喷射沉积SiC_P／Al-7Si复合材料的疲劳裂纹扩展

采用紧凑拉伸试样进行恒载和降K控制的拉--拉疲劳实验, 研究了喷射沉积SiC_p/Al-7Si复合材料及其基体的疲劳裂纹扩展行为. 通过金相显微镜和扫描电镜观察了复合材料及其基体的组织和疲劳裂纹扩展形貌, 研究了SiC颗粒对复合材料疲劳裂纹扩展机制的影响. 结果表明: 复合材料在任何相同的ΔK水平下其抗疲劳裂纹扩展能力优于基体材料, 并表现出较高的疲劳门槛值. 其原因是复合材料中裂纹裂尖遇到增强颗粒时, 裂纹发生偏转, 特别是SiC颗粒自身微裂纹萌生有效降低了裂纹尖端的应力强度因子, 复合材料的裂纹闭合效应也随之增大. 去除裂纹闭合效应的影响, 当有效应力因子ΔK_eff作为裂纹扩展的驱动力时, 复合材料的裂纹扩展速率却高于基体.     

Fatigue crack growth and wear behaviors in rheocast Al−Si−Mg alloys

Rheocast aluminum alloys, which consist of globular α-Al cells, refined grains and eutectic Si particle, were used to investigate fatigue crack growth and wear characteristics. The Si particles were systematically varied from coarse and acicular shapes to small and globular ones. At low ΔK fatigue crack growth rates decreased in samples consisting of acicular Si particles of large grain size, which induced a large amount of crack closure. Large and acicular Si particles were easily cracked and separated the particle/matrix interface, which promoted to fracture at smaller ΔK. On the other hand, small Si particles made fatigue crack grow, even at a high ΔK region, and increased the fracture toughness of the alloy. However, in the wear test, small eutectic Si particles were pulled out by friction force during sliding wear and the wear loss amount increased with increase in sliding distance.     

Fatigue crack growth in 2024 aluminum alloy with inhomogeneous solidification microstructure

This study reports the investigation of the fatigue crack growth behavior in 2024 aluminum alloy billets manufactured using the direct chill casting (DCC) and electromagnetic casting (EMC). It was expected that the billets produced from these different casting procedures would vary in terms of their microstructure. Therefore, they should exhibit different fatigue crack growth behavior. The results showed that, the structure of the middle center portion of both billets is not homogeneous and relatively coarse, while the structure of the center and edge portions is homogeneous compared with the structure of the middle center portion. As fatigue crack growth extended through many grains having different morphologies, it became unstable through the overall crack path, and the Paris zone almost disappeared. The effects of 5% rolling and commercially rolled DCC billets on the fatigue crack growth rate were also investigated. Striations were enhanced around Al₂Cu particles refined by commercial rolling, and fatigue crack growth was stable until large crack length.     

Thermal shock fatigue behavior of TiC/Al2O3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt. % TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference (ΔT) and number of thermal cycles (N) on fatigue crack growth (Δα). The mechanical properties and thermal fatigue resistance of TiC/Al2O3 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a "true" cycling effect (thermal fatigue). Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.     

LY12爆炸复合板疲劳性能研究

考察了ＬＹ１２爆炸复合板不同取向的拉伸强度、分层韧必及疲劳裂纹扩展行为。观测了疲劳长裂纹的扩展路径形态,并利用断裂力学理论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商界面性能对其疲劳性能具有重要影响,在ＬＹ１２爆炸复合板中,垂直板向Ｌ－Ｓ取向和Ｌ－Ｔ取向的疲劳裂纹在扩展过程中都发生了明显的止裂。     

