考虑塑性的钢轨表面疲劳微裂纹分析

目的 研究塑性条件下受重复轮轨荷载的钢轨表面的初始疲劳裂纹行为。方法 建立含表面微裂纹的钢轨二维有限元模型,通过耦合裂纹面对应节点达到无裂纹的效果。经过几次循环加载后取消节点耦合设置,达到出现裂纹的效果。分析随着轮轨循环加载钢轨的响应,计算残余应力强度因子,并利用渐进状态（随着循环次数的增加,裂纹尖端区新产生的塑性越来越小,裂尖小范围内塑性不再增加）下的应力强度因子计算初始疲劳裂纹扩展速率。结果 有限元模拟中,轮轨荷载循环加载在钢轨上表面,使其产生拉伸残余应力,且随着深度增加,拉伸残余应力越来越小。裂纹萌生后,不同角度的裂纹残余（KI）都随循环次数的增加而减小,但残余（KII）都随循环次数的增加而增大。受残余应力的影响,渐进状态下的钢轨表面初始微裂纹应力强度因子随裂纹角度（θ）的增加而增加。结论 钢轨表面的残余应力加快了初始微裂纹的扩展速率,降低了钢轨的使用寿命。     

轴重和摩擦力对轮轨接触疲劳的影响

目的 了解钢轨表面存在裂纹时的轮轨接触问题。 方法 采用有限元分析软件 ANSYS,获得不同裂纹位置的应力强度因子。 结果 裂纹在接触斑边缘的位置时,应力强度因子 K_I最大;随着轴重的增加,应力强度因子 K_I增加,而应力强度因子 K_II先增加,后减小;考虑摩擦力( μ = 0 . 3 ) 时,相对于无摩擦,K_I和 K_II都明显增加,且 K_II所占 K_I的比例提高了 15% 。 结论 车轮在钢轨上滚动时,轮轨间的轴重和摩擦力是影响钢轨疲劳的重要因素,轴重的提高会明显加剧钢轨疲劳,而摩擦力的影响建立在轴重的基础上,并使轴重的影响加剧。     

桥式起重机轮轨异常接触引起的轨道断裂分析

利用有限元程序ANSYS建立有微裂纹的三维实体模型,计算了桥式起重机轨道应力强度因子的大小,结合实验判断钢轨断裂情况。结果表明,轮压最大载荷工作时,随着时间推移轨道表面微裂纹会逐渐发生扩展,扩展积累会造成钢轨的断裂。发生车轮啃轨现象时,裂纹会扩展更加剧烈甚至发生瞬间断裂。     

应力特性对冲击磨损机理的影响

在固定正应力磨损条件下研究了切应力对材料表面失效和裂纹萌生位置的影响。试验结果表明,随着冲击角度从９０°到４５°（τ／σ增加）的变化,亚表层最大剪切应力增加,其位置从亚表层向表面迅速转移。τ／σ的变化不仅影响裂纹萌生及扩展的速度,而且影响裂纹萌生的位置。对冲击磨损表面的微观形貌、失效方式分析表明,随着冲击角度的变化冲击磨损机理表现为两种失效方式,即剥落和微观切削。     

T形接头承载角焊缝根部裂纹扩展角度

针对Q355B低合金钢T形接头承载角焊缝根部疲劳失效的扩展路径问题,提出了一种基于受力分析计算的等效应力强度因子法(KEQ法)预测根部裂纹的扩展角度. 经有限元模拟验证,其求解应力强度因子的最大误差小于5%. 与基于有限元分析计算的最大周向应力法(MCS法)和有效结构应力法(ETS法)相比较,并结合3种不同应力水平下弯曲疲劳试验结果发现,基于等效应力强度因子法、最大周向应力法与有效结构应力法求解的裂纹扩展角度分别为25.6°,25.9°和32.2°,相较于实际疲劳试验的根部裂纹扩展角度24°,误差分别为6.67%,7.92%和34.17%. 结果表明,基于KEQ法求解裂纹扩展角度准确度最高,更适于预测承受弯曲疲劳载荷下T形接头角焊缝根部裂纹的扩展角度.     

