Dynamic growth of tensile cracks by ductile and brittle fracture mechanisms in a viscoplastic material

In this paper, a finite element analysis of steady-state dynamic crack growth under Mode I, plane strain, small-scale yielding conditions is performed in a rate dependent plastic material characterized by the over-stress model. The main objective of the paper is to obtain theoretically the dependence of dynamic fracture toughness on crack speed. Crack propagation due to a ductile (micro-void) mechanism or a brittle (cleavage) mechanism, as well as transition from one mode to another are considered. The conversion from ductile to brittle has been observed experimentally but has received very little attention using analytical methods. Local fracture criteria based on strains and stresses are used to describe ductile and brittle fracture mechanisms. The results obtained in this paper are in general agreement with micro-structural observations of mode conversion during fracture initiation. Finally, the particular roles played by material rate sensitivity and inertia are examined in some detail.     

Fracture toughness of polycrystalline NiAl from finite-element analysis of miniaturized disk-bend test results

The controlled-flaw method in conjunction with the miniaturized disk-bend test (MDBT) was implemented to determine the fracture toughness of polycrystalline NiAl. This procedure was previously used to measure the fracture toughness of completely brittle materials, so the present research extends the method to a material that exhibits a small amount of ductility prior to failure. The controlled-flaw method is based on the placement of a Vickers indentation in the center of the tensile side of the disks. In the MDBT, the specimens are disks 3 mm in diameter, and in this investigation, the disks ranged from 194 to 367 μm in thickness. Fracture initiated at the corners of the indentations for indentation loads exceeding 39 N. The fracture toughness was determined from an analysis of the dependence of fracture stress, σ _f , on indentation load. In brittle materials, σ _f can be calculated from the measured load at fracture, but this is not possible when the specimen deforms plastically prior to failure. The finite-element program NIKE2D was therefore used to calculate the stress during plastic deformation, using data on the tensile behavior of NiAl to model its deformation as an inelastic cylindrically symmetric plate. The fracture toughness of polycrystalline NiAl was measured as 6.41±1.75 MPa√m, which agrees well with independently measured values for similarly processed material. The relatively large uncertainty is associated with scatter in the experimentally measured yield stresses. The results of this investigation demonstrate that the controlled-flaw method can be used in conjunction with the MDBT and finite-element modeling to provide a reasonable estimate of the fracture toughness of a material with limited ductility, provided fracture initiates at the corners of the indentation.     

Environmentally induced crack nucleation and brittle fracture

Brittle fracture is known to occur in ductile metals in special gaseous environments. Often, these gases form thin solid films on the metal. Computer simulations were used to study the propagation of cracks coated with thin elastically hard films in a ductile two-dimensional material. It was found that under certain conditions secondary crack nucleation occurred in the simulations and brittle fracture was observed. The dynamics of the crack nucleation and the detailed spatial distribution of local forces were examined. A model has been developed for this new mode of crack nucleation and brittle fracture. The role of the elastically hard film in suppressing dislocation generation and in initiating the secondary crack is discussed. This mode of initiating brittle fracture is entirely different from usual models for gaseous embrittlement.     

全层TiAl基合金室温断裂机制的研究

通过拉伸、压缩、弯曲实验分析研究了全层(FL)组织TiAl基合金的断裂机制。研究发现:拉伸和压缩时材料抵抗裂纹的扩展能力不同,抗压强度远高于抗拉强度,这是由于两者的变形及断裂机制不同。TiAl基合金拉伸断裂机制为脆性解理断裂,压缩变形断裂是剪应力和正应力共同作用下的断裂,是准解理断裂。TiAl基合金的缺口弯曲断裂方式也为解理断裂,其断裂过程是先在缺口处产生微裂纹,一旦裂纹在缺口根部产生,由于材料已积累足够的能量使得材料快速失稳解理断裂。     

Solute segregation and brittle fracture in an alloy steel

Using an Sb doped Ni-Cr steel as a model material, the mechanism of intergranular brittle fracture was studied by means of tests on smooth, notched, and precracked specimens, in conjunction with statistically analyzed, selected area Auger electron spectroscopy.The critical local stress for fracture of a grain boundary was determined as a function of the Sb concentration on that boundary. The Sb effect can be understood in terms of its influence on cohesive energy, which controls the plastic work associated with brittle fracture. The results indicate that the optimum method for assessing embrittlement behavior of such a steel with minimum ambiguity is to use a notched specimen at a fixed test temperature in the appropriate temperature range for that material. The conditions which govern brittle crack nucleation in steel and the factors which influence the plastic work are discussed.     

