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In this paper, a fatigue crack growth (FCG) behavior under variable amplitude loadings and for mixed modes is analyzed in terms of residual compressive stress zone. For that, the Chaboche non-linear kinematic hardening model is used to evaluate the crack behavior on AA2024-T3. At first, a series of FCG simulation is performed for a single edge notched tension specimen to highlight the influences of the crack length, the overload ratio, the stress ratio, the mean stress, the mixed mode, and the load sequence on the crack tip residual -stress zone under variable amplitude loadings. Besides, the size and shape of crack tip residual-stress zone are simulated for low/high, high/low block loadings and with/without overloads. Moreover, and according to the findings, a FCG code using the Python script and the extended finite element method (XFEM) Is improved. The developed code processes FCG behavior under variable amplitude loadings using the shape of residual stress distribution near the crack tip. The predicted results show a good agreement in simulating fatigue crack path under variable amplitude loadings with those published in the literature. The originality of this study is to simulate the fatigue crack path of cracked structures under variable amplitude loadings with the compressive residual stress model coupled with the XFEM for mode I and different mixed mode. [ABSTRACT FROM AUTHOR]

Paris' power law is commonly used to describe transverse thermal cracking propagation in asphalt concrete (AC) pavements. In its most common format, it relates the rate of change in the crack length per cycle to changes in either the stress intensity factor K or the energy release. The parameters defining this relationship for ACs can be obtained using the Texas cyclic Overlay Test (OTR). This paper describes two numerical approaches for simulating the OTR using the extended finite element method (XFEM). The first uses the direct low-cyclic fatigue (LCF) approach and the second uses the virtual crack closure technique (VCCT). Comparisons of the model predictions to laboratory OTR measurements for six mixtures suggest that the XFEM model coupled with VCCT is better suited for simulating the OTR. The model output includes the crack length and the energy release rate ERR (i.e. ${\rm \Delta }J_R$ Δ J R ) as cracks propagate. This data was used to fit the modified Paris' law parameters ${A}^{\prime}$ A ′ and ${n}^{\prime}$ n ′ as a function ${\rm \Delta }J_R$ Δ J R . Subsequently, the traditional A and n Paris' power law parameters were fitted by estimating the corresponding ΔK. Using this model, it was possible to efficiently estimate the AC cracking parameters using as input only the tensile modulus and the critical ERR. [ABSTRACT FROM AUTHOR]

• The CRTS Ⅲ slab track model with prefabricated cracks is established by extended finite element method. • The stress intensity factors (SIFs) at the tips of cracks on the track slab under train load are solved. • The effects of the crack length and depth on the SIFs of three types of cracks are discussed. • The variations of crack growth rate calculated by Paris' semi-empirical during service are analyzed. Microcracks may occur in Chinese Railway Track System type III (CRTS III) ballastless track slabs during service, and it is necessary to investigate their potential propagation under train load, which could lead to slab deterioration and affect the durability of the track structure. In this paper, based on the fracture mechanics theory, the stress intensity factors (SIFs) at the tips of transverse crack, longitudinal crack and vertical crack on the CRTS III track slabs under train load are solved by the extended finite element method (XFEM) in Abaqus software. The changes of SIFs at the crack tips during the passage of the train are analyzed, and the effects of the crack length and depth on the SIFs of the three types of cracks are discussed under train load. Crack growth rate is calculated using Paris' semi-empirical law to estimate slab life. The results show that the train load contributes to the propagation of transverse and vertical cracks in the vertical direction and inhibits the longitudinal cracks. When the middle of the sixth vehicle is above the crack, the SIFs at the tips of the crack reach their maximum values. The SIF of vertical cracks exceeds the fracture toughness at a certain depth. The fatigue load cycles calculated by Paris' semi-empirical law suggest that maintenance will not be required due to insufficient propagation under train load during service. [ABSTRACT FROM AUTHOR]

