An Engineering Approach to the Prediction of Creep Crack Growth

Abstract

This paper is concerned with assessing the integrity of cracked engineering components which operate at elevated temperatures. Fracture mechanics parameters are discussed for describing creep crack growth. A model is presented for expressing growth rate in terms of creep damage accumulation in a process zone ahead of the crack tip. Correlations are made with a broad range of materials exhibiting a wide spread of creep ductilities. It is found that individual propagation rates can be predicted with reasonable accuracy from a knowledge only of the material uni-axial creep ductility. An engineering creep crack growth assessment diagram is proposed which is independent of material properties but which is sensitive to the state of stress at the crack tip. Approximate bounds are presented for plane stress and plane strain situations and it is shown that crack growth rates about fifty times faster are expected under plane strain conditions than when plane stress prevails.

A unified European approach to high temperature defect assessment code and its incorporation in a knowledge base system

“…where o D and φ are material constants which can be measured experimentally using the NSW model [14] based on a creep zone model. Equation (7) can be approximated to…”

“…The initial stage of cracking prior to steady growth exhibits a transient phenomenon [12][13][14][15][16] due to the build up of damage at a crack tip prior to the onset of steady state behaviour. This can lead to an incubation (as shown in Fig.…”

“…For standard laboratory testing [12][13][14] where fracture mechanics specimens are employed to produce material crack growth properties at elevated temperatures, C* is derived experimentally using the loadline creep displacement rate [13] giving…”

“…for a specimen of thickness B n between side-grooves, width W and subjected to a load P. In this expression ∆ & is the experimentally measured load point creep displacement rate and F is a nondimensional factor which depends on the stress sensitivity of creep n and crack length a [12][13][14]. Most data bases for experimental creep crack growth use equation (3) for analysis but without proper validation between the two methods there may exist a clear discrepancy between results obtained from equation (2) and equation (3).…”

Effects of residual stress in creep crack growth analysis of cold bent tubes under internal pressure

“…Several parameters such as stress intensity factor, K [9], reference stress, σ ref [10] and C* [11] have been used to correlate creep crack growth rate data of cracked components at elevated temperatures. The correlations of steady state crack growth rate with K, reference stress and C* can be represented by straight lines of different slopes on log/log plots and expressed by power laws of the form…”