THE INFLUENCE OF SPECIMEN GEOMETRY ON NEAR THRESHOLD FATIGUE CRACK GROWTH

Abstract— The near threshold fatigue behavior of a nickel base superalloy and a wrought 2024 aluminum alloy is examined as a function of specimen geometry. Experimental results revealed that for a given value of Δ K in Region I, crack growth rates were observed to increase as the specimen loading became more symmetric with respect to the load line. Compact tension type specimens exhibited lower crack growth rates than the more symmetrically loaded center cracked tension specimens. Consideration of the observed discrepancies is given in terms of the near field crack tip stress distribution and deformation behavior of the material affecting crack advance.     

PREDICTING THE EFFECTS OF LOAD RATIO ON THE FATIGUE CRACK GROWTH RATE AND FATIGUE THRESHOLD

The effect of the load ratio, R, on fatigue crack growth behaviour is analysed on the basis of the recently proposed inelastic discrete asperities model. A wide range of load ratios, both positive and negative, are examined. Particular emphasis is placed on compressive excursions, i.e. negative R loadings. The inelastic discrete asperities model is a micro-mechanical analysis based on the plastic crushing of a single asperity (or multiple asperities) located on the crack face close to the crack tip and under dominantly plane strain conditions. Experimental data have indicated that the primary crack face contacts which obstruct closure are immediately adjacent to the crack tip, although segments of the crack face more distant from the crack tip are not neglected. However, the more distant asperities are a part of the past crack advance history which does not influence current behaviour. By use of this model, it is shown that the effect of the load ratio can be adequately predicted once some baseline information on mechanical material properties and surface roughness is provided. The model also provides useful trend information and explains many of the observed phenomena, e.g. the ‘saturation’ of the compressive underload effects. For a constant applied nominal stress intensity factor range, ΔK_nom , it is shown that the effective stress intensity factor range, ΔK_eff , initially decreases as the positive R decreases (corresponding to the increasing influence of closure), reaches a minimum around R = 0, and then starts increasing with negative R (corresponding to the plastic crushing of the asperities which reduces closure), eventually reaching a saturation level below ΔK_nom . Conversely, for an assumption of a constant ΔK_eff , the applied ΔK_nom increases as the positive load ratio decreases, reaching a maximum around R = 0, and then decreases with more negative R values, eventually reaching again a saturation level (above ΔK_eff ). It is also shown that the effect of material hardness can be directly analysed based on this model.     

INFLUENCE OF AN OVERLOAD ON THE FATIGUE CRACK GROWTH IN STEELS

Abstract— This study is concerned with the influence of a single-peak overload and the overload ratio on the subsequent rate of growth of a fatigue crack in steels. Retardation increases with increasing overload ratio.
The crack opening load was also measured during all tests. It is shown that the Elber's crack closure concept is not able to explain the effect of overloads. The importance of the material yield stress was evaluated by testing steels of different strength. It seems that the residual stress state induced by the overload is the major factor causing retardation. Two models are analyzed.     

THE EFFECT OF CRACK LENGTH ON FATIGUE THRESHOLD

This paper puts forward a new method for analysing the behaviour of very short fatigue cracks. A probability function is introduced into the definition of the growth threshold, which rationalises the scatter in experimental data produced using an aluminium bronze alloy. This probability function can be visualised in terms of the microstructure of the material.
It is shown that, in this material as in mild steels, fatigue crack initiation is not the critical stage. Initiation occurs relatively easily, but the cracks so formed may grow to only a few grain diameters in length before being arrested; thus it is the behaviour of cracks of this length which is critical in determining the fatigue strength of the material.
These observations, when combined with the probability functions, allow estimation of the probability of failure of a component or structure in service with greater confidence than the methods used at present.     

THE EFFECTS OF MOISTURE ON THE FATIGUE CRACK GROWTH BEHAVIOUR OF A LOW ALLOY STEEL NEAR THRESHOLD

Abstract— The influence of moisture on the fatigue crack growth behaviour near threshold of a 2NiCrMoV rotor steel has been investigated. At a high stress ratio ( R = 0.6), moisture enhances fatigue crack growth rates by approximately 60% compared with the growth rate in dry air. The effect would appear to be due to the influence of hydrogen which is confined to a volume of material at the crack tip considerably smaller than the plastic zone. At a low stress ratio ( R = 0.14), the growth rates in moist air can be very much lower than in dry air. This difference is closely associated with the formation of oxides on the fracture surface—moisture modifying the type and extent of oxidation which is observed. Observations of transient crack growth following environmental changes, suggest that fracture surface oxides within approximately 0.3 mm of the crack tip exert a strong retarding influence on crack growth although oxides up to at least 3 mm from the tip may also have some retarding effect.     

