Fatigue Crack Propagation Threshold Research Articles

Effect of ultrasonic impact treatment on the ultra high cycle fatigue properties of SMA490BW steel welded joints

Effect of ultrasonic impact treatment on ultra high cycle fatigue properties of SMA490BW steel welded joints was studied from the aspects of stress concentration of weld toe, residual stress distribution, and surface grain size. The results show that the ultrasonic impact treatment can significantly improve the high cycle and very high cycle fatigue strength of welded joints. After treated by ultrasonic impact, the stress concentration factor of weld toe is decreased by 19.1%. And residual tensile stress of weld toe is eliminated, which change into the beneficial residual compressive stress. Residual compressive stress increases the fatigue crack propagation threshold, inhibiting the expansion of early fatigue crack, so as to effectively prolong the fatigue life of welded joints. The surface grains of weld toe can be refined by ultrasonic impact treatment, and nanocrystalline structure with random orientation distribution can be obtained. Grain refinement and work-hardening effects cause the increase of microhardness on surface.

Thermo-mechanical fatigue property and life prediction of vermicular graphite iron

Thermo-mechanical fatigue (TMF) failure is the major problem of the cylinder head subjected to combined variations in temperature and loading during operation. This study mainly focuses on the TMF property and the life prediction of vermicular graphite iron (VGI). The ferrite clusters can be easily found from the microstructure and fractography images. Compared with the TMF experimental data testing at 125–400℃ and 125–500℃, significant cyclic hardening occurs in the former and slight hardening does in the latter. Depending on the difference in the damage mechanism between TMF and iso-thermal low-cycle fatigue (LCF), based on the hysteresis energy, a life prediction method has been proposed at first. By the minimum amount of LCF and TMF tests, the present method can predict the TMF life rapidly, accurately and cheaply. And based on the difference in the fatigue crack propagation thresholds between pearlite and ferrite, the fracture mechanism of TMF was also discussed.

Semi-probabilistic method for residual lifetime of aluminothermic welded rails with foot cracks

One of the most frequent and dangerous failure modes in continuous welded rails is fatigue crack propagation terminated by brittle fracture. Due to the brittleness of the weld material and HAZ and the scatter in its mechanical properties, a statistical approach is necessary. The paper deals with surface cracks at the foot base of aluminothermic welded rails, developing a probabilistic methodology for determining the day by day prospective failure probability. The investigations presented here comprise weld material characterization, simulation of fatigue crack propagation and finally the determination of the failure probability using the Monte Carlo method. The effect of various parameters, such as axle weight, initial crack size, residual stresses, fatigue crack propagation threshold and date of inspection were analyzed. The results show that, independent of the date of the last inspection, almost any failure event happens in wintertime. This is in accordance with practical experience. However, from the proposed analysis it is evident that the main parameter controlling rail fracture is not only the minimum local temperature, but the temperature range over the whole year. Finally, the results are compared to the standard rail classification method.

About the fatigue crack propagation threshold of metals as a design criterion – A review

The fatigue crack propagation threshold ΔKth is of paramount importance for any kind of damage tolerant design, but in contrast to this importance, the determination and application of the parameter is not on the firm ground as previously assumed. The paper discusses questions of its experimental determination as well as of its application to components. In both fields, new questions have been raised with the potential to challenge or modify long-standing knowledge. Against this background, the paper is an attempt to systematize the established knowledge as well as questions and open issues.

Near threshold fatigue crack propagation in railways’ steels: Comparison of two testing techniques

Mode I fatigue crack growth thresholds of steels for rail and wheel used in railways were determined according to two different procedures. Although fatigue crack propagation data in the Paris regime is usually straightforward to obtain, the same does not apply to the threshold regime, where crack growth rates are very small up to inexistent, and therefore tests are very time consuming. Furthermore, the very small fatigue crack growth rates involved in testing for threshold determination require very thorough and careful experimental procedures if extraneous effects – as for example interaction effects such as crack retardation or acceleration – are to be avoided. Two methods for evaluation of fatigue crack growth threshold value of the range of stress intensity factor were used: load shedding and constant maximum stress intensity factor with increasing load ratio R (R=minimum load/maximum load), taking into account the ASTM standard E647 prescriptions. The obtained results and lessons learned in conducting these tests are discussed, and the data generated is compared with data published.

