Structural Fatigue Research Articles

Structural analysis and fatigue prediction of harrow tines used in Canadian prairies

Structural analysis and fatigue prediction of harrow tines used in Canadian prairies

An Optimization Procedure to Convert Time‐Domain Complex Loadings to the Frequency Domain for Low‐Cost Fatigue Analysis

ABSTRACTThe time domain is the most widely chosen alternative in mechanical fatigue tests to completely represent random events. However, the main disadvantages of this approach are the long time required to complete the tests and the high cost of the complex equipment. On the other hand, fatigue test in the frequency domain is a potential alternative to overcome these disadvantages. That is, it requires less sophisticated equipment, because it uses only simple actuators instead of servo actuators. Moreover, the duration of the fatigue test is reduced, because the total length of the event is replaced by constant repetitions and constant damage. The objective of this work is to propose and develop a methodology that uses the results of structural fatigue as a starting point, and through an optimization problem solved by the genetic algorithm, it determines which loads/displacements caused them in the frequency domain. The purpose is to assist future developments in fatigue tests, supplanting tests in the time domain without losing the correlation in results.

Rapid dynamic load estimation procedure for lifting propellers in forward flight

AbstractLifting propellers operate at oblique inflow and thus encounter severe dynamic loads during forward flight, impacting structural integrity, fatigue, and vibration. Numerical optimization approaches consider aerodynamic, structural mechanical, and aeroacoustic aspects within preliminary design. To also account for dynamic loads during forward flight, a novel procedure allows their rapid estimation. Based on steady-state simulations combining strip theory and finite beam elements, aerodynamic excitation, damping, and stiffness are defined in the frequency domain. Loads are derived through a linear inflow model and quasi-steady aerodynamics. Damping and stiffness loads are linearized and transferred into matrix form to calculate the frequency response. The computationally expensive need for simulations in the time domain is thus avoided. Applicability extends to both fixed and variable pitch lifting propellers utilized in large multicopters for cargo or passenger transportation. Comparisons to time-marching simulations show good agreement with deviations of approximately 10 %. The analytical derivation yields physical insights to understand and reduce dynamic loads and their magnification due to resonance.

Interface compatibility and hysteresis in shape memory materials are affected by lattice distortions from applied stresses

Significant effort has been put into designing shape-memory materials that can survive many cycles without functional or structural fatigue. A component of the design process is the condition defining perfect interface compatibility between the austenite and martensite lattices (λ2=1). In this paper, we evaluate the traditional mathematical theories of martensite under applied stresses, which distort the lattice compatibility through elastic strains. In NiTi we find that elastic distortions resulting from applied stresses influence the interface compatibility to a degree of impacting the material's functional abilities. Combining our results with empirical relationships connecting interface compatibility to transformation hysteresis we show that the model matches reasonably to a number of experimental results in the literature in which hysteresis changes under applied loads. We also apply these theories to a shape-memory ceramic (zirconia), which suggests a large orientation-dependence and asymmetric behavior in tension and compression. In both systems, we find that variant selection plays a large role in whether interface compatibility will improve or worsen under stress.

Research on the influence of out-of-plane loads on the fatigue life of metal structures

Abstract For civil aircraft structures, in-plane tensile, compressive, shear, and their combined forms of loads are the usual loads on plate and shell structures, while bending moments, torsion, and other forms of loads are mainly borne by assemblies consisting of multiple parts. However, some plate and shell structures are subjected to in-plane loads while also being affected by out-of-plane loads. In order to study the difference between the fatigue performance of plate and shell structures under the influence of out-of-plane load and the fatigue performance under the effect of pure in-plane load, the axial tensile fatigue test considering the out-of-plane load is carried out, which demonstrates the negative effect of the out-of-plane load on the axial tensile fatigue strength of the structure. At the same time, a new structural fatigue life prediction method considering out-of-plane loads is established by combining finite element analysis, which can more accurately predict the structural fatigue life under the influence of out-of-plane loads and provide support for metal structure fatigue analysis.