EFFECTS OF AIR AND INERT ENVIRONMENTS ON THE NEAR THRESHOLD FATIGUE CRACK GROWTH BEHAVIOR OF ALLOY 718

The near threshold fatigue crack growth behavior of alloy 718 was studied in air and helium environments at room temperature and at 538°C. Tests were performed at 100 Hz and at load ratios of 0.1 and 0.5. At room temperature and at 538°C, the ΔK_th values in helium were lower than in air. The ΔK_th values in air decreased with increasing load ratio. These results can be explained with a model that involves the accumulation of oxide in the crack which enhances crack closure. In the air tests, the oxide build-up on the fatigue fracture surfaces at ΔK_th was of the order of magnitude as the crack tip opening displacement. In the helium tests, no significant build-up of oxide on the fracture surface at threshold was found.     

INFLUENCE OF TRANSFORMATION-INDUCED CRACK CLOSURE ON SLOW FATIGUE CRACK GROWTH UNDER VARIABLE AMPLITUDE LOADING

Abstract Fatigue crack growth under constant and random loading conditions was investigated for a metastable austenitic-bainitic steel in comparison with a ferritic chromium steel at very low crack growth rates. Experimentally determined random crack growth was compared with linear Miner calculations on the basis of constant amplitude results. It was found that the measured crack growth rates in transforming material are a factor of 10 lower than the calculated values, whereas the difference is only a factor of 2 for the ferritic steel. The reason for the pronounced crack growth retardation in the metastable alloy is transformation of part of the austenitic phase into martensite in the stress field of the crack tip, accompanied by a volume increase and, consequently, residual compressive stresses. Rare high load cycles in the random sequence increase the closure level, which then leads to pronounced retardation of fatigue crack growth for the numerous successive low amplitude cycles.     

THE EFFECT OF ENVIRONMENT ON CRACK CLOSURE AND FATIGUE THRESHOLD*

Abstract— The threshold value for fatigue crack growth of a medium carbon steel was increased when the test-environment was changed from air to an aggressive H₂S-containing brine. This increase in fatique threshold was shown to be caused by corrosion product-induced crack closure. Further, the fatigue threshold and crack closure level were shown to be dependent on the growth rate history in approaching threshold. The differences in fatigue crack growth rate and fatigue threshold resulting from test procedure and growth rate history were significantly reduced by employing the effective stress intensity concept.     

THE SIGNIFICANCE OF MEAN STRESS ON THE FATIGUE CRACK GROWTH THRESHOLD FOR MIXED MODE I + II LOADING

Abstract— The effect of mean stress on near threshold fatigue crack growth behaviour under mixed mode I+II loading has been studied in a structural steel BS4360 50D in laboratory air at room temperature. It was found that the branch crack threshold decreased significantly as R ratio ( P _min/ P _max) increased from 0.1 to 0.7. A simple model is proposed to predict the branch crack threshold behaviour for R -ratio sensitive materials. Further investigation is required to model the mode II dominant situations where the branch crack thresholds tend to converge on a high value.     

INFLUENCE OF TEMPERATURE ON FATIGUE CRACK GROWTH BEHAVIOUR OF STEELS AT ULTRASONIC FREQUENCY

Fatigue crack growth rates have been measured at 22 kHz for two types of carbon steel, between 20 and 500°C under symmetrical push-pull loading. The crack length was determined from the decrease in the resonant frequency of the specimen. For ?SN 412013 steel, an increased crack propagation resistance was observed between 250 and 300°C. The values of the constants C and n in the Paris equation, and K_a,th are dependent on temperature. The fatigue crack growth rate and C both increase with temperature, while n and K_a,th decrease with increasing temperature. Electron scanning and light microscopy have shown that intercrystalline fracture does not occur in ?SN 415313 steel at elevated temperature. Intercrystalline fracture was observed for specimens fatigued at 20°C and also in ?SN 412013 steel at temperatures of 200 and 500°C. The width of the plastic zone and the height of the surface relief around the fatigue crack increased with temperature.     

INFLUENCE OF SURFACE DEFORMATION AND ELECTROCHEMICAL VARIABLES ON CORROSION AND CORROSION FATIGUE CRACK DEVELOPMENT

This paper presents experimental data for a 0.2%C steel/artificial seawater system showing the influence of shear loading on corrosion response, via measurements of electrochemical variables, e.g. anodic/cathodic Tafel slopes and polarization resistance. Based on the results of these tests, several corrosion fatigue tests were conducted at different stress levels under potentiostatic control. Analysis of the results shows there to be a dependence of corrosion rate on the ratio of applied/yield strain and test frequency. In addition, the corrosion current associated with corrosion fatigue (CF) damage appears to be dependent upon the crack size, which in turn shows a relationship with fatigue crack growth rate. This paper sets out to determine the influence of stress on electrochemical parameters, i.e. free corrosion potential, E_corr , polarization resistance, R_p , anodic, τ_a and cathodic, τ_c Tafel constants. Based upon these results, it is found that a simple linear relationship between stress and corrosion damage does not exist. Furthermore, analysis of the corrosion current fluctuations during corrosion fatigue crack growth shows a minimum current coincident with the point at which a crack is growing at its slowest rate.     

