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The effect of shot peening (SP) on pre‐fatigue tested carburized steel was investigated. Plane‐bending fatigue tests were conducted on carburized CrMo steel that was pre‐fatigue tested and treated with SP. Fatigue cracks were induced by pre‐fatigue tests with a stress amplitude of σa = 1000 MPa, and the number of the pre‐fatigue test cycles was 2.0 × 104 (L5 test) and 4.0 × 104 (L15 test). We compared the fatigue lives of the fatigue‐damaged and smooth specimens without fatigue damage treated with the same SP. The fatigue life of the L5 tested specimens with SP was longer than that of the non‐fatigue damaged smooth specimens after SP. Therefore, the fatigue cracks introduced by the L5 test can be rendered harmless by SP. Furthermore, near‐surface material properties were evaluated and the mechanisms of rendering surface cracks harmless were investigated. [ABSTRACT FROM AUTHOR]

This paper presents a novel high‐resolution direct analysis of mating AA7050‐T7451 fracture surfaces at crack lengths from 0.1 to 1.3 mm (Kmax of 4.51 to 16.39 MPa√m) for the purpose of analyzing the fatigue growth mechanisms. The various fractographic features observed at these crack lengths were then compared to the known accelerative effects of an applied loading sequence that contained underloads at regular intervals. A correlation was found between some of the observed features and the local fatigue crack growth rate. This study provides new insight into the prominent mechanisms that dictate crack growth rate and ultimately should assist with developing crack growth prediction methods that more accurately represent the physics of the fatigue crack growth mechanisms. Highlights: Mating fatigue surfaces are compared at high resolution in AA7050‐T7451.Striation features are shown to significantly differ on mating surfaces.Fissuring from underloads is found to occur at all investigated crack lengths.This fissuring may be mechanistically linked to underload acceleration effects. [ABSTRACT FROM AUTHOR]

When the elastic‐brittle cracked body is subjected to compressive‐shear loading, not only does the phenomenon of wing crack initiation and propagation occur, but there is also a notable incidence of secondary crack initiation and propagation due to shear stress. Moreover, the shear fracture toughness (KIIc) predicted by the fracture criterion in the framework of classical fracture mechanics is usually smaller than the tensile fracture toughness (KIc), which is inconsistent with the actual observations. Therefore, this research firstly establishes a mechanical model of an elastic‐brittle cracked body under biaxial compression considering three kinds of crack parameters and materials' mechanical properties. Then, the stress field characteristics at the crack tip are analyzed. Next, a compression‐shear mixed fracture criterion is proposed. Finally, the effects of the lateral pressure coefficient (λ), critical plastic zone size (rc), friction coefficient (f), and deformation parameters of the crack surface on the failure modes at the crack tip are investigated. Highlights: A distinction is made between the failure and loading modes of the cracked rock mass.The stress field at the crack tip under compression and shear conditions is analyzed.A compression‐shear fracture criterion related to KIc and KIIc of materials is proposed.Conditions for the occurrence of shear failure at the crack tip are discussed. [ABSTRACT FROM AUTHOR]

Numerical simulation of fatigue crack growth (FCG) is presented, based on the application of the extended finite element method (XFEM). Couple of case studies are presented, as a review of recent experience of the Belgrade school of XFEM simulations, established at the Faculty of Mechanical Engineering. Case studies comprised Charpy specimen with initial crack at the notch tip, oil rig pipe with an axial surface crack, pressure vessels with a crack at the welded joint with connecting pipe, turbine shaft with a crack at the transition radius, optimization of a wing spar in respect to FCG, the effect of mesh size on fatigue life estimation of a stringer panel, fatigue life estimation of wing‐to‐fuselage connecting lug, geometry effect on fatigue life of orthopedic plates and fatigue life of hip implant. Based on these case studies, XFEM is discussed in respect to its accuracy and efficiency. It is concluded that XFEM is a versatile tool for simulation of FCG, providing an excellent option for precise and reliable fatigue life of a cracked component. [ABSTRACT FROM AUTHOR]

The stress intensity factor (SIF) distribution of a semi‐elliptical surface crack is often used for evaluating the fatigue strength of structures. Virtually exact distributions of SIFs FISE,FIISE,FIIISE can be provided along the crack front by solving the hypersingular integral equation of the body force method. In this paper, to create a very accurate SIF variation formula, the elliptical crack SIF solutions FIE, FIIE, FIIIE are used and the SIF ratios FISE/FIE, FIISE/FIIE, FIIISE/FIIIE are mainly focused. Paying attention to the corner point singularity, by applying the least squares method to the ratio FISE/FIE, FIISE/FIIE in the whole range of parametric angle β, FISE and FIISE formulas can be proposed. Instead, FIIISE formula can be proposed by applying the method of least squares to the ratio of FIIISE/FIIIE in the range β≥15° but applying directly to FIIISE in the range β≤15°. In this way, all of formulas proposed in this paper provide the SIFs with better than 1.0% accuracy. Highlights: Accurate SIF formulas for semi‐elliptical surface cracks were created within 1% error.Similar to solving the HIEM, closed form solutions of an elliptical crack were utilized.This formula assume the crack front intersects the free surface at an angle θS≅100.5°.Using singularity r‐0.5 along the crack front, the shape FISE(β)≈const. was investigated. [ABSTRACT FROM AUTHOR]

