSCB方法的冻结红砂岩动态断裂特性试验

采用红砂岩制作中心直裂纹半圆盘弯曲试样（Notched semi-circular bend, NSCB）,设置不同的负温温度对岩石试样预处理,随后利用改进后的分离式霍普金森杆（SHPB）实验系统开展动态试验.结果表明：岩石的断裂韧度存在明显的加载率效应,断裂韧度试验值随加载率的增加近似呈指数型增大;当加载率一定时,岩石断裂韧度由常温进入负温后先缓慢后快速增加,在–20℃时达到最大值,随着温度进一步降低,岩石断裂韧度快速减小.进一步对岩石破裂过程分析发现,不同温度下岩石的断裂过程基本一致,且裂纹扩展速度受温度影响较小.基于岩石断面的扫描电子显微镜结果分析岩石断裂模式为：负温下红砂岩的断裂以沿晶破裂和胶结物的撕裂为主,伴有少量的穿晶破裂现象,同时当温度降低至–25℃时,岩石内部微裂隙数量明显增多,说明负温对岩石具有劣化作用.最后探讨了温度对岩石内部结构的影响机制,对分析岩石断裂特性的低温效应具有一定参考意义.     

温度对不同尺寸砂岩巴西劈裂特性影响

为研究高温与尺寸效应耦合作用下的砂岩巴西劈裂特性,分别对经过25、200、400、600、800和1000 ℃高温处理后的标准砂岩试件进行巴西劈裂室内试验,并基于颗粒流软件开展不同尺寸高温砂岩巴西劈裂数值模拟,研究砂岩巴西劈裂强度及其劣化规律、孔隙率增加相对于裂纹扩展贯通的滞后性规律。研究结果表明:（1）在25～1000 ℃的温度范围和50～100 mm的直径范围内,温度与尺寸效应对砂岩巴西劈裂强度均有显著影响,且尺寸效应影响程度更大。在加热过程中,由于岩石内部首先发生热膨胀,然后在热应力作用下产生损伤,因此砂岩劈裂强度先有所增大,在400 ℃之后持续降低,劈裂强度下降约34.66%～35.10%;随着尺寸增大,岩石内部积聚的能量释放产生大量微裂隙,导致砂岩试样劈裂强度降低,下降约55.61%～56.99%。（2）砂岩巴西劈裂强度劣化幅值与其直径之间满足负指数函数关系,可用于预测不同尺寸高温砂岩的巴西劈裂强度。（3）砂岩在巴西劈裂过程中的孔隙率增加相对于裂隙扩展贯通滞后的荷载差值随温度升高以及尺寸增大而增大;考虑两因素的耦合作用,尺寸效应对荷载差值的影响程度随温度的升高而降低,温度对荷载差值的影响程度随砂岩尺寸的增大而降低。研究成果对火灾后顶板维护,初步预测顶板强度具有一定参考意义,也可为核废料处理、地热资源开发和深井工程等涉及高温和尺寸变化的岩体工程设计提供有益参考。      

γ-TiAl金属间化合物的压缩断裂行为研究

通过室温压缩试验,研究全片层γ-TiAl基合金在不同加载速度和不同卸载载荷下的压缩断裂行为。结果表明:随着加载速度的增加,γ-TiAl基合金试样的屈服强度及抗压强度相应增大;试样的最终断裂是通过裂纹的形核、扩展以及相互贯通而形成的,断裂面主要由剪应力形成的撕裂区和压应力形成的解理断裂区域组成,并且在不同加载速度下,断口也呈现出规律性的变化。在不同载荷加载-卸载-再加载的过程中,小载荷(4.67、9.42、18.94 k N)下卸载和加载的名义应力-名义应变曲线完全重合,大载荷(26.60、37.24、53.20 k N)下卸载后产生的不可逆应变依次增大;裂纹面密度随着卸载载荷的增大而逐渐增大,材料的损伤程度不断增加。     

