David W. Dean

Finite element analysis to assess the effect of initial plasticity on transient creep for defects under mechanical loading

Abstract Finite element (FE) transient creep analyses have been performed for geometries of differing constraint levels, namely compact and centre-cracked tension specimens and circumferentially-cracked cylinders. Elastic-creep and elastic–plastic-creep behaviour with the same stress exponents in power-law creep and plasticity were considered. The complete time history between initial transient creep response and steady state conditions has been analysed for a range of load levels. Estimates of J(t) and C(t) have been compared with computed values. The known asymptotic behaviour in the limits as time, t→0 and t→∞ has been confirmed. At intermediate times, estimation formulae are suggested which compare well with the FE results. The suggested expressions modify existing formulae in the literature and provide improved agreement with the FE results.

Creep Failure Simulations for 316H at 550°C

In this work a method to simulate failure due to creep is proposed using finite element damage analysis. The creep damage model is based on the creep ductility exhaustion concept. Incremental damage is defined by the ratio of incremental inelastic (plastic & creep) strain and multi-axial ductility. A simple linear damage summation rule is applied. When accumulated damage becomes unity, element stresses are reduced to almost zero to simulate progressive crack growth. The model is validated through comparison with experimental data on various sized compact tension, C(T), specimens of 316H stainless steel at 550 °C. The influence of the inelastic strain rate on the uniaxial ductility is considered. Good agreement is found between the simulated results and the experimental data.Copyright

Specimen Geometry Effects on Creep Crack Initiation and Growth in Parent Materials and Weldments

High temperature crack growth in weldments is of great practical concern in high temperature plant components. Cracking typically occurs in the heat affected zone (HAZ) and often propagates into adjacent parent material (PM). Recently, the importance of constraint effects on creep crack growth behaviour has been recognised and creep crack growth testing on a range of specimen geometries has been performed. Experimental crack growth testing has been performed at 550 °C on a range of fracture specimens using sections taken from a non-stress-relieved 316 steel weldment. These specimens include the compact tension, C(T), middle tension, M(T) and circumferentially cracked bar, CCB, geometries. Results are presented from two long-term creep crack growth (CCG) tests performed on M(T) weldment specimens and these are compared with available data on C(T) and CCB weldment specimens together with both long and short term tests on parent material for a range of specimen geometries. The creep crack initiation (CCI) and growth (CCG) behaviour from these tests has been analysed in terms of the C* parameter. As high levels of residual stress exist in non-stress-relieved weldments, the residual stresses remaining in the weldment specimens have therefore been quantified using the neutron diffraction technique. Long-term (low-load) tests are required on PM specimen to observe specimen constraint effects in 316 steel at 550 °C. When interpreted in terms of the C* parameter the CCG behavior of PM and Weldment materials follow the same trendline on low constraint geometries. However, significant difference is observed in the CCG behavior of PM and weldments on the high constraint C(T) geometry. Long term tests on C(T) specimen weldments are required to confirm the results found.Copyright

Creep Crack Growth under Complex Loading

Fitness-for-service assessments of components operating at high temperature often require estimates of creep crack growth in service. Although methods for calculating creep crack growth are given in R5 and other codes, these methods are limited in terms of their range of application and can be over-conservative. This paper extends the current methods in a number of areas. First, a general expression is derived for relaxation of a reference stress for combined primary and secondary loading and significant in-service crack growth. Second, the reference stress history is used to develop an estimate of the transient creep crack tip parameter, C(t) , for elastic-plastic-creep material behaviour for combined loading. This both extends current methods to more complex loading and reduces some known conservatism in these current methods. The general expression for C(t) then enables creep crack growth in service to be estimated using data collected from standard tests under mechanical loading. Finally, the paper presents some finite element calculations for defects in cylinders with welding residual stresses as validation for the new approaches.