Thermomechanical fatigue (TMF) crack growth of Alloy 718 when the cycle includes long high‐temperature tensile dwells is often accelerated because sustained‐load crack growth occurs during the dwells and a temperature affected zone (TAZ) develops ahead of a crack tip that reduces fatigue crack growth resistance upon application of a cycle after the long dwell. This paper presents new TMF and sustained‐load tests that aim to study and characterize the TAZ development and growth ahead of semi‐elliptical and single‐edge cracks. The influence of constraint on crack growth is assessed. This investigation focuses on a coarse‐grained (CG) Alloy 718 at 650°C. By comparing the crack growth behavior between a fine‐grained and CG microstructure after a long tensile dwell, a modification of the TAZ growth prediction model that includes the influence of grain size is proposed. Using this improved model for TAZ growth and acceleration, the TMF crack growth prediction of surface cracks is improved. Highlights: Temperature affected zone (TAZ) develops during tensile dwells in Alloy 718.The extent of TAZ is reduced when grains are larger.The extent of TAZ is also dependent on the constraint near the crack tip.A new grain size dependent TAZ growth model improves TMF predictions. [ABSTRACT FROM AUTHOR]

Lug is one of the most important lap joints in many engineering structures, especially airframes. Through‐thickness and part‐through cracks emanating from pin‐loaded lug hole are frequently encountered due to stress concentration. Stress intensity factors for various crack geometries are the prerequisite for damage tolerance design of attachment lug. In this paper, analytical 2D weight functions for through‐thickness crack in lug are derived considering the contact condition between the pin and lug. With the derived 2D weight function, a highly efficient 3D slice synthesis weight function method (SSWFM) is then developed to determine 3D SIFs of hole‐edge corner/surface crack in straight and tapered lugs under various pin loadings. The resulting 3D SIFs are verified by comparisons with the numerical FEM/Franc3D solutions and literature data. The SSWFM is about 1800 times faster than the numerical FEM/Franc3D and therefore will significantly enhance damage tolerance analysis capability for attachment lugs. Highlights: Accurate and wide‐range weight functions of pin‐loaded cracked lug are derived considering the contact constraint.A 3D weight function method for corner and surface cracks of lug is established.It is accurate and much faster than FEM for SIF calculation. [ABSTRACT FROM AUTHOR]

Four different types of specimens were prepared from the center and the periphery of two large IN706 forged discs of commercial scale, and low cycle fatigue (LCF) tests were conducted at 650°C. The IN706 specimens with lower LCF lives showed relatively large fraction of cleavage‐like fracture along linearly aligned η (Ni3Ti) precipitates in the area of crack propagation. The amount of η precipitates for those specimens with greater LCF lives was negligible, and the fracture mode of crack propagation was dominantly intergranular. Crack initiation was mainly by persistent slip band (PSB) cracking at the surface, and no notable difference was found for each specimen. The correlation between tensile properties, grain size, and LCF lives of IN706 at 650°C appeared to be not as significant as expected. Highlights: Low cycle fatigue (LCF) behavior of IN706 with different microstructure was studied at 650°C.The effect of tensile property and grain size was not critical on the LCF behavior of IN706 at 650°C.η precipitates provided an easy crack growth path, inducing cleavage‐like fracture mode on IN706. [ABSTRACT FROM AUTHOR]

The influence of trace elements variation and constraint conditions on welding hot cracking was investigated to reveal the cracking characteristics of nickel‐based alloy NS1402. The fishbone tests and full restraint welding tests have been conducted to determine the cracking ratio. The mechanical and metallurgical fracture mechanisms were elaborated simultaneously. And the low‐melting point liquid membrane, which was a key metallurgical factor, was found in cracking fracture surface. Numerical simulations were conducted to illustrate the effects of transverse tensile stress/strain during welding processing. Results show that the liquid membranes were composed of FeNi, FeP, Co3Fe7, etc., indicating that liquid membranes were developed to higher melting point by Co3Fe7 eutectics due to the addition of Co, and it contributes to reducing cracking susceptibility of NS1402. The cracking was ductility‐dip cracking (DDC), which propagated in the forms of intergranular or transgranular fracture. Thus, adjusting element content and degrees of constraint could control the cracking tendency. Highlights: The trace element Cobalt is added to control the welding hot cracking susceptibility.Cracking fracture mechanisms are elaborated from metallurgical and mechanical factors.The cracking characteristics were compared under different degrees of constraint. [ABSTRACT FROM AUTHOR]

A fracture mechanics model is developed to simulate cracking events that may have taken place at the root of a notch in a uniaxially loaded plate made of a two‐phase alloy during the fatigue crack initiation life. The roughness of the notch surface resembles microcracks of different sizes and locations at the notch root along the plate thickness, where material grains of different phases, sizes, and strengths are distributed at random. Potential notch surface cracking activities are anticipated during loading cycles until the emergence of a through‐thickness crack that propagates from the notch root on the plate surface along its width. Thus, the duration of the experimentally defined fatigue crack initiation at the notch root can be calculated. The model is applied to previously published experimental data on symmetrically U‐shaped ferritic‐pearlitic steel notched plates subjected to constant amplitude cyclic uniaxial stresses with different stress concentration factors. Plates with a random configuration were virtually tested. The comparison of experimental results with corresponding predictions validates the model. Highlights: This work develops a fracture mechanics model for the FCI at uniaxially loaded notches.Potential notch root surface cracking activities during the FCI period are considered.The notch FCI life is spent propagating microcracks resembling its surface roughness.The model predicts the effect of some factors that control the notch FCI behavior. [ABSTRACT FROM AUTHOR]

In this work, the thermal contact problem of a rigid flat punch sliding over functionally graded material (FGM) coating with a surface crack is investigated. The surface of a homogeneous isotropic substrate is ideally coated by FGM. The coefficient of friction on the contact surface is assumed to be constant, and dry Coulomb friction law is applied. The major purpose of this study is to compute the stress intensity factors at the tip of a surface crack under thermomechanical loading. In this perspective, the thermoelastic contact and the surface crack problems are modeled using finite element method. An iterative solution procedure based on finite elements is developed to solve the contact/crack problem until generated frictional heat reach the steady‐state condition. Obtained results are compared to those available in the open literature, and a good agreement is observed. Presented results involve stress intensity factors computed under various thermoelastic sliding conditions. Highlights: A surface crack problem in an FGM coating subjected to sliding contact is examined.Frictional heat due to punch has a remarkable influence on stress intensity factors.The iterative computational method is developed to determine stress intensity factors.Frictional heat and material properties significantly affect stress intensity factors. [ABSTRACT FROM AUTHOR]

