Simulate Fatigue Crack Research Articles

An Abaqus plug-in to simulate fatigue crack growth

Fatigue crack propagation is an important consideration in evaluating the design life of engineering components, especially in the energy and transport industries. Despite its importance, fatigue analyses are not usually supported by commercial Finite Element (FE) codes; in fact, most FE codes require the addition of costly plug-ins to perform fatigue crack growth simulations. Therefore, this paper introduces a new, freely distributed plug-in to simulate fatigue crack growth with the commercial FE code Abaqus. The plug-in includes five different fatigue crack growth models and relies on the extended FE method to simulate crack propagation. The plug-in is limited to 2D analyses, but covers all necessary steps for fatigue crack growth simulations, from creating the geometry to job submission and post-processing. The implementation of the plug-in is validated by comparing its predictions to analytical and experimental results. Finally, we hope that the simplicity of the plug-in and the fact that it is distributed freely will make it a useful simulation tool for industrial, research and educational purposes.

A phase field modeling approach of cyclic fatigue crack growth

Phase field modeling of fracture has been in the focus of research for over a decade now. The field has gained attention properly due to its benefiting features for the numerical simulations even for complex crack problems. The framework was so far applied to quasi static and dynamic fracture for brittle as well as for ductile materials with isotropic and also with anisotropic fracture resistance. However, fracture due to cyclic mechanical fatigue, which is a very important phenomenon regarding a safe, durable and also economical design of structures, is considered only recently in terms of phase field modeling. While in first phase field models the material’s fracture toughness becomes degraded to simulate fatigue crack growth, we present an alternative method within this work, where the driving force for the fatigue mechanism increases due to cyclic loading. This new contribution is governed by the evolution of fatigue damage, which can be approximated by a linear law, namely the Miner’s rule, for damage accumulation. The proposed model is able to predict nucleation as well as growth of a fatigue crack. Furthermore, by an assessment of crack growth rates obtained from several numerical simulations by a conventional approach for the description of fatigue crack growth, it is shown that the presented model is able to predict realistic behavior.

Structural integrity assessment of floating offshore wind turbine support structures

Floating offshore wind turbines are becoming more attractive to the wind industry due to their capability to operate larger turbines in deeper waters. The floating support structures are anchored to the seabed via mooring chains to impede the structure's unwanted movements. The combination of cyclic stresses and the corrosive marine environment makes the floating support structures vulnerable to corrosion pitting and subsequently fatigue crack initiation and propagation. In this study a framework is proposed to simulate fatigue crack growth from multiple corrosion pits at critical spots of the Spar-type floating support structures to examine the status of the crack during several years of operation. The proposed advanced fracture mechanics based approach provides a methodology to assess the integrity of the structure and subsequently plan for preventive or curative maintenance. The crack growth rate is examined for both singular and multiple cracks at different R ratios and for different stress levels using ABAQUS XFEM. Following numerical simulations, a sensitivity analysis is carried out using Crackwise software for different values of plate thickness, R ratio and initial crack size. The numerical results are discussed in terms of the corrosion pitting effects on fatigue life assessment of floating offshore wind turbines.

Development of a Cohesive Zone Model for Fatigue Crack Growth

In this study, fatigue damage is combined with a cohesive zone model to simulate fatigue crack growth along the interface between dissimilar materials under repeated loadings. An evolution equation for fatigue damage is considered for the degradation of materials in a failure process zone. The potential-based Park–Paulino–Roesler cohesive model is employed to correctly consider mixed modes in the relationships between cohesive tractions and crack opening separations. Numerical examples show that the present method can predict properly fatigue crack growth along the interface between dissimilar materials under repeated loadings.

