Reliability Fatigue Life Prediction Research Articles

Fatigue life modeling for a low alloy steel after long-term thermal aging

Fatigue failure of materials has a considerable impact on the safety of equipment in service. In this study, axially tensile and low cycle fatigue tests were conducted on a low alloy steel after accelerated thermal aged at 450 °C for 10,000 h. The experimental results indicate that the ultimate and yield strengths increase moderately, while the fatigue life of specimens experience a slight decrease in this circumstance. The fracture analysis demonstrates that the bainite breaking facilitates the fatigue crack initiation and propagation after thermal aging, which is accompanied by a decrease in plastic strain amplitude. Therefore, the plastic strain amplitude is considered as an indicator of thermal aging in fatigue life modeling for the low alloy steel. Finally, a novel life model that incorporates both aging time and temperature was proposed for rapid prediction of low cycle fatigue life. It is assumed that this model promotes reliable fatigue life prediction in low alloy steels under various thermal aging circumstances, as well as the extrapolation of fatigue performance of the material in service.

Optimizing dimension selection for rain flow counting in fatigue assessment of large-scale lattice wind turbine support structures: A comprehensive study and design guidance

The selection of dimension (d) for the rain flow counting matrix significantly influences the reliable prediction of fatigue life in wind turbine support structures. However, there are limited public reports on the impact of dimensions on the fatigue assessment of wind turbine structures. This study explores the influence of dimensions on the fatigue assessment process and outcomes, focusing on a large-scale ultra-high lattice wind turbine support structure from an engineering project. Through integrated load simulation, fatigue load time series for different design load cases (DLCs) are obtained for the overall structure. The rain flow counting method is utilized to derive the overall rain flow counting matrix (Markov_n) with varying dimensions. Subsequently, employing a multi-scale fatigue assessment method, the damage matrix (Markov_D), cumulative fatigue damage (D), and fatigue life are computed. A detailed exploration examines the impact of rain flow counting matrix dimensions on Markov_n, Markov_D, D, fatigue life, and calculation time (t). Furthermore, guidance is provided for selecting the optimal dimension for engineering design. The findings indicate that selecting dimensions that are too small results in the loss and merging of smaller mean and amplitude load values, thereby failing to accurately represent the load history. Moreover, a small dimension yields an excessively cautious estimation of cumulative fatigue damage (D) and underestimates the fatigue life, consequently inflating construction expenses.

New approaches for a reliable fatigue life prediction of powder metallurgy components using machine learning

AbstractUp to now, no consistent fatigue assessment approach of powder metallurgy (PM) components is available. For some materials and for some parameters, such as the density, a relationship to the fatigue strength is known; however, for other materials, such relationships are unknown. Based on an extensive data set with 828 test series, the present work addresses this problem by conceiving and applying five machine learning (ML)‐based approaches to increase the accuracy of the prediction of the fatigue life as well as to predict the scatter of unknown data as precisely as possible. With the elaborated procedure, on the one hand, a scatter range of can be achieved on completely unknown data. On the other hand, by using a newly defined loss function, the standard deviation of unknown data can be predicted very accurately. The findings provide the basis for further research on cost and efficiency optimized design of PM components through better estimation of fatigue life.

On the effect of environmental temperature on fracture fatigue entropy

Fracture behavior of carbon steel 1018 operating in low-temperature environments is investigated. Specifically, fully-reversed bending experimental tests are performed to examine the efficacy of applying the concept of Fracture Fatigue Entropy (FFE) for predicting fatigue life at different environmental temperatures. A large cooling chamber was constructed that houses the entire bending test rig and maintains the environmental temperature at the desired value down to −10 °C. It is shown that under conditions tested, fatigue life improves at lower operating temperatures. Results also reveal that FFE remains nearly constant and can be used for reliable prediction of fatigue life. Illustrative examples are provided to show the utility of the approach for prediction purposes.

