Fatigue Damage Process Research Articles

Effects of dent size on the evolution process of rolling contact fatigue damage on defective rail

Defects such as dents on the rail surface might induce severe rolling contact fatigue (RCF) and then threaten the running safety of railway vehicles. This study aims to explore the initiation and propagation of RCF cracks around dents with different sizes through observing both surfaces and cross-sections of the defective rail. The experiments are conducted on a wheel/rail twin-disc machine. Results indicate that the microstructure evolution process around the dent can be divided into three main stages, namely crack initiation, crack propagation and dent removal. Dents in different sizes act very similarly in each stage, including the position, the crack growth path and the final appearance after the dent has been fully worn off. However, dents in different sizes cause different lifetimes of crack initiation and different lasting times of dents at each stage. For the lager dent, cracks initiate later but propagate for a longer time period, so the dent size plays an important role in the RCF evolution process.

Fatigue damage identification of SiC coated needled C/SiC composite by acoustic emission

The fatigue damage process of SiC coated needled C/SiC composite specimen was monitored by acoustic emission (AE) under tension-tension cyclic loading. By analyzing the collected AE parameters of the composite, it is found that Kaiser effect enhances with the increase of stable cycles in the fatigue process. Moreover, multivariate K-means cluster analysis of AE parameters was carried out after the standardization of energy, amplitude, peak frequency and duration of AE signal. By comparing the objective function values of different number of clusters, and referring to the intra group variance and the variance between groups, the damage modes of the needled C/SiC composite are finally divided into four clusters, and the characteristics of AE parameters with different damage modes can be obtained. Furthermore, by referring to the microstructure characteristics of needled C/SiC composite, various damage modes at different fatigue stages were analyzed. In addition, the fracture morphology of the specimen was also observed by scanning electron microscope after fatigue fracture.

Stochastic fatigue damage in viscoelastic materials using probabilistic pseudo J-integral Paris' law

Fatigue damage of engineering materials severely affects the serviceability of their structures. It is impracticable to accurately predict fatigue damage process of engineering materials due to the variability of material properties, microstructure heterogeneity and others. This study aims to track the entire fatigue damage process of viscoelastic materials by coupling the variability with the fatigue damage mechanism. A typical viscoelastic material, asphalt binder, widely used in pavement engineering is selected for investigation in this study. A pseudo J-integral Paris' law model and probability theory are combined to establish a stochastic fatigue damage model for viscoelastic materials. Results show that the damage density can be determined by the apparent shear modulus and true shear modulus. The damage evolution rate is a function of material parameters (Paris’ law coefficients), apparent shear modulus, apparent shear strain amplitude and apparent phase angle. Then, a cumulative distribution function of loading time (TCDF) and damage density exceedance probability (DDEP) are derived and experimentally verified. Next, the stochastic fatigue damage model is proposed, which can track the entire fatigue damage process for viscoelastic materials, and it depends on the damage density, material parameters and variability parameters. The variability of minor damage can be used to predict the variability of severe damage based on the stochastic fatigue damage model.

Effect of Turning on the Surface Integrity and Fatigue Life of a TC11 Alloy in Very High Cycle Fatigue Regime

In this work, the effect of a turning process on fatigue performance of a Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) titanium alloy is studied in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. For this purpose, the surface characteristics including surface morphology, surface roughness and residual stress were investigated. Moreover, axial fatigue tests were conducted with an ultrasonic fatigue testing system working at a frequency of 20 kHz. The results show that the turning process deteriorated the fatigue properties in both HCF and VHCF regimes. The fatigue strength at 1 × 108 cycles of turned samples is approximately 6% lower than that of electropolished ones. Fracture surface observations indicate that turning marks play a crucial role in the fatigue damage process, especially in the crack initiation stage. It was observed that the crack of all the turned samples originated from turning marks. In addition, the compressive residual stress induced by the turning process played a more effective role in resisting crack propagation in the VHCF regime than in the HCF regime.

