Location Of Fatigue Crack Initiation Research Articles

Deformation mechanism and life prediction model of titanium alloy laser-arc hybrid welded joint during fatigue

The deformation mechanism and life prediction model of titanium alloy laser-arc hybrid welded joint during fatigue were studied. At the relatively small maximum cyclic stresses σmax (310 MPa to 350 MPa), the fatigue cracks initiated at the interface of lamellar α′ and needle-like α′ around the pore. At σmax=370MPa, the elongated lamellar α′ around the pore promoted fatigue crack propagation, leading to the formation of secondary cracks. At σmax=390MPa, the formation of two fatigue crack initiation locations and the occurrence of secondary cracks led to the maximum fatigue damage and the minimum fatigue life. In addition, the plastic deformation mainly occurred in β at σmax=310MPa, and it transformed into the phase interface of secondary α-β and granular β-α′ at σmax=350MPa. At σmax=390MPa, the main deformation forms were the cross-slip in β and the dislocations entanglement in α′. Finally, the fatigue life prediction model was established based on the equivalent cyclic stress, and the predicted fatigue life fell within a 3-fold error band.

Microstructure-sensitivity of CPFEM models on fretting fatigue crack initiation of AA2024-T351 alloy

The AA2024-T351 alloy is widely used in civil engineering, machinery, and aerospace industries for bolts, rivets, and thin plate components. When in service within these fields, the components are susceptible to performance degradation and fracture failure due to fretting fatigue. In this study, a crystal plastic finite element method (CPFEM) model incorporating microstructure sensitivity is employed to predict the fretting fatigue crack initiation (FFCI) location and lifetime. The submodel method is utilized to simulate various microstructures in the contact region of the specimen under fretting fatigue. This research focuses on two fatigue indicator parameters (FIPs): accumulated plastic slip p and FIPFS, used to determine the hotspots for crack nucleation on the contact surface and subsurface in fretting fatigue. The study explores the impact of different grain size and orientation on crack nucleation and analyzes how various microstructural characteristics influence the crack initiation lifetime. The findings indicate that the influence of grain orientation distribution in fretting fatigue on the initiation location of contact surface cracks is not significant, but it has a notable impact on subsurface cracks. Different grain textures combinations exhibit a more pronounced anisotropy based on the accumulated plastic slip p than FIPFS. Additionally, the effect of grain orientation distribution on the FFCI lifetime is influenced by the average grain size, showing remarkable influence within a specific range of grain sizes.

Comprehensive effect of simulated centrifugal force on characteristics of different extent notch-type foreign object damages

To systematically reveal effects of the centrifugal force on macro and micro characteristics and residual stress distributions of notch-type foreign object damages (FODs) on the leading edge of rotating titanium alloy fan/compressor blades, four series of notch-type damages with different depths were obtained under the same impact condition but four kinds of preloading conditions by using a light gas gun equipped with a 4-axial adjustable fixture. Macro and micro characteristics, stress concentrations and predicted numerical residual stress distributions of the notch-type damage impacted under the different preloading conditions were observed, calculated and compared. Finally, a qualitative analysis of fatigue crack initiation locations of FOD specimens was conducted by the comprehensive comparison of locations of stress concentrations, maximum residual stresses, micro damages such as micro cracks.

Estimation of the fatigue crack initiation site and life of Ti-6Al-4V alloy for two types of circle and slender crystal grains

In this study, the fatigue crack initiation behaviour and fatigue crack initiation life of the Ti-6Al-4 V alloy, which had circular and slender grains, were quantitatively evaluated based on slip line length and out-of-plane component of Schmidt factor. As the results, in crystal grains where fatigue cracks initiated, the slip system that contributes to crack initiation could be identified by considering the out-of-plane component of the Schmidt factor (SFsinΩ), and the fatigue cracks were initiated by the basal slip of the primally α phase in all specimens. Furthermore, regarding the prediction of the location of fatigue crack initiation, it is clear that the grains that are candidates for fatigue crack initiation could be estimated based on the relationship between the slip line length of the basal slip system that contributes to crack initiation in each grain and SFsinΩ. Finally, the macro fatigue crack initiation life could be estimated using a parameter of P that combines micro crack initiation driving force Pi and micro crack propagation driving force Pp.

