Fatigue Damage Process Research Articles

In-situ SEM and EBSD study on fretting fatigue crack initiation of a directionally solidified Ni-based superalloy

Fretting fatigue is an important cause of the failure of the turbine blade-disc dovetail joint in aero-engines. In this work in-situ scanning electron microscope (SEM) fretting fatigue experiments using dovetail shape specimens were conducted. The fretting fatigue damage process of Ni-based directionally solidified superalloys including slip trace evolution and crack initiation was captured. The misorientation and geometrically necessary dislocation (GND) density induced by fretting fatigue were quantified by electron back-scattered diffraction (EBSD). The significant correlation between crack path, slip trace, grain boundary microstructure and GND density was revealed. The EBSD analysis achieved good agreement with the in-situ SEM observation.

Fatigue damage evaluation of welded joints in steel bridge based on meso-damage mechanics

Welding is a rapid and flexible connection that facilitates the use of a broad range of steel bridge. However, fatigue cracks initiating from various welded connection details are common problems in steel bridges. Therefore, it is vital to accurately evaluate the fatigue damage evolution and fatigue life of welded joints in steel bridge. In this study, a fatigue damage evolution model based on the meso-damage mechanics was proposed to evaluate the fatigue damage of welded joints in steel bridge. The number density of micro-cracks was adopted as the damage variable in the fatigue evolution model. Finite element modelling and user material subroutine (UMAT) in ABAQUS were combined to simulate fatigue damage evolution of welded joints in steel bridge. The fatigue damage evolution model was embedded in UMAT while the UMAT was coupled with the finite element model of the welded joints under cyclic loading. Then, the proposed model was validated by fatigue tests. The fatigue damage evolution model was also used to investigate the fatigue damage evolution process, and fatigue life prediction of the typical welded joints specimens in orthotropic steel decks (OSDs) of steel bridge. The results indicate that the fatigue strengths of nominal stress method were estimated to be 65.7 MPa for typical welded joints. The fatigue cracks mainly initiating at the weld toe and propagating along the weld toe line. The simulation results indicate that the fatigue damage evolution model provides superior ability in evaluating fatigue damage process and fatigue life of welded joints in steel bridge.

Fretting Fatigue in Mechanical Joints: A Literature Review

Fretting fatigue is a fatigue damage process that occurs when two surfaces in contact with each other are subjected to relative micro-slip, causing a reduced fatigue life with respect to the plain fatigue case. Fretting has now been studied deeply for over 50 years, but still no univocal design approach has been universally accepted. This review presents a literature study that involves the three main types of mechanical joint affected by fretting fatigue (press-fitted shaft hub joints, dovetail joints, and bolted joints) aiming at giving insight into the progress made in terms of design guidelines for engineers.

Damage evolution and probabilistic strain-lifetime assessment of plain and fiber-reinforced concrete under compressive fatigue loading: Dual and integral phenomenological model

A phenomenological approach is proposed to describe and predict the random behaviour of the total strain in plain and fiber concrete under compressive fatigue loading. Twofold viewpoints are considered in the assessment of the fatigue damage process. The first one, acknowledged as the stochastic cumulative damage evolution in terms of the number of applied cycles is represented by a sample function, physically identified with the total specimen strain caused by matrix microcracking. The second one is ascribed to the intrinsic scatter of the total lifetime referred to as the prefixed ultimate fatigue strain limit state (not necessarily failure). The sample function, once normalized to the total fatigue lifetime, is identified as a cumulative distribution function (cdf) of the generalized extreme value (GEV) family, particularly as Weibull distribution. Besides the reliable probabilistic lifetime prediction for the material under compressive fatigue, the approach allows fatigue tests to be prematurely interrupted while the remaining evolution of the ε-N curve up to failure is fully restored from the fragment recorded. In this way, a remarkable time and cost reduction of the experimental program is achieved without detriment of data reliability.

Texture Characteristics of Polarized Thermal Images on Metal Surfaces in Fatigue Damage Process

Texture Characteristics of Polarized Thermal Images on Metal Surfaces in Fatigue Damage Process

Acoustic Emission Analysis during Fatigue Damage Process on Type 2 Accumulator Materials for Hydrogen Station

A research project is underway to apply the Acoustic Emission (AE) method to establish a non-destructive, non-opening, in-service inspection of high-pressure hydrogen Type 2 accumulators for hydrogen stations. The objective is to prevent hydrogen leakage due to fatigue crack initiation, propagation, and penetration in the steel liner of the accumulator caused by cyclic pressure loading. In this study, the relationship between fatigue crack initiation/growth and AE wave generation behavior is evaluated using a plate-type specimen that simulates the low-alloy steel liner of a pressure accumulator. As a result, AE generation is considered to capture the transition of a crack from Stage I to Stage II, and the usefulness of applying the AE method for in-service inspection of high-pressure hydrogen Type 2 accumulators is demonstrated.

