Ken U. Snowden

Modelling creep of pressure vessels with thermal gradients using Theta projection data

Pressure vessels are often exposed to through-wall temperature gradients. Thermal stresses occur in addition to pressure stresses. The resulting creep response is calculated using the Theta projection creep algorithm within a finite element code. It was found that the stress and temperature dependence of the creep response may lead to complex stress evolution.

Finite element analysis of creep using Theta projection data

The Theta projection creep algorithm has been implemented within a finite element code. This extends the predictive capability of Theta projection data to complex geometries, multiple material problems such as welded joints, and non-steady temperature conditions. Validation of the finite element methodology has been undertaken by re-modelling creep data based on the original Theta coefficients. The stress redistribution in a cylindrical pressure vessel was examined and compared to that predicted by the Norton equation. The effects of temperature variation were also modelled.

Creep Modeling of Welded Joints Using the Theta Projection Concept and Finite Element Analysis

Modeling of welded joints under creep conditions with finite element analysis was undertaken using the theta projection method. The results were compared to modeling based on a simple Norton law. Theta projection data extends the accuracy and predictive capability of finite element modeling of critical structures operating at high temperature and pressure. In some cases analyzed, it was found that the results diverged from those gained using a Norton law creep model.

Creep-Fatigue Prediction of Low Alloy Ferritic Steels Using a Strain Energy Based Methodology

The accumulation of creep-fatigue damage over time is the principal damage mechanism which will eventually lead to crack initiation in critical high temperature equipment. A model that calculates the creep damage under conditions of strain control has been developed that assumes on a macroscopic level that the energy dissipated in the material may be taken as a measure of the creep damage induced in the material. This then assumes that the creep damage is directly proportional to absorbed internal energy density. The model developed is derived from considerations of mechanistic cavity growth. The model makes use of already existing creep data and relatively easily determined fatigue data for estimation of life under non-steady state conditions. The predictions of the energy-density exhaustion approach are compared with the results of creep-fatigue tests on a low alloy ferritic steel 1/2Cr-1/2Mo-1/4V (CMV) and with creep-fatigue calculations using a number of current models. The predicted results of the energy-density model are found to have good correlation with the measured creep-fatigue lives.Copyright