The Significance of Crack Initiation Stage in Very High Cycle Fatigue of Steels

Different stages of the Very High Cycle Fatigue (VHCF) crack evolution in tool steels have been explored using a 20 kHz ultrasonic fatigue testing equipment. Extensive experimental data is presented describing VHCF behaviour, strength and crack initiating defects in an AISI H11 tool steel. Striation measurements are used to estimate fatigue crack growth rate, between 10^?8 and 10^?6 m/cycle, and the number of load cycles required for a crack to grow to critical dimensions. The growth of small fatigue cracks within the “fish‐eye” is shown to be distinctively different from the crack propagation behaviour of larger cracks. More importantly, the crack initiation stage is shown to determine the total fatigue life, which emphasizes the inherent difficulty to detect VHCF cracks prior to failure. Several mechanisms for initiation and early crack growth are possible. Some of them are discussed here: crack development by local accumulation of fatigue damage at the inclusion – matrix interface, hydrogen assisted crack growth and crack initiation by decohesion of carbides from the matrix.     

Prediction of Fatigue Lifetime under Multilevel Cyclic Loading Based on a Short Crack Growth Model in a Low Carbon Steel SAE 1017

This contribution deals with the initiation and growth of short fatigue cracks in cyclic loaded notched specimens made of the 0.15 wt% carbon steel SAE1017. On the basis of experimentally determined data, a damage model based on cyclic crack growth has been developed, which accounts for the anomalous behaviour of short fatigue cracks. In this model the initiation and propagation of a critical crack is regarded as damage. This approach allows to calculate fatigue life for constant amplitude tests as well as for multilevel tests and irregular loading. Deviations from Miner's rule, which have been often observed for two‐level tests, are attributed to the varying fraction of crack initiation and propagation phase for different loads. The inaccuracy of Miner's rule deduced from two‐level tests is of secondary importance for service life calculation when compared with the negligence of amplitudes below the fatigue limit. The proposed model yields shorter service life than the elemental version of Miner's rule.     

16Mn钢焊接件疲劳断裂机制研究

对16Mn钢对焊接头的焊缝及热影响区中疲劳裂纹的萌生与扩展机理进行了研究与讨论.结果发现疲裂纹的萌生与扩展与铁素体组织有关,焊接件疲劳断口很复杂,焊缝中的夹杂物对疲劳裂纹的萌生有重要影响.     

航空用超高强度钢中夹杂物导致疲劳裂纹萌生与扩展的微观行为

采用扫描电镜原位观测的方法,跟踪观察了低周疲劳载荷下航空用超高强度钢MA250中夹杂物导致裂纹萌生与扩展的微观行为,得到了MA250钢低周疲劳裂纹萌生与扩展的基本特性,讨论了精确测定MA250钢疲劳裂纹扩展速率的方法.     

Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel

The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.     

Fatigue Initiation Study of TMT Eutectoid Steel

Eutectoid steel and 1060 steel samples were given a variety of thermomechanical treatments (TMT), described in Table I, which varied the pearlite interlamellar spacing, the cementite and inclusion orientation, the degree of cold-work in the ferrite matrix, and (for 1060 steel only) the proeutectoid grain boundary ferrite network. These samples were then evaluated as to resistance to fatigue crack initiation. The TMT designated, G, involves a subcritical anneal and results in a partially recrystallized condition which shows not only excellent resistance to fatigue crack initiation in the near threshold region but is second only to a fine pearlite microstructure, B, in fatigue crack propagation threshold value. It is believed that the excellent properties of the TMT, G, are related to the formation of subgrains in the interlamellar ferrite. On a scale normalized to tensile strength,(ΔK/√ρ)/σ_u, fine oriented pearlite in a soft ferrite matrix (rapid up-quench TMT such as E, F) shows the best resistance to fatigue crack initiation. A proeutectoid ferrite grain boundary network is poor at resisting fatigue crack initiation but good at resisting fatigue crack propagation. It should be emphasized that the combined high resistance of the subcritical TMT (G) toboth fatigue crack initiation and propagation, coupled with its much easier implementation relative to similar microstructures produced by difficult rapid up-quench TMT's (E, F) make it a very promising approach to improving overall fatigue resistance.     

