TB6钛合金疲劳小裂纹扩展行为

为了研究TB6钛合金自然萌生小裂纹的扩展行为,针对单边缺口拉伸试样开展室温下不同应力比(R=0.1,0.5)的小裂纹扩展实验,采用复型法观测了小裂纹的萌生与扩展情况。结果表明:同一应力比下,随着应力等级的降低,小裂纹的萌生寿命由占全寿命的60%增加到80%,但应力等级对TB6钛合金小裂纹扩展速率没有明显影响。裂纹早期扩展速率受微观组织的影响大,TB6钛合金扩展速率转变临界值是200μm,一旦裂纹长度达到200μm,裂纹扩展速率将不受取向不同的晶界或晶粒影响而迅速提升。TB6钛合金疲劳小裂纹起源于试样缺口根部,所有试样的裂纹大部分为角裂纹,疲劳小裂纹萌生寿命占全寿命的绝大部分。     

TC11钛合金全范围疲劳裂纹扩展速率表征

通过疲劳裂纹扩展试验研究了TC11钛合金在不同取向及应力比下的疲劳裂纹扩展行为。结果表明:在相同应力强度因子的情况下,随着应力比的增大,TC11钛合金的疲劳裂纹扩展速率增大。通过TC11钛合金的拉伸性能对裂纹扩展门槛值进行估算,得到了可以对TC11钛合金进行全范围疲劳裂纹扩展速率预测的疲劳裂纹扩展模型,为进一步估算其疲劳寿命提供了依据。     

S135钻杆钢的拉扭复合加载疲劳行为

采用疲劳实验和回归分析相结合的方法,研究了S135钻杆钢在拉扭复合加载条件下的疲劳行为,并对疲劳断口进行了微观分析。结果表明:当τa/σeq=0.7时,由拉扭应力幅对应的当量应力表示的疲劳寿命公式可很好地描述S135钻杆钢的拉扭疲劳寿命规律;疲劳断口由疲劳源区、疲劳裂纹稳定扩展区和快速瞬断区组成,疲劳裂纹从试样表面萌生,并向试样内部扩展,且常为多疲劳源,不同疲劳源断口的连接和复合加载形成所谓的"屋脊"状特征;拉扭疲劳断裂试样裂纹源区的微观断口特征为明显的河流花样,裂纹扩展区的微观断口特征为疲劳条带与涟波状花样。     

ZTC4（Ti-6Al-4V）铸造钛合金的室温低周疲劳行为

研究了ZTC4钛合金应变控制的室温低周疲劳行为,对循环应力-应变和应变疲劳寿命数据进行了分析,通过双对数线性回归处理,得出了Manson-Coffin处理模型的疲劳参数。结果表明：ZTC4钛合金总应变幅在0.6%～0.8%时,材料存在轻度循环软化的现象;总应变幅为0.4%～0.5%时,循环初期表现出循环硬化的现象,而后循环软化。合金的疲劳裂纹萌生于试样表面,裂纹扩展区存在明显的疲劳条带,合金疲劳断口呈现韧性断裂特征。     

基于损伤力学的疲劳裂纹萌生及扩展规律研究

针对金属构件疲劳裂纹的萌生、扩展寿命及其规律等问题,应用损伤力学理论与有限元相结合的方法,建立了计算疲劳裂纹全寿命的统一模型。引入附加载荷法,通过MATLAB编程计算,实现了对刚度矩阵的连续计算,并给出了编程的流程。通过对单个单元的损伤计算,得到了单元从无损到破坏过程中等效应力的变化;通过计算各个构件损伤单元寿命,进而给出了金属构件总体疲劳寿命。分析得到了微裂纹萌生及扩展寿命占总体疲劳寿命的80%以上,并应用有限元软件ANSYS模拟给出了缺口件裂纹萌生及扩展过程。理论计算的结果与试验数据对比基本一致,验证了本工作方法的准确性。     