Numerical analysis of the cyclic mechanical damage of Li-ion battery electrode and experimental validation

Evidences have accumulated that the cyclic diffusion-induced stress within lithiation-delithiation process will result in the cyclically evolutive mechanical damage of battery electrode, which adversely affects the mechanical integrity as well as the performance of the Li-ion battery. In this work, the mechanical degradation of electrode under electrochemical-mechanical condition is innovatively evaluated as a fatigue damage process, governed by the interaction between diffusion behaviour and stress generation, and accumulated fatigue damage affected stress–strain response. Structural configuration of a layered electrode plate is modeled in finite element software ABAQUS and a set of user subroutines are developed to implement the proposed fatigue evaluation approach for battery electrode. The constructed approach is proved to be able to simulate multifarious categories of fatigue damage accumulation trends of battery electrode. The strategy to correlate the electrochemistry represented damage with mechanical fatigue damage are proposed. Experimental performance tests are conducted to parameterize the fatigue damage model within the assessment approach for electrode material LiNi0.5Mn0.3Co0.2O2 (NMC532). After parameterization, further circulating charging-discharging experiments and fatigue damage simulations with respect to different C-rate conditions are carried out to study the applicability of the proposed evaluation model as well as the assumption between electrochemical and mechanical deterioration. It is observed that the electrode surface adhering to electrolyte is more prone to fracture in the cycling operation. The present research work shows that it is available to apply the fatigue damage method to study the gradually mechanical failure of battery electrode under electrochemical-mechanical condition.

Microstructural influences on the high cycle fatigue life dispersion and damage mechanism in a metastable β titanium alloy

In this work, the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al (Ti-7333) alloy is investigated. Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1. Results show that the fatigue strength is closely related to the microstructure features, especially the αp percentage. The Ti-7333 alloy with a lower αp percentage exhibits a higher scatter in fatigue data. The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lower αp percentage. Crack initiation region shows the compound αp/β facets. Faceted αp particles show crystallographic orientation and morphology dependence characteristics. Crack-initiation was accompanied by faceting process across elongated αp particles or multiple adjacent αp particles. These particles generally oriented for basal slip result in near basal facets. Fatigue crack can also initiate at elongated αp particle well oriented for prismatic <a> slip. The β facet is in close correspondence to {110} or {112} plane with high Schmid factor. Based on the fracture observation and FIB-CS analysis, three classes of fatigue-critical microstructural configurations are deduced. A phenomenological model for the formation of αp facet in the bimodal microstructure is proposed. This work provides an insight into the fatigue damage process of the α precipitate strengthened metastable β titanium alloys.

The effects of grain morphology and crystallographic orientation on fatigue crack initiation in a metastable β titanium alloy Ti-7333

Introducing fine α particles into the soft β matrix contributes significantly to the strengthening of the metastable β titanium alloy microstructures. However, this microstructure design often leads to microcracks initiated at the α/β interface. In this work, fatigue crack-initiation (FCI) modes and FCI mechanisms at high-cycle fatigue regime in a metastable β titanium alloy, Ti–7Mo–3Nb–3Cr–3Al, are investigated by fracture surface observation and focused-ion-beam cross-section characterizations on a 2-D section through the faceted grains. Based on fracture observation, four categories of FCI modes were summarized from the given microstructures. Fatigue damage mechanism is non-unique and strongly dependent on microstructural constituent combinations although only subtle differences among them. FCI site revealed the compound primary-α (αp)/β facets or isolated αp facets. Elongated αp particle or multiple equiaxed αp particles favorably oriented for basal <a> slip are preferred crack-initiation sites, resulting in the formation of near basal facets. The β facet was in close correspondence to the {110} slip plane with high Schmid factor. The occurrence of the elongated αp facet is usually accompanied by the rugged β facet. Further, three classes of fatigue-critical microstructural configurations are deduced. This work provides an insight into the fatigue damage process of the α precipitate strengthened metastable β titanium alloys.

Magneto-mechanical coupling model of ferromagnetic materials under fatigue loading and its application in metal magnetic memory method

Fatigue damage of materials is generally evaluated by the degree of degradation of mechanical properties under fatigue loading. Existing experiments show that fatigue damage of ferromagnetic materials can also be evaluated utilizing changes in magnetization, but the quantitative relationship between magnetization changes and fatigue damage is still unclear. In this paper, a nonlinear magneto-mechanical coupling constitutive relation for ferromagnetic materials under fatigue loading is established based on the local equilibrium state of magnetization and the stress-strain response relationship in classical fatigue theory. The magnetic induction intensity of the ferromagnetic materials predicted by the present model increases with the number of cyclic loadings, and then tends to stabilize and finally increase rapidly, which is consistent with the existing experimental results. The model can also predict the evolution of complex magneto-mechanical coupling behavior under different loading times. In addition, the relationship between the evolution of spontaneous magnetic field (SMF) and fatigue damage for ferromagnetic materials is discussed in details. These results are helpful to understand the magneto-mechanical coupling behavior of ferromagnetic materials during fatigue damage process and also have important significance for the fatigue damage monitoring in practical engineering.

