Creep Crack Growth Rate Research Articles

Creep crack growth of GH3535 alloy at elevated temperatures

The behavior and micromechanism of creep crack growth (CCG) of GH3535 alloy at 650 to 750 °C have been investigated experimentally. The relationships among the effective stress intensity factor, fracture parameter C ⁎ , and CCG rate were obtained and the effect of temperature on the crack initiation time were analyzed. The results show that it is suitable to use the fracture parameter C ⁎ to correlate the CCG rate at different temperatures and that the CCG rate and fracture parameter C ⁎ have a good linear relationship in a narrow band in the log-log scale. This linear relationship does not depend on temperature when using the fracture parameter C ⁎ to correlate the crack initiation time. During crack propagation, intergranular propagation and dimple transgranular fracture occurred successively. The crack growth is found to be accompanied by oxidation. • Creep crack growth behavior of GH3535 allov used as structural material of TMSR at different temperatures was investigated. • The relationships between crack gowth rate crack initiation time and C* parameter are independent of temperature. • The propagation mechanism of GH3535 at different temperatures was found by observing the fracture and crack tip morphology. • Oxidation promotes the crack growth and the interaction between crack growth and oxidation is described.

Characterization of material constraint effects on creep crack growth for welded joints by a new constraint parameter [formula omitted

The effect of creep mismatch on the creep crack growth was studied with a wide range creep mismatch factors and mismatch widths. Moreover, the influences of cracked specimen geometry were considered. The creep crack growth was accelerated in an undermatch and slowed in an overmatch, influenced by the weld width and the specimen geometry. Furthermore, a new load-independent constraint parameter QM* was introduced to quantify the material constraint induced by material mismatch. QM* was employed to correlate with the normalized creep crack growth rate. This method could well demonstrate the variation of creep crack growth rate under different mismatched conditions.

Experimental investigation of creep crack growth behavior in the heat affected zone of boron added modified 9Cr–1Mo steel weld

Abstract This study aims to quantify the improvement in the creep crack growth (CCG) resistance in the heat-affected zone (HAZ) of boron added modified 9Cr–1Mo (P91B) steel weld at 898 K. The crack growth rates in the HAZ of P91B weld have been compared with the crack growth rates in HAZ of without boron P91 steel weld. The creep crack growth rates are estimated as per ASTM E1457 using 7 mm thick compact tension specimens with a notch in the HAZ and test duration 248h-11,000 h. The CCG rate ( a ˙ ) vs. C* correlation, commonly expressed as a ˙ = D ( C ∗ ) ϕ , has been established. In comparison, the crack growth rate in the HAZ of P91 weld is found to be threefold as compared to HAZ of P91B weld; with the difference in CCG rate increasing towards a higher C* range. Creep damage assessment study has been done in the vicinity of creep crack in HAZ of a P91B weld specimen tested for 11,000 h. The extent of cavitation, cavities link-up, and micro-crack formation in HAZ of P91B weld is found to be relatively lower than HAZ of P91 weld. Most of the cavities appear to be discrete in nature. This testifies improvement in the creep crack growth resistance in the HAZ of P91B weld due to controlled boron addition of about 100 ppm to modified 9Cr–1Mo steel.

Prediction of Reheat Cracking Behavior in a Service Exposed 316H Steam Header

Reheat cracking in an ex-service Type 316H stainless steel steam header component has been investigated in this study. The examined steam header was in service for 87,790[Formula: see text]h and the cracks in this component were found in the vicinity of the weld toe. The root cause of this type of failure was due to the welding residual stresses. The welding-induced residual stresses had been present in the header at the early stage of the operation and were released during service. In this paper, a novel technique has been proposed to simulate the residual stress distribution normal to the crack direction by applying remote fixed displacement boundary conditions in an axisymmetric model. This approach can simulate the presence of residual stresses in actual components without the need to develop full weld simulation to quantify them. The predicted residual stress levels and distributions normal to the crack direction have been found in good agreement with the measured residual stresses available in the literature for a similar header. The creep crack growth (CCG) rates have been characterized using the fracture mechanics [Formula: see text] parameter and estimated using predictive models.