基于分布位错法对涂层裂纹力学行为的研究

目的 在单轴拉伸载荷下,用理论方法求解弹性涂层中裂纹的力学性质和相互影响。方法 根据叠加原理,将问题分为2个子问题,使用分布位错原理求解裂纹问题,将裂纹建模为沿裂纹线分布的位错阵列,叠加后使用数值求解方法进行求解。结果 得到了不同涂层模量、不同裂纹长度下表面裂纹尖端的应力强度因子（SIF）和涂层界面应力。涂层与基底模量相差越大,表面裂纹越长,其界面应力越大。计算了不同方位下的微裂纹对表面裂纹的影响,给出了60°倾角微裂纹、2l/h=0.2和2l/h=0.04表面裂纹以及2a/h=0.01和2a/h=0.018表面裂纹的影响区域。分析了涂层内部倾斜裂纹对表面裂纹应力强度因子和扩展角的影响。内部倾斜裂纹尖端对表面裂纹尖端的等效应力强度因子（ESIF）有增强作用,两侧有减弱作用。结论 较硬涂层对表面裂纹的扩展有增强作用,裂纹越长,受涂层模量对其应力强度因子的影响越大。微裂纹对表面裂纹的影响跟微裂纹位置、方向、长度和表面裂纹长度有关。表面裂纹附近的倾斜裂纹对表面裂纹的扩展具有吸引作用。     

基于ABAQUS的某无人机机翼肋板裂纹扩展仿真

采用Python语言对ABAQUS进行二次开发,实现了对无人机机翼肋板裂纹扩展的模拟。试验将机翼肋板简化为二维平板结构,在肋板与梁的连接处放置裂纹,模拟不同初始角度的裂纹的扩展过程。结果表明:裂纹的扩展形态、扩展过程中的应力强度因子和裂纹扩展角的变化规律,与裂纹的初始角度有密切的关系;可分为两种情况:当裂纹初始角度为50°~70°时,裂纹沿着初始方向向前扩展,不发生方向的改变;当裂纹初始角度为-70°~40°时,裂纹最终向着翼肋的下表面扩展。该结果对研究机翼肋板裂纹的止裂以及裂纹扩展速率等有一定的参考价值。     

淬火残余应力对铝合金厚板裂纹应力强度因子及扩展趋势的影响

为探究淬火残余应力对铝合金厚板疲劳裂纹扩展的影响规律,建立7075铝合金厚板表面三维裂纹数值仿真模型。采用顺序热力耦合法求解淬火残余应力场,将残余应力场作为初始载荷条件求解裂纹应力强度因子,并与无残余应力场的应力强度因子值进行对比,研究两种条件下应力强度因子的分布规律和两者之间的异同;通过分析在初始淬火残余应力条件下不同半径裂纹受不同均匀拉应力荷载作用时的裂纹应力强度因子随裂纹位置角的演变曲线,探究淬火残余应力对裂纹扩展趋势的影响规律。结果表明,淬火残余应力的存在改变了铝合金厚板应力强度因子的分布规律和裂纹的扩展趋势,淬火残余应力使表层附近的裂纹扩展受到遏止,裂纹易于在厚度方向优先扩展。     

风机增速齿轮含初始裂纹扩展特性及寿命分析

针对某风机增速齿轮疲劳裂纹断裂问题，基于M积分法探究含初始三维裂纹的增速齿轮在裂纹扩展时的变化规律。根据断裂力学原理结合有限元原理分析计算，得出应力强度因子及疲劳扩展循环次数的变化规律。确定增速齿轮齿根受力最大位置后创建三维裂纹模型；通过改变齿根边缘三维裂纹纵向位置来探究三维裂纹在扩展过程中的应力强度因子及疲劳寿命变化。结果表明：随着裂纹扩展步数的增大，3组裂纹的应力强度因子 KⅠ 均增大，且齿根裂纹1应力强度因子一直保持最大；在直齿轮边缘的裂纹，越靠近齿根其疲劳寿命越小。     

涡轮盘中心孔三维疲劳裂纹扩展分析

基于有限元软件ABAQUS和三维裂纹扩展分析软件Franc3D,对涡轮盘中心孔三维疲劳裂纹扩展进行研究分析。首先,对平板试样表面裂纹进行裂纹扩展模拟计算研究,对比手册中Gross/Brown理论模型验证裂纹扩展应力强度因子数值模拟的准确性;其次,针对涡扇发动机涡轮盘结构,对轮盘不同外缘等效应力、转速情况的应力强度因子以及考虑初始缺陷的三维疲劳裂纹扩展寿命进行计算;最后,讨论发动机载荷差异对应力强度因子和裂纹扩展寿命影响规律。结果表明:在相同裂纹长度时,应力强度因子随着轮盘外缘等效应力和转速增加而增大,载荷越大疲劳寿命则越短,且裂纹越长,影响越大。为工程上三维裂纹扩展计算以及寿命评估提供参考。     

The effect of iodine content and specimen orientation on stress corrosion crack growth rate in Zircaloy-4

The influence of specimen orientation, stress intensity factor (K_I), and iodine concentration on the iodine-induced stress corrosion cracking growth rates in Zircaloy-4 was investigated in iodized methanol solutions at ambient temperature. When K_I is lower than 20 MPa.m^1/2, the intergranular and mixed intergranular/transgranular crack propagation rates increase linearly with (K_I − K_I,th), K_I,th being the onset of propagation stress intensity factor. The increase in iodine content induces an increase of the crack growth rate for a given K_I, and a decrease of the K_I,th. The specimen orientation is a second order parameter. A crack propagation law, depending on iodine content, is proposed.     