Fatigue characterization analysis of a submerged fishing farm platform through spectral-based fracture mechanics

This study proposes a general direct calculation approach for fatigue crack growth evaluation and life predictions through a spectral-based fracture mechanics method. First, equivalent damage accumulation rates are introduced to describe the crack propagation characteristics under short-term sea states, integrating wave spectrum and fracture mechanics. Subsequently, a general fatigue crack growth program is integrated to realize automatic crack growth, accurate stress intensity factor (SIF) calculation and reliable fatigue life prediction through coupling analysis of meter-scale structures and millimeter-scale cracks. Finally, a parameter study is implemented to explore the influence of wave direction and initial crack size/shape on the fatigue performance of the fishing farm platform. Results indicate that the remaining fatigue life is evidently longer under 90° wave direction than that under 0 and 45°, which can be well characterized through SIF transfer functions. Critical fatigue life decreases with the increase of crack size and aspect ratio with the shortest duration of 12.8 years, demonstrating that residual fatigue life is closely associated with initial crack state. Fatigue failure is more predominant at lower tubular joints with the critical fatigue life ranging from 15 to 30 years, indicating the self-developed program possesses good universality and applicability in dealing with fatigue issues.

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.

Effect of fiber hybrid mode on the tension–tension fatigue performance for the pultruded carbon/glass fiber reinforced polymer composite rod

In order to promote the application of fiber reinforced polymer (FRP) rod in bridge engineering, the carbon/glass fiber uniformly dispersed hybrid (UDH) rod with a diameter of 22 mm was developed as the bridge cable through the pultrusion technology. In the present paper, the tension–tension fatigue performances of hybrid rod were investigated with three stress levels of 0.33, 0.44, 0.60 and the fixed stress ratio of 0.4. The fatigue life, stiffness degradation and stress redistribution during the fatigue loading were obtained by linear variable differential transformer (LVDT) and digital image correlation (DIC) method. Furthermore, the fatigue failure mechanism was revealed by microstructure damage analysis. Through the fiber uniformly hybrid technology, the fatigue failure mode of hybrid rod transferred from shell-core debonding to uniformly carbon and glass fiber/resin interface splitting. The positive hybrid effect of fatigue performance was realized to obtain higher fatigue life, stiffness retention and residual mechanical properties. The reliable fatigue life prediction model was developed, and the recommended fatigue limit of present hybrid rod applied for bridge cable was 0.45 at the stress ratio of 0.4 to obtain more than two million fatigue cycles.

Composite Failure Modes at High Cyclic Fatigue Life Evaluation

Fibre reinforced polymer composite have been utilised in applications that require high strength-to-weight ratio and durability like automotive and spacecraft components. The literatures has indicated that there is a gap of knowledge in fatigue failure mechanism, and reliable prediction of fatigue life for glass and carbon fibre laminates. This study is to address experimentally the stress level dominated the composite failure mode and how to avoid stress concentration in fatigue design. In addition, it contributes to the scientific knowledge and to further increase the understanding the fatigue behaviour of composite materials structures. To accomplish this investigation goals, three types of materials were fabricated and tested; glass/epoxy, carbon/epoxy, and chopped glass/epoxy. Traditional hand layup technique for composite processing was used to fabricate the composite specimens. It involves manually positioning the reinforcement woven roving in an open mould and pouring, brushing, the resin onto the composites. This study details the experimental results of the ultimate tensile strength and high cycle fatigue, with a stress ratio of 0.1, using ASTM. Results showed that carbon fibre composite had the highest ultimate tensile strength. The power curves conducted from this paper were used to estimate the number of cycles which the material can endure.

Fatigue life prediction of spot-welded joints with a notch stress approach

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

A notch stress method for fatigue life prediction of spot-welded joints

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

Rapid fatigue life prediction for spot-welded joint of SUS301 L-DLT stainless steel and Q235B carbon steel based on energy dissipation

Spot welding of dissimilar materials can utilize the respective advantage comprehensively, of which reliable prediction of fatigue life is the key issue in the structure design and service process. Taking into account almost all the complex factors that have effects on the fatigue behavior such as load level, thickness, welding nugget diameter, vibrational frequency, and material properties, this article proposed an energy dissipation-based method that is able to predict the fatigue life for spot-welded dissimilar materials rapidly. In order to obtain the temperature gradient, the temperature variations of four-group spot-welded joint of SUS301 L-DLT stainless steel and Q235 carbon steel during high-cycle fatigue tests were monitored by thermal infrared scanner. Specifically, temperature variation disciplines of specimen surface were divided into four stages: temperature increase, temperature decrease, continuous steady increase in temperature, and ultimate drop after the fracture. The material constant C that a spot-welded joint of dissimilar material needs to reach fracture is 0.05425°C·mm3. When the specimen was applied higher than the fatigue limit, the highest error between experimental values and predicted values is 18.90%, and others are lower than 10%. Therefore, a good agreement was achieved in fatigue life prediction between the new method and the validation test results.