不锈钢在循环扭转载荷下近门槛值的疲劳裂纹扩展行为

研究了带环状预裂纹不锈钢圆棒试样在循环扭转载荷下、门槛值附近的疲劳裂纹扩展行为,用应力强度因子表征了裂纹扩展开始的门槛值．随着裂纹的扩展,裂纹扩展速率由于裂纹面的滑移接触而减小．通过外插裂纹扩展速率与裂纹长度之间的关系,可近似得到裂纹长度为零时无裂纹面滑移接触影响的裂纹扩展速率．施加的应力强度因子范围可分解为推动裂纹扩展的有效值和由于裂纹面的滑移接触而屏蔽掉的两部分．预测了疲劳裂纹的萌生和断裂极限,预测值和实验值相当一致．     

含有第二相的高强铝合金疲劳模型

基于疲劳裂纹尖端的应力和应变以及高强铝合金中不同尺度第二相性态对其延性的影响，建立了高强铝合金中粗大第二相、中间尺度第二相以及细小时效强化相性态与其疲劳裂纹扩展速率之间的多元非线性关系模型。结果表明：对于2024铝合金的疲劳扩展速率，该模型的预测趋势与他人的实验研究结果吻合良好。同时借助于对该模型的理论分析，提出了在确保高强铝合金强度不降低的前提下降低其疲劳裂纹扩展速率的优化方案。     

A537钢及工业纯铁在3.5%NaCl水溶液中腐蚀疲劳裂纹扩展机理的识别

本文根据外加电位对腐蚀疲劳裂纹扩展速率的影响,断口微观形貌特征及声发射活动性分析三种判别方法,研究了A537钢及工业纯铁在3.5%NaCl中性水溶液中腐蚀疲劳裂纹扩展机理。结果表明,扩展机理取决于外加电位值。对A537钢,外加电位在-800mV(SCE)以上时,裂纹扩展以阳极溶解控制为主,以下时以氢脆控制为主。对工业纯铁,对应的转换电位为-1000mV(SCE)。     

Detection of fatigue crack propagation with acoustic emission

Since its inception, acoustic emission has held great promise as a tool for detection and evaluation of damage in structures. Of particular concern is the occureence of fatigue manifested by the processes of crack initiation followed by crack growth to failure. Early research related acoustic emission parameters to physical variables, including load, stress, pressure etc. causing damage. In this paper, relationships relating acoustic emission to fatigue crack growth are developed. Acoustic emission equations, similar to Paris' law, are derived which allow determination of the stress intensity factor and the crack growth rate. These principles are embodied in a second-generation system which automatically and continuously determines these parameters. The approach is applied in the laboratory as well as to structures including bridges, air frames and transmission towers for electrical power.     

ZL101铝合金低周疲劳行为研究

研究了ZL101-T6铝合金的拉压低周疲劳行为,并重点分析了疲劳裂纹在材料中萌生与扩展的过程。通过扫描电镜和金相显微镜的观察分析表明:共晶硅相的断裂是材料低周疲劳裂纹萌生的主要方式,硅颗粒断裂的数量随疲劳循环周次的增加而增加,应变幅值的增加加快了硅颗粒的断裂速率,使疲劳裂纹的萌生速率加快;疲劳裂纹在循环初期主要通过断裂硅颗粒的互相连接进行扩展,随疲劳裂纹的长大,裂纹可穿过铝基体连接形成主裂纹并导致材料破坏,穿晶断裂为最终断裂的主要特征。     

In-situ monitoring of fatigue crack growth using high frequency guided waves

The development of fatigue cracks at fastener holes represents a common maintenance problem for aircraft. High frequency guided ultrasonic waves allow for the monitoring of critical areas without direct access to the defect location. During cyclic loading of tensile, aluminum specimens fatigue crack growth at the side of a fastener hole was monitored. The changes in the energy ratio of the baseline subtracted reflected guided wave signal due to the fatigue damage were monitored from a stand-off distance using standard ultrasonic pulse–echo measurement equipment. Good sensitivity for the detection and monitoring of fatigue crack growth was found.     

