Fatigue Life Of Structures Research Articles

A phase field model for fatigue fracture in piezoelectric solids: A residual controlled staggered scheme

Prediction of the fatigue life of piezoelectric devices and structures is of scientific and engineering significance. In this work, a phase field model for fatigue fracture in piezoelectric materials is developed by means of the Hamilton principle. Two classical phase field fracture models are unified in the present framework. A cumulative history variable is introduced to quantify the degradation of the fracture toughness under cyclic loading. The proposed model accounts for three representative electric boundary conditions on crack faces describing the fatigue fracture behaviors in different physical situations. The residual controlled staggered scheme is extended to address the fatigue fracture problems in the context of electro-elasticity. The present algorithm overcomes the convergence problem for the monolithic approach and shows higher accuracy with lower computational effort in comparison to the single-pass staggered scheme. Systematic numerical simulations are carried out in 2D and 3D cases. The effects of the electric boundary condition, external electric field, and the geometry of the crack tip on the fatigue fracture behaviors of piezoelectric solids have been discussed. The effect of the electric boundary conditions on the fatigue life varies depending on the direction of the external electric field. The present work is beneficial to assess the lifetimes of piezoelectric devices in practical applications.

Influence of pitting corrosion on the fatigue strength of offshore steel structures based on 3D surface scans

Support structures for offshore wind turbines and the corresponding transformer platforms are highly susceptible to corrosion. In particular, the phenomenon of pitting is crucial, as it leads to local stress concentrations and thus affects the fatigue life of structures. Despite corrosion protection systems, corrosion cannot be completely avoided, which can lead to pitting corrosion on the steel surface. This leads to fatigue life reduction, since the structures are exposed to high dynamic loads. Local stress concentrations can be considered in local concepts but so far, corrosion effects in local concepts are insufficiently defined. Hence, this paper aims to investigate the impact of pitting corrosion and the corresponding stress concentration on the fatigue life endurance of structural steel, used for offshore wind support structures. For this purpose, a total of 36 pre-corroded specimens with pitting were tested against fatigue failure and monitored with Digital Image Correlation. In addition, the specimens were scanned with a high-resolution 3D scanner and converted to numerical models by reverse engineering, to determine the stress concentrations on the surface. In most cases the hotspots from the numerical model coincide with the crack location detected with Digital Image Correlation. The notch effect has a significant impact on the crack location and crack path.

Effect of Seawater on the Mechanical Properties of PET Foam Sandwich Structure

Abstract The polyethylene terephthalate (PET) foam sandwich structure is widely used for ships and for studying the influence of the seawater environment on its mechanical properties as an important way to evaluate its safety performance. To study the effect of the seawater environment on the mechanical properties of the PET foam sandwich structure, this paper mainly studies the static properties, interlayer cracking properties, and fatigue failure properties of the PET foam sandwich structure under the simulated seawater environment at a constant temperature of 40°C. The compressive properties test, tensile properties test, three-point bending properties test, double cantilever beam test, and four-point test were carried out on PET foam specimens, glass fiber reinforced plastic (GFRP) panel specimens, and PET foam sandwich structure specimens with different immersion times. The multiphysics software COMSOL was used to model and analyze the mode I interlayer cracking properties of the PET foam sandwich structure. The comparative analysis of the experimental results shows that the seawater environment has an effect on the compression properties of PET foam, the tensile and flexural properties of GFRP panels, and the interlayer cracking properties of the PET foam sandwich structure, and as the immersion time is extended, the effect is more obvious. Seawater immersion can also lead to a reduction in the fatigue life and ultimate fatigue of the PET foam sandwich structure.

Prediction of flexural fatigue life and failure probability of normal weight concrete

Fatigue life has to be considered in the design of many concrete structures at various stress levels and stress ratios. Many flexural fatigue test results of plain normal-weight concrete are available in the literature and almost every set of test results provides different fatigue equations. It is necessary, though, to have a common fatigue equation to predict the design fatigue life of concrete structures under flexural load with reasonable accuracy. Therefore, a database of flexural fatigue test results was created for concrete with strengths ranging from 25 to 65 MPa; this database was used to derive new fatigue equations (Wöhler fatigue equation and S-N power relationship) for predicting the flexural fatigue life of normal-weight concrete. The concept of equivalent fatigue life was introduced to obtain a fatigue equation using the same stress ratio. A probabilistic analysis was also carried out to develop flexural fatigue equations that incorporate failure probabilities.