Lamellar orientation effect on fatigue crack propagation threshold in coarse grained Ti46Al8Nb

Cast Ti46Al8Nb with coarse lamellar microstructures was subject to fatigue crack propagation test at 650°C with R = 0·1. The notch region of each testpiece contains one or one dominant lamellar colony. A strong dependence of fatigue crack propagation threshold on lamellar orientation was found and the lamellar colonies with their interfaces parallel to the stress axis have the highest threshold and those 40–65° off the stress axis have the lowest threshold. It is elucidated that slip or twinning parallel to lamellar interface may interact with cross-lamella slip or twinning to help open up microcracks in front of the main crack.

Influence of Prior Austenite Grain Size on the Dispersion of Fatigue Crack Propagation Thresholds in Weld Metal

Fatigue crack propagation threshold is an important property for the structures with long life, such as nuclear turbine welded rotor. The fatigue crack propagation thresholds of multi-layer, multi-pass SAW weld metal of nuclear turbine rotor simulate are tested. But a big dispersion is found among test values, even at the same stress ratio . The dispersion of critical points between stable propagation zone and near-threshold zone is confirmed as an important reason. After the observation of microstructures around the critical points by backward inference method in different specimens, a good correspondence between the sizes of prior austenite grains and the maximum sizes of monotonic plastic zone at the crack tip is established. The inhomogeneous microstructures in SAW weld metal are the basic cause to dispersion of fatigue crack propagation thresholds at the same stress ratio.

Investigations on the Fatigue Crack Propagation Threshold in Very High Cycle Fatigue

Pariss law of fatigue crack propagation rate is well applied in the defect-tolerance fatigue approach. When carry out same approach in the very high cycle fatigue domain, the understanding of mechanism about fatigue crack propagation threshold which is obviously important, is helped. In the present work here, the fatigue crack propagation threshold of a surface crack for an Armco iron loaded in the VHCF regime was investigated by a new approach which combines the fracture surface analysis and the temperature recording on the surface during the test by an infra-red camera. The experiments were carried out on a sheet specimen under a 20 kHz ultrasonic frequency loading with IR images registration. Three stages of fatigue crack were identified with different mechanisms. It is found that the transition between initiation and crack propagation corresponds to the intrinsic fatigue threshold. It takes more than 99% of the gigacycle fatigue life to achieve this transition size.

Prediction of notch sensitivity effects in fatigue and in environmentally assisted cracking

AbstractSemi‐empirical notch sensitivity factors q have been used for a long time to quantify notch effects in fatigue design. Recently, this old concept has been mechanically modelled using sound stress analysis techniques, which properly consider the notch tip stress gradient influence on the fatigue behaviour of mechanically short cracks. This mechanical model properly calculates q values from the basic fatigue properties of the material, its fatigue limit and crack propagation threshold, considering all the characteristics of the notch geometry and of the loading, without the need for any adjustable parameter. This model's predictions have been validated by proper tests, and a criterion to accept tolerable short cracks has been proposed based on it. In this work, this criterion is extended to model notch sensitivity effects in environmentally assisted cracking conditions.

Analysis of the influence of defects on fatigue resistance of metallic components

The influence of defect on fatigue resistance of components is carried out by using a propagation threshold approach to estimate the high cycle fatigue resistance of components from data of intrinsic properties of the material. The Chapetti's threshold curve for fatigue crack propagation is used, which includes the short crack regime. The approach accounts for the effective driving force applied to the crack as the difference between the total driving force applied to the component, ΔK, and the threshold for fatigue crack propagation of the material, ΔK th . Application to analyze the influence of plate thickness on fatigue resistance of welded joints is presented.

Effect of submillimeter size holes on the fatigue limit of a high strength tool steel

ABSTRACTThe very high cycle fatigue and small fatigue crack growth behaviour of a generic tool steel material for diesel fuel injector application are described. The small crack growth tests for the tool steel material with and without the hardening heat treatment revealed the mechanisms of crack propagation and threshold behaviour. Based on the small fatigue crack propagation threshold value, an elastic plastic fracture mechanics methodology for the prediction of the endurance limit of specimens with submillimeter holes is proposed. The advantages of the new methodology are discussed in relation to existing methodologies for endurance limit prediction of specimens with small holes.

