Initiation Life Prediction Research Articles

Microstructural size effect on the notch fatigue behavior of a Ni-based superalloy using crystal plasticity modelling approach

Fatigue performances of critical structures are strongly affected by the microstructural (e.g. grain, defect, inclusion, etc.) size effect, and it is thus important to quantify their detrimental effect. In this work, a numerical procedure is constructed to quantify the influence of microstructure on the mechanical and fatigue behaviors of Ni-based superalloy GH4169. Specifically, by combining sub-modelling approach with crystal plasticity constitutive model, a dual-scale modelling approach is developed for studying grain-level mechanical behavior of Ni-based superalloy GH4169 notched components. In addition, the dislocation-based Tanaka-Mura-Wu model is applied for fatigue crack initiation life prediction. The Paris law is then utilized for fatigue crack propagation analysis based on the simulated short crack. To study the microstructural size effect on fatigue crack initiation and propagation behaviors, sub-models containing various grain orientation, grain size, defect size/shape are built and analysed. Finally, a series of fatigue tests on notched specimens of Ni-based superalloy GH4169 were carried out for method validation. Results indicate that the established dual-scale modelling approach and fatigue life prediction framework yields good agreement with experimental results.

Modeling of bidirectional chloride convection-diffusion for corrosion initiation life prediction of RC square piles under drying-wetting cycle

Modeling chloride penetration mechanism in unsaturated concrete is essential for corrosion initiation life prediction of marine reinforced concrete (RC) structures. This paper presents a bidirectional chloride convection-diffusion model for the chloride concentration distribution of RC square piles in the tidal and splash zone. The finite difference numerical method is used to solve the convection-diffusion governing equations with the drying-wetting cycle. The numerical model is validated by two experimental results and the analytical solution with constant surface chloride conditions. The peak value and convection depth in the chloride distribution curve of RC square piles are investigated. The results show that the periodic drying-wetting chloride environment is the dominant factor of the convection effect formed in the surface layer of the square piles. Moreover, a coupling effect is observed in the diagonal areas of square piles. The corrosion initiation life under a periodic drying-wetting chloride environment is shorter than that with constant surface chloride concentration. Furthermore, the sensitivity analysis results show that the diffusion coefficient, the concrete cover depth, and the variable rate of drying-wetting cycles are the critical parameters for predicting the corrosion initiation life of RC square piles under the drying-wetting cycle.

Experimental based time-dependent reliability assessment of corrosion initiation for PC girders in frozen marine environment

Chloride induced corrosion is the main reason for the deterioration of concrete structures in marine environments, and the prediction of corrosion initiation life is one of the greatest challenges for preventative maintenance. To conduct a corrosion initiation assessment of PC girders in frozen marine environment and support preventative maintenance, a time-dependent reliability assessment method is developed. Long-term exposure experiments and in-house tests are combined to calibrate the chloride diffusion model, and then a probabilistic diffusion model is proposed based on the randomness of the key model parameters. Through the analysis of the target reliability index of corrosion initiation, the risk-function based time-dependent reliability assessment method is developed. Finally, a case study is introduced to illustrate the application of the proposed method. It is found that the surface chloride concentration on concrete structures in frozen marine environments increases at the beginning of service life and it tends to be stabilize when the service time is around 9–10 years. The age reduction factor of chloride diffusion can be described as the linear function of cementitious composition. The target reliability indices of corrosion initiation are suggested as 2.46, 1.50 and 1.13 for the referenced periods of 1, 10 and 20 years. The corrosion initiation life is very sensitive to the tensile stress level of concrete, and it decreases linearly with the increasement in tensile stress when the stress is not higher than 2 MPa.

Fatigue crack initiation life prediction using different plasticity models in cold expanded AL-alloy plates utilized in double shear lap joints

