IN SITU OBSERVATION OF THE FRACTURE FOR A SPHEROIDIZED LOW CARBON STEEL

<正> 一、前言 应用透射电子显微镜(TEM)技术观察金属薄膜的断裂过程,最早的可能是Wilsdorf的金属Al薄膜的工作.以后,Forsyth和Wilson,Kuhlman-Wilsdorf等人分别对Al-4％Cu的合金,Cu单晶体薄膜中的裂纹扩展进行了TEM的直接观     

Quantitative evaluation of fracture toughness-microstructural relationships in alpha-beta titanium alloys

The fracture toughness of two alpha-beta titanium alloys containing an alpha platelet in a transformed beta matrix has been examined in terms of the microstructural parameters controlling the fracture initiation and propagation in the alloys. Equations have been formulated that show that the highest toughness values of both alloys were associated with the finest platelet spacings and the thickest alpha platelets. It is proposed that the fracture initiation process in both alloys is controlled by the distance between the platelets, the fracture toughness of the alloys being dependent on the distance between active centers of void nucleation, i.e., as a function of the alpha platelet thickness and spacing between the platelets. Seven models of ductile fracture relating fracture toughness to mechanical property and microstructural parameters have been compared in their ability to predict the toughness of the alloys after solution treatments, which produce varying platelet thickness and inter-platelet spacings. The principle has been adopted following Rice and Rosengren and Hutchinson (HRR)^[1,2] that there must be a 1/x energy singularity at the crack tip, which also prescribes the stress and strain distribution ahead of a crack tip. Any model not incorporating these requirements should be rejected.     

Experimental investigation of void coalescence in metallic sheets containing laser drilled holes

Although important, ductility remains difficult to predict and there is a tremendous need for more precise modelling. Progress in this field is hampered by a lack of quantitative experimental results to assess the validity of these models due to the stochastic nature of ductile fracture. In this paper, tensile tests have been carried out in a scanning electron microscope on model materials made of thin metallic sheets containing laser drilled holes. Depending on the material and hole configuration, different failure modes and strains are observed. The results show the importance of void spacing and orientation, constraining effects, materials yield stress and work hardening rate, and the competition between ductile fracture and shear localization. Finally, it is shown that the Thomason model for void coalescence is not appropriate for predicting fracture of the model material. However, the McClintock model for void growth, and the Brown and Embury and the McClintock models for void coalescence provide relatively good predictions.     

C1-controlled creep crack growth by grain boundary cavitation

Quasi steady-state creep crack growth is widely associated with the nucleation and growth of voids on grain boundaries ahead of the crack tip. In this paper, a micromechanics-based constitutive law is used to study the velocity-dependent fracture toughness of porous solids under extensive creep conditions. Void growth and coalescence in the fracture process zone is modeled by a nonlinear viscous microporous strip of cell elements. Under steady-state crack growth, two dissipative processes contribute to the macroscopic fracture toughness: the work of separation in the fracture process zone, and creep dissipation in the background material. Under extensive creep conditions, the competition between these two processes produces an inverted U-shaped C¹–velocity curve. The effects of rate sensitivity, initial porosity as well as hydrogen attack on fracture toughness are studied. The numerically simulated fracture toughness vs. crack velocity curves show good agreement with existing experimental results.     

Experimental and numerical analysis of toughness anisotropy in AA2139 Al-alloy sheet

Toughness anisotropy of AA2139 (Al–Cu–Mg) in T351 and T8 conditions has been studied via mechanical testing of smooth and notched specimens of different geometries, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms were investigated via scanning electron microscopy and synchrotron radiation computed tomography. Contributions to failure anisotropy are identified as: (i) anisotropic initial void shape and growth; (ii) plastic behaviour including isotropic/kinematic hardening and plastic anisotropy; and (iii) nucleation at a second population of second-phase particles leading to coalescence via narrow crack regions. A model based in part on the Gurson–Tvergaard–Needleman approach is constructed to describe and predict deformation behaviour, crack propagation and, in particular, toughness anisotropy. Model parameters are fitted using microstructural data and data on deformation and crack propagation for a range of small test samples. Its transferability has been shown by simulating tests of large M(T) samples.     

