双金属层合板垂直界面裂纹疲劳的扩展过程

采用四点弯曲加载方式进行奥氏体不锈钢／低碳锅炉钢双金属层合板垂直界面裂纹的疲劳扩展实验,研究了组元强度配合、爆炸焊接影响的区域性能（晶粒大小、形变强化、界面脱粘、独立塑性区等）对裂纹扩展行为的影响,以及垂直界面裂纹疲劳扩展的不同过程及其所对应的扩展机制．结果表明：由于强度错配,裂纹起始于高强度材料一倜时其疲劳扩展速率提高,而起始于低强度材料一倜时其疲劳扩展速率降低;当裂纹尖端接近界面时,界面区域的存在对上述两种情况下疲劳裂纹的扩展均起到了一定的屏蔽减速作用．     

颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和扩展行为的影响

通过原位扫描电子显微镜(SEM)研究了粉末冶金制备的Cu/WCp复合材料的疲劳裂纹萌生和扩展行为,分析了颗粒和微观结构对Cu/WCp复合材料疲劳裂纹萌生和早期扩展行为的影响。结果表明:疲劳微裂纹萌生于WCp颗粒和基体Cu的界面;微裂纹之间相互连接并形成主裂纹,当主裂纹和颗粒相遇时裂纹沿着颗粒界面扩展。在低应力强度因子幅ΔK区域疲劳小裂纹具有明显的"异常现象",并占据了全寿命的71%左右。疲劳小裂纹的早期扩展阶段易受局部微观结构和颗粒WCp的影响,扩展速率波动性较大,随机性较强;当小裂纹长度超过150μm时,裂纹扩展加快直至试样快速断裂。裂纹偏折、分叉和塑性尾迹降低了疲劳裂纹扩展速率,而颗粒界面脱粘则提高了复合材料的疲劳裂纹扩展速率。通过数值模拟也可以发现颗粒脱粘增大了材料的疲劳扩展驱动力,从而提高了疲劳裂纹扩展速率。     

橡胶夹层/复合材料粘接界面疲劳裂纹扩展行为的研究

用 型加载下的双悬臂夹层梁试样 ,以应变能释放率为裂纹扩展参量 ,研究橡胶夹层 /复合材料粘接界面疲劳裂纹的扩展行为。结果表明 ,循环载荷下的裂纹扩展速率对试验频率、载荷比、温度及橡胶夹层厚度反映较敏感。     

橡胶夹层／复合材料粘接界面疲劳裂纹扩展行为的研究

用Ⅰ型加载下的双悬臂夹层梁试样,以应变能释放率为裂纹扩展参量,研究橡胶夹层／复合材料粘接界面疲劳裂纹的扩展行为。结果表明,循环载荷下的裂纹扩展速率对试验频率、载荷比、温度及橡胶夹层厚度反映较敏感。     

软弱夹层对岩石爆破裂纹扩展影响的研究

为研究平行于炮孔的软弱夹层对岩石爆破裂纹扩展的影响，通过制作含不同软弱夹层的水泥砂浆试块进行爆破模拟试验，并用高速摄影仪对试块爆破裂纹扩展过程进行观测。同时，通过有限元分析软件ANSYS/LS-DYNA对含不同软弱夹层试块的爆破裂纹扩展起因进行数值模拟分析。结果表明：随着软弱夹层强度的降低，软弱夹层侧爆炸反射应力波强度逐渐增加、透射应力波强度逐渐降低，这使得软弱夹层侧裂纹扩展速度逐渐增加、裂纹扩展也越充分。同时，随着软弱夹层强度的降低，爆炸应力波在软弱夹层侧的衰减速度逐渐加快。试验结果与数值模拟结果具有较强的一致性，表明数值模拟对实践具有一定的指导意义。     

LY12爆炸复合板垂直界面疲劳性能研究

本文考察了LY12爆炸复合板不同取向的拉伸强度,分层韧性及垂直层合界面疲劳裂纹扩展行为.观测了垂直界面疲劳长裂纹的扩展路径形态,并利用断裂力学理论讨论了材料的层状结构与其疲劳性能之间的关系.结果表明,层合板中的层间界面性能对其疲劳性能具有重要影响,在LY12爆炸复合板中,垂直板面方向的疲劳裂纹在界面处发生了明显的止裂.     