This paper presents an application of the extended finite element method (XFEM) in the modeling and analysis of simultaneous cracks propagations in a seam casing pipe made of API J55 steel by high-frequency (HF) contact welding. The geometry used in simulations is pipe with axial crack subjected toconstant amplitude cyclic loads. Short theoretical background information is provided on the XFEM, as well as the demonstration of the method used for verification of computed stress intensity factors (SIFs). The obtained numerical results prove the efficiency of XFEM in the simulation of the axial cracks propagations in tube geometry. Some guidelines for improving the XFEM use in fatigue life predictions are also given. [ABSTRACT FROM AUTHOR]

High fatigue load, which exists widely in steel building structures, likely leads to brittle failure at the joints, supports, and so on. This can lead to the partial or total damage of the structure and even to cause the collapse of the whole structure. This article aims to provide a method to simulate high-cycle crack propagation in tubular joints, which is one of the most common types occurring in steel structures. Firstly, sixteen T-shaped tubular joint models under different load conditions and initial crack dimensions were built through the coordinate mapping method. Secondly, based on the extended finite element method (XFEM), an algorithm was developed by combining the secondary development in Abaqus and a quasistatic simulation method to simulate high-cycle crack growth in tubular joints under a constant amplitude. The results of the simulations were compared with experimental data. The study found that the surface stress calculated from the tubular joint models using the coordinate mapping method was close to the experimental data. Through the comparison of the crack propagation rate and the crack growth process between the simulation and experiment results, the simulation method was validated. When a crack penetrated the tube wall, the difference in the load cycles between the simulations and the experiment was 9.5%. The initial crack dimension had an impact on the crack propagation, with the decrease in the a/c and KII generally becoming the dominant factor with respect to the crack growth, while the fatigue life of the joints tended to increase. [ABSTRACT FROM AUTHOR]

This paper addresses the influence on the fatigue life induced by the implementation of a capillary-based structural health monitoring methodology, patented under the name eSHM. It consists in integrating structurally small and pressurized capillaries into the component, so that when a fatigue crack breaches the capillary network, it results in a leak flow to the open atmosphere and loss of pressure in the galleries which is detected by a pressure sensor. The novelty of the proposed system resides in the opportunity to locate the capillary according to the designer's need, as one resorts to additive manufacturing for the part production. However, the presence of these galleries in highly stressed regions raises concerns about crack initiation at the capillary itself and accelerated fatigue crack growth. This paper aims at the quantification of the influence the eSHM has on the fatigue behavior of the component and the determination whether this influence is significant or not. To that purpose, numerical simulations on a straight lug component, using the finite elements and eXtended Finite Elements Methods (XFEM), are performed. Various capillary sizes and shapes are assessed, so as to enable a general conclusion on the impact of the eSHM methodology in straight lugs. [ABSTRACT FROM AUTHOR]

Abstract In this paper, numerical analyses of fatigue crack growth in integral skin-stringer panel produced by means of laser beam welding (LBW) are presented along with fatigue results verified by previously conducted experiments. Skin-stringer panel is fail-safe structure widely used in aircraft industry, which requires a good estimate of fatigue life after crack initiation. In the past, experimental work was the only successful method for fatigue life estimation of complex damaged structure, but nowadays researches have powerful numerical tools, such as extended finite element method (XFEM), for three-dimensional crack growth analysis. In research presented here, XFEM was initially used for crack growth simulation in simple flat plate made of aluminium 6156 T6 and 6156 T4 for the purpose of procedure verification, since fatigue life for simple plate was known from experiment. Then, XFEM was used to simulate fatigue crack growth in the panel reinforced with four stringers. Fatigue life obtained on the basis of calculated stress intensity factors (SIFS) along the crack fronts on LBW skin-stringer panels was, as expected, significantly longer than experimentally measured life of simple flat plate. Finally, several numerical simulations were performed in order to analyse the influence of mesh size on accuracy of estimated fatigue life of damaged LBW skin panels. For that purpose, average element sizes of 1, 2 and 4 mm were used. Highlights • XFEM may play a significant role in structure fatigue life estimation. • Effect of mesh size on fatigue life estimation is evident, but not dominant. • Coarser mesh provides better agreement with experimental results. • Small changes in material properties significantly change fatigue life. [ABSTRACT FROM AUTHOR]

