Constant Amplitude Loading Conditions Research Articles

Experimental investigation and phenomenological modeling of fatigue crack growth in X80 pipeline steel under random loading

Ocean structures are subjected to random environmental loads during their service life. Under the action of random loads, fatigue crack growth within pipeline steel exhibits transient behaviors such as retardation or acceleration. This poses significant challenges in predicting the fatigue life of ocean structures under random loadings. To establish a predictive model for fatigue crack growth in X80 pipeline steel under random loadings, this study first conducts fatigue crack growth experiments under constant amplitude and random loading conditions. Then, based on deep learning methods and the fracture yield zone theory, a phenomenological model is developed to predict fatigue crack growth in X80 pipeline steel under random loading conditions. The results show that under random loading conditions, the fatigue crack growth curve of X80 pipeline steel maintains a relatively clear logarithmic-linear feature during the stable crack growth stage. However, due to the continuous variation of stress ratio with loading sequence, the fatigue crack growth curve exhibits strong fluctuation characteristics; The phenomenological model established in this study for predicting fatigue crack growth under random loading conditions can relatively accurately reflect the overall trend of fatigue crack growth under random loadings, with a maximum prediction error of less than 10%.

Crack Growth Simulations With SMART Method in Adhesively Bonded Joints

Due to the important advantages of adhesive joints, such as their suitability for multi-material designs, their use has been increasing in the last decade. Determining the fracture behavior of structural adhesive bondings is essential for structural durability. In crack propagation analyses, adaptive meshing has drawn considerable attention because of its improvements in terms of complex preprocessing and time management. This paper presents a recently introduced separating morphing and adaptive remeshing technology (SMART) innovative crack growth simulation for adhesively bonded joints, considering static and cyclic cases. For the static case, an R-curve was obtained for the bonding joints of carbon steel and Araldite 2015. For the cyclic case, the Carbon Fiber Reinforced Polymer (CFRP) bonding joints were analyzed under constant-amplitude loading conditions. The crack-propagation rate and the number of cycles were estimated. Crack propagation simulations were validated using experimental data. Acceptable agreement was achieved between the experimental and estimated results.

Experimental analysis of the plastic CTOD to characterize the variable amplitude fatigue crack growth in Grade 2 titanium samples

In this paper, the plastic component of the crack-tip opening displacement (CTODp) is experimentally evaluated to characterize the fatigue crack growth behavior in a Grade 2 titanium sample under single overload with constant amplitude loading conditions. The CTOD measurements were performed with the 2D-Digital Image Correlation (DIC) technique. The experimental results show a linear correlation between CTODp and fatigue crack growth rates, suggesting the independence of this relationship to load-interaction effects. Therefore, this study demonstrates that the CTODp can be a suitable crack-driving parameter to study crack propagation behavior under variable amplitude loading. Furthermore, it suggests that the fatigue crack growth process is primarily caused by cyclic plastic deformation.

Fatigue life prediction of aluminum 6061 alloy through experimental and numerical analysis under various stress ratios in axial tension-tension loading condition

Aluminum alloy 6061 is a versatile material widely used in aerospace, automotive, marine, and structural applications due to its mechanical strength, weldability, and corrosion resistance. Understanding and addressing fatigue is crucial for ensuring the reliable and safe performance of components made from this alloy. This study focused on predicting the fatigue life of aluminum 6061 at room temperature under axial loading conditions and evaluating the fatigue life at various stress ratios. The experimental S-N curve, derived from tension-tension fatigue loading tests, was instrumental in characterizing the material's fatigue behaviour under constant amplitude loading condition. The data obtained from these tests were then imported into ANSYS software, a widely recognized tool for finite element analysis and simulation. ANSYS was used to conduct a detailed numerical analysis to predict the fatigue life of aluminum 6061 at different stress ratios. The results from the experimental and numerical analyses exhibited good agreement, validating the accuracy of ANSYS software in predicting fatigue life at different stress ratios. This study demonstrated the effectiveness of employing a combined experimental and numerical approach for understanding fatigue behaviour in materials, particularly in terms of its applicability to different loading scenarios. The findings from this research provide valuable insights into the fatigue life of aluminum 6061 under diverse stress ratios, with the goal of enhancing material design and engineering processes. By leveraging the capabilities of ANSYS software in conjunction with experimental data, the study established a reliable methodology for predicting fatigue life, which is crucial for optimizing the performance and longevity of components subjected to cyclic loading in various industries.

