Fatigue Life In Presence Research Articles

Low-cycle fatigue performance and life prediction of a P/M nickel-based superalloy with artificial surface defect at elevated temperature

P/M nickel-based superalloys are widely used in turbine disks, defects such as inclusions and surface scratches inevitably arisen from manufacturing process will exert negative influences on the low-cycle fatigue performance of superalloys and make traditional life prediction method no longer reliable. The residual fatigue life in presence of defect is an important performance index for disc alloy in damage tolerance design criteria. In this work, LCF tests of FGH96 specimens with artificial surface defect were conducted at 600℃, 650℃, 700℃ and room temperature, the size of the defect is determined in accordance with the minimum flaw size which can be detected by NDT technique. The results show that the LCF life of the specimens with artificial surface defect decreases significantly with the increase of temperature and size of defect. Microscopic characterization technique SEM and EDS were executed to investigate the failure mechanism of the specimens, different mechanisms of crack initiation from artificial surface defect at room and elevated temperature were discussed. And then life prediction based on fracture mechanics was carried out by ABAQUS-FRANC3D co-simulation. This method allows that SIF can be conveniently calculated by finite element method and it is applicable to analysis of arbitrary engineering structure. Good life prediction results which fall within double dispersion band of experimental lives were obtained at elevated temperature, but the predicted life is excessively conservative at room temperature because the crack initiation process accounts for a large proportion of fatigue life. It seems that life prediction method based on fracture mechanics is more applicable to specimens with surface defect at elevated temperature due to the shorter crack initiation process. This work has reference significance to the damage tolerance analysis of turbine disks.

Assessment of Fatigue Life in Presence of Different Hole Geometries by X-FEM

Assessment of Fatigue Life in Presence of Different Hole Geometries by X-FEM

Fatigue life prediction considering mean stress effect based on random forests and kernel extreme learning machine

The mean stress effect plays a vital role in fatigue life analysis, affecting both macro-mechanical response and micro-crack evolution of materials. Even though semiempirical models are widely used in practice because of their simplicity, the mean stress effect for a broad range of materials and loading conditions may not be uniformly reflected due to the disorganized description of fatigue damage. To overcome such deficiency, two machine learning (ML)-based models, as useful alternatives to semiempirical models, are proposed to predict the fatigue life in presence of mean stresses using random forests and kernel extreme learning machine respectively. In the models, the monotonic, cyclic and fatigue properties as well as the cyclic stress–strain responses of materials are employed to map the fatigue life with mean stress effect. Also, hyperparameters are automatically optimized by the genetic algorithm/grid search method to avoid arbitrary procedures. A total of 354 experimental results generated for various materials and mean stress levels are collected to evaluate the prediction accuracy, performance stability and generalization ability of the proposed models. It is shown that these two proposed models, based on different prediction mechanisms, can both exhibit superior prediction performance over semiempirical models, and hold distinct prediction characteristics favorable for different application scenarios.

Crack closure effects during low cycle fatigue propagation in line pipe steel: An analysis with digital image correlation

Fatigue crack growth was investigated in a line pipe steel. Severe straining conditions, like those experienced by pipelines, were considered during the experiments. Crack closure effects were investigated during the tests by adopting an innovative technique based on digital image correlation. Experimentally measured crack closure levels were implemented in a crack propagation model based on elastic–plastic fracture mechanics. A modified formulation of ΔJeff, necessary to accurately describe crack propagation driving forces, is presented and employed to assess fatigue life in presence of high plastic strains.

Effect of notch on low cycle fatigue behaviour of 316 LN stainless steel

In the present investigation, effect of notch on low cycle fatigue behaviour of 316 LN stainless steel have been assessed by carrying out low cycle fatigue (LCF) tests on smooth and notched specimens of steel. The LCF experiments were carried out at strain amplitudes ranging from ±0.3 to ±1.0 % and at a strain rate of 3 × 10−3 s−1 at room temperature. The material exhibited prolonged softening after a few initial cycles of hardening followed by saturation and final failure. The gradual softening following initial hardening was not observed for the highest strain amplitude of 1 %. Fatigue life was found to decrease in presence of notch. The extent of decrease in fatigue life was higher for relatively sharper notch. Fatigue life of the notched specimens has been estimated based on the local plastic strain using finite element analysis and Neuber’s rule. The FE analysis approach was found to underestimate the fatigue life in presence of notch, whereas Neuber’s rule overestimated the fatigue life.