Particle filtering‐based adaptive training of neural networks for real‐time structural damage diagnosis and prognosis

Real-time failure diagnosis and prognosis play key roles in prognostics and health management of critical engineering assets. Physics-based (or empirical–phenomenological) and data-driven methods have been proposed in the literature to solve damage diagnosis and prognosis challenges. Among data-driven methods, neural networks have often successfully proven their regression capabilities for the complex, nonlinear problems typical of this field research field. Indeed, the need to resort to machine learning techniques (e.g., neural networks) becomes stronger when physics-based (or empirical–phenomenological) models are lacking for describing the degradation behavior of the systems under consideration. In this work, we propose to exploit the flexibility offered by neural networks in order to adaptively, and in real time, learn from the monitored structure and derive models for diagnosis and prognosis of structural components subject to damage processes. To this aim, neural networks are embedded within a particle filtering scheme, and the training of the network is performed sequentially and in real time, as observations become available during the damage evolution. The structural health monitoring tool is capable of sequentially updating itself using the observations gathered during the system's operation, avoiding the typical limitations associated to off-line training schemes, which are, in general, not capable of capturing possible changes in the dynamical degradation behavior, when not properly included in the training patterns. For validation and comparison purposes, the method is demonstrated on simulated fatigue crack growths and real crack evolution dynamics observed in a metallic aeronautical panel loaded with a changing-amplitude fatigue load, during an accelerated laboratory test.

Numerical analysis of fatigue crack growth of hip implant

Fatigue crack growth in hip implant has been analysed using the extended finite element method. Toward this aim, the three-dimensional finite element model is created and used in ABAQUS and Morfeo/crack add-in for ABAQUS to simulate fatigue crack growth from initial value up to the critical point. Initial crack dimensions are assumed according to the experience with failures of hip implants, whereas the final, critical value was estimated using data for fracture toughness obtained by experimental testing of hip implant made of Ti-6Al-4V alloy. Results for the equivalent stress intensity factor and number of cycles in each of 40 steps of fatigue crack growth are presented and analysed, indicating the point of unstable crack growth.

Modeling multicrack propagation by the fast multipole symmetric Galerkin BEM

The Fast Multipole Method coupled with the Symmetric Galerkin BEM is employed in this work to simulate fatigue crack growth. The resulted crack propagation code is accelerated with a fast matrix update, a parallel implementation and a sparse matrix format. By using multiple nodes, this code accommodates also multiple surface-breaking cracks. The numerical tests presented herein allow the propagation of multiple cracks in single or multilayer domains.

Fatigue crack propagation at aeronautic engine vane guides using the extended finite element method (XFEM)

This work presents the numerical modeling and validation of two different fatigue propagation tests that attempt to simulate the crack growth situation that takes place at aeronautic engine vane guides. These vanes, which are cylindrical skins of very thin thickness (few millimeters), are responsible for holding the static blades of both the compressor and the turbine. The union of discs and blades is a very conflictive zone due to high stresses and possible manufacturing defects. The first test designed for this work tries to reproduce crack growth at geometric discontinuities and sharp edges, such as the union between the disc and the blades. The second test tries to reproduce the crack growth situation that takes place in skins of very thin thickness, such as vane guides. First, the experimental set-ups as well as the experimental results are presented. Then, the numerical FE models and simulations -corresponding to the experimental tests that have been performed- are explained. Finally, the comparison between the experimental and numerical results is presented. Crack growth was controlled by optical microscopy and by progressive crack surface heat-tinting. For the numerical simulations, the Extended Finite Element Method (XFEM) implemented in Abaqus® 2017 software has been used. The comparison between the experimental and numerical results shows very good correlation regarding crack shape and number of cycles until failure. The capabilities of the XFEM-based LEFM approach to simulate fatigue crack growth in complex crack fronts are validated.

Nonlinear fatigue crack propagation in a baffle module of Wendelstein 7‐X under cyclic bending loads

AbstractSimulation of the fatigue crack propagation in a Wendelstein 7‐X baffle module is performed in this study using both a finite element method‐based software and the UniGrow nonlinear model for small‐scale yielding (SSY) conditions. Some experimental fatigue tests of several cracked baffle modules have been performed through a servo‐hydraulic machine. One of these experimental tests has been considered to simulate fatigue crack propagation in the baffle module. Before starting the experimental test, a first crack partly contained in the welding seam and partly in the steel pipe is found. Subsequently, owing to the applied load, the crack propagated both into the welding seam and into the steel pipe until the plastic zone in the near field attains SSY conditions. Finally, owing to the increase in the extension of the plastic zone, SSY conditions are not more valid, and the breakage of the steel pipe is produced by plastic collapse.

