Low Cycle Fatigue Crack Research Articles

Tube-burst Analysis of SA-213TP437H high Temperature Superheater

A SA-213TP437H high temperature superheater tube burst in a power plant. In this paper, combined with it’s runtime environment, macro examination, metallographic examination, electron microscope observation and micro area composition analysis were used to analyze the failure reason of the sample tube. The results show that: Due to the fine grain size of superheater tube, a large number of σ brittle phase are precipitated in the metallographic structure in lower temperature. During long-term running, the tube was affected form low cycle fatigue cracking by temperature stress coursed by the load change of generator set, start and stop and so on., which is a long-term over temperature tube burst. Although it is an example of misuse of ultra-fine grain steel pipe, it is recommend that the power plants should conduct spot-check of grain size for other SA-213TP437H high temperature superheater tube, and eliminate similar dangers of superheater tube burst.

Analysis of biaxial proportional low-cycle fatigue and biaxial accumulative plasticity of hull inclined-crack plate

Fracture induced failure of plates in hull structures subjected to in-plane biaxial low-cycle fatigue loading are generally a combination of accumulative plasticity and biaxial low-cycle fatigue damage. A biaxial low-cycle fatigue crack growth analysis of a hull structure that accounts for the accumulative plasticity effect can be suitable for the actual evaluation of the overall fracture induced failure of the hull structure in severe sea conditions. An analytical model of biaxial low-cycle fatigue crack propagation with a control parameter for the J integral was presented for hull inclined-crack plate. The test was conducted for cruciform specimens made of Q235 steel with an inclined crack to validate the presented analysis. The biaxial accumulative plasticity behaviour and the effects of the biaxiality and stress ratios were investigated by numerical analysis. The results of this study reveal a strong dependence of biaxial low-cycle fatigue crack propagation on biaxial accumulated plasticity.

Improved description of low-cycle fatigue behaviour of 316L steel under axial, torsional and combined loading using plastic J-integral

Low-cycle fatigue behaviour and fatigue crack kinetics of the 316L austenitic stainless steel were studied under cyclic axial, torsional and in-phase combined loading using hollow cylindrical (tubular) specimens with a small hole for crack initiation. The concept of plastic.J-integral was used, which was shown in previous studies to unify the crack growth rate data for several different materials. Dependencies of Jp on crack length were determined by extensive finite element modelling considering non-linear material behaviour according to the cyclic stress–strain curve. Locally deflected cracks were modelled in accordance with amplitudes of the axial and torsional components of combined loading. The measured crack growth rate diagrams for all types of loading and for various loading amplitudes were unified using amplitude of Jp. Fatigue lives under torsional loading were much longer than under axial loading for the same equivalent plastic strain amplitude, which was explained by higher crack driving forces in terms of Jp under axial loading than under torsional loading. Fatigue lives estimated by crack propagation based on a master curve in terms of Jp,a were in a good agreement with those obtained experimentally under all types of loading. The used concept can reduce the experimental program to obtaining of material data only for axial loading, which can then be used for prediction of behaviour under in-phase multiaxial loading. The von Mises formula for multiaxial low-cycle fatigue loading εeq,p2 = εp2 + γp2 / 3 was modified so that the fatigue lives under axial, torsional and combined loading were characterized in a matching way. Using the formula εp,Nf2 = εp2 + γp2 / 25, the fatigue life data fell on a single Coffin-Manson curve.