Cold expansion of fastener holes is a recognized technique for enhancing the fatigue life of holed plates utilized in detachable joints. This method involves introducing compressive residual stress around the holes, which has proven to be effective in improving the structural integrity and longevity of such components. In this research, the role of using different plasticity models in finite-element simulations of cold expansion (CE) in determining residual stress distribution and predicting the fatigue life of lap joints was investigated. Finite-element simulations were conducted to analyze the distribution of residual stress in cold-expanded Al-alloy 2024-T3 plates utilized in double-shear lap joints. Three time-independent plasticity models, specifically multilinear kinematic hardening, multilinear isotropic hardening (M.L.I.H) based on monotonic tensile tests, and nonlinear combined hardening models derived from saturated hysteresis stress and strain loops, were employed in the simulations. These models allowed for an accurate determination of the residual stress distribution in the plates. In finite-element simulations, two CE sizes of 1.5% and 4.7% were employed to create residual stresses. In the simulations, after creating residual stress by CE, remote sinusoidal loads are applied to the joints corresponding to the previously conducted fatigue tests in order to obtain stress and mechanical strain distributions. In order to predict the fatigue life, four different multiaxial fatigue criteria (Smith–Watson–Topper, Glinka, Kandil–Brown–Miller, and Fatemi–Socie) were employed, using the stress and strain distributions obtained from the finite element simulations with the different plasticity models. The simulations yielded varying stress and strain distribution results for the multilinear kinematic and M.L.I.H models, while the results of the nonlinear combined hardening model fell between the other two models. Notably, the fatigue life prediction based on the nonlinear combined hardening plasticity model closely matched the experimentally observed fatigue lives, with an absolute percentage deviation of 24.8%.

Crack initiation mechanisms and life prediction of GH4169 superalloy in the high cycle and very high cycle fatigue regime

In this study, the high cycle and very high cycle fatigue behavior of the GH4169 superalloy was investigated. The results of the fatigue tests demonstrated that failures in the high cycle fatigue regime were initiated from the material surface, whereas failures in the very high cycle fatigue regime had two competing modes of crack initiation: surface and internal. The fatigue fracture analysis revealed a quasi-cleavage fracture mode with facets for internal crack initiation. Additionally, the electron backscatter diffraction (EBSD) results indicated that the dislocations located at the internal twin boundaries of the material were preferentially activated under fatigue cyclic loading, resulting in significant strain localization at the twin boundaries. The {111} <110> slip systems with high Schmid factors had a higher probability of activation. The stress intensity factor (SIF) estimated based on rough area (RA) and facet sizes correspond to the fatigue long crack and short crack growth thresholds of the material, respectively. Finally, a modified FIP fatigue life prediction model was proposed based on the microstructure of crystallographic facets, and the results from experiment and the model prediction were found to be in good agreement.

Fatigue Life Prediction of Notched Details Using SWT Model and LEFM-Based Approach

The fatigue crack initiation life of unwelded steel components accounts for the majority of the total fatigue life, and the accurate prediction of it is of vital importance. In this study, a numerical model utilizing the extended finite element method (XFEM) and Smith-Watson-Topper (SWT) model is established to predict the fatigue crack initiation life of notched details extensively used in orthotropic steel deck bridges. Using the user subroutine UDMGINI in Abaqus, a new algorithm was proposed to calculate the damage parameter of SWT under high-cycle fatigue loads. The virtual crack-closure technique (VCCT) was introduced to monitor crack propagation. Nineteen tests were performed, and the results were used to validate the proposed algorithm and XFEM model. The simulation results show that the proposed XFEM model with UDMGINI and VCCT can reasonably predict the fatigue lives of the notched specimens within the regime of high-cycle fatigue with a load ratio of 0.1. The error for the prediction of fatigue initiation life ranges from -27.5% to 41.1%, and the prediction of total fatigue life has a good agreement with the experimental results with a scatter factor of around 2.

Oxidation-involved life prediction and damage assessment under generalized creep-fatigue loading conditions based on engineering damage mechanics

The object of this paper is to develop numerical procedures for creep-fatigue-oxidation life prediction and damage assessments based on engineering damage approach, in response to high-reliability and long-life design requirement that supports low-carbon and new-energy technologies. In order to achieve the prediction results in terms of cycle-dependent stress–strain responses, crack initiation life prediction and multi-damage evolutions, the generalized creep-fatigue loading conditions including tension-hold-only, compression-hold-only and tension-compression-hold-both, which are abbreviated as CP, PC, and CC types, are conducted for IN 718 at 650 °C. Results show that creep-fatigue deformation behaviors are well depicted through the evolutions of hysteresis loops, cyclic softening curves and stress responses. With incorporating into oxidation damage especially under long-life conditions, both error band and probability density function for life prediction are quantitatively improved. In addition, the cycle-dependent roles in multi-damage evolutions are clearly observed in a set of radar graphs, where fatigue, creep and oxidation damage are manifested as different evolutionary features. Finally, the technical route in the transition from deterministic to probabilistic multi-damage assessments is discussed based on the established creep-fatigue-oxidation diagram.

