Study of rolling contact fatigue of bearing steels in relation to various oxide inclusions

Flaking failure of rolling contact fatigue is known to originate from non-metallic inclusions, where fatigue cracks occur at subsurface region of materials. It is fundamental to understand the main factors governing the rolling contact fatigue life. Steel samples with different size and chemical composition of oxide inclusions were evaluated on rolling contact fatigue life. As a result,     

Evaluation of fatigue life of aluminium alloy wheels under bending loads

This paper presents the details of S–N curve for aluminium alloy (Al) A356.2‐T6 and fatigue life of alloy wheels under bending load of cornering fatigue test (CFT). Development of S–N curve has been carried out by conducting rotary bending fatigue test at different stress levels as per Standards IS 5075. The rotary bending fatigue test has been performed under constant amplitude fatigue loading. The CFT of the wheel in normal driving mode has been carried out as per the procedure given in Japanese Industrial Standard Disc Wheels (JIS D_4103). It has been observed from the test that the cracks are initiated at the spoke and hub joining area closer to spanner hole on the front face of the wheel. Fatigue life of the alloy wheel has been predicted by finite element analysis (FEA), simulating the realistic test conditions. From finite element analysis, it has been observed that the maximum stress occurs at the mounting face of the wheel. Further, it has been observed that there is significant difference between the computed fatigue life and experimental value. Parametric study has been carried out for reliable fatigue life estimation and proposed an appropriate safety factor for fatigue life estimation under rotary bending test.     

The effect of sulphide inclusions on fracture toughness and fatigue crack growth in 12 wt% Cr steels

The variation of plane strain fracture toughness and fatigue crack growth rates with sulphide inclusion content has been examined for four different sulphur levels in a 12 wt% Cr stainless steel. Charpy impact tests have also been conducted and the experimental results are correlated with the volume fraction of inclusions and the inter-inclusion spacing. The availability of the experimental results for the case of steam turbine blading is discussed with a hypothetical example problem.     

Application of an energy model for fatigue life prediction of construction steels under bending, torsion and synchronous bending and torsion

The paper contains a mathematical model of the material’s behaviour under cyclic loading taking into account the dynamics of the fatigue process, including the number of cycles to failure, induced by the mean stress value. The coefficients in the proposed model have been obtained from experimental tests under symmetrical and nonsymmetrical loading (with the stress ratio R=0). The proposed model has been used in order to modify an energy criterion with the aim of accounting for the influence of the mean stress on the fatigue life. The fatigue tests have been performed for structural steels 10HNAP and 18G2A subjected to cyclic bending, torsion and synchronous bending with torsion, by considering different values of the mean stress. A good agreement between the calculated and experimental results has been obtained.     

The crack growth behavior after overloading on rotating bending fatigue

Most of the crack propagation behavior relating to the variable amplitude load is conducted in flat specimens such as centered-crack specimens or compact test specimens with various thicknesses. For that reason, it is also important to understand the crack growth behavior relating to the overload on other geometry which differs from the previous investigation. In the present study, the crack growth behavior after being overloaded was investigated on a solid round bar. It was found that the size of diameter influences the crack growth behavior. The retardation or acceleration depends on the residual stress state in front of the crack tip. If the residual stress state developing was compressive, retardation was observed, and if the tensile residual stress state developed, the acceleration of the crack occurred after overloading. In addition, the shape of the component has to be taken into consideration for evaluating fatigue life in association with the crack growth.     

The effect of two-frequency low-cycle loading on the fatigue life of steels and welded joints

Some parts of nuclear power equipment (NPE) are subjected mostly to two-frequency loading during their service. Oscillations with a lower stress amplitude are superimposed on the basic slow loading with high stress (or strain) amplitude. The damage cumulation law, which is valid relatively well for random loading, seems to be less suitable for two-frequency loading. According to the design specifications [1] the service life of parts at two-frequency loading may be 10 to 20 times lower than that at single-frequency loading. The decrease in life depends on both the ratio of loading frequencies and amplitudes, and the material characteristics.Published inProblemy Prochnosti, Nos. 1–2, pp. 118–125, January–February, 1995.     