Hollow axles are critical safety components for high‐speed trains, and the operational safety of the axle is ensured by combining infinite life design and damage tolerance analysis based on ultrasonic testing. The establishment of the ultrasonic inspection interval requires the axle bench test, but it has been a challenge to obtain the initial crack at a determined depth. This paper proposes a method for predicting the transverse crack depth of axles by combining a full‐scale axle bench test with simulation analysis. The monitoring of crack depth is achieved by measuring the cracks on the axle surface. The crack propagation test was conducted to validate the prediction method with good results. The test results demonstrate the potential of crack monitoring methods in axle crack monitoring and provide new ideas for axle safety monitoring. Highlights: A method to analyze the evolutionary pattern of axle cracks is proposed.The morphology of axle cracks tends to be consistent and flattened gradually.The path of crack propagation is the path of the fastest crack area growth.Axle crack depth can be obtained in real time by the measurement of the crack length. [ABSTRACT FROM AUTHOR]

The behavior of small cracks in magnesium alloy is key to understanding very high cycle fatigue (VHCF), which is influenced by microstructure and deformation mechanism. This study explores how basal texture affects VHCF failure mechanisms in extruded magnesium alloys. Findings reveal that faceted and rough morphologies characterize fracture surfaces during crack initiation and early propagation, respectively. Basal texturing inhibits basal slipping during faceted‐like crack initiation while compression twinning dominates deformation during early propagation, contributing to rough zones. Stress intensity factor and plastic zone size analysis indicates significant influence of basal texture on VHCF failure mechanisms but with limited effect on stable propagation. Highlights: The influence of basal texture on fatigue failure in Mg alloys is investigated.SEM, EBSD, FIB, and TEM characterize mate texture, faceted/ribbon microstructure.TEM analyzes the formation mechanism of facet and rough fracture surface. [ABSTRACT FROM AUTHOR]

Surface initiated rolling contact fatigue (RCF) of gears and rolling bearings can lead to premature failure in mechanical transmission systems. The orientation of surface crack initiation closely depends on surface tractive force, while the mechanism leading to this dependence is still obscure. In this work, a numerical model is proposed to investigate the effect of friction on surface crack initiation in RCF. An inhomogeneous contact solver based on semi‐analytical method (SAM) is developed by combining the damaged‐coupled constitutive relation and the Eshelby's equivalent inclusion method. The damage accumulation and concurrent degradation of material properties are modeled with continuum damage mechanics (CDM). The effect of surface tractive force on the orientation of surface crack initiation is simulated and analyzed. Results indicate that the progressive evolution of damage under the cyclic loading is accompanied by the stress variation around the damaged area. The damaged area increases, and the crack initiation life decreases with an increasing coefficient of friction. Highlights: A new damage evolution model incorporating an inhomogeneous contact solver is developed.The possible causes of the orientation of surface crack affected by friction traction are analyzed.The simulated results of damage evolution and stress variation agree well with open literatures. [ABSTRACT FROM AUTHOR]

This work presents a comprehensive analytical solution for the mixed‐mode I/II stress intensity factor of a biaxially loaded plate that has a centrally straight inclined crack with frictional surfaces. The solution addresses every possible combination of coefficient of friction, biaxial ratio, crack length, and angle. The present results agree with the available corresponding finite element solutions. For the tension‐compression and compression‐tension biaxial loading, crack angles are recognized to achieve pure mode I, pure mode II, and mixed‐mode crack tip deformation in terms of stress biaxiality and friction coefficient. In the compression‐compression biaxial loading, only mode II crack tip deformation is operative. The ranges of crack angles necessary to produce dormant cracks are determined for the biaxial loading patterns, where a compressive stress is involved, as a function of the biaxial ratio and friction coefficient. It is possible to apply the present solution to cracked plates with two‐dimensional stresses that include a shear stress. Highlights: This work evaluates modes I/II stress intensity factor of central inclined frictional cracks in plates under biaxial loading.The solution addresses every possible combination of friction, biaxiality, and crack angle and length.The solution covers the three biaxial loading patterns that include a compressive stress.The solution can be applied to cracks in plates under complex 2D stresses that include a shear stress. [ABSTRACT FROM AUTHOR]

The fatigue properties of dog‐bone‐shaped specimens of FV520B‐I base metal and the welded joint up to 109 cycles were obtained. The S–N curves of the dog‐bone‐shaped specimens were lower than that of the funnel‐shaped specimens, and the S–N curves of welded joint specimens moved downward compared with that of base metal specimens. For all types of specimens, cracks initiating from the surface matrix and cracks initiating from subsurface inclusions were the main cracking modes in the high cycle regime and the very high cycle regime, respectively. The Murakami model and the modified model were used to predict the fatigue strength at 109 cycles. The parameter D* = NG / Ns was used to characterize the competition of different cracking modes, and the expression of the critical radius rc was obtained. Combined with the Gumbel distribution curve of the inclusion size, the probabilities of subsurface cracking and surface cracking were obtained. Highlights: The VHCF properties of dog‐bone‐shaped specimens of FV520B‐I were tested.The effects of specimen shape and welding on the VHCF behaviors and mechanisms were analyzed.The VHCF strength was predicted by Murakami model and the modified model.The parameter D* = NG / Ns was used to characterize the competition of different cracking modes. [ABSTRACT FROM AUTHOR]