Structural dynamic characteristics of vertical axis wind and tidal current turbines

Vertical axis turbines have received great attention in both offshore wind and tidal current energy communities considering their advantages of economic design and unidirectional operation. However, their commercialization process is rather slow compared with the development of horizontal axis turbines due to technical challenges resulting from higher loads from unsteady aero/hydrodynamic forces, centrifugal forces and gravity of the structures. These are mainly because, while the inherent unsteady tangential pulls and fatigue loads intensify the structural vibration, aggravating structural safety and fatigue life, the relationship between the geometric parameters and the structural responses of blades remains unclear. Therefore, in order to clarify this issue, in this study, we focus on the modal and transient analysis of vertical axis turbines using multi-body dynamics, geometrically exact beam theory and detached eddy simulation. We find that the natural frequency of the rotor is directly related to the ratio of blade length and arm length. The effect of arms cannot be ignored in the structural analysis of the turbine. Moreover, an increase of blade length intensifies the deformation and vibration of the turbine’s structure, making the turbine more prone to fatigue failure. This article is expected to extend the existing knowledge of wind and tidal current turbines and provide a reference for choosing proper design parameters and developing suitable-capacity vertical axis turbines.

Nonstationary Characteristics of Structural Response and Fatigue Life Calculation Under Base Stationary Excitation

ABSTRACTWhen a structure is randomly excited by its installation base, significant vibration stress may be caused within it, thus causing damage to the structure. The authors find in the vibration fatigue test that when the notched slender beam specimen is excited by stationary Gaussian random base movement, the measured acceleration response and strain response of the specimen have obvious nonstationary characteristics. The vibration responses of the specimen under Gaussian and non‐Gaussian random base excitations are calculated by the finite element method and corresponding vibration fatigue tests were completed. The nonstationary characteristics of the responses from calculations and experiments are analyzed. Finally, a method for calculating the fatigue life of the specimen under nonstationary response is given, and the calculated results are compared with the measured data. The results indicate that the method proposed in this article performed excellent calculation fatigue life with measured strain data.

Vortex-Induced Vibration Performance Analysis of Long-Span Sea-Crossing Bridges Using Unsupervised Clustering

Vortex-induced vibration is a type of wind-induced vibration occurring frequently in large-span sea-crossing bridges under relatively low wind speeds, posing a threat to the structural fatigue performance and driving comfort. Identifying the instantaneous occurrence moments of vortex-induced vibration is a prerequisite for establishing a data-driven prediction model for vortex-induced vibration, and it is of great significance for the monitoring and early warning of vortex-induced vibration performance in bridges. To automatically detect the occurrence moments of vortex-induced vibration and establish a correlation model between vortex-induced vibration amplitude and environmental factors, this study proposes a fuzzy C-means clustering-based classification method. In order to detect the occurrence moments of vortex-induced vibration more finely, only short-term or even instantaneous structural vibration indicators were selected and transformed for distribution as clustering features. The entire detection process could be carried out unsupervised, reducing the manual cost of obtaining vortex-induced vibration information from massive monitoring data. Finally, actual vortex-induced vibration test data from a certain overseas bridge was utilized to verify the feasibility of this method. Based on the classification results, the correlation between vortex-induced vibration amplitude and environmental variables was determined, providing valuable guidance for predicting vortex-induced vibration amplitudes.

Research on the structural performance and fatigue life analysis of commercial vehicle stabilizer bars

Research on the structural performance and fatigue life analysis of commercial vehicle stabilizer bars

Effect of Internal Technological Defects and Loading Waveform on Structural Composite Fatigue Life

This paper explores the effect of internal technological defects on the fatigue life of carbon fiber reinforced plastic (CFRP) under simple loading waveforms. One conducted experiments on CFRP specimens with embedded artificial defects, including delamination’s (circular dry-spot) and wrinkling. Following quasi-static tests, one developed a fatigue testing program using triangular and sine waveforms. The findings indicate that these simple waveforms do not significantly affect the fatigue resistance of defect-free CFRP or CFRP with internal technological defects. The presence of the wrinkling defect significantly diminishes the fatigue resistance of CFRP. However, constructing fatigue life curves in relative terms reveals that the reduction in fatigue properties is directly related to a decrease in strength properties. In the case of delamination (dry-spot) defects, a reduction in fatigue life by approximately 2.5 times was observed across the tested range.