Cu/WCp叠层功能梯度材料疲劳裂纹扩展速率的数值模拟

本研究基于FRANC2D(Fracture Analysis Code in 2 Dimensions)二维断裂分析有限元软件,并结合大型有限元软件ABAQUS,对Cu/WC_p双层及多层功能梯度材料的疲劳裂纹扩展进行数值模拟研究,控制疲劳裂纹沿不同梯度方向扩展,计算出裂纹扩展中的应力强度因子幅(ΔK),绘制出疲劳裂纹扩展速率曲线,以及裂纹扩展速率和裂尖距界面距离l的关系曲线(da/dN-l),将模拟计算结果与试验结果进行对比分析。研究表明:结合FRANC2D和ABAQUS的数值模拟方法,在比较复杂的叠层功能梯度材料有限元模型建立中具有很大的优势,可以快速地计算功能梯度材料的应力强度因子值;发现材料梯度层间界面的存在以及材料梯度含量的变化,对功能梯度材料裂纹扩展的整个阶段都存在很大影响;梯度层的数量对功能梯度材料的疲劳裂纹扩展速率也有一定影响。     

纤维增强复合材料界面疲劳裂纹扩展的模拟研究

脱粘是纤维增强复合材料界面裂纹扩展的主要表现形式。基于剪切筒模型和常用的实验加载方式,研究了纤维增强复合材料中纤维与基体界面在拉-拉循环荷载作用下的裂纹扩展。借助描述疲劳裂纹扩展的Paris公式,得到了疲劳裂纹扩展速率、扩展长度以及界面上摩擦系数与加载次数的关系。在分析中,考虑了疲劳加载引起的脱粘界面的损伤及损伤分布的不均匀性,同时还考虑了材料的泊松效应。     

疲劳裂纹在奥氏体／铁素体异种钢接接头中的扩展行为

采用三点弯曲试样研究了疲劳裂纹在奥氏体 /铁素体异种钢焊接接头中的扩展行为与显微组织的关系 ,测得疲劳裂纹在 Cr2 5 Ni13/ 13Cr Mo44异种钢焊接接头中的扩展速率 da/ d N,并且讨论了疲劳裂纹扩展与显微组织之间的关系。实验结果表明 ,疲劳裂纹在异种钢焊接接头熔合区中扩展的路径 ,是接头中韧性最低的热影响区过热区 ,裂纹在铁素体材料侧 ,跟随熔合线并平行于熔合线 5～ 2 5 μm扩展 ,而马氏体层对疲劳裂纹有较大的抗力 ,疲劳裂纹的扩展路径主要受组织韧性的控制。疲劳裂纹在 Cr2 5 Ni13/ 13Cr Mo44异种钢接头的扩展速率为 :da/ d N=7.0 7× 10 - 1 3(△ K ) 3.86 3     

两级载荷作用下复合材料界面的脱粘

研究了两级拉伸疲劳载荷作用下,纤维增强复合材料界面的脱粘。首先基于剪切筒模型,建立了求解纤维与基体应力的控制微分方程,并求得了相关解答。然后借助断裂力学中描述疲劳裂纹扩展的公式和能量耗散率理论,给出了界面脱粘长度、加载次数以及脱粘应力之间的关系式。最后通过实例模拟了两级拉伸疲劳载荷作用下的界面裂纹扩展,分析了界面疲劳裂纹扩展速率、脱粘长度在不同加载方式下的变化规律,以及材料泊松比的变化对界面脱粘的影响。从而为进一步研究工程结构的疲劳破坏和材料的最优设计提供一定的理论依据。      

Kurzrisswachstum bei mehrachsig nichtproportionaler Beanspruchung

Short fatigue crack growth under multiaxial nonproportional loading Initiation and short fatigue crack growth have been investigated under nonproportional cyclic loading. A critical plane approach based on fracture mechanics is used for modelling the fatigue process. A Paris‐type crack growth law, formulated using the effective cyclic J‐integral as crack driving force parameter, is integrated to give crack growth curves. Crack opening stresses and strains are calculated with approximation equations. Jiang's plasticity model is used to predict the stress‐strain path. The good agreement between model and real damage evolution is shown comparing experimentally determined crack growth curves, crack orientations, and life curves.     

