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ABSTRACT The desire to use additively manufactured (AM) parts to ensure the availability of military aircraft, and to build limited‐life unmanned aerial vehicles (drones), coupled with the United States Air Force (USAF) approach to the airworthiness certification of AM parts has focused attention on durability analysis/assessment, and hence on the growth of small cracks in AM parts. Previous studies have shown that laser powder fusion built (LPBF) Scalmalloy® has: i) A yield stress and an ultimate strength that are greater than that of AA2024‐T3 and comparable to that of AA7075‐T6; ii) A resistance to crack growth that is better than that of AA7075‐T6 and comparable to that of AA2024‐T3. However, since the ability to predict the durability of a part is essential for its airworthiness certification, the present paper illustrates how to perform a linear elastic fracture mechanics (LEFM)‐based durability assessment of Boeing Space, Intelligence and Weapon System (BSIWS) LPBF Scalmalloy®. The durability study includes specimens with both machined surfaces and surfaces left in the as‐built condition. As a result, it would appear that BISWS AM LPBF Scalmalloy® is an ideal candidate for building limited‐life AM replacement parts for fixed and rotary wing aircraft and drones. [ABSTRACT FROM AUTHOR]

Previous research has consistently found that introducing metastable retained austenite (RA) as a second phase retards the failure of steel under fatigue. However, the reasons for this benefit are not understood. Accordingly, the properties of RA most advantageous to resist fatigue are not known. Within this context, this paper examines the interaction between second-phase RA and short fatigue crack growth in a steel processed via quenching and partitioning, using quasi in situ electron backscatter diffraction experiments. Results show that most RA transforms into martensite under the plastic strain surrounding the crack. They also reveal various mechanisms whereby RA transformation delays short fatigue crack propagation; transformation-induced crack closure (TICC), crack deflection and branching, and roughness-induced crack closure (RICC). Crack deflection and branching are driven by a tendency of cracks to propagate towards transformed RA, which is against the previous assumptions in the literature. Furthermore, the impact of crack deflection/branching on retardation is more powerful than that of TICC acting alone. Microstructures including second-phase RA should avoid RA-lean areas and promote elongated RA grains, with relatively large size, and major axis normal to the preferential crack growth direction. Untransformed RA within the plastic zone (i.e. overstabilized) does not contribute to crack retardation.

[Display omitted] • First ever recommendations for the design of steel microstructures including second-phase retained austenite to resist fatigue crack propagation. • Short fatigue cracks grow towards second-phase retained austenite which transforms into martensite inside the plastic zone associated with the crack. • Retained austenite retards short fatigue crack growth by inducing crack deflection, branching and roughness-induced crack closure. • Large retained austenite grains with high aspect ratio and major axis parallel to the principal tensile stress promote fatigue toughness. Previous research has consistently found that introducing metastable retained austenite (RA) as a second phase retards the failure of steel under fatigue. However, the reasons for this benefit are not understood. Accordingly, the properties of RA most advantageous to resist fatigue are not known. Within this context, this paper examines the interaction between second-phase RA and short fatigue crack growth in a steel processed via quenching and partitioning, using quasi in situ electron backscatter diffraction experiments. Results show that most RA transforms into martensite under the plastic strain surrounding the crack. They also reveal various mechanisms whereby RA transformation delays short fatigue crack propagation; transformation-induced crack closure (TICC), crack deflection and branching, and roughness-induced crack closure (RICC). Crack deflection and branching are driven by a tendency of cracks to propagate towards transformed RA, which is against the previous assumptions in the literature. Furthermore, the impact of crack deflection/branching on retardation is more powerful than that of TICC acting alone. Microstructures including second-phase RA should avoid RA-lean areas and promote elongated RA grains, with relatively large size, and major axis normal to the preferential crack growth direction. Untransformed RA within the plastic zone (i.e. overstabilized) does not contribute to crack retardation. [ABSTRACT FROM AUTHOR]

A critical load (force) is estimated for a space with a penny-shaped crack in a case where a concentrated force acts on its faces along the circumference. It is shown that it is impossible to determine the ultimate force with the help of classical fracture criteria, so a structural strength criterion formulated for axisymmetric problems is used. A relationship between fracture toughness and crack size is investigated and a two-criteria fracture diagram is constructed. [ABSTRACT FROM AUTHOR]