Crystallographic evaluation of low cycle fatigue crack growth in a polycrystalline Ni based superalloy

We present the microscale fracture mechanics aspects of LCF cracks non-propagating till failure in the cylindrical specimens of Ni based superalloy Haynes 282 and attempt to unravel the underlying crystallographic factors. Two key parameters, Crack Tip Opening Angle (CTOA) and maximum tangential stress (θMTS) have been opted for characterization. CTOA variations along with a propagating crack, exhibit a non-linear decay followed by a stabilized regime and the fraction of the stabilized regime increases in lower strain amplitude where the fatigue life is more. Mixity of local KI and KII fields is directly proportional to θMTS and that has been assessed by measuring local deflections. The modal mixity has been found to be microstructure sensitive and an entirely statistical parameter. Different growth modes of these non-propagating cracks have been identified from the microstructural observations. The role of elasto-plastic incompatibility of neighboring grains has been addressed by conducting crystallographic analyses. There is a critical bound for Elastic Modulus (EM) and Schmid factor (SF) for the grains favoring subsurface crack propagation below which grains (mostly with high elastic compliance and low SF) are not participating in crack propagation. The favourable twin-matrix incompatibility of the microstructure has also been identified about the fatigue crack growth and twins in (211) plane is abundant in the cracked region. From the results of a detailed slip transfer analysis based on the Luster-Morris parameter (LMP), it is observed that there exists microstructurally controlled lower bounds that inhibits slip transfer (LMP value <0.1). Dislocation debris analysis using TEM has also been carried out to understand micromechanisms clearly. Both non coplanar and planar mode of dislocation activity has been observed and twin boundaries has been see to act as barrier to the planar dislocation motion.

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.

Responses of cracked stiffened plates to low-cycle fatigue loads

Low cycle fatigue (LCF) crack propagation experiments are conducted on 12 stiffened plate specimens with centrally located through cracks using MTS Dynamic Test System. The a-N curves of the specimens are obtained via crack propagation videos recorded by an electron microscope. Accumulated maximum crack opening displacement (AMCOD) is measured by an extensometer. Accumulated mean strain (AMS) is calculated from the loading rod displacement data recorded by the computer that controls the test machine. The effects of LCF loading on crack propagation, AMCOD and AMS are analyzed based on the experimental results. The parameter values in the general constitutive relation for fatigue crack growth are derived through curve fitting the da/dN-a results. Comparisons of AMCOD and elongation of the 12 specimens show that the two parameters are almost equal at all times; correlation between AMCOD and elongation is almost 1 for all 12 specimens. AMCOD is the main cause of elongation. The three responses (crack propagation, AMCOD and AMS) of the specimen to LCF loading occur simultaneously. Evaluating crack propagation length, AMCOD and AMS together can increase the accuracy of the evaluation of the ultimate strength of cracked structural elements in a ship.

Enhancements for technique of stable low cycle fatigue crack propagation simulation for gas turbine engine

Enhancements for technique of stable low cycle fatigue crack propagation simulation for gas turbine engine

Study of low-cycle fatigue crack growth behavior of central-cracked stiffened plates

This work is aims to study the low-cycle fatigue (LCF) crack growth behavior of central-cracked stiffened plates through experimental and numerical analysis methods. A central-cracked stiffened plate with two stiffeners is manufactured to test the influence of the stiffeners on the crack propagation in the base plate. The effects of loading amplitude, the stiffeners’ size and location on the crack growth life are investigated in the test. In numerical analysis, an elastic-plastic numerical model is developed to simulate the crack propagation of stiffened plates considering the influence of the plasticity-induced crack closure (PICC). The results show that PICC and the crack opening load can clearly reflect the crack arrest performance of the stiffener on the crack propagation in the plate. Increasing the stiffener stiffness or decreasing the space between the two stiffeners is beneficial to reduce the driving force acting on the crack tip, prolonging the crack growth life. This work is intended to contribute to a better understanding of the crack arrest performance of stiffened plates through the crack closure mechanism.

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.

