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Graphical abstract Highlights • Plastic strain gradients restrict crack-tip plasticity during an overload. • Restricted plastic deformation enhances post-overload crack growth rates. • Plastic strain gradients are key in the evolution of post-overload crack closure. • Plastic strain gradients do not affect steady-state fatigue crack growth rates. Abstract Fatigue crack growth is analyzed for steady state and overload conditions. The model combines an elastic-plastic strain gradient hardening material and an irreversible cohesive zone model. Plastic strain gradients are found not to affect steady-state fatigue crack growth but play a key role in the overload response. Plastic strain gradient hardening lowers plastic strain magnitudes and alters the spatial distribution of plastic strain. This leads to reduced crack closure and higher crack growth rates during and after the overload. Using classical plasticity alone to describe fatigue crack growth following an overload is thus nonconservative. [ABSTRACT FROM AUTHOR]

Elber found in the early 70s that fatigue cracks can close under tensile loads, and assumed that fatigue crack growth (FCG) would be controlled by Δ K eff = K max − K op , where K max and K op are the maximum and opening values of the stress intensity factor. This hypothesis can rationalize many transient effects observed under service loads, but it cannot explain many other effects like FCG retardation or arrest after overloads under high R = K min / K max , when K min > K op ; FCG at constant rates under highly variable Δ K eff ; cracks arrested at a given R that can reinitiate to grow at a lower R without changing their Δ K eff ; or the R -insensitivity of FCG in inert environments. Nevertheless, strip-yield models (SYM) based on Δ K eff ideas are more used for FCG life predictions than alternative models based on any other principles. To verify whether SYMs are indeed intrinsically better, their mechanics is used to predict FCG rates based both on Elber’s ideas and on the alternative view that FCG is instead due to damage accumulation ahead of the crack tip, which does not need the Δ K eff hypothesis or arbitrary data-fitting parameters. Despite based on conflicting principles, both models can reproduce quite well FCG data obtained under quasi-constant Δ K loading, a somewhat surprising result that deserves to be carefully analyzed. [ABSTRACT FROM AUTHOR]

The studies reported in the literature on the effect of stress ratio or mean stress on fatigue delamination growth in composites are critically reviewed in this paper. These studies are classified according to their mode of fracture. Majority of the studies lack a mechanistic approach to the delamination growth characterization and the effect of stress ratio. The researchers are divided in the use of a unified correlating parameter for the delamination growth. The mechanisms of fatigue crack growth in metals are often extended to the delamination growth characterization. [ABSTRACT FROM AUTHOR]

Highlights: [•] An analytical model is proposed for describing the growth of cracks of arbitrary length. [•] Newman’s crack opening function is modified to account for short crack effects. [•] Fictitious length scales are used to account for multiple crack closure mechanisms. [Copyright &y& Elsevier]

Abstract: A novel in situ scanning electron microscope (SEM) fatigue testing is proposed in this paper to investigate the fatigue crack growth mechanisms within one cyclic loading under plane stress conditions. The objectives of the experimental study are to verify the hypotheses of a small time scale fatigue crack growth model and to develop a new experimental methodology for the detailed mechanism investigation of fatigue crack growth. During the testing, one loading cycle is uniformly divided into a certain number of steps. At each step, high resolution images are taken around the crack tip region. Imaging analysis is used to quantify the crack growth kinetics and crack tip deformation behavior at any time instant in a loading cycle. Crack closure phenomenon is directly observed during the crack growth process in the current investigation. It is also observed that crack growth is not uniformly distributed within a loading cycle and only happens during a small portion of the loading path. Multiple mechanisms exist within one cyclic loading, which is not able to be captured using the classical cycle-based approaches. A detailed discussion is given based on the current investigation to explain some well-known issues in the classical fatigue theory. [Copyright &y& Elsevier]