Study on fatigue property and microstructure characteristics of welded nuclear power rotor with heavy section

Abstract The fatigue property and microstructure characteristics of the welded joint for nuclear power rotor with heavy section, were systematically reported in this paper. The welded joint microstructure is inhomogeneous for NiCrMoV rotor made by narrow gap submerged arc welding (NG-SAW), which could affect the properties in different zones of welded joint. As one of the important indicator to evaluate the running performance of welded rotor, the fatigue crack propagation behavior of the base metal (BM), weld metal (WM) and heat affected zone (HAZ) was comparatively studied. It was found that the fatigue crack propagation threshold (ΔKth) of BM was higher than that of WM and HAZ as stress ratio (R) was 0.1, but ΔKth was very close to each other as R increased. The microstructure, revealed by an optimized corrosive process, was granular bainite in WM and tempered martensite in HAZ, leading to their approximately equivalent resistance of fatigue crack propagation with BM. The experimental results showed that fatigue properties of welded joint for NiCrMoV rotor with heavy section could meet the design requirement, and also push NG-SAW into manufacturing large size rotor.

Is notch sensitivity a stress analysis problem?

Semi–empirical notch sensitivity factors q have been widely used to properly account for notch effects in fatigue design for a long time. However, the intrinsically empirical nature of this old concept can be avoided by modeling it using sound mechanical concepts that properly consider the influence of notch tip stress gradients on the growth behavior of mechanically short cracks. Moreover, this model requires only wellestablished mechanical properties, as it has no need for data-fitting or similar ill-defined empirical parameters. In this way, the q value can now be calculated considering the characteristics of the notch geometry and of the loading, as well as the basic mechanical properties of the material, such as its fatigue limit and crack propagation threshold, if the problem is fatigue, or its equivalent resistances to crack initiation and to crack propagation under corrosion conditions, if the problem is environmentally assisted or stress corrosion cracking. Predictions based on this purely mechanical model have been validated by proper tests both in the fatigue and in the SCC cases, indicating that notch sensitivity can indeed be treated as a stress analysis problem.

Estimation of Notch Sensitivity and Size Effect on Fatigue Resistance

This paper addresses the problem of high cycle fatigue resistance associated to notches and the role of short crack propagation in the fatigue notch sensitivity quantified by the notch factor kf. An integrated fracture mechanics approach is proposed to estimate the fatigue notch sensitivity, by including the effect of both blunt and sharp notches.Whether fatigue strength at a given life is controlled by crack initiation (very blunt notches, kf = kt), by microstructuraly short cracks (blunt notches, kf <kt), or by mechanically short crack propagation (sharp notches, kf <<kt), depends on the stress concentration kt, the notch length D and the material threshold to crack initiation ΔσeR, to short crack propagation ΔKth and to long crack propagation ΔKthR.The approach includes the prediction of the fatigue crack propagation threshold for short cracks, previously developed to analyze the short crack behavior in metallic materials with or without blunt notches, and is integrated adding the influence of sharp notches and accounting for the controlling parameters. It estimates the fatigue resistance of the component by comparing the threshold for fatigue crack propagation as a function of crack length, ΔKth, with the applied ΔK for the given configuration. Estimations for results reported in published bibliography are presented.The proposed fracture mechanics approach allows accounting for the effects of notch acuity, notch size and intrinsic material fatigue properties on fatigue notch sensitivity. It opens the door to a new simple method for predicting fatigue notch sensitivity and fatigue strength of components with geometric concentrators by using parameters that can be easily measured or estimated, without the necessity of any fitting parameter.

Short Fatigue Crack Propagation Threshold of Railway LZ50 Axle Steel

Collective short fatigue behaviour nearby material micro-structural barrier threshold is experimentally observed for the smooth specimens of Chinese railway LZ50 axle steel. Effective micro-cracks which result in the specimens’ failure were initiated from the material ferrite particles on the surfaces of the specimens. Four elements i.e. crack size, orientation angle, and the two crack tip orientation angles should be contained for each micro-crack. The collective micro-crack behaviour should be described by an equivalent dominant effective short fatigue crack (ESFC) concept using an ESFC theory. And an equivalent method is then developed with an equivalent growth driving energy concept. A random modeling of the equivalent dominant ESFC growth rates is further constructed for describing the random behaviour of the collective ESFCs nearby the material micro-structure barrier threshold. And the kinetic threshold is subsequently conducted from the modeling.