A continuum damage mechanics model for pit-to-crack transition in AA2024-T3

A Continuum Damage Mechanics model for corrosion−fatigue damage is presented to study the pit-to-crack initiation life in AA2024-T3. This study is based on the experimental observation of reduced fatigue life in presence of corrosive environment. The model is based on the assumption that stress field has effect on the corrosion rate, and corrosion-induced damage enhances fatigue damage. Model results show significant reduction in fatigue crack initiation life. The model takes into account the effect of stress and frequency on the pit-to-crack transition life. Bayesian updating method is used to account for uncertainties in the parameters of the model.

Classification of metallic alloys for fatigue damage accumulation: A conservative model under strain control for 304 stainless steels

Fully reversed uniaxial tests performed under total strain and stress control on 304 stainless steels specimens show that, under strain control the fatigue damage for High–Low (H–L) cycling is more significant than that using Miner’s rule, but under stress control opposite results are obtained. This has been attributed to opposite effects of pre-hardening under strain and stress control. Classical non linear damage accumulation models are not able to take into account this difference in sequence effect. Smith–Watson–Topper (SWT) and Fatemi–Socie (FS) criterion combined to linear damage accumulation can take into account this difference in sequence effect through the presence of maximum stress. However these models require an elastic–plastic constitutive law which is difficult to propose due to the presence of high cycle secondary hardening observed on 304 stainless steel. A conservative model for damage accumulation under variable amplitude strain control loading is thus proposed, which does not require a constitutive law. Linear damage accumulation is used, while sequence effect is taken into account using the elastic–plastic memory effect through cyclic strain–stress curves (CSSC) with pre-hardening. This modeling classifies metallic alloys in two groups for damage accumulation, with a stable (independent to pre-hardening) CSSC as for aluminum alloys and with an unstable (dependent to pre-hardening) one as for austenitic stainless steels. For the former case the modeling is identical to Miner’s rule. The modeling is approved based on a large number of tests on 304 stainless steel and is compared with SWT and FS models. In presence of mean stress the modeling permits in a qualitative way to explain the fact that tensile mean stresses in constant amplitude strain control tests are more detrimental than for constant amplitude stress control tests. Moreover it is shown that the SWT model is not always able to predict accurately the fatigue life in presence of a mean stress. Finally, it is concluded that for a 304 stainless steel, in order to take into account the mean stress in fatigue life, the mean stress effect has to be decomposed into two parts: maximum and “intrinsic” mean stress effects.

A stress invariant based spectral method to estimate fatigue life under multiaxial random loading

The present paper aims to propose a frequency-domain formulation of a stress invariant based multiaxial fatigue criterion, called “Projection by Projection” (PbP) approach, suitable for estimating fatigue life in presence of complex multiaxial loadings. The new proposed spectral method is first validated with experimental data from the literature, concerning bending-torsion random loading, showing a general accuracy. Numerical simulations are next used to generate a wide set of bending-torsion random loading with different correlation degrees, variance ratios and frequency content, in order to further validate the new proposed method. Simulations confirm a general quite good accuracy for PbP spectral method and also emphasize its better precision compared to another spectral method, namely the equivalent von Mises stress approach, developed by Preumont and co-workers.

Fracture Toughness and Pressure Vessel Performance

Criteria for predicting pressure vessel performance based on fracture toughness are reviewed in general terms. Experimental studies of small pressure vessels fabricated of high toughness, high strength steel 4330V (Mod + Si) are described. Data presented include fatigue life in presence of a small part-through-crack and burst properties of the fatigue cracked cylinders. Interpretation of the fatigue data is based on Paris’ relationship dadN=K4M. The failure stresses are discussed in relation to the stress elevating effect of local bulging on the apparent fracture toughness. The behavior observed in testing full scale high strength pressure vessels fabricated from materials having intermediate fracture toughness, namely, D6A steel at 200,000-psi yield strength and 300M steel at 230,000-psi yield strength and materials having limited fracture toughness, namely, twenty percent nickel maraging steel at 280,000-psi yield strength, are discussed in relation to the ratio of fracture toughness to plane-strain fracture toughness based on the part-through-crack model. Precautions necessary for fabrication and inspection to insure reliable performance are discussed.