A New Model for Predicting Fretting Fatigue Crack Initiation Life Based on Effective Slip Amplitude

In this study, a new model has been developed for fretting fatigue crack initiation life prediction based on the slip amplitude as a macroscopic feature of the contact interfaces. The main difference between the presented model and many other fretting fatigue life prediction models is the focus on the fretting specific characteristics. In this model, a damage parameter is combined with a damage severity factor to obtain a new fretting fatigue crack initiation life prediction parameter. To investigate the accuracy of this new parameter, a series of fretting fatigue tests have been conducted. Also, crack initiation life data from the literature were added to enhance parameter accuracy investigation. It is shown that the effective slip amplitude is an unbiased geometric-independent parameter in fretting fatigue crack initiation life prediction. Also, comparing prediction results from the effective slip amplitude with those of Smith-Watson-Topper and Ruiz parameters showed that the new parameter can outperform available fretting fatigue life prediction parameters.

A rolling contact fatigue health prediction model based on load interaction and damage energy accumulation

Rolling contact is one of the main contact modes of all kinds of mechanical equipment at present. This investigation studied the rolling contact fatigue behaviors under various slip ratios and surface roughness with the consideration of loading history, loading sequence, and mutual influence between different load levels. In this paper, a fatigue crack initiation life prediction model based on strain energy accumulation caused by slip dislocations is proposed. The effect of increasing slip dislocation density and friction resistance on strain energy accumulation under alternating load is studied. The experimental results show that compared with the existing models, the proposed model can effectively improve the prediction accuracy of contact fatigue life under variable stress loading.

A damage-based method to predict crack initiation lifetime of high-strength steel under proportional bending-torsional loadings

The fatigue life of specimen consists of crack initiation and crack propagation life. As the fracture toughness of high strength steel is low, the specimen will fail soon once crack appears. Therefore, the crack initiation life of high strength steel is considered to be its whole life. Based on the evolution of material fatigue damage and the critical plane method commonly used in multiaxial fatigue strength theory, a crack initiation life prediction method for multiaxial specimens is proposed in this paper. Firstly, a uniaxial nonlinear fatigue damage evolution equation is proposed based on the principles of thermodynamics and continuous damage mechanics. Then, a theoretical calculation method for determining the critical plane under multiaxial load is proposed, and the specific calculation process is given. After the critical plane is determined, the multiaxial fatigue damage parameter is constructed from the normal strain amplitude and shear strain amplitude on the critical plane, and a multiaxial nonlinear fatigue damage evolution equation is proposed by replacing the uniaxial damage parameter using the multiaxial damage parameter. Finally, the crack initiation life of fatigue specimens is predicted by using the proposed multiaxial nonlinear fatigue damage evolution equation, and the multiaxial fatigue tests of Q690D steel under different bending-torsion ratios and different amplitudes are validated. The comparison results show that the prediction error of the proposed method is within the two-fold dispersion band and better than that of Manson-Coffin method.

An Approach for Predicting the Low-Cycle-Fatigue Crack Initiation Life of Ultrafine-Grained Aluminum Alloy Considering Inhomogeneous Deformation and Microscale Multiaxial Strain.

In this paper, a low-cycle-fatigue (LCF) crack initiation life prediction approach that explicitly distinguishes nucleation and small crack propagation regimes is presented for ultrafine-grained (UFG) aluminum alloy by introducing two fatigue indicator parameters (FIPs) at the grain level. These two characterization parameters, the deformation inhomogeneity measured by the standard deviation of the dot product of normal stress and longitudinal strain and the microscale multiaxial strain considering the non-proportional cyclic additional hardening and mean strain effect, were proposed and respectively regarded as the driving forces for fatigue nucleation and small crack propagation. Then, the nucleation and small crack propagation lives were predicted by correlating these FIPs with statistical variables and cyclic J-integrals, respectively. By constructing a microstructure-based 3D polycrystalline finite element model with a free surface, a crystal plasticity finite element-based numerical simulation was carried out to quantify FIPs and clarify the role of crystallographic anisotropy in fatigue crack initiation. The numerical results reveal the following: (1) Nucleation is prone to occur on the surface of a material as a result of it having a higher inhomogeneous deformation than the interior of the material. (2) Compared with the experimental data, the LCF initiation life of UFG 6061 aluminum alloy could be predicted using the new parameters as FIPs. (3) The predicted results confirm the importance of considering the fatigue behavior of nucleation and small crack propagation with different deformation mechanisms for improving the fatigue crack initiation life prediction accuracy.

