Interaction Of High-cycle Fatigue Research Articles

Microstructurally-Informed Life Prediction Modeling of Combined High-Cycle Fatigue and Creep in a Ni-base Superalloy

The capability to accurately predict creep and high-cycle fatigue (HCF) lifetime is crucial to the successful design of an industrial gas turbine. In particular, turbine blades are subjected to extreme environments, where elevated temperatures and an array of mechanical and dynamic loads are present. Increased temperatures and aggressive designs are needed to meet next-generation efficiency and power output targets, further intensifying blade loading conditions and enabling new life-limiting concerns, such as the interaction of HCF and creep. The interaction of HCF and creep has not been fully investigated, and is not adequately captured in existing life prediction models. This lifing capability is needed to maintain current reliability standards in next-generation industrial gas turbine blades.The life-limiting interaction of creep and HCF is explored in this study using standard and pre-crept test specimens of a conventionally cast Alloy 247 LC material subjected to high temperature, high frequency loading until failure. The experimental data are obtained for two temperatures, three stress ratios, and a range of pre-creep strains, providing a comprehensive survey of synchronous and stepwise interactions. A microstructurally-informed life prediction model is created by leveraging existing principles with the experimental results and the findings of from a thorough post-test failure analysis.

Threshold damage-based fatigue life prediction of turbine blades under combined high and low cycle fatigue

Life prediction based on the damage mechanics of aero-engine turbine blades is crucial for performing their strength design and ensuring the operational reliability under combined high and low cycle fatigue (CCF) loadings. In view of this, a simple and efficient life prediction method is developed to consider the interaction of high cycle fatigue (HCF) and low cycle fatigue (LCF) without any additional material constants, and as a basis of that a new nonlinear damage accumulation model is proposed by introducing threshold damage of the component related to material type and current HCF damage. In order to validate the presented methodology, model predictions were compared with the experimental data sets of turbine blades and its alloy materials. The predicted results indicate that the simple life prediction model is capable of estimating the fatigue life quickly with an acceptable accuracy in contrast to other four existing ones. Moreover, the threshold damage-based method provides the higher prediction accuracy than others due to improved HCF damage determined by the damage threshold.

High-cycle fatigue interaction between soil foundation and concrete slab under moving wheel-type loads

Small-scale fatigue experiments were conducted to investigate failure modes of the coupled system of soil foundation and concrete slab subjected to wheel-type moving loads. Based on these experiments, the numerical fatigue simulation was validated by integrating constitutive models of cracked concrete and soil foundations. The finite element code used in this study is built by combining the multi-directional fixed crack model of concrete with the multi-yield surface plasticity for nonlinear shear and volumetric fatigue of soil. The computed deflections and failure modes of the pavement system show close correlation with experiments. To clarify the real working conditions, full-scale modeling was investigated as well. The results depict that the necessary and sufficient slab thickness varies nonlinearly according to the soil rigidity, and that the medium compacted soil foundation requires the thicker slabs rather than the current design value rooted in a beam theory on elastic foundation. Thus, a key factor to revise the design scheme is proposed in terms of the balanced slab thickness associated with the nonlinearity of soil foundation.

A Combined High and Low Cycle Fatigue Model for Life Prediction of Turbine Blades.

Combined high and low cycle fatigue (CCF) generally induces the failure of aircraft gas turbine attachments. Based on the aero-engine load spectrum, accurate assessment of fatigue damage due to the interaction of high cycle fatigue (HCF) resulting from high frequency vibrations and low cycle fatigue (LCF) from ground-air-ground engine cycles is of critical importance for ensuring structural integrity of engine components, like turbine blades. In this paper, the influence of combined damage accumulation on the expected CCF life are investigated for turbine blades. The CCF behavior of a turbine blade is usually studied by testing with four load-controlled parameters, including high cycle stress amplitude and frequency, and low cycle stress amplitude and frequency. According to this, a new damage accumulation model is proposed based on Miner’s rule to consider the coupled damage due to HCF-LCF interaction by introducing the four load parameters. Five experimental datasets of turbine blade alloys and turbine blades were introduced for model validation and comparison between the proposed Miner, Manson-Halford, and Trufyakov-Kovalchuk models. Results show that the proposed model provides more accurate predictions than others with lower mean and standard deviation values of model prediction errors.

Investigation of Cumulative Fatigue Damage Through Sequential Low Cycle Fatigue and High Cycle Fatigue Cycling at High Temperature for a Type 316LN Stainless Steel: Life-Prediction Techniques and Associated Mechanisms

Cumulative fatigue damage under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) cycling was investigated at 923 K (650 °C) by conducting HCF tests on specimens subjected to prior LCF cycling at various strain amplitudes. Remnant HCF lives were found to decrease drastically with increase in prior fatigue exposure as a result of strong LCF–HCF interactions. The rate of decrease in remnant lives varied as a function of the applied strain amplitude. A threshold damage in terms of prior LCF life-fraction was found, below which no significant LCF–HCF interaction takes place. Similarly, a critical damage during the LCF pre-cycling marking the highest degree of LCF–HCF interaction was identified which was found to depend on the applied strain amplitude. In view of the non-linear damage accumulation behavior, Miner’s linear damage rule proved to be highly non-conservative. Manson’s damage curve approach, suitably modified, was found to be a better alternative for predicting the remnant HCF life. The single constant (β) employed in the model, which reflects the damage accumulation of the material under two/multi-level loading conditions is derived from the regression analysis of the experimental results and validated further.

