Fatigue Life Prediction Method Research Articles

Multiaxial fatigue life prediction method based on the back-propagation neural network

The study was devoted to developing a reliable multiaxial fatigue life prediction method with the effect of additional cyclic hardening considered. Based on the original assumptions of traditional critical plane methods, a new characteristic plane (subcritical plane) was defined to describe the particularity of additional cyclic hardening under non-proportional loading condition. On the new defined subcritical plane, a new multiaxial fatigue damage control parameter containing the effect of additional hardening was also built, by which the dynamic path of stress spindle, combining material property and loading environment, was fully analysed. In addition, a multiaxial fatigue life prediction back-propagation neural network (BPNN), as an alternative to the traditional physical model, was proposed to calculate the fatigue life under multiaxial loading. To calculate the multiaxial fatigue life of different materials, a more simplified combination parameter σb/E for different types of materials was chosen as input parameter in BPNN training. The availability of the proposed method was validated by reasonable correlations with experimental data of six alloy steel materials and two Non alloy steel materials under diverse loading paths.

Research on multiaxial fatigue life of notched specimens based on Weibull distribution and Bayes estimation

In this study, based on the nominal stress method and the influence of stress gradient on fatigue life, a fatigue life prediction method combined with the Weibull distribution is proposed. Firstly, the strain energy density is defined as the damage parameter to determine the location of the critical plane. Then, the equivalent stress function is established by extracting the equivalent stress of the specific path on the critical plane, which is used to establish the modified nominal stress method by combining with the stress gradient factor. Secondly, the Weibull distribution is combined to establish a new fatigue life prediction model of the notched specimens under different failure probabilities. The accuracy of the model is verified with three materials, then the prediction life of the proposed model is compared with that of the SWT model, the FS model and the Manson-Coffin equation. The results show that the accuracy of the proposed model is higher than the other three models. Finally, the reliability of the proposed model is evaluated by the lower confidence limit (LCL) estimation and interval estimation to explore the feasibility of this method. In addition, the P-S-N curves under different failure probabilities are established to guide actual projects.

High cycle fatigue behavior of a selective laser melted Ti6Al4V alloy: Anisotropy, defects effect and life prediction

Selective laser melting (SLM) exhibits more and more utilization potentialities in today’s aviation industry. In order to ensure safe operation of SLM produced aero parts, it is important to have the fatigue performance well-characterized, and fatigue life prediction methods developed by considering the initial feature of this material. In this study, high cycle fatigue tests under stress ratios of −1, 0.1 and 0.5 at room temperature for the SLM Ti6Al4V alloy were carried out, and the fracture morphologies of fatigue specimens were observed. The results show that the subsurface pore is the main source of crack initiation for fatigue failure. By quantitative and statistical analysis of the pore characteristics in the crack source zone of all fractured specimens, it is revealed that the anisotropy and dispersion of fatigue performance are closely related to the difference of the shape, size, location and density of the pores in the crack source zone of specimens. The relationship between pore size and location, such as the ratio of radius to the distance to the specimen surface, plays an important role in estimating the fatigue life of the specimen. Aiming at the nature of defect-induced fatigue damage, the fatigue life of the SLM Ti6Al4V alloy was reasonably predicted using the fracture mechanics method based on the small-crack theory and the long crack growth data.

Research on fatigue life evaluation method of shafts based on small sample P–S–N

In view of the fact that there are few fatigue data in practical engineering, Weibull distribution is introduced to propose a fatigue life prediction method considering failure probability based on small samples. Firstly, a modified bootstrap sampling method is given by introducing random numbers between (0–1), and the small sample fatigue life data is expanded to obtain the correlation factor of Weibull distribution. Secondly, the relationship between equivalent stress and characteristic life is given by exploring the law between them. Meanwhile, combining with the Osgood equation and using the Weibull distribution to describe the fatigue life, an iterative procedure is established to obtain the predicted life, and a first-order reliability factor is introduced to improve the reliability of the iterative procedure to modify the iterative procedure. Finally, the P–S–N curves of different failure probabilities are given, and Cast iron and TC4 alloy are selected to evaluate the correctness and reliability of the proposed method in this study. The comparison results show that the predicted life obtained by the proposed method is within the double dispersion band, and possesses high accuracy. At the same time, the P–S–N given by the proposed method in this paper under different failure probabilities conforms to the actual law, which can provide a theoretical basis for solving practical engineering problems.

