Discrepancies of fatigue crack growth behaviour of API X65 steel

This paper discusses the fatigue life behaviour of the API X65 steel tested under constant amplitude loading in ambient temperature. The influence of repeated loading at a particular stress ratio would favour the initiation of fatigue cracks that would consequently affect the fatigue crack growth. The main objective of this paper is to evaluate the load ratio effects on fatigue crack growth rate, taking into account their statistical characteristics. The fatigue crack growth test was carried out using compact-tension specimen at different load ratios of 0.1, 0.4 and 0.7. The experimental results showed the crack growth rate was dependent on the applied load. It was observed that the load ratio effect was less significant in stable crack growth regions. The result was consistent with the multiple regression test obtained by the least square method at a significance level of 0.05. The empirical model of Paris and Walker was utilised to evaluate the effects of load ratio on the fatigue crack growth rates. The approximation of fatigue life lies between 10-25 % of error using conservative model and 6 % error using the Walker model. The experimental data was scattered within a factor-of-2 correlation line suggesting that the accuracy of the experimental data towards the estimated values was high.     

基于BS7910确定起重机械疲劳裂纹的容许尺寸

依据BS7910规范,编写了C语言迭代程序,以确定起重机械焊接接头在复杂受力状态下疲劳裂纹的容许尺寸,为安全检测和评估打下基础。以T型焊接接头为实例,计算了典型受力状态下表面裂纹容许尺寸,说明裂纹深度对起重机械安全的影响要大于裂纹长度的影响。     

Effect of sigma phase on near-threshold fatigue crack growth behavior and ultrasonic evaluation in AISI 316L stainless steel

In this work, we examined the influence of microstructural changes, such as an intermetallic sigma (??) phase, on the fatigue behavior of high-temperature aged AISI 316L stainless steel. Nondestructive ultrasonic test and fatigue crack growth tests were performed to determine the threshold stress intensity factor of these artificially aged specimens. Ultrasonic test results characterizing the microstructural changes were compared with those of the fatigue tests to propose an empirical formula capable of predicting the threshold stress intensity factor by a nondestructive method. We observed a strong correlation between the increase in the volume fraction of the ?? phase and the decrease of ??K_th. Ultrasonic velocity increased in response to the coarsening behavior of the ?? phase in the vicinity of the grain boundaries.     

The interpretation of quasistatic fatigue crack growth

An account of the quasistatic fatigue crack growth under cyclic loading and cyclic displacement is given in terms of the load-deflection (X − u) and fracture resistance-crack area (R − A) diagrams. Experimental results are reported in support of the concepts proposed.     

Effect of overloads on fatigue crack growth regularity within steel-molded pieces of freight cars

Experiment-calculated modeling of fatigue crack development of 20 GFL cast steel in railroad constructions irregularly loaded has been carried out. The loading of samples involves tensile overloads from the operational range of loading of freight car bogies. The calculation implies a complete fatigue failure diagram of the steel under study and a concept of the effective stress intensity factor at the overload-affected site. The parameters used in the calculation are determined from the results of tests. The calculation is correlated with the results of the experiment.     

加载频率对钢铁材料超长寿命疲劳性能的影响

使用82 Hz轴向加载疲劳试验机和20 kHz非间歇式加载的超声疲劳试验机,对比研究了超声加载频率对轴承钢GCr 15和35CrMo超长寿命疲劳性能的影响.断口观察表明:82 Hz轴向加载和20 kHz超声加载的GCr15试样,在全应力幅范围既发生表面破坏又发生由夹杂物引起的内部破坏;而35CrMo试样在两种频率下均为...     

Experimental investigation of crack growth behaviors and mechanical properties degradation during gear bending fatigue

Journal of Mechanical Science and Technology - Tooth bending fatigue remains as a fundamental bottleneck restricting the safety and reliability of modern high-performance gear. In this study, a...     

Numerical studies on crack growth in a steel tubular T-joint

The results of a numerical investigation on fatigue crack growth of an offshore tubular T-joint under the action of axial, in-plane and out-of-plane bending loads are presented in this paper. Extensive stress analysis has been carried out to determine the location of the hot spot stress along the brace—chord intersection for each load case. Semi-elliptical cracks with varying crack lengths and crack depths were introduced at the hot spot location by means of line spring elements for stress intensity factor evaluation. The line springs were properly constrained to prevent the problem of crack surface penetration. The stress intensity factors obtained are then used in a crack growth law for life estimation.     