Fatigue damage identification method of cement-based nanocomposites based on strain mode

In order to overcome the problems of low sensitivity, low accuracy and long time of traditional methods, a fatigue damage identification method for cement-based nanocomposites based on strain mode was designed in this paper. First, on the basis of calculating the stress concentration coefficient, the mode decomposition method is used to traverse the stress wave and obtain the new stress value. Then, the propagation process of material fatigue damage is separated by the number of load cycles, and the fatigue damage evolution equation is established. Finally, combined with the ultrasonic wave equation, the location and magnitude of fatigue damage are identified according to the position and amplitude of reflected wave. Experimental results show that the proposed method has the highest sensitivity of 0.944, the highest damage identification accuracy of 97.72%, and the minimum identification time is only 11s, indicating that the proposed method has high reliability and timeliness.

Experimental study on fatigue properties of Q420C steel welded joints at room temperature

To evaluate the fatigue life of Q420C steel welded joints, fatigue tests were performed on butt-welded joints, cross-butt-welded joints and cross-fillet-welded joints. The fatigue strength S-N curves of the joints were fitted. Then the formation and development of fatigue cracks were analyzed by fracture morphology. The fatigue properties of joints were compared with literature data and standard curves. The results showed that the fatigue test data of the three types of welded joints were roughly above the standard curves. Moreover, the fatigue strength of butt-welded joint was significantly higher than the calculated values of standards, indicating a large safety margin. The test value of cross-butt-welded joint was close to the calculated values of standards, so it is suggested to moderately reduce the standard values. In addition, all the three standards could well predict the fatigue life of cross-fillet-welded joint. The fracture morphology of the specimens showed the development process of fatigue damage, and the fatigue displacement curve and damage curve proved the formation of the fatigue fracture in specimens.

Multiresolution Feature Extraction of Surface Topography in Metal Fatigue Damage Process

Multiresolution Feature Extraction of Surface Topography in Metal Fatigue Damage Process

The Effects of Pits Size and Shape on Fatigue Crack Initiation

Generate and monitor the growth of corrosion pits on the surface of 316L stainless steel using the scanning droplet cell technique and assess the effects of varies factors such as pits size, pits shape, flow rate, and applied stress on pit initiation and growth. Configure of a test system to measure the pit and short fatigue crack growth behaviour, to investigate each stage of the corrosion fatigue damage process occurring at the material surface and compare this with the modified film surface using alternating voltage passivation process (AVPP). Understand the effects of corrosion on cracking behaviour during the different stages of fatigue crack growth.

Micromechanical Effects of Loading Frequency on Fatigue Fracture in Concrete

AbstractThis study investigated the effect of loading frequency on the fatigue damage process in concrete using digital imaging and acoustic emission techniques. It was found that the complex fatig...

Fatigue failure warning method for needled ceramic matrix composite by acoustic emission monitoring

Ceramic matrix composite is a kind of mechanical engineering material with excellent high temperature mechanical properties, which has been widely used in aircraft propulsion system and thermal protection system. Therefore, it is of great significance to study the fatigue failure of needled ceramic matrix composite. In this investigation, based on the realtime acoustic emission (AE) monitoring of needled C/SiC ceramic matrix composite, the characteristics of AE energy during the fatigue damage process were obtained. In addition, considering the emission coefficient of AE energy and the threshold value of AE energy in single cycle, a method for judging the imminent fatigue failure of needled composite was proposed. By comparing the cycle of failure warning by proposed method with the experimental fatigue life, the proposed method can provide fatigue failure warning near and before fatigue failure.

Fatigue-Induced Damage in High-Strength Concrete Microstructure.

A high-strength concrete subjected to compressive fatigue loading with two maximum stress levels was investigated and the behaviour was evaluated using the macroscopic damage indicators, strain and acoustic emission hits (AE-hits), combined with microstructural analyses utilising light microscopy and scanning electron microscopy (SEM). A clustering technique using Gaussian mixture modelling combined with a posterior probability of 0.80 was firstly applied to the AE-hits caused by compressive fatigue loading, leading to two clusters depending on the maximum stress level. Only a few cracks were visible in the microstructure using light microscopy and SEM, even in phase III of the strain development, which is shortly before failure. However, bluish impregnated areas in the mortar matrix of higher porosity or defects, changing due to the fatigue loading, were analysed. Indications were found that the fatigue damage process is continuously ongoing on a micro- or sub-microscale throughout the mortar matrix, which is difficult to observe on a mesoscale by imaging. Furthermore, the results indicate that two different damage mechanisms take place, which are pronounced depending on the maximum stress level. This might be due to diffuse and widespread compressive damage and localised tensile damage, as the findings documented in the literature suggest.

Mechanistic Characterization of Fatigue Damage Process and Failure Predictions of Asphalt Binders

AbstractTo better understand the fatigue damage characteristics of asphalt binders in a whole process, an energy-based mechanistic (EBM) approach was employed. A damage density is used to quantify ...

An Improved Nonlinear Cumulative Damage Model Considering the Influence of Load Sequence and Its Experimental Verification

According to the change characteristics in the toughness of the metal material during the fatigue damage process, the fatigue tests were carried out with the standard 18CrNiMo7-6 material. Scanning the fracture with an electron microscope explains the lack of linear cumulative damage in the mechanism. According to the obtained results, a nonlinear damage accumulation model which considered the loading sequence state under the toughness dissipation model was established. The recursive formula was devised under two-level. The fatigue test data verification of three metal materials showed that using this model to predict fatigue life is satisfactory and suitable for engineering applications.