Comparative study on numerical simulation methods of stress concentration calculation and fatigue life prediction for welds

The effective notch stress considers the influence of notch effect and solves the stress singularity in FEA by setting virtual notch radius. However, this method also has limitations such as inaccurate calculation results affected by virtual radius and modeling difficulty. The structural stress method based on fracture mechanics recommended in this paper has no such limitation, but also takes notch effect and stress singularity into account. To prove this, the engineering application ability of the two methods is compared through two cases. The results show that the structural stress method has better engineering applicability and is worth popularizing.

Fatigue Durability Analysis for Suspenders of Arch Bridge Subjected to Moving Vehicles in Southwest China

An analysis of the effect of vehicle loads on the fatigue life of local arch bridge suspenders in Southwest China was conducted by generating vehicle statistics, creating a fatigue vehicle model, and developing a fatigue life prediction method: First, the Midas model of an arch bridge was established, a random vehicle flow simulation was conducted in the Monte Carlo method, and the stress spectrum of the suspenders was calculated. Secondly, ANSYS software was used to estimate the fatigue life of the suspenders, and the accuracy of the proposed method was verified. Finally, the method was applied to the practical project of the Dafeng River Bridge in Southwest China. The results show that this method can predict the fatigue life of suspenders. It can be applied in the bridge health monitoring software system to realize real-time fatigue damage monitoring of the fatigue damage of suspenders, which could be used to predict the fatigue lives of bridges in Southwest China and provide references for the replacement of suspenders.

A hybrid approach of modified bootstrap and physics-based methods for probabilistic fatigue life prediction considering overload effects

A hybrid approach is proposed by combining the modified bootstrap and physics-based methods, which aims to achieve a reasonable and accurate probabilistic fatigue life prediction under arbitrary overload conditions based on constant amplitude fatigue data. The fatigue crack growth model and equivalent initial flaw size (EIFS) are employed to calculate the fatigue life. The crack closure mechanism is used to account for the loading interaction effects. Furthermore, the uncertainties of EIFS, coefficient of the fatigue model and overload intervals are considered. Fatigue data are collected by performing testing of aluminum alloy 7075-T6 under constant amplitude loading with/without overloads. The bootstrap method is modified by generating the expanding samples, which is utilized to quantify the dispersion of fatigue data. The resampling is conducted based on the expanding samples, instead of the original data directly obtained from the experiment. Eventually, the fatigue lives with different failure probabilities and confidence levels are calculated. In addition, other testing data sets of Al–Li alloy 2060 are also employed to verify the effectiveness and accuracy. Good agreements are observed.

Fatigue life prediction method for central hole of needled ceramic matrix composite

Based on the consideration of matrix porosity and representative volume element of needled structure, the macroscopic elastic constants of needled C/SiC ceramic matrix composite and the stress concentration factor of the hole-edge were calculated by the finite element method first. Then, based on the fatigue test data of the smooth and central hole specimens, the fatigue notch factors corresponding to different stress levels were obtained. In view of the different characteristics of the fatigue notch factor obtained under different limit lives, the relationship between the fatigue notch and stress concentration factors was established, and the influence of nominal stress on the fatigue notch factor was also considered. Based on this, a fatigue life prediction method for central hole of needled C/SiC composite was proposed. The proposed method can predict the fatigue life of central hole by using the S–N curve of smooth specimen and the stress concentration factor of central hole specimen. The comparison between predicted and experimental lives showed that the proposed method can better predict the fatigue life of central hole specimen for needled C/SiC composite.

