Showing papers in "Fatigue & Fracture of Engineering Materials & Structures in 2002"

Characteristic S-N properties of high-carbon-chromium-bearing steel under axial loading in long-life fatigue

Abstract: Owing to difficult economical conditions, machines and structures often have to be used beyond the design lifetime. In this study, fatigue properties of a bearing steel in the long-life region were experimentally examined under cyclic axial loading. The complicated S-N behaviour was well explained as a combination of S-N curves for surface-induced fracture and interior inclusion-induced fracture. Fish-eye marks were always observed on the fracture surfaces of specimens, which failed in the latter fracture mode, and an inclusion was found at the center of the fish-eye. Finally, it was found that the fatigue fracture of this steel in the long-life region occurred through the following three processes: (i) formation of the characteristic area as a fine granular area (FGA), (ii) crack propagation to form the fish-eye and (iii) rapid crack propagation to cause the catastrophic fracture.

An engineering model for prediction of rolling contact fatigue of railway wheels

Abstract: An engineering model for rolling contact fatigue (RCF) of railway wheels is developed. Three well-known types of fatigue in wheels – surface-initiated fatigue, subsurface-initiated fatigue and fatigue initiated at deep material defects – are accounted for. Fatigue impact is quantified by three fatigue indices expressed in analytical form. The model can easily be integrated in a multibody dynamics code without significantly increasing computational demands. A powerful tool for optimizing train–track configurations with respect to fatigue performance should result. In this paper, theoretical foundations, benefits and limitations of the model are presented. An example of a postprocessing analysis of data from a dynamic simulation of train–track interaction is given.

A bi-parametric Wöhler curve for high cycle multiaxial fatigue assessment

Abstract: This paper presents a method for estimating high-cycle fatigue strength under multiaxial loading conditions. The physical interpretation of the fatigue damage is based on the theory of cyclic deformation in single crystals. Such a theory is also used to single out those stress components which can be considered significant for crack nucleation and growth in the so-called Stage I regime. Fatigue life estimates are carried out by means of a modified Wohler curve which can be applied to both smooth and blunt notched components, subjected to either in-phase or out-of-phase loads. The modified Wohler curve plots the fatigue strength in terms of the maximum macroscopic shear stress amplitudes, the reference plane - where such amplitudes have to be evaluated - being thought of as coincident with the fatigue microcrack initiation plane. The position of the fatigue strength curve also depends on the stress component normal to such a plane and the phase angle as well. About 450 experimental data taken from the literature are used to check the accuracy of the method under multiaxial fatigue conditions.

Mechanism of fatigue failure in ultralong life regime

Abstract: The fatigue fracture surfaces of specimens of heat treated hard steels which failed in the regime of N =10 5 to 5 x 10 8 cycles , were investigated by optical microscopy and SEM. Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark and in the previous paper it was named the Optically Dark Area, ODA. The roughness inside ODA is larger than outside ODA. The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of ultra long life fatigue failure. Direct evidences of existence of hydrogen at the inclusion at fracture origin are presented. It is presumed that the ODA is made by the cyclic stress coupled with the hydrogen which is trapped by the inclusion at the fracture origin. To verify the influence of hydrogen, specimens containing different levels of hydrogen were prepared by different heat treatments. The results obtained by fatigue tests of these specimens suggest that the hydrogen trapped by inclusions is a crucial factor which causes the ultra long fatigue failure of high strength steels. Aspects of the double S-N curve are also discussed in terms of experimental methods, specimen size and statistical distribution of inclusions sizes.

Fatigue crack propagation behaviour derived from S–N data in very high cycle regime

Abstract: The crack propagation law was derived from the S-N data in the very high cycle fatigue of a bearing steel. The propagation rate, da/dN (m/cycle), of surface cracks was estimated to be a power function of the stress intensity range, ΔK (MPa√m) with the coefficient C s = 5.87 x 10- 13 and the exponent m s = 4.78. The threshold stress intensity range was 2.6MPa√m. The crack propagation from internal inclusions was divided into Stages I and II. For Stage I, the coefficient of the power law was C o = 3.44 x 10 -21 and the exponent m o = 14.2. The transition from Stage I to II took place at ΔK = 4.0 MPa√m. For Stage II, the coefficient was C i = 2.08 x 10 -14 and the exponent m i = 4.78. The specimen size and loading mode did not influence the surface fatigue life, while the internal fatigue life was shortened in larger specimens and under tension-compression loading. For ground specimens, the surface fatigue life was raised by the compressive residual stress, while reduced by the surface roughness introduced by grinding. For shot-peened specimens, fatigue fracture did not take place from the surface because of a high surface compressive residual stress. The internal fatigue life was reduced by the tensile residual stress existing in the interior of the specimens.

