Creep Crack Growth Data Research Articles

Characterization of creep-fatigue crack growth behavior under trapezoidal waveshape using Ct-parameter

An experimental study of creep-fatigue crack growth behavior of 1.25Cr-0.5Mo steel at 538°C (1000°F) under trapezoidal loading waveshapes has been carried out on compact type specimens. In creep-fatigue crack growth experiments, hold times ranged from 0 seconds to 24 hours with intermediate conditions of 10 seconds, 98 seconds, 10 minutes and 15 minutes. Time-dependent crack growth rate during the hold period, (da/dt)avg, is correlated with (Ct)avg. The values of (Ct)avg are estimated using the equation recently proposed for elastic-cyclic plastic-secondary creeping (EL-CPL-SC) materials. The (da/dt)avg vs. (Ct)avg data fall on a single trend which matches with the trend of da/dt vs. Ct creep crack growth data for the same material. A model is proposed for predicting both the creep crack growth behavior and the creep-fatigue crack growth behavior. The model is suitable for assessing the residual life and/or the safe inspection intervals of high-temperature components such as steam headers. Transition of crack tip damage patterns from oxidation to creep cavitation across the range of hold times examined is also discussed.

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.

The effect of porosity and γ-γ′ eutectic content on the fatigue behaviour of hydrogen charged PWA 1480: Gayda, J. Dreshfield, R.L. and Gabb, T.P. Scr. Metall. Mater.25 11 (Nov 1991) 2589–2594

Material pre-straining is known to have significant effects of the mechanical response and crack growth behaviour of steels. In this paper, the influence of material pre-straining on the subsequent creep-fatigue crack growth behaviour of Type 316H stainless steel at 550 °C has been examined by performing tests on compact tension specimens that were extracted from blocks uniformly pre-compressed at room temperature. Creep-fatigue crack growth tests on pre-compressed material were performed at the frequency of 0.01 Hz and R-ratio of 0.1. The crack growth data obtained from these experiments have been correlated with the C* and K fracture mechanics parameters and the results are compared with the existing creep crack growth data on the pre-compressed and as-received material at 550 °C. The results obtained have also been compared with the creep-fatigue data from experiments on weldments where the crack tip was located in the heat affected zone (HAZ). The crack growth behaviour in creep-fatigue tests on pre-compressed material has been found similar to that of HAZ material and are higher than that of the as-received material. Moreover, depending on the loading condition and frequency the crack growth data obtained from creep-fatigue tests on pre-compressed material may be characterized using C* or ΔK fracture mechanics parameters.

Evaluation of crack-tip parameters for characterizing creep crack growth: Results of the ASTM round-robin programme

The results of a round-robin programme to evaluate crack-tip parameters such as K, C* and Ct for characterizing creep crack growth behaviour are presented. The programme was sponsored by the E24 Committee on Fracture Mechanics Test Methods of the American Society for Testing and Materials (ASTM). The tests were made on a 1Cr-1Mo-0.25V turbine rotor steel (ASTM grade A470 class 8) and included specimens of two thicknesses from the compact type (CT) and the centre crack tension (CCT) geometries. The test temperatures were 538°C and 593°C (1000°F and 1100°F). All tests were found to be in the extensive creep regime where the C*-integral and the Ct parameter are identical. It was shown that all creep crack growth data from CT specimens correlated uniquely with the C*-integral or the Ct parameter and no unique correlation was found between creep crack growth rate and K. The creep crack growth rates in 6.3 mm (0.25 in) plane-sided CT specimens were about two to three times slower than those in 25.4 mm (1 in) th...

Methods of estimatingC

Several methods are available for estimating the creep fracture mechanics parameter C* used for describing creep crack growth in components and test specimens. A procedure is presented for determining C*, from experimental data, which makes use of limit analysis concepts and requires little knowledge of the creep strain behaviour of materials. Transient effects, during the early stages of cracking due to stress redistribution and a gradual build-up of creep damage at the crack tip, are discussed. It is shown that improved correlations of creep crack growth data are achieved when these effects are taken into account.

The VAMAS initiative on advanced materials and standards: A unified approach to creep crack growth measurement

The VAMAS (Versailles Agreement on Advanced Materials and Standards) Project, in providing a framework for international collaboration in pre-normative research, has enabled a unified approach to the measurement and interpretation of creep crack growth data. The concepts involved in the VAMAS initiative and the procedures adopted in defining agreed methods for determining creep crack growth rates are briefly described in this paper, which introduces a special issue of this journal devoted to creep crack growth.

Modelling crack growth for creep and fatigue loading

Estimates of creep crack growth in engineering components under steady load conditions are usually based on the application of fracture mechanics concepts. In particular the creep parameter C∗ has become widely used together with creep crack growth data obtained from laboratory tests. There are now a number of practical methods to utilise experimental data. For high temperature components, which are subjected to cyclic (fatigue) as well as creep loading, the estimation of the fracture mechanics parameters becomes much more difficult, and consequently the extent to which the growth of pre-existing cracks grow by creep and fatigue is difficult to quantify. In this paper the response of Type 316L stainless steel is examined. This material progressively strain hardens under reversed cyclic loading, and the creep behaviour also changes. Using uniaxial fatigue and creep results, fracture parameter maps are developed to establish the appropriate regimes for creep-fatigue crack growth. Using the maps a model is developed which can predict the combined effect of fatigue and creep on crack growth. The implications of the model are discussed in relation to the limitations of obtaining results from laboratory tests at short times, and the assessment of practical engineering components.