Multiaxial fatigue life prediction based on single defect for additively manufactured 316L

A multiaxial fatigue life prediction method based on single defect for 316L stainless steel processed by laser powder bed fusion is studied. Defects are detected in the crack initiation zone of more than 95% of the specimens. A method based on the small crack propagation theory is applied to predict fatigue life under uniaxial and pure torsional loading. The fatigue parameters obtained from the above method are substituted into the Modified Wöhler Curve Method to predict multiaxial fatigue life. The result shows that the predicted fatigue lives are in good agreement with the experimental results.

An Energy-Based Method for Lifetime Assessment on High-Strength-Steel Welded Joints under Different Pre-Strain Levels.

Pre-loading on engineering materials or structures may produce pre-strain, especially plastic strain, which would change the fatigue failure mechanism during their service time. In this paper, an energy-based method for fatigue life prediction on high-strength-steel welded joints under different pre-strain levels was presented. Tensile pre-strain at three pre-strain levels of 0.2%, 0.35% and 0.5% was performed on the specimens of the material Q345, and the cyclic stress and strain responses with pre-loading were compared with those without pre-loading at the same strain level. The experimental work showed that the plastic strain energy density of pre-strained welded joints was enlarged, while the elastic strain energy density of pre-strained welded joints was reduced. Then, based on the strain energy density method, a fatigue life estimation model of the high-strength-steel welded joints in consideration of pre-straining was proposed. The predicted results agreed well with the test data. Finally, the validity of the developed model was verified by the experimental data from TWIP steel Fe-18 Mn and complex-phase steel CP800.

Fatigue life prediction of gasoline storage tank considering varying current density and weld stress

In the field of corrosion fatigue crack research, how to couple corrosion and fatigue under various conditions is a problem worthy of discussion. According to the electrochemical corrosion principles and the superposition principles, studies have been made on the interaction between corrosion and fatigue, and the relationship model between the corrosion fatigue crack size and the corrosion fatigue remaining life in the Corrosion of Hydrogen evolution (COHE) fatigue long crack growth stage is established. The feasibility of the model is verified by using the experimental data. According to the specific structure of the dislocation butt weld of large storage tank, the stress at the crack is calculated. Based on the above model and the experimental data, the corrosion fatigue residual life of large storage tank is predicted. The feasibility of the model is verified, which provides a new method for corrosion fatigue life prediction of large storage tanks.

Design of a Practical Metal-Made Cold Isostatic Pressing (CIP) Chamber Using Finite Element Analysis

The fast development of deep-ocean engineering equipment requires more deep-ocean pressure chambers (DOPCs) with a large inner diameter and ultra-high-pressure (UHP). Using the pre-stressed wire-wound (PSWW) concept, cold isostatic pressing (CIP) chambers have become a new concept of DOPCs, which can provide 100% performance of materials in theory. This paper aims to provide a comprehensive design process for a practical metal-made CIP chamber. First, the generalized design equations are derived by considering the fact that the cylinder and wire have different Young’s moduli and Poisson’s ratios. Second, to verify the theory and the reliability of the CIP chamber, the authors proposed a series of FEA models based on ANSYS Mechanical, including a two-dimensional (2D) model with the thermal strain method (TSM) and a three-dimensional (3D) model with the direct method (DM). The relative errors of the pre-stress coefficient range from 0.17% to 5%. Finally, the crack growth path is predicted by using ANSYS’s Separating Morphing and Adaptive Remeshing Technology (SMART) algorithm, and the fatigue life is evaluated by using the unified fatigue life prediction (UFLP) method developed by the authors’ group. This paper provides a more valuable basis to the design of DOPCs as well as to the similar pressure vessels than the previous work.

A new cyclical generative adversarial network based data augmentation method for multiaxial fatigue life prediction

To meet the requirement of large experimental data for machine learning (ML) methods on multiaxial life prediction, a cyclical Generative Adversarial Network (named cGAN) integrating Fourier transformation and other semi-empirical equations, was proposed to augment data following physical knowledge. Two samples of each loading path can be augmented into hundreds of good quality samples. Consequently, the accuracy of ML methods for multiaxial fatigue life of 316L stainless steel can been improved by 35%-91%. The new method can definitely balance time cost of bigger sample size and prediction accuracy well.

