Simulation Of Fatigue Crack Growth Research Articles

A modified three-dimensional virtual crack closure technique for calculating stress intensity factors with arbitrarily shaped finite element mesh arrangements across the crack front

To obtain accurate stress intensity factors (SIFs) with arbitrarily shaped finite element mesh arrangements across the crack front, a modified three-dimensional virtual crack closure technique (modified 3D-VCCT) is proposed in this paper. This method is based on three-dimensional virtual crack closure technique (3D-VCCT) and finite element method. To increase the SIFs calculation accuracy for the crack with arbitrary shape, the length and shape of finite element meshes across the crack front are considered in this method. Then the SIFs with arbitrarily shaped finite element mesh arrangements across the crack front are calculated by three different methods, the modified 3D-VCCT in this paper, the extension of virtual crack closure-integral method proposed by Okada and the 3D-VCCT. The solutions of SIFs obtained by three methods are compared and the comparison results show the modified 3D-VCCT has better accuracy with arbitrarily shaped finite element mesh arrangements across the crack front. In addition, the simulations of fatigue crack growth are carried out by using the modified 3D-VCCT. Then the simulation results are compared with the test results, which shows good agreement.

Effect of residual stress redistribution and weld reinforcement geometry on fatigue crack growth of butt welded joints

Redistribution of residual stress during crack growth in butt welded joint of Al5083-H111 was studied. Furthermore, the effect of post weld heat treatment and removing the weld reinforcements on fatigue crack growth rate was investigated. Three dimensional simulations of welding process and fatigue crack growth were conducted. It was shown that removing the weld reinforcements causes an increase in the residual stress. A correlation between the fatigue crack growth rate data of welded joints can be obtained with that of the parent metal by considering the weld geometry and redistribution of residual stresses in calculation of stress intensity factor.

Numerical simulation of fatigue crack growth based on accumulated plastic strain

Fatigue crack growth (FCG) is simulated here by node release, which is made when the accumulated plastic strain reaches a critical value. The numerical procedure is very robust, showing a very fast stabilization and independence relatively to the load considered for node release. A critical accumulated plastic strain of 110% was obtained for the 2024-T351 aluminium alloy, comparing the experimental value of da/dN for a crack length of 26.5 mm and a stress ratio of 0.1, with plane strain numerical predictions. This critical value was used to predict da/dN for different crack lengths. The da/dN-ΔK curve was found to be linear in log-log plot with a slope m = 2.4, which is lower than the slope m = 3.6 presented by the experimental results. The difference is attributed to crack tip mechanisms activated at relatively high loads. The variation of stress ratio and stress state did not affect the da/dN-ΔK curves, which indicates that the effect of these parameters is not linked to crack tip plastic deformation. FCG rate was also predicted for the 7050-T6 aluminium alloy and the 18Ni300 maraging steel, and slopes m = 3.09 and 2.70, respectively, were obtained for da/dN-ΔK curves. The predictions obtained for the steel agreed well with experimental results.

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.

FASTIGUE: A computationally efficient approach for simulating discrete fatigue crack growth in large-scale structures

The renaissance of digital twin technology heralded by recent advancements in machine learning raises the demand for structural analysis tools for real time predictions of the structural performance and ultimately the remaining lifetime. This paper proposes a novel approach – FASTIGUE - for computationally super-efficient discrete fatigue crack growth analysis of large structures particularly for bondlines with high aspect ratios. The computational speed is considerably increased by outsourcing the finite element analysis into a pre-processing step in which the numerical model is solved for a comparatively small number of crack stages; providing the Stress Intensity Factors (SIFs) for a set of auxiliary crack tip location permutations. 3D surface fitting of the auxiliary data provides the SIF ranges as continuous functions of the crack lengths. The fatigue crack growth simulation is performed independently from finite element analysis by utilising the SIF-functions within an explicit growth prediction scheme. The method is applied to a trailing edge crack in a 14.3 m wind turbine blade model and further validated against an analytical solution. It is demonstrated that the computation speed outperforms conventional fatigue analysis approaches relying on update-and-rerun schemes.