Creep crack growth characterization of Type 316 stainless steel using miniature specimens

Remaining life estimation of components which experience elevated-temperature service requires knowledge of creep-crack-growth characteristics of the service-exposed material. In most cases only a small amount of material from in-service components is available for testing; thus, there is a need to design and benchmark test methods that use miniature creep-crack-growth specimens. In order to address this problem, a creep-crack-growth test methodology was developed using miniature single edge-notched tension specimens. The material used in this work was from a well characterized heat of Type 316 stainless steel, for which creep-crack growth data were previously obtained using conventional compact-tension type and center-cracked-tension type specimens. Good correlation was obtained between the crack growth rate and Cj, where Cj is the experimental counterpart of C*. The details of the analytic procedure for miniature specimens, particularly for small crack lengths, are presented. The current results agreed very well with those for large specimens as well as with creep-crack growth data obtained by other investigators.

Inclusion of primary creep in the estimation of the C t parameter

The problem of time-dependent fracture under transient creep conditions is investigated via finite element analyses of fracture specimens with stationary cracks. The constitutive models consist of linear elasticity with combinations of power-law secondary creep and two primary creep laws. Two proposed parameters are studied. One is a contour integral, C(t), which characterizes the crack tip singularity strength. The other one, Ct, is evaluated based on the load line deflection rate and has been used successfully in correlating experimental creep crack growth data.

Residual life techniques for plant life extension

According to industry estimates, by the year 1990, 15% of the total capacity and 75% of the reserve capacity of the electric power in the U.S.A. will be supplied by units which are over 30 years old. Under current procedures for life assessment, which embody severely conservative assumptions, a large number of the boilers will probably be retired at the end of the 30 years. Development of more accurate life assessment procedures may enable extension of their life by 20–30 years and result in an improvement of their availability by at least 1%. Although a number of post-service evaluation procedures have been applied by the industry, interpretation of the results is often clouded in uncertainties owing to a lack of a quantitative basis for relating observed damage to the remaining life. For instance, the progress of creep damage leading up to crack initiation in boiler and turbine steels needs to be quantitatively characterized in terms of materials and operational parameters. The effects of environment, stress state and cyclic operation on creep damage need to be considered. The validity of the life fraction rule for life assessment needs to be verified. For heavy section components, creep crack growth data should be generated for typical boiler steels, and generalized methodologies which rationalize crack growth in terms of readily measurable parameters must be developed. Research projects directed by the Electric Power Research Institute have made significant advances in a number of these areas. This paper is a progress report of the results from the various projects relating to boiler pressure parts operating in the creep regime.

Creep crack growth under transient conditions

The recent advances in time-dependent fracture mechanics concepts for characterizing creep crack growth rate under transient conditions are reviewed. At elevated temperatures, transients in the crack tip stress fields occur owing to stress relaxation during constrained creep due to primary and tertiary creep deformation, cyclic loading and crack growth. A unified crack tip parameter Ct which is formulated to account for all types of transient is described and evaluated. The currently available creep crack growth data support the validity of Ct. Under steady state conditions, Ct reduces to the familiar C∗ integral.

Elevated temperature creep-fatigue crack propagation in nickel-base alloys and 1 Cr-Mo-V steel

The crack growth behavior of several high temperature nickel-base alloys, under cyclic and static loading, is studied and reviewed. In the oxide dispersion strengthened (ODS) MA 6000 and MA 754 alloys, the high temperature crack propagation exhibited orientation dependence under cyclic as well as under static loading. The creep crack growth (CCG) behavior of cast nickel-base IN-738 and IN-939* superalloys at 850 °C could be characterized by the stress intensity factor,K 1. In the case of the alloy IN-901 at 500 °C and 600 °C,K 1 was found to be the relevant parameter to characterize the creep crack growth behavior. The energy rate line integral,C*, may be the appropriate loading parameter to describe the creep crack growth behavior of the nickel-iron base IN-800H alloy at 800 °C. The creep crack growth data of 1 Cr-Mo-V steel, with bainitic microstructure, at 550 °C could be correlated better by C * than byK 1.