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = −1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 105 cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean‐stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode‐I crack growth. Furthermore, the mean‐stress sensitivity is independent of loading condition and can be expressed by σwR=σwR=−1·1−R20.63 and τwR=τwR=−1·1−R20.63. Highlights: The uniaxial and torsional fatigue limit is equal for ultrahigh‐strength steels.The mean‐stress sensitivity is identical for uniaxial and torsional loading.The area‐parameter model predicts the fatigue limit in the presence of small defects.Ambient air is a chemically active environment for ultrahigh‐strength Ck45M steel. [ABSTRACT FROM AUTHOR]

Fatigue crack growth in Al 2024 and Al 7050 specimens was assessed experimentally and numerically. In the experimental investigation, various specimen configurations and loading conditions were used to achieve pure mode I, pure mode II, and mixed mode I/II, I/III, and I/II/III. Sample configurations were as follows: first, compact tension shear (CTS) specimen to reproduce pure mode‐I and pure initial mode‐II loading conditions; second, surface crack tension (SCT) specimen to analyze the behavior of semielliptical surface cracks in mode‐I and mixed‐mode conditions; and third, hollow cylindrical (HC) specimen with initial semielliptical surface crack to analyze fatigue crack growth under complex stress state. In the numerical study, finite element method (FEM) simulations were performed by means of FRANC3D and ABAQUS solver. The numerical predictions were validated with the experimental results. These predictions resulted to be accurate estimation of the crack growth across the broad range of the investigated loading conditions. [ABSTRACT FROM AUTHOR]

Tensile axial fatigue tests were carried out using carbon fiber‐reinforced thermoplastics (CFRTP) plate with a hole notch up to very high cycle fatigue (VHCF) regime, where number of loading cycles was 2 × 107. The fatigue test was periodically interrupted, and fatigue damage accumulation around the hole notch was non‐destructively detected by two positron annihilation methods. Positron annihilation lifetime spectroscopy was applied to obtain positron lifetime and Doppler broadening method to obtain S‐parameter. Both parameters increased with increasing fatigue loading cycles. Fatigue damage detected by two positron annihilation methods was attributed to the expansion of free volume in the polymer matrix. The estimated size of free volume was sub‐nanoscale according to the experimental value of the positron annihilation lifetime. SEM and μCT scan revealed that the macroscopic fatigue damages, such as surface cracking, transverse cracking and delamination, increased with increasing fatigue loading cycles, which indicated that sub‐nanoscale and macroscopic fatigue damages progressed simultaneously. Consequently, it can be concluded that the positron annihilation methods can be used for the non‐destructive fatigue damage detection of CFRTP. Highlights: Fatigue damage of CFRTP was detected by two kinds of positron annihilation methods.Positron lifetime and S‐parameter increased with increasing loading cycles up to 2 × 107.Microscale or nanoscale fatigue damages in CFRTP were related to macroscopic damages. [ABSTRACT FROM AUTHOR]

Two types of synchrotron radiation computed tomography (SR‐CT)—projection CT (micro‐CT) and phase‐contrast imaging CT (nano‐CT)—were used to observe internal fatigue cracks in (α + β) Ti‐6Al‐4V alloy. Micro‐CT detected cracks in the specimen at ~1 μm spatial resolution, and the nano‐CT provided magnified images at ~200 nm spatial resolution. The crack initiation sites were clarified as the α‐phase for both the surface and internal cracks; however, their opening behaviors differed. A sharp crack tip was observed in the surface crack, and the crack tip opening displacement (CTOD) increased with an increase in the applied load. By contrast, a blunted crack tip, similar to that of a crack in a vacuum, was observed for the internal crack, and its CTOD remained almost constant regardless of the applied load. These phenomena are likely to explain the different behaviors of surface and internal cracks, particularly the slower growth rate of internal cracks, which leads to a longer fatigue life in the very high cycle fatigue regime. Highlights: Small internal cracks with local microstructure were observed using nano‐CT in Ti‐6Al‐4V.Both surface and internal small cracks in Ti‐6Al‐4V initiated from the alpha phase.Surface cracks tips are sharper and more sensitive to external load than internal crack.Internal cracks have similar crack opening behaviors to those of cracks in a vacuum. [ABSTRACT FROM AUTHOR]

Field tests using an instrumented powered wheelset were performed to investigate fatigue damage accumulation in railway axles. Axle bending strains were measured and post‐processed to obtain axle stress spectra. Statistical analyses were used to investigate the variations of axle stress spectra due to changes in railway operation parameters. The study indicates that measured axle stress spectra can be modeled using truncated normal distributions, where the large majority of measured stress amplitudes are lower than 50 MPa. Stress cycles at higher amplitudes are affected by operation parameters such as track design, number of switches and crossings, and whether the wheelset is in a leading or trailing position. Variations in the obtained statistical distributions of axle stresses have been used as input for fatigue life analyses. It was concluded that fatigue damage can potentially initiate on axles suffering from corrosion or small surface cracks/scratches. [ABSTRACT FROM AUTHOR]

This work analytically derives closed form mathematical expressions for normalized mode I and mode II stress intensity factor (SIF) of Brazilian disk compressed with two diametric forces and confining pressure. The disk has central inclined crack with frictional surfaces. The influences of relative crack length, crack inclination, friction coefficient, and confining pressure are demonstrated. The present work defines the loading and geometric conditions to have a crack in the disk (1) operating in pure mode II loading, (2) with sticking surfaces, and (3) dormant. Mode II SIF of a crack with frictional surfaces decreases with the increase in the confining pressure and the friction coefficient. There is a close agreement between present results and relevant results found in the literature. Highlights: This work considers frictional CCBD compressed diametrically and with confining pressure.This work analytically derives closed form solutions for normalized modes I and II SIF.The influence of crack length and inclination, friction and pressure are recognized.Cracks (1) in pure mode II, (2) with sticking surfaces or (3) in dormant mode are defined. [ABSTRACT FROM AUTHOR]