试样几何因素对r值n值试验结果的影响

试验目的考核试样宽度、平行长度、过渡弧半径、标距等几何因素对r值n值试验结果的影响。试验条件 1)试验基本尺寸,如图1所示。2)尺寸测量工具标距一比长仪(分度值0.001mm)引伸计(最大应变示值误差≤0 .001) 3)测量方法 r值n值在同一试样上测定,所有结果均为3片试徉的平均值。r值用卸载结果计算,n值用不卸载结果根据拟合法计算。应     

S390级转向架用耐候钢的高温塑性

利用Gleeble 3500热模拟试验机对S390转向架用耐候钢的高温塑性进行测定。通过金相显微镜、扫描电镜及能谱仪对断口组织、断口形貌和析出物进行观察和分析。利用热膨胀仪测定S390耐候钢的临界相变温度,通过Thermal-Calc软件计算微合金元素的热力学析出温度。结果表明,在1 250~650 ℃范围内,存在3个区间,第Ⅰ脆性区在1 220 ℃以上,断裂形式是由S、O等元素偏析引起的沿晶断裂;第Ⅲ脆性区为980~650 ℃,断裂形式是由析出物钉扎晶界及先共析铁素体析出引起的沿晶断裂;在1 220~980 ℃第Ⅱ脆性区内,由于动态再结晶的发生,不出现脆性区,断裂形式为穿晶塑性断裂。实际生产过程中可避开脆性区间,以减轻S390耐候钢的裂纹倾向。     

Fracture Toughness of Wood Fiber Gypsum Panels from Size Effect Law

This paper addresses the size effect of inplane bending strength as well as Mode I fracture toughness and process zone length of wood fiber-reinforced gypsum panels. Wood fiber gypsum panels represent an incombustible short fiber composite material composed of recycled paper fibers embedded in a gypsum matrix. The material, which is used for sheathing and bracing of timber frame constructions, exhibits marked fracture softening supposedly resulting in a considerable size effect. In the paper presented, in a first step Ba?ant’s size effect law for quasi-brittle materials is derived. The parameters of this size effect law are then determined by means of nonlinear regression analysis applied to a test series with scaled single edge notched beam specimens. Detailed consideration is given to the adequacy of linear confidence intervals of the model parameters in comparison to nonlinear inferential results. Finally, the probability densities of fracture toughness and fracture process zone length are determined from the distributions of the size effect parameters by means of theory of random variables.     

Boundary Element Simulation of Size Effect for Quasi-Brittle Materials by Using One-Size Specimen

A multizone nonlinear boundary element method is used to simulate the size effect of quasi-brittle materials that are modeled as randomly dispersed particulate composites with randomly distributed microcracks located between the particles and matrix. Usually a series of geometrically similar specimens are used to describe the size effect of materials. In this paper, a one-size specimen method for determining the fracture energy Gf and effective process zone length cf is proposed and verified by numerical simulation. The specimen used for boundary element simulation is confined to only one shape and one size but with different initial crack lengths. A three-point-bend beam and a compact tension specimen are used to expand the range of brittleness number and therefore increase the degree of confidence of the results. Notchless specimens of the same size are also calculated and used to supplement the data of one-size notched specimens. The shape, size, and location of the fracture process zone caused by development of microcracking are observed. The results of Gf and cf obtained from the one-size specimen method are compared with a commonly used method and a good agreement is shown.     