The effect of periodic overloads on crack initiation, growth rate, and fatigue life, within a notch stress field representative of a turbine blade root fixing, has been investigated. Bend bars made from FV566 martensitic stainless steel were extracted from the root of ex‐service power plant turbine blades and representative notches introduced. These notched plain bend bars were loaded plastically in the low‐cycle fatigue regime and were tested with overloads up to 150% of the cyclic baseload every 150 baseload cycles. A periodic overload of 50% of the cyclic baseload increased the number of cycles to crack initiation and slightly retarded the crack growth rates of both short and long cracks, leading to a slight improvement in fatigue life. The results suggest that small overloads (less than 10%) are not expected to significantly impact fatigue lifetimes or service scheduling of components such as low‐pressure steam turbine blades. HIGHLIGHTS: Number of cycles to crack initiation was delayed due to overload cycles.Short crack growth rate in the notch was retarded due to overload cycles.Overall improvement in lifetime for notched samples when overload cycles applied.Increased resistance to fatigue due to compressive residual stress from overload cycles. [ABSTRACT FROM AUTHOR]

This work analytically calculates mixed-mode I/II stress intensity factors for an inclined central frictional crack in a Brazilian disc partially compressed with a uniform pressure. Wide-range controlling parameters are invoked. The analyzed parameters include the pressure angle, relative crack length, crack inclination, and friction coefficient between the two crack surfaces. For partially compressed discs, there exists a critical pressure angle at which a crack with any angle will experience pure II loading. This critical pressure angle is not affected by friction and decreases as the crack length increases. A crack of any angle becomes inactive once the pressure angle exceeds another critical value of pressure angle. This value decreases as the friction coefficient or/and crack length increases. The handling of frictional surfaces of short cracks is simplified. The analysis defines the conditions to have (1) the crack opened and (2) the two crack surfaces sticking. A close agreement is achieved with relevant findings in previous relevant investigations. The present results do not exist in relevant literature and are valuable in solving fracture mechanics problems of significance in relation to crack path in cracked brittle materials.

Overall damage tolerances of the heat‐treated 34CrMo4 steels having ferritic‐pearlitic, bainitic, and tempered‐martensitic microstructures were evaluated based on their threshold stress intensity factor prior to small crack propagation, fatigue strength, and fracture toughness under static loading. Kitagawa‐Takahashi diagrams were constructed to determine the limiting size of small crack propagation. The micromechanical effects of carbide morphology and phase distribution on quasi‐static and dynamic mechanical properties were also elaborated. Fractographic investigations were carried out on the notched fatigue test specimens to distinguish deterioration and deformation mechanism of the microstructure under reversed cyclic loads. Finally, improvements in the damage tolerance were discussed to present the advantages and disadvantages of each heat treatment procedure to minimize in‐service fatigue failures. [ABSTRACT FROM AUTHOR]

Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behavior of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behavior of human cortical bone which accurately assesses its toughness at appropriate size-scales. Here we use in-situ mechanical testing in the scanning electron microscope and x-ray computed tomography to examine how physiologically-pertinent short (

In this paper, the initiation and propagation behaviour of short fatigue cracks in nickel-base alloy Inconel 718 is studied extensively in the fatigue loading regime by ex-situ scanning electron microscopy on component-near notched specimens in the temperature range between 400 °C and 600 °C. To investigate the influence of different surface treatments, both specimens with polished and shot-peened surfaces were tested. For polished specimens, most fatigue cracks initiated at oxidised non-metallic inclusions, whereas for shot-peened specimens, the process-induced surface roughness also served as crack starters. It was found, that the observed scatter in lifetime for repetitive tests is mainly caused within the short crack fatigue regime below 50 μm, while cracks larger than 100 μm showed similar growth characteristics. • Ex-situ SEM crack initiation and crack propagation behaviour investigation of Inconel 718 at elevated temperatures. • Investigation of polished and shot-peened specimens. • Detailed information about crack initiation and growth behaviour. • Statistical evaluation of crack initiation sites (e.g. non-metallic inclusions or overlapping due to shot peening). • Evaluation and assessment of non-metallic inclusion distribution on fracture surface. [ABSTRACT FROM AUTHOR]

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 μm wide x 50 μm deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface.

