基于DISL方法的硬岩矿柱脆性破坏数值模拟

以板裂为表现形式的岩石脆性破坏行为是深埋硬岩岩体开挖卸荷造成的典型围岩破坏现象。在简要比较4种硬岩脆性破坏数值模拟方法的基础上,基于损伤启裂-板裂界限(Damage initiation and spalling limit,DISL)模型,借助FLAC^3D开展硬岩矿柱原位压缩及单轴压缩数值模拟,探讨DISL方法的适用性并进一步探究硬岩矿柱的脆性破坏过程及其特征。结果表明:原位压缩模拟中,当卸荷总时步大于某一临界数值时,矿柱两侧发生V形破坏,进而矿柱整体形成沙漏状;不同均质度下屈服破坏单元主要分布在V形破坏区内及周围,其分布范围受低围压区及张拉区范围的控制;V形剪切带主要以拉剪破坏为主,剪切带内部靠近矿柱边壁一侧,伴有张拉破坏单元;单轴压缩模拟中,峰后阶段矿柱两侧仍然产生V形破坏,并且张拉裂隙的形成范围受剪切带控制。     

TWIP钢的低周疲劳断裂机制

通过采用扫描电镜及透射电镜等手段,观察并研究了TWIP钢在低周单轴循环对称拉压载荷下的疲劳断裂后的显微组织。结果表明:TWIP钢矩形试样的疲劳裂纹一般萌生于角部,从表面萌生时可能表现为多个疲劳源。在低周疲劳变形过程中,TWIP钢不但产生了形变孪晶,还产生了大量的微条带,其实质为细微孪晶片层和驻留滑移带。疲劳裂纹主要萌生于微条带对晶界和孪晶界的撞击引发的孔洞。孔洞串连接起来成为裂纹,夹杂物促进了裂纹扩展。随着裂纹的扩展,试样的承载面积不断减小,最终发生快速的韧性断裂。     

STRESS CORROSION CRACKING OF AN ALUMINIUM ALLOY UNDER COMPRESSIVE STRESS

用WOL缺口压缩试样研究LC4铝合金在3.5％NaCl水溶液中的压应力腐蚀.有限元计算指出,试样加压缩位移时缺口附近有压应力集中.实验结果表明,当压缩位移大于临界值后产生压应力腐蚀裂纹. 实验测出压应力腐蚀裂纹的门槛值为K_(ISCC*)=27.6MPam~(1/2),等于拉应力腐蚀门槛值的3.5倍.在相同K_I下压应力腐蚀裂纹的孕育期比拉应力高一个数量级.压应力腐蚀时获得带有平行条纹花样的准解理断口,这和拉应力腐蚀的沿晶断口完全不同。     

压应力条件下铝合金的应力腐蚀

用WOL缺口压缩试样研究LC4铝合金在3.5％NaCl水溶液中的压应力腐蚀.有限元计算指出,试样加压缩位移时缺口附近有压应力集中.实验结果表明,当压缩位移大于临界值后产生压应力腐蚀裂纹. 实验测出压应力腐蚀裂纹的门槛值为K_(ISCC*)=27.6MPam~(1/2),等于拉应力腐蚀门槛值的3.5倍.在相同K_I下压应力腐蚀裂纹的孕育期比拉应力高一个数量级.压应力腐蚀时获得带有平行条纹花样的准解理断口,这和拉应力腐蚀的沿晶断口完全不同。     

层片状双相TiAl合金拉伸与压缩变形行为差异

本文研究了层片状双相TiAl合金的室温拉伸压缩变形行为与断裂行为,发现在拉压变形条件下,其室温塑性有显著差异.并且这种差异与裂纹扩展路径有关.在室温拉伸与压缩变形时,该合金的拉压屈服应力随外载与层片界面间的夹角φ的变化趋势一致,而拉压断裂应变εf随夹角φ的变化趋势正好相反.外载与层片界面垂直时(φ=90°),拉伸断裂应变最小(εf≈0);压缩断裂应变最大(εf≈38%).夹角φ减小时,拉伸断裂应变增加,压缩断裂应变减小.在拉、压变形时裂纹的扩展路径不同.     