Highlights • A continuum damage mechanics (CDM) and XFEM based methodology is developed for high cycle fatigue crack growth simulations. • A new damage model is proposed for the evaluation of fatigue crack growth life. • A new criterion is proposed based on the damage evolution to identify the appropriate definition of stress triaxiality. • A non-local CDM approach is implemented to reduce the mesh sensitivity. • The present methodology is found quite successful for fatigue crack growth simulations. Abstract In this paper, we have developed a continuum damage mechanics (CDM) based methodology for high cycle fatigue crack growth simulations. A fatigue damage law is proposed and implemented in the framework of extended finite element method (XFEM). A new criterion is proposed based on damage evolution to identify the appropriate definition of stress triaxiality for acquiring the constraint effect on the stress state correctly. Few mesh regularization schemes are also employed for reducing the mesh sensitivity in the results. Simulations are performed on fracture specimens of different materials subjected to constant amplitude fatigue loading. The fatigue life of a turbine disc is also predicted under constant amplitude loading. The results obtained from present methodology (CDM and XFEM) are found in good agreement with the published experimental results. These simulations highlight that the continuum damage mechanics is a simple and effective tool to perform crack growth simulations under high cycle fatigue conditions. [ABSTRACT FROM AUTHOR]

• The Johnson-cook damage model, traction separation and Paris law equations are incorporated in the XFEM model to predict the formation of spalling, crack propagation and fatigue life for the spur gear respectively. • 3D finite element analysis was performed to determine the fatigue life of the spur gear using with and without spall using different crack lengths and backup ratio. • Numerical modeling of spur gear developed and results showed that the increase in backup ratio from 0.3 to 3.3, an exponential increase in the fatigue life with and without spall. • The spalling depth obtained on the spur gears with backup ratio 0.3, 0.5, 1.0, and 3.3 are within the range of 20–100 μm. Spur gears are most commonly used to transmit power, and speed variation from one shaft to another shaft. Spalling more frequently occurs surface fault in the gears and sometimes it causes severe damage to the gear tooth. This paper focuses on the effect of spalling on the fatigue life of spur gear. The simulation study is conducted in two parts. In the first part, the Johnson-Cook Damage model is used for the spall generation on the tooth surface. After that, the spalling results are imported as input into the crack initiation /crack propagation model and calculated the rate of crack growth of fatigue crack is based on Paris law implemented in user-defined subroutine Abaqus software. User-defined subroutine code is also validated with the comparison of the numerical and experimental results available in the literature. Cracks with different initial lengths are placed in the deeper cavities of spalling and the crack propagation analysis is conducted using XFEM. The study is conducted for four different backup ratios with cracks of three different initial lengths. The fatigue crack growth life of spur gears is investigated using different crack lengths and backup ratios. Further, a comparison has been made between the fatigue life of a spur having a spalling fault with the fatigue life of spur gear that does not have spalling fault present on its surface. It has also been found in this study that spur gear having cracks originating from the spall region have a less fatigue life as compared to the spur gear without spalling. With the increase in backup ratio from 0.3 to 3.3, an exponential increase in fatigue life is observed. However, the fatigue life of spur gear without the presence of spalling fault has higher fatigue for all backup ratios. [ABSTRACT FROM AUTHOR]

This paper presents computational modeling of a crack growth path under mixed-mode loadings in linear elastic materials and investigates the influence of a hole on both fatigue crack propagation and fatigue life when subjected to constant amplitude loading conditions. Though the crack propagation is inevitable, the simulation specified the crack propagation path such that the critical structure domain was not exceeded. ANSYS Mechanical APDL 19.2 was introduced with the aid of a new feature in ANSYS: Smart Crack growth technology. It predicts the propagation direction and subsequent fatigue life for structural components using the extended finite element method (XFEM). The Paris law model was used to evaluate the mixed-mode fatigue life for both a modified four-point bending beam and a cracked plate with three holes under the linear elastic fracture mechanics (LEFM) assumption. Precise estimates of the stress intensity factors (SIFs), the trajectory of crack growth, and the fatigue life by an incremental crack propagation analysis were recorded. The findings of this analysis are confirmed in published works in terms of crack propagation trajectories under mixed-mode loading conditions. [ABSTRACT FROM AUTHOR]

