Low Cycle Fatigue Crack Initiation Research Articles

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Initiation and growth of fatigue cracks in sheets with U-shaped notches in the first and mixed modes of fracture

Evaluations and enhancements of fatigue crack initiation criteria for steels subjected to large shear deformations

While large accumulated plastic deformations occur in the rail surface layer where rolling contact fatigue cracks initiate, many available Low Cycle Fatigue (LCF) crack initiation criteria focus on small plastic strains. Accordingly, this paper evaluates available fatigue crack initiation criteria for highly shear-deformed R260 steels, reflecting the conditions in the surface layer of rails. Furthermore, modified crack initiation criteria are suggested. The evaluation is based on three different experiments: Large shear strain increments under varying axial loading (predeformation), strain-controlled LCF tests after some predeformation, and axial High Cycle Fatigue (HCF) experiments. For the predeformation, Finite Element (FE) simulations, with a large-strain plasticity model for cyclic and distortional hardening, provide predictions of the local stress and strain histories. A cross-validation procedure is used to assess the accuracy and reliability of both established and modified fatigue crack initiation criteria. The proposed modifications to one of the criteria show an improved fit to the experimental data. However, there is a tendency to overfitting, which can be improved by including more experimental data.

Low cycle fatigue behavior and crack initiation mechanism of Ni-based single crystal curved thin-walled blade simulator specimen with film cooling holes

A novel curved thin-walled blade simulator (CTWBS) specimens was used to investigate low cycle fatigue (LCF) behavior and crack initiation mechanism nearby film cooling holes (FCHs) in Ni-based single crystal superalloy. LCF tests were conducted under two loading stress levels at 700 °C and 1000 °C. Experimental results demonstrated that LCF lifetime reduced as the fatigue load increased at both temperatures. Crack modes were characterized on macroscopic fracture paths, and the analysis of fatigue crack early propagation revealed its microscopic mechanism. The fracture morphology and microstructure evolution also showed temperature dependence, and oxidation played a role at high temperature. The finite element calculation was based on the crystal plasticity theory considering the back stress and slip damage. The resolved shear stress (RSS) distribution law under two temperatures led to the identification of the octahedral slip system activation type respectively. In addition, the resulting fatigue damage increased in direct proportion to the slip systems activated numbers locally surrounding the FCH. The damage distribution around the FCH was in good agreement with the experimental observation. Finally, the temperature dependence of LCF crack initiation mechanism was discussed from three perspectives: crack nucleation around the FCHs, microstructure evolution and dislocation motion near crack.

Microstructural statistics for low-cycle fatigue crack initiation in α+β titanium alloys: A microstructure based RVE assessment

(0001) twist grain boundaries (BTGB) have been identified as critical microstructure configurations regarding fatigue crack initiation in titanium alloys. In the presently reported study, Ti-Al-V based alloys with different microstructures were tested in the low-cycle fatigue regime. Early cracking occurred at BTGB for all investigated alloys and microstructures. Microstructural statistics collected at crack initiation sites revealed a weak sensitivity to moderate differences in α and β stabilizers content and microstructural features. Criteria for the identification of crack initiation sites were then defined using this dataset. An automatic processing routine was applied on large-scale electron back-scattered diffraction maps to analyze the spatial distribution of BTGB susceptible to cracking. The low associated density implies that large microstructural regions, i.e., typically > 1 mm2, must be considered to include microstructural configurations prone to crack nucleation. It is likely to play a critical role in the high lifetime variability of Ti alloys.

A modified cyclic cohesive zone model for low-cycle fatigue crack initiation prediction for subsea pipelines under mode I loading

A modified cyclic cohesive zone model (CCZM) is developed for low-cycle fatigue crack initiation prediction for subsea pipelines. The traction and separation law in the model is modified by employing a piecewise function and linear softening for improving the computational convergence but maintaining the material nonlinearity. The validity of the modified model is evaluated with experimental results of two full-scale pipe specimens and also the results estimated by six published models. An engineering application is further presented to demonstrate the capability of the model and a parametric study is performed for investigating the impact of model parameters on model prediction.

