Creep Crack Incubation Research Articles

Creep Stress Relaxation and Crack Incubation in the Presence of Structural Elastic Follow-Up

This paper explores the behavior of steel components operating at high temperature subjected to combinations of applied and residual stresses. Two series of experiments are summarized that used Type 316H stainless steel samples tested at 550 °C. First, creep stress relaxation under displacement control with elastic follow-up is explored. Experimental results are compared with theoretical predictions and show that accounting for stress history during relaxation is necessary. A second set of experiments explores how combinations of applied and residual stress influence creep crack incubation, again with elastic follow-up. Incubation times with elastic follow-up close to conventional displacement control are longer than constant load.

The effects of long-range residual stress, elastic follow-up and applied load on creep crack incubation and material toughness

Creep crack incubation of Type 316H stainless steel at 550 °C is explored in this article. Fracture mechanics specimens, subjected to combinations of residual and applied loads and in the presence of elastic follow-up, are tested. The design of two new test rigs is described. The rigs introduce planned levels of elastic follow-up together with combined residual and applied loading conditions to the specimens. A series of high-temperature elastic–plastic and elastic–plastic–creep experiments are undertaken to compare the experimentally determined values of elastic follow-up with the theoretical values. A further series of fracture mechanics tests are performed to measure creep crack incubation and material toughness for samples subjected to constant load and for tests under combined loading with elastic follow-up. It is demonstrated that for tests subjected to the same initial reference stresses, longer incubation times are attained for elastic follow-up tests compared to constant load tests. Also, combined loading tests exhibit longer creep crack incubation times based on the same measured material toughness obtained from constant load tests. This suggests that not all the available strain energy provided by combined loading to a specimen at high temperature contributes to creep crack incubation.

Numerical investigation of creep crack initiation in P92 steel pipes with embedded spherical defects under internal pressure at 650 °C

Creep crack initiation position and incubation time of P92 steel pipes with embedded spherical defects at 650°C were investigated with finite element method. The function for the relationship between creep crack initiation position and the geometric parameters was obtained by fitting methods. The average equivalent stress was proposed to characterize the stress variation throughout the creep process and the mathematical relationships between it and the geometric parameters, internal pressures, creep crack incubation time were established through nonlinear regression. Finally, the proposed assessment function for creep crack incubation time was deduced from the above functions.

Effect of Elastic Follow-up on Creep Crack Incubation

Structural components operating at high temperature are often susceptible to the incubation and growth of cracks by creep. It is conventional to characterise this behaviour by subjecting laboratory specimens to a selection of constant loads and measuring the time taken for a pre-existing crack to extend a finite amount. In practice, structural components are subjected to external loading such that material at stress concentrations experiences boundary conditions that are a mixture of load and displacement control. Often these boundary conditions are associated with elastic follow-up. This paper describes a set of experiments undertaken to quantify the influence of elastic follow-up on creep crack incubation in a high carbon, high chromium, martensitic (P92) steel at 650°C. The experiments were performed using parallel bar test rigs designed in a previous research programme (Shirahatti et al [1,2]). The test rigs used modified compact tension C(T) specimens fitted adjacent to two parallel bars that introduced elastic follow-up as the C(T) specimen deformed.To complement the elastic follow-up experiments, a set of standard uniaxial round bar creep tests and constant load-controlled creep crack incubation tests were conducted. The uniaxial tests enabled observation of the elastic and creep response of the steel. These data were used as input to both analytical and finite element models of the elastic follow-up experiments.The crack incubation times obtained from the elastic follow-up experiments were found to be significantly longer than those for equivalent conventional constant load conditions. This was assumed to be a result of the load on the specimen relaxing during elastic follow-up. Finite element models of the experiments confirmed the effect of the load relaxation.