Risk assessment of oil drilling rig welded pipe based on structural integrity and life estimation

Risk assessment procedure based on structural integrity and life estimation has been introduced and applied to oil drilling rig welded pipe. The new concept based on application of structural integrity and life estimation to assess risk level according to probability and consequence of failure was used. Oil drilling rig welded pipe integrity has been assessed by simple application of the Failure Assessment Diagramme (FAD), i.e. by using the ratio of linear elastic fracture mechanics parameter, KI, and its critical value, KIc, in relation with the ratio of applied stress and its critical value, to define the point in FAD. Probability of failure is then taken according to the position of the point in the FAD, corresponding to oil drilling rig welded pipe data. Then, the FEM has been applied to calculate more precisely the strain and stress state and to get another, less conservative, risk assessment. Oil drilling rig welded pipe life has been estimated by simple integration of the Paris law, as well as by more complex simulation of fatigue crack growth, using the extended Finite Element Method (xFEM). Probability of failure is then taken as the ratio of the number of cycles for a given crack length and the number of cycles for the critical crack length. These analyses are useful for managers to make decision about further use of damaged pipes, based on data provided by engineers. Also, the procedure introduced here for risk assessment is not limited to oil drilling rig pipes, but it is rather a general one, applicable to any component with known geometry, material properties and loading data.

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.

High cycle fatigue behavior and life prediction for additively manufactured 17-4 PH stainless steel: Effect of sub-surface porosity and surface roughness

The high potential of additive manufacturing (AM) techniques offers novel opportunities and unexplored design freedom. However, typical internal defects and poor surface quality inherent to AM process not only cause a lower fatigue resistance, but also more scatter in fatigue data; thus, hindering adoption of AM to fatigue critical applications. This study investigates the effect of surface quality and sub-surface porosity on high cycle fatigue behavior of 17-4 precipitation hardening (PH) stainless steel (SS) fabricated using laser beam powder bed fusion (LB-PBF) process. Parts were fabricated in three conditions: net-shape (NS) specimens, oversized specimens, and cylindrical rods. The oversized specimens and cylindrical rods were, respectively, further shallow machined (SM) and deep machined (DM) to the dimensions and geometry of net-shape specimens. The population of defects was investigated via optical microscopy of polished sections, X-ray micro-CT scan analysis, and fractography of fracture surfaces after fatigue tests. The fatigue crack growth (FCG) properties were generated at three stress ratios of R=-1,0.1,0.7 to determine the Kitagawa-Takahashi diagram and propagation curve. The polished sections showed the presence of large sub-surface, close-to-surface pores in the NS specimens, while SM and DM conditions had smaller and more uniformly distributed porosity. Critical defects detected on the fracture surfaces were small pores in machined specimens, and relatively large surface irregularities in NS specimens. Machining process, both in SM and DM conditions, enhanced the fatigue performance of the material as compared to that of NS condition. However, in terms of level of machining allowance, no further enhancement in fatigue performance was observed for DM specimens as compared to that of SM ones. Fatigue assessment for both net-shape and machined conditions was obtained performing FCG simulations based on the typical surface features and volumetric defects. Simulation results yielded correct estimates for both net-shape and machined specimens.

FEM simulation of welded joint geometry influence on fatigue crack growth resistance

Fatigue behaviour of welded joints is investigated, in terms of welded joint geometry and the fatigue crack position. It is based on previous work which involved numerical simulation of fatigue crack growth in a welded joint made of micro-alloyed, low-carbon pressure vessel steel P460NL1, with the main focus on fatigue crack growth rate through different welded joint regions. The goal here was to change the size of the heat affected zone, as the region in which the fatigue crack initiated, and to compare the results obtained for new crack length values with the original ones, obtained by creating numerical models based on experimental data. A number of models were created, some of which simulated the case with a bigger heat affected zone (and, consequently, a smaller crack length in the weld metal), and other which simulated the case with a smaller heat affected zone. Due to the micro-structural differences between these two welded joint regions, noticeable differences appeared in the numbers of cycles obtained for each zone with varying fatigue crack lengths, as well as in the total number of cycles for both zones through which the crack propagated.

Estimation of stress intensity factors from crack flank displacements

In the present numerical investigation, the efficacy of a recently proposed displacement based method for estimation of mixed mode (I/II) stress intensity factors of 2D configurations are demonstrated using additional benchmark examples. The proposed displacement based technique is simple, efficient and robust and can be used for the fatigue crack growth simulations. This technique uses the crack opening and crack sliding displacements obtained from the finite element analysis to determine the mode I, mode II and mixed mode (I/II) stress intensity factors (SIFs). This technique can provide the sign of the mode II stress intensity factors which enables it to be used for the fatigue crack growth simulations involving numerous computational steps. Quarter point elements (QPEs) are employed around the crack tip. Convergence of the present method is studied and the performance is assessed by comparing the extracted SIFs using other methods such as displacement extrapolation, J-integral and interaction integral techniques. The present study indicates that very accurate values of SIFs can be computed using the proposed technique and the accuracy of the SIF solutions are of the order of the path independent integrals