疲劳裂纹扩展试验载荷谱加重方法研究

为探索缩短疲劳裂纹扩展试验周期的载荷谱处理方法,对多种裂纹扩展模型进行了综述,概括出一种通用的裂纹扩展预测模型表达式。提出了一种将峰谷值同时等比例放大的载荷谱加重方法,并推导出加重前后裂纹扩展寿命的转换关系。用一种随机谱和两种块谱的基准谱及对应四种比例加重谱进行了LY12CZ中心裂纹板裂纹扩展试验,用常幅载荷谱及其1.1倍加重谱进行了典型机身壁板结构裂纹扩展试验。试验结果表明载荷谱等比例加重方法可以有效缩短疲劳裂纹扩展试验周期而不改变结构破坏模式,而分析结果表明由该方法预测的裂纹扩展寿命与试验结果的误差在15%以内,满足工程精度要求。     

ON THE IMPACT FATIGUE CRACK GROWTH BEHAVIOUR OF METALLIC MATERIALS

The impact fatigue crack growth characteristics of a low carbon steel and an aluminium alloy were studied. An impact fatigue testing machine of the Hopkinson bar type was used in these experiments to conduct a series of crack growth tests under simple impact stresses. The following characteristics of impact fatigue crack growth behaviour were revealed: (1) crack growth rate is higher in impact fatigue than in non-impact fatigue; (2) crack opening ratio in impact fatigue takes a higher value than in non-impact fatigue; (3) crack tip plastic zone size is smaller in impact fatigue than in non-impact fatigue.     

纤维增强复合材料界面疲劳裂纹扩展的模拟研究

脱粘是纤维增强复合材料界面裂纹扩展的主要表现形式.基于剪切筒模型和常用的实验加载方式,研究了纤维增强复合材料中纤维与基体界面在拉-拉循环荷载作用下的裂纹扩展.借助描述疲劳裂纹扩展的Paris公式,得到了疲劳裂纹扩展速率、扩展长度以及界面上摩擦系数与加载次数的关系.在分析中,考虑了疲劳加载引起的脱粘界面的损伤及损伤分布的不均匀性,同时还考虑了材料的泊松效应.     

EFFECTS OF HYDROSTATIC PRESSURE ON THE RATE OF FATIGUE CRACK GROWTH

Abstract— Fatigue crack growth rate data have been obtained, for both positive and negative R stress ratios, in pressurised and unpressurised oil and in inert argon gas. Expected results show that for negative R the rate data should be correlated against the tensile stress intensity range. For tension-compression loading constant hydrostatic pressure has a detrimental effect increasing the tensile stress intensity range. For tension-tension loading constant hydrostatic pressure has a beneficial effect decreasing crack growth rates. A simple theoretical model has been proposed which describes these trends.     

THE INFLUENCE OF NOTCH PLASTICITY ON SHORT FATIGUE CRACK BEHAVIOUR

Abstract— A study has been made of fatigue crack formation and growth at the root of different notch profiles in a structural steel subjected to fully reversed tension-compression loading. The scale of stage I microstructural crack growth at notches decreased with increasing notch root strain and was comparable to the size of stage I cracks in shallow hourglass profile specimens at the same strain. Stage II crack growth rates were faster within the notch plastic field than in the elastic stress field of the bulk material.     

轴承钢疲劳裂纹扩展速率的研究

研究了轴承钢碳化物及晶粒细化对轴承钢疲劳寿命的影响。结果表明;细化轴承钢中的碳化物可以使其疲劳裂纹扩展速率下降,而同时细化轴承钢中的碳化物和晶粒,会使其疲劳裂纹扩展速率下降更明显。     

THE INFLUENCE OF MICROSTRUCTURE ON THE GROWTH OF SMALL FATIGUE CRACKS

Abstract— Recent experimental work on the growth of small fatigue cracks is surveyed and critically analyzed. It is shown that microcracks grow at anomalous rapid rates relative to large ones only when certain criteria, involving crack size, plastic zone size, and micro-structural element size, are met. Retardation and arrest of microcracks is found to correlate with microstructural element size, hence with crystallographic influence.