高炉煤气余压透平机叶片断裂失效分析

高炉煤气余压透平机在运行84d后突然出现故障,观察发现透平机叶片从根部断裂,对断裂叶片进行断口显微形貌、能谱和金相分析。结果表明:裂纹源处有很多在生产过程中形成的细小孔洞,这些孔洞处形成了应力集中并产生细小裂纹,裂纹不断长大、扩展,最终导致整个叶片产生疲劳断裂。     

不锈钢锻件的裂纹萌生机制与组织演变研究

研究了不锈钢锻件在变形过程中的组织演变规律和裂纹形核与扩展机理,采用应力三维度法对锻件断裂破坏过程进行研究。结果表明,在热拉伸过程中不锈钢锻件的裂纹产生主要是经过裂纹形核、长大和聚集过程,变形过程中缩颈区域的显微孔洞更倾向于在晶界处产生。单向拉伸过程中,应力三轴度在越靠近心部的区域越大,则孔洞或者微裂纹越容易形成断裂。采用应力三轴度法可以准确判断不锈钢锻件在高温变形过程中的裂纹萌生和扩展过程,对于预测不锈钢锻件在高温锻造过程中损伤破坏过程具有重要的作用。     

高强度钢压剪疲劳裂纹扩展的实验研究

滚珠轴承接触点附近的高压剪应力是引起轴承疲劳破坏的原因。为获得高强度钢在压剪疲劳加载下的断裂性态,通过轴向裂纹薄壁圆筒的压剪疲劳加载试验,研究了这类材料的疲劳破坏规律。结果表明,复合疲劳加载时门槛值对压应力分量并不敏感,但对扩散速率和临界扩展角却有明显影响。裂纹扩展速率随压应力增加而提高,裂纹较快达到失稳;裂纹扩散角则随压应力增加而减小,疲劳裂纹将向有利于Ⅰ型断裂的方向扩展。这表明轴承滚道内,任何平行于压应力的缺陷、微裂纹都将是十分危险的。     

微缺陷对B2-NiAl高温涂层材料力学性能及失效机理的影响

目的研究微缺陷对B_2-NiAl高温涂层材料在拉伸载荷作用下的变形行为和失效机理的影响。方法在考虑裂纹和孔洞等微缺陷的影响下,采用嵌入原子势函数(EAM)和分子动力学方法,模拟了完美B_2-NiAl涂层(Sample 1)、含中心对称微裂纹涂层(Sample 2)、含有中心微裂纹与单微孔洞涂层(Sample 3)和含有中心微裂纹与双微孔洞涂层(Sample 4)模型的失效过程,利用键对分析技术(CAN)、中心对称参数法(CSP)和径向分布函数(RDF),对涂层的变形过程和失效机理进行了分析表征。结果随着微缺陷的增加,高温材料的屈服应力会明显下降,弹性模量也会有所降低,屈服应变逐渐减小,但微孔洞会使涂层出现二次屈服现象。完美B_2-NiAl高温涂层在屈服后的位错和相变区域面积较小,分布比较均匀,在边界处萌发裂纹并沿(100)方向扩展,再沿着(111)方向扩展,直至失效。相比完美B_2-NiAl高温涂层,含有微裂纹和孔洞的涂层在屈服后的位错和相变区域较小,主要在裂纹尖端沿着111滑移系方向均匀分布,但其主裂纹沿着[100]方向扩展并导致断裂。结论微裂纹会降低B_2-NiAl高温涂层的强度,但微孔洞会提高其塑性。应力集中会导致微裂纹萌生并在裂纹尖端附近产生微孔洞,使其与主裂纹贯通直至失效,而位错塞积则是造成应力集中的主要原因。     

不同孔隙率规则多孔钛的变形失效特征

设计和制备了孔隙率不同的3组规则多孔钛试样,采用分离式霍普金森压杆实验装置,开展了应变率范围在600~2100 s^-1的单轴压缩实验,研究了加载过程中整个试样和局部孔洞的变形失效特征。结果表明,试样展示出2种典型的变形模式：压缩变形模式和连通断裂模式。压缩变形模式主要发生在低孔隙率的试样,试样的外表面胞壁出现局部塌陷,而连通断裂模式易发生在高孔隙率的试样中,常在试样的一个或多个孔层出现连通的断裂。另外,2种变形模式下的局部孔洞也呈现出不同的形状变化特征,且在孔洞最小曲率的位置都形成明显的应力集中,相应的应力集中因子随试样孔隙率的增大而增大。微结构分析显示：剪切带是试样失效的主要机制,试样的断口容易产生韧窝和韧性条纹。     

New fracture morphology of amorphous Fe78Si9B13 alloy

The tensile fracture morphology of a brittle amorphous Fe-based ribbon was investigated by scanning electron microscopy（SEM）. The fracture surface consists of mirror, mist and river-pattern zones with crack propagating. The formation of nanoscale damage cavity structure is a main characteristic morphology on the fracture surface. Approaching the fracture source in the mirror, these damage cavities assemble and form the nanoscale periodic striation patterns, which are neither Wallner lines nor crack front waves. At shear deformation stage, the apparent surface energy yf of amorphous Fe78Si9B13 ribbon is much smaller than that of less-brittle amorphous alloys, which indirectly indicates amorphous Fe7sSi9Bl3 ribbon is perfectly brittle. The crack branching appears at the moment of final fracture due to the high crack propagating velocity.     