XCQ16和20Mn2车轴用钢疲劳失效的微观机理

利用扫描电镜(SEM)对XCQ16和20Mn2两种车轴用钢疲劳断口和裂纹扩展断口进行分析,研究疲劳失效过程中的裂纹萌生及扩展机理.结果表明:裂纹均是从试样表面萌生,非金属夹杂和位错是主要的萌生机制;XCQ16钢在疲劳断口中部区的裂纹闭合效应大于20Mn2钢,裂纹扩展较慢;两者在裂纹扩展区的扩展机理不同,XCQ16钢属于塑性断裂,20Mn2钢属于解理断裂;在非金属夹杂上,XCQ16钢多含氧化物和硫化物的混合夹杂,20Mn2钢主要是氧化物夹杂,混合夹杂对材料的疲劳性能影响较小.     

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.     

氢致钢内部疲劳裂纹萌生和扩展的有限元分析

对氢致钢内部疲劳裂纹的萌生和扩展进行了数值模拟.首先用有限元法分析了氢在疲劳载荷作用下向钢中缺陷处扩散富集的过程,然后计算得到氢含量分布结果.根据夹杂理论将氢富集区视为在缺陷附近分布的弹性夹杂,用有限元法计算得到的氢含量场求出夹杂处的应力强度因子,进而建立疲劳裂纹萌生和扩展的判据.比较了在不同加载条件下氢致疲劳裂纹萌生和扩展的规律.用梯形法修正了Sofronis和McMeeking的瞬态扩散有限元公式,发现用梯形法可以缓解加载初期较高的浓度梯度和应力梯度引起的计算结果震荡的情况,这对于计算开裂判据是十分重要的.最后讨论了提高模拟精度和改进模型的方法.     

Effects of sodium chloride and shot peening on corrosion fatigue of AISI 6150 steel

Reversed-bending fatigue tests of quenched and tempered AISI 6150 steel were conducted in dry air and in aqueous 3 pct NaCl. The 3 pct NaCl environment drastically reduced fatigue life but two different shot peening treatments were found to improve the corrosion fatigue life over that of unpeened samples. Multiple fatigue crack initiation occurred at very distinct locations in both the unpeened and the peened specimens fatigued in 3 pct NaCl. Fatigue crack propagation from each initiation site occurred first on flat facets normal to the stress axis and then by a more ductile mechanism after the initiation facets had linked. The average size of the corrosion fatigue initiation facets in the peened specimens was much smaller than that of the unpeened specimens; however, the number of initiation sites was greater in the peened specimens. It is believed that the beneficial effect of the shot peening results from significantly reduced early fatigue crack propagation rates in the compressive residual stress layer at the surface. Formerly Research Assistant, Materials Department, University of Wisconsin-Milwaukee     

稀土对钢轨钢接触疲劳的影响

利用接触疲劳磨损试验机械钢轨／车轮接触疲劳作用,了钢轨钢接触疲劳表面区剖面的形貌,显微结构和硬度。结果表明,稀土可延缓钢表面疲劳裂纹的萌生和扩展,推迟表面剥离,减小裂纹贯穿角和裂纹稳定贯穿,缩小塑性化形范围和改善表面加工硬化效果。     

基于有限元方法的疲劳裂纹闭合行为

裂纹闭合行为将很大程度改变疲劳裂纹扩展行为。针对316L不锈钢,结合常幅加载和单个拉伸过载试验和动态数值模拟方法,对疲劳裂纹扩展行为中的裂纹闭合现象开展了一系列研究工作。详细对比了不同扩展阶段的裂纹闭合行为随裂纹长度、应力比和过载影响因素的变化,以及对裂纹扩展速率的影响。同时,研究了单个拉伸过载和裂纹闭合行为之间的内在联系和机理。结合裂纹闭合理论和有限元计算结果,等效应力强度因子被用来描述316L不锈钢的裂纹扩展过程,并提出316L不锈钢的裂纹扩展速率的预测模型。     

