Prediction of fatigue crack propagation life in notched members under variable amplitude loading

One of the interesting phenomenon in the study of fatigue crack propagation under variable amplitude load cycling is the crack growth retardation that normally occurs due to the application of a periodic overload. Fatigue crack growth rate under simple variable amplitude loading sequence incorporating period overloads is studied using single edge notched specimens of AISI304 stainless steel. Load interaction effects due to single and multiple overload have been addressed. Substantial retardation of fatigue crack growth rate is observed due to the introduction of periodic tensile overloads. Estimates of fatigue life have been obtained employing Wheeler model (using Paris and modified Paris equations) and Elber’s model. Analytical predictions are compared with experimental results. Results of these analytical fatigue life predictions show good agreement with the experimental fatigue life data. Fatigue crack propagation rates also have been evaluated from the fractographic study of fatigue striations seen on the fracture surface. Good agreement was found between the experimentally observed crack growth rates and the fatigue crack growth rates determined by the fractographic studies.     

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE CRACK GROWTH

The effect of proportional and non-proportional overloading on mode l fatigue crack growthhave been studied,and the influences of crack tip plastic zone,crack tip blunting as well ascrack closure were discussed.Proportional(model I)overloading may cause more seriouscrack growth retardation than non-proportional(mixed mode)overloading.Therefore,forestimating the fatigue life of engineering structures to simplify a real overload which may of-ten be non-proportional as a proportional one is not always safe.     

Fatigue properties of welded joints using steel with high resistance to fatigue crack growth

It has been generally recognized that the fatigue life of welded joints is little influenced by the strength of steels owing to the high-stress concentration and the tensile residual stress near the weld toe. In this paper, improvement of the fatigue life of welded joints using steel with high resistance to fatigue crack growth (ferrite/martensite (F/M) steel) is investigated. F/M steel has a microstructure with an elongated and banded martensite phase distributed in a ferrite matrix and a fatigue crack growth rate of about one-half to one-tenth in the thickness direction, compared with conventional steel. As a result, the fatigue life of an out-of-surface gusset-welded joint increases with the decrease of the fatigue crack growth rate. The fatigue life of welded joints using F/M steel with the highest resistance to fatigue crack growth increases to about twice that of joints using conventional steel. Whereas the fatigue crack growth rate decreases significantly, the fatigue life of welded joints increases only slightly. This can be attributed to the stress ratio independent of the fatigue crack growth rate. In other words, the fatigue crack growth rate of F/M steel increases with the increase of the stress ratio, approaching that of conventional steel. In the case of welded joints, even if a fatigue test is carried out at a low-stress ratio, the region near the weld toe is under a high-stress ratio due to tensile residual stress. Therefore, improvement of the fatigue life of welded joints becomes comparatively small so that the effect of fatigue crack retardation of F/M steel decreases.     

Role of structural parameters of ultra-fine grained Cu for its fatigue and crack growth behaviour

High cycle fatigue (HCF) life time curves and fatigue crack growth rates of bulk ultra-fine grained (UFG) copper deformed by high pressure torsion (HPT) were determined. Cu of two different purities as well as a bimodally structured HPT Cu were investigated. The results show increased HCF properties of the UFG materials compared to coarse grained (CG) Cu. Especially HPT Cu with lower purity shows enhanced fatigue resistance due to higher microstructural stability. Contrary, crack growth rates in HPT Cu are increased. In case of the high purity Cu, cyclic deformation induced coarsening of the UFG microstructure nearby the crack is found at threshold crack growth rates leading to a retardation of the fatigue crack propagation. Within these coarse grains typical fatigue surface slip marks as observed in CG Cu are found.     

