Creep Ductility Exhaustion Research Articles

Prediction of creep crack initiation time based on constraint parameters in specimens with different geometries

Based on two creep constraint parameters (R* and Ac), the constraint-dependent creep crack initiation (CCI) time equations of a Cr–Mo–V steel at 566 °C have been established over a wide range of C* region. The equations are independent on the choice of the reference specimen. Based on the two-parameter (C*-R* and C*-Ac) and the constraint-dependent CCI time equations, the CCI time for the specimens with different geometries and dimensions (with a wide range of constraint levels) has been predicted over a wide range of C* region. The predicted CCI time agrees well with experimental data available and CCI time of finite element simulations based on the creep ductility exhaustion model. The specimen geometry with high constraint strengthens the effect of out-of-plane constraint on the CCI time. The two constraint parameters R* and Ac can give similar CCI time prediction results. The two-parameter approaches with considering constraint effect may be applied in the CCI life assessment of high-temperature components (such as pressure pipes and vessels) under static loads in operating plants.

Creep-Ductility of High Temperature Steels: A Review

A number of measures of the creep-ductility of high temperature steels are reviewed with an ultimate focus on intrinsic creep-ductility. It is assumed that there will be a future requirement for the determination of long duration creep ductility values for design and product standards in the same way as there is currently for creep strength values. The determination of such information will require specialist modelling techniques to be applied to the complex nature of multi-temperature, multi-heat (multi-cast) data collations, and possible solutions are considered. In service, the exhaustion of creep-ductility is most likely to occur at stress concentrations, and for this, a knowledge of the multiaxial creep-ductility is required, and its relationship to uniaxial creep-ductility. Some practical applications requiring a knowledge of creep-ductility are reviewed.

The effect of creep strain rate on damage accumulation in Type 316H austenitic stainless steel

Many service components in power generation and aerospace industries operate at high temperatures and stresses that make them susceptible to creep deformation and damage. Their complex geometries and load multi-axiality are often treated only approximately in assessing their structural integrity via assessment codes that are based on standard creep tests. For example, the forward creep (defined here as constant load creep) test of round bars is not a true representation of the stress state that service components generally experience. The experiments conducted in this work used notched bar specimens to simulate the effect of stress triaxiality. The results from these experiments were then used to validate a well-established creep ductility exhaustion damage model. Although the damage model is largely based on uniaxial creep rupture tests, it has been previously adapted so that it can be applied to more complex states of stress. Rupture calculations were conducted prior to experimental testing to obtain an estimation of the duration of the experiments. The finite element simulation results, which utilised previously developed creep deformation and damage models, were then compared to the experimental data. It was shown that the model predicted the correct trend for the creep deformation and failure of the specimens and primary, secondary and tertiary creep behaviour of notched bars could be captured. The tests imply that the effective creep ductility was smaller at lower stresses, i.e., at slower strain rates creep strain was more damaging.

Prediction of creep crack initiation behavior considering constraint effects for cracked pipes

The creep crack initiation (CCI) location and time of axial surface cracks with different sizes in cracked pipes have been comparatively predicted by finite element calculations based on creep ductility exhaustion model and creep fracture mechanics considering constraint effects by using two new creep constraint parameters (R∗ and Ac). The effects of calculation methods of the parameters C∗, R∗ and Ac along the crack fronts on the prediction accuracy of CCI time have been analyzed. It has been found that with decreasing crack length, the CCI location changes from the deepest part to near surface part along the crack front, and the composite parameters composed of C∗ and constraint parameters can predict the CCI location. The CCI time of pipe cracks increases with decreasing crack depth and length due to the decrease of crack-tip constraint level. The excessive conservatism in conventional CCI life assessments using single parameter C∗ can be significantly reduced by using the two-parameter concept (C∗-R∗ and C∗-Ac) considering constraint effects. When the average values of C∗, R∗ and Ac along the crack fronts (instead of the values at the deepest locations of crack fronts) are calculated and used in CCI life predictions of cracked pipes, the prediction accuracy with respect to the finite element calculations based on creep ductility exhaustion model can be significantly improved. The C∗ values calculated by reference stress method can lead to extra conservatism in CCI life predictions.