Analysis of degradation process of a railway steel bridge in the final period of its operation

This paper presents the problem of assessing degradation process of a selected railway steel bridge in the final period of its operation. Changes in the technical condition of the analysed steel railway bridge in time on the basis of special inspections carried out in 2009–2014 are investigated. The methodology for conducting steel railway bridge assessment in an engineering approach is being presented taking into account conclusions and results from the subsequent special inspections. Moreover, the results of fatigue analysis and assessment of the steel railway bridge, which was put in service in 1933 are discussed. In order to conduct the fatigue analysis and assessment, the deflection of the structure under moving load is measured, as well as the geometry of the structural elements in the support zone is verified. The impact of rolling stock, in particular heavy freight transport, on the bridge degradation process is indicated. It is found that dynamic effects induced by the freight rolling stock reveal a big influence on the rate of fatigue damage process of the bridge. Additionally, the degradation of the considered railway bridge due to the effect of track misalignment is taken into account. Furthermore, the impact of corrosion damage and fatigue on the distribution of stresses in the bridge structure is also discussed.

Deformational behavior and damage mechanism of R-UHPFRC beam subjected to fatigue loading

The fatigue behavior of a reinforced UHPFRC (Ultra High Performance Fiber Reinforced Cementitious composite) T-shaped beam under four-point bending is investigated. The beam was subjected to a fatigue loading range equal to 49% of the static resistance and failed after 0.88 million cycles. It was instrumented with extensometers, strain gauges and distributed fiber optic sensors for strain monitoring. The fatigue process consists of three stages: with rapid, stable and again rapid growth of strains during 10%, 80% and 10% of total number of fatigue cycles, respectively. Except of the first 10%, this process takes place locally; therefore, it cannot be followed with the deflection measurement. During the stable stage, growth of strain occurs at minimum loading level in the fatigue cycle, indicating a fatigue damage process under tensile-compressive response of UHPFRC. Advanced fatigue crack propagation in the reinforcement bar determines the location of rupture of the beam. When the remaining cross-section of the rebar does not suffice to carry the tensile load, stress is transmitted to the encompassing UHPFRC causing its fast deterioration. Complete rupture of the rebar occurs only at the end of the test, when the beam collapses.

Ultrasonic and Eddy-Current Fatigue Monitoring of Austenitic Steel Welded Joints

We investigated the process of fatigue damage in welded joints made of austenitic steel by ultrasonic and eddy current types of testing. The development of an inhomogeneity in the distribution of the acoustic anisotropy parameter in the heat-affected zone and in the base material was studied. As a result of eddy current testing, the difference in the intensity of changes in the phase composition in the heat-affected zone and in the base material is shown. A linear relationship is obtained between the amplitude of the cycle deformation under regular loading in the low-cycle fatigue region and the critical value of the percentage of the magnetic phase in the base material. A method is proposed for assessing the fatigue damage to the material of welded joints in the heat-affected zone based on the results of eddy current testing.

Comparative study between S-N and fracture mechanics approach on reliability assessment of offshore wind turbine jacket foundations

This paper investigates from a structural reliability assessment (SRA) perspective the fatigue reliability using the S-N curve approach compared with the fracture mechanics (FM) approach for a typical welded offshore wind turbine (OWT) jacket support structure. A non-intrusive formulation was developed for an OWT jacket support structure in 50 m deep water, consisting of a sequence of steps. First, stochastic parametric 3D (three-dimensional) Finite Element Analysis (FEA) simulations are performed, taking into account stochastic variables such as wind loads, wave loads and soil properties using facilities within the software package ANSYS. Secondly, the FEA results are post-processed using an Artificial Neural Network (ANN) response surface modelling technique deriving the performance functions expressed in terms of stochastic variables. Finally, the First Order Reliability Method (FORM) is applied in calculating the reliability index values of components. The developed framework was applied to elucidate the fatigue damage process, including the small to long crack transition amongst other stages, for structural steels used for OWT jacket applications. The FM formulation investigated includes a crack growth formulation based on the bilinear crack growth law, considering both segments of the crack growth law as non-correlated and correlated in calculating the reliability index (RI). Sensitivity analysis results showed a strong dependence of the structure's reliability levels on the uncertainties of the crack growth law constants measured in terms of coefficient of variation (COV). Also investigated, was the reliability of the structure reassessed and updated in the presence of assumed structural health monitoring/ condition monitoring (SHM/CM) data. The results from the case study revealed that fracture reliability is highly sensitive to the initial crack size. It is recommended to apply the S-N curve method at the design stage while the FM approach applied towards the end of the design life as the structure approaches failure.

Performance Degradation Analysis and Life Prediction of the Fatigue Damage Process of High Strength Aluminum Alloy Using Acoustic Emission

Performance Degradation Analysis and Life Prediction of the Fatigue Damage Process of High Strength Aluminum Alloy Using Acoustic Emission