Effect of laser beam welded reinforcement on integral skin panel fatigue life

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.

Fatigue crack growth simulation of two non-coplanar embedded cracks using s-version finite element method

In this paper, the fatigue crack growth simulation of two non-coplanar embedded cracks using the s-version finite element method is presented, and the validity and reliability of the alignment rule for two non-coplanar cracks are evaluated. According to the previous numerical and experimental studies on two non-coplanar surface cracks, the simulated fatigue crack growth behavior is categorized into five patterns to discuss the criteria for the application of the alignment rule. The results suggest that the strength of interaction between the non-coplanar embedded cracks is similar to that between non-coplanar surface cracks. Finally, the interaction of the cracks is evaluated by the stress intensity factor, and the categorization of the fatigue crack growth behavior is discussed by the stress intensity factor. It can be found that the boundary corresponding to the criteria of the application of the alignment rule can be determined as the ratio of the stress intensity factor is 4%. Thus, instead of making a decision of the fatigue crack growth pattern based on the visual inspection, the ratio of the stress intensity factor can be used, and should give more quantitative evaluation of the interaction of two non-coplanar embedded cracks.

A cohesive XFEM model for simulating fatigue crack growth under mixed‐mode loading and overloading

SummaryStructures 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.

Fatigue crack growth simulation in adhesively bonded composite joints

AbstractA numerical model has been developed for simulating fatigue crack growth (debonding) in adhesively bonded composite joints subjected to mode‐I, mode‐II, and mixed‐mode I + II loading conditions. The model employs a cohesive zone model described by a modified bilinear traction‐separation law. Fatigue damage in the composite adherends is not considered in the model. To account for crack divergence and reduce sensitivity of numerical results on mesh density, a crack front detection algorithm based on the effective element's length was employed. The model is implemented as a user‐defined subroutine (UMAT) in the commercial FE code LS‐DYNA. The model's input parameters, in the form of a modified Paris law, and the validation data were obtained from experimental tests conducted by the authors. It was found that the model is able to successfully simulate crack growth in the regime of the experimental data.

A computational strategy for damage‐tolerant design of hollow shafts under mixed‐mode loading condition

AbstractThree‐dimensional numerical analyses, using the finite element method (FEM), have been adopted to simulate fatigue crack propagation in a hollow cylindrical specimen, under pure axial or combined axial‐torsion loading conditions. Specimens, made of Al alloys B95AT and D16T, have been experimentally tested under pure axial load and combined in‐phase constant amplitude axial and torsional loadings. The stress intensity factors (SIFs) have been calculated, according to the J‐integral approach, along the front of a part through crack, initiated in correspondence of the outer surface of a hollow cylindrical specimen. The crack path is evaluated by using the maximum energy release rate (MERR) criterion, whereas the Paris law is used to calculate crack growth rates. A numerical and experimental comparison of the results is presented, showing a good agreement in terms of crack growth rates and paths.

