High cycle fatigue crack propagation under random and constant amplitude loadings

This paper essentially summarises work carried out during the period 1963–1973 into the constant and random amplitude fatigue crack propagation performance of a mild steel at ambient temperature.Conventional fracture mechanics parameters are shown to describe the process of propagation under a variety of mean stress intensity conditions.Calculations of random amplitude propagation using ‘laws’ determined from the constant amplitude data are shown to be in agreement with experimental results.This agreement of calculation and experiment gives confidence in the use of these calculation principles for reactor applications outside direct experimentation.     

A study of crack closure under constant amplitude loading for 6063-T6 Al-alloy

Crack closure experiments were performed on 6063-T6 Al-alloy, using a COD gauge for various load ranges (Δp) and stress ratios, R. On the basis of the experimental results a model for effective stress intensity range ratio U was developed. This model was found to be a function of the stress ratio, R, and was fitted to existing constant amplitude crack propagation data for 6063-T6 Al-alloy. The crack closure load stabilized after 1 mm initial crack growth.     

Thermal fatigue crack growth experiments on austenitic steel plates

A thermal cycling fatigue crack growth experiment has been performed on austenitic stainless steel plates in which a semi-elliptical surface crack grows through a surface layer experiencing cyclic plasticity. Linear elastic fracture mechanics crack growth assessments over-predict the surface growth and under-predict the through thickness growth. An alternative crack growth analysis has been performed using a pseudo stress intensity factor which is closely related to a strain intensity factor. The pseudo stress used to derive the pseudo stress intensity factor is calculated from uncracked body inelastic finite element analysis. It is demonstrated that a much improved prediction of fatigue crack shape can be made provided that the effects of crack closure are incorporated into the analysis. The method is relevant to leak-before-break and leak tightness arguments where the accurate prediction of through-thickness crack growth is particularly important.     

Experimental observation of crack propagation in 6063-T6 al-alloy under constant amplitude loading

Crack propagation experiments were performed on 6063-T6 Al-alloy for various load ranges and stress ratios. Experimental results show that for a constant load range, the life of specimens decreased as stress ratio increased. At constant maximum load, the life of the specimen increased as the load ratio increased. The crack growth data were analysed in terms of ΔK_eff as a function of stress ratio R. The data covered R values from 0 to 0·5 and a good relationship was obtained for K_eff/K = 0·55 + 0·12 R². A crack growth rate equation was developed.     

Suppression of hydrogen-assisted fatigue crack growth in austenitic stainless steel by cavitation peening

In this paper we demonstrate the suppression of hydrogen-assisted fatigue crack growth in type 316L austenitic stainless steel by cavitation peening employing a cavitating jet in air. Plate bending fatigue tests on pre-cracked samples were conducted after cathodic hydrogen charging with and without cavitation peening. Without cavitation peening, the hydrogen effect on the crack growth behavior at low applied stress was clearly demonstrated compared with high applied stress in the fatigue test. The coalescence of sub-cracks and the main crack propagating from the pre-crack were observed in the hydrogen charged specimen. This phenomenon significantly accelerated the crack growth. This unexpected fracture was suppressed by introducing compressive residual stress by cavitation peening regardless of the length of processing time. In addition, lengthier treatment reduced the crack growth rate of the hydrogen charged specimen by 75% compared to an untreated one.     

Modelling crack growth for creep and fatigue loading

Estimates of creep crack growth in engineering components under steady load conditions are usually based on the application of fracture mechanics concepts. In particular the creep parameter C* has become widely used together with creep crack growth data obtained from laboratory tests. There are now a number of practical methods to utilise experimental data. For high temperature components, which are subjected to cyclic (fatigue) as well as creep loading, the estimation of the fracture mechanics parameters becomes much more difficult, and consequently the extent to which the growth of pre-existing cracks grow by creep and fatigue is difficult to quantify. In this paper the response of Type 316L stainless steel is examined. This material progressively strain hardens under reversed cyclic loading, and the creep behaviour also changes. Using uniaxial fatigue and creep results, fracture parameter maps are developed to establish the appropriate regimes for creep-fatigue crack growth. Using the maps a model is developed which can predict the combined effect of fatigue and creep on crack growth. The implications of the model are discussed in relation to the limitations of obtaining results from laboratory tests at short times, and the assessment of practical engineering components.     

Unstable crack growth in a component subject to constant applied loads. II: Effect of crack size and component width

When a crack grows slowly, by, for example, a stress corrosion mechanism, in a component that is subject to constant applied loads, it is possible that there will be a transition from slow growth to unstable non-environmentally-assisted (plastic) growth. Part I of this investigation¹ examined the plane strain deformation of a non-work-hardening solid containing two symmetrically situated deep cracks, and with tension of the small remaining ligament. The main conclusion was that the transition from stress corrosion crack growth to plastic crack growth, accompanied by instability, should not occur under constant load conditions until the general yield state is almost attained, even though a material's resistance to plastic crack growth might be only moderate.The present paper extends the earlier study by analysing the more general model of a solid of finite width containing two symmetrically situated cracks of equal depth. Irrespective of the crack size and solid width, it is shown that unstable plastic crack growth should not occur until the general yield state is almost attained, even if the plastic crack growth resistance is only moderate. The scope of the conclusion reached in Part I is therefore broadened, and the results therefore provide further support for the view that the occurrence of unstable plastic fracture under load control conditions approximately correlates with the attainment of a critical net-section stress across the remaining ligament. The results consequently provide additional underpinning for the currently used empirical net-section stress approach for predicting failure under load control conditions.     