Fatigue Life Analysis of Automotive Cast Iron Knuckle under Constant and Variable Amplitude Loading Conditions

The main aim of the present paper is to assess the fatigue lifetime of ductile cast iron knuckles as one of the critical components of an automotive steering system. To this end, a real driving path, including various maneuvers, such as acceleration, braking, cornering, and moving on various types of road roughness, was considered. Different load histories, which are applied on various joints of the component (i.e., lower control arm, steering linkage, and Macpherson strut), were extracted through Multi-Body Dynamics (MBD) analysis of a full vehicle model. The achievements of previous studies have proved that the steering knuckle fails from the steering linkage and due to the rotational motion. Therefore, only this destructive load history was considered in future analyses of the present study. The CAD model was created using Coordinate Measuring Machine (CMM) data and some corrections in CATIA software. Furthermore, transient dynamic analysis was performed, and the time history of von Misses equivalent stress was obtained at the root of the steering linkage (which is exactly the location of failure based on the laboratory data as well as finite element simulations validated by the author in the previous studies). To predict the fatigue life of a component, two different methodologies were considered. Firstly, some well-known criteria were employed for equalization of load spectrum to a Constant Amplitude Loading (CAL). Then, fatigue analysis under sinusoidal loading was performed. Secondly, the fatigue life of the component considering Variable Amplitude Loading (VAL) was predicted using the Critical Plane Method (CPM), employing the Rain-flow cycle counting technique, and utilizing the Miner–Palmgren damage accumulation rule. Eventually, to evaluate the prediction accuracy of different methodologies, the obtained results were compared with the full-scale axial variable amplitude fatigue test which was performed by the corresponding author. The results indicated that the prediction of variable amplitude fatigue lifetime by Finite Element (FE) analysis in the time domain has about 21% error compared to reality. Additionally, the relative error between the results obtained from two different methodologies is about 20%, which is acceptable due to the scattering of the fatigue phenomenon results, the complex geometry of the part, and the complicated loading.

Experimental investigation of high frequency pulse loading on fatigue crack growth in 5052-H32 series aluminum

In the marine industry, designers utilize 5052-H32 marine grade series aluminum for its strength to weight ratio, and the ability of this series to resist marine corrosive environments. During the operating lifetime of a marine vessel, it experiences constant and variable amplitude loading conditions from the sea environment, plus overloads/underloads in the form of high frequency pulses. These high frequency pulses can influence the fatigue crack growth behavior of the aluminum structure. Foundational models like Wheeler and Willenborg have previously been used to predict effects of underloads/overloads on fatigue crack growth, but have proven to be inadequate for all high frequency pulse loading conditions. In this paper, we coupled observations of the crack growth path obtained using an optical microscope with detailed measurements of surface strain fields using Digital Image Correlation (DIC) to elucidate the effects resulting from these high frequency pulses. An experimental test matrix was developed for applying pulses of varying amplitude and frequency to gain fundamental understanding of the characteristics of the high frequency pulses on crack growth behavior. Experimental results indicated there is a novel crack kinking behavior which results in retardation of the fatigue crack growth. Using DIC, we identified changes in the plastic zone ahead of the crack tip that mimic a hard inclusion, affecting not only the crack path, but also the crack opening displacements and subsequent crack growth rates. This work provides a foundation for understanding effects of high frequency pulses on crack growth to enable future development of a model incorporating the effects of the plastic zone on the growth mechanisms.

RESEARCH ON THE INFLUENCE OF GUSSET PLATE CONNECTIONS ON THE OUT-OF-PLANE STABILITY OF THE H-SHAPED STEEL BRACE

Experiments on 12 specimens of H-shaped steel bracing members with gusset plate connections (brace system) are conducted to study the effect of gusset plate on the out-of-plane rotational restraint and stability capacity of the brace. The experiments are under cyclic loading with constant-amplitude axial displacement, and the effect of clearance distance on the stability capacity of the brace is obtained. For the cyclic constant-amplitude loading condition, the degradation criterion of stability capacity of the brace system is obtained and the calculation formula of the post-buckling capacity is suggested. The finite element method is used to simulate the behaviors of specimens, and the simulated results are in good agreement with the experimental results. Moreover, a large-scale parameter analysis of the full-sized brace system is carried out. The results indicate that the brace slenderness ratio, the thickness of the gusset plate, the clearance distance and the brace angle have significant impacts on the out-of-plane rotational stiffness and stability capacity of the brace system. Based on the influence of these critical parameters on the rotational restraint of gusset plate and considering the size effect, the formulas for calculating the rotational stiffness of the gusset plate connection and the effective length factor of the brace are proposed, which can be used to easily obtain the stability capacity of the brace system.

