Creep Crack Growth Properties Research Articles

A novel methodology for determining creep crack initiation and growth properties using FEM with notched small punch specimens

Determination of material properties on creep crack initiation (CCI) and creep crack growth (CCG) using small punch tests is still a virgin field to be exploited. Spanish and Czech researchers have carried out pioneering work and put forward an approach to correlate the notched specimen with the standard CT specimen. The advantage is that, the formulas of C*-integral for CT specimen are available and can be used directly. However, building a big data pool with both SP and CT tests within the creep region is a daunting task and hence it is worthwhile to derive C*-integral formulas for notched specimens themselves. The research is novel in that it proposes a methodology to derive the key parameter C*-integral for notched small punch specimens directly without supporting known CT tests. A 3D finite element model (FEM) for through-wall notched specimen with different crack lengths is build, and a general formulas for stress intensity factor (SIF) K, and limit load PL are obtained to calculate C*-integral for notched specimen. Experiments on notched specimens are carried out for stainless steel Sanicro 25 at temperature 700 °C. By using general formulas derived in this study, the key parameter C*-integral is obtained. The relation between C*-integral and the creep initiation time ti can be drawn as a straight line in a double logarithmic coordinate. The relation between C*-integral and the creep crack rate ȧ appears to follow the same tendency. Material constants for CCI and CCG can be derived according to the power law using least square fitting. The proposed method is not fully developed and further validating experiments will be necessary to complete the research.

Creep crack growth properties of P91 and P22 welded joints

AbstractThe ferritic P22 and bainitic‐martenstic P91 steel welded joints operating at elevated temperatures are highly vulnerable to premature failure, because microstructure gradients can lead to cracking in the heat‐affected zone, commonly referred to as type IV cracking. Creep crack growth tests have been performed using compact tension specimens made of P91 and P22 weld joints at temperatures of 600 and 550 °C, respectively. The creep crack growth characterization through empirical equation of C* was the main fracture parameter for relating behaviour of component's regions. Results have shown that, for a specific value of C* parameter, the weld metal and heat‐affected zone had similar crack growth rates, which were about two times faster than base metal rates, because of lower creep strength that leads to a conclusion that the creep cracks will most likely initiate in the heat‐affected zone region.

Effects of HAZ widths on creep crack growth properties of welded joints

The finite element method (FEM) based on ductility exhaustion damage model was used to investigate the effects of heat-affected zone (HAZ) widths on creep crack growth (CCG) properties for the compact tension (CT) specimens with pre-crack located in the middle of softer and harder HAZs under the same initial load level. The results show for the welded joints with softer HAZ containing crack that the creep rupture life increases and CCG rate decreases with increasing HAZ width due to the decrease of material constraint effect. For the welded joints with harder HAZ containing crack, a second crack in soft material near interface generally forms, and the rupture life is mainly determined by the initiation time of the second crack. The CCG length in harder HAZ increases with increasing HAZ width. There exist proper widths of the harder HAZ which have the longest rupture life due to the lower stress triaxiality and later initiation of second crack in soft material near interface. The CCG properties of HAZs may be improved by adjusting HAZ widths and its creep properties by adopting proper welding methods or process. The HAZ widths and mismatches in creep properties should be fully considered in creep life assessments for the welded joints.

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.

Evaluation of Creep Crack Growth Rate of P92 Welds Using Fracture Mechanics Parameters

High Cr ferritic heat resisting steels have been widely used for boiler components in ultrasupercritical thermal power plants operated at about 600°C. In the welded joint of these steels, type-IV crack initates in the fine-grained heat affected zone during long-term use at high temperatures and their creep strength decreases. In this paper, creep properties and creep crack growth (CCG) properties of P92 welds are presented. The CCG tests are carried out using cross-welded compact tension C(T) specimens at several temperatures. The crack front was located within the fine-grained HAZ region to simulate type-IV cracking. Finite element analysis was conducted to simulate multiaxiality in welded joints and to compare experimental results. The constitutive behavior for these materials is described by a power-law creep model. C∗ and Q∗ parameters are used to evaluate CCG rate of P92 welds for comparison. C∗ parameters can characterize approximately 20% of the total life of CCG in P92 welds, and Q∗ parameters can characterize approximately 80% of the total life. Q∗ parameter is one of the useful parameters to predict CCG life in P92 welds.

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.

Prediction of Scatter in Creep Crack Growth Data from Creep Failure Strain Properties