Abstract: The overload retardation effect on fatigue crack growth rate (FCGR) in titanium alloy Ti–6Al–4V is studied. Synchrotron X-ray diffraction strain mapping of near-crack tip regions of pre-cracked fatigued samples is used to determine the effective stress intensity factors experienced by the crack tip. The effective stress intensity factor values are computed by finding the best match between the experimental strain maps and linear elastic fracture mechanics (LEFM) predictions. The dependence of the effective stress intensity factor, K, on the applied load is plotted, and an interpretation of the overload retardation effect is proposed. The present approach permits to reconcile the traditional LEFM fatigue crack propagation prediction and the experimental measurement of strain fields. [ABSTRACT FROM AUTHOR]

Abstract: A new fatigue crack growth formulation at the small time scale is proposed in this paper. This method is fundamentally different from the classical reversal-based fatigue analysis and is based on the incremental crack growth at any time instant within a cycle. It can be used for fatigue analysis at various time and length scales and is very convenient for the fatigue analysis under random variable-amplitude loadings without cycle-counting. Stress ratio effect is intrinsically considered in the proposed fatigue model since the stress state is directly used instead of using the cyclic stress range. In the proposed methodology, the reverse plastic zone concept is adopted to determine the lower integration limit during the time integral for crack length calculation. Model validation is performed using extensive experimental observations for various metallic materials under both constant amplitude and variable-amplitude loadings. Statistical error analysis is used to compare the proposed model with existing fatigue crack growth codes. Generally, very good correlations are observed between model predictions and experimental observations. [Copyright &y& Elsevier]

Abstract: Fatigue crack growth tests of three ferrite–pearlite steels with different size and spacing of pearlite particles, which were uniformly distributed in the ferrite matrix, were carried out to investigate the effect of microstructure on fatigue crack growth behavior in the Paris regime. The fatigue crack growth rates for the three materials did not coincide with each other, even when the crack growth curves were arranged by the effective stress intensity factor range. From in situ observations, crack tip stress shielding phenomena, such as interlocking and branching, were found on the crack wake, which enhanced fatigue crack growth resistance. A small size and spacing of pearlite particle seemed to induce small but frequent crack deflections, which resulted in crack closure phenomena. On the other hand, the large size of the pearlite particle seemed to induce stress shielding phenomena and, thus, contribute to high crack growth resistance, which was the main reason for the higher fatigue crack growth resistance of the large size and spacing of pearlite particles compared to the small size of pearlite particles. [Copyright &y& Elsevier]

Abstract: Three different methodologies for characterizing fatigue crack closure are compared using constant amplitude fatigue crack growth simulations. The adjusted compliance ratio (ACR), crack wake influence (CWI), and conventional Elber crack opening load approaches were considered, with the CWI approach reported in the literature generalized to allow a crack surface traction distribution. The ACR methodology consistently indicated lesser levels of crack closure or shielding, which were insensitive to the applied stress level, when compared with the CWI and opening load approaches for R >0. Closure levels using the Elber and CWI methodologies compared well for lower values of applied stress. For R =−1, the three methodologies resulted in similar levels of crack tip shielding. No evidence was observed that applied loading below the conventional crack opening load becomes more significant in the near-threshold regime. [Copyright &y& Elsevier]

A technique to in-situ measure the displacements in the vicinity of the crack tip during fatigue crack propagation has been developed. High-resolution images of the crack tip were taken continuously throughout the fatigue load cycles with a scanning electron microscope (SEM), and an image analysis program was used to determine the displacements at different positions with respect to the crack tip. The displacements were then used to determine crack shapes and compliance curves. The measured crack shapes show a general √ r dependence versus the distance to the crack tip. However, close to the crack tip the crack shape is clearly affected by plastic deformation, even in cases when small scale yielding prevails. The compliance curve measurements close to the crack tip can be used to determine the global stress level when the crack surfaces are separated, so that the exact opening and closure stresses can be determined. [ABSTRACT FROM AUTHOR]

