Surface crack growth of silicon nitride bearings under rolling contact fatigue

Surface crack growth of silicone nitride ceramic bearings under rolling contact fatigue has been investigated from the viewpoints of contact stresses (ring crack model) and fluid pressure (wedge effect model). The mechanisms of these two models have been investigated independently; however, it was impossible to separate the effects of contact stresses and fluid pressure on surface crack growth. In this paper the effects of contact stresses (ring crack model) on surface crack growth are investigated. In the ring crack model the crack growth is caused by contact stresses around the circumference of the contact circle. The growth of surface cracks located inside and outside the contact track was observed in order to obtain data from which we could reexamine the ring crack model. The outside cracks under rolling contact fatigue were propagated by contact stresses alone and also the inside cracks grew as slowly as the outside cracks. We concluded that the cracks are propagated by the single effect of contact stresses. Preliminary observations of surface crack growth showed that the cracks were unaffected by wear and residual stresses.     

Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si₃N₄SiC_W, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation.     

Cyclic fatigue of a silicon nitride: influence of frequency and fatigue mechanism

An investigation has been carried out on the slow crack growth behaviour of an advanced Si₃N₄ ceramic material at room temperature at different loading frequencies. The results clearly show a detrimental effect of cyclic loading on crack growth rate in terms of time and a reduced crack growth resistance with increasing cyclic frequency. Crack growth rates can be described by the Paris power-law expression for both static and cyclic loading, but the exponent n increases with decreasing loading frequency. Further support for the existence of mechanical fatigue in this material is provided from experiments involving alternate cyclic and static fatigue using the same specimen, which show substantial differences in crack growth rate in terms of time. Removal of crack wakes resulted in an unchanged crack growth rate under sustained load, which suggests that the crack wake does not play a key role in enhanced crack growth under cyclic loading. The likely crack growth mechanism is discussed.     

Effect of frequency on ambient temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite

A detailed study on a silicon nitride reinforced with silicon carbide whiskers has been undertaken on room temperature fatigue during static and dynamic loading at constant ΔK. It is shown that sub-critical crack growth rates are lower when the material experiences sustained far field loading than during cyclic far field loading. The increased crack growth rate during cyclic loading is attributed to a wedging effect within the crack wake causing an increase in the tensile stress and resultant increased micro-cracking ahead of the crack tip. This additional micro-structural damage leads to enhanced sub-critical crack growth rates during cyclic loading. The asperities that are responsible for the wedging effect are attributed to the isolation of small portions of material due to branching of small cracks and by degradation of the bridging SiC whiskers and Si₃N₄ grains within the crack wake.     

A model for fatigue crack growth with a variable stress intensity factor

A model for fatigue crack growth, similar to that of Majumdar and Morrow, is proposed where the crack growth rate is determined from the low cycle fatigue and cyclic stress-strain response of the material. The model is for a constant stress range at infinity, but does allow for a variable stress intensity factor due to the changing crack length. The study also includes an analysis of the strain range in the neighborhood of the crack tip. Further it is shown that the model predicts the critical stress intensity factor. A prediction of the crack growth rate is made for 2024-T351 aluminium, copper and CU-6.3 AL alloy and is compared to the experimental observations.     

Measuring stress intensity factors during fatigue crack growth using thermoelasticity

Thermoelastic stress analysis has been developed in recent years as a direct method of investigating the crack tip stresses in a structure under cyclic loading. This is a consequence of the fact that stress intensity factors obtained from thermoelastic experiments are determined from the cyclic stress field ahead of a fatigue crack, rather than inferred from measurement of the crack length and load range. In the present paper the results of fatigue crack growth tests performed on welded ferritic steel plates are reported. From the results it can be observed that the technique is sensitive to the effects of crack closure and the presence of tensile and compressive residual stresses due to welding.     

Effect of crystallization on room-temperature fatigue crack growth resistance in a SiC-reinforced silicon nitride composite

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Cyclic fatigue of reaction-bonded silicon nitride at elevated temperatures

Cyclic and static loading tests were performed on reaction-bonded silicon nitride from 1000–1400 °C in air. This porous, fine-grained material contained no glassy grain-boundary phase and exhibited no slow crack growth at room temperature. Under cyclic loading, the crack-growth behaviour at 1000 °C was similar to room-temperature results; however, at 1200 and 1400 °C crack-growth rates increased significantly. Under static loading, significant crack growth was detected at 1000 °C and increased with temperature. Most of the crack growth under cyclic loading was attributed to slow crack-growth mechanisms, but evidence of cyclic crack-growth mechanisms were also observed. Oxidation played a major role in crack-growth velocity at high temperature. This revised version was published online in November 2006 with corrections to the Cover Date.     