Creep ductility of 1CrMoV rotor steel

The creep ductility of 1CrMoV steels at a given temperature is high at high stresses responsible for high strain rates and ductile rupture, and low at lower stresses responsible for low strain rates and constrained cavity growth at grain boundaries. The magnitudes of ductility in the high and low stress regimes and the time to, and strain rate at, the transition between the two is determined by the chemical composition and the adopted quality heat treatment procedure of the steel. The basis for a material pedigree function forming part of a creep ductility model for 1CrMoV rotor steel is presented. Low ductility 1CrMoV steels are expected to be extremely notch sensitive. While this appears to be true for medium to high strength heats of the alloy, it is not necessarily the case for lower strength heats. There is not a simple inverse relationship between creep ductility and creep strength. Increasingly, creep-fatigue lifetime predictions for high temperature 1CrMoV power plant components subjected to thermo-mechanical transients employ a creep ductility exhaustion methodology to determine the creep damage accumulated per cycle. A creep ductility model of the type developed in the paper is suitable for forming the basis of such an approach.

New model for creep damage analysis and its application to creep crack growth simulations

A creep damage model from the micromechanics viewpoint is presented in the paper. In order to mitigate the difficulty of calibrating many parameters in the existing damage evolution models, a simple creep ductility exhaustion approach is employed to account for the accumulation of the creep damage. Two-dimensional and three-dimensional numerical analyses of creep crack growth based on the new creep-damage model and/or Liu-Murakami model are carried out. When the damage parameter reaches a critical value, the load carrying capacity of each damaged element approaches zero and thus crack growth can be characterised by a completely damaged element zone ahead of the initial crack tip. The finite element simulation results obtained are compared favourably with experimental data for the compact tension specimen for 316 stainless steel and bending cracked plate for T91 steel at elevated temperatures. The comparisons show the excellent capability of the proposed model in predicting the crack growth rate and progressive crack profiles. In addition, the influences of the plasticity and mesh size are discussed in this paper.

The Bree problem with different yield stresses on-load and off-load and application to creep ratcheting

The ratchet boundaries and ratchet strains are derived for the Bree problem and an elastic-perfectly plastic material with different yield stresses on-load and off-load. The Bree problem consists of a constant uniaxial primary membrane stress and a cycling thermal bending stress. The ratchet problem with differing yield stresses is also solved for a modified loading in which both the primary membrane and thermal bending stresses cycle in-phase. The analytic solutions for the ratchet boundaries are compared with the results of deploying the linear matching method (LMM) and excellent agreement is found. Whilst these results are of potential utility for purely elastic–plastic behaviour, since yield stresses will often differ at the two ends of the cycle, the solution is also proposed as a means of assessing creep ratcheting via a creep ductility exhaustion approach.

Creep crack growth in phosphorus alloyed oxygen free copper

Creep crack growth (CCG) has been studied in phosphorus-alloyed oxygen-free copper (Cu-OFP) at 22, 75, 175, and 215°C with compact tension (CT) specimens. At 175 and 215°C, the cracks grew about 10mm before final instantaneous failure. In contrast, there was no visible crack growth at 22 and 75°C. Strongly deformed grains were observed adjacent to the cracks at 175 and 215°C. Intergranular creep cavities were found around the cracks. At 22 and 75°C, deformed grains and some cavities as well as microcracks were observed close to the crack tip.A model for crack propagation based on creep damage formation has been formulated to interpret the test results. Rupture criteria based on both creep ductility exhaustion and grain boundary cavitation were taken into account. The contribution from the ductility exhaustion to the creep damage dominated at the lower two test temperatures whereas the contribution from grain boundary cavitation at the higher test temperatures. The model can describe the influence of temperature on the observed creep crack propagation. It can also account for the observed cavitation in a qualitative way.

Simulations of creep crack growth in 316 stainless steel using a novel creep-damage model

The paper provides an advanced creep model which gives reasonable descriptions of the effects of damage and stress state from micromechanics viewpoint. A modified creep ductility exhaustion approach is employed to calculate the creep damage. Based on the proposed creep-damage model, numerical analyses of creep crack growth are conducted with a failure simulation technique. The simulated results are compared favorably with available experimental data for compact tension and thumbnail crack specimens for 316 stainless steel tested at 600°C. The comparisons show the excellent capability of the proposed model in predicting the crack growth rate and progressive crack profiles.

Creep failure simulations of 316H at 550 °C: Part I – A method and validation

This paper proposes a method to simulate creep failure using finite element damage analysis. The creep damage model is based on the creep ductility exhaustion concept, and incremental damage is defined by the ratio of incremental creep strain and multi-axial creep ductility. A simple linear damage summation rule is applied and, when accumulated damage becomes unity, element stresses are reduced to zero to simulate progressive crack growth. For validation, simulated results are compared with experimental data for a compact tension specimen of 316H at 550 °C. Effects of the mesh size and scatter in uniaxial ductility are also investigated.