Crack Initiation Mechanism and Life Prediction of Ti60 Titanium Alloy Considering Stress Ratios Effect in Very High Cycle Fatigue Regime

Ultrasonic fatigue tests were performed on Ti60 titanium alloy up to a very high cycle fatigue (VHCF) regime at various stress ratios to investigate the characteristics. The S-N curves showed continuous declining trends with fatigue limits of 400, 144 and 130 MPa at 109 cycles corresponding to stress ratios of R = −1, 0.1 and 0.3, respectively. Fatigue cracks found to be initiated from the subsurface of the specimens in the VHCF regime, especially at high stress ratios. Two modified fatigue life prediction models based on fatigue crack initiation mechanisms for Ti60 titanium alloy in the VHCF regime were developed which showed good agreement with the experimental data.

Prediction of initiation and total life in fretting fatigue considering kinked cracks

This work proposes a methodology to compute fretting fatigue crack initiation and total life by iterative models calibrated with fatigue stress–strain-life curves. One of the main novelty of such models is the assumption that the crack initiation length is determined by the size of the critical distance. To separate initiation and propagation lives, a numerical approach with finite elements is used. The methodology incorporates not only the multiaxial non-proportional characteristic of the stress field in fretting problems, but also the stress gradient effect. Crack initiation path is obtained by means of the SWT parameter, and to validate it, the analysed results are compared with tests considering a cylindrical-flat configuration (Aluminium 7075-T651 alloy). Most of the life initiation estimates, as well as the total life ones, are within a scatter band of 3.

Prediction of low-cycle crack initiation life of powder superalloy FGH96 with inclusions based on damage mechanics

The effects of inclusions in powder superalloy FGH96 on low-cycle fatigue life were studied, and a low-cycle crack initiation life prediction model based on the theory of damage mechanics was proposed. The damage characterization parameter was proposed after the construction of damage evolution equations. Fatigue tests of the powder superalloy specimens with and without inclusion were conducted at 530 and 600 °C, and the model verification was carried out for specimens with elliptical, semi-elliptical, polygon and strip-shaped surface/subsurface inclusion. The stress analysis was performed by finite element simulation and the predicted life was calculated. The results showed a satisfying agreement between predicted and experimental life.

Evaluation and Prediction of Fatigue Damage Initiation Life of CFRP Quasi-isotropic Laminate in the Very High Cycle Region

Carbon fiber reinforce plastics (CFRP) have been applied to structural materials of blades of wind turbines and aircraft turbine engines. These blades may suffer from very high cycle fatigue up to more than 107 cycles. However, conventional fatigue testing methods consume longer period in excess of 1 year for obtaining only 1 fatigue data in the very high cycle region. In this study, axial ultrasonic fatigue tests were conducted up to 109–1010 cycles. The results showed that no fatigue limit was found up to 109 cycles. Fatigue damage initiation life was compared with results obtained by a hydraulic serve fatigue testing machine to discuss the strain rate dependency and the feasibility of the prediction of fatigue damage initiation life by using a modified Walker model.

A dual-scale modelling approach for creep-fatigue crack initiation life prediction of holed structure in a nickel-based superalloy

In this paper, a dual-scale modelling approach is developed to investigate creep-fatigue behavior and predict crack initiation life for holed structures under multi-axial stress state. The macro-scale simulation supplies local deformation histories to the dual-scale simulation as boundary conditions. In the dual-scale simulation process, the micro-mechanical behavior and damage evolution are described by using crystal plasticity. In order to validate the dual-scale simulation procedures, a series of creep-fatigue tests as well as the post-test characterizations were carried out for nickel-based Inconel 718 at 650 ℃. The detailed results of macro- and micro-scale simulations are presented in terms of stress–strain behavior, damage evolution and life prediction. Regarding the macro-scale simulations as the benchmark, it may provide an assistant support and precognition for the micro-scale damage calculation at higher cycles. The predicted cycle numbers to crack initiation are in agreement with the experimental ones. More advantages are manifested in the potential scientific and engineering significance for the dual-scale modelling approach.