Analysis of Biaxial Non-Proportional Low-Cycle Fatigue Crack Propagation Behavior of Hull Plate with an Inclined Crack Based on Accumulative Plasticity

Analysis of Biaxial Non-Proportional Low-Cycle Fatigue Crack Propagation Behavior of Hull Plate with an Inclined Crack Based on Accumulative Plasticity

The fatigue properties, microstructural evolution and crack behaviors of low-carbon carbide-free bainitic steel during low-cycle fatigue

The low-cycle fatigue properties, microstructural evolution and crack growth behaviors of a low-carbon carbide-free bainitic steel subjected to austempering at 300, 320 and 350 °C have been investigated. The results indicated that the fatigue lifetimes of the specimens increase with decreasing the austempering temperature at lower total strain amplitudes, but a comparable fatigue lifetime or even an opposite result is observed at higher total strain amplitudes. During fatigue loading, the blocky retained austenite with inhomogeneous carbon distribution partially transforms into martensite, and the transformation amount increases with the total strain amplitude, especially for the specimens austempered at high temperatures. The transformed martensite and the untransformed austenite satisfy a Kurdjumov-Sachs (K–S) orientation relationship, which can increase the cooperative deformation capacity between the both phases. However, with continuous cycling, more martensite is formed, and these newly formed martensite grains are more likely to form micro-voids/cracks due to their brittleness, which reduces the crack initiation lifetime, and become the fast path for crack propagation, which deteriorates the crack propagation lifetime. Alternatively, more crack branching, crack deflection and severely twisted bainitic ferrite laths are observed for the specimens austempered at high temperatures, which helps to delay/hinder crack growth and increase the crack propagation lifetime, especially at high total strain amplitudes. This suggests that the fatigue lifetime can be closely associated with the interactions of the crack initiation and propagation caused by mechanically-induced martensite from retained austenite.

Residual ultimate strength of ship cracked plates considering fatigue crack propagation under cyclic loads

ABSTRACT The main objective of this paper is to investigate the bearing capacity behaviour of cracked plates considering low-cycle fatigue (LCF) propagation under cyclic loads. The accumulative plastic damage after crack growth is treated as the residual stress and deformation of the plate. In this study, a cracked plate model is proposed to investigate the difference of its residual ultimate strength with preset crack and LCF crack. The present study has analysed the relationship between accumulated lateral deflection and residual stress of cracked plates in different cases. Its effect on the collapse behaviour of the cracked plate is also discussed. The influence factors such as crack length, tensile /compressive cyclic loads, maximum load, minimum load and crack location type are considered in the ultimate strength study of cracked plates. The numerical results clearly reveal the accumulative plastic damage and the bearing capacity behaviour of cracked plates under different cyclic loads.

Computational and fractographic study of the stable growth of low cycle fatigue cracks in the disk of an aircraft engine turbine under complex loading cycles

The results of a comprehensive computational and experimental study of the kinetics of low-cycle fatigue cracks (LCF) in a turbine disk made of EP741NP granular nickel alloy of an aircraft gas turbine engine under complex loading cycles (CLC) are presented. The configuration of crack fronts was reconstructed using light microscopy. Steps of the blocks of fatigue striations characterizing the crack increment under CLC at the stage of steady crack growth are measured using scanning electron microscopy during microfractographic analysis. The period of LCF crack steady growth is estimated and the reproducibility of the regularities of steady growth is demonstrated which testifies the capability of reliable prediction for LCF crack steady growth. The finite element modeling of the reconstructed crack fronts has been carried out. The values?? of the range of stress intensity factor at each crack front in the area of measuring the pitch of the striation blocks were calculated for the subcycles of complex loading cycles. Using the previously developed physically grounded mathematical model and calculation methodology, forecasting of stable growth of the LCF crack was carried out. The results of forecasting match the data of micro-fractographic analysis unlike the results of LCF crack growth prediction based on Paris law which differ significantly from experimental data.

Investigation on low cycle fatigue crack propagation in steel under fully random variable amplitude loading

Investigation on low cycle fatigue crack propagation in steel under fully random variable amplitude loading

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.