Manufacturing induced residual stress influence on the rolling contact fatigue life performance of lubricated silicon nitride bearing materials

Experimental results of the residual stress measurements on failed ceramic elements are presented in this paper. Residual stress measurements were performed through a small volume of irradiation. Residual stress relation to number of stress cycles, distance from induced ring crack, residual stress relation to contact path and measuring direction was studied. Residual stress measurements were performed on fatigue spall. A residual stress survey on the secondary cracks is also provided presented in this paper. Analysing the relationship of residual stress with rolling contact fatigue is an important study which will provide guidelines on the design process and manufacturing of these elements. Two compressive residual stress values of 213 and 228 MPa were recorded on either edge of the contact path, these values are higher compared to 84 MPa at the centre of the contact path. This phenomenon indicates that compressive residual stresses are relieved much faster during rolling contact fatigue at higher contact stress level. The spall depths in ceramic rolling contact bearing elements were found to be ranging from 100 to 148 μm. The variation of the compressive residual stress values is prominent at this depth. The decrease in the compressive residual stress values is registered in the spall region and demonstrates sub-surface damage in the region.     

A comparison of uniaxial and rotating bending fatigue tests on an acetal co-polymer

Using the same type of injection moulded specimen, uniaxial and rotating bending fatigue tests have been carried out and the results compared. In each type of testing conventional fatigue or thermal softening failures occurred depending on the loading conditions. Much higher cyclic frequencies could be used in rotating bending without causing thermal softening failure. Injection moulding produces a skin at the surface of the specimens which is more resistant to fatigue crack initiation than the internal structure. Since the maximum stress in bending is at the surface, the skin effect contributes to the much larger fatigue endurances observed in rotating bending. A sharp V-notch, a diametral hole or a moulded weld line in the specimens reduced endurances in both types of fatigue loading to various extents.     

Influence of non-metallic inclusions on fatigue life in the very high cycle fatigue regime

The present paper deals with the influence of non-metallic inclusions on fatigue life in the high cycle fatigue and the very high cycle fatigue regime. For that purpose, several castings of steel 42CrMo4 (AISI 4140, DIN EN 1.7225) were produced by using recently developed novel metal-melt filters. The specimens were tested in hot-isostatically pressed and heat treated condition. After fatigue failure every fracture surface was intensively investigated by scanning electron microscopy in order to define the type, the size, the chemical composition, the morphology and the location of the crack initiating discontinuity. Subsequently, Murakami’s √area model was used for the evaluation of the influence of non-metallic inclusions on the fatigue life. In the present investigation four common types of chemical compositions of crack initiating discontinuities were identified. Furthermore, four different internal failure types and their influence on the fatigue life in cast steel were investigated and described. Thus, the present contribution proposes a basic correlation determined from fatigue lives in case of various internal crack initiation types. The key parameters for fatigue life prediction in case of internal fatigue failure in the very high cycle fatigue regime are (i) the size of the crack initiating discontinuity, (ii) the inclusion depth and (iii) the crack initiating failure type.     

Structure analysis of aluminium silicon manganese nitride precipitates formed in grain-oriented electrical steels

We report a detailed structural and chemical characterisation of aluminium silicon manganese nitrides that act as grain growth inhibitors in industrially processed grain-oriented (GO) electrical steels. The compounds are characterised using energy dispersive X-ray spectrometry (EDX) and energy filtered transmission electron microscopy (EFTEM), while their crystal structures are analysed using X-ray diffraction (XRD) and TEM in electron diffraction (ED), dark-field, high-resolution and automated crystallographic orientation mapping (ACOM) modes. The chemical bonding character is determined using electron energy loss spectroscopy (EELS). Despite the wide variation in composition, all the precipitates exhibit a hexagonal close-packed (h.c.p.) crystal structure and lattice parameters of aluminium nitride. The EDX measurement of ~ 900 stoichiometrically different precipitates indicates intermediate structures between pure aluminium nitride and pure silicon manganese nitride, with a constant Si/Mn atomic ratio of ~ 4. It is demonstrated that aluminium and silicon are interchangeably precipitated with the same local arrangement, while both Mn^2 + and Mn^3 + are incorporated in the h.c.p. silicon nitride interstitial sites. The oxidation of the silicon manganese nitrides most likely originates from the incorporation of oxygen during the decarburisation annealing process, thus creating extended planar defects such as stacking faults and inversion domain boundaries. The chemical composition of the inhibitors may be written as (AlN)_x(SiMn_0.25N_yO_z)_1 − x with x ranging from 0 to 1.     