超声疲劳试验方法对S06钢疲劳性能及裂纹萌生机制的影响

采用超声疲劳试验方法和试样尺寸相近的高频疲劳试验方法对S06钢进行疲劳性能试验,将试验数据和试样断口形貌进行对比,研究超声疲劳试验方法对S06钢疲劳性能和裂纹萌生机制的影响.结果表明:在相同的应力水平下,超声加载频率下S06钢的疲劳寿命高于高频疲劳试验测得的疲劳寿命;超声疲劳试验中裂纹全部从表面萌生,而高频疲劳试样裂纹有表面萌生和内部萌生两种机制.分析了超声加载频率对S06钢疲劳性能和裂纹萌生机制产生影响的原因:对疲劳性能的影响与金属材料的晶体结构和裂纹尖端的化学反应有关,对裂纹萌生机制的影响与试样表层残余应力松弛有关.     

2024铝合金喷丸试件疲劳寿命试验及仿真研究

现有的喷丸材料疲劳性能研究扩展有限元模型没有考虑残余应力对裂纹扩展的影响。对2024铝合金的喷丸与未喷丸试样进行三弯疲劳试验,以明确喷丸工艺对试件疲劳寿命的强化作用。通过ABAQUS建立试件的二维平面应力模型,导入残余应力并利用扩展有限元法模拟循环载荷下裂纹的萌生与扩展,对比试验结果来验证该扩展有限元数值模型的正确性。最后基于该数值模型,改变载荷工况,研究不同载荷工况下残余应力对疲劳寿命的影响,得到喷丸残余应力强化作用与载荷工况的关系。结果表明:喷丸引入的残余应力可以有效地增强试件的疲劳寿命;过大的循环载荷可能造成喷丸残余应力发生松弛;在最大载荷不变的前提下,应力比越小,试件疲劳寿命越短;应力比越大,残余应力对疲劳寿命强化效果越明显。     

Ti-6Al-4V钛合金的疲劳裂纹扩展规律

针对熔模铸造Ti-6Al-4V钛合金的等幅疲劳裂纹扩展速率和疲劳裂纹扩展门槛值进行了研究。结果表明：该钛合金CT试样的疲劳裂纹扩展门槛值高于CCT试样的疲劳裂纹扩展门槛值,同一类试样的疲劳裂纹扩展门槛值随着应力比的增加呈下降趋势;疲劳裂纹扩展速率随着平均应力的增加以及应力水平的增加而增大;根据疲劳裂纹扩展试验数据拟合了Ti-6Al-4V钛合金Paris方程和Walker方程中的相关材料参数,以为材料的使用寿命评估及损伤容限设计提供参考。     

1240MPa级Ti-38644高锁螺栓的拉伸疲劳增寿机理

用扫描电镜和能谱观测分析Ti-38644高强钛合金高锁螺栓的拉伸疲劳断口,揭示了高锁螺栓的疲劳裂纹萌生和扩展的微观特征和疲劳增寿机理。结果表明,Ti-38644高强钛合金高锁螺栓的疲劳断口包括疲劳裂纹萌生区、扩展区和瞬断区:疲劳裂纹从螺栓头下圆角滚压薄弱部位表面萌生,随后在基体中呈放射性扩展;进入扩展区后裂纹的尺寸由微观扩展至宏观,以疲劳条带扩展机制为主,同时也存在解理断裂。头下圆角处的变形层对Ti-38644高锁螺栓的疲劳寿命有显著的影响,变形层使Ti-38644钛合金高锁螺栓的疲劳寿命明显提高。通过微观组织与疲劳寿命的对比,探讨了Ti-38644钛合金高锁螺栓疲劳强化的作用机理。     