Unfolding the early fatigue damage process for CFRP cross-ply laminates

This study investigates the early fatigue damage of cross-ply carbon/epoxy laminates. The aim is to unfold the damage accumulation process, understand the interaction between different damage mechanisms, and quantify their contribution to stiffness degradation. Tension-tension fatigue tests were performed, while edge observation and DIC technique monitored the damage evolution. It was found that different accumulation process and interactive levels between transverse matrix cracks and delamination exist for specimens with similar stiffness degradation. A linear increase of stiffness degradation was observed with the increase of matrix crack density, while the growing trend of stiffness degradation converged with the increase of delamination.

輪走行作用を受ける道路橋 RC 床版の疲労損傷過程に関する実験的研究

輪走行作用を受ける道路橋 RC 床版の疲労損傷過程に関する実験的研究

Fatigue damage of PBH shear connector of steel-concrete composite structure

Perfobond Leiste (PBL) shear connector is a widely-applied shear connector that rely on the concrete tenons and penetrating steel bars in the open hole steel ribs to resist slipping. Perfobond Hoop (PBH) shear connector uses the stirrups in the concrete structure to replace the independent penetrating straight reinforcement in the PBL shear connector, which has better spatial rigidness, and shear performance. In this paper, the static and fatigue tests of PBH shear connector are designed to analyze the static and fatigue failure characteristics and study the fatigue damage mechanism and fatigue damage evolution model of PBH shear connector. The study found that PBH shear connector has higher shear stiffness and fatigue properties than PBL shear connector. The concrete tenon in the open-hole steel rib plate is the key part of the PBH shear connector, and its failure form is different during static and fatigue failure. The fatigue shear stiffness of the PBH shear connector is shown as a jump reduction, which is different from the static shear stiffness as a smooth decline. The fatigue damage process of PBH shear connector is divided into three stages according to the failure mechanism: the bond failure stage of the steel-concrete composite interface, the crushing and migration stage of the concrete tenon, and the bending and failure stages of the penetrating steel bar. The three stages take up 10%, 80% and 10% of the fatigue life in turn. With the damage degree as the fatigue process variable, a two-line fatigue damage evolution model of the PBH shear connector is established, which can be used to predict the final interface slip value and fatigue life.

In situ measurements of deformation to fatigue failure on a flat-type divertor mockup under high heat flux loads by 3D digital image correlation

Plasma-facing components (PFCs) usually need to withstand extreme incident heat flux conditions in nuclear fusion engineering. In situ measurements of deformation to fatigue failure of PFCs under high heat flux (HHF) tests are significant and essential to understand their thermal-mechanical behaviors under servicing conditions; these measurements can provide first-hand and important information for evaluating and optimizing design performance and manufacturing techniques. However, unlike traditional contact measurements with strain gauges, the contactless optical measurement technique is rarely applied to measure or monitor the deformation to fatigue damage process of PFCs employed in HHF tests. In this work, a comprehensive HHF experimental platform was established by combining an electron gun scanning HHF heating system with a three-dimensional digital image correlation (3D-DIC) measurement system based on a vacuum chamber and specially designed optical windows and light sources. The 3D-DIC technology was utilized to measure the field deformation of a divertor mockup under cyclic HHF loads. Validation and qualification tests were conducted on the comprehensive experimental platform to ensure the performance of the platform and the accuracy of the 3D-DIC method. The thermal-induced field deformation of a flat-type divertor mockup under the conditions of 1000 HHF cycles at 10 MW m−2 and 300 HHF cycles at 20 MW m−2 using the 3D-DIC technique was then measured. The mechanical behavior of the accumulated plastic strain and fatigue debonding failure of the W/Cu interface due to periodic thermal stress were captured in situ by the measured strain curves and contours for the first time during HHF tests. The results demonstrate the feasibility and accuracy of the 3D-DIC technique for in situ fatigue deformation and damage strain measurements of PFCs during HHF tests. The proposed methods and technologies are expected to be applied to measure and monitor the servicing performance of PFCs under servicing conditions.