A study of the dynamics of high temperature brittle intergranular fracture by acoustic emission

This paper presents a study of high temperature brittle intergranular fracture (HTBIGF) in which acoustic emission (AE) data and fractography were used together to make deductions about the dynamics of fracture. The AE event rate was proportional to the rate of overall crack growth, and the average crack increment per AE event correlated well with spacing of striations observed on the fracture surface. The AE from HTBIGF was also compared with ambient temperature brittle intergranular fracture and high temperature ductile fracture. The HTBIGF generated about 50% of the emission of fully brittle fracture, and at least an order of magnitude more than that from ductile fracture. The observation of discrete bursts of AE of this magnitude, coupled with the striations on the fracture surface, indicates that HTBIGF takes place in discrete brittle steps. This is consistent with the segregation of sulphur as an embrittling species to the region of the crack tip, promoting stepwise decohesion, rather than a continuous crack growth process.     

Fracture toughness of brittle foams

Existing models for the fracture toughness of brittle cellular materials have two limitations: the crack is assumed to be large relative to the cell size and the modulus of rupture of the cell wall material is assumed to be constant. In the previous companion paper, we reanalyzed the fracture toughness using a finite element analysis for short cracks and assuming that the modulus of rupture of the cell wall material followed a Weibull distribution. The finite element analysis gave a reduction factor for the effective fracture toughness for short cracks which was independent of the cell geometry. The Weibull analysis showed that the effect of cell size on the fracture toughness of brittle honeycombs is dependent upon the Weibull modulus of the cell wall material. We now apply similar ideas to the analysis of the fracture toughness of brittle foams.     

Influence of Microcracking on Shear Localization

This work examines the influence of microcracking on a material’s tendency to shear localize under compressive loading. A two-dimensional (2D) finite-element framework with explicit crack representation using cohesive-element methodologies is employed. The influence of microcracking is examined by taking the fracture toughness of the cohesive elements as a free parameter. The simulations suggest that an optimum fracture toughness exists for promoting shear localization. This value corresponds to the limiting mode ?I fracture toughness, below which microscopic material defects lead to brittle compressive failure, as opposed to shear localization. While in the presence of confinement, this value is shown to be close to zero; in the absence of confinement, it is computed to be 28% of the shear band toughness for the specific case of ultrafine-grained tungsten. More generally, it is found that the ratio of mode?I fracture toughness to shear band toughness provides a crude indicator for predicting whether material defects are likely to lead to brittle failure or enhanced shear localization.     

The influence of Mo and Co on the embrittlement of an Fe-Ni-Mn alloy

In the “as rolled” condition an Fe-6 Ni-5 Mn maraging type alloy was found to be brittle exhibiting intergranular fractures. The addition of 2.5 pct Mo and 5.0 pct Mo increased the impact toughness of the “as rolled” material and changed the mode of brittle fracture to transgranular cleavage. The addition of 9 pct Co embrittled the alloy. On aging Mo and Co raised the peak hardness of the base Fe-6 Ni-5 Mn alloy, however, aging led to rapid embrittlement. The base alloy and an alloy containing 2.5 pct Mo showed brittle intergranular fractures on aging. The addition of 5 pct Mo gave rise to brittle transgranular cleavage fractures on aging at 450°C, but at temperatures less than 450°C there was always up to 20 pct intergranular fracture present in brittle fractures. At temperatures greater than 475°C brittle intergranular failure occurred in the 5 pct Mo alloy due to a grain boundary film of M₆C and Fe₂Mo. This paper is based upon a thesis submitted by D. R. Squires in partial fulfilment for a higher degree of CNAA at Sheffield Polytechnic.     

Physical metallurgy conditions for retaining the load-carrying ability of the gas-main pipeline metal during long-term operation

The causes and mechanisms of the failures in the operation of a gas-main pipeline that has been operating successfully for several decades are analyzed. Fractography is used to detect the presence of regions in a fracture surface that undergo brittle fracture at the stage of crack propagation. The temperature dependence of the impact toughness demonstrates that the appearance of such brittle fracture regions in the fracture surfaces forming during the operation of a gas-main pipeline is caused by high cold-shortness susceptibility of the pipeline metal. The reliability criterion for the gas-main pipeline metal consists in avoiding brittle fracture even in local volumes. The conditions that ensure the reliability of the gas-main pipeline metal consist in its low upper cold-shortness threshold temperature and high brittle fracture energy.     