A fatigue damage and residual strength model for unidirectional carbon/epoxy composites under on-axis tension-tension loadings

Fibre-reinforced composites experience a degradation of stiffness and strength during fatigue life. Understanding the reduction of these properties is fundamental to establish a reliable fatigue life prediction methodology. This work investigates the loss of stiffness and strength in advanced unidirectional carbon/epoxy laminates under on-axis tension-tension loads. A phenomenological stiffness-based fatigue model is formulated within the framework of continuum damage mechanics, where damage is described by the reduction of the in-plane longitudinal stiffness. The particularity of the model is to assume that the ultimate strain remains constant after fatigue damage. Thus, the residual strength model and the S-N curves are deduced from the residual stiffness model. This assumption reduces the experimental characterization of phenomenological-based approaches. The experimental challenges found in the fatigue experiments are also discussed. The accuracy of the model is verified by comparing the experimental data with the derived S-N curves.

Assessing reliability of fatigue indicator parameters for small crack growth via a probabilistic framework

Microstructurally small cracks exhibit large variability in their fatigue crack growth rate. It is accepted that the inherent variability in microstructural features is related to the uncertainty in the growth rate. However, due to (i) the lack of cycle-by-cycle experimental data, (ii) the complexity of the short crack growth phenomenon, and (iii) the incomplete physics of constitutive relationships, only empirical damage metrics have been postulated to describe the short crack driving force metric (SCDFM) at the mesoscale level. The identification of the SCDFM of polycrystalline engineering alloys is a critical need, in order to achieve more reliable fatigue life prediction and improve material design. In this work, the first steps in the development of a general probabilistic framework are presented, which uses experimental result as an input, retrieves missing experimental data through crystal plasticity (CP) simulations, and extracts correlations utilizing machine learning and Bayesian networks (BNs). More precisely, experimental results representing cycle-by-cycle data of a short crack growing through a beta-metastable titanium alloy, VST-55531, have been acquired via phase and diffraction contrast tomography. These results serve as an input for FFT-based CP simulations, which provide the micromechanical fields influenced by the presence of the crack, complementing the information available from the experiment. In order to assess the correlation between postulated SCDFM and experimental observations, the data is mined and analyzed utilizing BNs. Results show the ability of the framework to autonomously capture relevant correlations and the equivalence in the prediction capability of different postulated SCDFMs for the high cycle fatigue regime.

A fuzzy residual strength based fatigue life prediction method

The fatigue damage problems are frequently encountered in the design of civil engineering structures. A realistic and accurate fatigue life prediction is quite essential to ensure the safety of engineering design. However, constructing a reliable fatigue life prediction model can be quite challenging. The use of traditional deterministic approach in predicting the fatigue life is sometimes too dangerous in the real practical designs as the method itself contains a wide range of uncertain factors. In this paper, a new fatigue life prediction method is going to be proposed where the residual strength is been utilized. Several cumulative damage models, capable of predicting the fatigue life of a structural element, are considered. Based on Miner\'s rule, a randomized approach is developed from a deterministic equation. The residual strength is used in a one to one transformation methodology which is used for the derivation of the fatigue life. To arrive at more robust results, fuzzy sets are introduced to model the parameter uncertainties. This leads to a convoluted fuzzy based fatigue life prediction model. The developed model is illustrated in an example analysis. The calculated results are compared with real experimental data. The applicability of this approach for a required reliability level is also discussed.

Crack Growth Prediction under Variable Amplitude Loading Considering Elastic-Plastic Stress Field ahead of Crack Tip

Reliable prediction of fatigue life of the structural components under variable amplitude loading requires accurate computation of the residual stresses ahead of crack tip. In present study, a novel method of estimation of linear elastic stress field using concept of fictitious notch rounding is presented. The stress field is later used for evaluation of residual stress distribution. Corrective stress intensity factor is calculated from the residual stress field using modified weight function method. Corrective residual stress intensity factor is used to find the effective maximum stress intensity factor and effective stress intensity range. The values of root radius of aluminium alloys computed using proposed method is found to be closely matching with the values available in literature. Numerical investigation has been carried out to predict remaining life of plate panel with and without considering load interaction effects.

