INFLUENCE OF CONTACT LOADING ON FRETTING FATIGUE BEHAVIOUR

Abstract— Fretting induced cracking is commonly observed in industrial components that are in contact and are subjected to small oscillatory movements between them. Fretting causes a considerable reduction in fatigue strength. In this paper recent knowledge on the short and long crack growth behaviour is applied to estimate crack propagation and fatigue life in fretting. The model is based on mode I stress intensity factors with a threshold modified for short cracks. The predicted results are compared with experiments and the influence of the contact pressure is examined. A good correlation between predictions and experimental results are obtained for crack growth rates as well as fatigue lives in terms of number of cycles to failure. It is seen that the increase of fatigue life observed for contact pressures above a certain level can be predicted by the crack growth model.     

Fatigue strength of gas turbine compressor blades

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed into the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied considering the effect of the medium (sea water) and stress ratio in a cycle, as well as fatigue strength of newly-manufactured blades and those after being in operation. Specific features of fatigue crack propagation in blades have been considered and a method for predicting the life of blades with cracks has been justified.     

Fracture mechanics and scale effects in the fatigue of railway axles

Fatigue of railway axles is one of the basic problems of fatigue. However, in spite of the criticality of this component, modern approaches have not been used for addressing a critical revision of traditional design. The scope of this paper is to study the scale effects in fatigue limit and in crack growth rate for a high strength steel used for high speed railway axles.Fatigue limit tests on micro-notched specimens led to the determination of fatigue thresholds for small cracks of the examined steel. This allowed us to successfully analyse the `scale effect' and the fatigue strength of full-scale axles in terms of threshold stress for short cracks emanating from small non-metallic inclusions.A series of crack propagation tests on small scale specimens lead to the definition of an EPFM crack propagation model which has been successfully compared with propagation data on full-scale components. These results support the application of the crack propagation model for the determination of axle inspection intervals.     

Manifestation of Hydrogen and Low-Temperature Brittleness in Near-Threshold Cyclic Crack Resistance of Materials

We experimentally show that the realization of conditions of plane deformation at the tip of a fatigue crack is not sufficient for guaranteeing the unique dependence of the crack growth rate on the range of the stress intensity factor, which is explained by the effect of crack closure. We describe advantages and disadvantages of the effective range of the stress intensity factor as a parameter that determines the mechanical conditions for the propagation of a fatigue crack. We analyze the phenomenon of positive influence of strengthening factors (a decrease in the temperature of testing and hydrogenation) on the cyclic crack resistance of materials in a low-amplitude range of loading determined with regard for the effect of crack closure. The decrease in the crack growth rate and the increase in fatigue thresholds are intensified as the level of loading decreases and the ductility of materials increases. Differences in the influence of strengthening factors in low- and high-amplitude ranges of loading are explained by different mechanisms of fracture controlled by the shearing strength and the tensile strength, respectively. We give several examples of the mechanical behavior of materials that show the inversion of the influence of hydrogen on the resistance to fracture: fatigue fracture of smooth steel specimens in gaseous hydrogen, high-temperature corrosion fatigue of preliminary hydrogenated titanium alloys, and the influence of hydrogenation on the wear resistance of structural steels in the process of friction and cavitation and on the parameters of cutting of a tool steel.     

THE TWO THRESHOLDS OF FATIGUE BEHAVIOUR

Two limiting thresholds to fatigue crack propagation are discussed. The first threshold is related to the microstructural texture and this threshold may therefore be deemed a material-based threshold. The second threshold is mechanically-based, and is related only to the stress state at the tip of a substantial defect. The material-based threshold is characterized in terms of Microstructural Fracture Mechanics (MFM) and the mechanically-based threshold is characterized in terms of Linear Elastic Fracture Mechanics (LEFM). The former condition is important when considering the fatigue limit of materials and components, while the latter is more applicable to the fatigue limit of structures. The different factors which affect the two threshold conditions are briefly presented. Finally, this paper discusses aspects of MFM relevant to the fatigue resistance of metals and components.     

