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ABSTRACT Research on fatigue crack formation from a corroded 7075-T651 surface provides insight into the governing mechanical driving forces at microstructure-scale lengths that are intermediate between safe life and damage tolerant feature sizes. Crack surface marker-bands accurately quantify cycles ( Ni) to form a 10-20 μm fatigue crack emanating from both an isolated pit perimeter and EXCO corroded surface. The Ni decreases with increasing-applied stress. Fatigue crack formation involves a complex interaction of elastic stress concentration due to three-dimensional pit macro-topography coupled with local micro-topographic plastic strain concentration, further enhanced by microstructure (particularly sub-surface constituents). These driving force interactions lead to high variability in cycles to form a fatigue crack, but from an engineering perspective, a broadly corroded surface should contain an extreme group of features that are likely to drive the portion of life to form a crack to near 0. At low-applied stresses, crack formation can constitute a significant portion of life, which is predicted by coupling macro-pit and micro-feature elastic-plastic stress/strain concentrations from finite element analysis with empirical low-cycle fatigue life models. The presented experimental results provide a foundation to validate next-generation crack formation models and prognosis methods. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, the interaction between multiple cracks in crack growth direction is studied in an aluminium alloy under static and fatigue loading. Self similar as well as non-self-similar crack growth has been observed which depends on the relative crack positions defined by crack offset distance and crack tip distance. On the basis of experimental observations, the criterion for crack coalescence and crack growth direction are expressed in terms of the crack positions defined by crack offset and crack tip distances. The criterion presented in this study can be used to determine the limiting value of crack tip and crack offset distance and to determine the mode in which cracks coalesce during their growth process. Experimental results and crack interaction criterion presented under various crack positions and size conditions could be used to derive a new evaluation method of crack growth in multiple crack geometry. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper, a model using the energy approach was established to investigate the formation of fish-eye shape in the gigacycle fatigue failure. The model was used to predict the size of the fish-eye formed by a given subsurface inclusion. The predicted results were comparable with the fractography of SNCM439 steel and other experimental results of gigacycle fatigue available in the literature. The current model can capture the fish-eye shape and can be used to predict the life of crack initiation during fatigue failure. [ABSTRACT FROM AUTHOR]

ABSTRACT Sliding contact of a rigid rough surface with a semi-infinite medium including a horizontal subsurface crack was investigated by using linear elastic fracture mechanics and finite element method (FEM). The fractal geometry was used to characterize the rigid rough surface. The propagation of crack was studied with the shear and tensile stress intensity factors. The effect of surface roughness, crack length to depth ratio and friction at the contact and crack interfaces was investigated by using the FEM. It was shown that increasing friction coefficient at the contact interface increases both KII and KI. [ABSTRACT FROM AUTHOR]

To extend the predictive capability of existing crack growth models for fibre metal laminates under constant amplitude fatigue loading to variable-amplitude loading, further research on variable-amplitude fatigue mechanisms in fibre metal laminates is necessary. In response to this need, an experimental study into the effects of multiple overloads, underloads and various block-loading sequences on crack growth in the fibre metal laminate Glare was investigated. Crack growth retardation effects were observed in the tests; however, the magnitude of these effects was lower than seen in monolithic aluminium because of fibre bridging. As a result, predictions of the observed behaviour were attempted using an existing constant-amplitude fatigue crack growth model for Glare in combination with a linear damage accumulation law. [ABSTRACT FROM AUTHOR]

Fatigue crack closure is strongly influenced by the plastic wake developed because of the action of precedent cycles. It is related to the load sequence and is an additional difficulty for its determination. Finite-element method has been an alternative for the study of crack closure, commonly in constant-amplitude loading. However, such analyses are complex and computationally expensive, the length of the wake simulated being a critical parameter. In this paper, a comprehensive study of the effect of the plastic wake in fatigue crack closure is made. It is structured in two parts: one regarding the correct numerical modelling process and the other focussed on the influence of the wake under moderate variable-amplitude loading. After a brief description of the numerical methodology, results for different wake lengths are reported. The plastic wake is modelled with a new approach in which the final crack tip is fixed. Information regarding potential error is provided. Second, the load sequence effect is discussed. Crack closure is obtained for several load ratios, with wakes developed with different stress ratios. This permits the analysis of the influence of the load sequence over the crack-driving force compared with constant-amplitude loading. [ABSTRACT FROM AUTHOR]

