Creep Crack Growth Characteristics Research Articles

GS13 Cr-Mo-V鋼のクリープき裂成長特性に及ぼす結晶粒径効果

GS13 Cr-Mo-V鋼のクリープき裂成長特性に及ぼす結晶粒径効果

Characterization of incubation time on creep crack growth for weldments of P92

Most structures are mainly fabricated by welding which are likely to be regions of crack initiation and propagation. In such weldment, it is known that the multi-axial stress fields appear due to the plastic constraint induced by the differences in material micro-structure between the weld metal, heat affected zone (HAZ) and base metal. In the present study, the experiments of creep crack growth tests and the structural mechanical analyses of weldment were conducted to understand the effects of stress multi-axiality of weldment on the characteristics of creep crack growth, creep deformation and creep ductility. Additionally, to characterize the time of creep crack initiation up to the start of a brittle type creep crack growth, the distribution of stress multi-axiality ( TF ) through the base metal, fine-grain HAZ, coarse-grain HAZ to weld metal were investigated.

Fracture Toughness and Creep Crack Growth in Polypropylene Under Combined Modes i and ii Loading

Fracture toughness and creep crack growth characteristics under combined mode I and II loadings were studied using the compact tension shear (CTS) specimens of polypropylene. The KI - KII envelope for crack initiation was obtained under various combined mode loadings. The creep crack growth rates da/dt under combined mode I and mode II loadings can be correlated with a single effective stress intensity factor KIeff based on the combined mode fracture envelope.

Analysis of Creep Crack Growth by Intelligent Phased Array Ultrasonic Inspection

At high temperatures typical for service conditions in fossil power plants, the creep fracture is dominated by the formation, growth and coalescence of cavities. Using high temperature pipe materials, P92 and P122, the characteristics of creep crack growth were analyzed in this study according to the cavities. The characteristics of cavities play a critical role in creep crack propagation and load line displacement. The effect of the load line displacement rate(dv/dt) and crack growth rate(da/dt) on the da/dt–Ct relation of creep crack growth was evaluated at different temperatures and Ki(initial stress intensity factor) values. The number of cavities increased with increasing temperature and Ki. The crack growth rate and load line displacement rate increased with the increase in the cavity numbers. The kind and distribution of these internal flaws were investigated by an intelligent phased array ultrasonic method and they were utilized in deriving the relationship with the creep crack growth rate, which will predict the creep characteristics of these materials.

Type IV creep cavity accumulation and failure in steel welds

One region of concern for creep damage accumulation in steel welds is a narrow region adjacent to the parent material. This narrow region, called Type IV, consists of partially transformed material having a relatively small grain size. In this region creep arises from crack growth and creep cavitation. The conditions where one mechanism dominates failure are not clear. In this paper, results are presented from an extensive experimental programme using both cross-weld and simulated Type IV specimens, first to determine the mechanisms of strain accumulation in the narrow region, and second to determine creep crack growth characteristics. Creep cavitation occurred across the net section, and the numbers of cavities per unit area were quantified as a function of fractions of rupture life. When sufficient constraint was provided by cross-weld specimens, creep crack growth was promoted in the Type IV region. When constraint was low, the presence of a crack did not promote localised cavity accumulation and creep crack growth. It is suggested that cavitation is a consequence of grain boundary sliding leading to relaxation of constraint and multiaxial rupture governed by the von Mises stress.

Creep crack growth characteristics of polypropylene film at various temperatures

Creep crack growth rates were measured using centrally cracked tension specimens of thin polypropylene film at various temperatures and stress levels. The creep crack growth rates were correlated with the stress intensity factor. The experimental results showed that there is the region of the minimum constant crack growth rate which occupies more than 75% of the total creep failure life. This steady or constant creep crack growth rate depends on the test temperature and the initial stress intensity factor. The constant creep crack growth rate characteristics were analyzed on the basis of the Arrhenius type thermally activated process. It is found that creep crack growth behavior is closely related to the crack tip opening displacement and the creep zone size.

