Near-threshold fatigue crack growth behavior of 2195 aluminum-lithium-alloy—prediction of crack propagation direction and influence of stress ratio

Tensile properties and fatigue crack propagation behavior of a 2195-T8 Al-Li alloy were investigated at different stress ratios, with particular emphasis on their dependence on specimen orientation. Specimens with orientations of 0, 15, 30, 45, and 90 deg to the rolling direction were tested. The alloy contained a strong brass-type texture and a profuse distribution of platelike precipitates of T ₁ (Al₂CuLi) phase on {111} matrix planes. Both tensile strength and fatigue thresholds were found to be strongly dependent on the specimen orientation, with the lowest values observed along the direction at 45 deg to the rolling direction. The effect of stress ratio on fatigue threshold could generally be explained by a modified crack closure concept. The growth of fatigue crack in this alloy was found to exhibit a significant crystallographic cracking and especially macroscopic crack deflection. The specimens oriented in the L-T + 45 deg had the smallest deflection angle, while the specimens in the L-T and T-L orientations exhibited a large deflection angle. The dependence of the fatigue threshold on the specimen orientation could be rationalized by considering an equivalent fatigue threshold calculated from both mode I and mode II values due to the crack deflection. A four-step approach on the basis of Schmid’s law combined with specific crystallographic textures is proposed to predict the fatigue crack deflection angle. Good agreement between the theoretical prediction and experimental results was observed.     

Prediction of crack paths in WCCo alloys

A crack propagating through the WCCo microstructure has to choose between paths along the binder/carbide interface and paths across binder regions. The latter paths are selected when the crack enters a binder region at a large angle from the nearest carbide interface, while the interface paths are preferred by cracks entering at a small angle. A critical angle can be defined for the switch from one type of crack path to the other. Empirical data for the area fractions of the two crack paths in widely different WCCo alloys can be accounted for by a single critical angle, φ_c = 25°. Finite element analysis of the stress field in a region of binder enclosed between carbide grains shows that the preferred site for the growth of stress-induced microvoids will move from the carbide grain flanks to the interior of the binder region when the entry angle of the crack exceeds 24°. Thus the observation of a critical angle deciding the crack path is verified by the stress field analysis and given a physical explanation in terms of the most likely site for microvoid formation.     

Effect of α phase on fatigue crack growth of Ti-6242 alloy

Fatigue crack growth as a function ofαphase volume fraction in Ti-6Al-2Sn-4Zr-2Mo(Ti-6242)alloy was investigated using fatigue testing,optical microscopy,scanning electron microscopy,and transmission electron microscopy.Theα+βannealing treatments with different solid solution temperatures and cooling rates were conducted in order to tailor microstructure with differentαphase features in the Ti-6242 alloy,and fatigue crack growth mechanism was discussed after detailed microstructure characterization.The results showed that fatigue crack growth rate of Ti-6242 alloy decreased with the decrease in volume fraction of the primaryαphase(αp).Samples with a large-sizedαgrain microstructure treated at high solid solution temperature and slow cooling rate have lower fatigue crack growth rate.The appearance of secondaryαphase(αs)with the increase of solid solution temperature led to crack deflection.Moreover,a fatigue crack growth transition phenomenon was observed in the Paris regime of Ti-6242 alloy with 29.8% αp(typical bi-modal microstructure)and large-sizedαgrain microstructure,owing to the change of fatigue crack growth mechanism.     

Fracture behaviour of ceramic laminates in bending—I. Modelling of crack propagation

These two papers concern the fracture behaviour of specimens made up of cermic sheets, separated by thin interlayers, which act to deflect cracks and thus to prevent catastrophic failure of the specimen. The treatment is divided into two parts. In this first paper, the behaviour of this type of material during bending is quantitatively modelled, while the second paper compares predictions from the model with experimental data. The model is based on through-thickness cracks propagating when a critical stress is reached and interfacial cracks then advancing a distance dictated by the available energy. The variation in laminae strengths is modelled using a Monte Carlo method to determine the strength of successive laminae for a given Weibull modulus. The model is used to predict load/displacement plots and to explore the effects of changes in loading geometry and specimen variables, including Young's modulus, lamina strength, loading span, interfacial toughness, as well as lamina and sample thickness. A distinction is drawn between the energy actually absorbed in causing complete failure of the specimen as measured from the area under the load/displacement curve, and the amount of energy necessary to cause the crack propagation which occurred. These differ if the energy available to drive the interfacial cracks is more than sufficient for them to reach the ends of the specimen or if energy is dissipated elsewhere in the system. A criterion is derived by which specimens can be designed so as to minimise the difference between these two quantities. The significance of this concept in optimising the toughness of these laminated materials is briefly discussed.     

