Combined Low Cycle Fatigue Research Articles

Fatigue assessment of a notched member under combined LCF and HCF loading

Steam turbine last stage blades can experience a wide range of load spectra during their lifetime, which involves both Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) phenomena.In the paper a numerical model is proposed for considering this specific load spectrum in the fatigue life assessment of turbine blade components made of high strength precipitation-hardening steels.The method is based on cyclic isothermal stress and strain controlled fatigue test results of notched and un-notch specimen up to the HCF-regime. Moreover, stress controlled tests on notched specimens with different stress ratios were performed in order to evaluate the influence of mean stress and to derive a local Haigh diagram. The Highly Stressed Volume (HSV) has been used to transfer characteristic fatigue properties from specimens to a critical area (notched part) of a turbine blade. Furthermore, the real operating conditions of the component were used to allow the application of Palmgren-Miner damage rule.An ad-hoc MATLAB® software has been developed for the post-process of Finite Element Analysis (FEA) results, in particular for the automatic calculation of the HSV on the critical areas of the component. The proposed fatigue model has been finally applied on a real case study (blade dovetail) using appropriate safety margins for reliable design assessment.

Mechanisms and modelling of fatigue crack growth under combined low and high cycle fatigue loading

In this paper a mechanism-based model is presented, which is able to describe the evolution of microcracks under pure low cycle fatigue (LCF) and combined LCF and high cycle fatigue (HCF) loading conditions. In order to verify the model and to calibrate the model parameters, the crack length evolution of microcracks is followed at room temperature for pure LCF and combined LCF/HCF loading in a 10%-chromium steel. These studies reveal accelerated crack growth rates under LCF/HCF interaction as soon as a critical crack length is reached. The model is capable of accounting for this effect and needs only few parameters, including the threshold for fatigue crack growth, whose knowledge is crucial for the accuracy of the model.

Influence of foreign object damage on fatigue crack growth of gas turbine aerofoils under complex loading conditions

ABSTRACTForeign object damage (FOD) has been identified as one of the main life limiting factors for aeroengine blades, with the leading edge of aerofoils particularly susceptible. In this work, a generic edge ‘aerofoil’ geometry was utilized in a study of early fatigue crack growth behaviour due to FOD under low cycle fatigue (LCF), high cycle fatigue (HCF) and combined LCF and HCF loading conditions. Residual stresses due to FOD were analyzed using the finite element method. The longitudinal residual stress component along the crack path was introduced as a nodal temperature distribution, and used in the correction of the stress intensity factor range. The crack growth was monitored using a nanodirect current potential drop (DCPD) system and crack growth rates were correlated with the corrected stress intensity factor considering the residual stresses. The results were discussed with regard to the role of residual stresses in the characterization of fatigue crack growth. Small crack growth behaviour in FODed specimens was revealed only after the residual stresses were taken into account in the calculation of the stress intensity factor, a feature common to LCF, HCF and combined LCF + HCF loading conditions.

Crack growth in IMI 829 at 550°C under combined high and low cycle fatigue

Fatigue crack growth rates have been determined for the near-alpha titanium alloy IMI 829 at an elevated temperature of 550°C using corner notched test pieces. Combined low cycle fatigue (LCF) and high cycle fatigue (HCF) tests simulate stresses induced by the start-to-stop operation, and the in-flight vibrations, of aero-engines. Growth rates from these tests were compared to those from purely low cycle fatigue in order to determine the onset of HCF crack growth. This onset was safely predicted by the fatigue threshold value determined by the load-shed method for HCF cycles having a stress ratio of 0.90. The unsafe prediction for a stress ratio of 0.82 is connected with a HCF/LCF load interaction which is apparent in the vicinity of the onset condition, particularly in tests employing the higher ratio of HCF to LCF cycles. The introduction of superimposed high cycle fatigue was reflected in the appearance of the fatigue fracture surface.