On ‘multi-stage’ fatigue life diagrams and the relevant life-controlling mechanisms in ultrahigh-cycle fatigue

Abstract: In recent years, several fatigue studies on different metallic materials have indicated that fatigue failures can occur even at amplitudes below the conventional high-cycle fatigue (HCF) fatigue limit in the gigacycle or ultrahigh-cycle fatigue (UHCF) range (number of cycles to failure in excess of ca. 10 7 -10 8 ). In the latter case, fatigue failures were observed to originate from internal defects (non-metallic inclusions). The S-N curves displayed a multi-stage shape, sometimes with a second lower fatigue limit in the UHCF range. The present paper specifies the conditions under which internal and/or surface fatigue failure can occur at low load levels below the conventional HCF fatigue limit. The relevant fatigue thresholds are discussed for two classes of materials, namely pure single-phase metallic materials (type I) without internal defects and materials such as steels or cast materials (type II) with internal defects (inclusions or pores). In the case of type II materials, the probability of surface versus internal fatigue is discussed in terms of the volume density, size and location of the inclusions and the relevant cracking mechanisms. It is argued that, in both type I and type II materials, multi-stage S-N curves or Manson-Coffin plots can be expected under certain conditions.

Local texture and fatigue crack initiation in a Ti-6Al-4V titanium alloy

Abstract: Fatigue crack initiation was studied in a bimodal TA6V titanium alloy. A ghost structure inherited from the forging process, the scale of which is roughly 100 times the apparent grain size, was found to govern the initiation process. In these macrograins, that we have labelled macrozones, most of the primary alpha grains (αp) are found to display the same crystallographic orientation. Fatigue cracks are initiated on the basal plane or, if basal slip is difficult, on the prismatic plane. Thus in macrozones, where basal or prismatic slip is easy, numerous neighbouring tiny cracks appear over the whole macrozone, which have the size of the primary αp grains. In these macrozones the contribution of crack coalescence to crack growth is consequently very significant. On the contrary, if basal and prismatic slips are both difficult in the macrozone, no crack can be found in the corresponding macrozone. The crack initiation process is thus highly heterogeneous at the scale of the macrozone. Furthermore, this microstructure is found to induce a large scatter in the fatigue life of notched samples.

Very high‐cycle fatigue behaviour of shot‐peened high‐carbon–chromium bearing steel

Abstract: Effect of shot-peening on fatigue behaviour in the gigacycle regime was investigated in order to clarify the duplex S-N curve characteristics. Cantilever-type rotary bending fatigue tests were performed in laboratory air at room temperature by using hourglassshaped specimens of high-carbon-chromium bearing steel, JIS SUJ2. Fatigue crack initiation site changed from the surface of untreated specimen to the subsurface of the specimen because of hardening and compressive residual stress with shot-peening in the region of high-stress amplitude. On the other hand, no difference in fatigue life controlled by the subsurface crack initiation between untreated specimen and shot-peening one was observed in high-cycle region. It was suggested that the S-N curve corresponding to the internal fracture mode is inherent in the material, as compared with the S-N curve of surface fracture mode, which is affected by surface conditions, environmental conditions and so on. Subsurface crack initiation and propagation behaviour were discussed under the detailed measurement of crack initiation area and shape of the fish-eye fracture surface.