２・１／４Ｃｒ‐１Ｍｏ鋼のクリープき裂成長挙動の評価

Creep crack growth behavior of 21/4 Cr-1 Mo steel at 550°C was investigated by energy rate line integral, C* under conventional constant load control and deflection rate control. The same relationship of da/dt versus C*-integral was obtained by both of the control methods. The increasing deflection rate control test proposed in this paper was found to be a very efficient method because a wide range of creep crack growth data can be obtained stably on a single specimen. Various formulae to experimentally determine C*-integral of CT specimens were compared. Equation (3) considering the axial force as well as the bending effect agreed with the analytical results by the finite element method in the range of a/W≥0.5.The result of C*-integral estimation using the fully plastic solution, agreed with the experimental results in steady state creep conditions, when the plane strain solution was applied. Although the C*-integral approach with the fully plastic solution seems prospective for practical creep crack growth prediction, more investigations are needed under various conditions.

Experimental investigation of creep crack growth in a lead alloy

The results of 27 creep crack growth tests performed on four different specimen types (CT, 3PB, ECP, and CCP) are reported. A lead alloy, previously used as a creep ‘model material’ was used. Of the parameters commonly used to correlate creep crack growth data, the C∗ parameter was found to be the best for the lead alloy data. It is concluded that the lead alloy may be useful for ‘modelling’ the creep crack growth rates in components made of other materials with similar creep ductilities.

A generalized parametric approach to creep crack growth

The topic of creep crack growth has attracted increased attention in recent years. However, as yet no generally valid means for relating experimental data to a relatively simple correlating parameter such as K, the stress intensity factor, or C*, the creep equivalent to Rice's J-integral, has been established. In retrospect this lack of correlation may not be surprising in view of the variation in behavior of different alloys, e.g., creep brittle vs. creep ductile, as well as the uncertainty as to whether or not the assumed test conditions, usually steady state creep, are actually being realized in the process zone at the advancing crack tip. As an alternative a straightforward parametric approach has been proposed in which the load point deflection rate, ..delta.., is expressed as a function of the crack growth rate, a, and the load P in the following form. A correlation of creep crack growth data for the creep ductile aluminum alloy 6061 has been obtained in terms of a parameter which involves the deflection rate and bending moment for two types of pin-loaded specimens.

Creep crack growth in IN-738 and IN-939 nickel-base superalloys

The creep crack growth behaviour of cast IN-738 and IN-939 nickel-base superalloys was studied at various temperatures, but mainly at 1123 K. The applicability of the stress intensity factor K I and the path-independent integral C ∗ to correlate the creep crack growth data was investigated. It was found that for these alloys the stress intensity factor K I was more appropriate to use for data correlation. Microstructural investigation indicated that in these alloys the creep crack growth took place by the nucleation and growth of grain boundary cavities. An indirect support for this mechanism was found in the agreement between the calculated stress intensity exponent and the creep exponent. Also the activation energy for creep crack growth agreed with that for volume diffusion of nickel in NiCr alloy.

A Diffusive Crack-Growth Model for Creep Fracture

A grain-boundary creep-crack-growth model is presented based on the assumptions that a crack propagates along the grain boundary by a coupled process of surface and grain-boundary self-diffusion, the adjoining grains on either side of the boundary behave elastically, and steady state conditions prevail. Under the action of the applied stress, atoms on the crack surfaces are driven by surface diffusion toward the crack tip, from where they are deposited nonuniformly by grain-boundary diffusion along the grain interface so that the grain boundary opens up hi a wedge shape ahead of the advancing tip which, hi turn, produces a misfit residual stress field. The total grain-boundary normal stresses which are the sum of this misfit stress field and that due to applied stress as well as the boundary opening displacements due to materials deposition are solved from a singular integrodifferential equation to give the following equation relating K to v: where K is the applied mode I crack-tip stress intensity factor,Kmin=1.69 kg is the minimum K below which no crack growth is predicted, KG being the critical K based on the Griffith theory; ν is the fixed crack-tip velocity, and νmin is the minimum v for which K=Kmia. In terms of the conventional expression of ν∝Kn, the present model predicts the values of n varying from 12 to infinity. A comparison with a set of creep crack-growth data on Si-Al-O-N at 1400°C shows good agreement between theory and experiment. A detailed analysis of the energy balance for the present model is also presented which indicates that / or (1–ν2)K2/E is indeed the correct energy release rate during the crack growth, as is true in the theory of elastic fracture mechanics. However, the major portion of the energy released in the diffusion processes comes from work done by the normal stress in opening up the grain boundary to accommodate the diffused material rather than from strain energy released by the adjoining grains.

Characterizations of creep crack growth in 1 per cent Cr Mo V steel

Estimates of stress intensity factor, K, reference stress, σref, and creep parameter, C∗, have been made for compact tension (CT) and double cantilever beam (DCB) test-pieces containing side grooves. Limit analysis techniques were used to determine the latter two parameters. It is shown that the expressions developed for σref are sensitive to the collapse mode proposed, whereas those for C∗ are largely independent. Comparisons of predictions of creep crack growth data on CT and DCB specimens of a 1 per cent CrMoV steel in terms of K and σref have revealed different dependences for the two geometries, suggesting that neither parameter gives satisfactory correlations. Better overall agreement is obtained with the C∗ parameter, even though gross creep deformations were not observed. It is suggested that further improvement may be gained with this parameter if more accurate estimates of C∗, which allow the inclusion of elastic terms, are used.