Fatigue strength and life of compressor blades for marine gas turbine engines

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed on the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on the fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied taking into account the effect of the medium (sea water), stress ratio in a cycle, and programmed mode of loading. The authors also consider fatigue strength of newly-manufactured blades and those in operation, as well as specific features of fatigue crack propagation in blades. They have substantiated a method for predicting the life of blades with cracks. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 28–40, January–February, 1999.     

Propagation of fatigue cracks in mixed loading

A model is proposed of propagation of a fatigue crack which makes it possible to determine its growth rate at different ratios of K_I and K_II as well as during the variation of the stress ratio. The model can be used to determine the ratio of the stress intensity factors of the first and second kind corresponding to the threshold growth rate of the fatigue crack. The results of the calculations are compared with experimental data.Translated from Problemy Prochnosti, No. 3, pp. 3–8, March, 1990.     

Fracture mechanics assessment of railway axles: Experimental characterization and computation

The results of a joint research project aiming at developing validated fracture mechanics assessment procedures for railway axles are presented. Experimentally determined fatigue crack growth parameters for the commonly used axle steel 25CrMo4 (A4T) and the high strength steel 34CrNiMo6 are included in the range of stable crack propagation and near threshold. The results are employed for predicting fatigue crack growth for cracks initiating at the axle shaft. For the computational modelling of fatigue crack propagation a generally applicable solution for stress intensity factors has been derived. Furthermore, the influence of variable amplitude loading (block loading) on the crack propagation behaviour has been studied and is discussed. The computational results are in good agreement with experimental data determined on standard fracture mechanics specimens as well as down-scaled and geometrically similar axle specimens.     

AN ANALYSIS OF FRICTIONAL EFFECTS ON CYLINDRICAL MODE III FATIGUE CRACK PROPAGATION SPECIMENS

Abstract— A model predicting the magnitude of frictional effects from fracture surface roughness on mode III fatigue crack growth is presened. Analysis of published data indicates that fracture surface roughness of the order of micrometers or less is enough to account for mode III fatigue crack growth retardation observation for increasing crack lengths for growth at constant Δ K . The model suggests that high strength materials will exhibit a greater resistance to shear crack growth than low strength materials. It also suggests that the resistance to shear crack growth will be more prominent at low nominal applied shear stress. The results of the analysis suggest that the concept of similitude does apply to mode III fatigue crack growth when the effects of friction on the stress intensity factor are included.     

Environmental fatigue crack propagation in medium strength titanium sheet alloys

Results are reported for an investigation of environmental fatigue crack propagation resistance in four commercial titanium alloys of medium strength. The materials were IMI 130 (commercially pure titanium with low oxygen content), Ti-70 (commercially pure titanium with high oxygen content), IMI 230 (Ti-2.5 Cu) and Ti-5Al-2.5Sn. The environments were dry argon, normal air, distilled water and 3.5% aqueous NaCl. The conclusions were (1) the ranking of the materials in terms of conventional mechanical properties does not permit a ranking in terms of crack propagation resistance, (2) the material with the highest elastic moduli, Ti-5Al-2.5Sn, also had the best crack propagation resistance in the absence of stress corrosion, (3) there is a correspondence between the degree of isotropy of the static yield strength and the orientation dependence of crack propagation resistance, (4) for all the materials there was a trend of higher crack growth rates at similar ΔK values in the order; dry argon, air, distilled water, 3.5% aqueous NaCl, (5) in the aqueous environments only Ti-5Al-2.5Sn gave evidence of stress corrosion cracking.     

Influence of effective stress intensity factor range on mixed mode fatigue crack propagation

ABSTRACT The behaviour of fatigue crack propagation of rectangular spheroidal graphite cast iron plates, each consisting of an inclined semi‐elliptical crack, subjected to axial loading was investigated both experimentally and theoretically. The inclined angle of the crack with respect to the axis of loading varied between 0° and 90°. In the present investigation, the growth of the fatigue crack was monitored using the AC potential drop technique, and a series of modification factors, which allow accurate sizing of such defects, is recommended. The rate of fatigue crack propagation db/dN is postulated to be a function of the effective strain energy density factor range, ΔS_eff. Subsequently, this concept is applied to predict crack growth due to fatigue loads. The mixed mode crack growth criterion is discussed by comparing the experimental results with those obtained using the maximum stress and minimum strain energy density criteria. The threshold condition for nongrowth of the initial crack is established based on the experimental data.     