Research on dynamic response and fatigue life of tunnel bottom structure under coupled action of train load and groundwater

In this paper, based on a heavy-duty railway tunnel, the vibration characteristics and fatigue life of tunnel basement structure under the coupled action of train load and groundwater were studied by field tests and numerical simulation. The results showed that the field measurement and numerical simulation results were in good agreement, and the vertical dynamic stresses of the grade-IV and -V surrounding rock were attenuated significantly along the tunnel base. The attenuation rate to the bottom of the inverted arch were 88.6% and 87.0%, respectively. Thus, the inverted arch and the filling layer were the main carriers of the train dynamic load. When water damage occurred in the basement, the vertical displacement and vertical acceleration of the basement structure decreased due to the pore water pressure, but the principal stress increased significantly. Under conditions where the surrounding rocks of grade-IV and -V have water compared with no water, the lifespan of the inverted arch decreases by 21.07% and 29.28%, respectively. The worse the quality of the surrounding rock was, the greater the dynamic response and the shorter the life of the basement structure became.

Optimal Inspection Period for Structures Subjected to Fatigue

In this paper, an optimal inspection model for three-dimensional structures under random loading is presented from the view of reliability. Neuber method and finite element analysis as well as experiential stress intensity factor (SIF) were applied in the analysis of fatigue life. Based on the series system theory, the reliability function for the structural fatigue life is given. Treat the crack initiation life (CIL) as service life and the crack propagation life (CPL) as inspection period, the optimal inspection period was obtained under a given reliability. The proposed method is proved to be feasible by a numerical example, which involves a bracket under random loading.

System dynamics in structural strength and vibration fatigue life assessment of the swing bar for high‐speed maglev train

AbstractHigh‐speed maglev trains are subjected to severe dynamic loads, thus posing a failure hazard. It is necessary to account for the vehicle dynamics to improve the structural strength and fatigue life assessment approach under harsh routes and super high‐speed grades. As the most critical load‐carrying part between the vehicle body and levitation frames, the swing bar was taken as an example to demonstrate the significance of vehicle dynamics to integrate classical structural strength and fatigue life with the service conditions. A multiphysics‐coupled dynamic model of an alpha improvement scheme for an electromagnetic suspension maglev train capable of 600 km/h was established to investigate the complex dynamic loads and fatigue spectra. Using this model, the structural strength and fatigue life of the wrought swing bars were investigated. Results show only a slight effect on the structural strength and fatigue life of swing bars by the super high‐speed grades. The nonaxial bending moments caused by the uncompensated relative displacement between the vehicle body and bolsters are identified as the decisive factors. The minimum safety factor of the structural strength for wrought swing bars is 1.33, while the minimum fatigue life is 34 years. Both match the design requirements but are not conservative enough. Therefore, further verification and optimization are recommended to improve the design of swing bars.

Numerical simulation of reliability of single-sided/double-sided package interconnect structure under temperature cyclic load

PurposeThe purpose of this paper is to compare the single-sided packaging structure and double-sided packaging structure of high-power module and study the overall heat dissipation performance and reliability of the module.Design/methodology/approachIn this paper, the single-sided packaging structure and double-sided packaging structure of power module are designed based on Wolfspeed products. This paper is analyzed by finite element method. First, the heat dissipation performance of single-sided packaging structure and double-sided packaging structure is analyzed; second, the deformation and stress of single-sided packaging structure and double-sided packaging structure are compared and analyzed; and finally, the cumulative plastic deformation of single-sided packaging and double-sided packaging structures are compared and analyzed, and the fatigue life of the structure is calculated based on the plastic deformation.FindingsIn the heat transfer simulation, under the same power input, the heat dissipation performance of single-sided packaging structure is not as good as that of double-sided packaging structure. Under the reliability simulation of the same temperature cycle standard, the maximum equivalent stress of single-sided packaging structure is lower than that of double-sided packaging structure, but the fatigue life prediction based on plastic strain shows that the fatigue life of double-sided packaging structure is not different from that of single-sided packaging structure.Originality/valueThis paper creatively simulates the thermal characteristics and reliability of single-sided packaging structure and double-sided packaging structure and proves the advantages of double-sided packaging structure compared with single-sided packaging structure from the aspects of heat transfer performance and reliability.

