温度对GC-4钢CF裂纹扩展速率及断口形貌影响

一、前言 对于在常温下金属材料腐蚀疲劳(CF)的温度效应研究虽有所报道,但迄今为止,未见到对金属材料在3.5％NaCl溶液(SW)和蒸馏水(DW)这两种常见介质中的温度效应进行专门的对比研究。本文对GC-4(40CrMnSiMoVA)超高强度钢在SW与DW中CF裂纹扩展的温度效应和断口形貌进行了对比性的试验研究,并结合理论分析,得到了一些有用的研究结论。 二、材料与试验方法 试验用GC-4钢为电弧炉—电渣重熔钢棒,材料化学成分(wt％)为:C0.40,Si1.45,Mn0.90,Cr1.44,Mo0.50,V0.10,P0.07,S0.002。改锻后,取对CF较为敏感的T-L向加工成宽度W=50mm,厚度B=15mm的标准CT试样。试样热处理制度为920℃等温20min,300℃等温1h,200℃/20h除氢处理,获得贝氏体组织。材料基本力学性能为σ_b=1800MPa,σ_(0.2)=1321MPa,δ(％)=12.4,φ(％)=48.4。     

断口形貌几何特征定量分析

本文介绍了断口粗糙度与断口轮廓几何形态细节特征的定量分析方法,这一分析方法对于材料的失效分析,研究断裂的微观物理机制和设计高断裂阻力的材料等方面都具有实际应用价值。     

材料断口分形维数分析

本文分析了应用分形维数描述材料断口轮廓形貌几何特征的局限性。结果指出，分形维数对于材料断口粗糙度的变化是不敏感的，该参数与材料断口轮廓几何形态和断裂韧性之间不存在确定性的关系。     

材料断口分形研究现状及发展前景

讨论了分形几何应用于断裂研究的几个基本问题,主要包括断面的分形特征与分形测量,分维与断裂韧性的关系以及金属断裂的多种分形模型.进一步阐述了分形理论在金属断裂方面的应用前景.     

现代陶瓷刀具材料

文章回顾了现代陶瓷刀具材料的发展，强调了显微结构对刀具材料的机械性能和使用性能的影响，全面介绍了各种陶瓷刀具材料的组织和性能特点以及在高速切削加工时如何正确地选择和使用陶瓷材料。     

优质陶瓷刀具材料

陶瓷是未来的最新一类刀具材料,其潜力在于高速精加工操作范围宽以及对于难加工材料亦具有切削加工速率。陶瓷刀具材料的发展部分地归于70年代汽车、燃气透平和其他高温结构材料应用所新开发的高温结构陶瓷技术。优质陶瓷即氧化铝、部分稳定氧化锆(PSZ)、碳化硅及氮化硅,但是块状PSZ因硬度有限(努氏硬度～1500Kg/mm~2),不能用作机加工的切割刀具。此外,碳化硅也     

D6AC钢冲击断口形貌的分形研究

本文利用扫描电镜对含Mn$夹杂物和含ZrN夹杂物的D6AC钢的冲击断口形貌进行了观察与分析;利用数字图象法测定了试样的断口分形维数,揭示了冲击断口形貌和冲击韧性与夹杂物含量之间的内在联系,探讨了材料冲击断口形貌与分形维数的关系,拓宽了分形几何学在材料领域的应用范围.     

D6AC钢冲击断口形貌的分形研究

本文利用扫描电镜对含MnS夹杂物和含ZrN夹杂物的D6AC钢的冲击断口形貌进行了观察与分析;利用数字图象法测定了试样的断口分形维数,揭示了冲击断口形貌和冲击韧性与夹杂物含量之间的内在联系,探讨了材料冲击断口形貌与分形维数的关系,拓宽了分形几何学在材料领域的应用范围。     

明胶材料的力学性能及断口特征

研究了明胶、戊二醛（交联剂）和甘油（增塑剂）浓度对明胶材料力学性能的影响,并对明胶材料的拉钟断口进行了观察,研究表明,明胶材料的力学性能可在较大的范围内调整;随甘油浓度（即材料塑性）的变化,明胶材料的拉伸断口亦发生显著的变化。     

抽油杆断口失效分析及裂纹扩展期剩余强度预测

对油田中断裂的抽油杆进行宏观和微观的断口分析,分析引起抽油杆断裂的原因.结果表明,断裂原因部分包括静载破坏、人为损坏,多数属于疲劳破坏.对抽油杆裂纹扩展期的剩余强度进行了分析计算,以指导实际工程应用.     