Noise reducing elements filled with laminated safety glass (LSG) for use along railway lines

The main purpose of noise barriers is to shield the environment from unwanted sound. Along railway lines, noise barriers must also meet strict requirements for structural integrity, usability and fatigue safety. Furthermore, noise barriers should satisfy aesthetic criteria. Transparent noise barriers have less impact on the environment and are more widely accepted by the public. In Germany, PMMA (acrylic glass) is currently the only transparent material approved for railway noise reducing elements. Laminated safety glass (LSG) could be a viable alternative due to its high rigidity and transparency, comparatively low dirt accumulation and high chemical and physical resistance. However, it still needs to be demonstrated that LSG meets the strict requirements for use along railway lines. LSG is a type of laminated glass that has a splitter-binding intermediate layer of plastics. The multi-layered structure presents challenges regarding the proof of usability. MFPA Leipzig GmbH was commissioned by the German Centre for Rail Traffic Research at the Federal Railway Authority to develop and evaluate possible methods to prove the usability of LSG for noise barriers along railway lines. The paper presents the structure of the project, the main findings until now and the open challenges to be addressed.

Therapeutic radiation directly alters bone fatigue strength and microdamage accumulation

Radiotherapy (RTx) is an essential and efficacious oncologic treatment, however, post-RTx bone fragility fractures present a challenging clinical problem. Cancer survivors treated with RTx are at variable risk for these late-onset, complex fragility fractures. Little data exists regarding the effects of RTx on bone fatigue properties despite the likelihood of fatigue loading as a mechanism leading up to atraumatic fracture. In this study, femurs collected from adult male rats were irradiated ex vivo with a therapeutic dose of x-irradiation (20 Gy), and then fatigued using a three-point bend setup. Femurs positioned in an isotonic bath at room temperature were loaded to a range of prescribed initial strain levels (based on beam theory equations, prior to any fatigue damage) at 3 Hz in force control. The goals of this study were to determine the feasibility of assessing RTx-induced alterations in 1) femur fatigue strength, 2) structural microdamage (creep and stiffness), and 3) tissue damage (diffuse damage and/or linear microcracking). Mid-diaphyseal morphology and tissue mineral density were not different between the RTx and Sham groups (p ≥ 0.35). With increasing applied apparent strain, the number of cycles to failure was reduced for the RTx femurs when compared to the Sham femurs (treatment x εapp, p = 0.041). RTx femurs had a greater phase II (steady state) creep rate (p = 0.0462) compared to Sham femurs. For femurs that reached 500k cycles, the RTx group had greater diffuse damage area (p = 0.015) than the Sham. This study provides evidence that radiation at therapeutic doses can directly diminish bone fatigue properties. This loss of fatigue properties is associated with increased structural fatigue damage and diffuse microdamage, without alterations in morphology or tissue mineral density, indicating a reduction in bone quality.

Large Eddy simulation on the Lead-Bismuth Eutectic jet at reactor core outlet

The Lead-Bismuth Eutectic (LBE) Cooled Fast Reactor (LFR) represents a promising Generation-IV technology, anticipated to offer a sustainable, safe, and clean energy solution. LBE fluid from LFR assemblies varies in temperature, causing violent temperature oscillations during the jet mixing that may contribute to structural fatigue, threatening LFR safety. This study focuses on investigating the critical thermal–hydraulic phenomena occurring at the core outlet of the LFR by modeling the assembly head in a realistic reactor configuration. Large Eddy Simulation (LES) is employed to explore the LBE jet characteristics in a three-head model with varying inlet parameters. The analysis includes the fluctuation patterns of both the flow and temperature fields downstream of the assembly head. The results show that the influence area of the assembly head is mainly in the range of about 0–175 mm downstream of the outlet of the assembly head. The temperature fluctuation frequency is the highest near the assembly head, typically within 20 Hz, and the temperature fluctuation frequency in other areas is lower. These findings provide valuable insights for designing LFRs and understanding the flow and heat transfer behavior of liquid metals.

Comprehensive Assessment of Deep-Water Vessel Implosion Mechanisms: OceanGate's Titan Submersible Failure Sequence Explained

