基于晶体相场法研究缺口形貌对微裂纹扩展连通的影响

采用晶体相场方法研究金属微互连结构界面微裂纹在两种不同初始缺口区域形貌情况下的扩展连通过程。研究表明：微裂纹在两种初始缺口区域形貌下呈“树枝状”扩展;扩展过程中伴随着二次微裂纹和三次微裂纹的生长;微裂纹扩展面积占总面积的百分比随着演化时间的增加逐渐增大,主裂纹扩展面积占总扩展面积的百分比随着演化时间的增加而增大;初始缺口区域形貌为正方形的微裂纹扩展面积占总面积的百分比和主裂纹扩展面积占总扩展面积的百分比均高于初始缺口区域形貌为矩形的微裂纹扩展;初始缺口区域形貌的改变并不影响原子间距和原子波动周期。     

岩石裂纹扩展的力学分析

本文详细阐述和讨论了岩石裂纹分别在拉应力、剪应力作用以及拉剪应力共同作用下的扩展、闭合以及裂纹面上应力的传递问题。利用Barenblatt内聚力模型分析了裂纹顶端内聚力和裂纹面间摩擦力对裂纹扩展的阻尼作用, 讨论了在裂纹扩展过程中裂纹面间的接触条件、接触状态对裂纹扩展的影响。      

岩石裂纹扩展的力学分析

本文详细阐述和讨论了岩石裂纹分别在拉应力、剪应力作用以及拉剪应力共同作用下的扩展、闭合以及裂纹面上应力的传递问题。利用Ｂａｒｅｎｂｌａｔｔ内聚力模型分析了裂纹顶端内聚力和裂纹面间摩擦力对裂纹扩展的阻尼作用。讨论了在裂纹扩展过程中裂纹面间的接触条件、接触状态对裂纹扩展的影响。     

基于有限元方法的疲劳裂纹闭合行为

裂纹闭合行为将很大程度改变疲劳裂纹扩展行为。针对316L不锈钢,结合常幅加载和单个拉伸过载试验和动态数值模拟方法,对疲劳裂纹扩展行为中的裂纹闭合现象开展了一系列研究工作。详细对比了不同扩展阶段的裂纹闭合行为随裂纹长度、应力比和过载影响因素的变化,以及对裂纹扩展速率的影响。同时,研究了单个拉伸过载和裂纹闭合行为之间的内在联系和机理。结合裂纹闭合理论和有限元计算结果,等效应力强度因子被用来描述316L不锈钢的裂纹扩展过程,并提出316L不锈钢的裂纹扩展速率的预测模型。     

B级船板钢形变断裂过程的原位研究

 在扫描电子显微镜下观察分析了B级船板钢试样在原位拉伸过程中的断裂行为以及低温脆性断口中二次裂纹的扩展。结果表明：在拉伸变形过程中,微裂纹首先在试样缺口处形成,然后沿铁素体+珠光体界面扩展。加载过程中,多边形铁素体发生塑性变形,裂纹在基体内以“Z”字型扩展。在低温脆性断裂区,二次裂纹以穿晶方式通过铁素体基体,在扩展到珠光体区域时有时沿铁素体和珠光体的界面扩展,有时穿过珠光体区域扩展。     

预变形条件下纳观裂纹扩展的微观机制研究

采用晶体相场方法研究预变形条件下样品在单轴拉应变作用下的纳观裂纹扩展行为。通过观察裂纹演化,分析裂纹扩展过程的体系自由能变化、应力-应变曲线、裂纹周长曲线和裂纹面积分数曲线,发现:无预变形的样品,在拉应变作用下,当应变量达到临界值时,在裂口附近萌生出位错,使得应变集中的应变能得到释放。虽然位错伴随着裂尖扩展,没有观察到裂尖发射位错现象,裂纹呈解理扩展模式。预剪切变形为1%和2%的样品,在裂纹扩展的过程,左右裂尖各存在一个位错对,这两个位错沿着各自的滑移面交替滑移,使得裂尖沿着两个滑移方向交替扩展,裂纹呈锯齿状。预剪切变形为3%的样品,在裂口处发射位错,在变形滑移带上诱发生成孤立的空穴串,随后发展成空洞并连通成为主裂纹。在主裂纹之外,还出现空洞萌生二次裂纹,整个裂纹扩展呈韧性扩展模式。预剪切变形量的增加,有助于裂纹由脆性扩展转化成韧性扩展。     