Fatigue crack growth in a type 316 stainless steel and a 20 pet Cr/25 pet Ni/Nb stainless steel at elevated temperature

Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature propagation is transgranular and is controlled, to a large extent, by continuum plasticity effects at the crack tip. At elevated temperatures this simple process might be affected by oxidation and the tendency towards intergranular propagation. The elevated temperature fatigue crack propagation behavior of a 20/25/Nb stainless steel and a type 316 stainless steel is studied by optical measurement of the crack growth rate of artificially-induced notches under conditions where gross plastic straining is present in the bulk of the material. Tests conducted at ambient temperature are included for comparison with elevated temperature behavior. By reference to fatigue life data for smooth unnotched specimens, tested under identical conditions to the crack growth tests, an attempt is made to rationalize the roles of crack initiation and propagation in the fatigue process and indicate the relevance of crack growth data in predicting fatigue life data.     

Effect of residual magnesium content on thermal fatigue cracking behavior of high-silicon spheroidal graphite cast iron

This study investigates the thermal fatigue cracking behavior of high-silicon spheroidal graphite (SG) cast iron. Irons with different residual magnesium contents ranging from 0.038 to 0.066 wt pct are obtained by controlling the amount of spheroidizer. The repeated heating/cooling test is performed under cyclic heating in various temperatures ranging from 650 °C to 800 °C. Experimental results indicate that the thermal fatigue cracking resistance of high-silicon SG cast iron decreases with increasing residual magnesium content. The shortest period for crack initiation and the largest crack propagation rate of the specimens containing 0.054 and 0.060 wt pct residual magnesium contents are associated with heating temperatures of 700 °C and 750 °C. Heating temperatures outside this range can enhance the resistance to thermal fatigue crack initiation and propagation. When thermal fatigue cracking occurs, the cracks always initiate at the surface of the specimen. The major path of crack propagation is generally along the eutectic cell-wall region among the ferrite grain boundaries, which is the location of MgO inclusions agglomerating together. On the other hand, dynamic recrystallization of ferrite grains occurs when the thermal cycle exceeds a certain number after testing at 800 °C. Besides, dynamic recrystallization of the ferrite matrix suppresses the initiation and propagation of thermal fatigue cracking.     

Effect of Hot Deformation on Formation and Growth of Thermal Fatigue Crack in Chromium Wear Resistant Cast Iron

Chromium wear resistant cast iron is widelyusedin engineering, mining and power industry forits high strength,hardness and wear resistance .Inproduction process , some wear-resistant parts ser-ving in alternative stress due to rapid heating andcooling rate ofteninduces thermal fatigue andresultsinfailure .The badthermal fatigue property of chro-mium wear resistant cast iron is due to eutectic car-bides which distribute as continuous net in matrix.Recent researches showthat hot deformation can …     

基于动载荷谱的齿轮弯曲疲劳寿命预测

为了探讨齿轮弯曲疲劳寿命计算问题,将齿轮疲劳总寿命分为两个阶段,即疲劳裂纹萌生寿命和裂纹扩展寿命.通过ADAMS软件仿真实验齿轮的工作情况,使其接近真实状况,得到齿轮载荷谱.根据齿轮载荷谱,利用有限元ANSYS软件分析在齿轮齿根危险截面处的最大应力.采用断裂力学、雨流法和Miner疲劳损伤累积模型,对考虑动载荷情况下的齿轮弯曲疲劳寿命进行预测,推导了齿根裂纹萌生期和扩展期的疲劳寿命计算公式.在高频疲劳试验机上对算例齿轮进行了双齿脉动加载齿根弯曲疲劳寿命实验研究,理论计算结果与实验结果基本吻合,验证了本文理论分析的正确性.     