Experiments on crack propagation in a viscoelastic material

The problem of plane stress crack growth in a rectangular linearly viscoelastic plate under a pair of equal and opposite forces applied to the crack surface is studied experimentally. A fatigue crack is generated at the end of a slot cut on the specimen. After application of two opposite forces to the specimen with a fatigue crack, the crack grows gradually. It is found that under sufficiently small load, the speed of crack growth decreases as time increases, and crack arrest occurs finally. However when the applied load is sufficiently large, the crack grows with increasing speed until a catastrophic failure of the specimen takes place. Between these two extreme cases, the crack grows with an oscillatory speed. A theory based on the models of an infinite plate and double cantilevers is proposed for the explanation of the behaviour of crack growth under various loads. Our experimental results check fairly well with theoretical predictions.     

Plane crack propagation in a hyperelastic incompressible material

We propose a crack propagation criterion for hyperelastic materials (rubber type material) within the framework of plane elasticity in finite deformation. The criterion is based on the examination of the asymptotic elastic field near the crack tip prior to propagation. According to this criterion, the propagation will take place for a critical value of the strain energy density intensity factor. The kink angle, obtained by applying the criterion of maximum opening stress, will depend on the fracture tensile stress of the actual material. We propose to use a local iterative finite element method to compute the asymptotic quantities involved in the criterion at a reasonable cost. Examples of computation for some hyperelastic laws simulating the behavior of vulcanized rubber are presented.     

Bifurcation and propagation of a mixed-mode crack in a ductile material

The bifurcation and the propagation of a 2-D mixed-mode crack in a ductile material under static and cyclic loading were investigated in this work. A general methodology to study the crack bifurcation and the crack propagation was established. First, for a mixed-mode crack under static loading, a procedure was developed in order to evaluate the fracture type, the beginning of the crack growth, the crack growth angle and the crack growth path. This procedure was established on the basis of a set of criteria developed in the recent studies carried out by the authors [Li J, Zhang XB, Recho N. J-M^p based criteria for bifurcation assessment of a crack in elastic-plastic materials under mixed mode I-II loading. Engng Fract Mech 2004;71:329-43; Recho N, Ma S, Zhang XB, Pirodi A, Dalle Donne C. Criteria for mixed-mode fracture prediction in ductile material. In: 15th European conference on fracture, Stockholm, Sweden, August 2004]. A new criterion, by combining experimentation and numerical calculation, was developed in this work in order to predict the beginning of the crack growth. Second, in the case of cyclic loading, the crack growth path and crack grow rate are studied. A series of mixed-mode experiments on aluminium and steel specimens were carried out to analyse the effect of the mixed mode on the crack growth angle and the crack growth rate. On the basis of these experimental results, a fatigue crack growth model was proposed. The effect of the mixed mode on the crack growth rate is considered in this model. The numerical results of this model are in good agreement with the experimental results.     

Fatigue fracture toughness and crack propagation rate

Fatigue crack growth rate, da/dN, of two high strength steels were examined in a laboratory air at different stress ratios, covering almost the entire range of stress intensity, K, from nearly threshold value, Kth, to final fracture. The fatigue fracture toughness, Kfc, corresponding to the final fracture in fatigue, was also determined. The lower the Kfc, the higher da/dN and reduced Kth are revealed.This correlation was analyzed quantitatively based on the four parameter Weibull function. And the stress ratio dependency of the fatigue crack propagation curve can be cleared in a successful manner.The fatigue characteristic stress intensities, Ke and Kv, are proposed to define the transition behaviour in fatigue crack growth curve, from so called region 1 to 2, and from region 2 to 3, respectively. Especially the Kv valua can be specified to be the 0.63Kfc.