基于堆焊成形钛合金高周疲劳实验数据的R-S-N模型

针对应力比对疲劳寿命影响的问题,以TC18钛合金堆焊成形(利用多层堆焊的方法制备)试样为研究对象,进行了3种应力比(R=0.5、R=0.06、R=-1)的疲劳实验,得出相应的疲劳极限,应用"应力幅值寿命模型"和"三参数寿命模型"得到6条S-N曲线。根据裂纹扩展速率与疲劳寿命的积分关系,以两种疲劳寿命数学模型为基础,系统地研究了应力比(R)与疲劳寿命曲线(S-N)的关系,提出了考虑应力比的疲劳寿命(R-S-N)数学模型。根据本文提出的修正公式,建立了适用TC18钛合金堆焊成形材料的两种R-S-N数学模型,结果表明:用应力幅值寿命模型可对中等疲劳寿命区进行准确的预测,而三参数寿命模型更适合中长寿命区域的预测。提出的两种R-S-N数学模型与实验值吻合良好,并可在工程上预测任意应力比下的疲劳寿命曲线。     

Small fatigue crack growth behavior of titanium alloy TC4 at different stress ratios

In this paper, the small fatigue crack behavior of titanium alloy TC4 at different stress ratios was investigated. Single‐edge‐notch tension specimens were fatigued axially under a nominal maximum stress of 370 MPa at room temperature. Results indicate that fatigue cracks in TC4 initiate from the interface between α and β phases or within α phase. More than 90% of the total fatigue life is consumed in the small crack initiation and growth stages. The crack growth process of TC4 can be divided into three typical stages, ie, microstructurally small crack stage, physically small crack stage, and long crack stage. Although the stress ratio has a significant effect on the total fatigue life and crack initiation life at constant σ_max, its effect on crack growth rate is indistinguishable at R = ?0.1, 0.1, and 0.3 when crack growth rate is plotted as a function of ?K.     

Fretting fatigue behaviour of shot-peened Ti-6Al-4V at room and elevated temperatures

Fretting fatigue behaviour of shot‐peened titanium alloy, Ti‐6Al‐4V was investigated at room and elevated temperatures. Constant amplitude fretting fatigue tests were conducted over a wide range of maximum stresses, σ_max= 333 to 666 MPa with a stress ratio of R= 0.1 . Two infrared heaters, placed at the front and back of specimen, were used to heat and maintain temperature of the gage section of specimen at 260 °C. Residual stress measurements by X‐ray diffraction method before and after fretting test showed that residual compressive stress was relaxed during fretting fatigue. Elevated temperature induced more residual stress relaxation, which, in turn, decreased fretting fatigue life significantly at 260 °C. Finite element analysis (FEA) showed that the longitudinal tensile stress, σ_xx varied with the depth inside the specimen from contact surface during fretting fatigue and the largest σ_xx could exist away from the contact surface in a certain situation. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from FEA results to characterize fretting fatigue crack initiation behaviour. It showed that stress relaxation during test affected fretting fatigue life and location of crack initiation significantly. MSSR parameter also predicted crack initiation location, which matched with experimental observations and the number of cycles for crack initiation, which showed the appropriate trend with the experimental observations at both temperatures.     

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel in 3.5‐wt% NaCl solution

Corrosion fatigue behaviour and microstructural characterisation of G20Mn5QT cast steel are investigated in simulated seawater. Fractography is performed by using scanning electron microscopy (SEM). The macroscale fracture surface and microstructure of the failed specimen are acquired including the crack initiation, crack propagation, and pitting evolution. The maximum cyclic stress (S) versus number of cycles to failure (N) curves is derived by three‐parameter fatigue curve method. Fatigue life is predominantly controlled by the corrosion pitting‐induced crack initiation when tested in simulated seawater at lower stress levels. As the maximum cyclic stress is less than 185 MPa, the chloride ion erosion is the main influence factor, which affects the fatigue failure of the G20Mn5QT cast steel in simulated seawater.     