The behavior of short cracks that depart from elastoplastic notch tips is modeled to estimate the stresses required to initiate and to propagate cracks in notched structural components, and to evaluate the size of tolerable crack-like defects under general loading conditions. This analysis can model both fatigue and environmentally assisted cracking problems; can evaluate notch sensitivity in both cases; and can as well be used to establish design or acceptance criteria for tolerable non-propagating crack-like defects in such cases. The growth of short cracks is assumed driven by the applied stresses and by the stress gradient ahead the notch tip, and supported by the material resistances to crack initiation and to long crack propagation by fatigue or EAC. In the elastoplastic case, the stress gradient ahead of the notch tip is quantified by a J-field to consider the short crack behavior. The tolerable short crack predictions made by this model are evaluated by suitable fatigue and EAC tests of notched specimens specially designed to start nonpropagating cracks from the notch tips, both under elastic and elastoplastic conditions.

Fatigue damage is one of the leading causes for structural failure of long-span steel bridges, but fatigue damage evolution of a long-span steel bridge is very complex. This study proposes a concurrent multi-scale fatigue damage evolution simulation method for long-span steel bridges from micro short crack nucleation and growth to macro structural component damage until mega structural failure. As a case study, the fatigue damage evolution of the Stonecutters Bridge in Hong Kong under cyclic vehicle loading is finally simulated using the proposed method. It shows that the proposed method is computationally feasible even for such a large scale structure. The method can provide a clear picture how micro short cracks grow into macro fatigue damage of structural components and eventually lead to mega structural failure. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Resistance of CTB to dislocation slip was quantitatively evaluated by MD simulations. • Discrete dislocation formulation revealed the dependence of microscopic threshold on material nature and loading condition. • A multiscale model was established to describe FCG for short cracks. Abstract An atomistics-informed multiscale modeling approach was proposed to quantify the transgranular short crack growth during fatigue in polycrystalline metals. The approach solved the two scale inconsistency problems of the classical N-R model, replacing the arbitrary materials strength parameter by the atomistically determined ideal shear strength in the evaluation of both the fatigue crack growth (FCG) rate and the corresponding range of short-crack-featured fluctuations, and incorporating atomistically simulated Peierls stress into the discrete dislocation formulation to quantify the microscopic threshold of FCG through explicitly modeling the interactions between dislocations and grain boundaries. The model-predicted FCG results were compared with the experimental FCG data of GH4169 Ni-based superalloy, demonstrating improved predictive capability than the classical N-R model. Our study provides new physical insights and improved predictability towards accurate quantification of the short crack growth in polycrystalline metals. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The subsurface porosity controls the fatigue properties of the machined laser-welded Ti-6Al-4V butt joints. • The fish-eye pattern of the fracture surface consists of optically dark area and smooth area. • Crack-initiating pores are, on average, larger than X-ray-detected pores. • Environmental and short-crack effects are critical for fatigue-life assessment model. • Porosity distribution incorporated into the prediction model enables the prediction of the fatigue scatter range. Abstract The present paper focuses on the effect of inherent welding-induced defects on the high-cycle fatigue behaviour of laser-welded Ti-6Al-4V butt joints. The transition of the crack origin from the surface to the subsurface occurs upon removal of the surface stress concentrators. Under these circumstances, fatigue cracks nucleate at subsurface porosity and show a typical fish-eye pattern of fracture surface. A fatigue-life assessment model has been developed for internally flawed materials based on a fracture-mechanics approach, which takes effects of short-cracks into account. A novel approach for the simplified construction of the cyclic resistance curve of internal cracks is proposed herein. Using statistical methods, the experimentally determined porosity distribution has been incorporated into the model to predict the fatigue scatter range. The presented methodology can potentially be used to achieve a required reliability of the welded joints with respect to fatigue as a design criterion. [ABSTRACT FROM AUTHOR]

The work presents the procedure developed within the German research project IBESS , which allows for the fracture mechanics-based prediction of the fatigue strength of welded joints under constant amplitude loading. Based on the experimental observations of the crucial failure mechanisms, the approach focuses on the short crack propagation, where elastic-plastic fracture mechanics and the build-up of closure effects must be considered, as well as the variability of the local geometry at the weld toe and the modelling of multiple crack interaction. Analytical solutions are provided for the approximation of the through-thickness stress profiles at the weld toe and for the determination of the crack driving force in the form of a plasticity-corrected stress intensity factor range Δ K p . Proposals for the determination of the initial crack size and the crack closure factor are also included. The approach is validated against a large number of experimental data, which comprises fatigue tests on individual cracks monitored by heat-tinting and beach-marking techniques, as well as stress life curves. Three kinds of welded joints, two steels of significant different strength, two welding techniques and three stress ratios are considered. The results show that the procedure provides good estimations of the statistical distribution of the fatigue strength of welded joints both for the finite and infinite life regime. Furthermore, the predictions are compared with available benchmark data for structural steels. [ABSTRACT FROM AUTHOR]