Cyclic deformation of ferritic steel—II. Stage II crack propagation

The crack propagation process is dominated by the formation of ductile fatigue striations and the classical “one load cycle per striation” concept is only valid in a narrow crack propagation rate interval. However, over 4 orders of magnitudes in crack propagation rates the striation rates is independent of the ΔK, which implies that the number of cycles necessary to form one striation probably is greater than one. The similarities between the fatigue damage processes in the cyclic plastic zone and in plastic strain controlled specimens are documented in detail. The micromechanisms of crack propagation is related to a local plastic collapse of the dislocation substructure at the crack tip. A model for fatigue crack propagation rate predictions has been developed. The model is a refinement of the existing accumulated damage, LCF-models and the most important parameters taken into account are: the constant striation spacing, a realistic dynamic cyclic yield stress, the Coffin-Manson constants, a threshold plastic strain and a constant which coincide with the average grain size.     

Investigation of small fatigue cracks—II. A plasticity based model of small fatigue crack growth

Based on experimental evidence from periodic plastic zone size, (PZS), measurements on growing small fatigue cracks, PZS ahead of crack tips has been suggested as a correlating parameter for small crack growth. In this paper, the PZS of small cracks are calculated by incorporating an equivalent friction stress into the BCS model. The retardation of small crack growth is considered by comparing the stress concentration ahead of blocked plastic zones to the strength of grain boundary barriers. It is demonstrated that the model can predict the growth behaviour of small cracks well and also provides reasonable predictions of fatigue lives where they are dominated by small crack growth.     

The effects of dispersoids upon the micromechanisms of crack propagation in AlMgSi alloys

The effect of dispersoids containing Mn or Cr. in a series of peak-aged AlMgSi alloys, upon the crack-tip plastic zone size has been studied by the use of Selected Area Channelling Patterns (SACPs).In the absence of such dispersoids. the alloy fractures in a brittle intergranular fashion; when the particles are present, the alloys are ductile but still fracture in a predominantly intergranular manner but with an increase in the size of the plastic zone associated with the crack tip.It is suggested that the toughening effect of the dispersoid phase is indirect through its effect upon the matrix grain size and upon the distribution of slip. Fracture occurs because slip bands impinge on grain boundaries, causing a local strain concentration which at some critical value nucleates a void, thereby causing an increment of crack extension.     

A microscopic model of hydrogen-induced intergranular cracking—II. Steady state crack growth

A steady state model of hydrogen-induced intergranular crack growth has been developed. It is assumed that the steady state growth of an intergranular crack, screened by dislocations, is controlled by the grain boundary and crack surface diffusion processes of hydrogen. A series of second-order differential equations controlling the diffusion processes of hydrogen in the intergranular crack regions is solved numerically to determine the hydrogen distribution along a steadily moving crack and thereby the change of the ideal work of fracture. The relationship of the stress intensity required for the crack growth to the crack velocity is established as a function of several values of the grain boundary and crack surface diffusivities, the crack length, and the grain size. It is found that while the susceptibility to hydrogen-induced intergranular crack growth increases with increasing hydrogen diffusivities, it decreases with increasing crack length and grain size. The effect of grain size on the growth characteristics of hydrogen-induced intergranular cracking are discussed in terms of the direction change of the hydrogen flux along the moving crack.     

The crystallography of fatigue crack initiation in coarse grained astroloy at 20°C

The effects of crystallographic orientation on fatigue crack initiation has been examined for coarse-grained Astroloy at 20°C. Specimens were cycled by three-point bending at stress ranges between 5 and 95% of the proportional limit until fatigue cracks were detected. The crystallographic orientation of individual grains within which fatigue cracks initiated was determined by use of selected area electron channeling. Grains forming cracks were found to have surface normals near the 〈100〉, 〈011〉, and 〈113〉 directions. Conversely, grains which did not initiate cracks were not similarly grouped in orientation. Calculations of the Taylor factor using the Bishop-Hill approach revealed that fatigue crack initiation in Astroloy occurred at grains with low values of the Taylor factor.     

