Predict Fatigue Crack Initiation Research Articles

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Modeling the high‐cycle fatigue crack initiation in short fiber reinforced polymers with notches

AbstractIn this work, a new approach is proposed for predicting the life to crack initiation in Short Fiber Reinforced Polymers (SFRPs) under cyclic loadings in the presence of notches, cut‐outs and stress concentrations. A multiscale model is presented, encompassing two length scales for correlating the applied loads to the local stress fields in the matrix. These are used for defining an effective stress for predicting the life to crack initiation, accounting for the simultaneous influence of the notch geometry, the fiber content, orientation and distribution. The criterion was successfully validated on a large set of experimental results.Highlights A new model is proposed for predicting fatigue crack initiation in notched SFRPs The model is based on a multiscale procedure Prediction is based on local matrix stresses averaged in a structural volume The model accounts for fiber orientation, distribution and notch geometry

A computational framework based on 3D microstructure modelling to predict the mechanical behaviour of polycrystalline materials

In this work, a computational framework is proposed to predict the microstructure sensitive mechanical behaviour of polycrystalline materials i.e., fatigue crack initiation life and anisotropic deformation behaviour. A methodology is developed to generate 3D microstructural RVEs using Laguerre tessellation technique in Abaqus software at a particular location of the component. In any commercial software (Abaqus), the modelling of morphology of 3D polycrystalline microstructures is quite challenging due to the complex geometries of grains, and large number of interfaces or grain boundaries, however it can be achieved efficiently and automatically with the developed methodology. The continuum damage mechanics based elasto-plastic constitutive model is used to predict the microstructure sensitive fatigue crack initiation life of titanium alloy notched specimens. To capture an accurate fatigue response, damage coupled elasto-plastic constitutive equations are implemented via user defined material subroutine UMAT. The predicted fatigue crack initiation lives show good agreement with the experimental data. To further showcase the application of the developed framework, an anisotropic analysis is performed by generating 3D polycrystalline microstructure of β-titanium alloy. The developed methodology enhances the capabilities of commercial software in the field of microstructure modelling of complex problems.

A modified cyclic cohesive zone model for low-cycle fatigue crack initiation prediction for subsea pipelines under mode I loading

A modified cyclic cohesive zone model (CCZM) is developed for low-cycle fatigue crack initiation prediction for subsea pipelines. The traction and separation law in the model is modified by employing a piecewise function and linear softening for improving the computational convergence but maintaining the material nonlinearity. The validity of the modified model is evaluated with experimental results of two full-scale pipe specimens and also the results estimated by six published models. An engineering application is further presented to demonstrate the capability of the model and a parametric study is performed for investigating the impact of model parameters on model prediction.

Prediction of Fatigue Crack Initiation under Variable Amplitude Loading: Literature Review

Metallic materials are widely employed in engineering constructions, and one of the most common failure mechanisms in metals is fatigue failure. Even though metal fatigue has been studied for almost 160 years, many problems remain unsolved. Fatigue in metal occurs when the metallic material is subjected to varying loads, resulting in failure due to damage accumulation. The fatigue process consists of a buildup of damage that leads to crack initiation, followed by a period of crack growth until the critical flaw size is reached. The sum of a start phase and a propagation phase represents the total life. To better understand the fatigue phenomenon at its different stages and predict the fatigue life, various types of prediction models have been developed and reported in the literature. This paper reviewed the different models that include microstructure scale parameters that can be used to predict how fatigue cracks start under variable amplitude loading, including the Modified Tanaka-Mura Model, Acoustic Second Harmonic Generation, and the Probability of Crack Initiation on Defects. For perfect life prediction under variable amplitude loading, a stress-based approach, a strain-based approach, and a continuum damage mechanics approach are reviewed. The purpose of this paper is to get overview of the current state of approaches to the life prediction of fatigue crack initiation with variable amplitude. Finally, gaps in knowledge about the prediction of fatigue crack initiation under variable loading at high temperatures are pointed out.

Prediction of Fatigue Crack Initiation of 7075 Aluminum Alloy by Crystal Plasticity Simulation.

