Fatigue Initiation Life Research Articles

Mathematical modeling of the VHCF resonance loadings with a progressive damage accumulation

One of the most critical in-situ loading modes is resonant. Such loading conditions may lead to premature fatigue damage accumulation and failure. The progressive fatigue damage changes the natural frequency of a structure and causes a permanent modification of loading conditions. This paper considers the model of fatigue damage accumulation regarding its evolution and resonant loading parameters. To describe the evolution of material degradation due to fatigue a complex kinetic model of damage development and numerical algorithm are introduced. The model contains the evolution equation for the damage function depending on stress state. The algorithm is based on a special post proceeding of a numerical solution for an elastic solid mechanics problem that can be also applied for elastic-plastic case. Different loading conditions can be considered by choosing the method of solution for solid mechanics problem. This paper is focused on the comparison between quasi-static and resonant loading conditions used for damage function calculation. It was shown that use of resonant loading solution leads an earlier crack initiation and shorter fatigue life results compared to quasi-static solution. A spatial development of the fatigue damage zone is also different for these two cases. The normal crack opening mechanism is discussing in this paper.

Fatigue Life Prediction of Welded Girders with Flange Thickness Transitions and Cope Hole

AbstractThe paper presents an analytical and numerical approach for the fatigue life prediction using local concepts. A Two‐Stage‐Model is applied to a welded girder with thickness transition and cope hole to determine its fatigue life. The model follows the distinction between crack initiation and crack propagation according to the phenomenological background. Using the Strain‐Life‐Approach and the Crack Propagation Approach the local geometry and likely failure position influence the determined fatigue life for the initiation and propagation phase individually. The results are presented in terms of S‐N curves for initiation, propagation, and total fatigue life.

Evaluation of different drilling quality and fatigue life prediction of nickel-based superalloy using equivalent initial flaw size

The initial quality assessment and total fatigue life prediction of film cooling holes under different drilling processes have great impact on key structural components. In this paper, the initial fatigue quality of nickel-based superalloys with different drilling processes were evaluated and the fatigue life was predicted based on the equivalent initial flaw size (EIFS) theory. We raised the modified K-T diagram method based on intrinsic stress intensity factor threshold considering the small crack data. The fatigue crack growth test at room temperature is carried out on the single edge notched specimens made of nickel-based superalloy GH4169 under two different drilling processes (HEDM and FEDM) to quantitatively evaluate EIFS values. Then, a new expression of fatigue crack growth rate is proposed based on EIFS. Finally, the total fatigue life within 2 times error band is predicted by combining EIFS and fatigue crack growth rate, which confirms the validity of fatigue models and tests.

Buckling fatigue behavior of 2A97 Al-Li alloy stiffened panels under shear loading

In order to study buckling fatigue behavior and failure modes of 2A97 Al-Li alloy stiffened panels under shear loading, 1.2 mm and 1.5 mm thick 2A97 Al-Li alloy stiffened panels were designed and manufactured. The static and fatigue tests were performed under shear loading. The results indicate that the shear failure modes of stiffened panels are global buckling wave, skin cracks and rivets falling off. The effect of skin thickness on the buckling load is greater than that on the ultimate load. The fatigue failure processes of stiffened panels include gradual failure and catastrophic failure. The fatigue failure modes are complex and rivet falling off is most dangerous. Panels with cracks or broken rivets still has residual life. Once the skin tears, the panel will immediately fail. The total fatigue life and initial fatigue life of 1.5 mm stiffened panel are 28.87% and 59.88% higher than those of 1.2 mm stiffened panel respectively, while the residual life is 87.68% lower. The finite element models were established to simulate the buckling and post-buckling process of the stiffened panels, and the local stress spectrum of the fatigue dangerous point was obtained. Goodman's theory and nominal stress method was combined to predict the fatigue life and to explain the experimental findings.

Study on the Resonant Torsion Vibration in Hourglass Specimens under VHCF Loading

The problem of cyclic loading under pure torsion is an important problem for numerous different industrial applications, such as shafts, springs, blades, and others. The loading conditions for aircraft, railway, automobile industries are often corresponding to long fatigue life. The rough estimation shows that the number of cycles for springs in car engine is exceeds 108 cycle while blades for aircraft turbine may undergo more than 1010 loading cycles. Such, to provide a safety in service life for such elements a multiaxial fatigue failure criterion is needed to predict damage zone location and fatigue life. This paper is aimed to introduce the multi regime approach to predict fatigue damage zone evaluation and number of cycles to crack initiation and total fatigue life. The presented approach is based on damage theory supplemented by multiaxial failure criterion. The proposed model includes the algorithm for automatic determination the fatigue loading range (low-, high- or very-high cycle fatigue) and allows different crack opening modes. To show how to apply the proposed approach and verify the results of calculation the series of numerical simulation on hourglass specimen subjected to VHCF pure torsion is performed. The results of numerical simulation are compared to VHCF torsion tests.