High cycle thermal crazing has been observed in some residual heat removal (RHR) systems made of 304 stainless steel in PWR nuclear plants. This paper deals with two types of analyses including logical argumentation and simulation. Crack arrest in networks is demonstrated due to the presence of two cracks of the same depth in the network. This identical depth may be proved assuming that mean stress acts only on crack opening and that cracks are fully open during the load cycle before arrest. Weld residual stresses (WRS) are obtained by an axisymmetric simulation of welding on a tube with a chamfer. Axisymmetric and 3D parametric studies of crack growth on: representative sequences for variable amplitude thermal loading, fatigue crack growth threshold (FCGT), permanent mean stress, cyclic counting methods and WRS, are performed with Code_Aster software using XFEM methodology. The following results are obtained on crack depth versus time: the effect of WRS on crack growth cannot be determined by the initial WRS field in absence of crack, but by the associated stress intensity factor. Moreover the relation between crack arrest depth and WRS is analyzed. In the absence of FCGT Paris’s law may give a significant over-estimation of crack depth even if amplitudes of loading smaller than FCGT have not been considered. Appropriate depth versus time may be obtained using different values of FCGT, but axisymmetric simulations do not really show a possibility of arrest for shallow cracks in contrast with logical argumentations. [ABSTRACT FROM AUTHOR]

• A new simpler creep-fatigue interaction damage model is developed to predict the life creep fatigue crack growth life of specimen under different hold time conditions. • A new approach based on continuum damage mechanics and extended finite element method is developed to perform the creep-fatigue crack growth behavior. • Mixed mode CFCG simulations are performed to show the proficiency of developed coupled approach for solving complex problems. In this paper, a coupled continuum damage mechanics (CDM) and extended finite element method (XFEM) based methodology is developed to predict the crack growth life under creep-fatigue environment. To predict the life accurately, a new simple nonlinear creep-fatigue interaction (CFI) damage model is presented. In the combined methodology of CDM and XFEM, the XFEM computes stresses in cracked domain and the coined CFI damage equation determines specimen life under creep-fatigue environment. The simulations are performed for different hold time and compared with experiments. The simulated results confirm that proposed CFI damage model along with developed CDM-XFEM framework is a simple and effective methodology for determining creep-fatigue crack growth life efficiently. [ABSTRACT FROM AUTHOR]

• The surface corrosion methods affect the fatigue cracking modes of Inconel 617 at high temperature. • The small fatigue cracking behavior at 700 °C was estimated based on grain-scale strain field. • Small grains will lead to greater crack deflection and increase crack propagation resistance. • The small fatigue crack propagation path was predicted by CPFEM-XFEM model. • A prediction model of small fatigue crack growth rate and lifetime of Inconel 617 was proposed. Small fatigue crack initiation and propagation behavior of Inconel 617 was studied by in-situ SEM observation, electron back-scattered diffraction (EBSD), digital image correlation (DIC), crystal plasticity finite element method (CPFEM) and extended finite element method (XFEM) in this paper. The experimental results indicated that different surface corrosion methods used to display surface microstructure will affect the small fatigue cracking mode of Inconel 617. The local micro-scale strain field of Inconel 617 near the notch and crack tip was quantitatively measured by a new grain scale DIC method. The CPFEM-XFEM simulation model of small fatigue crack initiation and propagation characteristics of Inconel 617 was established. A new framework for small fatigue crack path and lifetime prediction at the polycrystalline scale was constructed, which includes the influences of slip system and grain boundary on small fatigue crack initiation and growth rate. [ABSTRACT FROM AUTHOR]

The extended finite element (XFEM) has been applied to investigate fatigue crack growth in engineering components containing different types of material irregularities. The standard approximations of finite element method are enriched with additional functions by employing the partition of unity approach. In order to keep track of discontinuities in the domain, level set method is employed. Fatigue crack growth in presence of different material irregularities, such as elliptical, rectangular, hexagonal and octagonal discontinuities, has been the main focus of the study. The effect of geometry, position and material properties of these discontinuities on fatigue behavior of the cracked specimen has been addressed. The fatigue life of the cracked component has been estimated by the generalized Paris law. The mixed mode stress intensity factors have been evaluated by the domain-based interaction integral approach. Several numerical problems are solved by XFEM to study the fatigue crack growth behavior of cracked specimens containing different types of material irregularities. [ABSTRACT FROM AUTHOR]