Exploring the applicability of the LICON methodology for the creep assessment of a dissimilar metal weld

The LICON methodology is an approach for predicting the lifetime of materials under creep loading conditions. The LICON method predicts long-time uniaxial creep strength using the results from several short duration creep crack incubation (CCI) tests in conjunction with the outcome of a mechanical analysis for the adopted multiaxial specimen geometry. The applicability of the methodology for long term creep strength prediction of martensitic 9%Cr and bainitic 1%Cr steels has already been demonstrated. This study has examined the applicability of the procedure for predicting long term uniaxial creep strengths for a dissimilar metal weld (DMW). It has required new developments to the original formulation. Application of the developed formulation for predicting uniaxial creep rupture behaviour of the investigated DMW shows an acceptable agreement with experimental observations which was not previously achievable.

The LICON methodology for predicting long-time uniaxial creep rupture strength of materials

The LICON methodology is an approach for predicting the lifetime of materials under creep loading conditions. The LICON method predicts long-time uniaxial creep strength using the results from several short duration creep crack incubation (CCI) tests in conjunction with the outcome of a mechanical analysis on the testpiece. This method was first applied to creep ductile, notch insensitive materials for which a reference stress solution was appropriate (e.g. advanced 9% Cr pipe steels). The original concept for the mechanical analysis part of the methodology was therefore to adopt reference stress solutions. This study explains the challenges involved in application of the reference stress based LICON method to creep brittle, notch sensitive materials and identifies the need for a careful implementation of advanced mechanical analyses (i.e. finite element analysis, FEA) for this application. Details are presented for the consideration of FEA in the LICON methodology, and the demonstration of a successful application of the FEA based LICON method for creep life assessment of a non-reference stress material, i.e. high creep strength (HCS) 1CrMoV steel.

Stress regime-dependent creep constitutive model considerations in finite element continuum damage mechanics

Structural analysis and the design of high-temperature components require the consideration of material inelastic deformation and damage accumulation characteristics for a wide range of stresses and temperatures. For many engineering alloys, the creep deformation/damage accumulation mechanism exhibited at high stresses are not the same as those at lower stresses. This article explains the importance of considering this stress regime dependency in the creep model formulations and introduces a new (primary–secondary–tertiary) creep model which considers a gradual change of creep deformation/damage accumulation mechanism with stress variation. Application of the new stress regime-dependent creep model formulation in finite element continuum damage mechanics to simulate creep deformation/damage accumulation in a series of creep crack incubation tests involving fracture mechanics compact tension specimens shows a good agreement with experimental observations which was not achievable by considering conventional single-regime creep model equations.

Creep constitutive model considerations for high-temperature finite element numerical simulations

Finite element modelling is increasingly used as an integral part of creep analyses for the integrity assessment of high-temperature structures. An important consideration in such finite element simulations is the constitutive model used to represent the creep strain response of the component material as a function of temperature, stress and time. There are a variety of creep models which can be chosen by the analyst for implementation in finite element codes. In this study, five different creep models have been fitted to a set of experimental uniaxial creep curves for a 1%CrMoV turbine rotor steel at 550 °C. Subsequently, the derived constitutive equations have been implemented in finite element model representations of a series of fracture mechanics compact tension specimens manufactured from the same heat of the steel and loaded at the same temperature. The outcome clearly demonstrates the potential sensitivity of high-temperature numerical analyses of structures to the type of creep model adopted, and to the scope of the experimental data from which the model is derived. This is shown by comparing the load point displacement records from a number of compact tension specimen creep crack incubation tests with the results of finite element simulations employing the different creep deformation models. FE calculated steady-state creep stress/strain distributions ahead of the notches of compact tension specimens can also exhibit a strong sensitivity to the type of creep model adopted. Prior benchmarking the effectiveness of finite element simulation procedures for critical high-temperature components with the selected material creep model equation is therefore strongly recommended.