Fracture behaviours of fracture toughness testing specimens with metallurgical heterogeneity along crack front

Safe use of welded structures is dependent on fracture mechanics properties of welded joints. In present research, high strength low alloyed HSLA steel in a quenched and tempered condition, corresponding to the grade HT 80, was used. The fluxo cored arc welding process (FCAW), with CO₂ as shielding gas, was used and two different tubular wires were selected. The aim of this paper is to analyse fracture behaviour of undermatched welded joints, and also to determine relevant parameters which contribute to higher critical values of fracture toughness. Towards this end three differently undermatched welded joints were analysed using results of testing the composite notched specimens with through thickness crack front positioned partly in the weld metal, partly in heat affected zone (HAZ) and partly in base material (BM).The presence of different microstructures along the pre‐crack fatigue front has an important effect on the critical crack tip opening displacement (CTOD). This value is the relevant parameter for safe service of welded structure. In the case of specimens with through thickness notch partly in the weld metal, partly in the heat affected zone and partly in the base material, i.e. using the composite notched specimen, fracture behaviour strongly depends on a partition of ductile base material, size and distribution of mismatching factor along vicinity of crack front. If local brittle zones occur in the process zone, ductile base metal can not prevent pop‐in instability, but it can reduce it to an insignificant level while the fracture toughness parameter is higher and the weakest link concept can not be applied.     

Elasto-Plastic Test of Q235 Steel Bending Beam With Cracking Resistance

More than 30 bending beams with rectangular cross-section and different thicknesses and heights were prepared from Q235 steel. The specimen dimensions were about 240 mm (length) × 60 mm (height) × 70 mm (thickness). Flaws were cut along its middle line with a wire cutter, with lengths ranging from 6 to 35 mm. Each specimen was tested with three-point bend loading, and a process curve was obtained between load and the displacement of the loading point, in order to analyze the fracture process when opening the crack. A deformation near the prefabricated crack was observed in the testing period, and the variation of the fracture characteristic parameters was analyzed for different sizes. For a comprehensive understanding of carbon steel fracture resistance behavior, its elasticity and plasticity were established by determining its Young's modulus and Poisson's ratio with an optical strain gauge. This gauge was also used for the loading process test. It was found that the fracture toughness varied with the dimensions, and the toughness of the elastic limit loading was almost constant. Using the relationship of crack resistance stress intensity factor and fracture criterion, the bearing capacity of the material structure could be estimated, which shows a good agreement with the experimental test data.     

红砂岩中矿物颗粒的塑性应变分析

为定量研究砂岩中矿物颗粒在载荷作用下的塑性应变和应力,揭示岩石内部赤铁矿颗粒的变形行为,基于弹塑性力学理论和张量分析,采用X射线CT对砂岩三维结构进行扫描（空间分辨率为4.6 μm）,分析岩石内部矿物颗粒的运移规律,并提出了岩石矿物颗粒变形梯度张量计算的方法研究其塑性应变。首先对砂岩矿物颗粒位移进行提取,并采用Non Local Means滤波算法对砂岩三维数字图像去噪;然后基于砂岩三维结构,对不同矿物组分进行分割,通过构造主轴应变的变形张量,计算砂岩矿物颗粒的变形梯度和应力、应变分量。结果表明：该滤波算法对砂岩CT图像和射束硬化现象具有显著改善效果。此外,基于X射线CT的砂岩原位测试结果显示,砂岩内部存在较复杂的变形行为和应力响应,且在断裂带和非断裂区域变形行为有着显著差异;岩石内部颗粒在Z轴方向受到压应力,而在XY平面受到拉伸应力的作用,同时矿物颗粒内部存在较大的塑性应变,且试样内部颗粒的应变和应力要远远大于试样宏观应变和应力。该方法对于揭示岩体内部结构和应力、应变状态演化过程具有重要作用。     

Toughening and weakening in ferroelectric ceramics by domain-switching process under mixed electric and mechanical loading

The structural reliability of ferroelectric ceramics calls for a better understanding of the mechanism of fracture. For a ferroelectric ceramic under mechanical and electric loadings, the intensified stress and electric fields in the vicinity of a crack tip lead to polarization switching. The switched zone induces an incompatible strain and, consequently, changes the stress intensities and the apparent fracture toughness. This article investigates the effects on fracture parameters of an initially poled ferroelectric ceramic induced by polarization switching in the case of small-scale switching. The changes of stress intensities and energy release rate at the crack tip induced by the switched zone are calculated when the specimen is under both electric and mechanical loadings. Our results show that a switched zone usually produces both mode I and mode II stress intensities at the crack tip, and the increments are strongly dependent on the initial poling direction and electric-field strength. With certain initial poling directions, the material could be toughened, whereas, with other directions, be weakened. Generally, high-magnitude electric fields (both positive and negative) decrease the material toughness when both the mechanical and electric fields at the crack tip are singular.     