This work presents the case study of a welded overhead crane runway girder. A full‐shell global finite element model was developed and validated using strain measurements. The global model drives solid submodels of different scales. Using load spectra based on real operational data, hot spot stress and XFEM‐based fracture mechanics simulations are performed to assess the fatigue properties of two critical joints. For the fracture mechanics‐based approach, it is shown that small cracks quickly converge to the same aspect ratio. Hence, the choice of the initial crack aspect ratio was found to be inconsequential. But the aspect ratio must not be assumed to be constant during the fatigue crack growth simulations. Finally, it is shown that the size at which a crack is transferred from one submodel to a subsequent is critical for the accuracy of the fatigue crack growth simulations. [ABSTRACT FROM AUTHOR]

• Fatigue crack growth rate (FCGR) for high-strength steel was studied experimentally. • FCGR under various overload ratio, overload interval, and load sequence was obtained. • FCGR under 'Storm model' and superimposed loadings was obtained. • Plastic zone around crack tip was obtained by digital image correlation (DIC) method. • FCGR was predicted by a modified Wheeler model combined with DIC method. In this paper, aimed at high strength steel for marine structures, a series of crack propagation tests were designed and carried out, and the influence of different load forms on crack growth has been investigated. At first, a series of fatigue crack propagation experiments were designed for compact tension specimens made of AH36 High-strength steel, and the influences of overload ratio, interval of overloads, and load sequence on fatigue crack growth rate were investigated. Besides, storm model loading and superimposed loading were applied to further investigate crack growth behavior under more complex loading condition. The research indicates fatigue crack growth behavior under variable amplitude loadings deviates apparently. The difference is mainly caused by the sequence of variable amplitude loadings and the retardation/acceleration when tensile/compressive overloads are applied. Moreover, digital image correlation technique was adopted to obtain the strain fields and residual strain around crack tip. The mechanism of fatigue crack growth behavior under variable amplitude loadings was investigated by analyzing the transformation of strain fields in crack tips. Extended finite element method to calculate the plastic zone size around crack tip was proposed and verified to be accurate and applicable. In the end, the plastic zone size obtained by digital image correlation technique, which shows better accuracy, was applied to improve the modified Wheeler model, and the fatigue crack growth behavior under variable amplitude loadings was predicted by the improved modified Wheeler model, and was in good agreement with experiment results. [ABSTRACT FROM AUTHOR]

• Numerical investigation of fatigue life of damaged wing-fuselage attachment lug. • Insertion of the corner cracks to the FE model of the lug which was not done before. • Comparison of the corner crack growth and through crack growth in the attachment lug. • Very good agreement of the fatigue life values obtained with XFEM and FEM. • High applicability of results in the aircraft maintenance business. The research presented in this paper is based on the use of extended finite element method (XFEM) for stress intensity factors (SIFs) determination in the case of fatigue crack presented in the wing-fuselage attachment lug used in light aerobatic aircraft. The presence of the through crack at the attachment lug hole is not allowed since its growth due to high dynamic loads is usually fast and can lead to catastrophic consequences. To demonstrate how dangerous the appearance of the crack could be, as well as to estimate the residual strength and fatigue life of cracked component, finite element model of attachment lug was made, and analyses were carried out using the maximum load that can occur during the flight (load factor n = 6). The predicted number of cycles to complete failure was – as expected – low, confirming the fact that attachment lugs must be designed using safe-life approach. [ABSTRACT FROM AUTHOR]