Experimental study on fatigue performance of M30 high-strength bolts in bolted spherical joints of grid structures

A grid structure with bolted spherical joints (BSJs) is one of the large-span space latticed structures and has been widely used in industrial buildings. Under the alternating actions of the suspension cranes, fatigue failure of BSJs is liable to occur in a high-strength bolt. In this paper, the constant amplitude fatigue tests of 39 40Cr steel alloy M30 high-strength bolts used in BSJs were carried out using an MTS fatigue testing machine. The corresponding fatigue S-N curves were developed with the stress range as the parameter, and the test results were discussed with the theoretical values of the current specifications. Then, by means of the finite element analysis, the stress state and fatigue failure position of bolts in BSJs in tension were discussed. The analysis of fracture morphology was performed to investigate the fatigue fracture characteristics. Finally, on basis of fatigue damage theory, a high-cycle fatigue damage model was adopted to describe the development process of fatigue damage.

Fatigue damage and creep characteristics of cement emulsified asphalt composite binder

Cement emulsified asphalt composite binder (simplified as CEACB) is a kind of viscoelastic material composed by cement, emulsified asphalt and various admixtures. Fatigue damage caused by dynamic loading affects the viscoelastic properties of CEACB. In this paper, evolution process of fatigue damage was analyzed by fatigue test. Based on the four-element model with five parameters and creep damage evolution equation, CEACB creep equation, single damage creep equation and double damage creep equation were established, and the effect of fatigue damage on the creep behavior of CEACB was analyzed. The damage rate of CEACB increases with the increase of stress level. With the increase of asphalt content, the sensitivity of fatigue properties change with stress level increases. With the increase of fatigue damage, instantaneous elastic deformation and creep rate increase in steady creep stage during creep process. The ability of CEACB to resist deformation is reduced by fatigue damage, and CEACB shows a tendency of softening. The damage of elastic and viscous properties of CEACB are different.

Microstructure-Sensitive Computational Estimates of Driving Forces for Surface Versus Subsurface Fatigue Crack Formation in Duplex Ti-6Al-4V and Al 7075-T6

Statistical realizations of three-dimensional digital microstructures with different crystallographic orientation distributions, grain shapes, and grain size distributions are subjected to uniaxial cyclic straining to compare the cases of bulk and free surface on driving forces for fatigue crack formation. Crystal plasticity finite element simulations are conducted using both fully periodic (more representative of the bulk) and traction-free (i.e., free surface) boundary conditions for duplex Ti-6Al-4V and rolled Al 7075-T6. Following elastic–plastic shakedown, mesoscale volume-averaged fatigue indicator parameters (FIPs) are computed within fatigue damage process zones of grains and are fit to known extreme value distributions (EVDs). Owing to differences in crystallographic slip symmetry, FIPs for fcc Al 7075-T6 statistically occur much closer to the traction-free surface than for hcp Ti-6Al-4V. Additionally, surface versus bulk EVDs of FIPs vary differently for the two material systems, indicating a coupled role of microstructure and surface proximity that had not been previously elucidated.

FE-SEM in situ observation of damage evolution in tension-compression fatigue of micro-sized single-crystal copper

While understructures formed by fatigue strain cycling, such as veins, ladders and cell structures, bring about extrusions/intrusions on the surface of bulk metal, micro-sized metal does not have space to form an understructure, implying that another fatigue mechanism must apply in micro-sized metal. To date, information obtained on this mechanism has been very limited because of experimental difficulties. In this project, we develop an experimental method of tension–compression high-cycle loading for a micro-sized metal specimen that allows in situ observations of the fatigue mechanism by a field emission scanning electron microscope (FE-SEM). A micro-sized single-crystal copper specimen with a single slip orientation is subjected to fully reversed cyclic loading, and the damage process is successfully observed in detail. The fatigue failure is caused by extrusions/intrusions formed along the primary slip system in a slip band consisting of numerous plates with thicknesses of 15–20 nm. Detailed observations of the evolution process suggests that they are formed by the multiplication and glide of dislocations without a conventional understructure, in contrast to the case for the bulk counterpart. The fatigue process can be classified into the following three stages: (A) Evolution stage of fatigue damage, (B) steady stage and (C) cracking stage. Surprisingly, the extrusions/intrusions initiate at the reverse loading of the first cycle and continuously grow in proportion to the number of strain cycles even for less than 1% of the overall fatigue damage process.