The 7075 aluminum alloy is a promising material for the aerospace industry due to its combination of light weight and high strength. This study proposed a method for predicting fatigue crack initiation of the 7075 aluminum alloy by crystal plasticity finite element analysis considering microstructures. In order to accurately predict the total fatigue life, it is necessary to calculate the number of cycles for fatigue crack initiation, small crack growth, and long crack growth. The long crack growth life can be estimated by the Paris law, but fatigue crack initiation and small crack growth are sensitive to the microstructures and have been difficult to predict. In this work, the microstructure of 7075 aluminum alloy was reconstructed based on experimental observations in the literature and crystal plasticity simulations were performed to calculate the elasto-plastic deformation behavior in the reconstructed polycrystalline model under cyclic deformation. The calculated local plastic strain was introduced into the crack initiation criterion (Tanaka and Mura, 1981) to predict fatigue crack initiation life. The predicted crack initiation life and crack morphology were in good agreement with the experimental results, indicating that the proposed method is effective in predicting fatigue crack initiation in aluminum alloys. From the obtained results, future issues regarding the prediction of fatigue crack initiation were discussed.

A microstructure-based fatigue model for additively manufactured Ti-6Al-4V, including the role of prior [formula omitted] boundaries

Microstructure-based models of additive manufactured (AM) Ti-6Al-4V should faithfully represent the unique microstructural features of these materials to provide a more thorough understanding of their role in the mechanical performance of the material. For AM Ti-6Al-4V, the prior β boundaries are likely sites of microscopic plastic strain localization, often leading to fatigue crack initiation. Within the context of crystal plasticity finite element methods, there is an existing gap in the current literature for creating synthetic microstructures of Ti-6Al-4V that capture both the prior β boundaries and α laths. This work focuses on generating such statistically equivalent microstructures, where the prior β boundaries and α laths are explicitly modeled. The framework can generate synthetic microstructures that consider one prior β grain or multiple β grains (and thus prior β grain boundaries) and their associated α laths forming a Widmanstätten microstructure, which are used as inputs for fatigue simulations. Within the fatigue model, the critical accumulated plastic strain energy density (APSED) is a metric used to predict the location and number of cycles of crack initiation. From a statistical average perspective, the presence of prior β grain boundaries was detrimental to the fatigue performance. The effectiveness of the prior β grain boundary in impeding slip, and therefore localizing APSED, is dependent on the loading conditions, prior β orientations, and α variants present in neighboring prior β grains. A higher percentage of failures occurred at the prior β grain boundary in the case of lower applied stress amplitudes, where the applied loads were below the percolation limit of the material and microplasticity was localized to a few locations relative to the microstructure. Additionally, for the case of crack initiation associated with the α variants within a given prior β grain, a combination of soft-hard α lath orientations resulted in sites of APSED localization and predicted fatigue crack initiation. The framework generated in this paper to create representative synthetic microstructures of the Widmanstätten microstructures associated with AM Ti-6Al-4V and the associated fatigue modeling enables trade-off studies between microstructural features and fatigue performance.

Fatigue crack initiation detection in ductile cast iron crankshaft under rotating bending fatigue test using the acoustic emission entropy method

The subject of this study was fatigue crack initiation detection in a ductile cast iron (EN-GJS-700–2) crankshaft. The specimens extracted from the crank web of a car crankshaft were exposed to the four-point rotating bending fatigue test with fully-reversed loading as per iSO-1143. The fatigue crack initiation estimation was performed by (1) using acoustic emission features, (2) utilizing the acoustic emission entropy method, and finally, (3) using crack density measured in field emission scanning electron microscopy images. In this research, the sampling rate in the acoustic emission features method was 1 million data per second, which was reduced to 10 data per second in the acoustic emission entropy method. Therefore, in the acoustic emission entropy method, the data volume was greatly reduced and it was much more practical and cost-effective for real-time health monitoring. As a remarkable issue, the average difference percentage between the Shannon entropy and the Kullback-Leibler relative entropy was 32% and 35%, respectively, compared to AE features for the fatigue crack initiation prediction.

