Degree Of Fatigue Damage Research Articles

Fatigue Cracking Characteristics of Asphalt Pavement Structure under Aging and Moisture Damage

Structural characteristics influence assessment of fatigue cracking behavior. In the assessment of asphalt pavements, the asphalt structure and practical conditions must be considered. This study analyzes changes in the elastic modulus of the pavement of different asphalt mixtures amid aging and moisture damage through fatigue cracking tests. A model to predict the tensile strain at the bottom of the pavement layer is developed through a structural analysis based on the material properties. The results are comparatively analyzed using the Mechanistic-Empirical Pavement Design Guide to predict the fatigue crack life. The test results indicate that moisture damage significantly influences the material properties of asphalt pavement and can accelerate pavement damage as the asphalt ages. The coefficient values of the proposed fatigue-life prediction model can be used to predict the fatigue life depending on the age of the asphalt and its moisture damage after aging. The degree of fatigue damage can be predicted by calculating the tensile strain using the regression equation and elastic modulus according to the aging and moisture damage.

Fatigue monitoring of coated steel belts by means of magnetic inspection

Coated steel belts (CSBs) are the primary load-bearing components of elevators. As a result, they may experience severe fatigue failure during their long-term service. In this study, a flexible fatigue fixture for CSBs is designed to simulate the CSB fatigue damage process for implementing a tensile fatigue test. The magnetic induction intensity signal on the surface of the CSB in a geomagnetic field environment is recorded using a high-precision weak magnetic sensor. Moreover, the CSB fatigue damage process is monitored online via the acquired magnetic induction intensity value. The fatigue test results indicate the following: the variation in the magnetic induction intensity signal curve can reflect the entire fatigue failure process when the CSB is under stress; the fatigue failure process of the steel wire inside the CSB does not occur smoothly as its progress occurs in stages; and the variation trend in the residual value between the magnetic induction intensity and fitting curves effectively indicates the degree of fatigue damage caused to the CSB.

Experiment and DEM simulation study on mechanical behaviors of shale under triaxial cyclic loading and unloading conditions

During shale gas exploration, natural fractures in shale reservoirs may be induced by cyclic loads frequently encountered in different geological processes, including tectonic movements, seismic actions, and artificial construction interference (vertical and horizontal wells drilling and cyclic hydraulic fracturing). In this paper, experiments and PFC2D simulations are conducted to investigate the mechanical response and mechanism of shale specimens under cyclic loading and unloading. Using the experimental and simulation results, the strength and deformation characteristics and strain energy and damage evolutions during the cyclic loading-unloading process are quantified and analyzed. The damage variable characterized by plastic deformation, deformation modulus, and dissipated energy is thoroughly analyzed. Based on the PFC2D simulation, the micro-crack distribution and evolution are further studied, and the results can reflect the experiments well and reveal the damage mechanism under cyclic loading. Furthermore, the experiments and simulations indicate that the degree of fatigue damage is closely related to the number of cycles the specimens undergo, and a small number of cycles may not have a distinct effect on the strength.

Study on magnetic memory detection of weld fatigue damage by using the relative entropy theory

In this paper, fatigue tests of different kinds of welds were carried out. The three magnetic components of MMM signal on the weld under different cycles were collected. The change law of MMM signal with the fatigue cycle was analyzed, and the relative entropy theory was introduced to establish the mapping relationship between the relative entropy and the fatigue cycle. The damage state of the weld was evaluated based on the change of the relative entropy. The results showed that the curves of initial MMM signal presented specific change law with the increase of fatigue cycle and a counter-trend change occurred at the local position after the weld failure. In addition, there was a good correspondence between the relative entropy with the fatigue stage of the weld. The value of relative entropy in the fatigue stable stage remained approximately constant and tended to be close to zero. In the final fatigue stage, the value of relative entropy increased exponentially. The relative entropy gradient and the magnetic index k were calculated to capture the time node of the deterioration of the weld more accurately and the k = 0.1 could be defined as the critical value to evaluated the stability of the weld. Hence, the relative entropy that was introduced could effectively characterize the fatigue damage degree of the weld, and realize the early warning of the weld damage.