Inclusion-controlled high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel

Axial loading fatigue tests were carried out to study the influence of inclusion on high cycle fatigue behavior of a high V alloyed powder metallurgy cold-working tool steel (AISI 11). The fatigue strength of 1538 MPa with endurance life of 10⁷ cycles were obtained by stair-case method. The fatigue specimens were also subjected to a constant maximum stress of 1650 MPa to investigate the relationship among inclusion origin size (10-30 μm), fish-eye size (70-130 μm) and fatigue life (10⁵-10⁷ cycles). The fatigue life was found to be dependent on the inclusion size and the crack propagating length. A compressive residual stress of 300-450 MPa turned out to be present at the specimen surface, and finally induced the interior failure mode. Further investigation into the correlation between stress intensity factors of inclusion origin and corresponding stages of fatigue crack growth and fatigue life revealed that the high cycle fatigue behavior was controlled by crack propagation. According to the fractographic investigation, two distinct zones were observed in fish-eye, representing Paris-Law and fast fatigue crack growth stage, respectively. Threshold stress intensity for crack propagation of 3.9 MPa√m was obtained from the well correlated line on the ΔK_I-log N_? graph. The fracture toughness can also be estimated by the mean value of stress intensity factor ranges for fish-eye.     

Analyses of the fatigue crack propagation process and stress ratio effects using the two parameter method

In situ SEM observations (Zhang JZ. A shear band decohesion model for small fatigue crack growth in an ultra-fine grain aluminium alloy. Eng Fract Mech 2000;65:665–81; Zhang JZ, Meng ZX. Direct high resolution in-site SEM observations of very small fatigue crack growth in the ultra fine grain aluminium alloy IN 9052. Script Mater 2004;50:825–28; Halliday MD, Poole P, Bowen P. New perspective on slip band decohesion as unifying fracture event during fatigue crack growth in both small and long cracks. Mater Sci Technol 1999;15:382–90) have revealed that fatigue crack propagation in aluminium alloys is caused by the shear band decohesion around the crack tip. The formation and cracking of the shear band is mainly caused by the plasticity generated in the loading part of a load cycle. This shear band decohesion process has been observed to occur in a continuous way over the time period during the loading part of a cycle. Based on this observation, in this study, a new parameter has been introduced to describe fatigue crack propagation rate. This new parameter, da/dS, defines the fatigue crack propagation rate with the change of the applied stress at any moment of a stress cycle. The relationship between this new parameter and the conventional da/dN parameter which describes fatigue crack propagation rate per stress cycle is given.Using this new parameter, it is proven that two loading parameters are necessary in order to accurately describe fatigue crack propagation rate per stress cycle, da/dN. An analysis is performed and a general fatigue crack propagation model is developed. This model has the ability to describe the four general type of fatigue crack propagation behaviours summarised by Vasudevan and Sadananda (Vasudevan AK, Sadananda K. Fatigue crack growth in advanced materials. In: Fatigue 96, Proceedings of the sixth international conference on fatigue and fatigue threshold, vol. 1. Oxford: Pergamon Press; 1996. p. 473–8).     

Resistance-curve method for predicting propagation threshold of short fatigue cracks at notches

A new resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks near the notch root. The resistance curve was constructed in terms of the experimentally determined threshold value of the maximum stress intensity factor which was the sum of the threshold effective stress intensity range ΔK_effth and the opening stress intensity factor K_opth The ΔK_effth value was constant, irrespective of crack length or notch geometry. The relation between K_opth and crack length was independent of notch geometry. The predicted effects of the notch-root radius and the notch depth on the propagation threshold of short fatigue cracks were compared with the experimental data obtained using center-notched specimens with various notch-root radii and single-edge notched specimens with various notch depths. Excellent agreement was obtained between predictions and experiments.     

Relation between the fatigue-failure diagram and fatigue curve

A method of converting the endurance curve to a fatigue-failure diagram is developed theoretically. The method consists in the solution of a system of three equations of the number of cycles of specimen failure for different levels of applied stress, as determined by the integration of the relationship between rate of crack growth and the stress-intensity factor, and also in the determination of three unknowns — the threshold stress-intensity factor and two parameters of the Paris equation. Confirmation of the method on the basis of published data on the endurance and crack resistance of steels subjected to cyclic loading indicated its applicability for practical calculations. Use of the method makes it possible to increase the informativeness of fatigue tests and to obtain a large amount of additional information on the results of numerous experimental investigations of the fatigue strength of materials.Translated from Problemy Prochnosti, No. 11, pp. 38–44, November, 1991.     