全层TiAl基合金缺口试样原位拉伸卸载试验的研究

通过对TiAl基合金不同类型的缺口试样进行原位拉伸卸载实验和相应的断裂表面观察,研究了TiAl基合金全层组织的断裂机理。研究发现:对于缺口试样,裂纹起裂于缺口根部,其断裂过程主要是主裂纹首先起裂、扩展并最后断裂。在整个断裂过程中断裂是穿层断裂和沿层断裂的混合体,裂纹路径较曲折。在拉伸过程中,试样产生微裂纹导致材料发生损伤,随后卸载再加载时,与先前相比,裂纹更易扩展。预损伤加快了裂纹的产生和扩展,使损伤进一步加重,促使材料抵抗裂纹产生、扩展的能力下降。     

高速列车支撑槽用6005A-T6铝合金腐蚀疲劳裂纹扩展行为研究

通过微观组织分析、裂纹扩展路径及断口表面形貌分析,研究了高速列车支撑槽用6005A-T6铝合金在3.5%NaCl溶液中的腐蚀疲劳裂纹扩展行为。结果表明:6005A-T6铝合金在3.5%NaCl溶液中的疲劳裂纹扩展速率较空气环境中快3倍左右;腐蚀环境中疲劳裂纹呈沿晶扩展,扩展过程中出现较多的沿晶二次裂纹。6005A-T6铝合金中较高腐蚀电位的第二相促进周围α-Al基体在3.5%NaCl溶液中的腐蚀,并在腐蚀过程中同时释放H2,氢和腐蚀环境的联合作用促进了6005A-T6铝合金在3.5%NaCl溶液中的疲劳裂纹扩展。     

基于XFEM的岩体卸荷过程裂纹起裂扩展规律研究

地下和边坡工程开挖常涉及岩体卸荷问题,采用ABAQUS软件中的扩展有限单元法(extended finite element method,XFEM)对开挖卸荷过程岩体内部裂纹的起裂扩展进行了模拟,通过计算裂纹尖端应力强度因子研究了其起裂特征,并探讨了起裂影响因素,通过记录裂纹扩展形态研究了其动态演化模式.结果表明,卸荷过程中卸荷速率越快,裂纹长度越长,倾角越大,其起裂越容易;并且裂纹面受到的正应力不断减小,剪应力不断增大,裂纹扩展主要由剪应力控制,这与理论分析结果一致.裂纹最终扩展演化形态也与物理试验相近,充分表明运用扩展有限单元法研究岩体裂纹问题的可靠性.      

奥氏体-贝氏体球墨铸铁断裂的微观过程及强韧化机理

用扫描电子显微镜及微拉伸台对奥氏体－贝氏体球墨铸铁裂纹萌生，扩展的微观过程进行了跟踪观察。结果发现：受拉时微裂纹首先在石墨－基体界面上萌生，并沿界面扩展，基体中的裂纹多数是沿贝氏体铁素体－奥氏体界面扩展，不同取向的基体组织可使裂纹偏转或分叉，主裂纹扩展过程中前方始终存在石墨－基体界面的开裂。此外，还根据实验结果进一步分析了奥氏体－贝氏体球墨铸铁的强韧化机理。     

Steady state vacancy concentration around a plastic crack

The vacancy chemical potential associated with the crack region and with the lattice dislocations representing the plastic zone are identified in terms of the energy of the dislocation configuration. In order to determine the steady state vacancy concentration, the configuration is considered as made up of several internal sources of stress. The diffusion equation under steady state is solved for a crystal containing each source of stress. Further, superposition of the concentration of vacancies around each source is used to determine the total concentration of vacancies for small scale deformation at the crack tip. On the other hand, for large scale deformation at the tip, matching boundary conditions are applied to determine the concentration in each region containing an internal source. Both the discrete dislocation and the single lattice dislocation representations of the crack are employed to determine the crack growth rate. The results are used to emphasize the influence of the plastic zone on the crack growth rate by vacancy diffusion mechanism. Formerly Assistant Professor, Department of Engineering Science and Mechanics, Tennessee Technological University. Cookeville, TN 38501.     