Detecting and evaluation of fatigue damage in concrete with industrial computed tomography technology

For effective maintenance of concrete structures, it is necessary to detect and evaluate the increasing fatigue damage. Fatigue loading tests under uniaxial compression was conducted on cylindrical concrete specimens with a diameter of 50 mm and a length of 100 mm. Spatial distribution of fatigue cracks within concrete was observed using industrial computed tomography (ICT) technology, the expanding process was summarized, and the internal fatigue crack ratio of concrete was accurately described. In addition, based on the quantitative description method of the overall distribution of fatigue cracks in concrete, the fatigue damage parameter D was defined which can better reflect the overall crack distribution of concrete fatigue. By comparing with the existing damage parameter in other researches, it can be indicated that the definition in this study can better characterize the evolution process of material fatigue damage of concrete under compression from the overall distribution of fatigue cracks. This method provided an effective opportunity to estimate fatigue damage status of concrete, and promoted the research of concrete fatigue from basic theory.

Nonlinear Fatigue Damage Model of Asphalt Mixture Based on Dynamic Modulus and Residual Strength Decay.

In order to describe the fatigue damage state of asphalt mixture more reasonably, direct tensile tests of the fatigue and the residual strength under stress levels of 1.00 MPa, 0.50 MPa and 0.25 MPa with five parallel tests were carried out. The trabecular specimens of AC-13C asphalt mixture (25 cm × 5 cm × 5 cm) were manufactured with Styrene-Butadiene-Styrene (SBS) modified asphalt, aggregate basalt and limestone mineral filler. The optimum asphalt-aggregate ratio was 5.2%. The dynamic modulus decay and the residual strength decay were termed as the damage variables to evaluate the fatigue damage process of asphalt mixtures, respectively. Based on the test results, the decay patterns of the dynamic modulus and the residual strength during fatigue tests under different stress states were revealed, and the model and the parameters of fatigue damage according to the corresponding decay patterns were obtained. Then, based on the assumption that the residual strength and dynamic modulus depend on the same damage state, the relationship between the two damage definitions was given, and the residual strength-dynamic modulus coupled model was established. The results showed that the residual strength-dynamic modulus coupled model could better describe the fatigue damage evolution law of asphalt mixture, and the parameter of this coupled model could be obtained by less residual strength tests. A modified formula for calculating the damage variables associated with residual strength and dynamic modulus was proposed based on the relationship between two kinds of damage variables.

Influence of Residual Stress on Fatigue Weak Areas and Simulation Analysis on Fatigue Properties Based on Continuous Performance of FSW Joints

The fatigue weak area of aluminum alloy for a friction stir-welded joint is investigated based on the hardness profile, the residual stress measurement and the simulation analysis of fatigue property. The maximum residual stresses appeared at the heat-affected zone of the joint in the fatigue damage process, which was consistent with the fracture location of the fatigue specimen. The fatigue joint model of continuous performance is established ignoring the original negative residual stress; considering that it will be relaxed soon when the joint is under tension-tension cyclic loading. The fatigue parameters of joint model is based on the static mechanical properties of the joint that obtained from the micro-tensile tests and four-point correlation method. The predicted results for the fatigue weak locations and fatigue lives based on the continuous performance joint model are closer to the fatigue experimental results by comparison with the simulation results of the partitioned performance joint model.

Stochastic analysis of fatigue of concrete bridges

A novel method is proposed in this work for the assessment of the remaining fatigue life and fatigue reliability of concrete bridges subjected to random loads. The fatigue reliability of a bridge is a function of the fatigue damage accumulation; a stochastic fatigue damage model (SFDM) with physical mechanism is introduced for deriving the fatigue damage process. In order to implement the probabilistic analysis, based on the probability density evolution method (PDEM), the generalised density evolution equation (GDEE) for the remaining fatigue life is developed. Finally, a prestressed concrete continuous beam bridge located in China is illustrated. The random fatigue load acting on the bridge is modelled as the compound Poisson process, and the simulation of the random load uses the stochastic harmonic function (SHF) method. To simplify the reliability analysis, an equivalent constant-amplitude (ECA) load process is introduced based on energy equivalence. By employing SFDM, the finite element analysis of the bridge under the fatigue loading is performed. Then, the fatigue damage accumulation process of the bridge under the fatigue loading is obtained. Through solving the probability density evolution equation for the remaining fatigue life, the probability density functions (PDFs) of the remaining fatigue life evolving with time is obtained. The fatigue reliability is then calculated by integrating the PDF of the corresponding remaining life.