A Numerical Strip-Yield Model for the Creep Crack Incubation in Steels

Abstract A numerical strip-yield model was developed to simulate creep crack incubation in heat-resistant steels. The model is based on a formulation proposed by Newman (Newman, J. C., Jr., “A Crack-Closure Model for Predicting Fatigue Crack Growth under Aircraft Spectrum Loading,” Methods and Models for Predicting Fatigue Crack Growth under Random Loading, ASTM STP 748, J. B. Chang and C. M. Hudson, Eds., ASTM International, West Conshohocken, PA, 1981, pp. 53–84) for fatigue crack growth under variable amplitude loading. The time evolution of the plastic deformation ahead of a crack loaded in tension is modeled using the Norton law for secondary creep stage, and the primary and tertiary creep stages are neglected. The model assumes a pre-existing crack in a specimen and models the behavior of the material prior to the beginning of crack propagation due to creep loading. The evolution with time of the crack-tip plastic zone, crack-tip opening displacement, and yield strength in the plastic zone are computed at constant temperature for center crack panels. Comparison with two previous strip-yield models and experimental data is performed, and good correlation is obtained for several Cr-Mo-V steels. This approach to modeling creep crack incubation has the potential to be applied to other types of cracked specimens under constant or variable amplitude loading.

Experience with using the LICON methodology for predicting long term creep behaviour in materials

The LICON methodology is a newly developed approach for predicting the lifetime of materials under creep loading condition. The LICON method has gained attention for application to newly developed materials, for which there is little existing creep-rupture data, and for their welded structures. The LICON method predicts long term uniaxial creep strength using the results from several short duration creep crack incubation tests in conjunction with the outcome of a mechanical analysis (e.g. involving finite element analysis, at least initially for a new material and/or multi-axial specimen geometry). In this study, the sensitivity of results from the LICON method to input conditions for the associated finite element analysis is investigated. Two important considerations for creep finite element analyses are the mesh configuration and the type of creep constitutive model used. In order to evaluate the sensitivity of the outcome of the approach to finite element mesh condition, twelve different configurations in terms of size, geometry and interpolating formulation are examined. Moreover, to examine the sensitivity of the outcome of the approach to the type of creep equation employed, three constitutive models have been evaluated. Finally, the sensitivity of the LICON approach to the evaluated conditions have been discussed with reference to the information contained in a comprehensive creep-rupture database available for a 1%CrMoV steel at 550 °C.

Using the results of creep crack incubation tests on CrMoV steel for predicting long time creep rupture properties

The results of a series of creep crack incubation (CCI) tests have been used to examine the effectiveness of the LICON methodology for predicting long duration uniaxial rupture strength of a CrMoV steel at 550° C. The study has revealed that effective long time predictions can be made, but only with the availability of additional information including: an awareness of the short and long time rupture mechanisms and the associated multi-axial rupture criteria obeyed by the material, the results of uniaxial and multi-axial creep tests, and a knowledge of the steady state creep stress conditions existing in the testpieces (structures) forming part of the evaluation. In order to apply the creep damage enhancement approach to the low alloy creep resistant steel, it has been necessary to establish the steady-state creep stress state in the compact tension testpiece geometry used in the investigation. The new evidence is reviewed.

Application of the LICON methodology to a low-alloy ferritic steel

An investigation to examine the application of the LICON methodology to a low-alloy ferritic steel is described. The study involves creep crack incubation testing of a 1% CrMoV steel at 550 °C and associated numerical and finite element analyses. It is demonstrated that the methodology may be applied to low-alloy ferritic steels, but with refinements to the way in which the concept was originally developed for the advanced 9% Cr steels.

Exploring the applicability of the LICON methodology for a 1%CrMoV steel

The results of a series of creep crack incubation (CCI) tests have been used to examine the effectiveness of the LICON methodology for predicting long duration uniaxial rupture properties of a 1%CrMoV steel at 550°C. It has been demonstrated that good predictions can be made using this approach but that, for this to be possible, a knowledge of additional information is also required including uniaxial creep strain data, uniaxial and multi-axial rupture time properties and the results of finite element analysis.In order to apply the creep damage enhancement approach to the low alloy creep resistant steel, it has been necessary to establish the steady-state creep stress state in the compact tension testpiece geometry used in the investigation. The new evidence is described.