Corrosion initiation life prediction of RC square piles based on a dual time-dependent bidirectional chloride diffusion model

An analytical solution for predicting the bidirectional chloride penetration in reinforced concrete (RC) square piles is proposed by considering the four sides of piles exposed to marine environments. This proposed solution considers the dual time-dependence of the surface chloride concentration and the chloride diffusion coefficient based on the Green function method. The distributions of the surface chloride concentrations for different marine zones along the altitude of square piles are introduced. The versatility of the proposed analytical solution is validated by comparison with the traditional solution considering different conditions and values of the surface chloride concentration and the chloride diffusion coefficient. Furthermore, the proposed solution is applied to predict the corrosion initiation life of RC square piles, and the main factors influencing the corrosion initiation life of piles are also analysed. The results show that the corrosion initiation life of piles in the tidal and splash zones is shortest, while that in the submerged and atmospheric zones becomes longer in turn. The study also indicates that the corrosion initiation life of RC square piles is sensitive to both the material and environmental parameters such as the concrete cover depth and the relative humidity.

A novel comprehensive evaluation method for the corrosion initiation life of RC hollow piles in chloride environments

A modified diffusion model of chloride ions in reinforced concrete (RC) hollow piles is proposed by taking into account the ageing of the pile concrete, loading level, environmental temperature and relative humidity. The corrosion initiation life of RC hollow piles is predicted by deterministic and probabilistic methods. In addition, by combining deterministic and probabilistic methods, a novel comprehensive method is proposed for evaluating the corrosion initiation life of RC hollow piles in chloride environments. A sensitivity analysis on the main influencing parameters is conducted for the prediction of the corrosion initiation life by the comprehensive method. Finally, the comparative analysis of three different prediction methods is also performed. The results show that the corrosion initiation life calculated by the comprehensive method decreases with increasing environmental temperature, relative humidity, water-cement ratio and initial surface chloride concentration, while it increases with increasing concrete cover depth and chloride threshold level. The comprehensive method is more reasonable than the deterministic and probabilistic methods for predicting the corrosion initiation life of RC hollow piles in chloride environments.

Evaluation of component‐similar rotor steel specimens with a local probabilistic approach for LCF

AbstractLately, a successful validation of a combined size and gradient effect modelling approach in a local probabilistic framework for LCF was published. A second validation study is here presented using LCF data of component‐similar specimens made of forged X12CrMoWVNbN10‐1‐1. It is found that a reasonable LCF crack initiation life prediction can be achieved by the local probabilistic model using single batch material parameters. However, the material parameters obtained from mixed batch LCF test data overestimate the actual material inherent LCF life scatter. Consequently, the size effect shift is pronounced too much, and the LCF life is predicted beyond crack initiation. Estimating the material parameters from a single batch data set instead results in a much less dispersed LCF life distribution and an approximately correct size effect shift. The predicted median Wöhler curve then corresponds to borehole‐specimen lives for initial cracks at the detection threshold. A reasonable prediction of technically relevant crack sizes in such inhomogeneous components requires additional modelling of stress gradient effects.

Effect of plastic deformation and fretting wear on the fretting fatigue of scaled railway axles

To investigate the effect of plastic deformation and fretting wear on the fretting fatigue of scaled railway axles, three fatigue assessment methodologies are developed, namely elastic-fatigue, elastoplastic-fatigue and wear-fatigue methodologies. Elastic material behaviour, elasto-plastic material behaviour and fretting wear are taken into consideration in these three methodologies, respectively. A comparison of the simulation and experimental results indicates that the profile evolution of the fretting scar is mainly caused by fretting wear. The wear-fatigue methodology provides a better prediction of the fretting scar width and crack initiation site than the elastic-fatigue and elastoplastic-fatigue methodologies. With respect to the crack initiation life prediction, a relatively accurate prediction is obtained on using the elastoplastic-fatigue and wear-fatigue methodologies in the short-life region, while the initiation life predicted using the elastoplastic-fatigue methodology is shorter than that predicted by the wear-fatigue methodology in the long-life region. The influence of the diameter ratio on the fretting fatigue of scaled railway axles is then studied. The obtained simulation results show that an increment in the diameter ratio can reduce the fretting wear and relative slip range by alleviating the stress concentration, which thereby improves the fatigue strength of scaled railway axles. There exists a critical value of diameter ratio between 1.092 and 1.116, below which the fretting scar width and relative slip range are highly influenced by variations in the diameter ratio. This is in agreement with published experimental results.