Finite element analysis of type IV cracking in 2.25Cr–1Mo steel weldment based on micro-mechanistic approach

Creep studies were carried out on 2.25Cr–1Mo steel base metal and its fusion-welded weldments at 823 K over the stress range 100–240 MPa. The weldment possessed lower creep rupture strength than the base metal due to type IV failure at the outer edge of the heat-affected zone (HAZ). Premature failure of the weldment was associated with pronounced creep cavitation accompanied with localized creep deformation in the soft intercritical region of the HAZ that was sandwiched between relatively higher creep deformation-resistant microstructural regions. The cavitation was associated with coarse intergranular precipitates in the intercritical region of the HAZ. The type IV cracking in the intercritical region of the HAZ was found to initiate deep inside the weldment and propagate towards the specimen surface. Finite element analysis of stress and strain distributions across the weldment was carried out considering the micro-mechanical strength inhomogeneity across it to explain the observed features of type IV cracking. The estimated higher von-Mises and principal stresses deep inside the intercritical region of the HAZ of the weldment led to the localized creep deformation and preferential cavity nucleation and growth, resulting in type IV failure of the weldment. The role of intergranular precipitate particles in the intercritical region of the HAZ in facilitating creep cavity nucleation by the exhaustion of creep ductility of the material close to the precipitate was corroborated from finite element analysis of stress and strain distribution around the precipitates.

Creep crack growth in a Cr-Mo-V type steel: experimental observation and prediction

In this study, the creep crack growth (CCG) properties and fracture mechanism of a Cr-Mo-V steel at 566 C in compact tension (CT) specimens were investigated, and the CCG rate was predicted by using the NSW model. The results show that the CCG rate measured by CT specimens is much lower than that predicted by the NSW model under plane-strain state. This means that the NSW model prediction for the CCG rate of the steel is over-conservative. In addition, the CCG rate da/dt versus C measured by the experiments shows the piecewise linear relation on log-log scale instead of a single linear relation predicted by the NSW model. The main reasons for these results are that the actual creep fracture mechanism of the steel and the actual creep crack tip stress field in the CT specimens have not been fully captured in the NSW model. The experimental observation shows that the creep crack propagates in a discontinuous way (step by step) at meso-scale, and the cracks at micro-scale are usually formed by the growth and coalescence of voids on grain boundaries. The NSW model based on the creep ductility exhaustion approach may not correctly describe this creep fracture process. In addition, the opening stress and triaxial stress ahead of crack tips calculated by three-dimensional finite element method is lower than those predicted by the HRR stress field which is used in the NSW model under plane-strain state. The use of the high HRR stress field will cause high CCG rates. The change in the creep fracture mechanism at micro-scale in different ranges of C may cause the piecewise linear relation between the da/dt and C . Therefore, it is necessary to study the actual CCG mechanism in a wide range of C and the actual creep crack tip stress field to establish accurate CCG prediction models.

The exhaustion of creep ductility in 1CrMoV steel

The calculation of creep damage due to secondary (self equilibrating) stresses for the lifetime assessment of critical locations in components subject to creep-fatigue loading at high temperatures may be determined in terms of ductility exhaustion. A new Spindler type stress-modified creep ductility exhaustion model has been developed for 1CrMoV steel in which the rupture ductility is dependent not only on creep strain rate, stress and temperature, but also on a material pedigree function. The paper covers the background to the development of the new model and demonstrates its capability for predicting the creep rupture ductility characteristics of heats of 1CrMoV at different positions in the alloy scatterband.

Typical failures in pyrolysis coils for ethylene cracking

AbstractThis paper points out the main failure mechanisms for tubes and outlet parts of pyrolysis furnace coils. In most cases there is a combination of factors which ultimately lead to the failure, e. g. carburization and creep ductility exhaustion. This results in bulging, bending and ovalisation of the tubes. Also, brittle fracture during furnace trips can result in large, longitudinal cracks on many tubes in the furnace.The metallurgical background of the combined action of carburization and creep ductility exhaustion will be explained. Dense and stable oxide scales on the bore surface lower the amount of catalytic coke and also the carbon ingress into the alloy matrix (carburization). Carburization is also determined by the nickel content of the matrix. Creep ductility exhaustion is determined by the number of cycles (start/stop‐ and decoke cycles) and the nature (or severity) of these cycles.Pyrolysis tube failures can be prevented by a combination of proper furnace operation, materials choice, regular inspections and good design.