Creep crack growth in P22 and P91 welds — overview from SOTA and HIDA projects

Creep crack growth (CCG) tests on two pressure vessel steels, P22 (2.25Cr1Mo) and P91 (9CrMoVNb) were carried out in two European Commission supported projects: SMT 2070 ‘SOTA’ and BE1702 ‘HIDA’. Tests in the SOTA project were conducted on the all-weld and cross-weld compact tension (CT) specimens; in the case of cross-weld specimens the electrical discharge machined (edm) notches were placed at various positions inside the HAZ to study the effect of notch position on CCG characteristics. These tests were conducted at 550°C (P22) and 600°C (P91). Tests in the HIDA project were carried out on the base metal and cross-weld CT specimens; in the case of cross-weld specimens the edm notches were placed in the centre of the HAZ. These tests were carried out at higher temperatures than those in the SOTA project i.e. at 565°C (P22) and 625°C (P91). In this project CCG tests were also carried out on large seam-welded pipes (feature specimens) with edm notches positioned in the base metal and in the centre of HAZ. The base and weld metals used in both projects were from the same cast/pedigree. This paper discusses the findings of the two projects and implications for crack growth in the weldments of these two steels.

Creep crack growth in polyethylene under combined mode I and mode II loading

Creep crack growth characteristics under various combined mode I and mode II loadings were studied using the compact tension shear (CTS) specimens of polyethylene. Creep crack growth rates da/dtunder combined mode I and mode II loading can be correlated with a single effective stress intensity factor KIeffderived from the combined — mode fracture toughness envelope. The steady state or constant crack growth rates which occupy the significant part of creep failure life increase with the increasing initial effective stress intensity factor.

Evaluation of Ct-parameter for Weld Interface Crack Considering Material Plastic Behavior

In this study, behavior of which is a well-known fracture parameter characterizing creep crack growth rate, is investigated for weld interface cracks. Finite element analyses were per formed for a C(T) specimen under constant loading condition for elastic-plastic-creeping materials. In modeling C(T) geometry, an interface was employed along the crack plane which simulated the interface between weld and base metals. The versus time relations were obtained under various creep constant combinations and plastic constant combinations for weld and base metals, respectively. A unified versus time curve is obtained by normalizing with and t with for all the cases of material constant variations.

Elevated temperature deformation and crack growth behavior in titanium alloy 6-2222

Elevated temperature tests were performed on titanium alloy 6-2222 (Ti 6-2222) sheet material to determine the creep deformation behavior and to evaluate the suitability of crack tip parameters for characterizing creep crack growth behavior in the temperature range of 350–450°C. At temperatures of 400°C and above, the material behaves essentially as a creep-ductile material in which the creep crack growth rate is characterized by the C t parameter. At 350°C, transition from creep-ductile to creep-brittle behavior was observed. A correlation which is independent of temperature for creep-ductile conditions in the range of 350–450°C between creep crack growth rate and C t is derived from these tests. The creep-brittle behavior observed at 350°C was in the transient range where no well-accepted crack tip parameter is available for characterizing the crack growth behavior. A methodology for predicting creep crack growth in Ti 6-2222 sheet structures at elevated temperature when creep-ductile behavior is observed is briefly discussed.

Transient crack growth behavior in aluminum alloys C415-T8 and 2519-T87

This research focused on the creep crack growth characteristics of two creep-brittle aluminum–copper alloys in sheet form, 2519-T87 (2.921 mm) and its chemical variant C415-T8 (2.286 mm), at 135 and 150°C. Transient crack growth behavior during which the creep crack growth rate does not correlate with K was observed in both aluminum alloys. By utilizing a transition time, t g, concept, the data obtained under transient crack growth conditions were separated from the quasi-steady-state crack growth data. In the quasi-steady-state regime, the creep crack growth rates were shown to best correlate with the stress intensity factor, K, for both alloys and test temperatures. The window for steady-state crack growth in creep-brittle materials is demonstrated to be very narrow, and the proposed condition for steady-state crack growth behavior is that V ̇ c / V ̇ ≤0.50 and that V ̇ c / V ̇ changes by less than 100% over a crack extension of 0.5 mm. K/ r q c is investigated as a crack tip parameter applicable to both transient and steady-state crack growth conditions; however, the experimental difficulties in determining the creep zone size were encountered and are discussed in the paper.