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。     

300M钢腐蚀疲劳裂纹萌生的超载特性

本文对３００Ｍ钢在空气与３．５％ＮａＣｌ溶液中疲劳裂纹萌生的周期超载和单次超载特性进行了试验研究与理论分析。试验结果表明，３．５％ＮａＣｌ溶液中，单次超载所致腐蚀疲劳裂纹迟滞萌生的迟滞强度远小于周期超载的对应值。３．５％ＮａＣｌ溶液与空气中，超载迟滞强度均随超载比增大或等效应力幅水平的提高而增大，在腐蚀疲劳裂纹萌生寿命估算中，只考虑单次超载的迟滞规律，会得出大大保守的寿命估计，只有将单次超载和周期     

Berechnung des Rißfortschrittsverhaltens geschweißter Proben aus Offshore-Stählen in Meerwasser zur Abschätzung der Lebensdauer von Offshore-Komponenten

Calculation of crack growth in welded specimens under seawater conditions in order to predict fatigue life of offshore components Welded components in the offshore field generally are designed that no cracks should occur. Under cyclic loading cracks must be taken in account, yet. In the research work described a suitable calculation method had to be proved to predict the crack growth behaviour of offshore components in seawater and under service-like conditions. Using Paris' law the crack growth behaviour of specimens which are similar to the component (welded V-shape specimens) may be predicted quite well in air, free corrosion, under cathodic protection (?860 mV Ag/AgCl) and under a variable load sequence. The use of Wheeler's model including crack retardation yielded in a significant higher crack propagation life and therefore unconservative predictions of fatigue life.     

十字接头恒幅和变载荷历程疲劳试验结果及预测

对十字接头分别进行恒辐轴向载荷和变载荷历程疲劳试验,接头疲劳裂纹通常开始发生在焊缝根部,但如果接头变形引起的弯曲应力较大,疲劳裂纹也可能发生在焊缝趾部。本研究试图找出影响焊接接头疲劳寿命各参数之间的相互关系,并利用作者引伸的焊接接头疲劳裂纹起始－扩展模型较精确地预测复杂焊件的疲劳寿命。该模型把疲劳裂纹起始寿命看成是疲劳裂纹萌生,早期生成并聚合成主疲劳裂纹的循环次数,利用应变控制疲劳数据和Ｐａｌｍｇ     

Prediction of Cold Dwell-Fatigue Crack Growth of Titanium Alloys

The dwell effect of the material can reduce the fatigue lives of titanium alloys at room temperature. A unified fatigue life prediction method developed by the authors' group is modified in this paper to predict dwell-fatigue crack growth taking into account the effects of dwell time and maximum stress. The modified model can be successfully used to predict the crack growth rate and calculate the fatigue life of different titanium alloys under pure fatigue and dwell-fatigue conditions. It is validated by comparing prediction results with the experimental data of several titanium alloys with different microstructures, dwell time, hydrogen contents, stress ratios and stress levels.     

Three-dimensional fatigue crack growth behavior in an aluminum alloy investigated with in situ high-resolution synchrotron X-ray microtomography

The fatigue crack propagation process in an Al–Mg–Si alloy was investigated using in situ high-resolution synchrotron radiation X-ray microtomography. Tomography datasets were obtained at periodic intervals throughout the 120,000 fatigue cycles. Three-dimensional rendering of the through-thickness crack shape indicates that in a number of regions the adjacent sides of two branched cracks tend to overlap with fatigue cycling and form a crack overlapping region. Measured crack growth rates in each tomographic slice show that crack growth retardation generally occurs in these crack overlapping regions. The through-thickness variation in crack tip opening displacement was also measured and was used to account for the observed crack propagation behavior. Crack morphologies were observed at different load levels in a fatigue cycle. The crack closure level varied for two selected regions comprising different overlapping cracks. The correlation of the crack growth rate with both crack opening and closure levels was discussed and interpreted.     

A probabilistic methodology for fatigue life prediction

This paper describes efforts in the probabilistic modelling of fatigue. The methodology proposed deals with crack nucleation from surface defects within a fracture mechanics framework. The methodology provides a quantitative understanding of the effects of bulk defects and their size distribution on the variation of fatigue life. It also considers the contributions of the variations of fatigue crack nucleation life and of crack growth resistance to the variation of fatigue life. A PM Ni-based superalloy Udimet 720 was used to evaluate the probabilistic methodology by an analytical solution and a Monte Carlo simulation. The model predictions were found to be in good agreement with the experimental results. However, it is found that Monte Carlo simulation provides a closer agreement with the experimental life distribution than the analytical solution. This is because the Monte Carlo simulation removes some untrue assumptions which have to be assumed to obtain the analytical solution.     