A high-fidelity crystal-plasticity finite element methodology for low-cycle fatigue using automatic electron backscatter diffraction scan conversion: Application to hot-rolled cobalt–chromium alloy

The common approach to crystal-plasticity finite element modeling for load-bearing prediction of metallic structures involves the simulation of simplified grain morphology and substructure detail. This paper details a methodology for predicting the structure–property effect of as-manufactured microstructure, including true grain morphology and orientation, on cyclic plasticity, and fatigue crack initiation in biomedical-grade CoCr alloy. The methodology generates high-fidelity crystal-plasticity finite element models, by directly converting measured electron backscatter diffraction metal microstructure grain maps into finite element microstructural models, and thus captures essential grain definition for improved microstructure–property analyses. This electron backscatter diffraction-based method for crystal-plasticity finite element model generation is shown to give approximately 10% improved agreement for fatigue life prediction, compared with the more commonly used Voronoi tessellation method. However, the added microstructural detail available in electron backscatter diffraction–crystal-plasticity finite element did not significantly alter the bulk stress–strain response prediction, compared to Voronoi tessellation–crystal-plasticity finite element. The new electron backscatter diffraction-based method within a strain-gradient crystal-plasticity finite element model is also applied to predict measured grain size effects for cyclic plasticity and fatigue crack initiation, and shows the concentration of geometrically necessary dislocations around true grain boundaries, with smaller grain samples exhibiting higher overall geometrically necessary dislocations concentrations. In addition, minimum model sizes for Voronoi tessellation–crystal-plasticity finite element and electron backscatter diffraction–crystal-plasticity finite element models are proposed for cyclic hysteresis and fatigue crack initiation prediction.

A phase-transformation based method coupled with entropy to predict fatigue crack initiation of metallic materials

Fatigue crack initiation can be regarded as a generalized phase transformation from the view of breaking energy barrier. A new phase transformation model is proposed to predict fatigue crack initiation based on the second law of thermodynamics. Evolution of dynamic energy during crack initiation and the damage under fatigue loading is considered combining with the entropy theory. The system becomes unstable when the Helmholtz free energy reaches the extremum value, which is adopted to represent fatigue crack initiation. The effects of grain size on the crack initiation and the asymptote of S-shaped S-N curve are discussed. The theoretical predictions are compared with the experimental data and show reasonable accuracy for different metallic materials.

Fatigue of X65 steel in the sour corrosive environment—A novel experimentation and analysis method for predicting fatigue crack initiation life from corrosion pits

AbstractOil and gas pipelines manufactured from API‐5L Grade X65 steel are generally subjected to cyclic loading and their internal surfaces are frequently exposed to corrosive sour fluids. Exposure of pipelines to these environments often leads to localized corrosion (pitting) and decreased fatigue life. Corrosion pits are geometrical discontinuities that may promote fatigue cracking by acting as stress raisers. In order to optimize asset inspection and repair scheduling, it is important to understand the fatigue behavior of X65 steel and in particular, the ability to predict the crack initiation from corrosion pit. To achieve this level of understanding, conducting fatigue tests in an environmental condition replicating the field environment is important. This paper presents the test protocol and results of environmental fatigue testing using bespoke laboratory apparatus to undertake in situ corrosion fatigue tests in a sour corrosive environment under uniaxial loading. The environment selected represent processes that are likely to occur at internal surfaces of oil and gas pipelines exposed to production fluids. The tests were carried out on smooth samples to obtain S‐N curve in this specific environment as well as on pre‐pitted samples. An electrochemical method is used to create corrosion pits on the samples. Also, a model is proposed to predict the crack initiation life from corrosion pit, using a local stress‐based technique, which has been validated by experimental test results. Post‐test fractography was carried out by scanning electron microscopy (SEM). The performance of our approach is demonstrated. The innovation is anticipated to encourage other workers to employ similar small‐scale tests requiring toxic and challenging test environments.