Visualization by X-ray tomography of void growth and coalescence leading to fracture in model materials

The literature contains many models for the process of void nucleation, growth and coalescence leading to ductile fracture. However, these models lack in-depth experimental validation, in part because void coalescence is difficult to capture experimentally. In this paper, an embedded array of holes is obtained by diffusion bonding a sheet filled with laser-drilled holes between two intact sheets. The experiments have been performed with both pure copper and Glidcop. Using X-ray computed tomography, we show that void growth and coalescence (or linkage) are well captured in both materials. The Brown and Embury model for void coalescence underestimates coalescence strains due to constraining effects. However, both the Rice and Tracey model for void growth and the Thomason model for void coalescence give good predictions for copper samples when stress triaxiality is considered. The Thomason model, however, fails to predict coalescence for the Glidcop samples; this is primarily due to secondary void nucleation.     

Effects of rate on crack growth in a rubber-modified epoxy

The rate-dependent fracture behavior of a 10-phr rubber-modified epoxy was investigated using double-cantilever-beam tests at various crosshead speeds. Dramatic rate effects were observed in the R-curve behavior and in the relationship between the applied energy-release rate and the crack velocity. Furthermore, a transition between fracture with toughening mechanisms operating (kinetic crack growth) and brittle behavior (dynamic crack growth) was observed. This transition depended on the crack velocity and applied energy-release rate. Such behavior is expected to depend on how the intrinsic toughness and/or the extrinsic toughening mechanisms are influenced by strain rate. It was shown that the size of the process zone was only weakly dependent on the crack velocity until the onset of dynamic fracture. Furthermore, the extent of void growth was virtually independent of the crack velocity in the kinetic regime. These results appear to rule out the notion that crack-tip shielding is significantly affected by rate effects in this rubber-modified epoxy. Rather, the rate effects may arise from a rate-dependent intrinsic toughness. It was observed that the intrinsic toughness decreased significantly with increasing crack velocity. The crack instability was shown to be associated with an abrupt cessation of the development of the process zone, with both cavitation and void growth being totally suppressed.     

Three-dimensional quantitative in situ study of crack initiation and propagation in AA6061 aluminum alloy sheets via synchrotron laminography and finite-element simulations

Ductile crack initiation and propagation in AA6061 aluminum alloy for a fatigue precrack have been studied in situ via synchrotron radiation computed laminography, a technique specifically developed for three-dimensional imaging of laterally extended sheet specimens with micrometer resolution. The influence of the microstructure, i.e. due to the presence of coarse Mg₂Si precipitates and iron-rich intermetallics, on the void nucleation process is investigated. Coarse Mg₂Si precipitates are found to play a preponderant role in the void nucleation and ductile fracture process. Void growth and void coalescence are then observed and quantified by three-dimensional image analysis during crack initiation and propagation. Parameters for a Gurson–Tvergaard–Needleman micromechanical damage model are identified experimentally and validated by finite-element simulations.     

Modeling vapor pressure effects on void rupture and crack growth resistance

The phenomenon of vapor pressure assisted void growth and rupture is studied. Plastic electronic packages absorb moisture which condenses within numerous micropores in the substrate, solder mask and die attach materials as well as near their interfaces. During reflow soldering, the condensed moisture vaporizes with the result that these micropores as well as interfaces are subjected to high vapor pressure. Under extreme conditions, our study suggests that vapor pressures can attain high enough levels to drive the voids to grow to rupture, thereby causing package failure. Under other conditions, residual/thermal stresses assisted by vapor pressure can cause crack growth within the polymeric materials as well as along interfaces. Vapor pressure effects on void growth have been incorporated into the Gurson model for porous ductile material. Using this model, a finite element study shows that the combination of high vapor pressure and high porosity is very detrimental to fracture toughness.     

Pore size scaling for enhanced fracture resistance of nanoporous polymer thin films

Poly(arylene) ether (PAE) polymer films containing controlled nanometer-sized pores are shown to exhibit increasing fracture resistance with porosity. Such surprising behavior is in stark contrast to widely reported behavior for the fracture toughness of porous solids, which decreases markedly with porosity. A ductile nano-void growth and coalescence fracture mechanics-based model is presented to rationalize the increase in fracture resistance of the voided polymer film. The model is shown to explain the behavior in terms of a specific scaling of the size of the pores with pore volume fraction. It is demonstrated that the pore size must increase with close to a linear dependence on the volume fraction in order to increase rather than decrease the fracture energy. Independent characterization of the pore size as a function of volume fraction is shown to confirm predictions made by the model. Implications for the optimum void size and volume fraction are considered for superior fracture resistance of the nanoporous films.     