Microporosity effects on cyclic plasticity and fatigue of LENS™-processed steel

Special microstructures of the newly developed Laser Engineered Net Shaping (LENS?)-processed steel induce a new variability in fatigue damage formation and evolution mechanisms. The microporosity and mechanism of fatigue damage formation and growth were invested using X-ray computed tomography and scanning electron microscopy. Systematic observations were made of the variations in the fracture surfaces according to three fatigue damage evolution stages: fatigue crack formation (incubation), microstructurally/physically small cracks, and long cracks. The fatigue crack was formed almost exclusively at the relatively large pores located at or near the specimen surface, with rare cases at incompletely melted power particles on the surface. Distributed cracks from large interior pores coalesced with each other in the microstructurally small crack regime to form the major critical crack that eventually fractured the specimen. This coalescence accelerated the fatigue crack growth, which in turn decreased the fatigue life but not significantly. In the long-crack regime, the fracture surface was rougher, but the overall surface tended to be perpendicular to the loading direction, indicating a Mode I type fracture. Cyclic strain-softening, with reduced strain-hardening, was also observed. The multistage fatigue model of McDowell et al. was used to capture the microstructure effects in the three fatigue damage evolution regimes, and the upper and lower bounds for the strain–life are predicted.     

Fatigue crack growth behaviors in Al−Si−Mg sand cast alloys

The fatigue crack growth behavior of Al−Si−Mg sand cast alloys has been investigated with reference to the effects of solidification structure and aging condition. Fatigue crack growth tests have been carried out under constant load amplitude and a stress ratio of R=0.1 using CT specimens. The amount of pores in the matrix was limited by performing HIP treatment. The pores tended to promote deflection of fatigue cracks, which decreased the fatigue crack growth rate at low ΔK regions and increased the number of cycles until final fatigue fracture. Refining and spheroidizing of eutectic Si particles increased the fatigue crack growth rates over a wide range of ΔK up to larger ΔK values. The difference of aging conditions significantly affected the da/dN-ΔK_eff relationship.     

Structural Performance of Inconel 625 Superalloy Brazed Joints

The purpose of this work was to investigate tensile and fatigue behaviors of Inconel 625 superalloy brazed joints after transient liquid-phase bonding process. Brazing was performed in a vacuum furnace using a nickel-based filler metal in a form of paste to join wrought Inconel 625 plates. Mechanical tests were carried out on single-lap joints under various lap distance-to-thickness ratios. The fatigue crack initiation and crack growth modes were examined via metallographic analysis, and the effect of local stress on fatigue life was assessed by finite element simulations. The fatigue results show that fatigue strength and endurance limit increase with overlap distance, leading to a relatively large scatter of results. Fatigue cracks nucleated in the high-stressed region of the weld fillets from brittle eutectic phases or from internal brazing cavities. The present work proposes to rationalize the results by using the local stress at the brazing fillet. When using this local stress, all fatigue-obtained results find themselves on a single S-N curve, providing a design curve for any joint configuration in fatigue solicitation.     

含铒铝合金显微组织对疲劳裂纹扩展的影响

对含铒铝合金板材进行了不同的热处理,分析了各板材的显微组织对疲劳裂纹扩展和力学性能的影响。结果表明:晶粒的长/径比越小,板材的疲劳裂纹扩展速率越慢;且随着疲劳裂纹扩展速率的降低,裂纹宽度增加且裂纹路径曲折;稀土元素Er的加入,在合金中形成Al_3(Er,Zr)第二相粒子,其强烈地钉扎位错,阻碍位错运动,以减少位错在晶界处的聚集产生的应力集中,降低板材的疲劳裂纹扩展速率。     

在役转轴表面疲劳裂纹的爬波检测

采用常规超声检测方法对在役转轴表面疲劳裂纹检测的效果很差。针对此类在役转轴疲劳裂纹产生的机理,结合超声爬波传播的特性,选用超声爬波对在役转轴产生的表面疲劳裂纹进行检测。对某规格的阶梯转轴进行的检测结果表明,超声爬波检测方法简便可行,能够有效地检测出转轴在役运行状态中出现的疲劳裂纹,因此可用于在役转轴表面疲劳裂纹的日常检测。     