渗铝复合激光冲击对321不锈钢腐蚀疲劳性能的影响EI北大核心CSCD

针对聚焦型太阳能热发电换热管材料在熔融铝硅环境中易发生腐蚀疲劳失效的问题,采用粉末包埋渗铝和激光冲击对AISI 321不锈钢进行表面改性,研究其熔融铝硅环境下腐蚀疲劳性能。结果表明,渗铝钢表面形成以FeAl金属间化合物为主的渗层,尽管能够有效地隔离基体和腐蚀介质,但是作为疲劳裂纹的形核源,会导致腐蚀疲劳寿命降低40%;经不同功率密度的激光冲击强化处理后,渗铝钢表面硬度显著提高,延缓了疲劳裂纹萌生,提升了耐蚀性,降低了腐蚀损伤的影响,疲劳损伤占据主导地位,使得渗铝钢腐蚀疲劳寿命提高100%~200%。     

Laser shock processing induced residual compression: Impact on predicted crack growth threshold performance

Design credit is not currently taken for laser shock processing (LSP) induced compressive residual stresses in damage tolerant design. The inclusion of these and other compressive stresses in design practice has the potential to dramatically increase predicted fatigue crack growth threshold performance and damage tolerant design life. In the current effort, Ti-6Al-4V coupons will be subjected to shot peening, glass bead peening, and high intensity laser shock processing. The in-depth residual stresses due to processing will be analyzed and then input into a linear elastic fracture mechanics analysis code to predict fatigue crack growth threshold performance. This analysis establishes both the utility and feasibility of incorporating LSP-induced compressive residual stresses into damage tolerant design practice.     

Fatigue properties of ferrite/bainite dual-phase X80 pipeline steel welded joints

Fatigue properties are important parameters for the safety design and security evaluation of pipelines. In this work, fatigue life and fatigue crack propagation of full-thickness X80 pipeline steel joints compared with the base metal (BM) was investigated. Full-thickness BM specimens showed superior fatigue life compared with that of welded joints. The fatigue crack initiation of full-thickness X80 welded joint specimens occurred at the outside weld toe and then grew inward until a fracture was formed. During fatigue crack growth in the heat-affected zone (HAZ), crack growth rate linearly increased with increased ΔK in each HAZ subregion. However, the change rate of fatigue crack growth rate (da/dN) differed among HAZ subregions. This difference was related to the variation in crack path and fracture mode because of the possible microstructural sensitivity of fatigue crack propagation behaviour.     

航空航天铝合金腐蚀疲劳研究进展

铝合金因具有高的比强度、比模量,好的加工性能及焊接性能,在航空航天领域应用广泛.而腐蚀疲劳是造成航空航天材料失效的重要原因之一,因其危害性高、破坏性强且难以提前预测等特点,受到了广泛关注.铝合金腐蚀疲劳问题一直是飞机日历寿命研究中的重点问题,随着可重复使用航天器理念的提出,多次空天往返和地面修复过程也使腐蚀疲劳问题在可重用航天器上不可忽视.综述了近年来航空航天铝合金腐蚀疲劳的研究现状,从航空铝合金腐蚀疲劳机理的角度,归纳了腐蚀疲劳裂纹萌生和扩展机制.从腐蚀疲劳环境模拟和腐蚀环境等效两方面,介绍了目前主要的实验室腐蚀疲劳试验技术.分别从材料因素、环境因素和力学因素,分析对腐蚀疲劳裂纹扩展及寿命的影响.重点关注了腐蚀疲劳交替形式下疲劳寿命的特点.提出了在多因素共同影响下的腐蚀疲劳裂纹扩展、损伤演化和寿命预测,以及加速腐蚀环境的当量等效.试验与模拟的有机结合,是今后铝合金腐蚀疲劳的重要发展方向.     