Aeromechanical Modeling of Rotating Fan Blades to Investigate High-Cycle and Low-Cycle Fatigue Interaction

Gas turbine engine components are subject to both low-cycle fatigue (LCF) and high-cycle fatigue (HCF) loads. To improve engine reliability, durability and maintenance, it is necessary to understand the interaction of LCF and HCF in these components, which can adversely affect the overall life of the engine while they are occurring simultaneously during a flight cycle. A fully coupled aeromechanical fluid–structure interaction (FSI) analysis in conjunction with a fracture mechanics analysis was numerically performed to predict the effect of representative fluctuating loads on the fatigue life of blisk fan blades. This was achieved by comparing an isolated rotor (IR) to a rotor in the presence of upstream inlet guide vanes (IGVs). A fracture mechanics analysis was used to combine the HCF loading spectrum with an LCF loading spectrum from a simplified engine flight cycle in order to determine the extent of the fatigue life reduction due to the interaction of the HCF and LCF loads occurring simultaneously. The results demonstrate the reduced fatigue life of the blades predicted by a combined loading of HCF and LCF cycles from a crack growth analysis, as compared to the effect of the individual cycles. In addition, the HCF aerodynamic forcing from the IGVs excited a higher natural frequency of vibration of the rotor blade, which was shown to have a detrimental effect on the fatigue life. The findings suggest that FSI, blade–row interaction and HCF/LCF interaction are important considerations when predicting blade life at the design stage of the engine. The lack of available experimental data to validate this problem emphasizes the utility of a numerical approach to first examine the physics of the problem and second to help establish the need for these complex experiments.

Modeling of fatigue damage under superimposed high-cycle and low-cycle fatigue loading for a cast aluminum alloy

This study investigates the interaction between high-cycle fatigue (HCF) and low-cycle fatigue (LCF) in cast aluminum alloys. Initially, the baseline HCF and LCF lives have been determined under constant amplitude loading; LCF–HCF interaction fatigue tests were subsequently carried out through the application of periodic underloads. To account for the inherent scatter in fatigue life, probabilistic fatigue life curves were employed to establish the baseline HCF and LCF fatigue lives. The interaction fatigue damage was found to increase with the stress amplitude of the HCF cycles. Larger underloads (LCF cycles) also resulted in larger interaction damage. The impact of different numbers of HCF cycles per block (between underloads) was also studied. It was also found out that there is an additional damage caused by frequent applications of LCF cycles, which is related to the damaged Si particles in the large reversed plastic zone generated by the LCF cycles. The evolution of the crack opening stress following the application of an underload has been proposed as the major mechanism for the load interaction effect. An interaction damage model is proposed to account for these observed effects.

Combined Cycle Fatigue of Gas Turbine Blade Materials at Elevated Temperature

Abstract: In gas turbine engines for aerospace propulsion, and in particular in the HP and LP stage blading where the highest temperatures are experienced, the assessment of the fatigue damage mechanisms due to the interaction of high cycle fatigue (due to vibration phenomena) and low cycle fatigue (due to ground‐air‐ground major engine cycling) are of paramount importance for the structural integrity of the components. In order to experimentally evaluate the material performance in this highly demanding environment, a number of combined cycle fatigue (CCF) and low cycle fatigue (LCF) tests with and without dwell have been carried out on an investment cast polycrystalline nickel superalloy, at temperatures up to 870 °C, on specimens with pre‐cracked coating. Comparison of the CCF and the LCF tests with dwell with conventional LCF tests is presented herein.

Interaction of high-cycle fatigue with high-temperature creep in superalloy single crystals

Abstract Smooth and notched specimens of superalloy single crystals CMSX-4 with 〈001〉 orientation were subjected to creep and creep/high-cycle-fatigue loading at temperatures of 800 and 850°C in air. Superposition of a cyclic stress component on the static stress component leads to a slowdown of the creep rate at small creep strains and to the formation of persistent slip bands running through both the γ matrix and the γ′ precipitates. Consequence of the formation of these zones is fatigue crack initiation at casting pores and shortening of lifetime at higher stress amplitudes.

Low and High Cycle Fatigue Interaction in 316L Stainless Steel

Abstract The interaction between initial low cycle fatigue (LCF) and high cycle fatigue (HCF) in 316L stainless steel is reported. Specimens were introduced to varying degrees of LCF and subsequently subjected to HCF until failure. LCF involves bulk plasticity where stress levels are usually above the yield strength of the material. On the other hand, HCF is predominantly elastic, and stress levels are below the yield strength of the material. Fatigue was carried out under strain control where two strain amplitudes in the LCF range with a common HCF strain amplitude were investigated. Results show that fatigue life decreased when specimens were introduced to increasing numbers of initial LCF. A linear life trend is observed for high numbers of LCF introduction, which deviates from linearity when lower numbers of LCF were introduced. It is therefore concluded that initial LCF causes substantial irreversible damage, subsequently reducing fatigue life in the HCF regime.

Interaction of high cycle fatigue with high temperature creep in two creep-resistant steels

The interaction of high cycle fatigue with high temperature creep was studied in bainitic steel of the type 2.25 Cr1Mo (pressure vessel steel) at 600 °C and in austenitic steel of the type AISI 304 at 700 °C. The cyclic stress component strongly affects both the minimum creep rate and the time to fracture. For a constant maximum stress in the stress cycle, σ max, the creep rate and the time to fracture depend on the stress ratio R = σ min/ σ max in a non-monotonous way. From the practical point of view it is important that even small vibrations superimposed on the static load increase the time to fracture substantially. The explanation of the effect of the cyclic stress component on the deformation processes is based on a composite model of dislocation structure.