Estimation of short crack propagation rates has proven to be a key factor in design and maintenance of metallic components subjected to loading conditions related to high cycle fatigue (HCF). Extrapolation of long crack fracture mechanics approaches fails to correlate short crack behaviour, since crack length dependence of the short crack propagation threshold is not accounted for. In this paper, a fracture mechanics approach for prediction of HCF behaviour involving short cracks is analysed. The method is defined through application of the resistance curve concept and a short crack propagation threshold prediction model. Different threshold estimation methods are reviewed and compared. Application of the proposed model is exemplified and results of predicted crack propagation rates and estimated fatigue lives are presented and discussed. [ABSTRACT FROM AUTHOR]

Fretting fatigue failure of press fitted railway axle-wheel assembly was presented. Size, distribution and propagation profiles of the circumferential fretting cracks on the full-scale axles were determined by magnetic particle and metallographic slicing methods. The distribution of multiaxial stress cycle along the press-fit seat was obtained by finite element analysis (FEA). The obtained stress path was used for interpretation of fractographic evidence collected at crack initiation sites and the crack propagation plane. Metallurgical and mechanical characterization of the axle material (34CrMo4) was made on the specimens sampled from the broken axles. The threshold conditions for propagation of small fretting cracks were determined by Kitagawa analysis and El-Haddad correction method. The results of the experimental study were compared with EA1N grade steel which is given as the reference axle material in the EN standards. The metallurgical factors affecting the fretting fatigue crack initiation and propagation were investigated. The causes of the examined axle failures were associated with the deteriorated mechanical properties of the axle material. [ABSTRACT FROM AUTHOR]

Whilst crack growth retardation following the application of an overload during variable amplitude loading is a well-known spectrum effect, the order of smaller amplitude cycles can also affect crack growth. One method of accounting for such spectrum effects in crack growth prediction algorithms is by correcting the estimated effectiveness of each cycle or group of cycles. Such predictions are usually verified by comparison to demonstrated total fatigue lives. These corrections have been shown to be suitable for cracks that are at least several millimetres long. However, for physically small cracks: here defined as less than a few mm in depth, such predictions are often far less accurate. In the case of aluminium alloy aircraft structures, where cracks typically grow approximately exponentially from small discontinuities, these predictive algorithms should also be evaluated for their ability to predict small crack growth rates, rather than total life alone. To examine the more subtle crack growth acceleration and retardation effects from individual load cycles in typical aircraft structural 7XXX aluminium alloys, this paper presents the results of quantitative fractography measurements of very small fatigue crack growth increments produced under the application of specially engineered spectra. One such spectrum that contained short segments of constant amplitude loads of various mean stresses, was designed to examine the extent of crack growth retardation, such as from closure, at a range of small crack lengths. Other spectrum effects were also explored through use of test spectra with constant amplitude cycles that were either separate to or embedded within small segments of variable amplitude loading. The local growth produced in each case was then measured and compared to predictions produced by two closure/retardation correction methods; based on wake and plastic zone related phenomena. The results showed that the prediction methods, although they made allowances for the presence of subtle retardations and accelerations, did not accurately predict these effects over the increments of growth examined. [ABSTRACT FROM AUTHOR]

Non-propagating fatigue crack lengths were analytically calculated under stress control conditions using plasticity-induced crack closure analysis with the Dugdale model. In addition, a non-dimensionalization method was applied in terms of the Burgers vector and a monotonic plastic zone size under small-scale yielding conditions, which was validated for various initial crack lengths and material properties. When the yield strength was increased, the non-propagating fatigue crack lengths were found to increase for a short crack and decrease for a long crack. The non-dimensionalization enabled the analytical derivation of a generalized non-propagating fatigue crack length, which can be utilized for fatigue designs. The material property controlling the threshold stress intensity factor range of mechanically and physically short-crack was discussed. [ABSTRACT FROM AUTHOR]

