Creep/fatigue crack growth testing standardisation assessment and validation for specimens containing weld mismatch

Abstract

Testing and modelling of creep and creep/fatigue in welded components are essential tools for improving safe lifing methodologies. Key factors which determine a successful methodology for modelling failure in engineering components are good testing data and the development of appropriate and accurate correlating parameters to treat the results in a the unified and verifiable manner. This work considers the first two aspects and identifies important issues and improvements in the development and standardisation in creep and creep/fatigue crack growth testing or welds. Under the auspices of the Versailles Agreement on Materials and Standards (VAMAS) committee a Code of Practice on creep crack growth of components has been developed. The procedure identifies methods of testing for non-standard, welded feature test components in VAMAS TWA31 Technical Working Area on Creep and Creep/Fatigue Crack Growth of weldments containing residual stress. The overview highlights the important elements in these pre-standardisation collaborative efforts by presenting the methods of analyses and example of their application to standard fracture mechanics weld and feature type plate and pipe specimens.This paper is part of a special issue on Creep-Fatigue Crack Development List of symbolsA, s o and _ e e 0 Creep Norton's law constants A Ave , s o , _ e e 0 Average creep Norton's law constants B n , W and a Net thickness, width and crack length/mm C* Creep contour integral/J m 22 h 21 C9 and m9 Paris Law parameters da/dN Fatigue crack growth rate per cycle mm/N D, D i Material creep cracking constants f Frequency/Hz J Plastic contour integral/J m 22 K Stress intensity factor/MPâm m Function of collapse load M Weld mismatch ratio n, N Creep and work hardening exponent R Minimum to maximum load ratio t R Time-to-rupture t g Elastic/plastic geometry function g LLD or g CMOD Loadline and crack opening g w, w i Material creep cracking index _ D D c Creep load line displacement rate/ mm h 21 s Applied stress/MPa s ref Reference stress/MPa _ e e Creep strain rate/1 h 21 _ e e Ave Average creep rate/mm h 21 e f Creep failure strain/1 _ e e ref Creep strain rate at s ref /1 h 21 IntroductionThe power generation industry is striving to meet criteria for clean and sustainable energy production by increasing efficiency while simultaneously decreasing levels of chemical emissions and pollutants. The efficiency of conventional steam and gas turbine power plant can be significantly improved by increasing the operating temperature, leading to reduced fuel consumption and lower levels of harmful emissions. With the trend towards higher operating temperatures and the competing need to extend the life of existing power plant, more accurate and reliable experimental data for use in improved predictions of component lifetimes at elevated temperatures are needed.In recent years a number of European and International collaborative programmes 1-7 have developed the testing and analysis methodologies as well as a number