砂岩Ⅰ型断裂的尺寸效应试验研究

通过对尺寸成等比例的砂岩试样进行三点弯曲试验,研究了I型断裂问题中试样尺寸对砂岩断裂参数的影响.结果表明:随着试样尺寸的增大,名义张拉强度减小,名义断裂韧度和破裂过程区长度增大,三者均为试样尺寸的函数.基于试验结果,引入有效破裂过程区长度cf,建立等效线弹性断裂力学尺寸效应模型,通过理论分析,发现其尺寸效应曲线介于理想脆性和理想塑性材料之间,体现出准脆性材料的尺寸效应特点,并计算得到了不同尺寸试样的破裂过程区长度等断裂参数.为验证该尺寸效应模型在砂岩中的正确性,采用数字图像匹配技术(DIC)测定相应尺寸砂岩试样的破裂过程区长度,测定结果和理论模型计算值相符.     

我国陶瓷片密封水嘴产品质量现状分析及改进建议

某工程中幕墙使用的热镀锌方管在焊接时出现大规模脆性断裂的现象，对其正常使用产生了严重影响，为了分析其脆断的原因，本文以断口试样为对象，借助扫描电子显微镜、能谱仪、直读光谱仪、万能材料试验机，对其进行了一系列的试验，并深入分析。试验结果表明，方管在镀锌之前已经存在裂纹，起到了断裂源的作用；3种方管试样中均有游离渗碳体出现，级别为4．5级，且呈网状分布在晶粒边界处，这种游离渗碳体组织一般是钢材在退火过程中形成的，是导致钢材发生脆性断裂的主要原因之一。     

硼铬微合金钢的脆性断裂失效分析

为了研究硼铬微合金钢的脆性断裂失效机理,以工厂履带拖拉机支重轮的中心轴脆性断裂为研究背景,通过对中心轴断口的观察以及不同位置的组织和晶粒度的对比,得出了硼铬微合金钢的断裂失效机理。利用FactSage和Thermo- Calc软件对TiN夹杂物的析出机理和控制进行理论计算。结果表明,钛合金的不完全溶解和钛、氮元素质量分数的不合理控制,导致大量的大尺寸硬质TiN夹杂物在凝固过程中析出,使得钛元素的加入没有起到很好地细化晶粒的效果,中心区域的铁素体＋奥氏体组织的晶粒度过于粗大。大尺寸TiN夹杂物作为裂纹源和心部粗大的晶粒导致了硼铬微合金钢的脆性断裂失效。热力学计算表明,TiN夹杂物在凝固过程中形成,低钛低氮和钛、氮质量分数的合理搭配可以有效推迟TiN夹杂物的析出,降低其尺寸。较小的夹杂物尺寸和细小的晶粒均可以有效增强材料抵抗裂纹扩展的能力。     

铝扣件脆断原因分析及处理方法

针对某企业工业纯铝扣件制品在弯折过程脆断的问题,考察了生产工艺中各工序的技术条件,分析论证了使其塑性降低的原因,判定氧化着色过程中除油及着色技术条件控制是主要影响因素。提出用低温长时间退火的方法改善其塑性,通过试验确定了退火工艺制度,使产品的变形性能达到了使用要求。     

Morphology of fractured domains in brittle fracture

We propose a thermodynamic approach, allowing one to determine the shape of the intensively fractured zones (IFZs), simultaneously with the distribution of stress and strain within the IFZ appearing in brittle fracture. The approach combines Gibbs’ variational paradigm of the theory of heterogeneous systems with Griffith’s variational paradigm of the theory of brittle fracture. We suggest some simple constitutive models for solid substances undergoing brittle fracture and solve some boundary value problems on nucleation of an isolated IFZ within an isotropic elastic material. This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California, under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committee.     