Modeling crack growth from pores under compressive loading with application to metallic glasses

The mechanism of the fatigue-crack growth is essential to understand the fatigue and fracture behavior of bulk metallic glasses (BMGs) and is thus critical to predict the service lifetime of BMGs as potential engineering structural materials. Experiments indicate that fracture under compressive loading exhibits distinct behaviors different from that under tensile loading. A typical compression failure may initiate from micro porosity where cracks propagate in a direction generally parallel to the loading axis. Micromechanical stress analysis shows that pores cause axial tensile microcracks emanating from the pore. A simplified computational model based on the linear elastic fracture mechanics (LEFM) is proposed to investigate crack initiation and subsequent propagation under compressive load, where the effect of crack closure on mode-I fracture is considered. The stable crack length is characterized by a dimensionless fracture-mechanics quantity required to attain the associated crack length. The behavior of crack growth is examined based on the stress-intensity-factor (SIF) calculation, and its dependence on the loading and lateral confinement conditions is discussed.     

Specific Features of the Nucleation and Growth of Fatigue Cracks in Steel under Cyclic Dynamic Compression

The processes of the fracture of 40Kh and U8 steels under cyclic dynamic compression are studied. It has been found that the main cause for the fracture of the cyclically compressed specimens is the propagation of cracks due to the effect of residual tensile stresses, which arise near the tips of the cracks at the stage of the unloading of the specimens. The growth rate of a crack has the maximum value at the initial stage of its propagation in the vicinity of the stress concentrator. As the crack propagates deep into the specimen, its growth rate decreases and depends only slightly on the real cross section of the specimen. The model of the process of the fatigue fracture of the steels under dynamic loading by a cyclically varied compressive force is proposed. It has been found that the high fatigue endurance is provided by tempering at 200°C for the 40Kh steel and at 300°C for the U8 steel.     

Thermal stresses in diamond coatings and their influence on coating wear and failure

Thermal stresses in diamond coatings deposited onto cemented carbide substrates are calculated using the finite element method. The thermo-elastic stress fields for some coating-substrate geometries are presented. The results are compared with experimental data on the tribological behaviour of diamond coatings. Residual stresses can explain many of the observed patterns of coating wear and failure. A model for the abrasive wear of brittle coatings under large compressive biaxial stresses is described. These stresses prevent cracks initiated at the surface to propagate towards the interface and may promote crack paths parallel to the interface, thus causing the formation of a smooth coating surface. Once the smooth appearance is reached it will become extremely hard to initiate and propagate cracks into the coating and consequently the wear rate becomes very low. Thus, large compressive residual stresses increase the already high wear resistance of diamond coatings. When diamond coatings are deposited onto substrate edges, intense concentrations of normal and shear stresses may lead to coating failure by interfacial spalling. These stresses are lowered by increasing the ratio r/h, where r is the edge radius and h is the coating thickness.     

The influence of cutting edge radius on the machined surface of brittle materials in simulated orthogonal machining

When cutting brittle materials, many surface cracks appear on the flat surface due to tensile stresses induced by the cutting tool. These cracks propagate into the substrate and will affect the fracture properties of a body. Crack spacings and the directions of crack propagation into glass were calculated numerically by applying the finite element method and using linear-elastic fracture mechanics. The calculated crack spacings were in the range of the experimental results. Stress intensity factors and crack extension angles were dependent on the edge radius and the load on the tool, and from these two factors the possible directions of crack propagation were calculated. The calculated propagation directions were in good agreement with the actual crack propagation directions.     

