Topology optimization considering fatigue life in the frequency domain

Fatigue assessment for jacket platforms is an indispensable practical issue. Because of the small-scale leg diameter, these structures are often drag dominated and wave-induced force in these structures can be tackled by using either linear or nonlinear form of Spectral Morison Equation. However, it is really complicated and difficult to incorporate nonlinear form of the Morison Equation to acquire the spectral density of the wave force, which is an important step of fatigue estimation. In this paper, in order to estimate the influence of nonlinear effect in wave force, fatigue assessments containing nonlinear effect for the fixed offshore structure are presented. Firstly, shallow-water jacket model locating at a water depth of 20 m is established and involved in calculation. Besides, for the sake of validating the effectiveness of the nonlinear term, the linear and nonlinear form of wave-induced force spectral densities are calculated by the Morison Equation in frequency domain. Secondly, the fatigue life of the jacket platform is assessed in time domain, where time-history of wave force can be obtained by transforming the linear or nonlinear wave force spectral densities from frequency domain to time domain. After the contrast of acquired fatigue life, the comparative results can indicate that the nonlinear drag force contributes a 14% fatigue damage to the total and the influences of the nonlinear term cannot be ignored for the jacket model.

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Time and frequency domain models for multiaxial fatigue life estimation under random loading

Frattura ed Integrità Strutturale ◽

10.3221/igf-esis.33.41 ◽

2015 ◽

Vol 9 (33) ◽

pp. 376-381 ◽

Cited By ~ 2

Author(s):

Andrea Carpinteri ◽

Andrea Spagnoli ◽

Camilla Ronchei ◽

Sabrina Vantadori

Keyword(s):

Fatigue Life ◽

Frequency Domain ◽

Multiaxial Fatigue ◽

Random Loading ◽

Life Estimation ◽

Fatigue Life Estimation ◽

Domain Models

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Application of Spectral Method for Vibration-Induced High-Cycle Fatigue Evaluation of an HP Turbine Blade

Volume 10B: Structures and Dynamics ◽

10.1115/gt2020-14477 ◽

2020 ◽

Author(s):

Iroizan Ubulom

Keyword(s):

Fatigue Life ◽

Frequency Domain ◽

Turbine Blade ◽

Euler Angle ◽

Mean Stress ◽

Critical Plane ◽

Life Estimation ◽

Fatigue Life Estimation ◽

The Mean ◽

Fully Coupled

Abstract A method of fluid-structure interaction coupling is implemented for a forced-response, vibration-induced fatigue life estimation of a high-pressure turbine blade. Two simulations approaches; a two-way (fully-coupled) and one-way (uncoupled) methods are implemented to investigate the influence of fluidsolid coupling on a turbine blade structural response. The fatigue analysis is performed using the frequency domain spectral moments estimated from the response power spectral density of the two simulation cases. The method is demonstrated in light of the time-domain method of the rainflow cycle counting method with mean stress correction. Correspondingly, the mean stress and multiaxiality effects are also accounted for in the frequency domain spectral approach. In the mean stress case, a multiplication coefficient is derived based on the Morrow equation, while the case of multiaxiality is based on a criterion which reduces the triaxial stress state to an equivalent uniaxial stress using the critical plane assumption. The analyses show that while the vibration-induced stress histories of both simulation approaches are stationary, they violate the assumption of normality of the frequency domain approaches. The stress history profile of both processes can be described as platykurtic with the distributions having less mass near its mean and in the tail region, as compared to a Gaussian distribution with an equal standard deviation. The fully-coupled method is right leaning with positive skewness while the uncoupled approach is left leaning with negative skewness. The directional orientation of the principal axes was also analyzed based on the Euler angle estimation. Although noticeable differences were found in the peak distribution of the normal stresses for both methods, the predicted Euler angle orientations were consistent in both cases, depicting a similar orientation of the critical plane during a crack initiation process. It is shown that the fatigue life estimation was conservative in the fully-coupled solution approach.

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