Reliability Prediction Method for Low-Cycle Fatigue Life of Compressor Disks Based on the Fusion of Simulation and Zero-Failure Data

Targeting reliability assessment problems with a small sample size and large amount of zero-failure data which appear in the life assessment tests of aero-engine compressor disks, a reliability assessment method and simulation test method were developed by fusing numerical simulation and zero-failure data based on the life reliability assessment method for zero-failure data. The developed method was verified by numerical examples, and further applied in the reliability analysis of compressor disks by combining the test and simulation results. The results indicated that fusing the test data with the simulation data increased the amount of information, improved the life prediction accuracy, and increased the lower confidence limit of the reliable life by 6.02% compared with the case where the test data alone were used.

Fatigue Life Prediction of Structures With Interval Uncertainty

Abstract A new method for reliable fatigue life prediction in metal structural components is developed, which quantifies uncertainties using interval variables. Using this crack-initiation-based method, first, the uncertainties in laboratory test data for the fatigue failure of a structural detail are enumerated. This uncertainty quantification is performed through an interval-based enveloping procedure that relates the interval stress ranges to the number of cycles to failure. This will lead to the construction of an interval S–N relationship. Next, the uncertainties in field test data are enumerated in the extremum values of each stress range, as intervals, leading to the construction of interval stress ranges. For both the laboratory and field data uncertainty analyses, the mean stress effects are considered. Next, the interval damage accumulated over the duration of the field data is determined using the constructed interval S–N relationship and the obtained interval stress ranges. Then, the interval existing damage and interval remaining life are determined. Finally, as a conservative measure, the minimum remaining fatigue life is obtained in which all uncertainties are considered. A numerical example illustrating the developed method is presented, and the results are compared with results obtained by both Monte Carlo simulation and optimization. Using this method, for the numerical example considered, it is shown that the results for bounds on the existing damage and the remaining fatigue life are sharp. Moreover, due to its set-based approach, the method is significantly more computationally efficient when compared with iterative procedures.

Research on Fatigue Life Prediction Method of Key Component of Turning Mechanism Based on Improved TCD

The main objective of this paper is to accurately obtain fatigue life prediction for the key components of a turning mechanism using the improved theory of critical distances (TCD). The irregularly shaped rotating arm is the central stressed part of the turning mechanism, which contains notches. It has been found that TCD achieves good results in predicting the fatigue strength or fatigue life of notched components with regular shape but is less commonly used for notched components with irregular shape. Therefore, TCD was improved and applied broadly to predict the fatigue life of an irregularly shaped rotating arm. Firstly, the notch depth and structure net width parameters were introduced into the low-order and low-accuracy classical TCD function to obtain a novel stress function with high computational efficiency and high accuracy, whereas the stress concentration factor was introduced to modify the length of critical distance. Secondly, the improved TCD was used to predict the fatigue strength of notched components with regular shape, and its accuracy was demonstrated by a fatigue experiment. Finally, the improved TCD was applied to predict the fatigue life of an irregularly shaped rotating arm. The deviation between prediction results and experimental results is less than 18%. The results demonstrate that the improved TCD can be applied effectively and accurately to predict the fatigue life of key components of turning mechanisms.

Misalignment effect on the fatigue failure behavior of load-carrying cruciform welded joints

This paper presents an analytical model based on the Strain Energy Density (SED) method to predict the high cycle fatigue failure behavior of load-carrying cruciform welded joints (LCWJ) weakened by axial and angular misalignments. Compared with the Hot Spot Stress (HSS) and Effective Notch Stress (ENS) methods recommended in standard codes, the SED values at weld toe and weld root were carried out systematically considering the welded details and the types of misalignments. Moreover, the analytical model was applied to estimate the magnitudes of fatigue indicators and illustrated the effects of misalignment on the fatigue failure transition region in LCWJ. It is validated effectively as a supplementary analysis method for fatigue life prediction and fatigue design of welded joints compared with the experimental data and published reports.

