Stress Evaluation Method by Frequency Response Function for Elbow Pipes Under Thermal Stratification

2018 ◽  
Author(s):  
Salman Alrakan ◽  
Hiroshi Kuribayashi ◽  
Naoto Kasahara
Keyword(s):  
Finite Element ◽  
Frequency Response ◽  
Fatigue Damage ◽  
Response Function ◽  
Thermal Fatigue ◽  
Thermal Stratification ◽  
Frequency Response Function ◽  
Generation Mechanism ◽  
Stress Generation ◽  
Fluid Temperature

In nuclear reactors, piping components are susceptible to thermal fatigue damage. This is due to the fluid temperature change along these pipelines that can generate repeated thermal loads. One of these loads is thermal stratification. Thermal stratification generates an oscillating stratified layer, which induce cyclic thermal stresses leading to fatigue damage. To evaluate thermal fatigue by thermal stratification, a frequency response function for straight pipes was developed. However, this function cannot evaluate elbow pipes under thermal stratification. Here, thermal stress generates due to bending moment that is generated by the horizontal portion unlike straight pipes. Furthermore, the elbow pipe can give rise to stress intensifications which can affect the peak stress values within the elbow. To understand the stress generation mechanism, Finite element analyses were performed. The study focused on the effect the frequency of the fluid oscillation on the stress generation mechanism. Based on the clarified mechanism, the frequency response function was improved to correspond to the thermal stratification at elbow pipes. Applicability of this function was validated through agreement with finite element simulation.

Thermal Stress Response to Boundary Oscillation Between Hot and Cold Fluid Temperature

2014 ◽  
Author(s):  
Kohei Soda ◽  
Takato Mizutani ◽  
Naoto Kasahara
Keyword(s):  
Thermal Stress ◽  
Frequency Response ◽  
Response Function ◽  
Thermal Fatigue ◽  
Thermal Stratification ◽  
Frequency Response Function ◽  
Nuclear Power ◽  
Fluid Temperature ◽  
Thermal Striping ◽  
High Cycle Thermal Fatigue

In nuclear power plants, high cycle thermal fatigue induced by temperature fluctuation of the coolant is one of frequent failure modes. To ensure the safety of nuclear power plant systems, it is important to prevent thermal fatigue failure. Typical causes of high cycle thermal fatigue are thermal striping at Tee-junction and thermal stratification oscillation. In order to evaluate thermal stress caused by thermal striping, a frequency response function has been developed. This function was derived from a heat transfer and thermal elastic theories, and can adequately evaluate thermal stress induced by temperature gradient into wall-thickness direction. However, this theoretical method cannot adequately evaluate thermal stress by thermal stratification oscillation, because this phenomenon has the fluid temperature distribution gradient along axial direction. To investigate the mechanism of thermal stress generated by oscillation of thermal stratification, two types of models were studied. In the first type, fluid temperature oscillates with sinusoidal history at the same location, and in the second one, the boundary layer of hot and cold fluid temperature moves with sinusoidal velocity. Through clarification of the stress generation mechanism, the frequency response function was improved to evaluate stress by the thermal stratification oscillation. Applicability of this function was verified through agreement with finite element simulations.

Change of modal parameters due to crack growth in a tripod tower platform

1993 ◽  
Vol 20 (5) ◽  
pp. 801-813 ◽  
Author(s):  
Yin Chen ◽  
A. S. J. Swamidas
Keyword(s):  
Finite Element ◽  
Frequency Response ◽  
Response Function ◽  
Frequency Response Function ◽  
Experimental Results ◽  
Modal Testing ◽  
Modal Parameters ◽  
Response Functions ◽  
Frequency Response Functions ◽  
Strain Frequency

Strain gauges, along with an accelerometer and a linear variable displacement transducer, were used in the modal testing to detect a crack in a tripod tower platform structure model. The experimental results showed that the frequency response function of the strain gauge located near the crack had the most sensitivity to cracking. It was observed that the amplitude of the strain frequency response function at resonant points had large changes (around 60% when the crack became a through-thickness crack) when the crack grew in size. By monitoring the change of modal parameters, especially the amplitude of the strain frequency response function near the critical area, it would be very easy to detect the damage that occurs in offshore structures. A numerical computation of the frequency response functions using finite element method was also performed and compared with the experimental results. A good consistency between these two sets of results has been found. All the calculations required for the experimental modal parameters and the finite element analysis were carried out using the computer program SDRC-IDEAS. Key words: modal testing, cracking, strain–displacement–acceleration frequency response functions, frequency–damping–amplitude changes.

A Stochastic Approach of Thermal Fatigue Crack Growth (LEFM) in Mixing Tees

2010 ◽  
Author(s):  
Vasile Radu ◽  
Elena Paffumi
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Frequency Response ◽  
Crack Growth ◽  
Response Function ◽  
Thermal Fatigue ◽  
Frequency Response Function ◽  
Narrow Band ◽  
Stochastic Approach ◽  
Thermal Fatigue Crack

The assessment of fatigue crack growth due to turbulent mixing of hot and cold coolants presents significant challenges, in particular to determine the thermal loading spectrum. Thermal striping is defined as a random temperature fluctuation produced by incomplete mixing of fluid streams at different temperatures, and it is essentially a random phenomenon in a temporal sense. The objective of this work is to develop a stochastic model to assess thermal fatigue crack growth in mixing tees, based on the power spectral density (PSD) of the temperature fluctuation at the inner pipe surface. Based on the analytical solution for temperature distribution through the wall thickness, obtained by means of Hankel transform, a frequency temperature response function is proposed, in the framework of single-input, single-output (SISO) methodology from random noise/signal theory under sinusoidal input. For the elastic thermal stresses distribution solutions, the magnitude of the frequency response function is first derived and checked against the prediction by FEA. The frequency response of the stress intensity factor (SIF) is obtained by a polynomial fitting of the stress profiles through the wall thickness at various instants of time. The variability in load is given by the statistical properties of thermal spectrum. The temperature spectrum is assumed to be given as a stationary normalized Gaussian narrow-band stochastic process, with constant PSD for a defined range of frequencies. The connection between SIF’s PSD and temperature’s PSD is assured with SIF frequency response function modulus. The frequency of the peaks of each magnitude for KI, which is supposed to be a stationary narrow-band Gaussian process, is characterized by the Rayleigh distribution, and, consequently, the expected value of crack growth rate in respect to cycles is obtained. The probabilities of failure are estimated by mean of the Monte Carlo methods considering a limit state function, which is based on the developed stochastic model. The results of the stochastic approach of thermal fatigue crack growth in mixing tees is completed with probabilistic input to account for the variability in the material characteristics, and finally an application is given to obtain the probability of mixing tees piping failure as function of time reference period.

Sign in / Sign up

Export Citation Format

Share Document