High-cycle rotating bending fatigue property in very long-life regime of high-strength steels

Abstract: In order to investigate the fatigue properties of high-strength steels in the very long-life regime up to over 10 9 cycles, cantilever-type rotating bending fatigue tests were carried out for two kinds of high-strength steels, SUJ2 and SNCM439, which were machined by grinding and finished by electropolishing after grinding. And also, the residual stress on the specimen surface of the ground specimen was examined by X-ray diffractometer in order to investigate effects of the residual stress on the fatigue properties. From the investigations, the S-N curves clearly have a tendency to decrease again in the longer-life range over 10 7 cycles for both types of specimen and for both steels. From observations of fracture surfaces, it was found that fatigue crack origins could be grouped into two types: (i) 'surface crack origin type' in the shorter-life regime and (ii) 'internal crack origin type' in the longer-life regime.

Fretting fatigue crack initiation mechanism in Ti–6Al–4V

Abstract: r Fretting fatigue crack initiation in titanium alloy, Ti-6Al-4V, was investigated experimentally and analytically by using finite element analysis (FEA). Various types of fretting pads were used in order to determine the effects of contact geometries. Crack initiation location and crack angle orientation along the contact surface were determined by using microscopy. Finite element analysis was used in order to obtain stress state for the experimental conditions used during fretting fatigue tests. These were then used in order to investigate several critical plane based multiaxial fatigue parameters. These parameters were evaluated based on their ability to predict crack initiation location, crack orientation angle along the contact surface and the number of cycles to fretting fatigue crack initiation independent of geometry of fretting pad. These predictions were compared with their experimental counterparts in order to characterize the role of normal and shear stresses on fretting fatigue crack initiation. From these comparisons, fretting fatigue crack initiation mechanism in the tested titanium alloy appears to be governed by shear stress on the critical plane. However, normal stress on the critical plane also seems to play a role in fretting fatigue life. At present, the individual contributions/importance of shear and normal stresses in the crack initiation appears to be unclear; however, it is clear that any critical plane describing fretting fatigue crack initiation behaviour independent of geometry needs to include components of both shear and normal stresses.

Fatigue design of structures under thermomechanical loadings

Abstract: This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.

The Equivalent Strain Energy Density approach re-formulated and applied to sharp V-shaped notches under localized and generalized plasticity

Abstract: In the case of a rounded notch, the stress and strain at the notch tip can be determined by the traditional Neuber rule or by the Equivalent Strain Energy Density (ESED) approach, as formulated by Glinka and Molski. In the latter case the elastoplastic strain energy density at the notch tip is thought of as coincident with that determined under purely elastic conditions. For sharply V-shaped notches this approach is not directly applicable, since the strain energy density at the notch tip tends toward infinity both for a material obeying an elastic law and a material obeying a power hardening law. By using the notch stress intensity factors, the present paper suggests a re-formulation of the ESED approach which is applied no longer at the notch tip but to a finite size circular sector surrounding the notch tip. In particular we have adopted the hypothesis that, under plane strain conditions, the value of the energy concentration due to the notch is constant and independent of the two constitutive laws. When small scale yielding conditions are present, such a hypothesis immediately results in the constancy of the strain energy averaged over the process volume. As a consequence, plastic notch stress intensity factors valid for sharp V-shaped notches can be predicted on the basis of the linear elastic stress distributions alone.

Micro‐mechanical modelling of ductile damage and tearing – results of a European numerical round robin

Abstract: Results of a European numerical round robin on the application of constitutive equations for ductile damage to simulate tearing of a ferritic steel are presented. The analysis of deformation and failure of a standard smooth tensile specimen is used to characterize the material and identify critical damage parameters for ductile tearing. Ductile crack growth in a C(T) specimen is then numerically simulated by applying the constitutive models of Gurson or Rousselier in order to predict a J R -curve. Although the tensile tests were satisfactorily described, two main problems arose. The first one is that the participants' results differ considerably from each other with respect to the critical damage parameter obtained in the first task and the predicted J R -curves in the second. The reasons were partially found in the strategies for parameter identification and the non-unique definitions for crack initiation and crack growth used in the simulations. Further reasons are expected in the individual realization of the damage model in the FE programmes or user subroutines. The second problem is a general discrepancy between the simulations and the experiment: most of the predicted J R -curves are too flat compared with the experimental one. Manifold conclusions can be drawn for future work aiming at improving the reliability of the numerical models and promoting their application in engineering practice.