Prestrain effects on creep ductility of a 316 stainless steel light forging

Specimens were machined from a section of 316 stainless steel header which was in service for approximately 30 000 h. Constant stress creep tests were performed at 450 MPa and 575°C on materials subjected to various levels of prestrain to determine the effect of prior deformation on subsequent creep behaviour. The results show that, provided the prestrain exceeds the initial strain on loading to the creep test conditions in the un-prestrained condition, a significant reduction in rupture life, minimum creep rate and creep ductility was obtained by increasing prestrain. The Monkman–Grant relationship proved to be a reliable indicator of the effect of prestrain on creep properties and showed that the reduced rupture lives were a consequence of the progressive exhaustion of creep ductility with prestrain.

Effects of simultaneous boron and nitrogen implantation on microhardness and fatigue properties of Fe13Cr15Ni alloys: Rao, G.R., Lee, E.H. and Chin, B.A. Metall. Trans. A (Jan. 1994) 25A (1), 193–202

The ratchet boundaries and ratchet strains are derived for the Bree problem and an elastic-perfectly plastic material with different yield stresses on-load and off-load. The Bree problem consists of a constant uniaxial primary membrane stress and a cycling thermal bending stress. The ratchet problem with differing yield stresses is also solved for a modified loading in which both the primary membrane and thermal bending stresses cycle in-phase. The analytic solutions for the ratchet boundaries are compared with the results of deploying the linear matching method (LMM) and excellent agreement is found. Whilst these results are of potential utility for purely elastic–plastic behaviour, since yield stresses will often differ at the two ends of the cycle, the solution is also proposed as a means of assessing creep ratcheting via a creep ductility exhaustion approach.

Cyclic hardening and substructure of Al-Mg alloys: Grinberg, N.M., Serdyuk, V.A., Gavrilkayo, A.M., Lychagin, D.V. and Kozlov, E.V. Mater. Sci. Eng. A May 1991 138A, (1), 49–61

The calculation of creep damage due to secondary (self equilibrating) stresses for the lifetime assessment of critical locations in components subject to creep-fatigue loading at high temperatures may be determined in terms of ductility exhaustion. A new Spindler type stress-modified creep ductility exhaustion model has been developed for 1CrMoV steel in which the rupture ductility is dependent not only on creep strain rate, stress and temperature, but also on a material pedigree function.The paper covers the background to the development of the new model and demonstrates its capability for predicting the creep rupture ductility characteristics of heats of 1CrMoV at different positions in the alloy scatterband.

Creep Crack Initiation and Propagation: Fracture Mechanics and Local Approach

Both the initiation and the propagation of macroscopic creep cracks have been studied in 316-L austenitic stainless steel, and, for comparison purposes, in 2219-T851 aluminum alloy. These alloys are, respectively, creep-ductile and creep-brittle. This difference in behavior is explained in terms of fracture mechanics concepts applied to creeping solids. The inability of fracture mechanics in providing unique correlations with K, C*, etc. . . for all the stages of both creep crack initiation and propagation is pointed out. Life prediction schemes using local rather than global fracture criteria are presented. A model based on creep ductility exhaustion concepts and the stress fields obtained by fracture mechanics is shown to provide good predictions for 316-L. Finite element analysis coupled to continuum damage mechanics is found to describe creep crack initiation in 2219-T851.

Creep crack growth behavior of type 316L steel

The creep crack growth behavior of a 316L type stainless steel was studied at 600°C and 650°C using widely different specimen geometries. A strong emphasis was laid on crack initiation, early crack growth, and slow crack growth rates ( a). The applicability of four load parameters, the stress intensity factor, the nominal stress, the reference stress, and the contour integral C * , was investigated. Finite element calculations were made to compare the experimental load line displacement rates with the calculated values. It is shown that it is possible to correlate time to initiation ( T i ) with C * . The obtained experimental relation is not in full agreement with a theoretical expression derived from models for creeping solids. The reasons for this discrepancy are discussed. Furthermore, it is observed that there is no unique correlation between a and any of the four investigated load parameters, especially at low crack growth rates ( a0 −3mm/h). At large crack growth rates ( a≳10 −2 mm/h) the apparent correlation between a and C * is essentially due to the fact that the displacement rate is controlled by crack growth and not by the overall creep behavior of the specimens. A simple model based upon the data on crack initiation and the creep ductility exhaustion concept is shown to give results in reasonable agreement with the experiments.