Digital Twin-driven framework for fatigue life prediction of steel bridges using a probabilistic multiscale model: Application to segmental orthotropic steel deck specimen

Accurate fatigue life prediction facilitates the fatigue maintenance of steel bridges. Since Digital Twin can simulate the lifecycle for physical objects at various scales, this study aims to provide a Digital Twin-driven framework for non-deterministic fatigue life prediction of steel bridges. A probabilistic multiscale model was developed to depict the fatigue evolution throughout the bridge lifecycle. The small crack initiation period was well described by the modified Fine and Bhat model considering microstructure uncertainties. After obtaining the critical model parameter via crystal plastic finite element simulation, the modified model was further calibrated using the assumed historical fatigue data in Digital Twin database. Based on the initiated half-penny-shaped small crack, the small crack initiation period was connected to the macrocrack extension period. Given the uncertainties of macrocrack propagation, the Paris’ law with random growth parameters was adopted. The Bayesian inference of the growth parameters realized the real-time calibration of the macrocrack growth model using Markov chain Monte Carlo simulation. The feasibility of the proposed framework was demonstrated through fatigue tests on a segmental steel deck specimen with mixed-mode deformed U-rib to diaphragm welded joints. The results show that the predicted fatigue initiation life and residual fatigue life are in good agreement with the experimentally observed life results. In summary, the proposed framework enhances our understanding of the fatigue evolution mechanism throughout the bridge lifecycle and provides an entirely new approach to accurately predict the fatigue life of steel bridges under various sources of uncertainties.

Low Cycle Fatigue Life Assessment Based on the Accumulated Plastic Strain Energy Density.

The accumulated plastic strain energy density at a dangerous point is studied to estimate the low cycle fatigue life that is composed of fatigue initiation life and fatigue crack propagation life. The modified Ramberg–Osgood constitutive relation is applied to characterize the stress–strain relationship of the strain-hardening material. The plastic strain energy density under uni-axial tension and cyclic load are derived, which are used as threshold and reference values, respectively. Then, a framework to assess the lives of fatigue initiation and fatigue crack propagation by accumulated plastic strain energy density is proposed. Finally, this method is applied to two types of aluminum alloy, LC9 and LY12 for low-cycle fatigue, and agreed well with the experiments.

Residual Stress Distribution Design for Gear Surfaces Based on Genetic Algorithm Optimization

The rolling contact fatigue of gear surfaces in a heavy loader gearbox is investigated under various working conditions using the critical plane-based multiaxial Fatemi–Socie criterion. The mechanism for residual stress to increase the fatigue initiation life is that the compressive residual stress has a negative normal component on the critical plane. Based on this mechanism, the genetic algorithm is used to search the optimum residual stress distribution that can maximize the fatigue initiation life for a wide range of working conditions. The optimum residual stress distribution is more effective in increasing the fatigue initiation life when the friction coefficient is larger than its critical value, above which the fatigue initiation moves from the subsurface to the surface. Finally, the effect on the fatigue initiation life when the residual stress distribution deviates from the optimum distribution is analyzed. A sound physical explanation for this effect is provided. This yields a useful guideline to design the residual stress distribution.

Influence of local residual stresses on fatigue crack initiation

The aim of the paper is to investigate the influence of compressive residual stresses on the fatigue initiation life (S-Nini) curve of AISI 1141 steel. The analysis has been performed by means of a micro-structurally-based fatigue crack initiation modelling approach. The focus is set on the local (surface) residual stresses typically imposed by a surface treatment technique known as shot -peening. By superimposing such stresses on stresses resulting from outer cyclic loading, it could be shown that an impact on the fatigue curve in the S-Nini diagram can be achieved.