Fatigue test data of duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue (VHCF) regime is presented. Fatigue testing was conducted using ultrasonic fatigue test equipment operating at 20 kHz under fully reversed tension-compression load condition. Scanning Electron Microscope (SEM) analysis of the fracture surfaces and external surfaces of failed specimens was conducted. Electron Channelling Contrast Imaging (ECCI) and Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens was done to analyse the accumulation of plastic fatigue damage and fatigue crack growth in the grains adjacent to the external surface and crack initiation site. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 was carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the Crystallographic Growth Region (CGR) showed features like grain boundaries and fatigue striations on the fracture surfaces. SEM analysis of the external surfaces of fatigue loaded specimens showed inhomogeneous accumulation of plastic fatigue damage. ECCI/EBSD analysis showed Persistent Slip Bands (PSBs) in ferrite grains in LDX 2101 grade but no PSBs were observed in any grains of 2304 SRG specimens. The barrier effect of grain and phase boundaries on short fatigue crack propagation was observed. TEM analysis of thin foils cut from the failed specimens of LDX 2101 showed stacking faults in austenite grains and they were seen to stop at the grain and phase boundaries. [ABSTRACT FROM AUTHOR]

The behavior of short cracks that depart from elastoplastic notch tips is modeled to estimate the stresses required to initiate and to propagate cracks in notched structural components, and to evaluate the size of tolerable crack-like defects under general loading conditions. This analysis can model both fatigue and environmentally assisted cracking problems; can evaluate notch sensitivity in both cases; and can as well be used to establish design or acceptance criteria for tolerable non-propagating crack-like defects in such cases. The growth of short cracks is assumed driven by the applied stresses and by the stress gradient ahead the notch tip, and supported by the material resistances to crack initiation and to long crack propagation by fatigue or EAC. In the elastoplastic case, the stress gradient ahead of the notch tip is quantified by a J-field to consider the short crack behavior. The tolerable short crack predictions made by this model are evaluated by suitable fatigue and EAC tests of notched specimens specially designed to start nonpropagating cracks from the notch tips, both under elastic and elastoplastic conditions. [ABSTRACT FROM AUTHOR]

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 μm wide x 50 μm deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface. [ABSTRACT FROM AUTHOR]

This paper shows an implementation of the Microstructural Finite Element Alternating Method (MFEAM) to calculate fatigue strength of engineering components subjected to complex loading conditions. These conditions include combination of primary cyclic loading and residual stress fields or localised stress distributions due to the contact of two solids. The alternating method uses the finite element method (FEM) to solve the boundary value problem of the un-cracked body, allowing treatment of arbitrary shaped components, in conjunction with a short crack model to account for the interaction of the crack with the microstructural barriers, implemented within the distributed dislocation technique (DDT) to assess the crack problem in an infinite medium. An iterative scheme between the FEM and the DDT solutions is proposed in order to predict the required applied load to propagate the crack in the finite size body. Comparisons of results with reported data in the literature show that the MFEAM can be used effectively to analyse finite size components and that it is capable of capturing the effect of localised stress concentration features on cyclic behaviour. [ABSTRACT FROM AUTHOR]

The fatigue behavior of metallic materials fabricated via additive manufacturing processes is currently not well understood and it has been the subject of many experimental investigations in recent years. In this paper we carried out a literature review about the fatigue strength of additively manufactured AlSi10Mg and Ti6Al4V, especially in terms of sensitivity to defects and inhomogeneities. The analysis shows that fatigue properties and key variables (heat treatment, defect size) are very similar to the ones of parts obtained with traditional manufacturing processes. These results confirm that defect tolerant design concepts can be adopted also for AM components. [ABSTRACT FROM AUTHOR]

The purpose of present work is to deepen the understanding of crack propagation, non-propagation, and fatigue limit. Quantitative prediction of fatigue crack non-propagation in constant amplitude loading is attempted through extensive 3D elastic–plastic finite element analysis of near-threshold propagation of short cracks nucleating from surface defects. Level of plasticity-induced crack closure is used to estimate the effective stress intensity factor range, which is evaluated in terms of non-propagation potential. It is shown that plasticity-induced crack closure alone is adequate in producing a local minimum in the development of effective stress intensity factor range, which gives a prerequisite to crack arrest. • Plasticity-induced closure quantification of a surface defect-initiated short crack. • By itself, plasticity-induced crack closure development can lead to crack arrest. • Minimum value of Δ K eff exhibits dependency of stress ratio and defect size. • Other closure mechanisms are also required to reach intrinsic threshold Δ K th,eff . [ABSTRACT FROM AUTHOR]