Features of fatigue crack development in cast steel parts of railroad structures

The paper presents the results of an experimental study on the features of fatigue crack development in 20GFL grade low-alloy steel of a freight car bolster with simulation of the operating process of loading. The experiment results are compared with calculations using the fatigue failure diagram for this steel.     

A probabilistic analysis on variability of fatigue crack growth using the markov chain

Understanding the stochastic properties of variability in fatigue crack growth is important to maintaining the reliability and safety of structures. In this study, a stochastic model is proposed to describe crack growth behavior considering the variability of fatigue crack growth rates due to the heterogeneity of material. Fatigue life distribution is then predicted based on this model To construct this model, fatigue tests are conducted on a high strength aluminum alloy 7075 T6 under constant stress intensity factor range control. The variability of fatigue crack growth rates is expressed by random variablesZ and Γ based on the variability of material constantsC andm of the Paris-Erdogan equation. The distribution of fatigue life under constant stress intensity factor ranges is evaluated by the stochastic Markov chain model based on the Paris-Erdogan equation. The merit of the proposed model is that only a small number of tests are required to determine this function, and fatigue life required to reach certain crack length at a given stress intensity factor range can be easily predicted. Department of Mechanical Design and Production Eng.     

轿卡货厢车顶盖疲劳失效分析及改进设计

利用有限元方法对一种轿卡货厢顶盖总成进行了静态分析和模态分析,探讨这种货厢车顶侧围外板前端翻边拐角部位疲劳裂纹失效的影响因素,并提出了加强改进方案,对原方案和改进方案进一步应力分析,阐述了结构改进的意义,其结果有效地指导了实际的车顶结构改进设计。     

16Mn钢焊接件疲劳裂纹扩展的微观特征研究

研究了在循环载荷作用下,16Mn钢焊接件疲劳裂纹的扩展情况.通过光学显微镜观察了裂纹扩展的微观特征,发现了裂纹在焊接结构不同部位的扩展路径方式并分析了显微组织对裂纹扩展的影响.裂纹在母材处为穿晶扩展,在热影响区和焊缝金属中的扩展为穿晶和沿晶混合型.     

Effect of local yield strength gradients on fatigue crack propagation

A new crack propagation effect is discussed analytically and demonstrated experimentally. It is shown that local one-dimensional variations in yield strength (strength gradients) can effect a major change in the rate of crack growth under conditions of constant amplitude cycled stress intensity at the crack tip as determined by linear elastic fracture mechanics. Rice's path independent integral is combined with certain fracture mechanics arguments to derive a crack propagation law which predicts that crack extension per cycle is modulated by strength gradients if they are present, in proportion to their slope. Experimental data are developed for two aluminum alloys which demonstrate the effect conclusively. The importance of these results as they apply to real engineering components where strength gradients are known to occur because of casting, forging, rolling, joining, etc. is discussed. The strength gradient effect is shown to be discrete and separate from other effects including the influence of local levels of strength.     

Effect of silicon nitride coating thickness on fatigue of silicon microbeams

In this paper, two silicon nitride layers with thickness, 0.2 and 0.4 μm, are coated onto single crystal silicon (SCS) in order to achieve Si₃N₄/Si cantilever microbeams. The effect of LPCVD silicon nitride surface coatings on fatigue properties of SCS cantilever microbeams is investigated. Fatigue testing is conducted at both 40 Hz and 100 Hz. Typical S–N (strain amplitude–fatigue cycle) curves of the beams are achieved and correlated fatigue failure modes are investigated. It is found that thinner Si₃N₄ coating of 0.2 μm results in better fatigue lives of Si₃N₄/Si beams than thicker Si₃N₄ coating of 0.4 μm. Both thinner and thicker coated beams have major fatigue crack planes along {1 1 1} planes; however, thicker coated beams possess specific failure mode of delamination, which is not found in thinner coated beams. Delamination reduces the reinforcing effect of thicker Si₃N₄ coating and leads to its shorter fatigue life. For thicker coated beams, fatigue life at 100 Hz is longer than that at 40 Hz. The mechanism for delamination and the effect of cyclic frequency is investigated, and factors for better fatigue life are proposed.     