Initiation and Propagation Characteristics of Multi-Source Fatigue Crack in Superalloy Disk

The multi-source fatigue crack growth characteristics of a turbine disk were studied via high-speed rotating low cycle fatigue test, fracture analysis and crack growth simulation analysis. The results show that the fatigue initiation life N0 of Superalloy disk and its simulation parts is less dispersive and similar, and the S-N0 curve of the material is not conservative to predict the N0 of the disk. The fracture of eccentric hole of disk is multi-source fatigue fracture, and the surface characteristic of the crack L/D decreases rapidly from 7.5 to 2, which is obviously different from the single source fatigue crack growth. Because of the long and thin stress distribution at the eccentric hole and the small dispersivity of GH698 crack initiation life, it's relatively easy to crack on the plane A of the eccentric hole, which is prone to multi-source fatigue. At the stage of a ≥ 1.23 mm, the reverse life of crack propagation fracture of 2# and 5# eccentric holes is consistent, and the error between the simulation value of single source fatigue crack propagation life and the reverse life is below 5%. In the stage of 0.61 mm ≤ a < 1.23 mm, the simulation crack growth rate of 5# eccentric hole of multi-source is close to that of fracture inversion. Accurate simulation analysis of multi-source crack growth needs to define the initial crack accurately, at the same time, consider the influence of existing fatigue damage on crack growth.

Crack initiation mechanism and fatigue life of titanium alloy Ti–6Al–2Sn–2Zr–3Mo-X: Effects of stress ratio and loading frequency

In this paper, the fatigue behavior of Ti–6Al–2Sn–2Zr–3Mo-X alloy with basketweave microstructure is investigated at stress ratios R = −1, −0.5, 0.1 and 0.3 and loading frequencies f = 0.4 Hz, 4 Hz and 35 Hz. It is shown that the Ti–6Al–2Sn–2Zr–3Mo-X alloy presents two different crack initiation mechanisms. One is due to the cleavage of α grains and the other is due to the microstructure inhomogeneity and deformation incompatibility. For the former case, the fracture surface exhibits facet feature in the crack initiation region. For the latter case, the crack nucleates and initiates at α grains or interfaces and the fracture surface presents no facet feature in the crack initiation region. The higher stress ratio and the lower frequency tend to induce the higher probability for the occurrence of multiple crack initiation mode. Moreover, the paper indicates that the effect of stress ratio on fatigue life could be expressed as Nf = A [(1–R)/2]lσma,R, where A, l and m are material parameters. The scatter of fatigue life is related to both the stress amplitude and the loading frequency. For relative higher stress amplitude, the fatigue life and the frequency at R = −1 could be correlated by a linear relation in log-log scale.

New Crack Arresting Methodology Using Structural Optimizing Technique. (Dept. M. ( Production ) )

The stop-hole method is a simple and economic repair technique widely used to retard or even to stop the propagation of a fatigue crack in structural components that cannot be replaced immediately after the detection of the crack. Its principle is to drill a circular hole at or close to the crack tip to transform the crack into a notch, reducing in this way its stress concentration effect. In the present study, the stop-hole method was investigated with creating a non circular hole. The aim of the present work is to obtain an optimum stop hole shape that gives maximum fatigue crack initiation life. Fatigue crack initiation life depends at least on the crack size and on the hole diameter. An optimization technique had been used and a finite element program had been built to find this optimum shape of stop hole. It was found that the fatigue life obtained by using the optimum hole shape ranges from 2 to 9 times the fatigue life obtained by using the circular holes depending on the initial hole size and initial crack length. The effect of this optimum hole shape on crack reinitiating life for different specimen geometries has been studied. It was found that the optimum hole shape increased the initial fatigue life for all specimens used whatever its geometry was. A global optimum hole shape (which is practically suitable for all geometries ) is defined. Global optimum hole shape helps engineers to use this global hole shape directly in practice without carrying out any calculation much like the use of the stop hole size found in the literature. Opposite to the traditional stress concentration factor minimization problem where the nominal area remains constant during optimization, here it is allowed to vary nominal area using design variables resulting in decreasing of the nominal stress in addition to decreasing of the stress concentration factor. This leads to higher fatigue life compared to previous studies.

Rapid initiation and growth life characterization of additively manufactured alloy 718 through compliance monitoring

Additive manufacturing offers significant design versatility which can be leveraged for complex geometries and microstructural tailoring through the manipulation of numerous process parameters. Structurally critical components require significant material characterization and qualification for each material iteration. Traditional methods for characterizing fatigue performance of materials is expensive and time consuming, so new test methods to rapidly quantify important fatigue characteristics and behavior is necessary to supplement standard data and increase the production of fatigue data for advanced alloys. Toward this end, specimen compliance is initially investigated to detect crack initiation in AM components under fatigue loading. Initial results indicate that this method detects crack initiation earlier and cheaper than optical or thermal imaging methods. The observations made in this investigation provide valuable insight on the dependence of fatigue initiation and growth life on applied stress and pore size which is critical for designing for AM components.