• Plastic notch effects in fatigue strength due to their tolerance to non-propagating short cracks. • Consequences of plastic fatigue stress concentration factor on SN curves. • The Theory of Critical Distances (TCD) as a simplification of the stress gradient method. Elastoplastic (EP) stress gradient factors (SGF) ahead of notch tips are used to evaluate the actual notch effects in fatigue strength due to their tolerance to non-propagating short cracks, which depends on the notch stress concentration factor, on the stress gradient ahead it, and on the material. Considering the significant role of local plasticity in the growth behavior of short cracks within the notch plastic zone, a sound mechanical methodology is proposed to account for the effect of EP stress and strain fields in the actual fatigue stress concentration factor value, which includes its tolerance to short cracks. To validate the proposed methodology, life predictions based on numerical EP SGF are compared to experimental SN data of notched specimens made of different materials tested under uniaxial loads collected from the literature. [ABSTRACT FROM AUTHOR]

The fatigue life of ductile metals is controlled mainly by the propagation life of a short surface crack; to clarify the growth behavior of short cracks is crucial factor to the safe design of smooth surface members. However, little has been reported on the growth behavior of short surface cracks in thermal-chemical treated duralumin. In the present study, stress-controlled fatigue tests (under rotating bending load) for 70 hours artificial aged duralumin 7075-T6 without chemical treatment case (1) and with chemical treatment case (2). The growth behavior of short cracks was monitored by a plastic replication technique and the surface damage (short crack propagation) during cyclic stressing was observed by optical microscopy. The physical background of fatigue damage for case (1) and case (2) was discussed from the viewpoints of the initiation and growth behavior of short cracks.

The paper contains results of an investigation of effects of shot peening on growth of physically short fatigue cracks in a 2.4 mm thick sheet of an aircraft V-95 Al-alloy (a type of a 7075 alloy), clad with a 7072 Al-alloy (Al-Zn1). Using an improved and verified experimental methodology, particularly direct current potential drop (DCPD) method, short fatigue cracks of the length from 0.2 mm to more than 3 mm, most of them between 0.8 - 1.5 mm, were prepared under high cycle fatigue repeated tensile loading of the constant nominal stress range 160 MPa, load asymmetry R 0. Edges of the specimens with already existing short fatigue cracks were then shot peened using two different groups of parameters. Some reference specimens without cracks were shot peened with the same parameters. Microhardness measurement and informative measurement of residual stresses near the peened edges were carried out. Significant retardation of crack growth was observed particularly in case of cracks shorter than 2 mm. The work was completed by a unique measurement of actual crack growth rate throughout the peened area with residual stresses starting with crack length less than 0.2 mm. Different stages of the growth are discussed.

• A laboratory-scale solution for the tracking of a surface crack path in the short fatigue crack regime under different cyclic loading conditions has been proposed, based on the introduction of an artificial defect produced by means of the focused ion beam (FIB) technology. • Such a strategy is an alternative to other ones that imply either post-mortem microstructural examination of fatigue specimens or a limited flexibility concerning the operational conditions. • The effect of the local microstructure of an advanced steel, nanostructured bainite, on fatigue crack initiation and short crack growth has been revealed. For a full exploitation of novel steels in applications under mechanical cyclic loading, it is vital to understand the crack-microstructure interactions. In this work, the fatigue behaviour of a nanostructured bainitic steel in the short crack regime has been studied considering the structural integrity of the material and its polycrystalline nature. For that purpose, a specimen containing an artificial defect has been designed and manufactured, subjected to interrupted fatigue testing, and examined by crystallography-sensitive techniques. Results show that, depending on the loading conditions, different crystallographic boundaries can play an important role in the crack path and crack growth rate. [ABSTRACT FROM AUTHOR]