Stress-induced martensitic transformation in metastable austenitic stainless steels: Effect on fatigue crack growth rate

This paper addresses the influence of cyclic stress-induced martensitic transformation on fatigue crack growth rates in metastable austenitic stainless steels. At low applied stress and mean stress values in AISI type 301 stainless steel, fatigue crack growth rate is substantially retarded due to a cyclic stress-induced γ-α′ and γ-ε martensitic transformation occurring at the crack-tip plastic zone. It is suggested that the transformation products produce a compressive residual stress at the tip of the fatigue crack, which essentially lowers the effective stress intensity and hence retards the fatigue crack growth rate. At high applied stress or mean stress values, fatigue crack growth rates in AISI type 301 steels become almost equal to those of stable AISI type 302 alloy. As the amount of transformed products increases (with an increase in applied or mean stress), the strain-hardening effect brought about by the transformed martensite phase appears to accelerate fatigue crack growth, offsetting the contribution from the compressive residual stress produced by the positive volume change of γ → α′ or ε transformation.     

Fatigue crack growth resistance of SiC<Subscript>p</Subscript> reinforced Al alloys: Effects of particle size,particle volume fraction,and matrix strength

The main aim of this work was to study the effects of particle size, particle volume fraction, and matrix strength on the long fatigue crack growth resistance of two different grades of Al alloys (Al2124-T1 and Al6061-T1) reinforced with SiC particles. Basically, it was found that an increase in particle volume fraction and particle size increases the fatigue crack growth resistance at near threshold and Paris regimen, with matrix strength having a smaller effect. Near final failure, the stronger and more brittle composites are affected more by static modes of failure as the applied maximum stress intensity factor (K _max) approaches mode I plane strain fracture toughness (K _IC).     

Effect of various heat and surface treatment conditions on the fatigue behavior of SKD11 steel

Effects of various heat treatment processes, including Q/L-T, Q/H-T, Q/L-T + Ti-nitriding, Q/H-T + Ti-nitriding, and Q/CT/H-T on the S-N fatigue behavior of SKD11 steel were investigated. Ti-nitriding is a modified nitriding process that entails a small addition of approximately 2 g to 20 g of electrolyzed fine, metallic titanium in a salt bath. In the present work, depending on the applied stress and surface conditions, different crack initiators were operative, resulting in three types of crack initiation. Multi-site crack initiation around the specimen surface, observed only for the Ti-nitrided specimens at a high stress range, decreased the resistance to S-N fatigue by reducing the crack initiation cycles, Ni. At low and intermediate stress ranges, the cracked carbide particles located near the surface or internally served as crack initiators for all the specimens, regardless of whether Ti-nitriding was applied, resulting in similar resistances to S-N fatigue. Different heat treatment parameters, such as tempering temperature and cryogenic treatment, did not affect the fatigue behavior of SKD11 steel to any considerable degree. The fatigue behavior of SKD11 specimens with different heat and surface treatment conditions is discussed based on micrographic and fractographic observations.     

Accumulation of microstructural damage due to fatigue of high-strength aluminum alloys

The microstructural features of aluminum alloy 7050-T7451 in the vicinity of fatigue cracks and on the crack path were studied to determine which of these features influence fatigue crack propagation. The studies included characterization of the full spectrum of microstructural and fracture surface features— from the largest (e.g., roughness and grain type) to the smallest (e.g., second-phase particles and dislocations). Of all the features studied, only the second-phase particles were shown to have a definite influence by causing crack deflection. The number of particles encountered by the fatigue cracks were significantly higher than the expected average. The fatigue crack path was predominately transgranular, and there was no change in the dislocation and precipitation structures in the crack-affected zone.