Creep deformation and crack growth behavior of a single-crystal nickel-base superalloy

Uniaxial creep deformation and crack growth data are presented on the single-crystal nickel-base superalloy SC16, which is a candidate material for industrial gas turbine applications. All testing was performed at 900 °C. The uniaxial experiments were conducted with the loading direction aligned approximately along the [001] crystallographic axis of the material. Under these conditions, a small primary region followed by mainly tertiary creep was obtained, and failure initiated from cracks at interdendritic pores. The crack growth experiments were performed on single-edge notch tension specimens and compact tension test pieces containing deep side grooves to examine state-of-stress effects. A selection of crystallographic orientations was also examined. Little effect of stress state and orientation was obtained. It has been found that the creep crack growth characteristics of the alloy can be predicted satisfactorily from a model of the accumulation of damage at a crack tip using the creep fracture mechanics parameter C* and assuming plane stress conditions.

Creep crack growth in power plant materials

Economic considerations have made it desirable to extend the 30 to 40 year operating life of power plants by another 10 to 20 years. Crack growth at elevated temperatures is an important consideration in estimating the remaining life, determining operating conditions and deciding inspection criteria and intervals for power plant materials. This paper presents an overview of high-temperature crack growth phenomenon in such materials. The focus is on various techniques used for characterizing creep crack growth (CCG) and creep-fatigue crack growth (CFCG) in high-temperature materials. The collection of data, their analysis and the interpretation of results is discussed in detail, especially for CFCG laboratory testing. The discussion is primarily focussed on creep-ductile materials such as those used in power plant applications. Special considerations for elevated temperature crack growth in weldments are also presented. Finally, the application of these concepts to the life prediction of power plant components is also discussed.

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...

An alternative correlating parameter for creep crack growth rate and its application: Derivation of the parameterQ

The parameter Q* has been proposed for correlating or characterizing creep crack growth rate. The creep crack growth rate has been expressed in terms of an activation process equation, and the parameter Q* was defined as the exponent of the exponential in the thermal activation process equation for da/dt, in which the activation energy is expressed in terms of free energy. CrMoV steel was used for VAMAS round-robin tests, and other materials such as SUS 304 and the Nl-base alloy of Hastelloy-XR were used. For these materials good correlations are obtained. An analytical conclusion about the effect of thickness on crack growth is described.

Crack propagation behavior under creep conditions

The creep crack propagation behavior of a Cr−Mo−V rotor steel has been investigated at 538°C using time-dependent fracture mechanics concepts. The creep crack propagation lives and the creep deflection rates of double-edge-notched (DEN) specimens were estimated using previous data from compact-type specimens. The predicted crack growth lives and deflection rates compared favorably with the experimental data. When plotted as a function of the C t parameter, the experimentally determined creep crack propagation rates of DEN specimens were found to be in agreement with those of compact and center-crack-tension specimens. These results provide further experimental verification for the validity of the C t parameter for characterizing creep crack growth behavior. Some discrepancies between the predicted and the observed behavior are attributed to primary creep deformation behavior which was not considered in estimating the value of C t .

