Introduction of SCRI model for creep rupture life assessment

A “service condition-creep rupture property” interference model (SCRI model for short) based on Z parameter method is proposed to predict the reliability of creep rupture life for high temperature components. The dispersibility of creep rupture property and the fluctuation of service conditions such as servicing temperature and stress are considered in this model. The scattering of creep rupture data is generally supported by normal distribution. Monte-Carlo method is applied to evaluate the reliability at a certain operating condition by simulating the fluctuation of service temperature and stress. HK40 steel has been used as an example to demonstrate the influence of creep rupture property dispersibility and operating condition fluctuation on the result of reliability evaluation.     

A creep life assessment method for boiler pipes using small punch creep test

The small punch creep (SPC) test is considered as a highly useful method for creep life assessment for high temperature plant components. SPC uses miniature-sized specimens and does not cause any serious sampling damages, and its assessment accuracy is at a high level. However, in applying the SPC test to the residual creep life assessment of the boiler in service, there are some issues to be studied. In order to apply SPC test to the residual creep life assessment of the 2.25Cr–1Mo steel boiler pipe, the relationship between uniaxial creep stress and the SPC test load has been studied.     

Integrity assessment for a tubeplate using the linear matching method

A series of numerical procedures have been presented recently for the integrity assessment of structures based upon the Linear Matching Method. A typical example of a holed plate has been used to verify these procedures for the evaluation of plastic and creep behaviours of components. In this paper, a more complex 2D tubeplate at the outlet from a typical AGR heat exchanger is analysed for the shakedown limit, reverse plasticity, ratchet limit and creep relaxation based on the application of the Linear Matching Method for a thorough case study. Both a constant material yield stress and a temperature-dependent yield stress are adopted for the evaluation of the ratchet limit. For the evaluation of accumulated creep strains, flow stresses and elastic follow-up factors with differing dwell times at the steady cyclic state, a monotonic creep computation is performed, where the start-of-dwell stress is the rapid cycle creep solution at the beginning of the dwell period. An estimation of the tubeplate lifetime is then obtained by the evaluation of fatigue and creep endurances.     

New ECCC assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91)

A first assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91) was performed by ECCC in 1995. The results were included in the European standard EN 10216. Due to a significant increase of test data and test duration it was decided in 2005 to make a re-assessment of the extended database. Different procedures have been used independently by different assessors. The method with the best overall fit of the data set has found to be the ISO CRD method. This is characterized by a two steps procedure: in the first step the mean isotherms are evaluated from the test data, afterwards the evaluated isotherms are used for averaging by a Manson–Haferd master-curve. The results have been chosen as the basis to specify long term creep rupture strength values in a new ECCC data sheet for X10CrMoVNb9-1 (Grade 91).     

An axisymmetric method of creep analysis for primary and secondary creep

A general axisymmetric method for elastic–plastic analysis was previously proposed by Jahed and Dubey [ASME J Pressure Vessels Technol 119 (1997) 264]. In the present work the method is extended to the time domain. General rate type governing equations are derived and solved in terms of rate of change of displacement as a function of rate of change in loading. Different types of loading, such as internal and external pressure, centrifugal loading and temperature gradient, are considered. To derive specific equations and employ the proposed formulation, the problem of an inhomogeneous non-uniform rotating disc is worked out. Primary and secondary creep behaviour is predicted using the proposed method and results are compared to FEM results. The problem of creep in pressurized vessels is also solved. Several numerical examples show the effectiveness and robustness of the proposed method.     

Hardness testing as a means for creep assessment

The mechanics of hardness testing during creep is investigated using Kachanov principles. Time dependent relationships between indenter geometry and load which are derived reveal that the initial indenter velocity is a measurable quantity which enables the rupture characteristics of a creeping material to be determined. Initial experimental results show reasonable agreement with predicted values.     

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.     

Steady-state creep reference rupture stresses for internally pressurised pipes for use in life prediction

In this paper, the steady-state reference rupture stresses are presented for a wide geometry range of internally pressurised pipes, under a closed-end condition, in order to investigate the applicability of the approach in life prediction. Creep damage mechanics finite element (FE) calculations were performed to obtain accurate failure lives and to determine the skeletal point rupture stresses, using material constants for two low alloy ferritic steels. Analytical solutions were used to derive steady-state reference rupture stresses, from which the effects of pipe dimensions and the tri-axial stress state parameter can be assessed. The accuracy of the life predictions was assessed, for a number of different stress levels and the design rule based on the mean diameter hoop stress. The results confirm the validity of the steady-state reference rupture stress in life prediction for a wide range of pipe geometries and tri-axial stress state parameter.     