Low cycle fatigue damage model and sensitivity analysis of fatigue crack initiation by finite element approach

To meet the design requirements, different types of defects are often machined on the surface of fatigue components. Local stress concentration formed at the notch accelerate the initiation of fatigue crack, therefore greatly shorten the service lives of such components. Based on the theory of continuous damage mechanics and the principle of irreversible thermodynamics, the damage evolution model of low cycle fatigue is investigated. By programming the damage evolution model as UMAT subroutine and coupling it to ABAQUS, the fatigue damage and crack initiation life of notched P92 steel samples under specific loads are simulated, and the crack initiation location is determined. Furthermore, the damage evolution and crack initiation sensitivity of notch morphology are considered. The results show that the crack initiation occurs easily in the notch root where the damage is greatest and the plastic strain accumulates fastest under cyclic loading. The fatigue damage accumulates slowly at the initial stage, but the damage accumulates rapidly after the cumulative damage reaches a critical value. The fatigue damage evolution and fatigue initiation life are very sensitive to the notch morphology parameters. The notch morphology need to be carefully analyzed, to improve the fatigue life of the notched samples.

Experimental investigation of fatigue behavior for adhesively-bonded GFRP/steel joints

The fatigue behavior of an adhesively bonded glass fiber-reinforced polymer (GFRP)/steel double-lap joint (DLJ) under cyclic tensile loading with load ratio R = 0.1 was experimentally studied. Results in terms of failure mode, F-N curve, stiffness degradation law, crack initiation and propagation laws were obtained. Linear and nonlinear models of stiffness degradation and a mathematical model of the crack growth rate were developed for the adhesive GFRP/steel DLJ under fatigue loading. The fatigue performance of adhesive GFRP/steel DLJ was compared with that of adhesive GFRP/GFRP DLJ to identify the differences between these behaviors. The results demonstrated that the stiffness degradation law of the adhesive GFRP/steel DLJ can be objectively and accurately characterized by the nonlinear stiffness degradation model in terms of the equivalent shear stiffness. The crack initiation life and the ratio between the crack initiation life and fatigue life increased with the deceasing maximum applied load. The fatigue life of the GFRP/steel DLJ was more sensitive to the variation in the maximum applied load compared with that of the GFRP/GFRP DLJ. Moreover, the decaying rate of stiffness and the interfacial crack propagation rate for the GFRP/steel DLJ under fatigue loading were larger than those for the GFRP/GFRP DLJ.

Crack Initiation and Propagation Fatigue Life of Ultra High-Strength Steel Butt Joints

The division of the total fatigue life into different stages such as crack initiation and propagation is an important issue in regard to an improved fatigue assessment especially for high-strength welded joints. The transition between these stages is fluent, whereas the threshold between the two phases is referred to as technical crack initiation. This work presents a procedure to track crack initiation and propagation during fatigue tests of ultra high-strength steel welded joints. The method utilizes digital image correlation to calculate a distortion field of the specimens’ surface enabling the identification and measurement of cracks along the weld toe arising during the fatigue test. Hence, technical crack initiation of each specimen can be derived. An evaluation for ten ultra high-strength steel butt joints reveals, that for this superior strength steel grade more than 50% of fatigue life is spent up to a crack depth of 0.5 mm, which can be defined as initial crack. Furthermore, a notch-stress based fatigue assessment of these specimens considering the actual weld topography and crack initiation and propagation phase is performed. The results point out that two phase models considering both phases enable an increased accuracy of service life assessment.

The relationship between microstructure, stiffness and compressive fatigue life of equine subchondral bone