A Probabilistic Fatigue Framework to Enable Location-Specific Lifing for Critical Thermo-mechanical Engineering Applications

The present paper describes a probabilistic framework to predict the fatigue life and failure mode under various thermo-mechanical loading conditions. Specifically, inclusion- and matrix-driven competing failure modes are examined within nickel-based superalloys. The critical accumulated plastic strain energy density (APSED) is employed as a unified metric to predict fatigue crack initiation in metals, which is favorable due to the usage of a single unknown parameter and its capability to predict failure across loading conditions and failure modes. In this research, we characterize the temperature-dependent variation of the critical APSED using a Bayesian inference framework and predict the competing failure modes in a coarse grain variant of RR1000 with varying strain range and temperature. The critical APSED appears to decrease along a vertically reflected sigmoidal curve with increasing temperature. Further, (a) the prediction of a failure mode, (b) failure mode associated with the minimum life, and (c) the change in the location associated with the matrix-driven failure mode with increasing temperature and decreasing strain range are consistent with the experimentally observed trends in RR1000, as well as other Nickel-based superalloys, documented in the literature. Finally, for each simulated loading condition, the uncertainty in the fatigue life is quantified as a prediction interval computed based on a 95% confidence level of the critical APSED and the computed APSED from simulations. The overall framework provides a promising step towards microstructural-based fatigue life determination of components and enables a location-specific lifing approach.

Fatigue Behavior of Additively Manufactured IN718 with Columnar Grains

Fatigue failure of polycrystalline materials is often dominated by crack initiation processes, which are strongly influenced by salient features existing in the microstructure. Herein, a crystal plasticity framework based on the finite element method is used to quantitatively assess the mechanistic drivers existing in columnar IN718 polycrystals for fatigue crack nucleation. To that end, microstructurally differing polycrystalline samples are subjected to strain‐controlled cyclic loading at ambient conditions to elucidate the microstructural mechanisms of fatigue damage. 2D grain structures obtained from the cellular automata (CA) simulations of the additive manufacturing of IN718 are utilized to construct representative 3D microstructures for use in computational analyses. A criterion related to the stored energy density (SED) is used to predict the scatter in fatigue crack nucleation life. This criterion presents a unique and consistent approach toward predicting fatigue crack initiation at the microstructural scale. It is demonstrated that SED is necessary and sufficient to drive crack nucleation in low cycle fatigue. The results show that grain boundaries are the preferred crack initiation sites.

Modelling the influence of the microstructure on the high cycle fatigue crack initiation in short fibre reinforced thermoplastics

In this work, a new criterion to predict fatigue crack initiation in plain Short Fibre Reinforced Thermoplastics is proposed. Several experimental observations in the literature showed that the initiation of a macro crack results from the accumulation of damage in the matrix at the micro-scale. Accordingly, in the proposed criterion the life to crack initiation is predicted through an effective stress acting in the most critical regions in the matrix (close to the fibre ends and lateral surfaces). More precisely, the effective stress is evaluated within a control volume defined as a certain percentage of the matrix volume subjected to the highest stresses. A multi-scale (macro-meso-micro) procedure is adopted for estimating the local stress fields in the polymer. The criterion was proved to successfully predict the influence of the material microstructure (fibre content and orientation distribution) and load orientation.

Low cycle fatigue crack initiation in Ti-5Al-5Mo-5V-3Cr in relation to local crystallographic orientations

Low cycle fatigue crack initiation was investigated in a Ti-5Al-5Mo-5V-3Cr alloy with a bimodal microstructure. In particular, the crystallographic orientations of both α and β phases at crack initiation sites were characterized. Crack formation is shown to generally proceed along prismatic slip bands in primary α grains. A key role of the interplay between elastic and plastic anisotropies of both α and β phases was evidenced as the crystallographic orientation of the surrounding β matrix is a critical parameter as well. Different lengthscales have thus to be considered for an accurate prediction of fatigue crack initiation in near-β titanium alloys.