连铸板坯内裂纹的检验分析

对连铸板坯内裂纹试样进行了金相及扫描电镜分析，发现裂纹主要发生在粗大的等轴晶区，并伴有大量的疏松和缩孔；沿拉坯方向裂纹是连续的；距板坯窄面的距离由50mm变为110mm时，铸坯的凝固时间不一致，裂纹表面的硫、磷含量逐渐增大．裂纹断口形貌由塑性断口变为凝固收缩断口。     

Crack Healing in a Low-Carbon Steel Under Hot Plastic Deformation

The behavior of internal crack healing in low-carbon steel samples undergoing hot plastic deformation was investigated using the MMS 200 thermo-mechanical simulator. The characterization of cracks after plastic deformation was analyzed using scanning electron microscopy under different heating temperatures, reduction ratios, numbers of deformation passes, strain rates, and holding time durations. It was found that the degree of crack healing increases with increasing heating temperature, reduction ratio, and holding time duration, and with decreasing number of deformation passes and strain rate.     

A Mössbauer spectrometry study of the mechanical transformation of precipitated austenite in 6Ni steel

A previous study suggested that precipitated austenite in “QLT? 6Ni steel is sufficiently stable to avoid transforming to martensite in the path of a propagating crack. Impact specimens of “QLT? 6Ni steel with differing geometries were broken at 77 K and 290 K. The depth of transformation of the austenite below the fracture surfaces was measured by backscatter electron Mössbauer spectrometry. The depth of transformation correlated with the impact energy and with the depth of the plastic zone, but was mostly independent of temperature. It is concluded that the martensitic transformation of precipitated austenite occurs in the plastic deformation ahead of the crack, and in ductile fracture the precipitated austenite will not remain to interfere directly with crack propagation.     

Hydrogen effects on brittle fracture of the titanium aluminide alloy Ti-24Al-11Nb

Varying amounts of hydrogen were dissolved in the titanium aluminide alloy Ti-24Al-llNb (atomic percent). Virtually all of this hydrogen probably precipitated as hydride on cooling because the terminal solubility in the dominant Ti₃Al phase is very low at room temperature. Although the yield strength (YS) increased, the ultimate tensile strength (UTS), ductility, fracture stress in notched bend bars, and fracture toughness decreased with increasing amounts of hydride. The strength and fracture properties, for all hydride contents, did not change with testing speed below about 5 to 50 mm/min but decreased steeply for speeds greater than that. The presence of hydride decreased the critical value of testing speed by about an order of magnitude. Brittle cracks in bluntly notched bend bars, with or without hydride, nucleated at the notch root or at a distance below the root which was less than one fifth of the distance to the peak stress location. This result suggests that the cleavagelike cracking in this material is not controlled by normal stress alone but has some dependence on the applied strain. The fracture surfaces of notched or precracked specimens, with or without hydride, consisted entirely of cleavagelike fracture, but these cracks exhibited stable crack propagation. This permitted both the measurement of crack resistance or R curves and also observation of the initiation and propagation of the crack with increasing K_I. The results showed that cracks initiated discontinuously at characteristic sites within the plastic zone and along the slip bands when the plastic deformation ahead of the precrack developed to a particular and reproducible extent. Literature cleavage models were compared to results for the present tests. WU-YANG CHU, Formerly Visiting Professor, Carnegie Mellon University,     

A Mössbauer spectrometry study of the mechanical transformation of precipitated austenite in 6Ni steel

A previous study suggested that precipitated austenite in “QLT” 6Ni steel is sufficiently stable to avoid transforming to martensite in the path of a propagating crack. Impact specimens of “QLT” 6Ni steel with differing geometries were broken at 77 K and 290 K. The depth of transformation of the austenite below the fracture surfaces was measured by backscatter electron Mössbauer spectrometry. The depth of transformation correlated with the impact energy and with the depth of the plastic zone, but was mostly independent of temperature. It is concluded that the martensitic transformation of precipitated austenite occurs in the plastic deformation ahead of the crack, and in ductile fracture the precipitated austenite will not remain to interfere directly with crack propagation.     