Low Cycle Fatigue Crack Damage Behavior of TC21 Titanium Alloy with Basketweave Microstructure

Low cycle fatigue (LCF) crack initiation, propagation and damage behaviors of TC21 alloy with basketweave microstructure were investigated. The process of LCF damage was observed by a long-focus optical microscopic imaging system, and fatigue crack propagation was analyzed through in-situ SEM fatigue. The results indicated that LCF crack damage displayed different sensitivity to cyclic stress. LCF microcracks initiated from slip bands and propagated through the microcrack coalescences at high stress, while LCF cracks tended to initiate at the αL/β interface and connect with these interface microcracks. Furthermore, the LCF damage model was established on the basis of Lemaitre damage theory. When the maximum stress exceeded yield stress, LCF damage increased sharply and fatigue life decreased significantly, which agreed with experiment data.

Influence of microstructure on strain controlled low cycle fatigue crack initiation and propagation of Ti-55531 alloy

Low cycle fatigue (LCF) crack initiation and propagation behavior of Ti-55531 alloy with lamellar and bimodal microstructures were comparatively investigated at room temperature. Results indicated that the two microstructures displayed different sensitivity to the change of cyclic strain, which resulted in significantly different cyclic deformation, fatigue crack nucleation, and crack propagation behavior of two microstructures. Finally, it was found that cyclic lives of two microstructures were entirely different. Moreover, besides slips and deformation twins, stacking faults promoting cracking of αs/β interface may be another important LCF microcrack nucleation mechanism of Ti alloys, which hasn’t been revealed by other researchers.

A probabilistic model for low-cycle fatigue crack initiation under variable load cycles

A probabilistic low-cycle fatigue (LCF) model, ProbLCF-VA, is proposed in this work to describe the scatter of the LCF crack initiation life for homogeneous materials subjected to variable or random load cycles. This model can be implemented either to estimate the probabilistic cumulative damage for a given load history consisting of variable cyclic loads, or to predict the probabilistic LCF crack initiation if distributional estimates of a random load spectrum are available. More importantly, the proposed model is capable of describing certain prevailing effects such as load interaction and small cycle contributions, which are very commonly observed in fatigue tests with variable amplitude cyclic loads but can hardly be described by other models with physical and mathematical foundations. Model predictions are illustrated with multiple examples and compared to those from classical cumulative damage rules. The result shows a good qualitative match between the model prediction and test data in the literature on load interaction effects.

A physically-based method for predicting high temperature fatigue crack initiation in P91 welded steel

A methodology is presented for physically-based prediction of high temperature fatigue crack initiation in 9Cr steels, with a focus on material inhomogeneity due to welding-induced metallurgical transformations. A modified form of the Tanaka-Mura model for slip band formation under cyclically-softening conditions is implemented in conjunction with a physically-based unified cyclic viscoplasticity constitutive model. The physically-based constitutive model accounts for the key strengthening mechanisms, including precipitate hardening and hierarchical grain boundary strengthening, successfully predicting cyclic softening in 9Cr steels. A five-material, finite element model of a P91 cross-weld test specimen, calibrated using the physically-based yield strength and constitutive models, successfully predicts the measured detrimental effect of welding on high temperature low-cycle fatigue crack initiation for P91 cross weld tests, via the modified Tanaka-Mura model. A key finding of the current work is the requirement to adopt an energy-based Tanaka-Mura method to account for cyclic softening in 9Cr steels, with packet size as the critical length-scale for slip band formation. The work demonstrates the significant effect of highly flexible operation on the service life of welded connections.

A microstructure‐based homogenization model for predicting the low‐cycle fatigue initiation life of GH4169 superalloy

AbstractA microstructure‐based homogenization model is proposed for simulating the cyclic plasticity and predicting the low‐cycle fatigue (LCF) crack initiation life of GH4169 superalloy. Classical crystal plastic model (CPM) with a simple softening model is used at the grain level. Then, the transition from grain level to polycrystal level is based on the conservation of virtual work between the two levels. The Eshelby's formulation is applied in the model. Especially, local influences of grain interactions are considered by introducing the external Eshelby's tensor. Relatively precise macroresults and microresults as the finite element method can be provided by the present model with less computational cost. Grain volume averaged fatigue indicator parameters (FIPs) with considering the effect of inclusions are formed to predict the LCF crack initiation life, and a fold‐line fitting model is proposed to substitute for the cycle‐by‐cycle simulation. Predicted lives fit well with the experimental data for both the strain loading and stress loading simulations. Scatter of the life can also be predicted by the model and overwhelming influences of the incubation stage on the variability of LCF initiation life can be captured. It is shown that the inclusions and the inhomogeneous plastic strain are responsible for the scatter of the incubation stage.