Study on Fatigue Behavior of S135 Steel and Titanium Alloy Drillpipes: Experiment and Modeling

Summary This paper studies the fatigue failure behavior of S135 steel and titanium alloy drillpipe, tries to reveal the mechanism of fatigue fracture, and evaluates the degree of fatigue damage. First, the chemical composition, tensile test, and microstructure analysis of two types of pipes were carried out, and the difference in microstructure affects its macroscopic fatigue performance. Second, the stress vs. fatigue life (S-N) curve equation and fatigue limit of S135 steel and titanium alloy drillpipes in air and mud were obtained through the fatigue test. The equations of the S-N curves in air are NS135−air=9.45×1037(S+1776)−9.3 and NTi−air=1.41×1028(S+1124)−6.7, while in mud, they are NS135−mud=6.68×1034(S+551)−9.6and NTi−mud=5.74×1032(S+973)−8.3. Scanning electron microscope analysis results show that the S135 steel drillpipe presents brittle fracture, while titanium alloy drillpipe presents mixed fracture of quasi-cleavage and dimple. Finally, a case study of actual horizontal well was carried out to evaluate the fatigue damage of two drillpipes; compared with conventional steel drillpipe, titanium alloy drillpipe can effectively resist fatigue damage in the drilling fluid environment. The fatigue test results and corrosion fatigue mechanism analysis in this paper can provide data and theoretical guidance for the study of drillpipe fatigue life.

A methodical approach to determining the damage characteristics of cyclically loaded samples of metal structures

The article deals with an actual task of developing engineering methods for predicting the durability of modern long-life machines should be based on the methodology for determining the degree of fatigue damage to materials and structural elements. In the development of such a methodology, the work proposes to use modern optical and computer tools that allow analyzing the parameters of the deformed surface of metal structure samples and, on this basis, to estimate the residual resource of the metal structure. The purpose of the article was to substantiate a methodical approach for determining the limit state of damage using photometric analysis of the topography of the micro-deformed surface of cyclically loaded metal structure samples. To analyze the fatigue damage characteristics of metals and alloys, an experimental information system was developed based on the method of coherent optical scanning of the deformed surface of metal structure samples with a resolution of 0.2 μm/px. The obtained specklograms of the photodiode matrix, corresponding to the two-dimensional image of the micro-deformed surface of the sample in shades of gray 0-255, were computer analyzed. This made it possible to construct diagrams of the kinetics of the process of accumulation of the amplitude of the deformation microrelief of the surface of the laboratory samples for the studied steels St 45 and St 20. It is shown that the kinetics of speckle brightness obtained in the work by the correlation method, which corresponds to the evolution of the accumulation of fatigue damage on the surface of the studied structural steel samples, is characterized by a nonlinear function, which is consistent with the results of deformation microrelief studies obtained by other methods. The proposed methodology for analyzing the discrete distribution of specklogram brightness can be used to predict the onset of the limit state of a metal structure before failure due to fatigue

Study on Fatigue Spectrum Analysis and Reliability Analysis of Multilayer Flexible Riser

Multilayer composite flexible risers have been widely used in engineering. However, this type of structure is complex, as there are influences between layers. Moreover, a range of uncertain factors need to be considered in fatigue analysis. Therefore, it is difficult to perform the fatigue analysis research of multilayer flexible risers. In this paper, the fatigue spectrum analysis and reliability analysis method of a nine-layer flexible riser structure are proposed, and a complete fatigue and reliability analysis process for multilayer structures is developed. The theoretical basis of the fatigue spectrum analysis method is introduced, and the calculation program is described. Finite element software is used to analyze the stress of the multilayer flexible riser under the influence of the upper platform structure movement and the ocean current. Moreover, the stress response of the riser structure of each layer is obtained. According to this, the irregular wave load is simulated by the method of random number simulation, and the stress response spectrum is formed. Then, the appropriate S-N curve is selected to calculate the fatigue damage degree of each layer, and the fatigue damage nephogram is displayed, so as to analyze the structural fatigue damage. Finally, the uncertainty in the process of fatigue damage calculation is analyzed. According to the results, the methods of multilayer riser analysis are summarized and the future research directions are put forward.