Evolution of surface roughness of notched steel details under fatigue loading

A fatigue crack initiates on the surface of the steel members, which can be attributed to the extrusions and intrusions caused by cyclic loading. The surface roughness parameters, including the statistical surface roughness and maximum surface roughness parameters, can characterize the increase in extrusion and intrusion. Six notched specimens were loaded with cyclic loading, and the evolution of the surface roughness parameters was monitored during the test to explore the characteristics of the surface roughness during the fatigue loading procedure in the high-cycle fatigue regime and clarify the effects of the initial surface finish and load ratio on the surface roughness evolution. Based on the test results, it was found that the 3D average surface roughness Sa and 3D root-mean-square surface roughness Sq were preferable for detecting the onset of fatigue cracks compared with the 2D average surface roughness Ra and 2D root-mean-square surface roughness Rq. The 3D maximum valley depth, Sv, was an effective indicator of the fatigue crack initiation location, except for compression-dominant fatigue loading. The evolution patterns of the 3D average surface roughness, Sa, and 3D root-mean-square surface roughness Sq during fatigue loading can be classified into three phases: instant response, damage accumulation, and crack-induced response. In addition, surface polishing is recommended for evaluation using 2D surface roughness parameters. However, it is not mandatory for evaluations using 3D surface roughness parameters. The effects of typical load ratios (0.1, −1, and 10) on the evolution pattern of the 3D statistical surface roughness parameters, especially in the instant response phase, were also investigated.

Failure Mechanism Research on Bending Fretting Fatigue of 6061-T6 Aluminum Alloy by Experiment and Finite Element Method.

The fatigue failure mechanism of bending fretting for cyclic softening material 6061-T6 aluminum alloy was researched by experiment and finite element method. The influence of cyclic load on bending fretting fatigue was researched and the damage characteristics under different cycles was discussed experimentally though SEM images. In the simulation, a normal load transformation method was employed to obtain a simplified two-dimensional model used for simulating the bending fretting fatigue from a three-dimensional model. An advanced constitutive equation with the Abdel-Ohno rule and isotropic hardening evolution was transplanted into ABAQUS by UMAT subroutine to consider the ratchetting behavior and cyclic softening characteristics. The peak stain distributions under various cyclic loads were discussed. Additionally, the bending fretting fatigue lives and crack initiation locations referring to a critical volume method were estimated using the Smith-Watson-Topper critical plane approach and reasonable results were obtained.

A deep learning approach to predict fretting fatigue crack initiation location

Fretting fatigue is a destructive failure that usually occurs in the parts or components in the aviation field. One of the important research topics in fretting fatigue is the prediction of crack initiation location. This paper proposed a new prediction methodology of crack initiation location, which is a deep learning model based on the statistical bootstrapping technique and finite element method (FEM). In this work, two kinds of deep learning models, namely Model-I and Model-II, are established based on different input features, and with the crack initiation location as output feature. The input features of Model-I are the contact load, the tangential load, the cyclic axial stress, and the radius of the fretting pad. In Model-II, one additional feature, namely the half contact-width, is considered. Firstly, three different normalization methods (Max-min, Zero-mean, and Nonlinear) are compared. It is found that the prediction of deep learning models based on the Zero-mean is the most accurate and stable. Then, through the study of sample size reliability, it is found that the prediction accuracy gradually increases initially then becomes stable with the increase of the sampling number. The estimated intervals also gradually become narrowed and then stable. Further, another highlight from the comparison between the performance of Model-I and Model-II is that with the physical-mechanical reasoning factor as additional input features, a more stable and accurate estimated crack initiation location can be obtained from deep learning models. The range of estimated intervals is also effectively narrowed. As an alternative to FEM, the computational efficiency of deep learning is at least 30% higher. The range of estimated intervals for crack initiation location are limited to 250 μm and the errors between predictions and target values are lower than 5%. Therefore, the deep learning method proposed in this paper can effectively and efficiently predict fretting fatigue crack initiation location.