高速列车制动盘SiCp／A356颗粒增强铝基复合材料的热疲劳性能研究

通过SiCp／A356颗粒增强复合材料切口试样在20℃-300℃循环下的热疲劳试验,获得热疲劳裂纹形成寿命与试样切口半径及厚度等几何尺寸的关系。采用热弹塑性有限元法模拟热疲劳试验中试样切口根部的应力．应变响应,进而揭示出残余应力形成机制。结合热疲劳试验的裂纹形成寿命与有限元模拟的应力．应变响应,建立起考虑平均应力影响的...     

Mode I cracks subjected to large T-stresses

There are several criteria for predicting brittle fracture in mode I and mixed mode loading. In this paper, the modified maximum tangential stress criterion originally proposed for mixed mode loading, is employed to study theoretically brittle fracture for mode I cracks. In particular, the effect of the non-singular term of stress, often known as the T-stress, on the angle of initiation of fracture and the onset of crack growth is explored. The T-stress component of the tangential stress vanishes along the crack line. Therefore, it is often postulated for linear elastic materials that the effect of T-stress on mode I brittle fracture can be ignored. However, it is shown here that the maximum tangential stress is no longer along the line of initial crack when the T-stress exceeds a critical value. Thus, a deviation in the angle of initiation of fracture can be expected for specimens having a large T-stress. It is shown that the deviation angle increases for larger values of T-stress. Theoretical results show that the apparent fracture toughness decreases significantly when a deviation in angle occurs. Earlier experimental results are used to corroborate the findings. The effect of large T-stresses is also explored for a crack specimen undergoing moderate scale yielding. The elastic-plastic investigation is conducted using finite element analysis. The finite element results reveal a similar deviation in the angle of maximum tangential stress for small to moderate scale yielding.     

A shear stress-based parameter for fretting fatigue crack initiation

The purpose of this study was to investigate the fretting fatigue crack initiation behaviour of titanium alloy, Ti–6Al–4V. Fretting contact conditions were varied by using different geometries of the fretting pad. Applied forces were also varied to obtain fretting fatigue crack initiation lives in both the low- and high-cycle fatigue regimes. Fretting fatigue specimens were examined to determine the crack location and the crack angle orientation along the contact surface. Salient features of fretting fatigue experiments were modelled and analysed with finite element analysis. Computed results of the finite element analyses were used to formulate a shear stress-based parameter to predict the fretting fatigue crack initiation life, location and orientation. Comparison of the analytical and experimental results showed that fretting fatigue crack initiation was governed by the maximum shear stress, and therefore a parameter involving the maximum shear stress range on the critical plane with the correction factor for the local mean stress or stress ratio effect was found to be effective in characterizing the fretting fatigue crack initiation behaviour in titanium alloy, Ti–6Al–4V.     

CORROSION FATIGUE FRACTURE BEHAVIOUR OF A SiC WHISKERALUMINIUM MATRIX COMPOSITE UNDER COMBINED TENSIONTORSION LOADING

We describe an investigation into the fatigue fracture behaviour under combined tension–torsion loading of a SiC whisker-reinforced A6061 aluminium alloy fabricated by a squeeze casting process. Special attention was paid to the environmental effects on fatigue fracture behaviour. Tests were conducted on both the composite and its unreinforced matrix material, A6061-T6, under load-controlled conditions with a constant value of the combined stress ratio, α = τ_max /σ_max in laboratory air or in a 3.5% NaCl solution at the free corrosion potential. The corrosion fatigue strength of both the matrix and composite was less in the solution than in air. The dominating mechanical factor that determined the fatigue strength in air was either the maximum principal stress or the von Mises-type equivalent stress, depending on the combined stress ratio. However, in the 3.5% NaCl solution, the corrosion fatigue strength of both materials was determined by the maximum principal stress, irrespective of the combined stress ratio. In the case of the matrix material, crack initiation occurred by a brittle facet normal to the principal stress due to hydrogen embrittlement. However, in the composite material, the crack was initiated not at the brittle facet, but at a corrosion pit formed on the specimen surface. At the bottom of the pit, a crack normal to the principal stress was nucleated and propagated, resulting in final failure. Pitting corrosion was nucleated at an early stage of fatigue life, i.e. about 1% of total fatigue life. However, crack initiation at the bottom of a pit was close to the terminal stage, i.e. about 70% or more of total fatigue life. The dominating factor which determined crack initiation at a pit was the Mode I stress intensity factor obtained by assuming the pit to be a sharp crack. Initiation and propagation due to pitting corrosion and crack growth were closely examined, and the fatigue fracture mechanisms and influence of the 3.5% NaCl solution on fatigue strength of the composite and matrix under combined tension–torsion loading were examined in detail.     