Micromechanical modelling of damage and fracture of ductile materials

Abstract: A micromechanical model of ductile damage by void nucleation, growth and coalescence is widely and successfully applied to describe phenomena of ductile tearing. The model’s fundamental principles, and especially the constitutive equations of Gurson, Tvegaard and Needleman (GTN-model), are briefly described. Some of the material parameters of the GTN-model are calibrated by performing cell model calculations, which is a method of determining the structural behaviour of a single void in a plastic material. The approach is used to study the dependence of material strength and toughness on microstructural features of nodular cast iron. The numerical simulations were realized within the FE-program ABAQUS by a user-supplied material model.

High cycle fatigue mechanisms in a cast AM60B magnesium alloy

Abstract: We examine micromechanisms of fatigue crack initiation and growth in a cast AM60B magnesium alloy by relating dendrite cell size and porosity under different strain amplitudes in high cycle fatigue conditions. Fatigue cracks formed at casting pores within the specimen and near the surface, depending on the relative pore sizes. When the pore that initiated the fatigue crack decreased from approximately 110 μm to 80 μm, the fatigue life increased two times. After initiation, the fatigue cracks grew through two distinct stages before final overload specimen failure. At low maximum crack tip driving forces (K max 2.3 MPam), the fatigue crack propagated primarily through the β-Al 12 Mg 17 particle laden interdendritic regions. Based on these observations, any proposed mechanism-based fatigue model for cast Mg alloys must incorporate the change in growth mechanisms for different applied maximum stress intensity factors, in addition to the effect of pore size on the propensity to form a fatigue crack.

Specific features of high-cycle and ultra-high-cycle fatigue

Abstract: Several specific features of high-cycle fatigue (HCF) and ultra-high-cycle fatigue (UHCF) are discussed both on the basis of direct experimental results and on the basis of speculations extrapolating HCF data to the UHCF regions. The following points are dealt with: (i) Extent and distribution of cyclic plastic deformation. (ii) Sensitivity of the fatigue behaviour in the HCF and UHCF regions to the start-up procedure of the test and to the preceding stress-strain history. (iii) Role of persistent slip bands (PSBs) and the microcrack initiation in the HCF and UHCF regions. (iv) Dependence of the notch sensitivity on the amplitude of cycling. (v) Effect of high number of very small amplitude cycles on creep behaviour of superalloy single crystals.

A non‐local theory applied to high cycle multiaxial fatigue

Abstract: The stress gradient effect on the fatigue limit is an important factor which has to be taken into account for an efficient transfer of fatigue data from laboratory tests to design of industrial components. A short review of some multiaxial high cycle fatigue criteria considering this effect is presented. On the basis of the two local mesoscopic approaches of Papadopoulos, two new non-local high cycle multiaxial fatigue criteria are developed. These proposals are based on the concept of volume influencing fatigue crack initiation. Their predictions are compared with experimental multiaxial fatigue data on four materials (a mild steel, two high strength steels and a spheroidal graphite cast iron). The accuracy of the two local Papadopoulos criteria and of the non-local proposals are compared and discussed, together with the physical interpretation of the threshold defining the volume influencing fatigue crack initiation.

Step loading for very high cycle fatigue

Abstract: The conventional method for determining fatigue strength at high cycle counts is to either extrapolate S-N data from lower cycle counts or to generate S-N data in the high cycle count regime using special high frequency test machines. In the process of generating S-N data, runout tests often occur where no failure is obtained within the allotted time for a test. Such data points are normally indicated on an S-N plot with an arrow, but no information is available as to whether failure would ever occur. As an alternative, and as a method for getting a data point for every test, a step-loading method may be employed. In this paper, some of the unique test equipment being applied to the study of high cycle fatigue in our laboratory are reviewed. Data are presented on the long-life fatigue behaviour of Ti-6Al-4V which validate both the step-loading methodology and indicate the trend of fatigue strength as a function of cycle number in the very high cycle fatigue regime. The absence of coaxing is demonstrated by comparing S-N data with step-loading data. Applicability of high frequency testing to the determination of fatigue crack growth thresholds is also demonstrated.