Effect of material plasticity on fatigue crack propagation under complex stress state

The maximum energy release rate criterion, i.e., G _max criterion, is commonly used for crack propagation analysis. However, this fracture criterion is based on the elastic macroscopic strength of materials. In the present investigation, a modification has been made to G _max criterion to implement the consideration of the plastic strain energy. This criterion is extended to study the fatigue crack growth characteristics of mixed mode cracks in steel pipes. To predict crack propagation due to fatigue loads, a new elasto-plastic energy model is presented. This new model includes the effects of material properties like strain hardening exponent n, yield strength σ_y and fracture toughness and stress intensity factor ranges. The results obtained are compared with those obtained using the commonly employed crack growth law and the experimental data.     

Prediction of short fatigue crack growth of Ti‐6Al‐4V

It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.     

Fatigue fracture of AMg6N alloy under loading at sound frequencies

Cyclic crack growth resistance tests of AMg6N alloy under loading at frequencies of 20 to 10 kHz have demonstrated that the rate of fatigue crack propagation decreases with increasing frequency and the threshold stress intensity factors increase exponentially with the frequency of strain cycling. Fractographic observations of fracture surfaces of the specimens have revealed that an increase in the loading frequency is accompanied by a decrease in the fatigue striation spacing and in the size of the striation microzones by intnsifying the processes of secondary cracking and the formation of fretting products. This leads to a decrease in the rate of fatigue crack propagation and an increase in the threshold values of the stress intensity factors. An increase in the asymmetry of the loading cycles reduces the contribution of delamination and the formation of fretting products to the process of fracture of the alloy and results in a smaller fraction of the striation relief and in an earlier occurrence of the elements of quasistatic fracture by dimples, which is the cause of the reduction in characteristics of the cyclic crack growth resistance under asymmetrical loading. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 94–105, March–April, 1999.     

HIGH CYCLE FATIGUE AND THRESHOLD BEHAVIOUR OF POWDER METALLURGICAL Mo AND Mo-ALLOYS

Abstract A detailed characterization of the room temperature fatigue properties of powder metallurgical Mo, Mo–W and Ti–Zr–Mo (TZM) alloys is presented. In particular the factors affecting fatigue crack nucleation and growth behaviour are described. Fatigue tests were carried out by conventional rotating-bending and compared with results from a time-saving 20 kHz resonance push-pull test method. Fatigue strength data were determined by a statistical evaluation of test results from a sufficiently large number of specimens. The results show an increase in fatigue strength with alloying additions. Fatigue cracks were observed nucleating at highly localized slip bands at the specimen surface with the fatigue crack zones comprising only a small fraction of the total specimen cross-section. Fatigue crack growth rates at low stress intensities and threshold stress intensity values for crack growth were determined for a stress ratio of R =– 1 using a 20 kHz resonance test method. These latter values were found to be sensitively dependent on microstructure, composition and processing history.     

Brief Review and Reconsideration of Fatigue Crack Closure Effect in Materials

The state-of the-art of fatigue crack closure effect, involving the mechanisms, measurements,models, evaluation methods, innuencing factors and existing problems is briefly reviewed. Attention is concentrated on a new consideration of closure effect and a re-definition of the effective d riving force for the propagation of a fatigue crack. A theoretical basis fOr measuring the closure effect is established. The proposed closure concept takes into account the role of the lower portion of loading cycle below the opening load. The conventionaI closure concept is proven to be correct only for some extreme cases which are practically non-existent fOr engineering materials. A miniature strain gauge tech nique is developed to measure the crack closure. Stressratio-change tests at threshold level clearly indicate the shortcomings of the conventional closure concept, and the validity of the newly-proposed closure concept. The application of the proposed concept can give rise to a new method for the determination of the effective fatigue threshold of materials. If the fatigue threshold is measured at a sufficiently negative stress ratio (R相似文献