Abstract Procedures are available for determining the creep crack growth (CCG) properties of materials experimentally in terms of the creep fracture mechanics parameter C*. When these properties have not been measured, predictions can be made from models of the cracking process. These models show that creep crack growth rate at a given value of C* is most sensitive to the creep ductility ɛ*f appropriate to the state of stress at a crack tip. For plane stress conditions, it is found that ɛ*f can be taken to be equal to the uniaxial creep ductility ɛf of a material and for plane strain to be given by ɛf/30. Hence, in this paper, uniaxial creep properties on type 316 L(N) austenitic stainless steel and two 9 %Cr steels, designated P91 and P92, in the temperature range 500–750°C are reported and scatter in the data identified from statistical analysis. For each steel, the dependence of creep ductility on stress and temperature is determined. It has been found that none of the rupture time (tR), creep strain rate (˙ε), and net section stress (σnet), parameters has a better relationship with the creep failure strain. Hence, none of these parameters characterizes better than the other the creep ductility's behavior. Additionally, it has been observed that although the creep failure strain of 316 L(N) is independent of these parameters, it increases with temperature. In contrast, the creep failure strain of P91 is slightly sensitive to σnet but is overcompensated by the scatter and so can be assumed stress independent. The creep failure strain of P91 is seen to be also insensitive to temperature. For P92, ɛf goes from a lower to an upper shelf through a transition region. Thus the best solution to characterize the failure strain behavior is to use a polynomial solution. Creep crack growth properties obtained on cracked compact tension and C-shaped specimens are also presented. Probabilistic analysis, based on Monte Carlo simulation methods, is employed to predict scatter in the creep crack growth data from scatter in the uniaxial creep properties. In most instances, close agreement is found between the experimentally measured scatter in the cracking results and that predicted from the scatter obtained from creep ductility data.

Estimation of Creep Crack Growth Properties Using Circumferential Notched Round Bar Specimen for 12CrWCoB Rotor Steel

Most heat resisting materials in structural components are used under multi-axial stress conditions and under such conditions ductile materials often exhibit brittle manner and low creep ductility at elevated temperature. Creep crack initiation and growth properties are also affected by multi-axial stress and it is important to evaluate these effects when laboratory data are applied to structural components. Creep crack growth tests using circumferential notched round bar specimens are a simple method to investigate multi-axial stress effects without using complicated test facilities. Creep crack growth tests have been performed using a 12CrWCoB turbine rotor steel. In order to investigate the effects of multi-axial stress on creep crack growth properties, the tests were conducted for various notch depths at 650°C. The circumferential notched round bar specimen showed brittle crack growth behaviour under multi-axial stress conditions. Creep crack growth rate was characterized in terms of the C* parameter. A 12CrWCoB turbine rotor steel has been tested using circumferential notched round bar specimens with different multi-axiality. Circumferential notched round bar specimens show increased brittle creep crack growth behaviour due to the multi-axial stress condition. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multi-axial conditions.

Experimental and numerical study on the relationship between creep crack growth properties and fracture mechanisms

Evaluation of creep crack growth properties taking microscopic aspects into account is effective for developing more accurate life prediction of structural components. The present study investigated the relationship between creep crack growth properties and microscopic fracture aspects for austenitic alloy 800H and 316 stainless steel. The growth rate of wedge-type intergranular and transgranular creep crack could be characterized by creep ductility. Creep damages formed ahead of the void-type crack tip accelerated the crack growth rate. Based on these experimental results, a three-dimensional finite element method (FEM) code, which simulates creep crack growth, has been developed. The effect of creep ductility on da/dt vs C* relations could be simulated based on the critical strain criteria. The diffusion of vacancies toward crack tip would accelerate the crack growth under creep conditions. The change of vacancy concentration during creep was computed for a three-dimensional compact-type (CT) specimen model by solving the diffusive equation under multiaxial stress field. The experimental results that crack growth was accelerated by creep damages formed ahead of the crack tip could be successfully simulated.

Creep crack growth properties for 12CrWCoB rotor steel using circular notched specimens

Most heat resisting materials in structural components are used under multiaxial stress conditions. It is important to evaluate the effect of multiaxial stress conditions on initiation and growth of creep cracks, when laboratory data are applied to structural components. The creep crack growth test using a circular notched bar (circumferentially cracked round bar) specimen is a simple method to investigate multiaxial stress effects without using complicated test facilities. Creep crack growth tests have been performed using 12CrWCoB turbine rotor steel. In order to investigate the effects of multiaxial stress and specimen size on creep crack growth properties, the tests were conducted for various notch depths and specimen diameters. The circular notched specimens showed brittle crack growth behaviour under multiaxial stress conditions. Creep crack growth rate was characterized in terms of the C ∗ parameter. The creep crack growth rate, for the same C ∗ value, increased with increase in the initial notch depth, i.e. as the multiaxiality increased. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multiaxial conditions.

Creep crack growth properties for 12CrWCoB rotor steel using circular notched specimens

Most heat resisting materials in structural components are used under multiaxial stress conditions. It is important to evaluate the effect of multiaxial stress conditions on initiation and growth of creep cracks, when laboratory data are applied to structural components. The creep crack growth test using a circular notched bar (circumferentially cracked round bar) specimen is a simple method to investigate multiaxial stress effects without using complicated test facilities. Creep crack growth tests have been performed using 12CrWCoB turbine rotor steel. In order to investigate the effects of multiaxial stress and specimen size on creep crack growth properties, the tests were conducted for various notch depths and specimen diameters. The circular notched specimens showed brittle crack growth behaviour under multiaxial stress conditions. Creep crack growth rate was characterized in terms of the C ∗ parameter. The creep crack growth rate, for the same C ∗ value, increased with increase in the initial notch depth, i.e. as the multiaxiality increased. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multiaxial conditions.