Prediction of threshold stress intensity for fatigue crack growth using a dislocation model

It has been shown that the ratio of threshold stress intensity for fatigue crack growth to the shear modulus is nearly a constant for many materials. This implies that fatigue crack growth is related to some fundamental phenomenon occurring at the crack tip. In the following a dislocation model has been developed to predict the threshold stress intensity. It is shown that the stress intensity can be related to the stress necessary to nucleate a dislocation at the crack tip. The most important outcome of the present analysis is that the threshold stress intensity depends more on the elastic modulus rather than on any other material property in agreement with many experimental results.

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Résumé On a démontré que le rapport de l'intensité de seuil de la contrainte provoquant une fissuration par un accroissement de la fissuration par fatigue au module de cisaillement est sensiblement une constante pour de nombreux matériaux. Ceci implique que la croissance d'une fissure de fatigue est reliée à certains phénoménes fondamentaux qui se produisent à l'extrémité d'une fissure. Dans le mémoire, on développe un modèle de dislocation qui permet de prédire l'intensité critique de la contrainte. On montre que l'intensité de la contrainte peut être mise en relation avec la contrainte nécessaire pour créer une dislocation à l'extrémité d'une fissure. La conséquence la plus importante de cette analyse est que l'intensité critique de seuil dépend davantage du module d'élasticité que de toutes autres propriétés du matériau et ce en accord avec de nombreux résultats expérimentaux.

     

Flaking failure originating from a single surface crack in silicon nitride under rolling contact fatigue

Flaking failure caused by surface cracks of silicon nitride ceramic bearings has been investigated from the viewpoint of the ring crack model. However, the relation between surface and subsurface cracks under rolling contact fatigue is not fully understood. In this investigation subsurface cracks branching from an initial surface crack were observed in detail, and the process of flaking failure was investigated. The specimens were observed prior to the separation of the surface layers and it was found that the initial surface cracks grew vertically to the surfaces and did not curve as predicted by the ring crack model. Subsurface cracks branched from the single surface cracks and grew in a direction parallel to the surface. They grew in both the same and the opposite directions to the ball movement, with small upward and downward branches. These subsurface cracks grew prior to the semi‐circular surface cracks. From these observations it was concluded that the flaking failures are not caused directly by the surface cracks, but by the subsurface cracks that branch from them.     

A plasticity-corrected stress intensity factor for fatigue crack growth in ductile materials under cyclic compression

For prediction of the fatigue crack growth (FCG) behavior under cyclic compression, a plasticity-corrected stress intensity factor (PC-SIF) range ΔK_pc is proposed on the basis of plastic zone toughening theory. The FCG behaviors in cyclic compression, and the effects of load ratio, preloading and mean load, are well predicted by this new mechanical driving force parameter. Comparisons with experimental data showed that the proposed PC-SIF range ΔK_pc is an effective single mechanical parameter capable of describing the FCG behavior under different cyclic compressive loading conditions.     

Phenomenology of the effective stress intensity related to fatigue crack growth thresholds

The fatigue crack growth threshold conditions for effective stress intensity amplitude are examined using simple phenomenological models for crack face interference and internal stresses. We show that behaviors correlating with all pure fatigue classifications can be generated from a single ‘ideal fatigue’ behavior by accounting for internal stress and crack face interference. The possible threshold or near-threshold manifestations of an intrinsic K_MAX threshold, independent of effective ΔK effects, are discussed.     

Cyclic fatigue of hot isostatically pressed silicon nitride at elevated temperatures

Cyclic fatigue properties of a hot isostatically pressed silicon nitride were investigated at 1150, 1260 and 1370 °C in ambient air. The uniaxial tensile tests were conducted under various cyclic loading wave forms and frequencies. The correlation of stress-life relations between cyclic and static fatigue results was evaluated. At 1150–1370 °C, cyclic loading caused less damage than static loading, as evidenced by the longer failure time under cyclic loading versus static loading with the same maximum applied stresses. The cyclic loading effect was more pronounced in high frequency tests at 1260 and 1370 °C and might be related to the viscoelastic behaviour of the intergranular phase. Microstructural analyses and macroscopic cyclic stress-strain and strain-time relations indicated that cyclic loading/unloading may inhibit the normal accumulation of creep damage.     

Cyclic analysis of a propagating crack and its correlation with fatigue crack growth

Stress and strain field of a propagating fatigue crack and the resulting crack opening and closing behavior were analysed. It was found that a propagating fatigue crack was closed at tensile external loads due to the cyclically induced residual stresses. Strain range value $\text{Δ?}{}_{\mathrm{y}}$ in the vicinity of the crack tip was found to be closely related with the effective stress intensity factor range $\text{ΔK}{}_{\mathrm{eff}}$ which was determined on the basts of the analytical crack opening and closing behavior at its tip. Application of this analysis to the non-propagating fatigue crack problem and the fatigue crack propagation problems under variable stress amplitude conditions revealed that both $\text{Δ?}{}_{\mathrm{y}}$ and $\text{ΔK}{}_{\mathrm{eff}}$ were essential parameters governing fatigue crack growth rate.