Effect of thermal barrier coatings on the fatigue behavior of a single crystal nickel-based superalloy: Mechanism and lifetime modeling

Coatings are typically used to protect engineering materials from oxidation, corrosion, and erosion. However, it also changes the surface condition of substrate materials and causes a mechanical mismatch between the substrate and coatings. This study investigated the effect of thermal barrier coatings (TBCs) on the fatigue behavior of a second-generation single crystal (SX) Ni-based superalloy. The experimental results showed that the impact of the TBCs on the fatigue lifetime was loading-dependent. The coating was beneficial under low stresses but had no effect under high stresses. The scanning electron microscope (SEM) observation showed that the TBCs did not change the fatigue crack initiation location. For most coated and uncoated samples, the cracks are more likely to generate from the internal defects in the substrate. In addition, an equivalent strain energy density-based lifetime prediction model was proposed for coated samples. The prediction capability of the proposed model agreed well with experimental results. This study could provide insight into the failure mechanism for TBCs-coated single-crystal superalloys.

Significant improvement in fatigue life of titanium alloy induced by superlattice coating

The purpose of this paper is to investigate the effect of the TiN coating and the TiN/AlN superlattice coating on the fatigue performance of titanium alloys. The effect of the coating on the fatigue property of the titanium alloy is studied through the coating-substrate interaction. The results show that the TiN/AlN superlattice coating with high hardness and fracture toughness inhibits strain localization on the surface, changes the location of fatigue crack initiation and improves the fatigue strength at 107 cycles by 23.7%. Dislocation redistribution and “secondary slip” are found in the titanium alloy to eliminate stress concentration.

A Comparison of Two Methods Modeling High-Temperature Fatigue Crack Initiation in Ferrite–Pearlite Steel

High-temperature fatigue tests are carried out to investigate the fatigue crack initiation behavior of ferrite–pearlite steel. Two approaches to modeling fatigue crack initiation under high temperatures in ferrite–pearlite steel are developed and compared in this study. In the first approach, basal energy is introduced as the criterion for crack initiation. Various initial basal energy values are assigned to each potential location. The increments in basal energy after each cycle can be calculated. In the second approach, a crystal plasticity model is developed, combined with the cyclic hardening/softening effect and three fatigue indicator parameters (two of them are adopted from previous works and one is developed in this work). Results indicate that both approaches are capable of predicting the fatigue crack initiation location and the corresponding life. The first approach requires a lower computational cost but has many limitations, and the second approach is suitable for calculating mesoscale stress and strain distributions but with a higher computational cost.

Effect of wear debris on fretting fatigue crack initiation

Both wear and fatigue occur in fretting condition, and they interact with one another during the whole process. Fretting fatigue is commonly analysed without considering the effect of wear in partial slip regime, although wear affects the lifetime of crack initiation. This paper investigates, for the first time, the effect of wear debris on fretting fatigue crack initiation. To investigate the effect of debris, first fretting wear characteristics in partial slip regime are analysed for loading conditions. Then, the effect of wear on fretting fatigue crack initiation is investigated using Ruiz parameters and critical plane methods without considering the debris effect. Through the results, we can see that loading conditions affect the wear profiles in different ways. Moreover, wear has a significant effect on the fatigue in partial slip regime without considering debris especially on the crack initiation location. Finally, considering wear debris in the analysis, its effect on critical plane parameters is investigated. It is found that by considering the wear debris effect, the fretting fatigue crack initiation location is shifted towards the trailing edge. The predictions of both crack initiation location and lifetime show a good agreement with the experimental data.

The high-cycle fatigue properties of selective laser melted Inconel 718 at room and elevated temperatures

Aero-engine turbine disk is always subjected to cyclic loading at room and elevated temperatures. Therefore, as one of the typical high-temperature materials, selective laser melted Inconel 718 superalloy was chosen to perform high-cycle fatigue tests at 25 °C, 450 °C and 650 °C and investigate the effect of temperature on fatigue damage mechanism. The locations of fatigue crack initiation are surface and subsurface defects at room temperature, and become subsurface and internal facets at elevated temperatures. An analyzed stress intensity factor and El-Haddad model were performed to assess the effect of defects on fatigue performance at room temperature. By means of electron back scattered diffraction, the fatigue damage mechanisms at elevated temperatures were elucidated. Based on the general relationship between fatigue and tensile strengths of metallic materials, an accurately quantitative relationship among fatigue strength and temperature was proposed.