Fatigue Crack Growth Analysis with Extended Finite Element for 3D Linear Elastic Material

This paper presents computational modeling of a crack growth path under mixed-mode loadings in linear elastic materials and investigates the influence of a hole on both fatigue crack propagation and fatigue life when subjected to constant amplitude loading conditions. Though the crack propagation is inevitable, the simulation specified the crack propagation path such that the critical structure domain was not exceeded. ANSYS Mechanical APDL 19.2 was introduced with the aid of a new feature in ANSYS: Smart Crack growth technology. It predicts the propagation direction and subsequent fatigue life for structural components using the extended finite element method (XFEM). The Paris law model was used to evaluate the mixed-mode fatigue life for both a modified four-point bending beam and a cracked plate with three holes under the linear elastic fracture mechanics (LEFM) assumption. Precise estimates of the stress intensity factors (SIFs), the trajectory of crack growth, and the fatigue life by an incremental crack propagation analysis were recorded. The findings of this analysis are confirmed in published works in terms of crack propagation trajectories under mixed-mode loading conditions.

Fatigue analysis of the friction stir welded AA6061-T6 aluminum alloy

Investigation of mechanical and fatigue behavior is necessary to find out the long-term durability of the welded joints. The current work focuses on the mechanical and fatigue behavior of the joint formed when AA6061-T6 alloy plates were friction stir welded (FSWed) then joint was also analyzed as compared with parent material. The tensile strength and microhardness profile were obtained by the uniaxial tensile test and Vickers hardness test respectively. The maximum yield strength and ultimate tensile strength of the FSWed specimens were found to be 207.2 N mm−2 and 264 N mm−2 which are 72.2% and 80.3% of the parent metal respectively. The minimum value of hardness in FSWed specimen was equivalent to 63% of the parent material. The fatigue tests were carried out in a constant amplitude loading condition at a positive load ratio (R = 0.1). The limit of maximum stress applied during the fatigue tests was determined based on the results of the tensile tests. Microstructural and scanning electron microscope (SEM) examination of welded specimens was also carried out to understand the effect of FSW on microstructural development, commence of fatigue crack, and the way the crack spreads. The outcomes show the formation of a fine-grained microstructure in the center of the weldment. Results of the fatigue tests show that the fatigue strength of both types of configurations i.e. FSWed and parent material were almost the same at the low-stress level. At high-stress level, parent material exhibits superior fatigue strength than FSWed specimens. An effort has also been made to relate the fatigue resistance to the microstructural observations of the welded specimens.

Influence of load spectrum on contact fatigue damage of a case carburized wind turbine gear

Contact fatigue failures are becoming increasingly prominent in heavy-duty gears due to the remarkable effects on service lives and reliabilities of associated machines such as wind turbines. The contact fatigue estimation of such gears is complicated due to the coupling effect of mechanical properties, lubrication and stochastic wind loadings. A numerical model was established to evaluate the contact fatigue damage evolution of a megawatt level wind turbine carburized gear pair based on the measured wind load spectrum. The elastohydrodynamic lubrication (EHL) theory was employed to compute the gear contact pressure and subsequently, the stress responses were obtained efficiently through the discrete convolute, fast Fourier transformation (DC-FFT) algorithm. The hardness and the residual stress gradients were captured via the Vickers hardness test and X-ray diffraction method, respectively. The Dang Van multiaxial fatigue criterion and the Basquin equation were adopted to estimate the fatigue lives under stochastic loadings, based on which the contact pressure-life curve was further profiled. Finally, the gear damage accumulation under this load spectrum was evaluated via the linear Palmgren-Miner rule. Results indicate that both the subsurface and transition area failures should be considered in gear design. The fatigue life prediction should consider the load spectrum rather than solely rely on a constant amplitude loading condition.

Energy dissipation and damage evolution for dynamic fracture of marble subjected to freeze-thaw and multiple level compressive fatigue loading

For rock engineering in cold regions, rock is often subjected to coupled fatigue conditions of freeze-thaw (F-T) and stress disturbance. Rock fracture evolution and energy mechanism under room temperature and constant stress amplitude loading condition have been widely investigated. Yet the rock energy dissipation and damage evolution characteristics subjected to multiple level cyclic loading conditions are not well understood. In this work, multiple level cyclic compressive loading experiments were conducted using GCTS RTR 2000 rock mechanics system on marble with F-T treatment of 0, 20, 40 and 60 cycles. The fracture evolution and energy dissipation mechanism were analyzed as well as the damage evolution characteristics. The results indicate that F-T treatment strongly influences the fatigue mechanical behaviors of marble, with both fatigue strength and strain energy decreases and irreversible volumetric deformation increases with increasing F-T cycles. The incremental rate of dissipated energy becomes faster as cyclic loading level grows. In addition, a F-T-fatigue loads coupling damage variable was proposed by using the input total strain energy and the dissipated strain energy to describe the rock damage evolution after F-T treatment and experiencing fatigue loading. Moreover, a damage evolution model was first established based on the obtained coupling damage variable to describe the two-phase damage accumulation characteristics. Damage accumulation curve presents a first steady increase and then faster increase trend, the damage evolution model can good fit the experimental data.