Crack path morphology in dual-phase steel

Intercritical heat treatment (ICHT) and thermomechanical processing (TMP) were used on steel having 0.16% C to vary the morphology, distribution of ferrite, and martensite phases, in order to study the resistance to fatigue crack propagation and crack path morphology in dual-phase steel. A crack growth rate has been determined at ∼10⁻¹⁰ to 10⁻³ m per cycle in ICHT and TMP samples. The tortuous morphology of the crack path was observed in unrolled materials, which resulted in reduction of the crack driving force from crack deflection and increased the ΔK _th. In thermomechanical processed materials, the crack tended to cross the martensite and the crack path become less circuitous, resulting in decrease a threshold stress intensity factor (ΔK _th) as compared with unrolled material.     

A comparison of SCC velocity measurements under conditions of constant load and constant displacement

The SCC velocity for 2024-T351 aluminium alloy in aqueous 3% NaCl solution was measured employing DCB specimens oriented for crack propagation in the SL and ST orientations. For lower values of the nominal or applied stress intensity factor, K_app, the results obtained under constant load (increasing K_app) and constant load-point displacement (decreasing K_app) agree well. For higher values of K_app, significantly lower crack velocities can be obtained in constant displacement tests, as a result of greater crack branching. Procedures that can minimize the difference between K_app and the effective stress intensity factor at the crack tip are discussed.     

Hot corrosion cracking of stainless steel

Investigations have been carried out to study the hot corrosion cracking and crack propagation in 304 type stainless steel at 700°C. Kerosene was used as the base fuel to which CS₂ was added in controlled quantities to obtain different sulphur contents (2.5% and 5%). Crack propagation tests were carried out at three stress levels. It has been observed that an increase in the sulphur content of the fuel reduces the crack initiation time, increases the crack propagation rate and decreases the rupture life. The crack growth rate plotted against the LEFM (linear elastic fracture mechanics) parameter K resulted in a sigmoidal type of relation, which could be well correlated by a model based on COD approach. The threshold stress intensity factor K₀ is reduced with increasing sulphur content. Fractographic studies by SEM and light microscope revealed crack propagation through grain boundaries, which were weakened by the formation of low melting sulphides.     

Nano-indentation fracture test of Pb(Zr0.52Ti0.48)O3 ferroelectric thin films

In this paper, we propose an elastic groundsill beam model with piezoelectric effect considered to assess the interfacial adhesion of ferroelectrics thin films, complemented and validated by nano-indentation fracture test on Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films. It was observed that the hardness and elastic modulus of thin films depend on the indentation depth. It was also observed from the load-indentation depth curves and atomic force microscopy (AFM) images that the fracture failure of PZT thin films induced by nano-indentations can be divided into three typical stages: no damage, bulging and spallation. The delamination of thin film systems was modeled as an interfacial crack propagation problem, with the energy release rate determined from the elastic groundsill beam model. Good agreement was observed between the indentation load and the radius of the largest imprint. For PZT thin films deposited on single Si substrate with thickness of 350 nm and 450 nm, the energy release rates per unit new crack area are in the range of 3.4~52.4 J/m² and the phase angles are constant of 13.4°. The corresponding mode I and mode II stress intensity factors are in the range of K_I=0.4–1.6MPa·m^1/2 and K_II=0.6–2.2MPa·m^1/2.     

Crack growth in a welded microalloyed steel under sulfide stress cracking conditions

In this work, the hydrogen sulfide stress-corrosion cracking (SSC) susceptibility of a welded API X-80 pipeline was investigated. For this purpose, steel welding was carried out normal to the rolling direction using a 60° single V-joint design. After welding, compact modified-wedge opening loading (M-WOL) fracture mechanics specimens were machined and loaded to an applied stress intensity factor, K_I, of 27 to 53 MPa√m. This was followed by specimen exposure to H₂S saturated synthetic seawater. Each of the M-WOL specimens contained the typical microstructures developed during welding, such as the weld metal (WM), base metal (BM), and heat affected zone (HAZ). No attempt was made to establish a unique K_ISCC for crack arrest because its significance was not clear. Qualitatively, the experimental outcome indicated that in mode I loading under a K_I of 40.3 MPa√m only the base metal region underwent SSC. Apparently, active anodic dissolution of the crack tip started the growth process, but it was followed by a transition to hydrogen induced cracking. At an applied K_I of 55 MPa√m and under similar exposure times, crack growth in the base metal was discontinuous and tended to follow the grain boundaries. Moreover, the HAZ exhibited the least SSC susceptibility as inferred from the relatively short crack propagation lengths (0.829 mm). In this case, it was found that the crack path was highly tortuous due to the presence of acicular ferrite and a refined grain structure. The most SSC susceptible condition was found in the weld metal where crack lengths of up to 4.2 mm developed. In this case, the presence of a relatively coarse dendritic structure coupled with interdendritic segregation provided a weak path for crack propagation.     