SCC fracture location shifting affected by stress-controlled fatigue damage of NiCrMoV steel welded joints

The influence of stress-controlled fatigue (SCF) damage on the fracture location shifting of welded joints for NiCrMoV steels in the corrosive solution of 3.5wt.% NaCl is systematically investigated by electrochemical measurements, immersion tests, slow strain rate tensile (SSRT) tests, and fractural morphology observations in the present work. The results show that both the yield strength (YS) and the ultimate tensile strength (UTS) of welded joints are increased after the SCF tests. The susceptibility of stress corrosion cracking (SCC) and galvanic corrosion of the SCF damaged welded joints are higher than that of the as-received specimens. In addition, the fracture locations of welded joints after SSRT tests in the corrosive solution are shifted with the degree of SCF damage. Different degrees of fatigue damage are accumulated in the three zones of welded joints during the cyclic deformation, which could change the competitive relation of the most susceptive SCC zone between the fusion zone and the middle of WM.

Application of SBF Test to Fatigue Damage Assessment of Type 316 Steel

Abstract A new fatigue testing technique, the small bulge fatigue (SBF) test, was recently developed by some of the present authors. In this technique, a cyclic oil pressure is alternatively applied to both surfaces of a small disk-type specimen (8 mm in diameter) with flat and concave surfaces. This SBF test is a kind of biaxial fatigue test as well as the disk bending fatigue (DBF) test. In this study, damaged specimens were subjected to the SBF test to examine the adaptability of this test to the fatigue damage assessment of actual components. To prepare these damaged specimens, Type 316 austenitic stainless steel was subjected to the DBF test using large disk specimens (87 mm in diameter) by considering that most of components were generally operating under multiaxial loading condition. The test was stopped at several stages of the fatigue life. Then, the SBF test specimens were taken from the surfaces of the interrupted DBF test specimens. It was expected that fatigue life of the SBF tests corresponded to the degrees of fatigue damage induced by the DBF test. The SBF test results showed that the fatigue life of the damaged SBF specimen, which contained microcracks formed by the DBF test, was only one tenth as long as that of the undamaged specimen. It was noteworthy that some microcracks introduced by the DBF test were opened and became clearly visible after the SBF test, although they were closed and not visible just after the DBF tests.

Predicting the power module cumulative damage degree in new energy vehicle: Improved Manson model

The cumulative fatigue damage degree is a core parameter used to evaluate the service life of an IGBT module and accurately calculate the cumulative fatigue damage degree. This study is necessary to accurately evaluate the cumulative fatigue damage of insulated gate bipolar transistor (IGBT) power module in drivetrain system to improve the reliability of stable operation of electric vehicles. Especially, the improved Manson model calculates the cumulative fatigue damage of IGBT more accurately. The reliability of IGBT module is effectively evaluated to make the effective operation and maintenance strategy of drivetrain system in electric vehicles, and improving the reliability of electric vehicles. The limitations of the Manson model were considered, and an improved Manson model was proposed and the spectrum of temperature load was obtained by the rain-flow counting method of junction temperature-time curve. The cumulative fatigue damage of IGBT module was calculated using the improved Manson and Miner models. The results revealed that in situations 1 and 2, the total damage calculated by the improved Manson model was 230.25% and 8.39% higher than that of the Miner model. The working period was segmented. The results indicate that the damage degree calculated with non-segmentation was greater than that with segmentation. The total damage calculated by the improved Manson and Miner model was closer to the true value than that calculated by the Miner model.