Holistic Approach on the Research of Yielding, Creep and Fatigue Crack Growth Rate of Metals Based on Simplified Model of Dislocation Group Dynamics

The simplified model of numerical analyses of discrete dislocation motion and emission from a stressed source was applied to predict the yield stress, dislocation creep, and fatigue crack growth rate of metals dominated by dislocation motion. The results obtained by these numerical analyses enabled us to link various dynamical effects on the yield stress, dislocation creep, and fatigue crack growth rate with the experimental results of macroscopic phenomena, as well as to link them with theoretical results obtained by the concept of static, continuously distributed infinitesimal dislocations for the equilibrium state under low strain or stress rate conditions. This will be useful to holistic research approaches with concern for time and space scales, that is, in a time scale ranging from results under high strain rate condition to those under static or low strain rate condition, and in a space scale ranging from meso-scale to macro-scale mechanics. The originality of results obtained by these analyses were found by deriving the analytical formulations of number of dislocation emitted from a stressed source and a local dynamic stress intensity factor at the pile-up site of dislocations as a function of applied stress or stress rate and temperature material constants. This enabled us to develop the predictive law of yield stress, creep deformation rate, and fatigue crack growth rate of metals dominated by dislocation motion. Especially, yielding phenomena such as the stress rate and grain size dependence of yield stress and the delayed time of yielding were clarified as a holistic phenomenon composed of sequential processes of dislocation release from a solute atom, dislocation group moving, and stress concentration by pile-up at the grain boundary.

Evaluation of the creep crack growth behavior in 9Cr–1Mo steel under different stress conditions

Creep crack growth (CCG) behaviors of 9Cr–1Mo heat resistant steel under different stress states, such as the different initial crack lengths, two/three dimensional stress states, different side groove depths and angles are investigated by the numerical method. The results show that decreasing the initial crack length can increase the specimen life. Affected by the higher stress tri-axiality in the specimen, the side groove specimen life is greater than that the plain CT specimen. Crack front straightness extent increases with the increasing of value of d/B0 or magnitude of the α. The side groove depth of d/B0 = 0.1 and side groove angle of α = 80° meet the crack front straightness requirement, and it can be used to predict the CCG rate of the 9Cr–1Mo steel. The CCG rate of the plane strain specimen is greater than that the plane stress specimen for both the same loading condition of initial stress intensity factor and reference stress. On the whole, all the CCG rate data are dropped into the 95% prediction band of fitting curve, demonstrating that the CCG rate of 9Cr–1Mo heat resistant steel can be reasonably calculated by C*.

Characterization of creep constraint effect for brazed joint specimens at crack tip by new constraint parameter As

In this study, a new creep constraint parameter As of brazed joint was proposed based on existing constraint parameters R* and Ac. The proposed parameter As is superior than R* and Ac because the constraint parameter As is not affected by the selection of creep steady time and distance from crack tip. And the load independence of constraint parameter As for different constraint levels are all superior than that of parameter R* and Ac. Based on the relationships between creep crack growth (CCG) rate and constraint parameter predicted by single-material method, the superiority to characterize constraint effect of constraint parameter As is clarified. The CCG rate equation dependent constraint of brazed joints is established based on the defined parameter. The CCG data predicted by established constraint dependent CCG rate equation have a good accordance with experiment. The established CCG rate equation dependent constraint can predict the CCG rate at different constraint levels very well.

Creep fracture behavior of the Crofer 22 APU for the interconnect of solid oxide fuel cell under different temperatures

Creep damage and creep crack growth (CCG) behavior of Crofer 22 APU as interconnect of solid oxide fuel cell at different temperatures are investigated by the continuum damage mechanics method through CT specimen. It is illustrated that the fracture time of CT specimen reduces with temperature increasing at the same stress level, they are 1297 h, 42 h, 1.8 h and 0.25 h, respectively, at 650, 700, 750 and 800 °C with stress level of 2.73 MPa⋅m. The slopes of creep fracture mechanics parameter C* to CCG rate a˙ (C*- rate of a) are nearly the same at 650 and 700 °C. The slope of C*- a˙ at 650 and 700 °C is smaller than that at 750 and 800 °C. It is concluded that the temperature effect on CCG behavior of Crofer 22 APU is more evident than that the stress effect. All the data of C*- a˙ fall within the area of 95% prediction band, and the obtained equation of C* and CCG rate is a˙=3.8e2C∗0.91. Therefore, the a˙ of Crofer 22 APU can be predicted under different stress levels and temperatures.