Determination Method of a Proper Small-Load-Omitting Criterion Based on the Extreme Load

A crucial step to obtain a reliable fatigue life prediction is to determine a proper small load threshold below which the cycles at small loads or stresses with high frequency causing little fatigue damage are truncated from the original load time history. By taking both the peak over threshold theory and the endurance limit threshold into account, a proper small load threshold is proposed in this paper. The optimal threshold should be high enough to remove the high-frequency small cycles and low enough to minimize the loss of the fatigue damage which maybe be truncated by the empirical small-load omitting threshold. Based on this proper threshold, the fatigue life prediction will be more reliable.

Diversity of damage evolution during cyclic loading at very high numbers of cycles

Over the last decade, it has been shown for a number of metals that failure occurs even beyond the classical fatigue limit. High frequency testing techniques make it possible to conduct fatigue tests up to 109 or even 1011 cycles for various alloys, ranging from aluminium to high strength steels. As a consequence, the characterisation of fatigue life and damage mechanisms at N>107 cycles [very high cycle fatigue (VHCF)] has become a major research issue. Fatigue life in this regime is dominated by crack initiation. With the overall strain being in the purely elastic range, microstructural features acting as stress raisers lead to localised and inhomogeneously distributed irreversible deformation. Hence, microstructural discontinuities become the leading features controlling fatigue life at very high numbers of cycles. The present survey will focus on dislocation arrangement, grain orientation, grain size and surface roughening and their implications on the VHCF behaviour for selected virtually defect free metals, thus providing a sound basis for a detailed understanding of the relevant deformation and damage evolution mechanisms. It will also focus on the VHCF behaviour of materials representing a ‘transition’ between non-defect related damage evolution and defect based crack initiation, thus pointing out the complexity of damage evolution in the VHCF range. In this context, the term defect is limited to hard non-metallic inclusions, which can be found, among others, in high strength steels as well as pores in casting materials, both dominating the VHCF behaviour of these material types. In contrast, second phases, precipitates or intermetallic particles are considered as irregularities of the microstructure and will not be classified as defects. The current review will show that a true understanding of the VHCF behaviour requires a careful differential analysis of the possible microstructural features leading to localised plastic deformation and that not only crack initiation, but also crack growth behaviour analysis is essential to gain a sound basis for a reliable fatigue life prediction.

Effective strain damage model associated with finite element modelling and experimental validation

An efficient computational effort is required for sufficient analysis during component design stage to avoid expensive modifications during the later stages of the manufacturing process. A combination of the effective strain damage (ESD) model based FORTRAN code with finite element software was proposed to predict fatigue life under random loadings. The ESD model parameters were extracted from the experimental and finite element analysis representing the strain history for the most damaging node in an automobile lower suspension arm. The ESD model provides reliable fatigue life prediction under random loadings with only 12% minimum difference comparing to the experimental estimations.

A Method of Simulating Computation for the Fatigue Life Prediction

The fatigue life prediction and reliability analysis of dynamic systems under random excitement are important topics in modern engineering design. However, the stress power spectral density and the formulation of fatigue life prediction of a component in a dynamic system must be known when one predicts its fatigue life. A method of simulating computation for the fatigue life of a dynamic structure is presented. The method is based on the concept of unit load stress matrix. According to it, the relationship between the stress power spectral density of a structure in a system and the response spectra of the system can be established. Based on this, the formulation of the fatigue life prediction which is decided by the random stress process is obtained. With this method, the stress power spectral density simulation and the reliability fatigue life prediction for the CW-200 type vehicle truck are made. The method is suitable not only for the running dynamic structure but also for newly designed dynamic structures especially.

Evaluation of fatigue life of aluminum alloy wheels under radial loads

This paper is concerned with generation of S–N curve for aluminum alloy (Al) A356.2-T6 and estimation of fatigue life under radial fatigue load. The S–N curve is developed by conducting tests at different stress levels under constant amplitude loading. Tests are conducted on alloy wheels for fatigue life evaluation under radial loads. Finite element analysis (FEA) is carried out by simulating the test conditions to analyze stress distribution and fatigue life of the alloy wheels. It is observed from analysis that the prediction of fatigue life using FEA is found to be in close agreement with the corresponding experimental observations. An attempt has been made by conducting parametric study to suggest a suitable safety factor for reliable fatigue life prediction.