In order to accurately assess creep crack initiation (CCI) and growth (CCG) life for elevated temperature cracked structures, both out‐of‐plane and in‐plane constraints need to be considered, and the unified creep constraint parameter solutions should be investigated. In this work, the new unified creep constraint parameter Ac values for plate structures with various surface crack sizes subjected to tensile and bending loads have been investigated and obtained using three‐dimensional (3D) finite element (FE) analysis. The effects of material creep properties, crack sizes, loading modes, and plate thickness on the parameter Ac have also been analyzed. The estimation equation of Ac for materials with different creep exponent n has been fitted. The parameter Ac solutions can be used in CCI and CCG life evaluation of the C*‐Ac two‐parameter approach for cracked plates subjected to tensile and bending loads. Highlights: The Ac solutions for cracked plates under tensile and bending loads are investigated.The equations for estimating Ac for materials with different creep exponent n are given.The effects of the crack size, loading mode and plate geometry on Ac are analyzed.The constraint‐dependent CCG rate and CCI time of cracked plates are investigated. [ABSTRACT FROM AUTHOR]

A multilayer overlay coating system containing an intermediate intermetallic layer is an architecture expected to show good fatigue resistance. Experimental characterization and modeling simulations were carried out to classify the different crack initiation mechanisms occurring during fatigue of this coating system (2IML coating system) and to reveal how changes in the layer architecture lead to fatigue improvement. Surface crack initiation seems to play the dominated role in the fatigue failure of this coating system as over 55% of observed cracks were surface cracks. Fatigue improvement is achieved by increasing surface initiation resistance via decreasing the IML‐Top layer thickness. However, subsurface initiation mechanisms inhibit the improvement (dominated by surface initiation mechanism) achieved by locating the IML‐Top layer closer to the top surface. Optimizing the design of 2IML multilayer coatings based on the fatigue performance should eliminate the unreacted nickel layer in the intermediate IML‐Top layer, increase the subsurface interface strength, and decrease the density of voids occurring in the annealing process. Highlights: Mechanical properties of materials in layered coatingsFatigue failure mechanisms in layered coatings composed by harder and softer materialsEffect of each mechanism on fatigue performance with varying architectural parameters [ABSTRACT FROM AUTHOR]

This work determines analytically normalized mode I and mode II stress intensity factor (SIF) of a Brazilian disc having a central inclined crack with smooth surfaces. Two basic types of loading are considered: (1) partial compression with uniform pressure and (2) two‐force diametric compression with confining pressure. The present analysis is based on a widely used published solution of the SIFs of the cracked disc as a result of applying two‐force diametric compression. Wide‐range parameters controlling the behavior of the disc are considered. The parameters involve relative crack length, crack inclination, half‐pressure angle, and confining pressure. The conditions required for transmitting from mixed‐mode I/II loading to pure mode II loading are well defined. Good agreement has been generally obtained with relevant results found in the literature. Highlights: This work derives analyses to evaluate modes I/II SIF of frictionless CCBD.This work considers two basic loading types with wide‐range controlling parameters.The analyzed CCBD is compressed with (1) partial uniform pressure and (2) confining pressure.The analyses define the conditions to transmit from mode I/II to mode II loading. [ABSTRACT FROM AUTHOR]

A semianalytical model is introduced to assess rolling contact fatigue problems in railway applications. The constitutive law is based on the nonlinear kinematic and isotropic hardening model of Chaboche–Lemaitre, which allows the cyclic elastoplastic strain under the contact surface to be evaluated. The much higher computational effectiveness in comparison with finite element (FE) analyses is quantified. The Dang Van multiaxial fatigue criterion is implemented to evaluate the rolling contact fatigue in the subsurface elastic region where cracking is relatively rare but more dangerous than surface cracks. The influence of the presence of sulfides in the wheel matrix in decreasing fatigue strength is also assessed by means of Murakami's approach. The model is used to compare conditions under small‐scale twin‐disk experiments to full‐scale wheel/rail contact conditions. It is found that, for the same Hertzian pressure, the small‐scale contact is more conservative in that it causes a deeper plasticized layer as compared with the elliptical full‐scale contact. In the investigated cases, crack initiation is also not expected according to Dang Van criterion in neither of the studied contact conditions. Highlights: A semianalytical model is developed to study the rail–wheel contact problem.Full‐scale rail–wheel contact and small‐scale twin‐disk contact problems are compared.The line contact problem experiences a deeper plasticized layer.The subsurface crack initiation is assessed using the Dang Van fatigue criterion. [ABSTRACT FROM AUTHOR]

In order to predict and assess creep life for plate structures with semi‐elliptic surface cracks under high temperature condition, the accurate calculation of the creep fracture mechanics parameter C* is a critical step. In this paper, the effects of crack sizes, plate geometries, and material creep properties on the parameter C* have been investigated under tensile and bending loads by extensive finite element analyses. Based on the results, the creep influence functions Hc for calculating C* values were obtained and fitted into equations for surface cracks in plates under both loads. The equations have been verified by finite element calculations. The C* solutions were obtained through these equations which are suitable for wide ranges of crack sizes, plate geometries, and materials. [ABSTRACT FROM AUTHOR]

Surface cracks are one of common forms of flaws in thin‐walled structures such as pressure vessels, oil, and gas pipelines. Accurate evaluation of the growth driving force of such surface cracks is important for integrity analyses of these structures. In this study, the combined effect of the depth and the length of a given surface crack under tension was analyzed using combined elastoplastic finite element analysis (EPFEA) and crack propagation experiments with selected crack shapes. Based on the consideration of the distribution profile of the J integral as the crack growth driving force along crack front, the crack growth stability with different crack shapes was analyzed. Finite element analysis (FEA) results showed that the growth from partial to through‐wall penetration is affected by the shape of the initial crack. As shown by the distribution profile of the J integral along the crack front, the location(s) of the maximum J, that is, the highest crack driving force, is (are) found to vary with the crack shape and development. [ABSTRACT FROM AUTHOR]