Effects of micro-scale corrosion damage features and local microstructure on fatigue crack initiation location

The fatigue resistance of aerospace structures is degraded by the presence of corrosion damage. This work characterizes the micro-scale (<50 µm) corrosion morphologies and the underlying microstructure of AA7050 that impact fatigue crack formation. Corroded specimens were cycled to failure using a fracture surface marking spectrum. Pre-test corrosion characterization, fractography and EBSD were performed. No corrosion damage or microstructure metric independently correlates to the location of fatigue crack initiation from corroded surfaces. However, there is a modest correlation between the grain that initiates the fatigue crack and grains that have a high misorientation angle with respect to adjacent grains.

A multiaxial fatigue life prediction method for metallic material under combined random vibration loading and mean stress loading in the frequency domain

A spectral method for the determination of the critical plane is proposed for metallic material under random vibration loadings based on the criterion of maximum shear stress amplitude, where the direction of critical plane is obtained by solving the maximum value of a function using mathematical optimization method and the fatigue critical location of the structure can be determined. A multiaxial fatigue damage parameter is proposed for combined random vibration loading and mean stress loading cases based on Susmel’s criterion, which is a Power Spectral Density (PSD) of the equivalent shear stress and can be adopted for fatigue life evaluation. The fatigue life estimation of structures under multiaxial random vibration loadings with mean stresses in the frequency domain could be carried out based on this damage parameter constructed at the fatigue critical location with the spectral method for random vibration fatigue life. The multiaxial random vibration fatigue experiment with mean stress loadings is conducted on the notched plate specimens made of 2297-T87 aviation grade aluminum alloy, and the initiation sites of the fatigue cracks as well as the experimental fatigue lives are compared with those calculated by the proposed models, where satisfactory prediction capabilities on both the fatigue crack initiation locations and fatigue lives of the models are demonstrated.

Effect of Laser Shock Peening on Fretting Fatigue Life of TC11 Titanium Alloy.

The purpose of this paper is to investigate the performance of laser shock peening (LSP) subjected to fretting fatigue with TC11 titanium alloy specimens and pads. Three laser power densities (3.2 GW/cm2, 4.8 GW/cm2 and 6.4 GW/cm2) of LSP were chosen and tested using manufactured fretting fatigue apparatus. The experimental results show that the LSP surface treatment significantly improves the fretting fatigue lives of the fretting specimens, and the fretting fatigue life increases most when the laser power density is 4.8 GW/cm2. It is also found that with the increase of the laser power density, the fatigue crack initiation location tends to move from the surface to the interior of the specimen.

Effect of cutting process on the residual stress and fatigue life of the welded joint treated by ultrasonic impact treatment

To obtain the final shape and ensure quality in the manufacturing of welded structural components, a cutting process is usually applied to remove the weld edges. Comparative tests were conducted with replicas of non-load-carrying cruciform joints in the as-welded and ultrasonic impact peened conditions, which were before or after cutting process at the two ends. Those contained the measurement of three types of residual stress distributions of various weld lengths by the cut-release method and relevant fatigue tests. The locations of fatigue crack initiation and fracture surfaces were also determined. The experimental results demonstrated that in the welded joint peened by the ultrasonic impact treatment, the welding residual stress component among the superimposed residual stresses relaxes after cutting. Owing to the original compressive welding residual stress in the peened welded joint, the cutting process has a distinct effect on the change of the superimposed residual stress distribution near the cutting edges. Furthermore, the cutting process reduces the fatigue life improvement of the peened welded joint and transfers the positions of the fatigue crack source. For the fatigue life improvement of welded structures, arranging ultrasonic impact treatment after the cutting process is better. Besides, the compressive superimposed residual stress in the UIT-welded joint will not relax completely under cyclic fatigue loading.

Direct comparison of microstructure-sensitive fatigue crack initiation via crystal plasticity simulations and in situ high-energy X-ray experiments

The qualification of engineering materials requires extensive testing of the time-dependent performance of the material, namely the fatigue behavior. Model-based approaches for determining the fatigue behavior have presented a tangible step towards complementing and reducing the overall number of physical tests necessary to qualify a material for use in application. Yet, prior to the adoption of the model-based approaches, the model needs to be thoroughly validated, presenting challenges, including substantiating the model's ability to capture the correct physics for crack initiation, which is difficult provided the multiple length-scales of the problem. In this paper, a methodology and demonstration for validating the location of microstructure-sensitive fatigue crack initiation as predicted by crystal plasticity finite element (CPFE) simulations, using high-energy X-ray diffraction and tomography experiments are presented. Realistic 3D microstructural models are created for the material of interest, IN718 (produced via additive manufacturing), with different twin instantiations, based on the experimental data for use in the CPFE simulations. The location of failure predicted using the extreme values of failure metrics (plastic strain accumulation and plastic strain energy density) resulted in an unambiguous one-to-one correlation with the experimentally observed location of crack-initiation for the models with statistical twin instantiations.