Assessment methods of material ageing using Sdobyrev''s creep rupture model

The physical aspects of the activation energy, in higher and high temperatures, of the metal creep process were examined. The research results of creep-rupture in a uniaxial stress state and the criterion of creep-rupture in biaxial stress states, at two temperatures, are then presented. For these studies creep-rupture, taking case iron as an example the energy and pseudoenergy activation was determined. For complex stress states the criterion of creep-rupture was taken to be Sdobyrev's, i.e. σ_red = σ₁ β + (1 − β)σ_i, where: σ₁-maximal principal stress, σ_i-stress intensity, β-material constant (at variable temperature β = β(T)). The methods of assessment of the material ageing grade are given in percentages of ageing of new material in the following mechanical properties: 1) creep strength in uniaxial stress state, 2) activation energy in uniaxial stress state, 3) criterion creep strength in complex stress states, 4) activation pseudoenergy in complex stress states. The methods 1) and 3) are the relatively simplest because they result from experimental investigations only at nominal temperature of the structure work, however, for methods 2) and 4) it is necessary to perform the experimental investigations at least at two temperatures.     

Effect of specimen size and shape on creep rupture behavior of creep strength enhanced ferritic steel welds

The creep strength enhanced ferritic (CSEF) steels such as Grades 91, 92, 122, 911, 23 and 24 have become very important key materials for high efficiency fossil-fired power plants for last decades, however the long-term creep rupture strength and strength reduction in welds due to Type IV failure of these steels are serious problem to be urgently resolved. In order to use CSEF steel welds safely setting new weld strength reduction factors have internationally discussed. For instance ASME Boiler and Pressure Vessel Code Committee recently developed a new creep strength reduction factors for CSEF steels intercritically post-weld heat treated to be 0.5 based upon the creep rupture data obtained for standard creep specimens. However it is needed to make further consideration on the specimen size/shape effect on the creep strength of the welds to determine more appropriate weld strength reduction factors and joint influence factors. Present report provides comprehensive creep rupture test results of the specimens with various size and shape, and full size components dealing with creep rupture location/behavior and specimen size/shape effect on the creep rupture strength of CSEF steel welds.     

Effective metal temperature in creep life assessment

In assessing remaining creep life of high temperature components, the thermal history, as expressed by the ‘effective metal temperature’ is an important input. This temperature can be derived from quantifiable thermally induced changes such as growth of internal scale or coarsening of carbide particles in the microstructure. Conventional practice is to use this temperature to predict the consequences of a different thermally activated process, i.e. creep. Under isothermal steady-state conditions, this approach is valid. Under the variable loading conditions typical of operating plants, however, effective temperatures for different processes may not be identical. The paper addresses this issue, comparing effective metal temperatures for creep, oxidation and carbide coarsening under several forms of variable loading.     

Robust methods of life assessment during creep

The paper presents a simplified, yet robust analysis procedure for structural problems occurring in the high temperature regime where the combined effects of permanent creep deformation and progressive material degradation must be taken into account. The method described here is based on the continuum damage concepts proposed by Kachanov and Rabotnov in the late fifties and early sixties, and on the reference stress notions that originated in the late sixties. These ingredients are combined with well-tried numerical techniques such as finite element procedures to form a basis for rational creep/damage analyses. The excessive computer capabilities required for this type of analysis, together with the scarcity and uncertainty of material data, have led to the need to de-couple the creep stress analysis from the material degradation. This approach gives the designer adequate results with accuracies commensurate with the inherent uncertainties in material data and service loadings, and within acceptable computing time limits. At the same time the robust approach gives the designer the ability to establish which effects are most important whereas the coupled analysis does not. The development of the method is described in detail in the paper and is illustrated with an example of a pressure vessel nozzle. Monte Carlo simulations methods for dealing with randomness of material and geometrical parameters are illustrated in other examples.     

Reliability-based assessment of polyethylene pipe creep lifetime

Lifetime management of underground pipelines is mandatory for safe hydrocarbon transmission and distribution systems. The use of high-density polyethylene tubes subjected to internal pressure, external loading and environmental variations requires a reliability study in order to define the service limits and the optimal operating conditions. In service, the time-dependent phenomena, especially creep, take place during the pipe lifetime, leading to significant strength reduction. In this work, the reliability-based assessment of pipe lifetime models is carried out, in order to propose a probabilistic methodology for lifetime model selection and to determine the pipe safety levels as well as the most important parameters for pipeline reliability. This study is enhanced by parametric analysis on pipe configuration, gas pressure and operating temperature.     