Repairing steel beams with different notch levels using composite materials

Three notch levels were used to simulate fatigue crack propagation in the steel beams. The three levels are a complete loss of the tension flange of the simply supported beam adding 10.9% web loss; notch level #1, a complete loss of the flange adding 21.74% web loss; notch level #2 and a complete loss of flange adding 32.26% web loss; notch level #3. Nineteen beams were tested under two-point bending static loading up to failure. An intact beam and three notched beams with the three notch levels at the mid-span are the control specimens. Five specimens from each notch levels were repaired with different configurations. These configurations include attaching CFRP to the tension flange, covering 18 and 28 mm heights of the beam webs plus the flange, bonding CFRP to the flange and applying GFRP strips around the beam section and increasing the number of CFRP layers from two to three layers. A finite element (FE) program was used to simulate the beams. By wrapping CFRP to cover 18 mm web height, the difference between the peak load values of the repaired beams with notch level #1 and 2 was not increased than 1.8% while this difference reached 7.7% between repaired beams with notch level #1 and 3. Additionally, clear differences in mid-span deflection and energy absorption values between repaired beams with CFRP attached on the tension flange only and repaired beams with CFRP bonded on the flange and wrapping GFRP strips around the section of the beam were appeared in the cases of the three notch levels.

Fatigue crack propagation in complex stress fields: Experiments and numerical simulations using the Extended Finite Element Method (XFEM)

This work presents the numerical simulation and validation of a fatigue propagation test of a semi-elliptical crack located at the side of the rectangular section of a beam subjected to four-point bending. For most common fatigue test configurations there are equations that allow calculating the stress intensity factors (SIFs). However, no solution is provided if the crack is located on any of the lateral sides of the rectangular section, since one part of the crack is located in the tractive zone while the other is at the compressive zone. In these cases, it is necessary to use alternative methods. The Extended Finite Element Method (XFEM) provides a new alternative for the calculation of SIFs, and to simulate crack propagation, by using special interpolation functions. Furthermore, XFEM-based LEFM approach offers the advantage of performing crack growth analysis without the need for updating the mesh (re-meshing). The experimental tests have been carried out in an Instron 8874 biaxial testing machine. Crack growth was controlled by optical microscopy and by progressive crack surface heat tinting. For the numerical simulations, the Extended Finite Element Method (XFEM) implemented in the Abaqus® 2017 software has been used. The comparison between the experimental and numerical results shows very good correlation regarding crack shape and number of cycles to failure. The capabilities of the XFEM-based LEFM approach to simulate fatigue crack growth in complex crack fronts are validated.

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

AbstractThe paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.

THE EFFECT OF CRITICAL TRACTION IN COHESIVE ZONE MODEL FOR FATIGUE CRACK GROWTH RETARDATIO

A cohesive zone model for simulation of fatigue crack growth is presented. The cohesive zone model is one of many alternative approaches used to simulate fatigue crack growth. The model incorporates a relationship between cohesive traction and separation in the zone ahead of a crack tip. The model introduces irreversibility into the constitutive relationships by means of damage accumulation with cyclic loading. The traction-separation relationship underpinning the cohesive zone model is not required to follow a predetermined path, but is dependent on irreversibility introduced by decreasing a critical cohesive traction parameter. The approach can simulate fatigue crack growth without the need for re-meshing and caters for single overloading. This study shows the retardation phenomenon occurring in elastic plastic-materials due to single overloading. Increasing the value of critical cohesive traction increases the extent of plastic zone at the crack tip which causes the fatigue crack growth to retard. Plastic materials can generate a significant plastic zone at the crack which is shown to be well captured by the cohesive zone model approach.

Validation of a peridynamic model for fatigue cracking

Fatigue cracking in homogeneous and in composite materials is studied using a peridynamic model. We introduce a set of critical damage factors that improve stability and efficiency of a recently introduced peridynamic model for fatigue cracking. We perform convergence studies in terms of the nonlocal region size for a modified compact tension test which leads to curved fatigue crack paths. The experimental crack paths are strongly influenced by the location of a hole in the specimen, and the peridynamic results capture this sensitivity well. Fatigue lifetimes obtained by the peridynamic model are in good agreement with experiments for different crack growth rates in different cycle ranges. Different from methods based on classical continuum mechanics, the peridynamic fatigue crack model does not require additional criteria to guide crack or damage growth. In particular, we use the peridynamic fatigue model, without any modifications, to simulate fatigue crack growth in a two-phase composite in which several crack initiation points exist and where fatigue crack paths interact in complex ways.

Fatigue life prediction of casing welded pipes by using the extended finite element method

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.