Fatigue crack propagation under cyclic thermal stress

Structures used at elevated temperature subject to severe cyclic thermal stress. Therefore, accurate prediction procedures for thermal fatigue crack growth should be applied to rationalise component flaw assessment. Fatigue crack propagation tests under thermal stress were carried out using an modified type 316 stainless steel (316FR), which is a candidate material for the fast reactor in Japan. Thermal stress of the tests was generated by cyclically changed temperature distribution through thickness in a plate by induction heating and air-cooling. Numerical analysis was also carried out to examine the applicability of the J integral under cyclic thermal stress. The J integral under elasto-plastic condition under thermal stress is close to the elastically calculated J integral. Prediction by J integral tends to be conservative for deeper cracks, and modification of the J integral value using crack opening ratio gives good agreement with the experimental crack growth.     

Lebensdauerberechnung festgewalzter Kerbproben unter Axialbelastung auf der Basis eines Rißfortschrittskonzepts

Axial fatigue life calculation of fillet rolled specimens by means of a crack growth model Fillet rolling is a method which significantly improves the fatigue strength of members. Residual compressive stresses induced in the surface layer during the fillet rolling process are able to retard or prevent crack propagation. An elastic‐plastic on the J‐integral based crack growth model considering the crack opening and closure phenomenon in nonhomogeneous plastic stress fields is described. Experimentally determined crack growth curves and fracture fatigue life curves at constant amplitude loading were used to verify the developed model.     

An incremental formulation for the prediction of two-dimensional fatigue crack growth with curved paths

This paper presents a new incremental formulation for predicting the curved growth paths of two-dimensional fatigue cracks. The displacement and traction boundary integral equations (BIEs) are employed to calculate responses of a linear elastic cracked body. The Paris law and the principle of local symmetry are adopted for defining the growth rate and direction of a fatigue crack, respectively. The three governing equations, i.e. the BIEs, the Paris law and the local symmetry condition, are non-linear with respect to the crack growth path and unknowns on the boundary. Iterative forms of three governing equations are derived to solve problems of the fatigue crack growth by the Newton–Raphson method. The incremental crack path is modelled as a parabola defined by the crack-tip position, and the trapezoidal rule is employed to integrate the Paris law. The validity of the proposed method is demonstrated by two numerical examples of plates with an edge crack. Copyright © 2007 John Wiley & Sons, Ltd.     

A computational strategy for damage‐tolerant design of hollow shafts under mixed‐mode loading condition

Three‐dimensional numerical analyses, using the finite element method (FEM), have been adopted to simulate fatigue crack propagation in a hollow cylindrical specimen, under pure axial or combined axial‐torsion loading conditions. Specimens, made of Al alloys B95AT and D16T, have been experimentally tested under pure axial load and combined in‐phase constant amplitude axial and torsional loadings. The stress intensity factors (SIFs) have been calculated, according to the J‐integral approach, along the front of a part through crack, initiated in correspondence of the outer surface of a hollow cylindrical specimen. The crack path is evaluated by using the maximum energy release rate (MERR) criterion, whereas the Paris law is used to calculate crack growth rates. A numerical and experimental comparison of the results is presented, showing a good agreement in terms of crack growth rates and paths.     

Effects of non-proportional loading paths on the orientation of fatigue crack path

Fatigue crack path prediction and crack arrest are very important for structural safety. In real engineering structures, there are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path, etc. In this paper, both experimental and numerical methods are applied to study the effects of loading path on crack orientations. Experiments were conducted on a biaxial testing machine, using specimens made of two steels: 42CrMo4 and CK45 (equivalent to AISI 1045), with six different biaxial loading paths. Fractographical analyses of the plane of the stage I crack propagation were carried out and the crack orientations were measured using optical microscopy. The multiaxial fatigue models, such as the critical plane models and also the energy‐based critical plane models, were applied for predicting the orientation of the critical plane. Comparisons of the predicted orientation of the damage plane with the experimental observations show that the shear‐based multiaxial fatigue models provide good predictions for stage I crack growth for the ductile materials studied in this paper.     