A paste comprising fine alumina particles was applied on surfaces of steel plate specimens and a welded joint specimen, and the effects of the paste on restraint and visual detection of fatigue crack growth were experimentally investigated by performing fatigue tests with in situ observations by a harge oupled evice (CCD) microscope and through various kinds of fractographic observations and elemental analyses using an scanning electron microscope (SEM) and an electron probe microanalyser (EPMA). As a result, the crack growth rate in the plate specimen was drastically retarded by the wedge effect of the alumina particles, and a 311% increase in failure life was produced on an average. The restraint of crack opening displacement (COD) value by the wedge materials was estimated by elastic finite element (FE) analyses, and the results were compared with the experimental data. Similar crack growth restraint effects were also observed on the welded joint specimen, producing a 117% increase in failure life. In the fatigue tests of the specimens on which the alumina paste was applied, a remarkable black colour developed in the white alumina paste along the paths of crack propagation, facilitating the visual detection of the cracks. An analysis using an X-ray diffractometer showed that the black matter in the paste consists of fine debris derived from the base metal. [ABSTRACT FROM AUTHOR]

This paper examines fatigue crack paths in coarse-grained magnesium alloys. Visualization of the microstructure by etching and continuous crack monitoring with a microscope provide evidence of the crack-grain interaction at different fatigue crack growth regimes. Near-threshold fatigue crack growth promotes the development of rough surfaces by the activation of single slip mechanisms and crack deflection. TheR-ratio effect on near-threshold results is discussed in the light of the partial crack-closure model. The role of grain boundaries and grain-to-grain crack deflections on fatigue crack growth rates is discussed. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study related to the influence of different maximum load, Pmax, levels on crack closure, and on fatigue crack growth, at a negative stress ratio, R=−1. It focuses on crack tip plasticity effects and on roughness effects on crack closure. Measurements of roughness and of crack opening loads are made, in order to verify their influence on crack propagation rate. It is confirmed that, at negative stress ratios, crack closure changes with Pmax for the same R ratio. It is also confirmed that roughness is not a relevant mechanism at negative stress ratios. It is demonstrated in this study that the crack closure concept is not adequate to explain crack propagation rate as a function of Pmax at R=−1. In its place, a concept based on plasticity and on cyclic plastic properties, or on internal stresses, should be used to explain fatigue crack propagation. [Copyright &y& Elsevier]

Crack closure is the most used mechanism to model thickness and load interaction effects on fatigue crack propagation. But assuming it is the only mechanism is equivalent to suppose that the rate of fatigue crack growth da/dN is primarily dependent on ΔKeff=Kmax−Kop, not on ΔK. But this assumption would imply that the normal practice of using da/dN×ΔK curves measured under plane-stress conditions (without considering crack closure) to predict the fatigue life of components working under plane-strain could lead to highly non-conservative errors, because the expected fatigue life of “thin” (plane-stress dominated) structures could be much higher than the life of “thick” (plane-strain dominated) ones, when both work under the same stress intensity range and load ratio. However, crack closure cannot be used to explain the overload-induced retardation effects found in this work under plane-strain, where both crack arrest and delays were associated to an increase in ΔKeff. These results indicate that the dominant role of crack closure in the modeling of fatigue crack growth should be reviewed. [Copyright &y& Elsevier]

Results are presented for fatigue crack growth at elevated temperatures during both constant amplitude and variable amplitude loading. A careful experimental investigation is presented to show that the concept of the effective stress-intensity factor range ΔKIeff can be used to eliminate the load ratio effect on fatigue data and produce one single set of crack growth data. The fatigue crack propagation data corrected for crack closure is then used for comparison of mean crack propagation rates for variable amplitude loads with simple time histories. It is concluded that measured crack closure can not fully explain the discrepancies between measured and predicted propagation rates. A reduction factor fitted to the experimental data could be used to successfully correlate the results. [Copyright &y& Elsevier]