复合材料高周弯曲疲劳试验与寿命预测

利用一阶弯曲共振现象,开展了复合材料悬臂梁高周弯曲疲劳试验。为了取代传统的金属疲劳理论,根据复合材料疲劳损伤渐进扩展的特点,发展了新的数值方法应用于复合材料的疲劳分析。研究局部疲劳损伤模型和周期跳跃技术,开发了复合材料悬臂梁高周弯曲疲劳的半解析法Matlab疲劳损伤分析程序;另一方面,通过开发UMAT子程序,实现了疲劳损伤模型和周期跳跃技术在商业有限元软件ABAQUS中的应用。分别使用半解析法和有限元法分析复合材料悬臂梁高周弯曲疲劳的损伤累积破坏过程,预测了其高周弯曲疲劳寿命,数值预测结果与试验结果较好吻合。     

The effect of laser power density on the fatigue life of laser-shock-peened 7050 aluminium alloy

Laser shock peening (LSP) is an innovative surface treatment method that can result in significant improvement in the fatigue life of many metallic components. The process produces very little or no surface profile modification while producing a considerably deeper compressive residual stress layer than traditional shot peening operations. The work discussed here was designed to: (a) quantify the fatigue life improvement achieved by LSP in a typical high strength aircraft aluminium alloy and (b) identify any technological risks associated with its use. It is shown that when LSP conditions are optimal for the material and specimen configuration, a —three to four times increase in fatigue life over the as-machined specimens could be achieved for a representative fighter aircraft loading spectrum when applied at a representative load level. However, if the process parameters are not optimal for the material investigated here, fatigue lives of LSP treated specimens may be reduced instead of increased due to the occurrence of internal cracking. This paper details the effect of laser power density on fatigue life of 7050-T7451 aluminium alloy by experimental and numerical analysis.     

Effect of oxide additive in silicon nitride on interfacial structure and strength of silicon nitride joints brazed with aluminium

Silicon nitride ceramics with Y₂O₃ and Al₂O₃ as sintering additives were brazed with aluminium, and the brazed strength and the interfacial structure of the joints were compared with those of the joints made of additive-free silicon nitride ceramics. It is concluded that the additives in silicon nitride ceramics take part in the interfacial reaction, make the reaction layer thicker, and hence increase the brazed strength greatly.     

Simulated extreme value fatigue sensitivity to inclusions and pores in martensitic gear steels

The objective of this work is to model the sensitivity of high cycle fatigue resistance of secondary hardening martensitic gear steels to variability in extrinsic inhomogeneities such as primary inclusions, and pores, coupled with intrinsic microstructure variability. A simplified approach is presented to quantify the variability in the driving force for fatigue crack formation in the matrix at non-metallic inclusions and pores in lath martensitic gear steels using a three-dimensional crystal plasticity constitutive model. The utility of a simulation-based strategy for exploring sensitivity of minimum fatigue lifetime (low probability of failure) to microstructure lies in its inherent capability to consider parametric simulations of hundreds of inclusions and microstructures in contrast to limited numbers of physical experiments. Experiments are used to calibrate the polycrystalline cyclic stress–strain response and mean (50% probability) fatigue crack formation life. Several remote loading conditions are considered in the high cycle fatigue (HCF) regime relevant to typical gear applications. Idealized inhomogenieties (spherical) in the form of hard (Al₂O₃), soft inclusions (La₂O₂S), and pores are systematically investigated in this parametric computational study. Relations between remote loading conditions and local plasticity are discussed as a function of stress amplitude and microstructure. The maximum plastic shear strain range is used in the modified form of Fatemi–Socie parameter evaluated at the grain scale as a measure of the driving force for fatigue crack formation (nucleation and early growth to lengths on the order of several times the average grain size). Multiple realizations of the polycrystal microstructure are considered to obtain a statistical distribution of this fatigue indicator parameter (FIP). The results are used to construct an extreme value Gumbel distribution of the FIPs for the selected microstructures. Subsequently, a computational micromechanics based minimum life estimate that corresponds to 1% fatigue crack formation probability is calculated.