Numerical Analysis of Fatigue Crack Growth Path and Life Predictions for Linear Elastic Material.

The main objective of this work was to present a numerical modelling of crack growth path in linear elastic materials under mixed-mode loadings, as well as to study the effect of presence of a hole on fatigue crack propagation and fatigue life in a modified compact tension specimen under constant amplitude loading condition. The ANSYS Mechanical APDL 19.2 is implemented for accurate prediction of the crack propagation paths and the associated fatigue life under constant amplitude loading conditions using a new feature in ANSYS which is the smart crack growth technique. The Paris law model has been employed for the evaluation of the mixed-mode fatigue life for the modified compact tension specimen (MCTS) with different configuration of MCTS under the linear elastic fracture mechanics (LEFM) assumption. The approach involves accurate evaluation of stress intensity factors (SIFs), path of crack growth and a fatigue life evaluation through an incremental crack extension analysis. Fatigue crack growth results indicate that the fatigue crack has always been attracted to the hole, so either it can only curve its path and propagate towards the hole, or it can only float from the hole and grow further once the hole has been lost. In terms of trajectories of crack propagation under mixed-mode load conditions, the results of this study are validated with several crack propagation experiments published in literature showing the similar observations. Accurate results of the predicted fatigue life were achieved compared to the two-dimensional data performed by other researchers.

Application of the S-N Curve Mean Stress Correction Model in Terms of Fatigue Life Estimation for Random Torsional Loading for Selected Aluminum Alloys.

The paper presents the experimental fatigue test results for cyclic constant amplitude loading conditions for the case of the torsion of the PA4 (AW-6082-T6), PA6 (AW-2017A-T4) and PA7 (AW-2024-T3) aluminum alloy for a drilled diabolo type test specimen. The tests have been performed for the stress asymmetry ratios R = −1, R = −0.7, R = −0.5 and R = −0.3. The experimental results have been used in the process of a fatigue life estimation performed for a random generated narrowband stress signal with a zero and a non-zero global mean stress value. The calculations have been performed within the time domain with the use of the rainflow cycle counting method and the Palmgren−Miner damage hypothesis. The mean stress compensation has been performed with the S-N curve mean stress model proposed by Niesłony and Böhm. The model has been modified in terms of torsional loading conditions. In order to obtain an appropriate R = 0 ratio S-N curve fatigue strength amplitude, the Smith−Watson−Topper model was used and compared with literature fatigue strength amplitudes. The presented solution extends the use of the correction model in terms of the torsional loading condition in order to obtain new S-N curves for other R values on the basis of the R = −1 results. The work includes the computational results for new fatigue curves with and without the mean stress effect correction. The results of the computations show that the mean stress effect plays a major role in the fatigue life assessment of the tested aluminum alloys and that the method can be used to assess the fatigue life under random conditions.

A novel approach to model fretting-fatigue in multiaxial and non-proportional loading conditions

This article proposes a new method for determining the behavior and service life of contact areas of parts subjected to fretting-fatigue. The first objective was to be able to predict the service life of contact parts with various curvature radii from fretting-fatigue tests on laboratory test geometries such as cylinder-plane. The second objective was to be able to predict the service life for variable amplitude and multiaxial loading conditions from laboratory tests in constant amplitude loading conditions.

Mixed mode (I + II, I + III) fatigue crack growth rate description in P355NL1 and 18G2A steel using new energy parameter based on J-integral approach

In this paper, the problem of fatigue crack growth description under mixed mode loading condition is discussed. A new ΔS* crack driving force is proposed for mixed-mode loading conditions. For experimental verification, two types of low-carbon steels were used (P355NL1 and 18G2A /EU S355/). Mixed mode (I + II) fatigue crack growth rate experiment performed for R = 0.05 and R = 0.5 confirmed the usefulness of ΔS* parameter as a crack driving force. The similar effect was observed in the case of 18G2A (S355) steel and mixed mode (I + III) experiment for R = 0, −0.5, −1. On the contrary to the force factor ΔK or energy parameter ΔJ, the ΔS* approach allows to describe the kinetics of fatigue crack growth independently from stress ratio R. Based on experimental data it is worth to note that the usefulness of mentioned parameter can be reflected in further mathematical models of fatigue crack growth for both constant and variable amplitude loading conditions.