• Corresponding relationship between heat diffusion equation and seepage diffusion equation. • An analogy method to compute fatigue fracture in steam turbine by using ABAQUS. • An analysis process for turboset to estimate the remaining thermal-fatigue life. • To supply a structural assessment and lifetime prediction for pressure vessels. Natural crack propagation analysis in thermal-fatigue fracture problem is a challenging problem. Reliable predictions for the crack path and residual live of structure provide essential information for the structural integrity assessment and repairment. Various conventional predictive fatigue models have been proposed to evaluate the remaining life, but most of them can't build an explicit relation between damage degree and fatigue life. In recent years, the advanced techniques on crack propagation analyses were proposed, which were used to simulate the fatigue crack growth, but no coupling thermal-fracture function existed in the commercial finite element codes. Fortunately, the extended finite element method (XFEM) in ABAQUS has the coupling permeation diffusion-fracture model. In this paper, an analogy method from coupling permeation diffusion-fracture to coupling thermal-fatigue is proposed to compute fatigue fracture, which is applied for predicting the fatigue life in a specific steam turbine by directly using ABAQUS. Typical fatigue experiments by compact specimens of cast iron are carried out for the fatigue parameters measurement. By using analogy method and Paris' Law, an entire analysis process for the turbine units is built to estimate the thermal-fatigue life. The simulation results predicting remaining life and analyzing fatigue crack growth for the pressurized cylinder are agreed well with the measured engineering data. [ABSTRACT FROM AUTHOR]

The present study investigates the behaviour of propagating cracks in engineering materials containing different material irregularities like bi-material interfaces and holes by employing enriched numerical techniques. Three enriched techniques like the extended finite element method, the element free Galerkin method and the Coupled finite element-element free Galerkin method have been used for analysis. The enriched techniques provide strong numerical tools for modelling different discontinuities independent of the mesh or the grid due to which the problems of conformal meshing, mesh adaption and remeshing do not occur. The level set method has been used to represent different types of discontinuities present in the domain. Several numerical problems, involving fatigue crack growth in presence of bi-material interfaces and holes, are solved to illustrate the applicability, efficiency and accuracy of the three enriched techniques in modelling such problems. [ABSTRACT FROM AUTHOR]

Fluctuated loadings from currents, waves and sea ground motions are observed on offshore steel pipelines, and they will result in small cracks to propagate continuously and cause unexpected damage to offshore/geotechnical infrastructures. In spite of the availability of efficient techniques and high-power computers for solving crack problems, investigations on the fatigue life of offshore pipelines with 3D interacting cracks are still rarely found in open literature. In the current study, systematic numerical investigations are performed on fatigue crack growth behaviours of offshore pipelines containing coplanar and non-coplanar cracks. Extended finite element method (XFEM) is adopted to simulate the fatigue crack growth. The qualitative validations of numerical results are made for certain cases with available experimental results. Parametric studies are conducted to investigate the influences of various important parameters on fatigue crack growth. The results will be helpful to assess the fatigue behaviours of steel pipeline with 3D interacting cracks. [ABSTRACT FROM AUTHOR]

Summary: Structures are subjected to cyclic loads that can vary in direction and magnitude, causing constant amplitude mode I simulations to be too simplistic. This study presents a new approach for fatigue crack propagation in ductile materials that can capture mixed‐mode loading and overloading. The extended finite element method is used to deal with arbitrary crack paths. Furthermore, adaptive meshing is applied to minimize computation time. A fracture process zone ahead of the physical crack tip is represented by means of cohesive tractions from which the energy release rate, and thus the stress intensity factor can be extracted for an elastic‐plastic material. The approach is therefore compatible with the Paris equation, which is an empirical relation to compute the fatigue crack growth rate. Two different models to compute the cohesive tractions are compared. First, a cohesive zone model with a static cohesive law is used. The second model is based on the interfacial thick level set method in which tractions follow from a given damage profile. Both models show good agreement with a mode I analytical relation and a mixed‐mode experiment. Furthermore, it is shown that the presented models can capture crack growth retardation as a result of an overload. [ABSTRACT FROM AUTHOR]

The extended finite element (XFEM) method has been used to simulate fatigue crack growth in casing pipe, made of API J55 steel by high-frequency welding, in order estimate its structural integrity and life. Based on the critical value of stress intensity factor KIc, measured in different regions of welded joint, the crack was located in the base metal as the region with the lowest resistance to crack initiation and propagation. The XFEM was first applied to the 3 point bending specimens to verify numerical results with the experimental ones. After successful verification, the XFEM was used to simulate fatigue crack growth, position axially in the pipe, and estimate its remaining life. [ABSTRACT FROM AUTHOR]