Influence of different load models on gear crack path shapes and fatigue lives

A computational model for determination of the service life of gears with regard to bending fatigue at gear tooth root is presented. In conventional fatigue models of the gear tooth root, it is usual to approximate actual gear load with a pulsating force acting at the highest point of the single tooth contact. However, in actual gear operation, the magnitude as well as the position of the force changes as the gear rotates. A study to determine the effect of moving gear tooth load on the gear service life is performed. The fatigue process leading to tooth breakage is divided into crack‐initiation and crack‐propagation period. The critical plane damage model has been used to determine the number of stress cycles required for the fatigue crack initiation. The finite‐element method and linear elastic fracture mechanics theories are then used for the further simulation of the fatigue crack growth.     

Experimental investigation on low cycle fatigue and fracture behaviour of a notched Ni‐based superalloy at elevated temperature

In order to investigate the effects of stress concentration on low cycle fatigue properties and fracture behaviour of a nickel‐based powder metallurgy superalloy, FGH97, at elevated temperature, the low cycle fatigue tests have been conducted with semi‐circular and semi‐elliptical single‐edge notched plate specimens at 550 and 700 °C. The results show that the fatigue life of the notched specimen decreases with the increase of stress concentration factor and the fatigue crack initiation life evidently decreases because of the defect located in the stress concentration zone. Moreover, the plastic deformation induced by notch stress concentration affects the initial crack occurrence zone. The angle α of the crack occurrence zone is within ±10° of notch bisector for semi‐circular notched specimens and ±20° for semi‐elliptical notched specimens. The crack propagation rate decreases to a minimum at a certain length, D, and then increases with the growth of the crack. The crack propagation rate of the semi‐elliptical notched specimen decelerates at a faster rate than that of the semi‐circular notched specimen because of the increase of the notch plasticity gradient. The crack length, D, is affected by both the applied load and the notch plasticity gradient. In addition, the fracture mechanism is shown to transition from transgranular to intergranular as temperature increases from 550 to 700 °C, which would accelerate crack propagation and reduce the fatigue life.     

Analysis of the crack growth propagation process under mixed-mode loading

In the present paper, a computational model for crack growth analysis under Mode I/II conditions is formulated. The focus is on two issues – crack path simulation and fatigue life estimation. The finite element method is used together with the maximum principal stress criterion and the crack growth rate equation based on the equivalent stress intensity factor. To determine the mixed-mode stress intensity factors, quarter-point (Q-P) singular finite elements are employed. For verification purposes, a plate with crack emanating from the edge of a hole is examined. The crack path of the plate made of 2024 T3 Al Alloy is investigated experimentally and simulated by using the finite element method with the maximum tangential stress criterion. Then, the validation of the procedure is illustrated by applying the numerical evaluation of the curvilinear crack propagation in the polymethyl methacrylate (PMMA) beam and the Arcan specimen made of Al Alloy for which experimental results are available in the literature. In order to estimate fatigue life up to failure of the plate with crack emanating from the edge of a hole, the polynomial expression is evaluated for the equivalent stress intensity factor using values of stress intensity factors obtained from the finite element analysis. Additionally, the fatigue life up to failure of the Arcan specimen is analyzed for different loading angles and compared with experimental data. Excellent correlations between the computed and experimental results are obtained.