A method for the transformation of creep constitutive equations

A method has been developed that allows the material constants of complex creep constitutive equations to be determined from the knowledge of the constitutive equations of a base material, and a set of normalized material property ratios. These ratios are based on the global features of creep behaviour, such as minimum creep strain rates, lifetimes and failure strains, which may be determined, or estimated, without the need for detailed records of the creep strain which are not available in many circumstances due to the expense, or difficulty, of testing. The normalized material property ratios are formed between the global features of the creep behaviour for a non-base material, for which the material constants are to be determined, and a base material, for which the constitutive equations and their constants are accurately known. It is shown how these normalized material property ratios may be combined into a mathematical function that, when minimized, yields the constitutive constants of the non-base material. The method has been developed for a set of constitutive equations which incorporates three state variables to describe the creep behaviour of a ferritic steel. Two extrapolations of these constitutive equations to different temperatures are presented as a verification of the method. As an example, the method is used to determine the constitutive equations for the Type IV material region of a ferritic steel weldment from a knowledge of the constitutive equations for the parent ferritic steel. Finally discussed are the wider uses and implications of the method.     

Life estimation of pressurised pipe bends using steady-state creep reference rupture stresses

Steady-state reference rupture stresses were obtained for a range of 90° pipe bends, subjected to internal pressure only, using simplified 2D axisymmetric finite element (FE) models. The bends were considered to be circular in shape and not include any ovality. Creep damage FE analyses were performed to obtain realistic failure lives and to determine the skeletal point rupture stresses, using the material properties, obtained at 640 °C, for a service-exposed CrMoV pipe steel. The effects of the normalised pipe bend dimension on the reference rupture stresses are presented. The results obtained confirm the validity of the use of the steady-state reference rupture stress in life estimation for a wide range of pressurised pipe bend geometries. The life predictions were compared with those of the corresponding straight pipes and their relevance considered.     

An approximate and computationally efficient algorithm for computing reference stress for creep life assessment

One of the powerful methods that are currently available for the design and life assessment of components that operate within the creep range is the reference stress (RS) method. However, for problems for which the RS is not available from existing solutions, one usually needs to use a non-linear finite element method which is normally iterative, time-consuming and computationally expensive. An efficient and effective method for computing an approximate value for RS is described that combines a lower-bound theorem and finite-element discretization. The resulted quadratic programming is solved by an active set algorithm. The verification and application of the proposed method are also described.     

Remaining creep life estimation by strain assessment on plant

By utilising a generalised damage parameter first introduced in the Rabotnov-Kachanov equations for tertiary creep a model is proposed for remanent creep life prediction based on in situ strain assessment. It is shown that for relatively ductile materials the rupture life can be accurately related to a single strain or strain rate measurement without a knowledge of the rupture strain. Materials data are required only in the form of the minimum creep rate-rupture life product. Considerable evidence suggests that the latter is approximately constant for relatively low stresses in a range of materials so that the specific creep response of the component material is not required. It is also demonstrated that an assessment of strain at more than one stage of the life negates the need for materials data. Consideration is given to the effect of multiaxial stressing and the model is applied to the life prediction of low alloy ferritic steel tubes and pipes.     

Damage assessment method of P91 steel welded tube under internal pressure creep based on void growth simulation

Development of creep damage assessment methods for longitudinal welded piping of P91 steel is important and an urgent subject to maintain reliable operation of boilers in ultra super critical thermal power plants. Internal pressure creep tests were conducted on P91 steel longitudinal welded tubes to characterize the evolution of creep damage in a heat-affected zone (HAZ) of the longitudinal welded pipe. Failure occurred at a heat-affected zone without significant macroscopic deformation. It was found that initiation of creep voids had concentrated at mid-thickness region rather than surface. Three-dimensional finite element (FE) creep analysis of the creep tested specimens was conducted to identify stress and creep strain distribution within the specimen during creep. Finite element creep analysis results indicated that triaxial tensile stress yielded at the mid-thickness region of the HAZ. It was suggested that the triaxial stress state caused acceleration of the creep damage evolution in the heat-affected zone resulting in internal failure of the tube specimens. Void growth behavior in the heat-affected zone was well predicted with the previously proposed void growth simulation method by introducing void initiation function to the method. A “limited strain” was defined as rupture criterion and dependency of the maximum stress and multiaxiality on the “limited strain” was derived by the void growth simulation. Creep damage distribution in the HAZ under the internal creep test was calculated by proposed damage assessment method.