Initiation and early growth of fatigue cracks in an aerospace aluminium alloy

Abstract: Material imperfections usually play a substantial role in the early stages of fatigue cracking. This article presents some observations concerning fatigue crack initiating flaws and early crack growth in 7050-T7451 aluminium alloy specimens and in full-scale fatigue test articles with a production surface finish. Equivalent initial flaw size (EIFS) approaches used to evaluate the fatigue implications of metallurgical, manufacturing and service-induced features were refined by using quantitative fractography to acquire detailed information on the early crack growth behaviour of individual cracks; the crack growth observations were employed in a simple crack growth model developed for use in analysing service crack growth. The use of observed crack growth behaviour reduces the variability which is inherent in EIFS approaches which rely on modelling the whole of fatigue life, and which can dominate EIFS methods. The observations of realistic initial flaws also highlighted some of the significant factors in the fatigue life-determining early fatigue growth phase, such as surface treatment processes. Although inclusions are often regarded as the single most common type of initiating flaw, processes which include etching can lead to etch pitting of grain boundaries with significant fatigue life implications.

An approach to size effect in fatigue of metals using fractal theories

Abstract: As was experimentally observed by several authors, the fatigue strength of metallic materials decreases with increasing the specimen size. Such a decrease can be remarkable for very large structures like, for example, big cargo ships (some hundred meters long) transporting oil or other goods. Size effect in fatigue is herein explained by considering the fractal nature of the reacting cross sections of structures, that is, the renormalized fatigue strength is represented by a force amplitude acting on a surface with a fractal dimension lower than 2, where such a dimensional decrement depends on a self-similar weakening of the material ligament, owing to the presence of cracks, defects, voids and so forth (microscopic level). However, this decrement tends to progressively disappear with increasing the structure size (macroscopic level), i.e. the effect of the material microstructure on the macroscopic fatigue behaviour gradually vanishes for structures large enough with respect to a characteristic microstructural size, this phenomenon being defined as multifractality. A multifractal scaling law for fatigue limit of metals is proposed, and some experimental results are examined in order to show how to apply the theoretical approach presented.

Crack closure and the fatigue-crack propagation threshold as a function of load ratio

Abstract: The phenomenon of crack closure, which involves the premature closing of fatigue cracks during the unloading portion of a fatigue cycle resulting in the development of crack-tip shielding due to crack wedging, has become widely accepted as a critical mechanism influencing many aspects of the behaviour of fatigue cracks in metallic materials; these include effects of load ratio, variable-amplitude loading, crack size, microstructure, environment and the magnitude of the fatigue threshold. Recently, however, the significance of crack closure has been questioned and alternative suggestions made for many of these phenomena, e.g. the effect of the load ratio (i.e. the ratio R of the minimum to maximum loads) on threshold behaviour. In the light of this, the present paper provides evidence to rebut the assertion that crack closure is an insignificant process. Particular attention is given to the effect of crack closure on the threshold level as a function of load ratio.

Three‐dimensional finite element simulation of crack extension in aluminium alloy 2024fc

Abstract: A simulation of 3D crack extension using a cohesive zone model (CZM) has been carried out for a side-grooved compact tension specimen and a surface-crack tension specimen of aluminium 2024FC. Detailed finite element calculations were conducted by assuming crack extension only along the crack plane (mode I). For comparison, a 2D plane strain simulation is also presented. Load, displacement and crack extension histories are predicted and compared with the experiment. It is shown that the 2D approximation appears to agree reasonably well with experimental results, and that the 3D calculation gives very good agreement with test data. The determination of the CZM parameters is also discussed. Numerical results show that the CZM is a workable computational model which involves only a few microstructurally motivated phenomenological parameters for crack extension simulation.