In order to reach the best numerical properties with the numerical manifold method (NMM), uniform finite element meshes are always favorite while constructing mathematical covers, where all the elements are congruent. In the presence of steep gradients or strong singularities, in principle, the locally-defined special functions can be added into the NMM space by means of the partition of unity, but they are not available to those complex problems with heterogeneity or nonlinearity, necessitating local refinement on uniform meshes. This is believed to be one of the most important open issues in NMM. In this study multilayer covers are proposed to solve this issue. In addition to the first layer cover which is the global cover and covers the whole problem domain, the second and higher layer covers with smaller elements, called local covers, are used to cover those local regions with steep gradients or strong singularities. The global cover and the local covers have their own partition of unity, and they all participate in the approximation to the solution. Being advantageous over the existing procedures, the proposed approach is easy to deal with any arbitrary-layer hanging nodes with no need to construct super-elements with variable number of edge nodes or introduce the Lagrange multipliers to enforce the continuity between small and big elements. With no limitation to cover layers, meanwhile, the creation of an even error distribution over the whole problem domain is significantly facilitated. Some typical examples with steep gradients or strong singularities are analyzed to demonstrate the capacity of the proposed approach. [ABSTRACT FROM AUTHOR]

Samples of S355 steel with different initial surface roughnesses were treated via ultrasonic peening. The micro characteristics and fatigue properties of samples that underwent ultrasonic peening treatment (UPT) were evaluated. Our results indicate that in addition to residual compressive stress, the UPT process might also introduce short cracks, known as fold defects at the surface of metal samples. Fold defects were observed in all samples that underwent UPT, with the extent of the defects depending on both the initial surface roughness of the sample and the UPT time. When UPT time and surface roughness were optimized, the fatigue life was double that of untreated steel. This may be due to the formation of small fold defects and the introduction of compressive stress in the metal. However, long treating times led to a significant reduction in the fatigue life to levels lower than those for untreated samples. We hypothesize that extended UPT times can induce an excess of fold defects with complex morphologies and large dimensions, leading to the formation of cracks where sample failure can occur. [ABSTRACT FROM AUTHOR]

Fatigue crack growth at room temperature and its relation to the local microstructure is studied for four different γ-TiAl-alloys with microstructures ranging from coarse and fully lamellar to fine and partly lamellar. It is shown that the number of cycles to failure depends strongly on the efficiency of the first barrier to crack extension, as crack growth rates may increase rapidly once this barrier has been breached by a specific crack. The crack extension behaviour for two typical barriers (colony boundary and twin boundary) is studied using high-resolution optical and scanning electron microscopy. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microstructurally small fatigue crack growth (FCG) rate is studied by DIC. • FCG rate retardation is associated with shear strain localisations. • Grain boundaries control interaction between strain localization and FCG. • Shear strain localisations lead to formation of grain sub-structures. • Intense cross slip facilitates the overcoming of the strain localisation zone. Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain localisations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retardation events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant. [ABSTRACT FROM AUTHOR]

Fatigue crack growth was investigated in a line pipe steel. Severe straining conditions, like those experienced by pipelines, were considered during the experiments. Crack closure effects were investigated during the tests by adopting an innovative technique based on digital image correlation. Experimentally measured crack closure levels were implemented in a crack propagation model based on elastic–plastic fracture mechanics. A modified formulation of Δ J eff , necessary to accurately describe crack propagation driving forces, is presented and employed to assess fatigue life in presence of high plastic strains. [ABSTRACT FROM AUTHOR]

In a number of previous papers, the authors have proposed a model for fracture mechanics based prediction of the S-N characteristics of metallic components with large microstructural defects. Here, an extension to materials that do not show large defects onto the fracture surfaces is provided. In such cases, an approach based on a so-called cyclic R-curve analysis is proposed for the determination of the initial flaw size, which has to be used in the calculation of fatigue crack propagation. The principle is explained and demonstrated by a first application to a welded joint. [ABSTRACT FROM AUTHOR]

During VHCF loading of duplex steels fatigue damage starts within the softer austenite phase. Depending on the spatial mismatch between the slip systems of neighboring grains, intergranular or transgranular fatigue cracks are initiated. If these cracks can propagate or are blocked depends on the interactions with the local microstructure. These interactions have been studied by ultrasonic fatigue testing in combination with electron microscopy and synchrotron diffraction experiments. It was found that the strength of the first microstructural barrier is decisive for VHCF life. The barrier strength as a function of the microstructural arrangement has been implemented in a micromechanical finite element approach that shall be used to predict the VHCF life or the existence of a fatigue limit for given microstructure parameters. [ABSTRACT FROM AUTHOR]