激光冲击强化对7050铝合金小孔结构残余应力和疲劳性能的影响

研究了激光冲击强化对7050 T7451铝合金小孔结构显微硬度、残余应力和疲劳性能的影响。结果表明:当激光能量为30 J、光斑直径ø4 mm,冲击2次时,7050 T7451铝合金显微硬度显著提高,表层硬度相对于母材提高约12%且硬化层深度可达1 mm;残余压应力幅值超过300 MPa,影响深度可达约1 mm,明显大于喷丸强化残余应力影响层深度。激光冲击诱导的残余压应力可提高疲劳裂纹的萌生抗力,其较深的残余压应力层则有利于延长裂纹的扩展寿命。激光冲击强化后小孔结构疲劳寿命相对于母材提高了4.7~17.6倍,且其疲劳寿命增益及稳定性明显优于喷丸强化。     

Damage tolerance of wrought alloy 718 Ni- Fe-base superalloy

The influence of a modified heat treatment (MHT) and the standard heat treatment (SHT) on the damage tolerance of alloy 718 turbine disk material has been studied over a range of temperatures— from room temperature to 650 °. The influence of these heat treatments on creep, low-cycle fatigue (LCF), notch sensitivity, cyclic stability, and fatigue crack growth rate (FCGR) properties has been studied. The microstructure developed through the MHT sequence is shown to be damage tolerant over the temperature range studied. Shot peening leads to a marked improvement in the LCF crack initiation life of the MHT material relative to the SHT material at 650 °. Serrated grain boundaries formed through controlled precipitation of grain-boundary 5 phase are beneficial to elevated- temperature FCGRs. The S-phase precipitates formed at an angle to the grain boundaries do not make the material notch sensitive.     

A NEW FATIGUE-CREEP LIFE PREDICTION METHODOLOGY

在“等效裂纹”概念及裂纹扩展理论基础上,从微裂纹扩展导致材料破坏的角度出发, 得到了一种新的疲劳蠕变寿命预测模型.该模型在处理微裂纹扩展时考虑了时间无关疲劳以及时间相关静蠕变、循环蠕变的影响.时间无关疲劳裂纹扩展采用Tomkins模型,时间相关蠕变裂纹扩展采用C*控制参量. 用该寿命预测模型对1.25Cr0.5Mo钢540 ℃应力 控制下疲劳蠕变寿命进行了预测, 预测结果与实验结果符合较好.     

Biaxial Fatigue Life Predicted by Crack Growth Analysis in Various Material Microstructures Modeled by Voronoi-Polygons

Fatigue life is affected by the crack growth behavior that depends on the material microstructure as well as the stress biaxiality. By considering such effects on crack growth, a numerical procedure for predicting failure life in biaxial fatigue of materials with different microstructures was proposed in this study. Such a procedure will be helpful in the material design for higher performance of fatigue resistance in a material. The microstructure of a material was first modeled using Voronoi-polygons, and the crack initiation was analyzed as the result of slip-band formation in individual grains in the modeled microstructure. In the analysis, stress states in individual grains were randomized so that the average stress state should be equivalent to the bulk stress state. An algorithm for the crack growth analysis was established as a competition between the crack-coalescence growth and the propagation as a single crack. The failure life was statistically predicted based on the crack growth behavior simulated for 40 distinct microstructural configurations, which were generated by randomizing shapes of Voronoi-polygons for the same material. By applying the proposed procedure, simulations were conducted for experimental conditions of fatigue tests, which had been conducted under axial, torsional, and combined loading modes using circumferentially notched specimens of pure copper, medium carbon steel, and (α + β) and β titanium alloys. In this case, 40 different failure-lives were obtained for each combination of material and loading mode. It was revealed that the failure lives observed in experiments were almost covered by the life-ranges between the minimum and the maximum lives given in simulation. Statistical characteristics in simulated life-distributions were investigated using Weibull distribution function and its related statistical parameters.     

Effect of corrosive environment on the fatigue crack initiation and propagation behavior of Al 5454-H32

Fatigue behavior of aluminum alloy 5454- H32 was studied under laboratory air and 3 % NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3 % NaCl environment during fatigue loading drastically reduced alloy fatigue life. The deleterious effect was pronounced in both types of specimens in the long- life regions, where the fatigue lives were lowered by as much as a factor of 10. However, the sharply notched specimens showed only a modest reduction in fatigue life in corrosive environment. The severe influence of the corrosive environment in the long- life (low- stress) regime cannot be explained merely by the early initiation of the fatigue crack from surface pits; the environmental contribution in the early crack growth regime must also be considered an important factor. Fracture surface studies revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in Al 5454- H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be operative in the small crack growth region. This could have enhanced the early crack growth rate and thus contributed to the lower fatigue life in the long- cycle region.