Mechanical properties of track-asphalt concrete-subgrade bed structure system under both temperature and train loading

Asphalt concrete has good viscoelasticity, impermeability and durability, which is very suitable for the requirements of high-speed railway substructure. The stress and deformation characteristics of track-asphalt concrete-subgrade system under periodic temperature and train loading were studied. The results indicate that the fatigue life of asphalt concrete structure is much longer than the actual number of train actions. The temperature effect is the main reason for forced tension of asphalt concrete layer at the base plate joints. Laying 1.2 m composite geomembrane on both sides of structural joint can reduce the tensile strain by 60% and effectively improve the stress state.

Creating an asset management plan for traffic signal structures through interactive explorations of wind induced fatigue damage

This paper presents an asset management framework for remote monitoring of traffic signal structures for fatigue damage under wind loads, using historical wind measurements from nearby weather stations. Estimate wind profiles developed from the wind data are used to calculate fatigue damage at welded connections for a wide variety of common traffic pole configurations. The analysis is embedded within a novel remote monitoring application that performs the fatigue life analysis in real time for a user defined location and traffic pole configuration. Such a framework is the first of its kind that allows operators to make real time asset management decisions though interactive explorations of the relationship between traffic pole configuration and fatigue life due to local wind conditions. Results are demonstrated using several traffic poles located around Logan, Utah, which has complex topography that creates drastic changes in wind conditions over short geographical distances. The results illustrate the ability of the remote monitoring application to detect small-scale differences in wind conditions that result in significant differences in the estimated fatigue life of traffic structures within communities. Such results highlight the need for, and feasibility of developing site-specific asset management frameworks at a much finer spatial resolution than previous attempts.

Analytical-Based application software for Estimating remaining fatigue life of Non-Cantilevered sign structures

This paper presents the development of an application framework software for accurately predicting the consumed fatigue life of overhead sign highway structures. The software was designed as a package tool to utilize a 45-year wind model and the finite element software Staad Pro platform in generating a multitude of simulations while adapting their results into a fatigue calculation engine. The applicability and validity of the developed environment in evaluating the fatigue life of structures was tested by blindly running the simulations for different structures. Given that, the remaining time span of a secondary column brace was predicted to be exhausted for a structure in Wichita, KS and the follow-up physical inspection confirmed that. Thus, it is believed that this present tool could be confidently used in predicting the initiation of fatigue cracks to alert the governmental agencies to take a proactive role in preventing the further collapse of these types of structures.

Experimental and Numerical Investigation on Fatigue Properties of Carbon Fiber Cross-Ply Laminates in Hygrothermal Environments.

The fatigue properties of composite materials are degraded seriously in hygrothermal environments, so taking into account their influence is very important when evaluating the fatigue life of composite structures. Tensile fatigue experiments of carbon fiber reinforced resin composite cross-ply laminates were conducted in room temperature/dry (RTD), cool temperature/dry (CTD) and elevated temperature/wet (ETW) conditions. The S-N curves and fatigue failure modes of the cross-ply laminates were obtained in three conditions. On this basis, a finite element model was established to discuss the influence of temperature and moisture content on the fatigue properties, as well as a method for determining environmental factors of fatigue life of cross-ply laminates was established. The results show that the saturation moisture absorption and temperature have a significant influence on the tensile fatigue properties of cross-ply laminates. The high-cycle fatigue property is weakened significantly by the saturation moisture absorption and high temperature, but the low-cycle fatigue properties were strengthened in cool temperature conditions. The delamination failure mode in ETW is the most severe, presenting with an obvious necking phenomenon. The influence of temperature has a greater effect than that of moisture content, but moisture absorption would play its affect obviously when temperature exceeds 40 °C.