Stresses State and Mechanical Behaviors of the Green Body During Die Compaction and Ejection Process

Finite element simulations for metal powder compaction of a clutch plate were performed to examine the stresses during compaction, unloading, and ejection. To describe the mechanical behavior of compacted green body, a modified density-dependent Drucker-Prager Cap model was utilized to predict the stress and density distribution of the compacted clutch plate during loading and ejection stages. The results indicate that maximum tensile principal stress was a main driving force for the tensile crack initiation during ejection stage, and shear stress may be another driving force in both compaction and ejection stages for shear crack initiation. There were peak value of the stresses during ejection stage, and the stresses are in compressive state only during compaction stage. Therefore, the tensile crack initiation is not possible during compaction except shear crack. Hoop stress in the clutch plate is of less contribution to the crack initiation during compaction, unloading and ejection. Study of criteria of the crack initiation and fracture is necessary in order to obtain uniform density and crack-free components in the manufacturing of metal powder compaction.     

Fatigue properties of rolled AZ31B magnesium alloy plate

Fatigue strength, crack initiation and propagation behavior of rolled AZ31B magnesium alloy plate were investigated. Axial tension-compression fatigue tests were carried out with cylindrical smooth specimens. Two types of specimens were machined with the loading axis parallel (L-specimen) and perpendicular (T-specimen) to rolling direction. Monotonic compressive 0.2% proof stress, tensile strength and tensile elongation were similar for both specimens. On the other hand, monotonic tensile 0.2% proof stress of the L-specimen was slightly higher than that of the T-specimen. Moreover, monotonic compressive 0.2% proof stresses were lower than tensile ones for both specimens. The fatigue strengths of 107 cycles of the L- and T-specimens were 95 and 85 MPa, respectively. Compared with the monotonic compressive 0.2% proof stresses, the fatigue strengths were higher for both specimens. In other words, the fatigue crack did not initiate and propagate even though deformation twins were formed in compressive stress under the cyclic tension-compression loading. The fatigue crack initiated at early stage of the fatigue life in low cycle regime regardless of specimen direction. The crack growth rate of the L-specimen was slightly lower than of the T-specimen. Consequently, the fatigue lives of the L-specimen were longer than those of the T-specimen in low cycle regime.     

High-temperature deformation of ZrB2 ceramics with WC additive in four-point bending

This work investigated high-temperature four-point bending deformation of hot-pressed ZrB₂ ceramics with WC additive at 1900 °C for 0–60 min in argon atmosphere under a static load of 25 MPa. After 60 min bending deformation, about 8% strain was achieved without apparent macroscopic cracks, suggestive of superplastic behavior at 1900 °C. The tensile side and the compressive side of the deformed samples exhibited different microstructural characteristics. On the tensile side, many cavities were observed at the multigrain junctions and the fracture surface was relatively flat. On the compressive side very few cavities occurred and some large cracks were present on the fracture surface. The presence of a secondary W-containing phase formed by the addition of WC, prohibited the coalescence of cavities by its plastic deformation.     

A novel limiting strain energy strength theory

With applied dislocation theory, the effects of shear and normal stresses on the slide and climb motions at the same section of a crystal were analyzed. And, based on the synergetic effect of both normal and shear strain specific energies, the concept of the total equivalent strain specific energy (TESSE) at an oblique section and a new strength theory named as limiting strain energy strength theory (LSEST) were proposed. As for isotropic materials, the plastic yielding or brittle fracture of under uniaxial stress state would occur when the maximum TESSE reached the strain specific energy, also the expressions on the equivalent stresses and a function of failure of the LSEST under different principal stress states were obtained. Relationship formulas among the tensile, compressive and shear yield strengths for plastic metals were derived. These theoretical predictions, according to the LSEST, were consistent very well with experiment results of tensile, compressive and torsion tests of three plastic metals and other experiment results from open literatures. This novel LSEST might also help for strength calculation of other materials.     

氢对锆基块体非晶合金形变和开裂的影响

通过充氢和未充氢缺口拉伸试样和三点弯曲试样在SEM下的原位加载,研究了氢对Zr65Al7．5Ni10Cu17．5块体非晶合金形变和开裂过程的影响,结果表明,无论是否有氢,块体非晶的剪切带发展到临界尺寸,剪切裂纹就沿剪切带形核、扩展,它一旦张开就导致快速的断裂,断口边缘观察到的无特征区是剪切带,而不是剪切裂纹断口;剪切断口形貌和拉伸断口形貌没有本质区别,只有当长时间充氢才能形成氢鼓泡,如鼓泡很小或尚未形成,则氢对剪切带以及裂纹的形核、扩展没有明显影响;如存在较大的氢鼓泡,则当剪切带尚未充分发展时微裂纹就形核,导致低应力脆断。