Comparison of different approaches for estimation of creep crack initiation

In large components such as rotors defects due to manufacturing processes have to be taken into account and crack assessments based on findings of non-destructive evaluation are necessary. Approaches are used in remaining life estimations, for example: • Time Dependent Failure Assessment Diagram (TDFAD), • Two Criteria Diagram (2CD) and • Nikbin–Smith–Webster-Model (NSW-Model). The TDFAD approach is currently being developed within the R5 procedures as an alternative to conventional methods for predicting incubation and the early stages of Creep Crack growth. A key requirement of TDFAD approaches is the evaluation of a time dependent creep toughness, denoted K c mat . The 2CD approach has been developed independently in Germany to assess Creep crack incubation in ferritic steels. This approach uses crack tip and ligament damage parameters, R K and R σ , respectively. Furthermore the NSW-Model is employed for the estimation of creep crack initiation by using the creep fracture mechanics parameter C ∗. Calculations and used parameters were compared for a ferritic 1CrMoV-steel.

Developments in the use of creep crack initiation for design and performance assessment

In large components such as rotors defects due to manufacturing processes have to be taken into account and crack assessments based on findings of non-destructive evaluation are necessary. Similar approaches are used in remaining life estimations. The Time Dependent Failure Assessment Diagram (TDFAD) approach is currently being developed within the R5 procedures as an alternative to conventional methods for predicting incubation and the early stages of creep crack growth. A similar Two Criteria Diagram approach has been developed independently in Germany to assess creep crack incubation in ferritic steels. Both approaches have been investigated in a study initiated by ECCC Working Group 1.2 “Creep Crack Initiation”. Calculations and parameters used for the analysis such as reference stresses and fracture mechanics parameters have been compared for two materials, a ferritic 1CrMoV steel and an austenitic steel of type 316H.

Developments in the use of creep crack initiation for design and performance assessment

In large components such as rotors defects due to manufacturing processes have to be taken into account and crack assessments based on findings of non-destructive evaluation are necessary. Similar approaches are used in remaining life estimations. The Time Dependent Failure Assessment Diagram (TDFAD) approach is currently being developed within the R5 procedures as an alternative to conventional methods for predicting incubation and the early stages of creep crack growth. A similar Two Criteria Diagram approach has been developed independently in Germany to assess creep crack incubation in ferritic steels. Both approaches have been investigated in a study initiated by ECCC Working Group 1.2 “Creep Crack Initiation”. Calculations and parameters used for the analysis such as reference stresses and fracture mechanics parameters have been compared for two materials, a ferritic 1CrMoV steel and an austenitic steel of type 316H.

THE USE OF A FAILURE ASSESSMENT DIAGRAM FOR INITIATION AND PROPAGATION OF DEFECTS AT HIGH TEMPERATURES

AbstractThe use of a failure assessment diagram, of the type in the R 6 defect assessment procedure, is investigated for creep crack growth under steady loading. While a detailed approach based on C* remains attractive for appreciable creep crack growth, it is shown that a simplified approach can be formulated for limited crack extension. A failure assessment diagram is derived based on the option 2 curve of R 6 using isochronous stress‐strain data. The inclusion of elastic strains in the isochronous data covers stress redistribution effects.Equations are given which enable a toughness, Kmat, for assessments at temperatures in the creep range to be evaluated from creep crack incubation and growth data presented in terms of C*. The toughness, Kmat, replaces the fracture toughness used in low temperature R 6 assessments. Thermal stresses can be included in assessments by evaluating the stress intensity factor for the combined thermal and mechanical loading. A formula is given which enables the effect of thermal stresses to be reduced when creep strains are sufficient to relax out part or all of the thermal stress.

Strip yield models of creep crack incubation and growth

The strip yield model of Dugdale and Bilby, Cottrell and Swinden is applied to analyse the incubation and growth of creep cracks. This extends previous work by Vitek (on incubation) and by Heaton and Chan (following To) on growth. Following both Vitek and Heaton and Chan, the main criterion of crack growth used is a COD-based one. The crack is assumed to grow at constant crack opening displacement (COD) φ=φc and the creep strain ∈t at the crack tip is assumed to be φ/h, where the gauge length is a material constant. The COD-based theory indicates that the incubation and failure timest i,t fare controlled by the stress intensity factorK at large stresses, but by the net stress at very low stresses. A weakness of the theory is that it neglects primary-creep effects: these have to be absorbed by modifying the secondary-creep index. If this is done, then comparison with two independent experiments shows moderately good agreement with the COD-based theory for botht iandt f.