Fatigue crack propagation behaviour of steels with different microstructures

The fatigue crack propagation behaviour of steels with different microstructures was investigated. The results show that the Paris constant m values of the different steel microstructures have a linear relationship with the yield strengths for the same or approximate material composition, and that the ratio of the fatigue crack propagation threshold value to fracture toughness (ΔKth0/Kc) has a linear relationship with both the m values and the yield strengths. The crack propagation rates at the Paris regime were governed by the yield strengths, whereas the crack propagation paths at the threshold regime were governed by the microstructures.

Researches on the Fatigue Crack Propagation of Pipeline Steel

Researches and industrialization of pipeline steel fatigue crack propagation are summarized, especially the X60 and X70 pipeline steel after mechanical damage and in the synthetic soil solution. The results show that the fatigue crack propagation of the pipeline steel has the similar law after mechanical damage or in the synthetic soil solution. It is very clearly that the fatigue crack propagation is deeply depending on the ΔK, and there are all have threshold characters. The mechanical damage and the synthetic soil solution can increase the fatigue crack propagation rate, decrease the fatigue crack propagation threshold ΔKth, accelerate crack propagation rate and shorten the service life of the pipe.

Fatigue Crack Propagation Analysis of X60 Pipeline Steel Pipelines with Defects

It is impossible to keep pipelines free from defects in the manufacturing, installation and servicing processes. In this paper, pre-tension deformation is used to imitate the defects of the part of material and the electric method is used to measure the distance of the fatigue crack. Experimental investigations were carried out on the fatigue crack propagation characteristics of X60 pipeline steel after different degrees of pre-tension deformation Test results show that the fatigue crack propagation was augmented with the pre-tension deformation increase, especially the crack propagation at the near threshold section. The results of quantitative analysis show, the pre-tension deformation reduced the fatigue crack propagation threshold and increased the crack propagation coefficient; consequently the fatigue performance of the part of mechanical damnification was degenerate.

A simple model to predict the very high cycle fatigue resistance of steels

This work deals with the very high cycle fatigue behavior of high strength steels where fracture origins are mostly at non-metallic inclusions. From the analysis of several features involved in the crack initiation and propagation processes, and since the fatigue resistance usually observed for metals is mainly due to a microstructural threshold for pure fatigue crack propagation, the following expression is proposed to estimate the internal fatigue resistance for a given fatigue life for fractures produced by cracks initiated from an internal inclusion: σ e Int = 444 Δ K th n R i where σ e Int is given in MPa, the inclusion radio R i is in μ m, n is a dimensionless factor and the pure fatigue crack propagation threshold ΔK th is equal to the lower value given by the following expressions: Δ K th = 4 . 10 - 3 ( H V + 120 ) a 1 / 3 Δ K th - 1 = - 0.0038 σ u + 15.5 where the pure fatigue crack propagation thresholds Δ K th (function of crack length) and Δ K th −1 (a constant value for a given tensile strength or hardness) are in MPa m 1/2, the crack length a in μm, the Vicker harness H V in kgf/mm 2 and the ultimate tensile strength σ u in MPa. Such estimation becomes very important in the assessment of the fatigue resistance of components subjected to very high cycle fatigue, as this might become a very expensive and time consuming task.

More insights into fatigue crack growth from experiments on steels and aluminium alloys—Thresholds

The paper deals with the influence of the R-ratio on the fatigue crack propagation threshold of different metallic materials. Based up on threshold data of steels, aluminium-alloys and two particulate-reinforced aluminium-alloys the threshold behaviour has been analyzed and a new model to describe the threshold behaviour has been deduced. With this model, which contains two quantities that can be deduced from special fatigue crack propagation tests, the threshold behaviour of different kind of materials can be explained. Finally, the existence of the threshold can be attributed to specific elastic parameters of the materials (shear modulus) and their microstructure.