Fracture process of a low carbon low alloy steel relevant to charpy toughness at ductile-brittle fracture transition region

The fracture process that determines the Charpy energy at the ductile-brittle transition region was investigated by means of the instrumented Charpy test and fractographic analysis with a low carbon low alloy steel subjected to different control-rolling conditions. The decomposition of a Charpy energy into the energies dissipated in the course of the notch-tip blunting, stable crack growth, and brittle crack propagation is unique irrespective of the testing temperatures and specimen series. Toughness level can be divided into four regions according to the pre-dominating fracture process. The temperature dependence of toughness and effects of the an-isotropy of a specimen originates in the brittle fracture initiation stage rather than the resistance against the notch-tip blunting or stable crack growth. From fractographic examination referring to the stress analyses, it is discussed that the brittle fracture initiation is controlled by the local deformation microstructures in the plastic zone together with the stress field ahead of the notch or the stable crack front. Formerly Graduate Student, Department of Materials Science and Engineering, Waseda University     

Anisotropy of fracture toughness in textured titanium-6Al-4V alloy

Fracture toughness tests have been carried out on a wide range of specimen orientations from textured Ti-6A1-4V alloy plate. The tests were instrumented to allow measurement of crack extension as well as stress intensity factors and COD data. Preferred orientation of the dominant alpha phase was found to influence not only the toughness parameters but also the fracture surface and propensity to shear lip formation. The anisotropic nature of plasticity in the material was demonstrated by direct measurement of yield loci and by the variable shapes of plastic zones around crack tips. Results are discussed in terms of a qualitative analysis of slip systems at the crack tip and also in a semiquantitative anisotropic upper bound slip line treatment. It is found that stable crack extension starts at low values ofK and COD for specimens in which plastic flow can occur readily at the crack tip but toughness, assessed as resistance to unstable crack propagation, is greatest in these cases. The magnitude of microstructure induced anisotropy is shown to be much smaller than that originating in preferred crystallographic orientation. Troughlike features on some fracture surfaces were found to be associated with regions of particularly strong local texture. Formerly with the Department of Physical Metallurgy, University of Birmingham     

压下对重轨钢大方坯内裂纹敏感性的影响

为研究压下对连铸坯内部裂纹产生的影响,利用ABAQUS有限元软件建立了230 mm×280 mm断面大方坯压下数学模型。通过压下模型对重轨钢连铸坯压下过程进行热力耦合模拟计算,对压下过程中产生的内部裂纹进行了预测。首先,对连铸坯不同中心固相率为0.3～0.7的温度场进行计算;然后,利用压下模型计算了连铸坯中心固相率0.3～0.7时凝固前沿的等效塑性应变。研究结果表明,在连铸坯中心固相率为0.3～0.7的位置处分别施加7 mm压下量进行压下,连铸坯凝固前沿等效塑性应变未超过临界等效塑性应变（0.4%）,连铸坯未出现内裂纹;同时,对连铸坯在中心固相率为0.6位置处进行了不同压下量的研究,研究结果表明,当连铸坯压下量超过7 mm时,凝固前沿的等效塑性应变超过临界塑性应变（0.4%）,连铸坯出现内裂纹,并且压下量越大,连铸坯内裂纹越严重。同时,工业试验结果与模型计算结果基本吻合,验证了模型计算的准确性。      

Thermal stress and strain in a metal matrix

The stresses and strains, induced by coefficient of thermal expansion (CTE) mismatch, are analyzed for a metal matrix composite (MMC) with a spherical reinforcement particle. The spherical reinforcement particle is found to be in a hydrostatic stress state and remains in the elastic state. The stresses and strains are largest, and plastic deformation occurs in the matrix adjacent to the reinforcement particle. Accordingly, the reinforcement particle/matrix interface becomes a potential crack initiation site under thermal cycling. The critical internal pressure for plastic deformation is less than two-thirds of the yield stress of the matrix material and decreases with increasing range of thermal cycle.