The multiaxial fatigue behavior of Ti6Al4V samples, made by selective laser melting (SLM) process, was investigated in this work. Fatigue tests were carried out under three different loading conditions: (i) tensile, (ii) torsion, and (iii) in‐phase axial–torsional. A modified damage model, based on the Sines criterion, is proposed and used to analyze proportional multiaxial fatigue data. Preliminary results showed that the proposed version provides a better prediction with respect to the Sines model. Infrared thermography and digital image correlation were also used to capture both temperature and strain evolutions during fatigue tests. Fatigue results highlighted different failure modes due to surface crack initiation and internal crack initiation, as also demonstrated by scanning electron microscopy of the fracture surface. [ABSTRACT FROM AUTHOR]

The present work is an extension of a previously developed fracture mechanics cracking damage model and highlights the ability of that model to predict the fatigue lifetime of un‐notched round specimens made of a ferrite–pearlite 0.4C‐70/30 carbon steel in the cases of (a) two‐step fully reversed axial loading with low‐to‐high and high‐to‐low sequences and (b) repeated application of fully reversed two‐step axial loading blocks. This model numerically simulates the collective behavior of growing short fatigue cracks originating from the specimen surface. The surface roughness is assumed to resemble microcracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes, and strengths are randomly distributed over that circumference. Possible activities of surface cracks are predicted against loading cycles till a fracture occurs. Published experimental data on ferritic‐pearlitic steel specimens in fully reversed variable amplitude loading are utilized. Different specimen tests are randomly configured and simulated. The present predictions are in fair agreement with the corresponding experimental results. [ABSTRACT FROM AUTHOR]

A Failure Assessment Diagram (FAD), developed for rolling contact, was specifically formulated for railway wheel steels. The FAD allows combining a multiaxial fatigue criterion with the linear elastic fracture mechanics concept; in particular, it was used to assess the fatigue failure in wet contact, related to the pressurization of the fluid entrapped in surface cracks. The approach was applied to the results of previous bi‐disc tests carried out on three railway wheel steels, subjected first to a dry rolling‐sliding contact step, then to a wet contact step. The crack size distribution after the dry contact step was evaluated by a statistical approach; subsequently, the stress intensity factor during the wet contact step was estimated by finite elements. The results of the FAD agreed with the damage observed on the cross section of the specimens at the end of the tests. Furthermore, the FAD was used to determine the maximum allowable crack to prevent rolling contact fatigue, this way showing its potential as a damage tolerant approach. [ABSTRACT FROM AUTHOR]

Recently, surface crack nondamaging technology has been developed to render harmless surface cracks that can reduce the fatigue limit of metals by 20–60%. This is due to the effects of compressive residual stress induced by shot peening. To improve the reliability of high‐strength steel equipment designed against fatigue limit using surface crack nondamaging technology, the following characteristics were determined: (1) the effects of surface crack aspect ratio on maximum harmless crack depth (ahlm) at high‐strength steel for three residual stress distributions; (2) the relationship between the maximum allowable crack depth (acrN) and ahlm assuming the value of safety factor; (3) the relationship between minimum detectable crack depth via nondestructive inspection (aNDI) and ahlm, assuming that nondestructive inspection could detect the crack with sufficient probability; and (4) the relationships between ahlm, aNDI, and acrN. [ABSTRACT FROM AUTHOR]

Some of the fatigue tests performed using the standard compact tension (CT) and a non‐standard specimen made of rolled 7075 aluminium alloy exhibit fatigue crack growth (FCG) lagging in a small region along the crack front. Through‐thickness microstructural evaluation shows that material grains in this region did not flatten as much as other regions. In the non‐standard specimen, surface cracks are either grown under fatigue loading or broken under monotonically increasing quasi‐static loads at different crack sizes. The aforementioned lagging also exists in a narrow region of 3‐D FCG for specimens with microstructural through‐thickness non‐uniformity. A more important feature for this type of specimen with surface crack is the deflection of fast fracture direction into the grain interfaces, namely from L‐T orientation to S‐L and S‐T directions. It is proved that this is due to significant levels of second principal stresses near the free surface for small cracks and lower fracture toughness of the material in S‐L and S‐T directions. [ABSTRACT FROM AUTHOR]

The present study aims to investigate the extent to which the fatigue behaviour of laser beam‐welded AA6056‐T6 butt joints with an already existing crack can be improved through the application of laser shock peening. Ultrasonic testing was utilized for in situ (nondestructive) measurement of fatigue crack growth during the fatigue test. This procedure allowed the preparation of welded specimens with surface fatigue cracks with a depth of approximately 1.2 mm. The precracked specimens showed a 20% reduction in the fatigue limit compared with specimens without cracks in the as‐welded condition. Through the application of laser shock peening on the surfaces of the precracked specimens, it was possible to recover the fatigue life to the level of the specimens tested in the as‐welded condition. The results of this study show that laser shock peening is a very promising technique to recover the fatigue life of welded joints with surface cracks, which can be detected by nondestructive testing. [ABSTRACT FROM AUTHOR]

In this paper, the weight function was extracted at the deepest point of a semielliptical circumferential crack. The crack is assumed to exist on the outer surface of the cylinder. For this purpose, the three‐dimensional finite element method was accomplished to specify two reference loads, which are indispensable for determining the weight function. The verification study confirms the accuracy of the derived weight function under prescribed mechanical loading on the crack surfaces. There is consistency among the solution results compared with those in the literature. The second part describes the application of the weight function for the thermal boundary conditions. Steady‐state thermal stress intensity factors are demonstrated using the weight function and presented as a closed‐form solution. The results were compared with the finite element data on the special case of thermal loading, and good agreement is obtained. [ABSTRACT FROM AUTHOR]