The dependence of fatigue crack growth on hydrogen in warm-rolled 316 austenitic stainless steel

The fatigue crack growth rate of warm-rolled AISI 316 austenitic stainless steel was investigated by controlling rolling strain and temperature in argon and hydrogen gas atmospheres. The fatigue crack growth rates of warm-rolled 316 specimens tested in hydrogen decreased with increasing rolling temperature, especially 400 °C. By controlling the deformation temperature and strain, the influences of microstructure (including dislocation structure, deformation twins and α′ martensite) and its evolution on hydrogen-induced degradation of mechanical properties were separately discussed. Deformation twins deceased and dislocations became more uniform with the increase in rolling temperature, inhibiting the formation of dynamic α′ martensite during the crack propagation. In the cold-rolled 316 specimens, deformation twins accelerated hydrogen-induced crack growth due to the α′ martensitic transformation at the crack tip. In the warm-rolled specimens, the formation of α′ martensite around the crack tip was completely inhibited, which greatly reduced the fatigue crack growth rate in hydrogen atmosphere.     

Investigation of ‘ageing’ of pressure vessel steel during the growth of a fatigue crack

The study of cyclic strain embrittlement during the growth of fatigue cracks in a pressure vessel steel used in Swedish boiling water reactors, A533B, emanated from observations made in Germany of stepwise crack growth during fatigue. It was demonstrated that stepwise crack growth does not occur in the investigated batch of A533B. No indication of embrittlement was found.     

Unstable crack growth in a component subject to constant applied loads: The interaction between plasticity and geometrical effects

When a crack grows slowly by, for example, a stress corrosion mechanism, in a component that is subject to constant applied loads, it is possible that there will be a transition from slow growth to unstable nonenvironmentally-assisted growth. Earlier work has shown that, even though a material's resistance to nonenvironmentally-assisted crack growth may be only moderate, the transition should not occur unless plastic deformation traverses the ligament between the crack front and the opposite free surface, i.e. unless general yield occurs. The earlier work analyzed the model of an edge crack in a semi-infinite solid, and compared the extent of spread of plasticity with the solid width in the real situation, thereby neglecting the interaction between the plastic zone and the boundary opposite the crack in the real situation. This paper extends the scope of the earlier investigation by analyzing a model in which the interaction between plasticity and geometrical effects is accounted for in an accurate manner. Though it is shown that unstable nonenvironmentally-assisted growth can occur before general yield, such growth occurs so close to general yield that the earlier work's conclusions are essentially unchanged. The implications of the conclusions are discussed in relation to the empirical net-section stress approach for predicting failure in ductile materials.     

疲劳裂纹预测的灰方法

借助灰色理论,建立了预测疲劳裂纹扩展的灰色模型,并应用此模型预测了某不锈钢构件腐蚀疲劳裂纹的扩展,得到了较高精度的预测结果,为疲劳裂纹预测提供了一种简易而可靠的新途径。     

The stability of crack growth in pipes subject to tensile loads

This paper presents a simple analysis for the stability of crack growth in 304 stainless steel pipes subject to tensile loads. The model of two identical part-through and part-circumference cracks, symmetrically situated with regard to the pipe cross-section, is examined for crack stability under displacement control tensile loading. Irrespective of the crack depth, the instability condition for a wide range of crack lengths, i.e. except for very short cracks and long cracks, is: $\text{2σ}{}_0\text{L}\text{πER}{}_{\mathrm{\chi }}\text{≡}\text{2L}\text{πR}\text{·}\text{1}\text{T}{}_{\mathrm{MAT}}\text{> 1}$ where σ₀ is the flow stress, E is Young's modulus, L is the pipe length, R is the pipe radius, χ is the crack tip opening angle, CTOA, associated with the crack growth and T_MAT is the material's tearing modulus. With a CTOA of 20° (i.e. T_MAT ～ 200), $\text{L}\text{R}$ must exceed 300 for instability. Since this number is far in excess of the $\text{L}\text{R}$ values for typical piping systems, the stability of cracks in pipes subject to tensile loads is essentially demonstrated.     

Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔK_th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔK_th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔK_th in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔK_th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔK_th in the hydrogen gas environment.     