This study outlines the physical mechanisms involved in a submersible implosion and analyzes the loss of the Titan submersible (‘sub’) that occurred on 18 June 2023 during a mission to visit the wreck of the Titanic. Titan’s collapse mechanisms at the moment of implosion are described in detail and outer hull fracturing rate, subsequent implosion rate, accompanying heat release and other key processes are quantified. Plausible causes of the hull’s leak leading up to critical loss of the sub's hermetic closure are reviewed using test results made publicly available by the U.S. Marine Board of Investigation. Their data indicated that the bond interfaces between the individual layers of the carbon fiber hull (Hull V2) were critically compromised due to manufacturing defects, voids, porosity, and inadequate adhesive integrity, resulting in significant delamination. Analysis of data from the real-time hull health monitoring system, revealed acoustic anomalies and strain shifts, pointing toward increasing structural fatigue, which went unaddressed prior to the fatal dive. The implosion process can be characterized by an instantaneous collapse of the air volume within the hull under extreme external pressure: even the tiniest leak would lead to destruction of the vessel's structural integrity. The destruction was the more devastating, because it was accompanied by a secondary explosion due to the heat exchange between the collapsing air volume and the ambient sea water. While the collapsing air was phase-changed into a supercritical state, the generated heat caused the adjacent seawater to evaporate and expand. Hull pieces fragmented by the initial implosion were strewn around during the secondary explosion phase, which ceased rapidly as the steam condensed back into seawater once again. The Titan incident underscores the urgent need for improved design standards, rigorous quality control in manufacturing, and enhanced real-time monitoring to prevent similar failures of future deep-sea exploration vehicles.

An aircraft structural risk assessment method considering fatigue crack propagation based on fatigue damage diagnosis and prognosis

An aircraft structural risk assessment method based on fatigue damage diagnosis and prognosis has been developed, considering fatigue crack propagation. The process is divided into three stages: initial crack diagnosis, crack diagnosis, and prediction, utilizing Monte Carlo simulation. Using 2024 aluminum alloy specimens with central holes, the study indicates that in the initial crack diagnosis stage, an inspection standard with a Single Flight Probability of Failure (SFPOF) less than 10-7 and a threshold method enhances structural fatigue crack diagnosis. In the crack diagnosis and prediction stages, iterative updates using Gaussian Process Regression (GPR) within a Dynamic Bayesian Network (DBN) improve crack propagation prediction and risk assessment accuracy. The diagnostic interval significantly impacts SFPOF, with an optimized interval balancing accuracy and computation time. Simplified and precise K value calculation methods enhance efficiency and accuracy. The method reduces costs and improves risk assessment accuracy, providing new insights for SPHM-based aircraft structural risk assessment.

Bandgap Control of Local Resonance Sandwich Meta-Plate with Cantilevered Archimedean Spiral Beam-Mass Resonators

In order to attenuate the various low-frequency vibrations that may cause structural fatigue and damage, impact human health and affect work efficiency, this paper presents a novel local resonance metamaterial sandwich meta-plate structure containing a cantilever spiral beam-mass resonator, and investigates vibration reduction effects of it within the obtained low-frequency bandgap range. For this purpose, a theoretical model of the low-frequency resonator being composed by a cantilever Archimedean spiral beam and cylindrical mass is established. The natural frequency of it is calculated by solving the Euler-Lagrange equation of the resonator. The accuracy and reliability of the theoretical model are verified through COMSOL simulation. The stress distribution of five types of cantilever spiral beam mass resonators (i.e. Archimedes spiral, triangle spiral, square spiral, pentagon spiral and hexagon spiral) and different cross-sectional shapes (rectangular, square, circular, diamond and triangular) are analyzed. Then, a unit cell model of a sandwich meta-plate with a cantilever Archimedean spiral beam resonator is selected. Hamilton’s principle is used to deduce the bandgap range under infinite periods, and the theoretical solutions are also verified. The parameter optimization of the substrate plate and resonator are also studied. Moreover, the effects of the various structural variables on the bandgap range are systematically explored. Finally, the dynamic responses of the sandwich meta-plate composed of [Formula: see text]-unit cells under different excitation frequencies, boundary conditions and structural damping are analyzed.

Wind turbine foundation ring fatigue reliability analysis under wind-load using SCADA system

Under the action of wind load, the foundation ring of the fan will generate stress concentration and alternating stress, leading to fatigue failure. Based on the average wind speed obtained from the supervisory control and data acquisition (SCADA) system, the orthogonal expansion method of random pulsating wind field and the number theory point selection method are used to calculate and simulate the corresponding pulsating wind speed time series, and then calculate the wind speed time series and wind load time series. Based on the ABAQUS finite element software, a model of a 5 MW wind turbine is established. The modal analysis is used to obtain the modal vibration pattern and the intrinsic frequency to verify the reasonableness of the structural modeling. Subsequently, the wind load time series is applied to the wind turbine model to obtain the stress time series using instantaneous modal dynamic analysis (Modal dynamics). The stress time series at the location of maximum stress concentration in the foundation ring is extracted, and the stress time series with the same stress amplitude is obtained through conversion, which has Wiener characteristics. After obtaining the stress amplitude using the rainflow counting method, the fatigue reliability is calculated based on the structural fatigue reliability analysis method considering the threshold crossing duration of the random process. This research holds reference significance for the calculation of fatigue reliability under approximate working conditions.