Feasibility of early fatigue damage evaluation using the Neutron diffraction method

One of the most successful applications of the Neutron Diffraction (ND) method is the evaluation of residual stress, specifically in welded structures, and many examples exists in the literature. The present study explores the feasibility of applying the ND method to the evaluation of early high-cycle fatigue damage (i.e. the damage prior to formation of fatigue micro- or macro-cracks). In metals and advanced alloys early fatigue damage is normally associated with the accumulation of irreversible and highly localised micro-plastic strains. These strains change the micro-strain/stress field on various scale levels. In this study we attempt to measure these changes applying the ND method to G350 steel fatigue samples, which have been relieved from residual stress and subjected to various degrees of high-cycle fatigue damage. Multiple measurements of the strain/stress field in each sample have been undertaken using the ND method with an incoming beam of 0.5×0.5 mm2. The outcomes demonstrate that it is feasible to evaluate severe fatigue damage using the ND method, and, in general, the severity of the fatigue damage correlates relatively well with the averaged hydrostatic component of the residual stresses measured by the ND. However, more accurate evaluation may require higher spatial resolution (smaller gauge length) and, possibly, a larger number of measurement points to improve the quality of the experimental data.

The characterization of bovine compact bone fatigue damage using terahertz spectroscopy

Fatigue can cause cracks to propagate from the micro- to the macroscale, which results in a decrease of Young’s modulus of the bone. Non-destructive measurements of bone fatigue damage are of great importance for bone quality assessment and fracture prevention. Unfortunately, there is still a lack of effective nondestructive methods sensitive to the initial deterioration during damage accumulation, particularly in the field of orthopedics and biomechanics. In this study, terahertz spectroscopy was adopted to evaluate microscale bone damage. Specifically, the refractive index and Young’s modulus of bone samples subjected to different degrees of fatigue damage were tested at a fixed area. Both parameters are found to decrease in two stages under cycled fatigue loading, which is attributed to the initial onset and subsequent development of microdamage during fatigue loading. The change in refractive index reflects the accumulation of fatigue damage as well as the decrease in Young’s modulus.

Fatigue Behavior of Heavy-Haul Railway Prestressed Concrete Beams Based on Vehicle-Bridge Coupling Vibration.

Due to the demand for increasing trainload and enhancing some existing heavy-haul railways, the low reserve value of bearing capacity is a problem for the 32 m-span simply supported beam. The fatigue behavior of prestressed concrete beams in a heavy-haul railway loaded by 33 t and larger axle weight of trains was experimentally investigated. The experimental results of the fatigue behaviors, including fatigue deformation, crack propagation behavior, and strains of classical materials were obtained and analyzed. A fatigue behavior assessment model was established to investigate the residual stiffness and yield point degradation of the beams loaded by the trainload. The effects of train fatigue cycles and prestress loss on the residual stiffness and yield point degradation models of the beams were analyzed. The results indicated that the crack development process had three stages during the fatigue process: the derivative stage, gradual development stage, and fatigue failure stage. Trainload was the main external factor influencing the fatigue behavior of prestressed concrete beams. The increase in trainload accelerated the degradation rate of the residual stiffness of the beams and yield point, reducing the fatigue life. The prestressing strand was primarily used to delay the concrete cracking in the tension zone. When the beam was not cracked, the prestressed concrete beam showed good fatigue performance, and the degree of prestressing did not affect the fatigue life of the beams. When the maximum fatigue load exceeded the cracking load of the beam, prestress loss in beams became a critical issue that accelerated the degradation rate of fatigue strength and reduced fatigue life. The higher the fatigue damage degree, the more pronounced the effect of prestress loss on the fatigue strength of the beams. The fatigue failure of prestressed concrete beams occurred in the bottom tensile steel bar. Therefore, when the trainload of a heavy-haul railway is greater than the cracking load of the beam, it is recommended to strengthen the beam by prestressing and strictly control the trainload to avoid yield failure.