Multiaxial fatigue life prediction method considering notch effect and non-proportional hardening

Engineering components with geometric discontinuous structures generally appear notch effect, which might eventually induce fatigue failure. In this study, a novel multiaxial fatigue prediction model is established for notched components by incorporating the notch effect and non-proportional hardening. Firstly, according to the notch effect, a computational procure of damage parameters is presented by combining the notion of stress intensity with finite element simulations based on the critical plane method. Secondly, considering the non-proportional hardening effect of path modes and material properties, a new modified non-proportional hardening factor is proposed. Particularly, the relationship between the relative stress gradient and the fatigue life is investigated under different stress concentration factors. Finally, with the help of the Manson-Coffin equation, a multiaxial fatigue life prediction model is developed subjected to multiaxial proportional/non-proportional loadings. Experimental data of medium steel En8 and GH4169 alloys are utilized to evaluate and validate the proposed model as well as four other classical models (FS model, MSWT model, Yu model and CXH model). The result indicates that the proposed model yields a higher accuracy on multiaxial fatigue life than other four models, and the majority of prediction results are within the ±2 life factor.

Three-dimensional nonlinear vibration model and fatigue failure mechanism of deepwater test pipe

In deepwater test condition, the riser–test pipe (tubing string) system (RTS) is subject to the vortex-induced effect on riser, flow-induced effect on test pipe and longitudinal/transverse coupling effect, which is prone to buckling deformation, fatigue fracture and friction perforation. To resolve this, the three-dimensional (3D) nonlinear vibration model of deepwater RTS is established using the micro-finite method, energy method and Hamilton variational principle. Based on the elastic–plastic contact collision theory, the nonlinear contact load calculation method between riser and test pipe is proposed. Compared with experimental measurement results, calculation results using the proposed vibration model in this study and the single tubing vibration model in our recent work, the correctness and effectiveness of the proposed vibration model of the deepwater RTS are verified. Meanwhile, the cumulative damage theory is used to establish the fatigue life prediction method of test pipe. Based on that, the influences of outflow velocity, internal flow velocity, significant wave height, as well as top tension coefficient on the fatigue life of test pipe are systematically analyzed. The results demonstrate that, first, with the increase in outflow velocity, the maximum alternating stress and the annual fatigue damage rate increased. The location where fatigue failure of the test pipe is easy to occur at the upper “one third” and the bottom of test pipe are easy to occur fatigue failure. Second, with the increase in internal flow velocity, the “one third damage effect” of the test pipe will decrease, and the “bottom damage effect” of the test pipe increased that needs the attention of field operators. Third, during field operation, it is necessary to properly configure the top tension coefficient so that there can be a certain relaxation between the riser and the test pipe, so as to cause transverse vibration and consume some axial energy and load. The study led to a theoretical method for safety evaluation and a practical approach for effectively improving the fatigue life of deepwater test pipe.

Fatigue life prediction method for subsea wellhead welds based on the nonlinear fatigue accumulation model

In drilling and completion conditions, subsea wellheads are exposed to enormous alternating bending moment loads from the riser due to complex environmental conditions, causing fatigue damage. The joint weld between the high-pressure (HP) wellhead and the casing is especially vulnerable to fatigue, creating the risk of operational failure and accidents. Although fatigue analysis is vital, the traditional S–N curve method is often not accurate enough. This paper proposed an analytical technique to examine subsea wellhead fatigue by combining subsea wellhead fatigue decoupled analysis with a nonlinear fatigue accumulation theoretical model. This method was used to assess subsea wellhead weld fatigue in a gas field in China's Bohai Sea during drilling and completion. The bending moment load spectrum at the top of the HP wellhead was obtained via global dynamic analysis. The bending moment-stress and bending moment-strain response curves of the welds were extracted using local quasi-static analysis. Finally, the theoretical nonlinear fatigue accumulation model and traditional S–N curve method were used to evaluate subsea wellhead weld fatigue, respectively. The results indicated that the fatigue damage caused by drilling was more significant than completion, while the fatigue damage at the weld between the HP wellhead and the 16″ casing exceeded that of the weld between the low-pressure (LP) housing and the 30″ conductor. Furthermore, the technique proposed in this paper could be used to assess more extensive fatigue damage than when employing the S–N curve method, indicating that the results were more conservative and more suitable for practical engineering applications.