Variations of VHCF Characteristics by Microstructure of a Spring Steel to UNSM Treatment

<i>The bainitic structure resulting from the austempering of spring steel exhibits high strength and ductility. On the other hand, there appears to be no study on the effects of very high cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of this bainite structure. Therefore, this study compared the fatigue properties of VHCF using spring steel with bainitic and martensitic structures and bearing steel data. This study analyzed the characteristics of microstructure transformation associated with the heat treatment cycles and studied and evaluated the fatigue strength characteristics because of the UNSM in terms of fracture mechanics method and fracture surface analysis through electron backscatter diffraction, scanning electron microscopy fracture analysis, and energy-dispersive spectroscopy analysis. The fatigue limit of UNSM-treated spring steel was improved significantly by approximately 33% to 50% compared to the fatigue test results of the untreated material in the VHCF. In the long life range of bainized spring steel, fish-eye cracks appear in the form of the fine granular area, and fatigue cracks occur in the form of fish-eye cracks that occur in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.</i>

Structural fatigue crack propagation simulation and life prediction based on improved XFEM-VCCT

To simulate structural crack propagation and predict fatigue life, the extended finite element method (XFEM) combined with the virtual crack closure technique (VCCT) is adopted in this paper. Firstly, the underlying principles of the XFEM-VCCT framework are elaborated comprehensively, mainly including the calculation of crack tip energy release rate based on VCCT, the simulation of element cracking utilizing the phantom nodes, and the computation of structural responses under cyclic loading through the direct cyclic analysis. In addition, to calculate the crack propagation length, an interpolation method to obtain the crack tip coordinates is developed based on tracking and locating the crack by the level set functions. Meanwhile, to compensate the defect that the fatigue life is often overestimated when dealing with the complex mode crack in complex structure through XFEM-VCCT, a simple improved algorithm based on the average rate concept is proposed without altering the XFEM-VCCT framework. Based on specific examples, the necessity and accuracy of the improved algorithm are fully verified by comparing with the original method, and the fatigue life predicted by the improved algorithm is more consistent with reality. Finally, this method is successfully applied to the simulation and analyses for a typical ship stiffened plate structure, demonstrating good engineering applicability.

Effect of combined thermomechanical treatment on structure, mechanical properties, electrochemical behavior and functional corrosion fatigue of biodegradable Fe-30Mn-5Si alloy

In present study, the combined thermomechanical treatment (TMT), including the radial-shear rolling (RSR) at 900 °C followed by longitudinal rolling (RSR+LR) at 700 °C, was applied to biodegradable Fe-30Mn-5Si (wt%) alloy. The effect of these TMTs on the microstructure, phase composition, mechanical properties (ultimate tensile strength UTS, apparent yield stress σ0.2, relative elongation to failure δ, Young's modulus E), electrochemical behavior and functional corrosion-fatigue behavior in Hanks’ solution were studied. It was found that RSR leads to the formation of a statically recrystallized γ-austenite structure with an average grain size of 10±3 and 20±3 µm in the peripheral and central zones, respectively. The RSR+LR results in a mixture of dynamically recrystallized and dynamically polygonized structures with the average grain size of 5±1 µm. Moreover, RSR and RSR+LR lead to the formation of a single-phase FCC γ-austenite state compared to reference heat treatment (RHT), where microstructure is characterized by a two-phase state of FCC γ-austenite and HCP ε-martensite with an average grain size of 200–300 µm. Based on static mechanical tensile tests to failure, it was established that RSR and RSR+LR lead to a significant increase of UTS up to 770±21 and 935±55 MPa, σ0.2 up to 300±38 and 490±43 MPa, and δ up to 28±2 and 19±10 %, while maintaining acceptable of E value about 165±29 and 150±8 GPa, respectively as compared to 460±41 MPa, 210±91 MPa, 11±3 %, and 140±14 GPa after RHT. The electrochemical studies showed that single-phase structure (FCC γ-austenite) formed after RSR and RSR+LR leads to the non-critical biodegradation rate lowering as compared to the two-phase state (γ-austenite and ε-martensite) after RHT. Besides RSR and RSR+LR lead to significant improvement of functional corrosion-fatigue life in Hanks’ solution.