Compressive Pre-Strain Effects on the Creep and Crack Growth Behaviour of 316H Stainless Steel

The effects of compressive plastic pre-strain on the creep deformation and crack growth behaviour of Type 316H stainless steel have been examined. Creep crack growth (CCG) tests have been performed on compact tension specimens of material which had been uniformly pre-strained by 4% and 8% in compression at room temperature. The CCG behaviour of the pre-compressed material has been interpreted in terms of the creep fracture mechanics parameter C* and compared with that of a significant data set of as-received (un-compressed) specimens and with CCG models. All creep testing has been performed at a temperature of 550 °C. High CCG rates, for a given value of C* have been observed for the pre-compressed material, compared with those of as-received material and these data follow the same trends as the long-term CCG data for as-received material. These observations are explained in terms of specimen constraint effects and variations in creep ductility.Copyright

R5 Procedures for Assessing the High Temperature Response of Structures: Current Status and Future Developments

The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, there are specific procedures for assessing creep-fatigue crack initiation in initially defect-free components (Volume 2/3) and for assessing components containing defects (Volume 4/5). This paper first describes in outline the current R5 Volume 2/3 and Volume 4/5 procedures. Attention is then focused on recent and proposed future developments in these procedures.Copyright

Recent Developments in the R5 Volume 2/3 Procedures for Assessing Creep-Fatigue Initiation in Defect-Free Components Operating at High Temperatures

The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, there are specific procedures for assessing creep-fatigue crack initiation in initially defect-free components (Volume 2/3) and procedures for assessing components containing defects (Volume 4/5).Recent developments in the R5 Volume 2/3 procedure for assessing creep-fatigue initiation in defect-free components include improved methods for predicting creep damage and a new approach for assessing weldments. The improved methods for predicting creep damage include the option of using the stress-modified ductility exhaustion (SMDE) approach as an alternative to the existing ductility exhaustion approach, together with improved methods for assessing compressive dwells. The new approach for assessing weldments involves splitting the existing Fatigue Strength Reduction Factor (FSRF) into a Weldment Endurance Reduction (WER), which accounts for reduced fatigue endurance due to weld imperfections, and a Weldment Strain Enhancement Factor (WSEF), which accounts for material mismatch and local geometry effects.This paper briefly outlines the current R5 Volume 2/3 procedures and then focuses on these two significant recent developments in the procedures.Copyright

The Influence of Creep-Fatigue Interaction on High Temperature Crack Growth in 316 Steel Weldments

High temperature crack growth in weldments is of great concern and generally occurs along the boundary between the heat affected zone (HAZ) and parent material (PM) of welded components in high temperature plants. Static creep and low frequency cyclic crack growth tests have been performed on compact tension, C(T), specimens manufactured from sections taken from a 316 steel weldment at 550 °C, where the crack tip was located along the fusion line within the HAZ. The data has been analyzed in terms of both the creep-brittle and creep-ductile crack tip parameters, K and C*, respectively. The cyclic test results have been compared to static creep crack growth tests on 316 steel weldments and homogeneous parent material specimens, and to crack growth models. The cracking rates of the cyclic crack growth tests are found to be higher than that of the of the static load creep crack growth tests on weldments. The data may be bounded by the high frequency fatigue and the static load creep crack growth predictions. However, further work is required to establish the fatigue and subsequently the creep component of the cyclic load crack growth tests on weldments.Copyright

Mismatch Effect of Creep Properties on Steady-State Stresses for Welded Straight Pipes: Quantification and Application

This paper describes steady-state stresses on welded straight pipes with the heat-affected zone (HAZ) using detailed two dimensional elastic creep finite element analyses. In our previous studies [9,10], it was found that the mis-match effect in creep on steady-state stresses within the weld metal for a various branch junction could be uniquely quantified by the mis-match factor, defined as a function of creep exponent and constants. The present study expands the findings to be applicable for welded straight pipes, not only the most widely used but also relatively simple compared to branch junctions. To see the effects of mis-match in creep properties and weld configuration, the parametric studies have been performed for various mismatched creep properties and three different groove angles, respectively. Internal pressure, tension and its combined cases are applied to investigate the effect of the loading mode. It is found that steady-state creep stresses can be quantified as mis-match factor and creep exponent. In conclusion, validation of the findings is presented and discussed through an application to the CMV pipe.Copyright

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