Fatigue of a glass bead blasted nickel-base superalloy

Room temperature fatigue crack initiation and propagation in the wrought nickel-base superalloy Udimet 700 were investigated with electropolished and glass bead blasted material. Cracks were found to initiate at the surface along coherent annealing twin boundaries oriented for maximum in-plane shear stress in both the electropolished and glass bead blasted conditions even though bead blasting more than doubled the fatigue strength. This increase was found to result from an enhanced crack initiation resistance, but even more importantly from a very pronounced retardation of early Stage I crack propagation by the residual compressive surface stress induced in glass bead blasting.     

拉伸与低周疲劳载荷作用下夹杂物特征参数对航空用超高强度钢中裂纹萌生与扩展的影响

为了研究非金属夹杂物对航空用超高强度钢性能的影响,采用扫描电镜原位观测的方法,跟踪观察了拉伸和低周疲劳载荷作用下两种航空用超高强度钢中不同种类、形态和尺寸的夹杂物导致裂纹萌生与扩展的微观行为.结果表明,对于单个TiN和AlN夹杂,在拉伸载荷作用下,裂纹均首先在夹杂内部萌生.夹杂面积越大,夹杂内萌生的裂纹条数越多,第1条裂纹萌生所需的应力越小.在疲劳载荷作用下,对于单个TiN夹杂,裂纹也首先萌生于夹杂内部.但对于以点链状形式存在的AlN夹杂,无论是在拉伸还是疲劳载荷作用下,裂纹均首先在点链状夹杂内部两夹杂之间的母材中萌生,然后沿点链状夹杂向两侧扩展.以点链状形式存在的夹杂对材料疲劳性能的危害比单个夹杂严重得多,夹杂物对材料疲劳性能的危害远大于对拉伸性能的危害.     

Microstructural examination of

Fatigue tests were performed to examine how microstructural conditioning influences crack initiation and propagation in SA508 class 3 low-carbon steel. A 3-mm-long crack was introduced in compact tension (CT) fatigue test specimens under four different loads in order to obtain crack tip plastic zones at different stress intensity factor ranges, ΔK = 18, 36, 54, and 72 MPa√m. The microstructure of the plastic zones around the crack tip were examined by trans- mission electron microscopy (TEM) and selected area electron diffraction (SAD). Micro- orientation of the dislocation cells in the plastic zones of all of the CT samples increased to 4 deg from the level of an as-received sample. Four-point bending fatigue tests were performed for plate shape samples with a large cyclic strain range. The SAD value of the bending samples was also 4 deg in the damaged area where cracks already initiated at an early stage of the fatigue process. These test results indicate that the microstructural conditioning is a prerequisite for the fatigue crack initiation and propagation in SA508. These observations may lead to better under- standing of how fatigue initiation processes transit to cracks.     

The slip character and low cycle fatigue behaviour: The influence of F.C.C. twinning and strain-induced F.C.C. → H.C.P. martensitic transformation

The influence of slip character on the low cycle fatigue behaviour was investigated in the CoNi system at room temperature. Three alloys of increasing stacking fault energy (SFE) were used: Co31Ni (SFE ∼- 12 mJm⁻²) which deforms by slip and f.c.c. → h.c.p. strain-induced martensitic transformation, Co33Ni (SFE = 15 mJm⁻²) which exhibits slip and twinning and Co45Ni (SFE = 45mJm⁻²) which deforms only by slip with easy cross-slip. Push-pull low cycle fatigue tests were conducted under plastic strain control up to a few 10⁴ cycles. The number of cycles to fracture was found to increase with decreasing SFE which promotes planar deformation mechanisms: the life obtained in Co31 Ni and Co33Ni alloys is respectively about 6 and 3 times higher than that of Co45Ni alloy.Measurements of striation spacings on the fracture surfaces have enabled to show that the influence of twinning on fatigue life is mainly due to a large increase of the initiation period before stage II crack propagation. This behaviour was associated with a difference in crack initiation sites along twin or h.c.p. platelets where there is a strain localization in low SFE alloys or along grain boundaries in the high SFE alloy. The increase of crack initiation period was explained on the basis of a reduced stage I crack propagation rate in the alloys exhibiting planar deformation mechanisms.