Three-dimensional fatigue crack growth simulation of embedded cracks using s-version FEM

PurposeSince the most of structures and structural components suffers from cyclic loadings, the study on the fatigue failure due to the crack growth has a great importance. The purpose of this paper is to present a three-dimensional fatigue crack growth simulation of embedded cracks using s-version finite element method (SFEM). Using the numerical results, the validity of the fitness-for-service (FFS) code evaluation method is verified.Design/methodology/approachIn this paper, three-dimensional fatigue crack propagation analysis of embedded cracks is performed using the SFEM. SFEM is a numerical analysis method in which the shape of the structure is represented by a global mesh, and cracks are modeled by local meshes independently. The independent global and local meshes are superimposed to obtain the displacement solution of the problem simultaneously.FindingsThe fatigue crack growth of arbitrary shape of cracks is slow compared to that of the simplified circular crack and the crack approximated based on the FFS code of the Japan Society of Mechanical Engineers (JSME). The results tell us that the FFS code of JSME can provide a conservative evaluation of the fatigue crack growth and the residual life time.Originality/valueThis paper presents a three-dimensional fatigue crack growth simulation of embedded cracks using SFEM. Using this method, it is possible to apply mixed mode loads to complex shaped cracks that are closer to realistic conditions.

Non‐linear fatigue propagation of multiple cracks in an aluminium metal matrix composite (AlMMC) with silicon‐carbide fibre reinforcement

The fatigue crack growth (FCG) in a mesoscale metal matrix composite (MMC) sample has been simulated by the analyses of a 3D finite elements (FEs) model of a sample volume cell of the composite. Several FCG numerical simulations have been performed on MMC specimens made of aluminium (Al) alloy and silicon carbide (SiC). First, in the microscale, the homogenization of the single MMC unit cell has been performed in order to find the crack initiation sites. Subsequently, in the mesoscale, a tensile remote load with appropriate boundary conditions has been applied on the MMC unit volume. Simulation of FCG has been carried out using the Paris law and two non-linear laws for small-scale yielding (SSY), namely, the modified Kujawski-Ellyin (KE) and UniGrow laws. Finally, the FCG rates provided by numerical simulations have been compared with each other.

Numerical simulation of quasi-static and fatigue debonding growth in adhesively bonded composite joints containing bolts as crack stoppers

ABSTRACT In this paper, the cohesive zone modeling (CZM) method has been employed to simulate static and fatigue debonding growth in adhesively bonded composite joints containing bolts as crack stoppers. The cracked-lap shear specimen (CLS) for static and fatigue loads and the wide single-lap shear (WSLS) structural element for static load have been modeled. In both cases, mixed-mode I+ II loading conditions are present in the bondline. Bonded and bonded/bolted configurations of the specimens have been analysed in order to evaluate the efficiency of bolts as crack stoppers. Numerical simulations have been performed using the LS-DYNA FE code. For the static debonding simulation, a standard bilinear traction-separation law has been applied while for the simulation of fatigue debonding growth a modified traction-separation law has been implemented using experimental data derived from fatigue tests. The modification has been realized by means of a user-defined subroutine (UMAT) having the capacity to account for crack divergence by making use of a crack front detection algorithm based on effective element’s length. A parametric study on the effect of distance between bolts and crack-tip has been performed. Additionally, the WSLS structural element has been subjected to impact load aiming to create a realistic initial disbond scenario. The residual strength of the element has been predicted by applying a quasi-static load after impact. The numerical results reveal that the presence of bolts under certain circumstances can cause crack growth retardation, under both static and fatigue loading conditions. This study emphasizes on the importance of fatigue crack growth simulation and aims to contribute to the numerical design of crack stopping features as a means of complying towards certification of adhesive bonding in primary composite aircraft structures.

Mixed mode fatigue crack growth studies in AISI 316 stainless steel

Damage tolerance assessment requires the accurate prediction of fatigue crack growth lives. Numerical simulation techniques using mixed mode Paris' law and various equivalent stress intensity factor models (ΔKeq) are widely employed for fatigue life prediction. In the present work, mixed mode (I/II) fatigue crack growth experiments are performed using the compact tension shear specimens made of AISI 316 austenitic stainless steel for various loading angles. Finite element fatigue crack growth simulations are carried out, and the effect of ΔKeq model in fatigue life prediction is studied. To achieve this, a three parameter double exponential type best fit is proposed for fitting the experimental mixed mode crack length vs fatigue life. The performance and capability of various selected models are assessed by comparing the predicted life with the experimental results. Fractographic studies at different stages of crack propagation for different loading angles are also presented to aid the above assessments. Based on the overall consistent performance, Irwin's and Tanaka models are predicting life close to the experimental data and Richard's and Yan's models provide conservative solutions.