有限要素法によるクリープき裂進展シミュレーション

A finite element method was applied to a generation phase simulation of creep crack growth. Experimental data on creep crack growth in a 1Cr-1Mo-1/4V steel compact tension specimen were numerically simulated using a node-release technique and the variations of various fracture mechanics parameters such as CTOA, J, C* and T* during creep crack growth were calculated. The path -dependencies of the integral parameters J, C* and T* were also obtained to examine whether or not they could characterize the stress field near the tip of a crack propagating under creep condition. The following conclusions were obtained from the present analysis.(1) The J integral shows strong path-dependency during creep crack growth, so that it does not characterize creep crack growth.(2) The C* integral shows path-dependency to some extent during creep crack growth even in the case of Norton type steady state creep law. Strictly speaking, we cannot use it as a fracture mechanics parameter characterizing creep crack growth. It is, however, useful from the practical viewpoint because it correlates well the rate of creep crack growth.(3) The T* integral shows good path-independency during creep crack growth. Therefore, it is a candidate for a fracture mechanics parameter characterizing creep crack growth.

Investigation of Creep/Fatigue Interaction on Crack Growth in a Titanium Aluminide Alloy

Crack growth under sustained load and sustained load with superimposed fatigue cycles at elevated temperatures in a titanium-aluminide alloy (Ti3Al) was investigated. The objectives were to determine the creep crack growth characteristics and to evaluate the applicability of linear cumulative damage modeling to the Ti3Al alloy. The linear elastic stress intensity factor, K, was used as a correlating parameter for all the tests. Sustained load tests were used to characterize the time-dependent crack growth behavior at elevated temperatures. The test results showed that sustained load crack growth rate and fracture toughness were relatively insensitive to temperature between 700 and 800°C. Crack growth rates were only a factor of five apart between the slowest and fastest growth rates over this range of temperatures. Several sustained load tests with periodic fatigue cycles (hold-time tests) were used to test the applicability of linear cumulative damage modeling. The model was developed using data from the sustained load and a baseline fatigue test at 750°C. Crack growth rates calculated using the model were accurate for a fatigue cycle with a ten minute hold time, but were up to 2 times less than the growth rates for shorter hold-time tests based on summation of the sustained load and fatigue crack growth rates only. A mixed-mode correction factor added to the model produced more accurate results.

Creep crack growth in copper containing pre-implanted grain boundary cavities

The creep crack growth behavior of pre-damaged Cu was studied at 773 K using precracked, single-edge notched specimens under various loading conditions. The specimens were predamaged prior to creep crack growth testing using an experimental technique for implanting water vapor bubbles to produce a uniform distribution of cavities along the grain boundaries. The characteristics of creep crack growth were investigated through the correlation of crack growth rate with various loading parameters such as J, C ∗ h and C∗ . It was found that the crack growth rate of pre-damaged Cu is much higher than that of initially undamaged Cu at the same crack length and for the same loading. The results also show that the crack growth behavior of pre-damaged Cu with the given initial damage level is of the same form as that of initially undamaged Cu. At an early stage, when the damage evolution is not severe, crack growth is characterized by the continuum mechanical approach combined with the constitutive damage equations with a C ∗ stress field as a remote boundary condition. As the damage level increases, the degree of crack tip stress singularity decreases and creep damage mechanisms have more influence on the crack growth. The time required to reach this stage is much shorter for the pre-damaged Cu than for the initially undamaged Cu. A load parameter map obtainable from various characteristic time calculations proved to be useful for predicting the macroscopic crack growth behavior.

Low temperature (360°C) creep crack growth characteristics of a CMn steel

Creep crack growth characteristics of a CMn steel in the cold-worked condition have been studied at 360°C. These have been compared with the characteristics of non-cold-worked (i.e. normalized) material the data for which were available from a parallel research programme carried out elsewhere. Analysis of the data on unstrained material showed that the notch rupture life of this material can be predicted from its uniaxial creep life on the basis of a reference plane stress. The present work showed that cold work results in an increase in uniaxial creep life with a proportional decrease in notch rupture life, thus invalidating the use of reference stress for life prediction. In this case, the validity of other correlating parameters was investigated. This showed that the initiation or no-crack-growth period (the specimens had exhibited initiation periods of between 30% and 40% of life) could be described as a function of the linear elastic stress intensity factor K while the crack growth rate could be described by the correlating parameter C ∗.