Although methods for predicting the growth and coalescence of cracks have been introduced, uncertainties persist in structures with multiple short cracks. Quantitative fractographic methods were used to study the way in which multiple short cracks in AA7050-T7451 of varying sizes on the same and on different planes grew under low amplitude cyclic loading. The accuracy of existing theoretical methods for predicting the growth and coalescence of multiple cracks was evaluated. Experimental results corroborate the findings that growth of the ‘largest’ crack dominated the fatigue life of a specimen with very little influence from relatively shorter cracks. [ABSTRACT FROM AUTHOR]

Crack propagation in full plastic zones is one of the main factors for life assessment of a component subjected to high strain concentrations in critical regions. Residual life evaluation can be obtained considering a crack growing from component first load cycle: life assessment for those components parts where Low Cycle Fatigue (LCF) design is adopted, especially for those critical regions where high stress concentrations cause cyclic yielding of the material, can be made only considering an appropriate crack growth model. The scope of this paper is to discuss the application of existing models to three different types of quenched and tempered structural steel. In particular, a series of experimental tests in LCF regime at room and high temperature (up to T = 500 ° C ) were performed in order to determine ∊ - N diagrams and to determine crack growth rates in the LCF regime. During fatigue tests interruptions, crack advancement was detected using the plastic replica technique with a thin foil of acetate. Experimental results were examined in terms of crack growth rates and a series of models based on elastic–plastic J -Integral. Results showed that at high temperature there is a significant increase of crack growth rates respect to models usually valid at room temperature. This increase is related to the presence of a diffused damage ahead of the crack tip. [ABSTRACT FROM AUTHOR]

A period of crack growth “retardation” after coalescence of a pair of cracks, propagating under constant amplitude cyclic loading, has been found in previous studies. In this study, quantitative fractography has been used to examine the evolving shapes and sizes of the fracture area of a newly coalesced crack. The size of the “remaining” area required for two coalesced cracks to form a single semi elliptical crack was found to be associated with the period of crack growth “retardation”. Based on this relationship, a new fracture area method has been formulated in which accounts for the period of crack growth “retardation” of a coalesced crack. The crack growth of the newly coalesced crack predicted using the new method agreed well with experimental data. [ABSTRACT FROM AUTHOR]

The threshold stress intensity factor range Δ K th was analytically calculated under stress control conditions using the plasticity-induced crack closure analysis with the Dugdale model. Before obtaining Δ K th , the accuracy of the method was verified. The characteristics of the crack closure behavior were discussed by specifying the small-scale yielding (SSY) and large-scale yielding (LSY). The analysis results of Δ K th were discussed systematically by non-dimensionalizing them with the Dugdale model. Under the SSY condition, Δ K th was dependent on Young’s modulus and Poisson’s ratio. Under the LSY condition, Δ K th increased when the initial crack length or yield strain increased. [ABSTRACT FROM AUTHOR]

Semi-empirical notch sensitivity factors q have been used for a long time to quantify notch effects in fatigue design. Recently, this old concept has been mechanically modelled using sound stress analysis techniques, which properly consider the notch tip stress gradient influence on the fatigue behaviour of mechanically short cracks. This mechanical model properly calculates q values from the basic fatigue properties of the material, its fatigue limit and crack propagation threshold, considering all the characteristics of the notch geometry and of the loading, without the need for any adjustable parameter. This model's predictions have been validated by proper tests, and a criterion to accept tolerable short cracks has been proposed based on it. In this work, this criterion is extended to model notch sensitivity effects in environmentally assisted cracking conditions. [ABSTRACT FROM AUTHOR]

The Kitagawa–Takahashi diagram in its commonly used form allows to predict, for cracks of given length and stress range, the allowable stress range for infinite life. However, caution is advised if a crack emanates not directly from the plane surface but from a sharp, crack-like notch instead. In this contribution, it is shown that taking the crack length equal to the total flaw depth (sum of notch depth and crack length) gives non-conservative results. Based on a simple mechanical model, a 3-dimensional Kitagawa–Takahashi diagram considering the build-up of crack growth resistance as well as the influence of the notch depth is developed. Comparison of model predictions and experimental results shows good agreement. [ABSTRACT FROM AUTHOR]

This paper has been focused on the modern theoretical and experimental investigations on fatigue of metals which are now largely concentrated on three important areas: fatigue crack nucleation, short crack growth and long crack propagation. The main differences concerning the short cracks and long cracks mechanical investigation analysis have been presented. [ABSTRACT FROM AUTHOR]