This paper proposes a local stress concept to evaluate the fretting fatigue limit for contact edge cracks. A unique S–N curve based on the local stress could be obtained for a contact edge crack irrespective of mechanical factors such as contact pressure, relative slip, contact length, specimen size and loading type. The analytical background for the local stress concept was studied using FEM analysis. It was shown that the local stress uniquely determined the Δ K change due to crack growth as well as the stress distribution near the contact edge. The condition that determined the fretting fatigue limit was predicted by combining the Δ K change due to crack growth and the Δ Kth for a short crack. The formation of a non-propagating crack at the fatigue limit was predicted by the model and it was experimentally confirmed by a long-life fretting fatigue test. [ABSTRACT FROM AUTHOR]

High-cycle fatigue properties were investigated for Ti–5% Al–2.5% Sn ELI alloy with a mean α grain size of 80 μm, which had been used for liquid hydrogen turbo-pumps of Japanese-built launch vehicles. At cryogenic temperatures, the fatigue strength in high-cycle region did not increase in proportion to increments of the ultimate tensile strength and the fatigue strengths at around 106 cycles were about 300 MPa independent of test temperatures. Fatigue cracks initiated in the specimen interior independent of the test temperatures of 4 K, 77 K and 293 K. At 4 K and 77 K, several crystallographic facet-like structures were formed at crack initiation sites. On the other hand, there were no facet-like structures that could be clearly identified at the crack initiation sites at 293 K. Low fatigue strengths in longer-life region at cryogenic temperatures could be attributable to the formation of large sub-surface crack initiation sites, where large facet-like structure are formed. [ABSTRACT FROM AUTHOR]

It is well known that microcracking in brittle materials results in a reduction of the stress intensity factor (SIF) and energy release rate (ERR). The reduced SIF or ERR represents crack tip shielding which is of significant interest to micromechanics and material science researchers. However, the effect of microcracking on the SIF and ERR is a complicated subject even for isotropic homogeneous materials, and becomes much more formidable in case of interface cracks in bonded dissimilar solids. To unravel the micromechanics of interface crack tip shielding in bonded dissimilar anisotropic solids, an interface crack interacting with arbitrarily oriented subinterface microcracks in bonded dissimilar anisotropic materials is studied. After deducing the fundamental solutions for a subinterface crack under concentrated normal and tangential tractions, the present interaction problem is reduced to a system of integral equations which is then solved numerically. A J-integral analysis is then performed with special attention focused on the J2-integral in a local coordinate system attached to the microcracks. Theoretical and numerical results reassert the conservation law of the J-integral derived for isotropic materials also to be valid for bonded dissimilar anisotropic materials. It is further concluded that there is a wastage when the remote J-integral transmits across the microcracking zone from infinity to the interface macrocrack tip. In order to highlight the influence of microstructure on the interfacial crack tip stress field, the crack tip SIF and ERR in several typical cases are presented. It is interesting to note that the Mode I SIF at the interface crack tip is quite different from the ERR in bonded dissimilar anisotropic materials. [ABSTRACT FROM AUTHOR]

This paper proposes an engineering method suitable for predicting the fatigue limit of both plain and notched components subjected to uniaxial as well as to multiaxial fatigue loadings. Initially, some well-known concepts formalized by considering the cracking behaviour of metallic material under uniaxial cyclic loads have been extended to multiaxial fatigue situations. This theoretical extension allowed us to form the hypothesis that fatigue limits can be estimated by considering the linear–elastic stress state calculated at the centre of the structural volume. This volume was assumed to be the zone where all the main physical processes take place in fatigue limit conditions. The size of the structural volume was demonstrated to be constant, that is, independent from the applied loading type, but different for different materials. Predictions have been made by Susmel and Lazzarin's multiaxial fatigue criterion, applied using the linear–elastic stress state determined at the centre of the structural volume. The accuracy of this method has been checked by using a number of data sets taken from the literature and generated by testing notch specimens both under uniaxial and multiaxial fatigue loadings. Our approach is demonstrated to be a powerful engineering tool for predicting the fatigue limit of notch components, independently of material, stress concentration feature and applied load type. In particular, it allowed us to perform predictions within an error interval of about ±25% in stress, even though some material mechanical properties were either estimated or taken from different sources. [ABSTRACT FROM AUTHOR]