Effects of microstructure on the short fatigue crack initiation and propagation characteristics of biomedical α/β titanium alloys

This article presents the results of a study of the effects of microstructure on the fatigue strength and the short fatigue crack initiation and propagation characteristics of a biomedical α/β titanium alloy, Ti-6Al-7Nb. The results are compared to those obtained from a Ti-6Al-4V extra-low interstitial (ELI) alloy. Fatigue crack initiation occurs mainly at primary α grain boundaries in an equiaxed α structure, whereas, in a Widmanst?tten α structure, initiation occurs within the α colonies and prior β grains, where α plates are inclined at around 45 deg to the stress-axis direction. In an equiaxed α structure, the short fatigue crack initiation and propagation life, where the length of the crack (a) is in a microstructurally short fatigue-crack regime (2a < 50 μm), occupies around 50 pct of the total fatigue life. On the other hand, the fatigue crack in a Widmanst?tten α structure initiates at very early stages of fatigue, and, therefore, the fatigue crack-initiation life occupies a few percentages of the total fatigue life in an α structure. Then, the short fatigue crack propagates rapidly and is arrested at the grain boundaries of α colonies or prior β grains for a relatively long period, until the short crack passes through the boundaries to specimen failure. Therefore, the short fatigue crack-arrest life occupies more than 90 pct of the total fatigue life in a Widmanst?tten α structure. These trends are similar between the Ti-6Al-7Nb and Ti-6Al-4V ELI alloys and biomedical α/β titanium alloys. The total fatigue life for the Ti-6Al-7Nb alloy with an equiaxed α structure is changed by the volume fraction of primary α phase and the cooling rate after solution treatment. By increasing the volume fraction of the primary α phase from 0 to 70 pct, the fatigue limit of the Ti-6Al-7Nb alloy is raised. Changing the cooling rate after solution treatment by switching from air cooling to water quenching improves the fatigue limit of the Ti-6Al-7Nb alloy significantly.     

Effects of grain size and precipitate size on the fatigue crack growth behavior of alloy 718 at 427 °C

The fatigue crack growth rates of four Alloy 718 microstructures comprising a two-by-two matrix of grain size and γ" precipitate size were determined in air at 427 °C and 0.33 Hz. For a stress ratio of 0.05, slower Region II rates were obtained for coarse-grained microstructures, independent of γ" size, and microstructures with large γ", independent of grain size. In the near-threshold regime, the coarse-grained microstructures again showed slower growth rates (higher ΔK_th), whereas the effect of γ" size was mixed. Deformation modes were studied using scanning and transmission electron microscopy. Fatigue deformation resulted in the formation of slip bands which were longer for coarse-grained microstructures and typically spaced farther apart and more planar for large γ micro-structures. The concepts of dislocation reversibility and slip band strain localization were used to explain the microstructural effects. Fatigue morphologies and cyclic constitutive behavior were con-sistent with the observed deformation modes. For a stress ratio of 0.75, the effects of grain size and γ" size were essentially identical to those observed for a stress ratio of 0.05. This indicated that roughness-induced closure had a minimal influence on the differences that were observed in fatigue crack growth behavior.     

Fatigue crack propagation in oil environments— II. A model for crack closure induced by viscous fluids

Several mechanisms have recently been identified for fatigue crack closure. The current work examines the concept of crack closure induced by the hydrodynamic wedging action of a viscous fluid within an advancing crack during cyclic loading. Unlike previous analyses, the model considers both the hydrodynamics of the pressure distribution within the crack and the kinetics of the penetration of the fluid into the crack. In addition, the results are presented in fracture mechanics terminology, and expressed as an effective (near tip) stress intensity range. Analyses involving both “full-” and “partial-penetration” of the viscous fluid inside the crack are utilized to rationalize the influence of viscosity on fatigue crack propagation in dehumidified oil environments, described in Part I of this paper. The roles of stress intensity range, crack size and frequency on the development of such fluid-induced crack closure are also examined.