Probabilistic modelling of pitting corrosion and its impact on stress concentrations in steel structures in the offshore wind energy

Supporting structures for offshore wind turbines and the appropriate transformer platforms are highly susceptible to corrosion. Especially the phenomenon of pitting corrosion is very crucial, since this leads to local stress concentrations and hence has an impact on the fatigue life of marine structures. Despite corrosion protection systems, corrosion cannot be completely avoided, which can lead to pitting corrosion on the steel surface. Stress concentrations from pitting corrosion are not explicitly considered within the design of offshore structures. This is only possible by using local fatigue concepts, which requires a realistic modelling of corroded steel surfaces and the corresponding stress concentrations. Hence, in this work, three different models for evaluating the stress concentrations were developed: single-pit model, double-pit model and random fields model. The results were compared to the stress concentrations of a real corroded surface, which was analysed with the reverse engineering method. In all models, the probability-based determination of the stress concentration was the main focus in order to consider the characteristic scatter in pitting corrosion. The comparison of the three models with the real surface shows a good agreement with the double-pit model. It becomes evident, that the single-pit and the random fields model are not sufficient for modelling the stress concentrations. However, derived stress concentrations from the double-pit model and the real surface model can be utilized for risk and reliability-based analysis with local fatigue strength concepts.

Increasing the fatigue life of offshore wind turbine jacket structures using yaw stiffness and damping

In this study, an appropriate proportion of yaw stiffness and damping is proposed to greatly improve the fatigue life at the tubular joints of offshore wind turbine (OWT) jacket support structures. The fatigue analyses using the DTU 10-MW OWT jacket support structure under 1148 IEC-61400-3 loads indicated that the torsion of rotor blades under turbulent wind conditions is the most important load leading to fatigue damage to tubular joints. Other loads, such as the moments generated from rotor blades and wave loads, are less important than the torsion. A Guyan reduction two-degrees-of-freedom model and eleven full finite element models indicated that low stiffness and large damping of the yaw system can significantly increase the fatigue life of tubular joints, and the yaw angular velocity will not increase. However, it is necessary to address the restrictions in the yaw rotation and to avoid Fn3P resonance (Integer multiples of 3P) with the first natural frequency of the structural vertical rotation.

A multi-axial and high-cycle fatigue life prediction model based on critical plane criterion

Accurate prediction of fatigue failure behavior and fatigue life of structures or materials under multi-axial loading are important for engineering application. In this work, a multi-axial and high-cycle fatigue failure model based on the critical plane criterion is proposed with considering the influences of material property and loading path on the initiation and growth of fatigue cracks. A material parameter, defined as the ratio of tensile yield strength σy and torsional yield strength τy, is introduced to describe the mechanical properties of the material. For σy/τy<3, it is considered that the crack growth under the multi-axial fatigue load is mainly controlled by the maximum normal stress. While for σy/τy>3, it is considered that the crack growth under the multi-axial fatigue load is mainly controlled by the maximum shear stress. Experimental results of three kinds of material were applied for model validation. For all materials, most predictions are within the ±2 times scatter band of fatigue life and only few data are around ±3 times scatter band. Good agreements of the predicted fatigue life by the model with experimental results illustrated the model applicability. The model provides meaningful tool for predicting multi-axial fatigue life of materials under different loading parameters.

Improving the fatigue life of printed structures using stochastic variations

Additive manufacturing allows designers to create geometries that would not be possible or economical to manufacture using traditional manufacturing processes. Production with these technologies does, however, introduce a large amount of variation and additional unknowns. These random variations from idealized geometry or material properties can harm the performance of the design. The current work presents an approach to improve the fatigue life of such structures, and simultaneously reduce its influence from random variations in local thickness. Following an initial numerical study, the results are experimentally validated. Experimental results show a significant improvement in fatigue life in the redesigned sample with a tailored thickness distribution.

The Crack Resolution Required to Ensure Fatigue Life of a Rescue Submarine’s Transfer Skirt

Transfer skirt (TS) is a structure with spherical asymmetric sectors, which is connected to a rescue submarine and used to transfer crew between two submarines in the depth of the sea. When this structure is connected to the disabled submarine in deep, it is under violent stresses that when repeated, cause fatigue damage at sensitive regions such as welded joints. In this article, based on the Paris crack growth equation and fracture mechanics criteria in BS 7910 standard, a code is developed, by which the fatigue life of the transfer skirt structure containing various initial crack sizes is evaluated. Stress analysis is performed in order to acquire the stress distribution and to determine the stress concentration spots at the welded joints. At last, regarding the relationship between crack length and prospected life under various conditions, the minimum resolution required for examination instruments to identify defects in the welds is resolved. It seems that a permissible size of an initial crack in the welds of a transfer skirt enduring 50000 fatigue cycles is the utmost 0.65mm.