Energy-based method of fatigue damage cumulation

The study presents the results of low-cycle fatigue tests on AW-2024 aluminium alloy specimens under constant amplitude and multistage loading conditions. As expected, significant hardening of the material was observed in both cases. The tests confirmed that cyclic properties of AW-2024 alloy under multistage loading conditions can be represented based on the results of tests under constant amplitude conditions for the same alloy. The basis for a new fatigue damage cumulation method was formulated based on a comparative analysis of the test results. The main feature of the proposed method consists in including into the analysis the cyclic hardening of AW-2024 alloy during fatigue life calculation. An experimental verification of the method proved its practical applicability. A comparative analysis of the results for a standard Palmgren-Miner hypothesis and the new approach demonstrated the advantages of using the new method.

Effect of quality control parameter variations on the fatigue performance of aluminum friction stir welded joints

This paper describes fatigue tests performed on 6061-T651 and 5083-H321 aluminum friction stir welded joints with dimensions and loading conditions typical for structural applications. Butt and lap joint details with various defects intentionally introduced were tested under tension-only constant and variable amplitude loading conditions. In this paper, the fatigue test results are presented along with supporting metallurgical and nonlinear fracture mechanics analyses. Based on this work, it is concluded that kissing bond defects on the order of 0.3–1.0 mm can result in a significant fatigue life reduction and a shift in the failure mode to the weld root. The investigated toe-flash defect had less of an effect on fatigue performance. The lap joint did not perform as well as the butt joint detail.

Assessment and propagation of mechanical property uncertainties in fatigue life prediction of composite laminates

A probabilistic model for estimating the fatigue life of laminated composite materials considering the uncertainty in their mechanical properties is developed. The uncertainty in the material properties is determined from fatigue coupon tests. Based on this uncertainty, probabilistic constant life diagrams are developed which can efficiently estimate probabilistic ɛ-N curves at any load level and stress ratio. The probabilistic ɛ-N curve information is used in a reliability analysis for fatigue limit state proposed for estimating the probability of failure of composite laminates under variable amplitude loading cycles. Fatigue life predictions of unidirectional and multi-directional glass/epoxy laminates are carried out to validate the proposed model against experimental data. The probabilistic fatigue behavior of laminates is analyzed under constant amplitude loading conditions as well as under both repeated block tests and spectral fatigue using the WISPER, WISPERX, and NEW WISPER load sequences for wind turbine blades.

Quantification of fatigue damage in carbon fiber composite laminates through image processing

The quantification of damage in carbon fiber reinforced plastic composite laminates subjected with post-impact fatigue loading conditions using digital image processing technique is presented in this paper. The damage progression in the post-impact, post-cycling under constant amplitude and variable amplitude fatigue loading conditions are quantified through image analysis of the X-ray computed tomography (CT) scan images. The estimated damage is compared with that of bare-impacted specimens to understand the effect of fatigue loading on damage progression. The results are correlated with the stiffness degradation that was estimated from fatigue tests; a good correlation is found between damage volume and stiffness degradation.

Multiaxial variable amplitude fatigue life analysis using the critical plane approach, Part I: Un-notched specimen experiments and life estimations

Despite its importance in many engineering applications, fatigue life assessment is still not well understood under multiaxial variable amplitude loading conditions. Although critical plane-based analysis approaches have been shown to work well for constant amplitude multiaxial loading, results for variable amplitude life estimations have been mixed. This is likely due to the consideration of additional factors in a variable amplitude analysis, which can include load history effects, varying degrees of loading non-proportionality, changes in mean stress, cycle counting, and fatigue damage accumulation. In order to better understand the impact that each of these factors can have on life estimation accuracy, this study compares experimental results for both un-notched and notched specimens of 2024-T3 aluminum alloy to variable amplitude life estimations computed using a critical plane-based analysis approach. A modified version of the Fatemi-Socie parameter was used to calculate fatigue damage, while different analysis procedures were implemented to study the effect of transient material deformation behavior and crack initiation definition on life estimation accuracy. Part I of this two part paper presents the experimental results and analyses for un-notched specimen tests, while part II applies the same life assessment techniques to situations involving notched specimens. While the best analysis procedures investigated were able to estimate most un-notched specimen fatigue lives within a factor of 3 of experimental results, life estimations had a general tendency to be non-conservative and were not as accurate as those obtained under constant amplitude loading conditions.