On phase field modeling in the context of cyclic mechanical fatigue

AbstractIn this work the phenomenon of cyclic fatigue is integrated within a phase field model for brittle fracture by means of an additional contribution in the internal energy formulation in order to achieve a dependency of the crack evolution on the number of load cycles. Results of numerical fatigue crack growth simulations are presented.

Thermo-electro-mechanical fatigue crack growth simulation in piezoelectric solids using XFEM approach

Cracks and multiple voids resulting from manufacturing processes interact with each other and severely affect the fracture phenomenon of piezoelectric components. These geometrical discontinuities degrade the service performance and lower the reliability of the piezoelectric devices. Therefore, the present work predicts fatigue-cracking behavior of piezo-electric structures in existence of multiple geometrical discontinuities under cyclic thermal-electrical-mechanical loads using extended finite element method (XFEM). Thermal interaction integrals approach and Stroh formalism are applied for estimating stress intensity factors (SIFs). Modified hoop SIF criterion has been utilized to predict crack growth direction under quasi-static fatigue loading conditions.

Numerical simulation of distortion-induced fatigue crack growth using extended finite element method

Aiming to investigate the propagation behaviour of distortion-induced fatigue cracks in steel bridge web gaps, multi-scale numerical analysis models were built based on fracture mechanics theory and extended finite element method (XFEM), combining with the full-scale fatigue tests data. Propagation behaviours of representative fatigue cracks in vertical stiffener web gaps and horizontal gusset plate web gaps were analysed. Finite element models of welds connecting web, vertical stiffener and horizontal gusset plate were built, and the welding residual stresses of such details were analysed. Significant transverse welding residual tensile stresses exist at stiffener web weld toes for web gap details. Residual stress measurements were conducted, and the crack shape and the propagation direction path were basically the same with that in the numerical simulation, indicating that the numerical simulation results were relatively reliable. Furthermore, the welding residual stress fields were considered in the crack propagation analysis models. Representative fatigue cracks at web gaps are Mode I leading mixed-mode cracks of Modes I, II and III. Crack propagation considering welding residual stress has faster propagation rate and is more consistent with fatigue test results. The welding residual stress cannot be ignored for analysis and assessment of distortion-induced fatigue cracks in steel bridges.

Surface crack modelling in an engine compressor disc

This work presents two distinct procedures to perform fatigue crack-growth simulations in a high pressure compressor disc of a turbofan engine.Two distinct load cases were separately considered as applied to the compressor disc under analysis: one load case was used to replicate a full-size experimental fatigue bench test, whereas the other was used to simulate the real in service operating conditions of the engine.Two distinct approaches were adopted for the simulations: a full FEM approach and a particular variant of a FEM-DBEM submodelling strategy. It is worth mentioning that, with the latter approach, the only needed boundary conditions for the DBEM submodel analysis are tractions applied on the crack faces; such tractions are derived by the global FEM analysis.A sound agreement between numerical and experimental outcomes was obtained.

Specimen Level and Component Level Simulations of Fatigue Crack Growth Behavior under Cyclic Bending

Power plant piping system subjected to many cyclic loading conditions leads to fatigue loading and thereby crack initiation in the piping system. Demonstrating Leak-Before-Break (LBB) in the design stage itself plays an important role in ensuring the safety of the Sodium-cooled Fast Reactors (SFR) system. The design code guidelines as per RCC MRx A16 in LBB demonstration is conservative in nature. Conducting experiments for each size of the piping systems are expensive and time-consuming. In this paper present, the efforts taken to develop reliable numerical methods to demonstrate Fatigue Crack Growth (FCG). The geometries selected for the FCG analysis include both specimen level as well as full-scale component. The results show that KI estimate for the plate specimen increased with the increase of ‘a/t’ ratio up to 0.6 and then decreases, whereas, in case of the pipe bend, it reduced beyond the ‘a/t’ ratio of 0.4. This early reduction is due to the influence of global compressive stress present ahead of the crack growth. Thus, component level FCG simulation is essential for the accurate demonstration of LBB. FCG simulation has numerically demonstrated with better engineering accuracy for both specimen level as well as component level geometries

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.