The influence of waveform on the fatigue crack growth behaviour of SA508 cl III RPV steel in various environments

This paper describes the effect of waveform, sinusoidal and positive sawtooth, on the fatigue crack growth behaviour of SA508 cl III RPV steel in various environments under ambient conditions. Positive sawtooth waveform consistently gave faster crack growth rates than sine waveform although the amount varied depending on the environment and in some cases frequency. Of the two basic mechanisms suggested for environmentally enhanced crack growth, namely, dissolution controlled and hydrogen induced cracking, the latter can more readily explain the results of the present investigation. The longer period of increasing tensile load for the positive sawtooth resulted in increased hydrogen transport to the region of greatest triaxial stress.     

Fatigue crack growth calculations for a vessel internal corner crack under repeated thermal transients

A problem of current fundamental and industrial interest is the structural integrity of pressure vessels containing defects when subjected to repeated thermal shock at constant internal pressure. The component considered is a nozzle to a hemispherical pressure vessel intersection containing a defect at the internal corner. A range of 12 cases of thermal shock loads characterised in severity by two dimensionless parameters, the Biot (B) and Fourier (F) numbers, were applied. Estimation of remaining life for each case was carried out based on the PD6493 procedure. Numerical modelling of the crack growth from the quadrant shaped crack showed that more severe shocks accelerate crack growth while less severe shock leads to slow growth or eventual crack arrest. The effect of the Biot and Fourier parameters on the remaining life of the component was quantified. A methodology that comprises of heat transfer, thermal stress and fracture mechanics of defect assessment is applied to a problem of industrial interest.     

Fatigue crack growth in multi-pass butt-welded joints of mild steel

Fatigue crack growth rate properties obtained by testing multi-pass butt-welded joints in the through-the-thickness direction are presented along with a characterization of the mild steel base material. Edge-notched four-point-bending specimens are used to investigate R-ratio, specimen geometry and post-weld heat-treatment effects on fatigue crack growth rates. The pervasive influence of residual stresses on welded joint fatigue testing using the fracture mechanics approach is also discussed. For these multi-pass joints, conservative fatigue crack growth rates are obtained with post-weld heat-treated specimens.     

Effects of hydrogen on fatigue crack growth behavior of austenitic stainless steels

The effect of hydrogen on fatigue crack growth behavior of three stainless steels has been investigated from the viewpoint of microscopic fatigue mechanisms, martensitic transformation and hydrogen content. Fatigue crack growth rates in the hydrogen-charged SUS304 and SUS316 were accelerated with respect to crack growth rates in uncharged specimens. The crack growth rate in the hydrogen-charged SUS316L was only slightly higher than that in the uncharged SUS316L. Martensitic transformation on the fatigue fracture surfaces was detected using X-ray diffraction both in the hydrogen-charged and uncharged specimens of SUS304, SUS316 and SUS316L. Materials with increased tendency for martensitic transformation also showed increased acceleration in fatigue crack growth rate due to hydrogen. It was concluded that martensitic transformation in the vicinity of the fatigue crack tip increased the local diffusion of hydrogen thus increasing crack growth rate.     

The near-threshold high R-ratio fatigue crack growth characteristics of SA508 cl III reactor pressure vessel steel

This paper describes the effect of frequency and environment on the near-threshold fatigue crack growth behaviour of SA508 cl III reactor pressure vessel (RPV) steel. The study has shown that in the near-threshold regime microstructure and environment markedly affect fatigue crack growth behaviour. In an aqueous environment, fatigue crack growth behaviour became even more sensitive to microstructure, and the fatigue crack growth rate increased by a factor of four in the case of the 3 Hz test, while that for the 0·3 Hz test was increased by a factor of approximately sixteen. This environmental enhancement manifested itself in the form of intergranular failure. For the 0·3 Hz test the percentage intergranular failure decreased from 18% to <1% with an increase in ΔK level. The transition from microstructure-sensitive to microstructure-insensitive occurs when the cyclic plastic zone size is of the order of the prior austenite grain size.     

曲轴疲劳裂纹扩展可靠性分析的工程方法

以DF4B型机车柴油机球墨铸铁曲轴为研究对象 ,计算了在试验谱下的应力分布 ,提出了一种计算疲劳裂纹扩展可靠寿命的工程方法 ,并对曲轴裂纹扩展可靠寿命进行了计算 ,进而对柴油机曲轴的检修期提出了合理建议。     

Effect of weld metal properties on fatigue crack growth behaviour of gas tungsten arc welded AISI 409M grade ferritic stainless steel joints

The effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on fatigue crack growth behaviour of the gas tungsten arc welded ferritic stainless steel joints was investigated. Rolled plates of 4 mm thickness were used as the base material for preparing single ‘V’ butt welded joints. Centre cracked tensile (CCT) specimens were prepared to evaluate fatigue crack growth behaviour. Servo hydraulic controlled fatigue testing machine was used to evaluate the fatigue crack growth behaviour of the welded joints. From this investigation, it was found that the joints fabricated by duplex stainless steel filler metal showed superior fatigue crack growth resistance compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Higher yield strength, hardness and relatively higher toughness may be the reasons for superior fatigue performance of the joints fabricated by duplex stainless steel filler metal.