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Résumé On a étudié la vitesse de propagation de fissure en fatigue da/dN de deux aciers à haute résistance dans un atmosphère de laboratoire sous des sollicitations couvrant toute la gamme des intensités de contraintes variables K, depuis une valeur voisine de la valeur du seuil Kth jusqu'à celle correspondant à rupture finale.La ténacité à la rupture par fatigue Kfe correspondant à la rupture finale par fatigue a été également déterminée. II s'avère que plus Kfe est faible, plus élevée est da/dN et plus Kth est réduite. Cette correlation est analysée quantitativement en se basant sur la fonction de Weibull à quatre paramètres. On peut ainsi clarifier la manière dont le rapport de contraintes influe les courbes de propagation des fissures de fatigue.On propose de définir pas les facteurs caractéristiques d'intensité de contrainte Ke et Kv les comportements de transition de la courbe de vitesse de propagation de la fissure entre respectivement les régions dénommées 1 et 2, et 2 et 3.En particulier, on peut spécifier que la valeur Kv vaut 0,63 Kfe.

     

Interfacial fracture characteristic and crack propagation of thermal barrier coatings under tensile conditions at elevated temperatures

Thermal barrier coatings (TBCs) have been extensively used in aircraft engines for improved durability and performance for more than fifteen years. In this paper, thermal barrier coating system with plasma sprayed zirconia bonded by a MCrAlY layer to SUS304 stainless steel substrate was performed under tensile tests at 1000°C. The crack nucleation, propagation behavior of the ceramic coatings in as received and oxidized conditions were observed by high-performance camera and discussed in detail. The relationship of the transverse crack numbers in the ceramic coating and tensile strain was recorded and used to describe crack propagation mechanism of thermal barrier coatings. It was found that the fracture/spallation locations of air plasma sprayed (APS) thermal barrier coating system mainly located within the ceramic coating close to the bond coat interface by scanning electron microscope (SEM) and energy dispersive X-Ray (EDX). The energy release rate and interface fracture toughness of APS TBCs system were evaluated by the aid of Suo–Hutchinson model. The calculations revealed that the energy release rate and fracture toughness ranged, respectively, from 22.15 J m⁻² to 37.8 J m⁻² and from 0.9 MPa m^1/2 to 1.5 MPa m^1/2. The results agree well with other experimental results.     

Fractal effects of crack propagation on dynamic stress intensity factors and crack velocities

Crack extension paths are often irregular, producing rough fracture surfaces which have a fractal geometry. In this paper, crack tip motion along a fractal crack trace is analysed. A fractal kinking model of the crack extension path is established to describe irregular crack growth. A formula is derived to describe the effects of fractal crack propagation on the dynamic stress intensity factor and on crack velocity. The ratio of the dynamic stress intensity factor to the applied stress intensity factor K(L(D, t), V)/K(L(t), 0), is a function of apparent crack velocity V_o, microstructure parameter d/a (grain size/crack increment step length), fractal dimension D, and fractal kinking angle of crack extension path . For fractal crack propagation, the apparent (or measured) crack velocity V_o, cannot approach the Rayleigh wave speed Cr. Why V_o is significantly lower than Cr in dynamic fracture experiments can be explained by the effects of fractal crack propagation. The dynamic stress intensity factor and apparent crack velocity are strongly affected by the microstructure parameter (d/a), fractal dimension D, and fractal kinking angle of crack extension path . This is in good agreement with experimental findings.     

Thermal fracture as a framework for quasi-static crack propagation

We address analytically and numerically the problem of crack path prediction in the model system of a crack propagating under thermal loading. We show that one can explain the instability from a straight to a wavy crack propagation by using only the principle of local symmetry and the Griffith criterion. We then argue that the calculations of the stress intensity factors can be combined with the standard crack propagation criteria to obtain the evolution equation for the crack tip within any loading configuration. The theoretical results of the thermal crack problem agree with the numerical simulations we performed using a phase field model. Moreover, it turns out that the phase-field model allows to clarify the nature of the transition between straight and oscillatory cracks which is shown to be supercritical.