A 2D peridynamic model for fatigue crack initiation of railheads

Due to the use of PDEs (Partial Differential Equations), classical continuum mechanics is difficult to solve the problems of discontinuous deformation. Using integral equations, peridynamic theory can overcome this difficulty. A 2D method for predicting fatigue crack initiation of railheads was presented based on the peridynamic theory. First, the reasonable values of the model parameters were analyzed. Second, the accuracy of the method was verified through comparisons with the results of classical continuum mechanics and existing conclusions in the literature. Finally, the effects of wheel-rail rolling-sliding status, friction coefficient, and wheel load on fatigue crack initiation were analyzed and discussed.

Evaluation of different strain-based damage criteria for predicting the fatigue life of friction stir spot-welded joints under multi-axial loading conditions

In this article, fatigue life of dissimilar friction stir spot welds in Cross-Tension and Lap-Shear specimens is predicted using a number of fatigue damage criteria. The results revealed a relatively good correlation between predicted fatigue crack initiation lives and experimental data. Also, it has been shown that in the whole range of applied load levels, the least discrepancy between predicted fatigue lives and experimental data is related to the Smith–Watson–Topper criterion. Finally, an excellent agreement was found in predicting the location of the crack tips using the Fatemi–Socie criterion in both types of specimens.

Predicting fatigue crack initiation from coupled microstructure and corrosion morphology effects

The onset of fatigue crack initiation is driven by the microstructure and the corrosion morphology; however, these mechanisms are rarely studied synergistically. In this work, characterizations of the microstructural features and the corrosion morphology resulting from two different environmental exposures are instantiated into crystal plasticity models. Fatigue indicator parameters (FIPs) are calculated from the micromechanical fields to quantify failure. The FIPs compare favorably to predict the experimentally observed location of fatigue crack initiation. These results show the potential behind analyzing environmentally-assisted fatigue crack initiation from a multivariable perspective.

Low-cycle fatigue life prediction of a polycrystalline nickel-base superalloy using crystal plasticity modelling approach

A crystal plasticity model is developed to predict the cyclic plasticity during the low-cycle fatigue of GH4169 superalloy. Accumulated plastic slip and energy dissipation as fatigue indicator parameters (FIPs) are used to predict fatigue crack initiation and the fatigue life until failure. Results show that fatigue damage is most likely to initiate at triple points and grain boundaries where severe plastic slip and energy dissipation are present. The predicted fatigue life until failure is within the scatter band of factor 2 when compared with experimental data for the total strain amplitudes ranging from 0.8% to 2.4%. Microscopically, the adjacent grain arrangements and their interactions account for the stress concentration. In addition, different sets of grain orientations with the same total grain numbers of 150 were generated using the present model. Results show that different sets have significant influence on the distribution of stresses between each individual grain at the meso-scale, although little effect is found on the macroscopic length-scale.

Micromechanical Modelling of the Influence of Strain Ratio on Fatigue Crack Initiation in a Martensitic Steel-A Comparison of Different Fatigue Indicator Parameters.

Micromechanical fatigue lifetime predictions, in particular for the high cycle fatigue regime, require an appropriate modelling of mean stress effects in order to account for lifetime reducing positive mean stresses. Focus of this micromechanical study is the comparison of three selected fatigue indicator parameters (FIPs), with respect to their applicability to different total strain ratios. In this work, investigations are performed on the modelling and prediction of the fatigue crack initiation life of the martensitic high-strength steel SAE 4150 for two different total strain ratios. First, multiple martensitic statistical volume elements (SVEs) are generated by multiscale Voronoi tessellations. Micromechanical fatigue simulations are then performed on these SVEs by means of a crystal plasticity model to obtain microstructure dependent fatigue responses. In order to account for the material specific fatigue damage zone, a non-local homogenisation scheme for the FIPs is introduced for lath martensitic microstructures. The numerical results of the different non-local FIPs are compared with experimental fatigue crack initiation results for two different total strain ratios. It is concluded that the multiaxial fatigue criteria proposed by Fatemi-Socie is superior for predicting fatigue crack initiation life to the energy dissipation criteria and the accumulated plastic slip criteria for the investigated total strain ratios.