A Strain-based continuum damage model for low cycle fatigue under different strain ratios

In this paper, a strain-based damage model is developed to account the effect of different strain ratios on the fatigue failure. Thereafter, continuum damage mechanics and extended finite element method based framework is proposed to simulate the low cycle fatigue crack growth simulations. The proposed damage model is imposed in the developed framework to evaluate the fatigue life of cracked specimen. Chaboche mixed hardening model is implemented to determine the stress–strain accurately in the cracked domain. The acquired stress-strains are incorporated in the proposed damage model to estimate the remaining fatigue life. The computed strain life curves at different strain ratios as well as crack growth curves are compared with the experimental results available in literature. The simulated results are found in good agreement with the experimental observations. This work ensures that the continuum damage mechanics based damage law is an excellent and readily applicable method to acquire the effect of strain ratios on the strain life. This work also provides comprehensive and significant modeling insights for computing the low cycle fatigue crack growth efficiently.

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 &lt; 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.

Inclusion-induced fatigue crack initiation in powder bed fusion of Alloy 718

Abstract Fatigue crack initiation of Alloy 718 additively manufactured via electron beam-powder bed fusion (EB-PBF) process was investigated. The melt parameters were chosen to achieve sufficient energy input and minimize process-induced defects. A line offset of 200 µm with enough line energy was used, leading to the formation of wide and deep melt pools. This strategy facilitated the formation of equiaxed grains at the melt pools bottom, and short columnar grains within the melt pools aligned parallel to the build direction. The mixed grain morphology and texture were retained after various thermal post-treatments, including heat treatment (HT), hot isostatic pressing (HIP), and HIP-HT. Micron-sized non-metallic inclusions in the feedstock powder, such as Al-rich oxide and titanium nitride clustered during the EB-PBF process, and remained intact during the post-treatments. Low cycle fatigue cracks mainly originated from the non-metallic inclusions found near the surface of the test specimens. HIPing was able to remove a portion of the internal defects, including round-shaped and shrinkage pores; therefore, a small fatigue life enhancement was observed in HIP-HT compared to HT.

The investigation of low-cycle fatigue crack propagation of 16 Mn eccentric bar based on the acoustic emission technique

Compared to the traditional cropping technology, low-cycle fatigue cropping can reduce load and improve cross-section quality. Geometric factors of V-shaped notch and processing parameters (such as load amplitude and load frequency) influence the cropping efficiency and section quality. By changing the factors, the sufficient efficiency of cropping and quality of cross-section get together. However, the influences of material defects, geometric parameters, loading frequency and other factors are different in each stage of the whole cropping process. If the effects of the parameters on the each stage of cropping process are clear, the higher productivity and the better cross-section quality will be obtained by applying suitable parameter on each stage of cropping process. To investigate the effects of eccentric ratio on each stage of low –cycle fatigue cropping process, a suitable monitoring method is needed. This study proposes acoustic emission (AE) technique to detect the low-cycle fatigue cropping process of 16 Mn eccentric bar (The bar prefabricated eccentric notches. The prefabricated notch improve the efficiency of cropping). The parameters of signals such as counts and kurtosis during the low–cycle fatigue process are obtained. According to the counts changes over time, the process of the 16 Mn metal bar cropping can be divided into three stages: the crack initiation stage, the crack propagation stage, and the fracture stage. Based on the cumulative counts, the eccentric ratio’s influence on the time of each cropping process stage is obtained. The time of the crack initiation stage and the final fracture stage doesn’t increase with the eccentric ratio. The time of the propagation stage influenced by the eccentric ratio greatly. Besides, the eccentric ratio’s influence on the cross-section quality is studied by using an advanced optical microscope system. The cross-section quality was influenced by the eccentric ratio significantly. The results of the paper indicate the acoustic emission (AE) monitoring technique is a useful method to detect the process of low-cycle fatigue cropping. Especially, it provide effective information to investigate the effects of the notch eccentric ratio during the low-cycle fatigue cropping process. The eccentric ratio’s effects on each stage of low-cycle fatigue cropping process offer guidance to improve low-stress fatigue cropping efficiency and cross-section quality.