Environmental considerations for fatigue cracking

Abstract: An essential aim for the study of fatigue cracking is the development of science-based methods for reliability and service life assessments as a part of the life-cycle design and management of engineered systems. In this context, a critical comparison between a mechanistically based probability and the more traditional, empirically based statistical approaches for reliability analysis and life prediction is presented. The comparison serves to highlight the need for quantitative, mechanistic understanding and modeling and for the adoption of a multidisciplinary approach. Current understanding of the influences of gaseous and aqueous environments on fatigue cracking, particularly, with respect to the effects of frequency and temperature, is briefly summarized, and special considerations for fatigue at very' high frequencies are discussed. The efficacy of the mechanistically based probability approach is illustrated through a comparison of the estimated and observed distribution of corrosion and corrosion fatigue damage in Boeing 707 aircraft that had been in commercial service for 24years.

The effect of solution heat-treatment time on the fatigue properties of an Al-Si-Mg casting alloy

Abstract: Experiments have been carried out to investigate whether reducing the solution heat-treatment time of Al-7Si-0.6Mg castings from the currently recommended values adversely affects their fatigue properties. Fatigue endurance tests have been carried out in zero-tension (R=0) and measurements made of the casting defects that initiated the fatigue cracks. The work has been limited to stresses that produce a fatigue life of ∼10 5 cycles and to two solution-treatment times (8 and 4 h). Two statistical techniques have been applied to the fatigue life data and no effect of solution heat-treatment time was detected at a confidence level of better than 95%. Similarly, no effect of cyclic test frequency could be detected for tests carried out at 1 and 60 Hz. The conclusions are confirmed by an analysis of the relation between fatigue life and the size of the casting defects that initiated fatigue failure. The scatter in fatigue lives is related to the scatter in the sizes of casting defects in the specimens. It is clear that there is a potential for considerable savings in heat-treatment costs for castings of the size and shape chosen for the study.

Effect on fatigue crack growth of interactions between overloads

Abstract: Various types of interactions between overloads were studied in a 0.38% C low carbon steel. The retarding effect due to consecutive overloads is found to increase with the number of overloads, until it reaches a maximum. Similarly, it is found that a critical distance between overloads ensures the highest retarding effect, while shorter or longer spacing are less efficient for retarding crack growth. These effects are successfully explained using FEM calculations of the effective stress intensity factor. The kinematic hardening of the alloy, which is very efficient in ferritic-pearlitic steels, is shown to be mostly responsible for those effects. Taking into account the amplitude of kinematic hardening allows qualitative explanation of the observed effects. The order of application of the cycles during variable amplitude fatigue is thus important and should be taken into account for predicting fatigue lives.

A rolling contact fatigue crack driven by squeeze fluid film

Abstract: A new model of surface breaking rolling contact fatigue (RCF) crack driven by a coupled action of a squeeze oil film built up in the crack interior and a pressure exerted at the external contact interface was developed. The model can be applied to the ‘nominally dry’ contact couples with an occasional presence of liquid in the crack interior (wheel/rail contact) as well as to the elastohydrodynamic lubrication (EHL) conditions. In the first case, the contact load is a result of solid/solid interaction and can be determined by solving the FE contact problem, but the liquid contained in the crack interior forms a thin film between the crack faces changing their interaction into the type of liquid/solid. This liquid is being periodically squeezed under contact load and acts as a ‘squeeze film’ known from the lubrication theory. In the second case, the liquid (lubricating oil) is permanently present in the contact area and consequently in the vicinity of the crack mouth. This creates conditions for filling the crack with oil. Similarly as in the first case, the ‘squeeze oil film’ is built between the crack faces. The contact load in this case results from a liquid/solid interaction and can be approximated by the pressure and traction distributions obtained from the numerical solution of the elastohydrodynamic contact problem. In both cases the model can be used to determine the Linear Elastic Fracture Mechanics (LEFM) crack tip stress intensity histories during cyclic loading and consequently to predict the crack growth rate and direction. An example of applying the model to the EHL case is given to explain the mechanisms and phenomena leading to the crack front loading. The cycle of rolling a roller over the crack was numerically simulated to obtain the mixed mode (I and II) SIF histories. In the analysis, the EHD pressure and traction were determined through the full solution of the EHD line contact problem accounting for the presence of a crack, whilst the pressure in the crack was found with the use of the wedge shaped squeeze oil film (SOF) model. Possible effects of the mode I and mode II stress intensity cycles on crack growth rate and direction are discussed. The solution indicates high pressure in the neighbourhood of the crack tip, exerted on the crack faces by the squeeze oil film. This leads to the ranges of the mode I and mode II SIF variations, which are larger than for the ‘dry’ and ‘fluid entrapment’ models, and can be an explanation for the crack growth rate observed in practice