Effect of triaxiality on void growth and coalescence in model materials investigated by X-ray tomography

Void growth and coalescence/linkage, which play significant roles during ductile fracture processes, are strongly influenced by stress triaxiality in a deforming solid. The stress state can be changed by cutting notches in a tensile sample. In the current paper, void growth and linkage of an artificial void array embedded in a notched model material was studied by X-ray computed tomography, coupled with in situ tensile deformation. The cross-sectional shape of the tensile specimens was square, and a pair of notches was cut along only one direction. Thus, the lateral principal stress does not have an isotropic distribution: the principal stress along the notch direction is considered to be higher. This technique allowed us to explore the entire process of growth and linkage events of a void array embedded in a metal matrix. The notch effect creates a marked acceleration in void growth, leading to a large reduction in the linkage strains, as compared with similarly fabricated unnotched samples. The standard models for coalescence could not provide consistent predictions of the measured notch effect.     

Size-Dependent Rupture Strain of Elastically Stretchable Metal Conductors

Experiments show that the rupture strain of gold conductors on elastomers decreases as the conductors are made long and narrow. Rupture is caused by the irreversible coalescence of microcracks into one long crack. A mechanics model identifies a critical crack length ?(cr), above which the long crack propagates across the entire conductor width. ?(cr) depends on the fracture toughness of the gold film and the width of the conductor. The model provides guidance for the design of highly stretchable conductors.     

X60管线钢非穿透裂纹体的断裂研究

对X60管线钢进行了不同形状的表面裂纹、单边和双边角裂纹等非穿透裂纹形式的断裂实验,结合断理解理论分析和断口宏微观分析,探讨了穿透裂纹的断裂实验数据用于一般三维形态裂纹体断裂的可行性,结果发现,裂纹在起裂后并不连续扩展,而是严重钝化,发生明显预缩后才失稳扩展;各种非穿透裂纹体断裂过程中,由于裂纹在起始和不同扩展阶段与厚度方向的相对方位不同,导致三维约束效应与分层裂纹的耦合效应不同;各理解纹形式下的起裂临界应力强度因子值比穿透理解纹的平均值低9．6％－45％,导致三维约束效与分层裂纹的耦合效应不同;各裂纹形式下的起裂临界应力强度因子值比穿透理裂纹的平均值低9．6％－45％,研究表明基于穿透裂纹实验数据得到的名义断裂韧性不是客观的断裂评定指标,而是随几何形状、尺寸、理解纹取向等变量而变化,因而不能作为管道断裂评定的控制参量。     

Microstructural model of intergranular fracture during tensile tests

A microstructural model of intergranular fracture in textured materials is presented. In this model, the material is represented by a two-dimensional microstructure with non-regular polygonal grains which represents material's texture and grain shape measured in experiments or calculated from Monte Carlo simulations. The grain boundary character, grain boundary energy, and fracture stress are assigned to each grain boundary according the grain boundary character distribution. Intergranular fracture susceptibility is analyzed by defining the probability of finding a continuous path along the grain boundaries which are intrinsically susceptible to fracture. In this analysis the orientations of the grain boundary with respect to the applied or residual tensile stress axis is considered. The probability of intergranular fracture for each grain boundary depends on the intergranular fracture resistance, the interface orientation relative to the stress axis, and a value of the tensile stress acting on the grain boundary. The crack arrest distance and the fracture toughness are calculated in terms of the frequency of low-energy grain boundaries, fracture stress of low-energy grain boundary, angle distribution of grain boundary interfaces, and anisotropy of grain shape. The results indicate that the fracture toughness increases and the crack arrest distance decreases dramatically with increasing the frequency of the low-energy grain boundaries. Lowering the grain boundary energy can improve the fracture toughness and decrease the crack arrest distance. The angle distribution of grain boundary interfaces and the grain shape factor are also very effective in controlling the fracture toughness. High fracture toughness of polycrystalline materials is related to the presence of a high frequency of low-energy boundaries which are resistant to fracture. The best fracture toughness for brittle materials can be achieved by controlling the frequencies of the low-energy grain boundaries, the grain boundary character, and the boundary inclination.     