Numerical analysis for predicting the rolling contact fatigue crack initiation in a railway wheel steel

This paper concerns the modelling of the rolling contact fatigue of a railway wheel steel, which is simulated with moving Hertzian contact pressure. Parametric studies are carried out with a two-dimensional elastic-plastic finite element model of a part of a wheel containing defects. Several parameters, namely the size and shape of material defects, the load magnitude and the friction coefficient are varied to investigate their effect on the railway wheel fatigue damage. Defects or small friction coefficient are a plausible explanation to the initiation of deep subsurface fatigue cracks.     

Fatigue crack growth behavior in ultrafine grained low carbon steel

Ultrafine grained (UFG) low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating the effect of load ratio on the fatigue crack growth rate. Fatigue crack growth resistance and threshold of UFG steel were lower than that of as-received coarse grained steel. It was attributed to the less tortuous crack path. The UFG steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase of R ratio. The R ratio effect on crack growth rates and ΔK_th was basically indistinguishable at lower load ratio (R>0.3), compared to other alloys, which indicates that contribution of the crack closure vanishes. The crack growth rate curve for UFG steel exhibited a longer linear extension to the lower growth rate regime than that for the coarse grained as-received steel.     

Influence of coating thickness on rolling contact fatigue of alumina ceramics thermally sprayed on steel roller

The influence of the sprayed layer thickness on rolling contact fatigue of a thermally sprayed alumina ceramics with a nominal composition of Al₂O₃–2.3 mass% TiO₂ was investigated using a two-roller test machine under pure rolling contact condition with oil lubricant. The influence of undercoating of sprayed nickel-based alloy on rolling contact fatigue was also investigated. Thicknesses of the ceramics-sprayed layer of tested rollers were 0.2, 0.5 and 1.0 mm. The failure mode of sprayed rollers was spalling caused by subsurface cracking. In the case of sprayed rollers without undercoating, rolling contact fatigue strength of rollers with 0.2 mm thickness sprayed layer was the smallest. Rolling contact fatigue strength of sprayed roller with 0.2 mm thickness sprayed layer was improved by undercoating. In case the failure depth was small as compared with the thickness of sprayed layer, effect of undercoating on the rolling contact fatigue strength was little. The depths where the maximum values of subsurface shear stresses occurred, almost corresponded to the observed depth of spalling cracks.     

Some remarks on the engineering application of the fatigue crack growth approach under nonzero mean loads

The well-known fatigue crack growth (FCG) curves are two-parameter dependents. The range of the stress intensity factor ?K and the load ratio R are the parameters normally used for describing these curves. For engineering purposes, the mathematical representation of these curves should be integrated between the initial and final crack sizes in order to obtain the safety factors for stresses and life. First of all, it is necessary to reduce the dependence of the FCG curves to only one parameter. ?K is almost always selected and, in these conditions, considered as the crack driving force. Using experimental data from literature, the present paper shows how to perform multiple regression analyses using the traditional Walker approach and the more recent unified approach. The correlations so obtained are graphically analyzed in three dimensions. Numerical examples of crack growth analysis for cracks growing under nominal stresses of constant amplitude in smooth and notched geometries are performed, assuming an identical material component as that of the available experimental data. The resulting curves of crack size versus number of cycles (a vs. N) are then compared. The two models give approximately the same (a vs. N) curves in both geometries. Differences between the behaviors of the (a vs. N) curves in smooth and notched geometries are highlighted, and the reasons for these particular behaviors are discussed.     

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

This paper presents bi-criteria formulation of a tolerance allocation model for an interchangeable assembly to simultaneously evolve suitable combination of manufacturing facility in multiple facility shaft-hole production environments of medium- and large-scale industries and tolerances to complement the need of customers. An Exhaustive Search Procedure is used to obtain the optimal solution for small and medium size problems and simulated annealing algorithm is used for large size problems. The usefulness of the Pareto front in manufacturing tolerance allocation decisions is demonstrated with three case study problems. The effect of process capability of shaft-hole assembly manufactured from alternative manufacturing machines and the optimality is analyzed in three cases to understand their criticality in decision-making. The models discussed in this paper could be useful for medium- and large-scale manufacturing industries, where there will be a variety of manufacturing facilities (specifications, capabilities, models, and types) for making both shaft-hole assembly and play a key role to meet the tolerance and cost requirements of different customers. This paper further discusses how this formulation and methodologies can be used for two hole and two shaft assemblies and multiple shaft-hole assemblies. Finally, the paper ends with highlighting directions of future research avenues in the shaft-hole assembly.