Cracks starting at surfaces will grow under fatigue loading conditions both along the surface and in the thickness directions of the component geometry. In those cases where the crack grows through the thickness, the fatigue crack may transition to a corresponding through crack geometry. While the fatigue crack growth behaviour of both the surface flaws and complete through cracks are well understood, the method for modelling the process by which they transition from one to the other is not. This paper seeks to bring greater clarity and understanding to the transition process by implementing a transition method and developing the associated codes and equations to do so based on careful consideration of boundary conditions, experimental data, and finite element simulations. [ABSTRACT FROM AUTHOR]

A fracture mechanics numerical model is developed to simulate the collective behavior of growing short fatigue cracks originating from the surface of unnotched round specimens made of a two‐phase alloy. The specimen surface roughness is considered resembling microcracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes, and strengths are randomly distributed over that circumference. Variations in mechanical and microstructural features of grains are randomly distributed. Possible activities of surface cracks are predicted against loading cycles till either fracture occurs or all existing cracks become nonpropagating. The material's S‐N curve and fatigue limit can, thus, be assessed. Published experimental data on ferritic‐pearlitic steel specimens in push‐pull constant amplitude loading (CAL) were utilized. Different specimens were randomly configured and virtually tested. Comparison of experimental results and corresponding predictions validates the model, which, further, recognizes the effect of surface roughness, specimen size, and mean stress on lives. [ABSTRACT FROM AUTHOR]

Cracking is an important phenomenon in the failure of geomaterials. The prediction of crack paths is difficult and challenging because of the randomness and uncertainty in the cracking behaviors of geomaterials. In this paper, to predict crack paths based on 2D X‐ray computed tomography (CT) images, a digital energy grade‐based approach is proposed, and the corresponding energy principle is established. Excellent consistency of crack paths is found between the predicted crack path and the real crack path of the specimen. The numerical results indicate that the proposed approach provides a useful way to predict cracking paths in geomaterials. Meanwhile, the stress and displacement fields before and after the specimen fails can be obtained by combining the proposed method with finite element (FE) analysis. In total, this proposed method can be applied not only to monitoring the health of but also to the stability analysis of engineering structures during engineering activities. [ABSTRACT FROM AUTHOR]

This paper presents a robust online‐offline model for the prediction of crack propagation under complex in‐phase biaxial fatigue loading in the presence of overloads of different magnitudes. The online prognosis model comprises a combination of finite element analysis and data‐driven regression to predict the crack propagation under constant loading, while the offline model is trained using experimental data to inform the post‐overload crack growth retardation behavior to the online model. The developed methodology is validated by conducting biaxial fatigue experiments using aluminum AA7075‐T651 alloy cruciform specimens. A close correlation is observed between the experimental results and model predictions. The results show that the model successfully predicts the crack retardation behavior under the influence of overloads with different magnitudes occurring at different stages of fatigue crack growth. Error analysis is conducted to investigate the sensitivities of the number of training points and crack increments to the prediction accuracy. In addition, the error propagation with respect to the crack length is studied, which provides constructive suggestions for further model improvement. [ABSTRACT FROM AUTHOR]

In this paper, a simple, robust, and an efficient technique has been proposed for accurate estimation of mixed mode (I/II) notch stress intensity factors (NSIFs) of sharp V‐notched configurations using finite element notch opening and sliding displacements at the selected number of nodes along the notch flanks. Unlike the crack problems, displacement field is rarely employed in the notch problems due to complexities introduced by the presence of rigid body displacements. One of the main emphasis of the present work is to neatly bypass these rigid body displacements and develop a simple approach for accurate computation of the NSIFs so that it can be easily incorporated in the existing code. Several benchmark problems have been analyzed. The results obtained using the present method show excellent agreement with the solutions available in the literature. Some new results have also been reported in the present work. [ABSTRACT FROM AUTHOR]

In order to assess the structural integrity of tubular members or pipes containing circumferential through‐wall cracks, their stress intensity factor solutions are required. While stress intensity factors for tension and bending are available, few solutions exist for the case of torsion, even though these components may also be subjected to torque. In this paper, the finite element method is used to compute the stress intensity factors for this geometry under tension and torsion. Shell elements are employed to compute the results for thin shells by the means of the displacement extrapolation technique. The computed results indicate that the available analytical solution for torsional loading, which is based on shallow shell theory, is nonconservative for long cracks in thin shells. Shallow shell theory is in general not applicable to long cracks, and the present work is therefore able to provide solutions for a wider range of crack lengths than what is currently available. [ABSTRACT FROM AUTHOR]

An infinite quasi‐orthotropic plane with a cracked circular hole under tensile loading at infinity is studied analytically. To this end, complex variable theory of Muskhelishvili is used. In addition, to obtain analytical functions, a new conformal mapping is proposed and expanded to series expressions. Stress intensity factors (SIFs) for two unequal cracks emanating from a circular hole are obtained. To validate the analytical SIFs in a quasi‐orthotropic plane, the results are compared with FEM and the results of isotropic plane. The SIFs for small cracks in a quasi‐orthotropic and an isotropic plane are different, because of difference between stress concentrations in points which cracks emanate from the hole. However, the results of quasi‐orthotropic plane converge to isotropic plane for the large cracks. Therefore, the SIFs of the large cracks in a quasi‐orthotropic plane can be replaced by the results of the center crack with equivalent length in an isotropic plane. [ABSTRACT FROM AUTHOR]

This work presents an analytical study on the stress fields induced by cracks nucleated at the tip of radiused notches under antiplane shear and torsion loadings. Closed‐form solutions for both shallow and deep notches are derived leveraging a combination of proper curvilinear coordinate systems and the complex potential method for antiplane elasticity. Based on the exact stress field solutions, convenient expressions for the stress intensity factors are derived. The accuracy of all the analytical formulations is verified by comparison with the results from a bulk of numerical analyses. [ABSTRACT FROM AUTHOR]