Mechanism study on subcritical crack growth of flabby and intricate ore rock

1 Introduction Because of the effects of diagenetic process and tectonic movement, lots of jointed plane of weakness with different contacted characteristics exist in the practicable rock mass. The deformation and fracture mechanisms of the rock mass are…     

Anisotropic stress corrosion cracking behaviour of prestressing steel

This paper evaluates the anisotropic stress corrosion cracking behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to stress corrosion cracking tests in aqueous environments using a constant strain technique and precracked three point bend specimens to measure the crack growth rate da/dt as a function of the stress intensity factor K_I under hydrogen embrittlement environmental conditions (pH = 12.5 E = − 1200 mV SCE). While the hot rolled bar presents an isotropic stress corrosion cracking behaviour associated with mode I crack growth, the cold drawn wire exhibits a change in crack propagation direction approaching that of the wire axis (cold drawing direction) and producing mixed mode crack growth. This anisotropic stress corrosion cracking behaviour is a consequence of manufacturing, since cold drawing affects the microstructure of the material and produces a preferential orientation of the pearlite lamellae aligned parallel to the wire axis. The differences of crack growth rate as a function of the crack propagation direction are discussed.     

Effects of stress ratio on fatigue crack propagation properties of submicron-thick free-standing copper films

The dominant mechanics and mechanisms of fatigue crack propagation in ca. 500 nm thick free-standing copper films were evaluated at the submicron level using fatigue crack propagation experiments at three stress ratios, R = 0.1, 0.5 and 0.8. Fatigue cracking initiated at the notch root and propagated stably under cyclic loading. The fatigue crack propagation rate (da/dN) vs. stress intensity factor range (ΔK) relation was dependent on the stress ratio R;da/dN, increases with increasing R. Plots of da/dN vs. the maximum stress intensity factor (K_max) exhibited coincident features in the high-K_max region (K_max ? 4.5 MPa m^1/2) irrespective of R, indicating that K_max is the dominant factor in fatigue crack propagation. In this region, the fatigue crack propagated in tensile fracture mode irrespective of the R value. The region ahead of the fatigue crack tip is plastically stretched by tensile deformation, causing necking deformation in the thickness direction and consequent chisel-point fracture. In contrast, in the low-K_max region (K_max < 4.5 MPa m^1/2), the da/dN vs. K_max function assumes higher values with decreasing R; in this region, the fracture mechanism depends on R. At the higher R value (R = 0.8), the fatigue crack propagates in the tensile fracture mode similar to that in the high-K_max region. On the other hand, at the lower R values (R = 0.1 and 0.5), a characteristic mechanism of fatigue crack propagation appears: within several grains, intrusions/extrusions form ahead of the crack tip along the Σ3 twin boundaries, and the fatigue crack propagates preferentially through the intrusions/extrusions.     

Discrete dislocation plasticity modeling of short cracks in single crystals

The mode-I crack growth behavior of geometrically similar edge-cracked single crystal specimens of varying size subject to both monotonic and cyclic axial loading is analyzed using discrete dislocation dynamics. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation incorporated through a set of constitutive rules. The fracture properties are specified through an irreversible cohesive relation. Under monotonic loading conditions, with the applied stress below the yield strength of the uncracked specimen, the initiation of crack growth is found to be governed by the mode-I stress intensity factor, calculated from the applied stress, with the value of K_init decreasing slightly with crack size due to the reduction in shielding associated with dislocations near a free surface. Under cyclic loading, the fatigue threshold is ΔK-governed for sufficiently long cracks. Below a critical crack size the value of ΔK_I at the fatigue threshold is found to decrease substantially with crack size and progressive cyclic crack growth occurs even when K_max is less than that required for the initiation of crack crack growth in an elastic solid. The reduction in the fatigue threshold with crack size is associated with a progressive increase in internal stress under cyclic loading. However, for sufficiently small cracks, the dislocation structure generated is sparse and the internal stresses and plastic dissipation associated with this structure alone are not sufficient to drive fatigue crack growth.