Subchondral bone injuries often precede articular cartilage damage in osteoarthritis and are common in thoroughbred racehorses due to the accumulation of fatigue damage from high speed racing and training. Thus, racehorses provide a model to investigate the role of subchondral bone in joint disease. We assessed the association of horse and racing related factors and micro-CT based micromorphology of three separate subchondral bone layers with the initial stiffness and compressive fatigue life of bone plugs. Furthermore, we investigated three different definitions of fatigue failure of subchondral bone during compressive fatigue testing. Initial stiffness was 2,362 ± 443 MPa (mean ± standard deviation). Median compressive fatigue life during cyclic loading to -78 MPa was 16,879 (range 210 to 57,064). Subchondral bone stiffness increased over a median of 24% (range 3%–42%) of fatigue life to a maximum of 3,614 ± 635 MPa. Compressive fatigue life was positively associated with bone volume fraction in the deeper layers of subchondral bone, maximal stiffness, and the number of cycles to maximal stiffness. Initial stiffness was positively associated with tissue mineral density in the deeper layers and bone volume fraction in the superficial layer. Most specimens with a fatigue life of less than 5,500 cycles fractured grossly before reaching 30% reduction of maximal stiffness. Cycles to 10% reduction of maximal stiffness correlated strongly with cycles to lowest recorded stiffness at gross fracture and thus is a valid alternative failure definition for compressive fatigue testing of subchondral bone. Our results show that subchondral bone sclerosis as a result of high speed exercise and measured as bone volume fraction is positively associated with compressive fatigue life and thus has a protective effect on subchondral bone. Further research is required to reconcile this finding with the common collocation of fatigue damage in sclerotic subchondral bone of racehorses.

The effect of a critical micro-void defect on fretting fatigue crack initiation in heterogeneous material using a multiscale approach

Abstract Fretting fatigue problems are common in the aerospace and automotive industries, which can seriously affect the fatigue response of the material and the stability of the overall structure. In this paper, the effect of the size, location of single critical micro-void on the fretting fatigue initiation life of heterogeneous materials is studied by numerical simulations and modified critical plane criterion. The theory of critical distance (TCD) is used to consider stress gradients near the crack initiation points. It is shown that when the critical micro-void is located at different positions inside the sample, there is a weakened area similar to the half butterfly wings. This means that when the micro-voids are located in this area, the crack initiation life will be less than that in the case of homogenous material. Furthermore, there are also two enhancement zones. When the micro-voids are located in these two zones, the fretting fatigue life can be increased, that is, at a certain depth below the contact edge and at a certain distance to the right of the contact edge.

Aggregate gradation effect on the fatigue performance of recycled asphalt mixtures

This paper presents the details of a laboratory study on the evaluation of the fatigue characteristics of asphalt mixtures containing Recycled Asphalt Pavement (RAP) material using controlled stress indirect tensile fatigue test (ITFT). The effect of different aggregate gradations on the fatigue performance of RAP mixes is the main focus of the study as this helps us to identify the proportion of RAP that can be used from fatigue performance consideration. Two dense mixes and three gap graded mixes, both with different percentages of RAP, were investigated. VG 30 viscosity grade virgin binder was used. The fatigue test was conducted at 25°C. Laboratory fatigue life determined under stress controlled mode increased with increase in the RAP content for all the five aggregate gradations. The corresponding ‘Initial strain vs fatigue life’ trends suggest a decrease (for any specific strain level) in the fatigue life with an increase in the RAP content. However, analysis of the fatigue performance of typical pavement sections with different RAP contents do not suggest any significant detrimental effect of increasing the RAP content for pavements with thick bituminous layers. Use of open graded mixes permits the use of larger binder contents and higher RAP contents in the mix. Fatigue lives of RAP mixes are longer for gap graded mixes compared to dense graded mixes. The fatigue lives of RAP mixes are found to correlate very well with fatigue parameters of the virgin-RAP binder blends prepared in suitable proportions.

Application of critical distances to fatigue at pores

AbstractThis work applies Taylor's theory of critical distance to quantify the effect of defects on fatigue initiation in an additively manufactured metal. We focus on hollow pores that are ideal spherical, prolate, and oblate spheroids isolated in an otherwise homogeneous linear‐elastic material. These conditions support the development of exact solutions using the exterior Eshelby tensor for a pore in a remote, arbitrary stress field. For spheres, this solution process admits simple closed‐form solutions for principal stresses disturbed by the pore. For prolate and oblate spheroids, we present the solutions as graphical curves showing stress variations under uniaxial tension. This report then extends the analysis to determine the effect of defects on a parametric, power‐law stress range vs fatigue life model. By propagating the distributed stress fields through this model, this study demonstrates the effect of pore size, pore shape, stress, and parametric fatigue properties on the life reduction due to porosity. These results suggest several approaches to increasing fatigue lives in porous materials, eg, reducing the pore size, promoting spherical pores, and increasing the microstructural parameter (comparable to the El Haddad parameter). Results presented in this work may be useful to inform trends of fatigue strength and fatigue initiation lives in metallic alloys with limited porosity, eg, additively manufactured materials that have been HIP'ed.