Effect of aging state on corrosion fatigue properties of 7075 aluminum alloy

The corrosion fatigue performance of Al alloy components is crucial for their engineering applications. This study focuses on the corrosion fatigue properties of 7075 Al alloys in three different aging states, i.e. over-aging (OA), peak-aging (PA) and under-aging (UA) states. The results show that the fatigue damage mechanism of the Al alloy is greatly affected by the stress level. In the high-stress region where the cyclic number is short, the fatigue damage is mainly caused by dislocations accumulation and stress concentration on the grain boundary. Compared with the UA state, the wave slip dislocations at the OA state are easy to accumulate and annihilate at the second phase boundaries, which reduces the fatigue damage degree at the grain boundaries. However, in the low-stress range where the cyclic number is large, the corrosion fatigue life of the Al alloy is mainly controlled by pitting-induced crack initiation. For the OA state, the spacing and size of the precipitated phase are large, so it is easy to trigger corrosive pitting, which reduces the corrosion fatigue performance of the alloy.

The estimation method of normalized Nonlinear Output Frequency Response Functions with only response signals under stochastic excitation

The nonlinearity degree of the system can be sensitively revealed by the methods based on Nonlinear Output Frequency Response Functions (NOFRFs). However, there is a lack of methods based on NOFRFs that work with only response signals under stochastic excitation, which limits the application of NOFRFs in practical engineering. In this study, a new concept of normalized NOFRFs is proposed, and a corresponding estimation method that employs only stochastic output response is established. Firstly, a method of estimating NOFRFs by Power Spectral Density (PSD) is established, and therefrom the normalized NOFRFs and a corresponding nonlinearity index are proposed. Then, by taking the PSD of the excitation signal as the input and the PSD of the response signal as the output, a general estimation method of normalized NOFRFs is built, and an output measured only estimation method of normalized NOFRFs is further deduced and established. In these two estimation methods, the assumption of white noise excitation is not required, and the estimated results are insensitive to the change of excitation intensity. Finally, the experiments of fatigue damage detection of three-point bending fatigue steel plate specimens and reducer box cases under stochastic excitation are carried out. The results demonstrate that, compared with the NOFRFs, the degree of fatigue damage can be effectively revealed by the normalized NOFRFs with output.

The Characterization of Fatigue Damage of 316L Stainless Steel Parts Formed by Selective Laser Melting with Harmonic Generation Technique.

Fatigue damage is the main reason for the failure of parts formed by selective laser melting (SLM) technology. This paper presents a high-order, harmonic, and nonlinear ultrasonic testing system for monitoring the generation and evolution of fatigue damage in SLM 316L stainless steel parts. The results demonstrate that the normalized ultrasonic, nonlinear coefficients show a significant dependence on the degree of fatigue damage of the tested specimen and that the normalized, ultrasonic, and nonlinear coefficients are effective in characterizing the degree of fatigue damage in SLM 316L stainless steel parts. Transmission electron microscope (TEM) and scanning electron microscope (SEM) analyses show that the variation in the normalized, ultrasonic, nonlinear coefficients reflect the generation and evolution process of dislocation and crack in the fatigue process of SLM 316L stainless steel specimens, and reveal the fatigue damage mechanism of SLM 316L stainless steel parts.

Applications of continuum fatigue risk monitoring in riser connectors system integrity management

This paper discusses the effects of marine riser connectors (MRCs) on fatigue failure risk using multi-coupling models. When MRCs are subjected to vortex-induced fatigue, a risk early warning strategy is used for effective risk monitoring as a preventive measure for the operation of MRCs. To ensure safe marine oil and gas production and to improve fatigue failure prevention and control, research on offshore structural fatigue risk failure theory criteria and evaluation and the vibration fatigue characteristics of MRCs over time and frequency is characterized. By coupling data-driven technological means, including rain-flow counting (RFC), Hilbert–Huang transform (HHT), power spectral density (PSD), artificial neural network (ANN), risk-based inspection (RBI), and risk management measures (RMM) to assess the fatigue risk characteristics of in-service members in special offshore environments. The fluid-structure interaction characteristics of MRCs and a load spectrum are analysed by empirical mode decomposition (EMD) to determine the changes in the components of an intrinsic mode function (IMF) and investigate the degree of fatigue damage. The fatigue damage of MRCs is predicted using the PSD of the IMF and the RFC of the IMF-coupled ANN time series prediction model. The results show that using any IMF component or combined component data as input to the ANN and training set facilitates cross-validation of the analysis. When the input layer and a hidden layer of the ANN are optimized and properly trained, the predicted performance is suitable. Through early warnings and coordination, the spread of fatigue failure risk trends can be predicted, and warning information is issued accordingly.