The evolution of the stress–strain fields near a stationary crack tip under cyclic loading at selected R-ratios has been studied in a detailed elastic–plastic finite element analysis. The material behaviour was described by a full constitutive model of cyclic plasticity with both kinematic and isotropic hardening variables. Whilst the stress/strain range remains mostly constant during the cyclic loading and scales with the external load range, progressive accumulation of tensile strain occurs, particularly at high R-ratios. These results may be of significance for the characterization of crack growth, particularly near the fatigue threshold. Elastic–plastic finite element simulations of advancing fatigue cracks were carried out under plane-stress, plane-strain and generalized plane-strain conditions in a compact tension specimen. Physical contact of the crack flanks was observed in plane stress but not in the plane-strain and generalized plane-strain conditions. The lack of crack closure in plane strain was found to be independent of the material studied. Significant crack closure was observed under plane-stress conditions, where a displacement method was used to obtain the actual stress intensity variation during a loading cycle in the presence of crack closure. The results reveal no direct correlation between the attenuation in the stress intensity factor range estimated by the conventional compliance method and that determined by the displacement method. This finding seems to cast some doubts on the validity of the current practice in crack-closure measurement, and indeed on the role of plasticity-induced crack closure in the reduction of the applied stress intensity factor range. [ABSTRACT FROM AUTHOR]

Linear elastic fracture mechanics (LEFM) is the science frequently used to understand the stable and progressive fatigue crack growth that often occurs in engineering components under varying applied stress. The stress intensity factor (SIF) is its basis and describes the stress state at the crack tip. This can be used with the appropriate material properties to calculate the rate at which the crack will propagate in a linear elastic manner. Unfortunately, the SIF is difficult to compute or measure, particularly if the crack is situated in a complex three-dimensional geometry or subjected to a non-simple stress state. This is because the SIF is not only a function of the crack and component geometry but is also dependent on the applied stress field. In the last 20 years, the SIF weight function has gained prominence as a method for calculating and presenting SIFs independent of applied stress. This paper demonstrates that the real promise of the SIF weight Function lies in its use to rapidly generate SIF solutions for cracks in complex geometries by simple composition of geometric influences from reference constituent solutions. [ABSTRACT FROM AUTHOR]

The tear resistance behaviour of Al–7% Si–0.4% Mg cast alloys was examined using Kahn-type tear test specimens. Tests were performed for two permanent mould casts with an ordinary dendrite structure and a semi-liquid die cast with a globular cell and fine grain structure. The microstructure of the two permanent mould casts was controlled by the cooling rates and the addition of Ti elements. Tear resistance was evaluated by the ‘pop-in’ stress, the energies required for crack initiation, UEi and the crack propagation, UEp. Special attention was paid to an effective microstructural parameter for tear resistance improvement. Pop-in, indicating sudden crack extension and arrest, was observed in all specimens. Homogeneous deformation occurs near the notch tip of the semi-liquid die cast, characterized by a refined grain structure. Refinement of the grain size is more effective than that of the dendrite cell size or eutectic Si particle size to increase the energy for crack initiation. Unit propagation energy, UEp, can be converted into a critical stress intensity factor, Kc, which in the semi-liquid die cast was improved due to an increased amount of slant or shear fracture surface. [ABSTRACT FROM AUTHOR]

The ferrite phase of cast duplex stainless steels becomes embrittled after thermal ageing, leading to a significant decrease in fracture properties. In the present paper, the influence of ageing and solidification structure on the fatigue crack growth rates (FCGRs) and on the fatigue crack growth mechanisms in a cast duplex stainless steel is studied. FCGRs measured at room temperature increase slightly after ageing at 400 °C, due to ferrite cleavage and to the resulting irregular shape of the crack front. The crack propagates without any preferential path by successive ruptures of ferrite and austenite phases. The macroscopic crack propagation plane depends on the crystallographic orientation of the ferrite grain. Secondary cracks can appear due to the complex solidification structure. This in turn influences the FCGR. The fatigue crack closure level decreases with increasing ageing. This can be explained by a decrease in the kinematic cyclic hardening of these materials. [ABSTRACT FROM AUTHOR]

Recent manufacturing of advanced ceramics has developed phase transformation and cracks tilted or bridged to enhance ceramic-inherent toughness. However, main crack and near-tip microcracks interaction is another prominent means in toughening of the ceramic materials. In this study, mechanics of discrete model combined with alternative iterating numerical technique is developed and applied in assessment of main–microcrack interaction. Consequences of main–microcrack interaction can either shield or amplify the resulting main crack stress intensity factor, which accounts for the increase or decrease of ceramic toughness. Numerical outcomes show good agreement with the available solutions in the literatures; in addition results also reveal that the toughness shielding or amplification is dependent on the type of loadings as well as location/orientation of the microcrack. Nevertheless, residual normal stress plays an important role in shielding/amplification region trade-off. This work may provide a useful quantitative tool in ceramic design and a valuable insight into main–micro interaction phenomenon. [ABSTRACT FROM AUTHOR]

Calculates the stress intensity factors for deep semi-elliptical cracks in flat plates. Values of the stress intensity factors; Aspect ratios; Application of uniform, linear, parabolic or cubic stress distributions to the crack face.