Closure of plane‐strain cracks under large‐scale yielding conditions

Abstract: This paper presents computational and theoretical investigations of the plasticity-induced crack-closure of a plane-strain crack under large-scale yielding conditions Solutions of the crack-tip opening displacements for a stationary crack and a growing fatigue crack have been obtained using the finite element method The self-similar crack-closure model has been extended to the plane strain case by introducing two plastic constraint factors: one for tension yielding and the other for compression yielding These two plastic constraint factors are identified by matching the model predictions with the computational results It is shown that the first constraint factor decreases rapidly with the applied stress while the second constraint factor is approximately equal to unity The findings of this study allow the cohesive-zone based crack-closure model to be extended to plane-strain cracks, especially under large-scale yielding conditions

A quantitative model for predicting the morphology of surface initiated rolling contact fatigue cracks in back‐up roll steels

Abstract: A previously presented qualitative model for the rolling contact fatigue and spalling failure of back-up rolls has been quantified in terms of crack lengths and growth directions. The morphologies of surface initiated fatigue cracks have been predicted using published data on the mode I and mode II thresholds in low carbon and roll steels, respectively, and the theoretical determination of the mode I and mode II stress intensity factors at the tips of the inclined surface cracks. The predictions have been validated by using the results of the metallographic examination of rolling contact fatigue cracks produced in test discs used in experimental simulations and the examination of spalled material from a back-up roll.

Influence of atmospheric moisture on slow fatigue crack growth at ultrasonic frequency in aluminium and magnesium alloys

Abstract: The deleterious influence of atmospheric moisture on the fatigue properties of an aluminium wrought alloy AlZnMgCu1.5-T6, an aluminium cast alloy AlSi9Cu3 and magnesium cast alloys AM60 hp, AZ91 hp and AS21 hp has been studied at a cycling frequency of 20kHz. Atmospheric moisture accelerates fatigue crack growth and decreases the threshold stress intensities to 55-75% of the respective values in vacuum. In ambient air, fatigue crack growth rates were up to two decades higher than those in vacuum. Accelerated crack growth was found at propagation rates below about 2 x 10 -9 m cycle -1 in aluminium alloys and below about 3 x 10 -8 m cycle -1 in magnesium alloys. As the threshold regime is approached, fatigue cracks in ambient air either propagate at a minimum mean growth rate on average of approximately one lattice spacing per cycle or they stop propagating, whereas mean growth rates of 10 -12 m cycle -1 were found in vacuum. Crack initiation and slow fatigue crack growth mainly determine lifetimes in the high cycle regime, and endurance data obtained at ultrasonic frequency in ambient air of 40-60% relative humidity are similar to lifetimes measured at conventional frequencies.

A low-cycle fatigue life prediction model of ultrafine-grained metals

Abstract: A (high strain) low-cycle fatigue (LCF) life prediction model of ultrafine-grained (UFG) metals has been proposed. The microstructure of a UFG metal is treated as a two-phase 'composite' consisting of the 'soft' matrix (all the grain interiors) and the 'hard' reinforcement (all the grain boundaries). The dislocation strengthening of the grain interiors is considered as the major strengthening mechanism in the case of UFG metals. The proposed model is based upon the assumption that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the UFG metal takes place. In high-strain LCF conditions, the fatigue-damaged zone is described as the region in which the local cyclic stress level approaches the ultimate tensile strength of the UFG metal, with the plastic strain localization caused by a dislocation sliding-off process within it. The fatigue crack growth rate is directly correlated to the range of the crack-tip opening displacement. The empirical Coffin-Manson and Basquin relationships are derived theoretically and compared with experimental fatigue data obtained on UFG copper (99.99%) at room temperature under both strain and stress control. Good agreement is found between the model and the experimental data. It is remarkable that, although the model is essentially formulated for high strains (LCF), it is also found to be applicable at low strains in the high-cycle fatigue (HCF) regime.