An improved crack growth model of corrosion fatigue for steel in artificial seawater

Corrosion will greatly reduce the fatigue life of steel structures. Based on the Paris equation and continuum damage mechanics, an improved crack growth model is obtained by introducing corrosion damage into the Paris equation. The derivation and calculation procedure of the improved Paris equation are discussed in detail. The proposed model was used to calculate the corrosion fatigue crack growth length of S690 in 3.5 wt% NaCl solution. The model demonstrates good accuracy and predicts the corrosion fatigue life of T-welded joint specimens in artificial seawater.

Estimation of Overall Fatigue Life of Jack-up Leg Structure

Jack-up platforms are designed to work in three conditions: Transit, Preloading and Operating. The fatigue life of the jack-up platforms in operating condition will be determined to be similar to that of offshore fixed steel structures. In preloading conditions, fatigue damage is usually ignored. Up to now, the fatigue damage of the jack-up leg structure in transit conditions has been calculated at approximately 20% of the total fatigue damage of the jack-up leg structure in two conditions (transit and operating). The approximate calculation method is usually accepted by consultants and register agencies. If the approximation is used, the fatigue life of the jack-up leg structure will be calculated only as for the jacket structure of the fixed offshore structure, with 80% of the allowable fatigue life based on standards. The approximation proved to have many disadvantages: the different travel times of each jack-up rig cannot be mentioned; hot spots that need to be maintained during the transit condition have not been pointed out; it is difficult to guarantee the safety of the jack-up leg structures in the transit condition. In order to overcome the limitations of the approximation method, this paper will propose a method to predict the overall fatigue life of the jack-up leg structure in three main problems. Firstly, we use the analysis method of fatigue of fixed steel offshore structures for jack-up leg structures in operating conditions. Secondly, we suggest a method to analyze the fatigue of the structures in transit conditions. Herein, motion analysis and determination of inertia forces on the leg structure are performed by the Boundary Element Model (BEM) in SACS software. Then the inertia forces are assigned to a Finite Element Model (FEM) in SACS to decide the internal forces of the structures. Hotspot stresses are determined by combining nominal stress from FE analysis results with a concentrated stress factor from the analysis of joint local models in the ANSYS program. Then, fatigue damage and fatigue life of hotspots of the structure are determined in the transit condition. Finally, a formula is suggested to determine total fatigue damage in operating conditions and transit conditions with different cases in relation to different fraction factors. These results are used to predict fatigue life corresponding to the most dangerous cases of structural joints. These new suggestions are applied to fatigue analysis for jack-up Tam Dao 05. Currently, the Tam Dao 05 platform has been operating in the Vietnam East Sea. Doi: 10.28991/CEJ-2022-08-03-06 Full Text: PDF

Parametric study on SCF distribution along the weld toe of internally ring-stiffened two-planar tubular KK joints under axial loading

The internal ring stiffeners have been commonly introduced in tubular joints to improve the ultimate strength and fatigue life of offshore structures. However, for internally ring-stiffened multi-planar tubular joints which are frequently used in practical application, few investigations about stress concentration factor (SCF) in the joints have been reported so far and no parametric equation of the SCF is currently available for fatigue design. In this paper, the effects of geometrical parameters on the SCF distribution along the weld toe of the internally ring-stiffened two-planar tubular KK joints under axial loading are investigated by performing a set of three-dimensional (3D) FE stress analyses. Firstly, based on the requirements of AWS D1.1-2010, an approximate model of the actual weld profile is proposed. Then, several 3D FE models of various tubular joints are established to verify the proposed model. Subsequently, the effects of multi-planarity and stiffeners on the SCF in two-planar tubular KK joints are investigated. Additionally, the influence of geometrical parameters on both the SCF and the SCF distribution of the joints under axial loading is studied. Finally, SCF parametric equations for fatigue design of the internally ring-stiffened two-planar tubular KK joints under axial loading are developed from nonlinear regression analysis.