Research on Fatigue Performance of the Rib Beam Bridge Carriageway Slab Based on Cumulative Damage Theory

The purpose of this paper is to study the development law of fatigue damage of the carriageway slab of reinforced concrete ribbed girder bridge and to provide theoretical support for carriageway slab design. The fatigue test of reinforced concrete rib beam bridge carriageway slabs was conducted. Based on the material fatigue damage theory, the fatigue damage model was established by ABAQUS software. The fatigue performance, failure mechanism, and the effect of fatigue level on the fatigue performance of the carriageway slab were studied. The experimental research results show that when the slab of the rib beam bridge was fatigued, and the radial crack appeared at the bottom of the slab, which belonged to punching shear failure. The fatigue failure process was divided into three stages as follows: in the initial stage of fatigue, the deflection and reinforcement strain increased linearly with fatigue times, and in the stable development stage, the bottom deflection and reinforcement strain increased steady progression. With the increase in fatigue cycles, the fatigue accumulation damage was gradually accumulated, and the stiffness of the carriageway slab gradually decreases. In the rapid development stage, the deflection, reinforcement strain, and crack growth speed were significantly increased. According to the fatigue damage model, the simulation analysis results show that the accumulated damage to the bridge carriageway was more serious with the increase in fatigue load level. The fatigue damage degree was lower when the design fatigue load level is not greater than 0.55.

Effect of aging state on fatigue property of wrought aluminum alloys

The high-cycle fatigue (HCF) behaviors of wrought Al alloys are vitally important for their industrial application. To optimize the fatigue property of the wrought Al alloy, the effects of aging conditions on the fatigue properties of 7N01, 7075 and 2024 Al alloys were studied. The results show that the overaged samples have the lowest tensile strength among the three aging states, but their fatigue strength is the highest. Further investigations on the dislocation morphologies indicates that the dislocations are easy to accumulate and annihilate at the second phase boundaries for the overaged state, which reduces the fatigue damage degree at the grain boundaries. Besides, the discontinuous distribution of the grain boundary precipitates and the increased width of the precipitate-free zone also contributed to the improved fatigue property of the overaged state. Those results should be significantly important for the optimization of their fatigue properties and the industrial application of wrought Al alloys.

Research on Residual Life Estimation Method for KMN Steel Based on Nonlinear Ultrasonic Testing

The testing of KMN steel bending fatigue with different cycles was carried out using a nonlinear ultrasonic detector to obtain its nonlinear coefficient. The experimental results show that the nonlinear coefficient first increases and then decreases with an increase in fatigue cycles. The relationship between the propagation of the micro-cracks inside the material and the nonlinear coefficient was researched by microscopic analysis in the dangerous position of the specimens. As the fatigue cycles increase, the microstructure of the specimen gradually deteriorates and cracks occur, which proves that nonlinear ultrasonic detection can be used to characterize the initiation of micro-cracks in the early fatigue stages of the material and that the nonlinear coefficient β of the material can be used to reflect the fatigue damage degree and fatigue life of the interior of the material. An analysis of the numerical statistics of the fatigue cracks inside the specimens was carried out, and the extreme value of fatigue cracks was calculated using the Gumbel distribution. An empirical formula for the nonlinear coefficient and crack growth size of KMN steel was established and then a method for estimating the fatigue life of KMN steel based on nonlinear ultrasonic testing was proposed.