Surface Integrity of TA19 Notched Simulated Blades with Laser Shock Peening and Its Effect on Fatigue Strength

To reveal the fatigue strengthening mechanism of TA19 notched simulated blades with laser shock peening (LSP), surface integrity and fatigue strength were investigated. The surface morphology, residual stress, near-surface microstructure, fatigue strength and fatigue fracture morphology were analyzed by surface profiler, x-ray diffraction (XRD), transmission electron microscopy (TEM), QRG-100 servo-hydraulic fatigue test machine and scanning electron microscope (SEM). Results indicated that LSP induced surface micro-dents plastic deformations with a few microns in depth, surface compressive residual stresses and surface nano-grains of TA19 notched simulated blades. Compared with the as-received material, fatigue strengths of TA19 notched simulated blades were improved by 162% for LSP-1 and 218% for LSP-3. In addition, fatigue crack initiation (FCI) locations reduced after LSP, fatigue crack growth (FCG) rate decreased after LSP, and secondary cracks were observed in the FCI and FCG regions of fatigue fracture morphologies after LSP. Fatigue strengthening mechanism of TA19 notched simulated blades with LSP was compressive residual stresses and refined grains. Results could be beneficial to the application of LSP process in civil engines blades.

Fatigue failure and optimization of double-sided weld in orthotropic steel bridge decks

Double-sided weld is an effective way to improve the fatigue performance of orthotropic steel bridge decks (OSBD). However, the fatigue performance and optimization design methods of double-sided welds connecting the U-rib and the deck in OSBD require further study. In this paper, the finite element modeling method suitable for the notch stress approach in double-sided welds is discussed, and its variance from that of the single-sided weld is revealed. All possible fatigue failure modes of double-sided welds, the transfer phenomenon of fatigue crack initiation location and the change of fatigue failure modes under different vehicle loading conditions are revealed. It is further discussed that how the change of the deck plate thickness and the weld foot length alters the fatigue failure mode under the same loading condition. According to the above analysis, in order to obtain the optimal fatigue performance of double-sided welds, an optimal design method based on the fatigue damage factor per unit time under real vehicle loading conditions is proposed. The analysis results show that the fatigue failure modes of the double-sided welds of the U-rib-to-deck welded joint in the OSBD is determined by three factors: loading condition, deck plate thickness and weld foot length. According to the optimization factor proposed in this paper, the thickness of the deck plate and the length of the weld foot can be reasonably determined under different traffic loading conditions, so that the fatigue performance of the rib-to-deck joint in the OSBD is optimized.

Investigation of the fatigue life of AISI304 stainless steel round rod with small hole subjected to laser shot peening

The fatigue properties of AISI304 stainless steel round rod with small hole subjected to laser shot peening (LSP) were investigated. Laser shot peening was performed for rods with and without sacrificial layer, respectively. After laser treatment, the surface morphologies of peened and unpeened regions were observed, and residual stress was measured by the X-ray diffraction method. Fatigue test was employed to conduct fatigue tension test on laser-treated and untreated rods. The fracture micro-topography was detected using a scanning electron microscope. The results show that micro-dents appear on the surface of the peened rod coated with sacrificial layer. The surface of the rod irradiated directly by laser is ablated, and tensile residual stresses are generated on the irradiated region. The fatigue lives of the laser-treated specimens with and without sacrificial layer are 1.2 and 1.1 times, respectively, compared with that of the laser-untreated specimen. The location of fatigue crack initiation is transformed from the hole edge to the wall of rod after LSP, and the average fatigue striation spacing of the processed rod with sacrificial overlay is narrowest, and that of the unprocessed rod is widest. Furthermore, the size and depth of the dimples on the fatigue fracture region of the laser-peened rod with sacrificial overlay are largest and deepest, which is closely related to the severe plastic deformation in the samples with LSP.