挖掘机斗杆断裂的力学分析

对挖掘机斗在不同工况条件下受力状况进行了分析，并通过测试，将１５ＭｎＶ和ＨＱ８０钢动态力学性能进行了对比，认为斗杆早期断是由所用材料的动态断裂特性所决定的，测试结果证明：ＨＱ８０钢抗脆断裂优于１５ＭｎＶ钢。     

微合金钢第Ⅲ脆性区的形成机理

为研究微合金钢第Ⅲ脆性区形成机理及其影响因素,控制连铸坯的表面裂纹,采用Gleeble热力模拟机测定了S355微合金钢在不同温度下的抗拉强度及断面收缩率。使用扫描电镜对拉伸断口进行观察分析,同时采用透射电镜对析出物进行观察分析。在此基础上对拉伸试样进行金相实验,对第二相析出进行热力学计算,分析了组织状态及第二相析出规律对脆性区的影响。结果表明,在第Ⅲ脆性区(660~850℃)内,拉伸断口呈冰糖状,韧窝较浅,形貌表现为沿晶脆性断裂。铁素体网膜沿奥氏体晶界优先析出、第二相沿晶界析出是第Ⅲ脆性区形成的主要原因。     

The fatigue of pseudoelastic single crystals of α-CuAINi

The fatigue life of single crystal α-CuAINi has been studied on pseudoelastic cyclic loading. In general the fatigue life was quite poor with most specimens having less than 3000 cycles to failure. The fatigue life decreased significantly with increasing stress level. However, the fatigue failure was due primarily to stress-induced martensite formation, since if the stress level on cycling was not sufficient to form martensite, the specimen did not fail. The fatigue life appeared to be largely independent of percent strain, crystal orientation and environment of testing. Surface preparation, however, was very important with an electropolished specimen having a much longer life than an abraded one. Fatigue cracks grew only in the final few hundred cycles of the life of the specimen. Cracks initially grew in the direction of growth of the stress-induced martensite, approximately at 45 deg to the tensile axis. Final failure was due to brittle fracture caused by stress concentration at the tip of the fatigue crack, the α-CuAlNi being very brittle. Scanning electron micrographs of the fracture showed no fatigue striations but rather river markings spreading out from the point of nucleation of the fatigue crack, characteristic of a brittle material.     

Statistical analysis of cleavage fracture ahead of sharp cracks and rounded notches

Mechanisms of initiation and unstable propagation of transgranular cleavage cracks are compared for brittle fracture ahead of sharp cracks and rounded notches, e.g. for fatigue pre-cracks and Charpy V-notches, respectively, in standard toughness specimens. The comparison is made over a range of temperatures, from the lower shelf into the ductile/brittle transition region, for a single phase material containing a known distribution of particles where weakest link statistics can be used to model the onset of catastrophic failure. Using linear and nonlinear elastic solutions for the stress distribution ahead of a sharp crack, and slip-line field solutions, modified for a power hardening material, for the rounded notch, statistical modelling is employed to define the critical dimensions ahead of the crack or notch tip where initial cracking events are most probable. The analysis provides an interpretation of the role of stress gradient in governing microscopic fracture behavior. Predictions are evaluated by comparison with experimental results on the low temperature flow, Charpy V-notch and plane strain fracture toughness behavior of a low carbon mild steel with simple ferrite/grain boundary carbide microstructures.     

Experimental and Numerical Simulations of Jointed Rock Block Strength under Uniaxial Loading

To simulate brittle rocks, a mixture of sand, plaster of paris, and water was used as a model material. Thin galvanized sheets were used to create joints in blocks made out of the model material. To investigate the failure modes and strength, 30 × 12.5 × 8.6 cm jointed model material blocks having different joint geometry configurations were subjected to uniaxial compressive loading. Results indicated three failure modes: (1) tensile failure through intact material; (2) combined shear and tensile failure or only shear failure on joints; and (3) mixed failure of the above two modes depending on the joint geometry. The fracture tensor component in a certain direction quantifies the directional effect of the joint geometry, including number of fracture sets, fracture density, and probability distributions for size and orientation of these fracture sets. Results obtained from the experiments were used to develop a strongly nonlinear relation between the fracture tensor component and the jointed block strength. The laboratory experiments conducted on jointed model material blocks were simulated numerically using the Universal Distinct Element Code (UDEC). With careful selection of suitable material constitutive models for intact model material and model joints, and accurate estimation and calibration of mechanical parameters of the constitutive models through a combination of laboratory testing and numerical simulations of the intact model material and model joints separately, it was possible to obtain a good agreement between the laboratory experimental and distinct element numerical results.