Behaviour of short cracks in alloy 800HT under biaxial fatigue

Biaxial in phase fatigue tests were carried out on thin walled tube specimens of alloy 800HT at ambient temperature. The loading modes included tension, torsion, and combined tension—torsion with a tensile/shear plastic strain range ratio Δ?p/Δγp = 3^1/2. The influence of effective strain amplitudes and biaxiality on the initial growth of fatigue cracks was investigated using the replica technique. The results indicated that the loading conditions strongly affected the growth rates of short cracks. In torsion the cracks grew significantly more slowly than under axial or biaxial loading. A mean tensile stress perpendicular to the shear crack promoted its growth and reduced the fatigue life. The growth of the cracks could be described by the ΔJ integral for axial and biaxial loading; the integration predicted the fatigue life under axial and biaxial loading correctly. However, significantly conservative lifetime predictions were obtained for pure torsional loading since ΔJ does not include crack closure and crack surface rubbing.

MST/3234     

THE EFFECT OF GRAIN SIZE ON THE FATIGUE OF COMMERCIALLY PURE ALUMINIUM

Abstract— Fully reversed uniaxial fatigue tests were performed on polished hour-glass specimens of commercially pure aluminium with three different grain sizes, in order to examine the effect of grain size on fatigue. The growth of surface cracks was monitored by a plastic replication method. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. Multiple crack initiation and growth is a feature of the fatigue of aluminium, while the grain size influences the specific detail of crack coalescence. Crack path deviation is greatest in the coarse grained microstructure and crack surface roughness is more pronounced. SEM fractography reveals that crack initiation and early crack growth takes place along crystallographic slip planes, and that fatigue striations, characteristic of stage II cracking, extend to the very edge of the specimen section, suggesting extensive crack tip blunting.     

室温大气环境下峰时效态3J21合金疲劳行为

在MTS万能实验机上对室温大气环境下峰时效态3J21合金的疲劳行为进行研究,并采用扫描电镜(SEM)对宏观断口及微观断口进行分析。结果表明,峰时效态3J21合金的疲劳裂纹主要呈穿晶扩展,沿晶扩展的所占比例较小,疲劳裂纹萌生寿命较低,扩展路径比较平直,扩展速率较大,裂纹扩展抗力较小,疲劳寿命较低;峰时效态3J21合金疲劳断口由疲劳源、疲劳裂纹扩展区和瞬断区组成。在低速扩展区,峰时效态断口呈现冰糖状花样,在中速扩展区未看到长的疲劳条纹,仅发现个别细且短小的疲劳条纹,瞬断区可观察到二次裂纹、准解理和韧窝。     

On the interactions between strain accumulation, microstructure, and fatigue crack behavior

Fatigue crack growth is a complex process that involves interactions between many elements ranging across several length scales. This work provides an in-depth, experimental study of fatigue crack growth and the relationships between four of these elements: strain field, microstructure, crack path, and crack growth rate. Multiple data sets were acquired for fatigue crack growth in a nickel-based superalloy, Hastelloy X. Electron backscatter diffraction was used to acquire microstructural information, scanning electron microscopy was used to identify locations of slip bands and crack path, and optical microscopy was used to measure crack growth rates and to acquire images for multiscale digital image correlation (DIC). Plastic strain accumulation associated with fatigue crack growth was measured at the grain level using DIC. An ex situ technique provided sub-grain level resolution to measure strain variations within individual grains while an in situ technique over the same regions showed the evolution of strain with crack propagation. All of these data sets were spatially aligned to allow direct, full-field comparisons among the variables. This in-depth analysis of fatigue crack behavior elucidates several relationships among the four elements mentioned above. Near the crack tip, lobes of elevated strain propagated with the crack tip plastic zone. Behind the crack tip, in the plastic wake, significant inhomogeneities were observed and related to grain geometry and orientation. Grain structure was shown to affect the crack path and the crack growth rate locally, although the global crack growth rate was relatively constant as predicted by the Paris law for loading with a constant stress intensity factor. Some dependency of crack growth rate on local strain and crack path was also found. The experimental comparisons of grain structure, strain field, and crack growth behavior shown in this work provide insight into the fatigue crack growth process at the sub-grain and multi-grain scale.