Onset of void coalescence in uniaxial tension studied by continuous X-ray tomography

Void growth and coalescence in model materials containing a pre-existing three-dimensional void array were studied by X-ray computed tomography coupled with in situ uniaxial tensile deformation. A newly developed continuous tomography technique was employed to capture the onset of coalescence. Using a picosecond laser machining system and a diffusion bonding technique, model materials with different void geometries were prepared. By implementing continuous tomography, the plastic strain at the onset of void coalescence was measured (instead of simple linkage) for the first time. The plastic strains at the onset of void coalescence were compared with the existing void coalescence models. Finite-element (FE) simulations were performed to study the influences of void shape (sphere, cylinder, tapered-cylinder) on the void growth behavior. This study shows that the coalescence models developed by Thomason and later extended by Pardoen and Hutchinson provide accurate predictions of coalescence strain when the voids are aligned normal to the tensile axis. However, offsets can induce shear effects that lower the coalescence strain in a manner not predicted by the models. Two-dimensional plane-strain FE simulations were also used to explore the influence of shear localization between two misaligned coalescing voids on ductility. These demonstrate the nature of the effect.     

On the influence of void clusters on void growth and coalescence during ductile fracture

Based on the behavior of a three-void cluster embedded within a representative volume element, this study utilizes three-dimensional finite element analyses to examine the sensitivity of void growth and coalescence to strain hardening, multiaxial stress state and inter-void spacing. The strain-induced growth of voids within the cluster is accelerated when the voids are closely spaced in a low strain-hardening material subject to high levels of stress triaxiality. Far-field deformation causes strain to concentrate within the inter-void ligament, and the resulting behavior induces a load–loss response of the inter-void region. Based on the load–loss criterion for the onset of void coalescence, the results show that coalescence is accelerated by increasing stress triaxiality and decreasing strain hardening and inter-void spacing. A straightforward analysis is then presented that relates void coalescence to (a) the strain-hardening exponent and (b) the dependence of the plastic constraint factor within the inter-void ligament on strain, the latter being sensitive to far-field stress triaxiality and void geometry.     

Evolution of voids during ductile crack propagation in an aluminium alloy sheet toughness test studied by synchrotron radiation computed tomography

The anisotropy of fracture toughness in AA2139 (Al–Cu–Mg) alloy sheet has been investigated via synchrotron radiation computed tomography of arrested cracks in Kahn tear test pieces for different loading cases. The three-dimensional distribution and morphology of pores and defects in the as-received state are seen to be anisotropic, with chains of voids and void elongation in the L (longitudinal) direction. For toughness testing in L–T orientation (T is long transverse), voids ahead of the crack grow and link in the L direction. In T–L tests, voids ahead of the crack tip also grow in the loading direction, although a high degree of alignment is retained in the L direction. The present work provides quantitative microstructural data that can be used as input for and validation of recent idealized model formulations, and it is shown that the measured void dimensions and evolution are consistent with measured toughness anisotropy.     

Ti17合金网篮组织的断裂韧性及其预测模型研究

研究了Ti17钛合金网篮组织的断裂韧性及断裂行为,发现裂纹扩展路径曲折度(外因)及沿着裂纹扩展路径所消耗的塑性功(内因)对断裂韧性有显著影响,好的塑性及曲折的裂纹扩展路径有利于提高合金的断裂韧性。基于能量原理,建立了综合考虑内因和外因的断裂韧性预测模型,模型预测精度较高,误差在6%以内。分析模型发现,对网篮组织断裂韧性起决定作用的是其本征塑性功,占80%以上,裂纹扩展路径的贡献较小,在20%以内。     

FCC晶体中晶体取向对孔洞长大和聚合的影响

通过编制率相关有限元用户子程序，采用包含一个和两个球形孔洞的单胞探求了FCC晶体中晶体取向对孔洞长大和聚合的影响。计算结果表明：晶体取向对孔洞长大的影响较大，孔洞的形状和长大方向与晶体取向密切相关：由于变形不均匀，孔洞在晶界处产生尖角，易形成裂纹。由于约束较少，孔洞周围和两孔洞间的区域塑性变形较大，晶体的转动和滑移主要集中在孔洞周围以及两孔洞间的区域。     

Crack propagation behavior in TiAl–Nb single and Bi-PST crystals

The fracture toughness of directional solidified Ti–(45,47)Al–3Nb, Ti–(45,47)Al–3Nb–0.2Si–0.1C, Ti–(45,47)Al–3Nb–0.3Si–0.2C type I alloys and their contribution to crack growth resistance of TiAl–Nb alloys were studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Fracture toughness of type I specimens (in which crack propagates against lamellae boundaries) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. The toughness for type I specimens was controlled by α₂–α₂ spacing in which the delamination-type separation occurred. Compared to 47Al alloys, α₂–α₂ spacing in 45Al alloys increased by decreasing Al content, therefore, fracture toughness increased rapidly. These results are discussed and the ability to improve toughness by changing Al content, Si and C addition in TiAl–Nb alloys produced by directional solidification is suggested.