Concrete Fracture Process Zone Characterization with Fiber Optics

The characterization of the fracture process zone inside a cracked concrete specimen is investigated through optical fiber technology. Fiber Bragg gratings are used to measure the strains in the close vicinity of the crack front, thus enabling us (for the first time) to directly measure those deformations inside the concrete specimen. A detailed discussion of the experimental technique is first provided. Then, following the experimental measurements, test results are presented and discussed. Preliminary results indicate that there is a narrow zone on each side of the crack front with inelastic residual strains, and another where strains increase only in the presence of a neighboring crack tip. The width of the process zone is found to be about three times the maximum aggregate size.     

Damage effect on the fracture toughness of nodular cast iron: Part-II. Damage zone characterization ahead of a crack tip

In order to understand the fracture toughness of nodular cast iron, the damage zone was studied by Scanning Electron Microscope (SEM) observations of the polished surface of a CT 25 specimen before and after ductile tearing. Damage is defined as decohesion at the graphite/matrix interface. It is shown that the damage zone is very large in nodular cast iron (almost throughout the whole remaining ligament ahead of the crack tip), so linear elastic fracture mechanics (LEFM) are not valid for small specimens. The size of the damage zone was calculated analytically by introducing a damage initiation criterion which was based both on observations of the debonding of the interface between matrix and graphite nodules and on measurements of the pressure sensitivity of cast iron. To take into account the actual boundary conditions, the damage zone was also calculated by numerical modeling using the modified Gurson’s model and by considering the nodular cast iron as a porous material. The calculated results led to good agreement with the damage zone observations. Plane stress and plane strain calculations yielded nearly the same size plastic zone. This result is opposite to those obtained for fully dense materials.     

Heat Treatment Optimization and Properties Correlation for H11-Type Hot-Work Tool Steel

The aim of this research was to determine the effect of vacuum-heat-treatment process parameters on the material properties and their correlations for low-Si-content AISI H11-type hot-work tool steel using a single Circumferentially Notched and fatigue Pre-cracked Tensile Bar (CNPTB) test specimen. The work was also focused on the potential of the proposed approach for designing advanced tempering diagrams and optimizing the vacuum heat treatment and design of forming tools. The results show that the CNPTB specimen allows a simultaneous determination and correlation of multiple properties for hot-work tool steels, with the compression and bending strength both increasing with hardness, and the strain-hardening exponent and bending strain increasing with the fracture toughness. On the other hand, the best machinability and surface quality of the hardened hot-work tool steel are obtained for hardness values between 46 and 50 HRC and a fracture toughness below 60 MPa√m.     

Random Lattice-Particle Simulation of Statistical Size Effect in Quasi-Brittle Structures Failing at Crack Initiation

The modeling of statistical size effect of concrete structures that fail at crack initiation is studied, with special attention to the interaction between the autocorrelation length and the size of the failure zone. The mechanical failure of concrete is modeled by a network of axial springs with degrading stiffness. The heterogeneity of concrete is idealized by spatial variation of the tensile strength and fracture energy. As an example, the direct tensile test in plane stress, with the size range of 1:20, is simulated. Furthermore, simulations of a four-point bending test with varying bending span are compared to the experimental results reported in the literature. The interaction of the autocorrelation length and the size of the failure zone is identified as a key parameter for the modeling of statistical size effect.