Investigation of thermal high cycle and low cycle fatigue mechanisms of thick thermal barrier coatings

Thick thermal barrier coating systems in a diesel engine experience severe thermal low cycle fatigue (LCF) and high cycle fatigue (HCF) during engine operation. In this paper, the mechanisms of fatigue crack initiation and propagation in a ZrO 2–8wt%Y 2O 3 thermal barrier coating, under simulated engine thermal LCF and HCF conditions, are investigated using a high power CO 2 laser. Experiments showed that the combined LCF–HCF tests induced more severe coating surface cracking, microspallation and accelerated crack growth, as compared to the pure LCF test. Lateral crack branching and the ceramic/bond coat interface delaminations were also facilitated by HCF thermal loads, even in the absence of severe interfacial oxidation. Fatigue damage at crack wake surfaces, due to such phenomena as asperity/debris contact induced cracking and splat pull-out bending during cycling, was observed especially for the combined LCF–HCF tests. It is found that the failure associated with LCF is closely related to coating sintering and creep at high temperatures, which induce tensile stresses in the coating after cooling. The failure associated with HCF process, however, is mainly associated with a surface wedging mechanism. The interaction between the LCF, HCF and ceramic coating creep, and the relative importance of LCF and HCF in crack propagation are also discussed based on the experimental evidence.

Investigation of thermal fatigue behavior of thermal barrier coating systems

In the present study, the mechanisms of fatigue crack initiation and propagation, and of coating failure under thermal loads that simulate those in diesel engines are investigated. Surface cracks initiate early and grow continuously under thermal low cycle fatigue (LCF) and high cycle fatigue (HCF) stresses. It is found that, in the absence of interfacial oxidation, the failure associated with LCF is closely related to coating sintering and creep at high temperatures. Significant LCF and HCF interactions have been observed in the thermal fatigue tests. The fatigue crack growth rate in the ceramic coating strongly depends on the characteristic HCF cycle number, N * hcf , which is defined as the number of HCF cycles per LCF cycle. The crack growth rate is increased from 0.36 μm/LCF cycle for a pure LCF test to 2.8 μm/LCF cycle for a combined LCF and HCF test at N * HCF about 20 000, A surface wedging model has been proposed to account for the HCF crack growth in the coating systems. This mechanism predicts that the HCF damage effect increases with heat flux and thus with increasing surface temperature swing, thermal expansion coefficient and elastic modulus of the ceramic coating, as well as with the HCF interacting depth. Good correlation has been found between the analysis and experimental evidence.

Fatigue crack growth characteristics of ARALL-1. Interim report, August 1987–January 1988: Ruschau, J. J. University of Dayton Report No AD-A196 185/3/XAB May 1988

Thick thermal barrier coating systems in a diesel engine experience severe thermal low cycle fatigue (LCF) and high cycle fatigue (HCF) during engine operation. In this paper, the mechanisms of fatigue crack initiation and propagation in a ZrO2–8wt%Y2O3 thermal barrier coating, under simulated engine thermal LCF and HCF conditions, are investigated using a high power CO2 laser. Experiments showed that the combined LCF–HCF tests induced more severe coating surface cracking, microspallation and accelerated crack growth, as compared to the pure LCF test. Lateral crack branching and the ceramic/bond coat interface delaminations were also facilitated by HCF thermal loads, even in the absence of severe interfacial oxidation. Fatigue damage at crack wake surfaces, due to such phenomena as asperity/debris contact induced cracking and splat pull-out bending during cycling, was observed especially for the combined LCF–HCF tests. It is found that the failure associated with LCF is closely related to coating sintering and creep at high temperatures, which induce tensile stresses in the coating after cooling. The failure associated with HCF process, however, is mainly associated with a surface wedging mechanism. The interaction between the LCF, HCF and ceramic coating creep, and the relative importance of LCF and HCF in crack propagation are also discussed based on the experimental evidence.