The important role of lug attachments in mechanical structures makes them one of the most critical parts in the industries. Hence, the estimation of their fracture characteristic (ie, stress intensity factor) under various loadings is very crucial. Here, a 2D weight function is developed which not only calculate the stress intensity factors on each point of the front of quarter‐elliptical planar cracks in straight lugs but also could be used for those cracks subjected to 2D stress distributions. For this, a series of finite element analyses for a reference loading is conducted, and the weight function is subsequently extracted. Comparison between results obtained through the proposed weight function with those available in the literature reveals good agreements. The effects of various geometrical parameters have been studied on stress intensity factors which affirms the unique capability of the proposed 2D weight function in its applicability to a wide range of common lug geometries. [ABSTRACT FROM AUTHOR]

In experimental fatigue tests, it may be necessary to identify the onset of crack initiation, which is often defined at a given (short) crack length. Different experimental techniques are available to estimate the initiated crack size, one of which is the direct current potential drop technique. In this paper, the calibration curves reporting the potential drop change as a function of the crack depth have been derived by means of 3D electrical finite element analyses. Driven by previous experimental observations, two different crack shapes have been considered: (i) circumferential and (ii) semi‐elliptical surface cracks. Dealing with circumferential cracks, the effects of the three‐dimensional distribution of the electric current density and the temperature have been investigated. Concerning semi‐elliptical surface cracks, the effects of the crack aspect ratio and the potential probes locations have been analysed. Finally, the obtained calibration curves have been validated against experimental measurements, obtained by fatigue testing notched specimens under a selection of loading conditions. [ABSTRACT FROM AUTHOR]

Abstract: The effect of welding residual stress on surface fatigue crack propagation and fracture mechanics parameters of intersecting orthogonal cracks is discussed. The effect of welding residual stresses on fatigue propagation of the internal surface crack is analysed in the case of welded joint of the pipeline. To predict fatigue crack propagation, the modified Forman law taking into account the nonsingular T‐stress as the crack‐tip constraint has been employed. It is shown by means of numerical simulation (the software ANSYS and MATLAB) that welding residual stress and crack‐tip constraint effects lead to the semielliptical form of final configuration of the surface fatigue crack. Fracture mechanics parameters for intersecting surface cracks, namely, circular and semielliptical cracks in welded joint of the pipeline are also analysed by numerical simulation. It was concluded that welding residual stress intensifies the effect of intersecting orthogonal surface cracks on fracture mechanics parameters including the crack‐tip constraint. [ABSTRACT FROM AUTHOR]

Abstract: The full‐field of a fatigue crack, ahead and behind the crack tip, has been examined in this work using Digital Image Correlation. We measured, in situ, the surface crack opening displacements at selected locations behind the crack tip and the evolution of the near‐tip displacements and strains ahead of the crack tip, during loading and unloading in a model material of a standard compact tension specimen. In addition, the stress intensity factor for the compact tension specimen was also estimated from the Williams' series expansion by using the displacements obtained from Digital Image Correlation as a function of applied load. The results present a complete picture of the crack tip field at the selected load levels, where events both ahead and behind of the crack tip were studied for the first time in crack driving and attenuation effects. We hope that the results provide some fundamental insights into the full‐field behaviour of a crack tip under cyclic loading conditions and shed light on the phenomenon of crack closure and its relevance to fatigue crack driving force ΔK under small‐scale yielding conditions. [ABSTRACT FROM AUTHOR]

Abstract: In recent years, digital image correlation has been widely used in the analysis of crack problems. This review will examine digital image correlation, as a full‐field measurement technique, in the studies of crack tip mechanical behaviour under cyclic loading conditions. In particular, topics including determination of fracture mechanics parameters and evaluation of crack closure will be discussed. Micromechanical aspects of crack growth under cyclic loading will also be explored in crack driving and attenuation effects. [ABSTRACT FROM AUTHOR]

Abstract: Theoretical 2‐parameter C*‐Q* approaches and numerical simulation were conducted to investigate the creep crack initiation (CCI) time and the effect of constraints induced by the geometrical sizes of pipelines with circumferential surface cracks. The theoretical enhancement model of the C*‐Q* approaches under the transient creep condition, which considered the load‐independent constraint parameter Q*, was proposed to predict the CCI time around the crack front. The results revealed that the distribution regulation of Q* along the crack front for circumferential internal surface cracks and external surface cracks was similar. The maximum constraint level occurred near the deepest crack front part for cracks with small a/c (a/c < 0.4), while it occurred near the free surface for cracks with large a/c (a/c > 0.4). The constraint values at the same position (2Φ/π) increased with the increasing of the crack depth when a/c kept constant. In addition, the circumferential internal surface cracks of pipelines were proved more dangerous than the external surface cracks with the same geometrical size. Furthermore, the CCI times were decided by the peak values of constraint, or the CCI firstly occurred at the position where the constraint level was maximum. Additionally, the variation of hydrostatic stresses, triaxiality, and multiaxial strain factor considering the constraint parameter Q* was discussed. The suitability of the analytical C*‐Q* approaches was verified to predict CCI. The comparison of CCI times between the analytical approach and the BS 7910 as well as the finite element results demonstrated that the solutions under stress intensity factor–Riedel‐Rice control (initially by stress intensity factor and then by transient creep stress or Riedel‐Rice conditions)—were more accurate when internal pressure P < 18 MPa, but the solutions under Hutchinson‐Rice‐Rosengren–Riedel‐Rice control (initially by plastic Hutchinson‐Rice‐Rosengren control and then by Riedel‐Rice conditions) were more appropriate when P > 18 MPa. Finally, the accuracy of C*‐Q* 2‐parameter approaches and the suitability of finite element simulation were validated. [ABSTRACT FROM AUTHOR]