Evaluation of creep crack growth properties using circular notched specimens

It is important to evaluate the effect of multiaxial stress conditions on initiation and growth of creep cracks, when the laboratory data are subsequently applied to structural components under the same or similar stress state. The round robin tests of creep crack growth using circular notched specimens of 1CrMoV steel at 538 and 594 °C and 12CrWCoB steel at 650 °C were conducted by the Japanese VAMAS TWA25 group. The effect of notch depth and specimen size, i.e. stress multiaxiality on crack growth properties was investigated. The test procedure including criteria for crack length measurement by electric potential drop was established. The circular notched specimens fractured intergranularly and showed different crack growth behaviour from that of a CT specimen due to the multiaxial stress field. The creep crack growth rate for the same C* value increased as the ratio of the notch depth to specimen diameter, i.e. stress multiaxiality increased. The Q* evaluation method based on the thermally activated process can also be applied to the circular notched specimen.

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.

Effects of pretest aging on creep crack growth properties of type 308 austenitic stainless steel weld metals

Type 308 austenitic stainless steel weld metals made by SMAW and FCAW were used in order to quantitatively determine the effects of bismuth segregation and sigma phase precipitation as a result of high temperature pretest aging on creep crack growth properties. Tests were conducted using CT-type specimens and the data derived were arranged according to the parameter C ∗. The creep crack growth rate is significantly higher in cases where the metal contains a large amount of bismuth and where sigma phases have adequately precipitated.

ＴｉＡｌ金属間化合物のクリープき裂成長試験方法に関する検討とクリープき裂成長特性

VAMAS project which aims to establish the standard test method of creep crack growth for creep brittle advanced materials is being conducted. The round robin tests on TiAl intermetallic compound with fully lamellar structure were carried out. Based on the results of round robin, the recommended test method was proposed. Fatigue pre-cracking was not applicable to this material as a standard method because fatigue crack deviated along the lamellar structure. In the present study, the machined V-notch specimen was recommended based on the experimental results and stress distribution analysis ahead of the V-notch. Creep crack growth properties and creep fracture mechanism of the present TiAl were clarified. Creep crack growth properties depending on test temperatures were also investigated in terms of the creep fracture mechanism for this material.

Comparison Between Crack Growth in Fracture Mechanics Specimens and Feature Component Tests Carried Out in a Low-Alloy Steel

In both power generation plants and the chemical industries, there is a need to assess the significance of defects which may exist in high-temperature equipment operating in the creep range. This paper examines the methods of analysis used in laboratory creep crack growth data and their relevance to crack growth data derived from feature component tests which best simulate actual components under controlled testing condition. The material examined was a 214 Cr 1 Mo steel in the new condition at 550 and 600°C. The creep crack growth properties were determined on compact tension specimens. The data were compared with representative crack growth data from feature test components. These consisted of cracked rings, thick-walled cylinders, and thin-walled tubes containing axial or circumferential defects under combinations of axial and internal pressure loading. Little influence of size or temperature on the measured crack propagation rates was observed when the results were plotted against the creep fracture mechanics parameter C*. This is shown to be because the relevant condition had little effect on the appropriate crack tip creep ductilities of the material. Good correspondence was observed between the compact tension and the feature component tests, suggesting the feasibility of the C* method for predicting short-term laboratory tests using different geometries. [S0094-9930(00)01001-5]

Creep crack growth in low alloy steel weldments

Developments in the determination and analytical representation of creep crack growth property data during the past 30 years are reviewed. The testing and data analysis of weldments involve additional complexities, and these are appraised with respect to low alloy steel weldments. Creep crack initiation and growth properties are dependent on creep deformation and rupture ductility characteristics. Consideration is given to the relationship between these properties using data determined for a ½Cr½ Mo¼V/2½CrMo pipe joint.

高温強度 １Ｃｒ‐Ｍｏ‐Ｖ鋼のクリープき裂成長特性に及ぼす試験片板厚の影響

Creep crack growth tests on very large 5T-CT specimens of 1Cr-Mo-V steel have been conducted in order to clarify the specimen size effect on creep crack growth properties and to contribute to the standardization of test method in VAMAS project. The creep crack growth rate could be correlated with C* parameter except for the initial stage of crack growth. In this stage, crack was considered to propagate under small-scale creep conditions. In the accelerating stage, the crack growth rate was independent of specimen width but was dependent on specimen thickness. As the specimen thickness increased, the creep crack growth rate increased due to the deformation constraint. The estimation method of displacement rate for CT specimens from the creep strain curve of round bar specimens was also discussed.

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.

Creep crack growth laws in heat-resistant steels

The conditions of various fundamental fields' domination at the creep crack tip have been considered. The creep crack growth properties and criteria are discussed. The relation between the creep crack growth kinetics and mechanisms have been studied for heat-resistant turbine steels over a wide test temperature and load range. In recent years much attention has been paid to the theoretical analysis of creep crack growth, though experimental investigations were generally performed for a comparatively narrow temperature and test time interval. The paper is concerned with the criteria and basic laws of creep crack growth. The results of our experimental investigation of the crack kinetics and crack growth mechanisms in heat-resistant steels in wide temperature and test time ranges are also presented.