Low cycle fatigue crack initiation in Ti-5Al-5Mo-5V-3Cr in relation to local crystallographic orientations

Low cycle fatigue crack initiation was investigated in a Ti-5Al-5Mo-5V-3Cr alloy with a bimodal microstructure. In particular, the crystallographic orientations of both α and β phases at crack initiation sites were characterized. Crack formation is shown to generally proceed along prismatic slip bands in primary α grains. A key role of the interplay between elastic and plastic anisotropies of both α and β phases was evidenced as the crystallographic orientation of the surrounding β matrix is a critical parameter as well. Different lengthscales have thus to be considered for an accurate prediction of fatigue crack initiation in near-β titanium alloys.

Low-cycle fatigue crack initiation and propagation from controlled surface imperfections in nuclear steels

The impact of a controlled surface imperfection on the low-cycle fatigue life in two different steels is investigated. After introduction of a single imperfection with a depth varying from 50 to 350 µm in cylindrical samples, fatigue tests were conducted under fully-reversed total axial strain control in air at ambient temperature. It is shown that the fatigue life is significantly reduced, even in presence of small imperfections. In addition, the use of the potential drop methods provided an assesment of the initiation life and the determination of crack growth rates. Two characteristic crack growth domains were thus identified.

Comparative analyses of the tensile and damage tolerance properties of tri-modal microstructure to widmanstätten and bimodal microstructures of TA15 titanium alloy

For a two-phase titanium alloy, the tensile and damage tolerance properties exhibited in tri-modal microstructure were investigated and compared to the same properties exhibited in widmanstätten and bimodal microstructures. These investigations and comparisons were conducted by analyzing the microstructure characteristics and their influences on deformation behavior and fracture features. The results show that the tri-modal microstructure exhibits superior strength and plasticity that are close to bimodal microstructure and remarkably higher than the widmanstätten microstructure. The underlying reason is that the multiple types of interfaces and refined grains of tri-modal and bimodal microstructures produce greater interface strengthening and better overall deformation homogeneity than the widmanstätten microstructure. Furthermore, the better deformation homogeneity also contributes to suppressing the low-cycle fatigue crack initiation of tri-modal microstructure. Besides, the unique lamellar α in tri-modal microstructure with disordered distributions on both geometric and crystallographic orientations effectively hinders the fatigue crack propagation. Thus, the low-cycle fatigue life (the number of reversals to failure) of tri-modal microstructure is the longest, which is 3.93 times as long as that of widmanstätten microstructure and 1.12 times as long as that of bimodal microstructure. For the fracture toughness, the fracture crack paths of bimodal and tri-modal microstructures with the small sized grains present far lower fluctuation and tortuosity compared to those of widmanstätten microstructure, which means lower energy consumption. However, the crack path of tri-modal microstructure is still more deflected than that of bimodal microstructure due to the contribution of the disorderly distributed lamellar α, which increases the energy consumption to some extent. Thus, the fracture toughness of tri-modal microstructure is lower than the widmanstätten microstructure but higher than the bimodal microstructure. These results indicate that tri-modal microstructure exhibits more excellently combined tensile and damage tolerance properties compared to the other two microstructures.

Low-cycle fatigue crack initiation life of hull-notched plate considering short crack effect and accumulative plastic damage

When studying the low-cycle fatigue crack initiation life of notched plates, the effects of fatigue short crack and accumulative plastic damage in the vicinity of the notch should be considered. The low-cycle fatigue crack initiation life of notched plates has been revised in the analysis process by considering the short crack initiation and expansion of notch stress concentration area. By determining the accumulative plastic deformation in the vicinity of a notch under low-cycle fatigue loads, a predictive model of low-cycle fatigue crack initiation life was presented for notched plates, and the relative influence factors were quantitatively analyzed. A comparison study verified that the results obtained from the presented calculation model correlate quite well with those from the existing experiments. Some important conclusions were drawn from the study on considering the effects of short crack and accumulative plastic damage. The presented method may be used for predicting the low-cycle fatigue crack initiation life of ship plate.

Refined Analysis of Fatigue Crack Initiation Life of Beam-to-Column Welded Connections of Steel Frame under Strong Earthquake

This paper presents a refined analysis for evaluating low-cycle fatigue crack initiation life of welded beam-to-column connections of steel frame structures under strong earthquake excitation. To consider different length scales between typical beam and column components as well as a few crucial beam-to-column welded connections, a multiscale finite element (FE) model having three different length scales is formulated. The model can accurately analyze the inelastic seismic response of a steel frame and then obtain in detail elastoplastic stress and strain field near the welded zone of the connections. It is found that the welded zone is subjected to multiaxial nonproportional loading during strong ground motion and the elastoplastic stress-strain field of the welded zone is three-dimensional. Then, using the correlation of the Fatemi-Socie (FS) parameter versus fatigue life obtained by the experimental crack initiation fatigue data of the structural steel weldment subjected to multiaxial loading, the refined evaluation approach of fatigue crack initiation life is developed based on the equivalent plastic strain at fatigue critical position of beam end seams of crucial welded connections when the steel frame is subjected to the strong earthquake excitation.

A novel simulation for the design of a low cycle fatigue experimental testing programme

This paper proposes an innovative concept for the design of an experimental testing programme suitable for causing Low Cycle Fatigue crack initiation in a bespoke complex notched specimen. This technique is referred to as the Reversed Plasticity Domain Method and utilises a novel combination of the Linear Matching Method and the Bree Interaction diagram. This is the first time these techniques have been combined in this way for the calculation of the design loads of industrial components. This investigation displays the capabilities of this technique for an industrial application and demonstrates its key advantages for the design of an experimental testing programme for a highly complex test specimen.

Effect of fiber cross section geometry on cyclic plastic behavior of continuous fiber reinforced aluminum matrix composites

This paper investigates the cyclic plastic behavior of continuous fiber-reinforced aluminum matrix composites (CFAMCs) with different shape of fiber cross section arranged in a square packing geometry. The 2D micromechanical FEM models, composed of elastic undamaged reinforcement perfectly bonded to an elastic-perfectly plastic matrix with a volume fraction equal to 30%, are subjected to off-axis constant macro stress and a cyclic temperature history. under such load conditions, the matrix undergoes large internal inelastic deformations potentially leading to internal crack initiation as well as macroscopic ratcheting. The computational method, the Linear Matching Method (LMM), is used throughout the analysis for the direct evaluation of shakedown, alternating plasticity and ratcheting behaviors. The effect of the matrix yield stress thermal degradation upon two common design limits, i.e., the reverse plasticity limit and the ratchet limit, is also investigated and discussed, including its influence on the off-axis low cycle fatigue crack initiation.

CDM Approach Applied to Crack Initiation and Propagation under Variable Amplitude

In this paper, the low cycle fatigue crack initiation life was regarded as a process of damage accumulation and a damage accumulation model was established based on the Continuum Damage Mechanics. By the model, we analyzed how the variable amplitude applied at the crack initiation stage influenced the low cycle fatigue life of high temperature materials. With the parameters of GQGH4169 alloy at room temperature, we determined the specific values of damage parameters by finite element method and numerical analysis method. Then, the crack initiation life predictions were carried out. The results show that using this approach can not only predict the crack initiation life of CT specimen accurately, but also reflect a definite influence of variable amplitude on the crack propagation life combining with the Paris Law, and the test costs reduced consequently.

Ni基一方向凝固超合金の高温低サイクル疲労き裂発生に対する結晶特性の影響評価

Ni基一方向凝固超合金の高温低サイクル疲労き裂発生に対する結晶特性の影響評価

Application of the cyclic life additivity concept for the determination of service life of aero engine disks made from powder alloys

An approach to the estimation of cyclic life of aero engine disks made from powder alloys on the basis of the concept of cyclic durability additivity is proposed. Within the framework of this approach the durability of aero engine disks includes both the service life prior to low-cycle fatigue crack initiation and safe crack propagation life which is calculated as the crack stable growth period. Complex studies including the analysis of metallurgical defects in the areas of low-cycle fatigue crack initiation in disks of aircraft gas turbine engines, microfractographic reconstitution of crack kinetics and mathematical modeling of the disk stress-strain condition have been conducted. On the basis of these studies the connection between 1) the size, location and type of the defects, 2) the crack free life prior to low-cycle crack initiation caused by these defects, 3) the period of the crack growth and 4) the characteristics of stress-strain condition and the temperature of the disk in the area of crack initiation has been established. Comparative analysis of defects in granular nickel alloys produced by various technologies (for various maximum size of the grains) has been carried out. The maximum size of the defects and their number in a unit volume are shown to decrease considerably as the grain size decreases. The number of defects in critical areas of engine disks, however, remains quite large, even for the minimal size of the grains. The expedience of applying the deterministic approach for the calculation of disk survivability is shown.