Fractal scaling and crack-size effects on creep crack growth

Scaling effects on the creep crack growth behaviour are investigated by analyzing the results of compact tension (CT) tests on different-sized notched steel specimens appearing in the literature. Creep crack growth rate data are correlated to the elastic stress-intensity factor in terms of a Paris-type law, da∕dt = C0Kq, where C0 turns out to be a crack-size dependent coefficient of proportionality. Considering specimens with the same loading configuration (CT) and the same thickness, the observed crack-size effect on the creep crack growth rate is discussed on the basis of self-similarity considerations, and geometrically interpreted in terms of fractal tortuosity of the crack profile. A size-independent formulation of the creep crack growth law correlating renormalized quantities is finally deduced and confirmed by the experimental results.

Numerical investigation of creep crack growth in plastically graded materials using C(t) and XFEM

In this work, a numerical methodology is proposed to predict the creep response and creep crack growth for functionally graded material/plastically graded material in the extended finite element method framework. The stress relaxation and redistribution as a result of creep are incorporated into the analysis of graded material. The Ct-integral (which includes all creep stages from small-scale creep to extensive creep) based relation analogous to Paris Law is employed to calculate the creep crack growth rate. The numerical difficulties faced at the time of simulation, such as strain-energy rate density derivative and maintaining plastic/creep irreversibility during crack growth are well addressed in this paper. The strain-energy rate density derivative is evaluated by the surface approximation. The plastic/creep irreversibility is maintained by the appropriate data transfer and null step analysis scheme. The crack growth direction is decided by the maximum circumferential stress criterion. The proposed methodology is first validated with the experimental results for homogeneous material and later on, it is implemented to calculate the crack growth for various cases of gradation. The capabilities of the present scheme are demonstrated by performing the mixed-mode crack growth simulations at the specimen and component level.

A novel method for load line displacement rate partitioning in creep crack growth tests on Type 316H stainless steel

Characterising the creep crack growth behaviour of Type 316H stainless steel is vital in obtaining accurate predictions for the lifetime of high temperature components, for example in UK advanced gas cooled reactors. The correlation between creep crack growth rates and the fracture mechanics parameter C∗, considered to govern the crack growth process, is obtained from creep crack growth tests. The C∗ parameter is experimentally determined using an expression which requires knowledge of the load line displacement rate due to creep. Historically this has been calculated by subtracting values for the elastic and plastic contributions to the load line displacement, obtained from available solutions, from the total experimentally measured load line displacement. However, the solutions available to determine the plastic contribution rely on generating a power-law fit to uniaxial tensile data, which is difficult to accomplish accurately over a large stress range. In addition, these expressions cannot account for strain history effects during crack growth. Consequently the elastic and plastic contributions are often erroneously large and can even be in excess of the experimental total load line displacement. A novel technique has been proposed to provide improved estimates of the creep contribution to the load line displacement rates during creep crack growth tests. This technique employs finite element analysis that incorporates material specific uniaxial tensile test data to simulate crack growth in an experimental test. A single elastic-plastic-creep simulation is used to determine the separate elastic-plastic and creep contributions to the load line displacement, meaning that, unlike historic analyses, creep stress relaxation and strain history effects can now be accounted for. The results have demonstrated that advanced predictions of the creep contributions to the load line displacement can be obtained using this technique.

Prediksi Laju Perambatan Retak Melar pada Baja Austenitik 316L Menggunakan Data Uji Melar dan Metode NSW

In this study, an experimental creep crack growth rate (CCGR) of austenitic steel AISI 316L was predicted from the creep constant values and Nibkin Smith Webster (NSW) method. The experimental CCGR data were obtained from the CCG test under four different load conditions ranging from 6000 to 7000 N at 525 o C. The creep constants, A and n were obtained from the uniaxial creep rupture test under various load conditions, ranging from 180 to 225 Mpa at 525 o C. The creep constants were determined both from the minimum and average creep strain rate data. By applying the creep constants and NSW method, the predicted CCGR curve was generated and compared with the experimental CCGR curve. The result showed that the predicted CCGR curve based on the minimum creep strain rate data was found to be close to the experimental CCGR curve. The significant portion of creep crack growth’s lifetime of austenitic steel AISI 316L was occupied by the crack propagation (steady stage) period rather than crack initiation and/or fracture periods. In addition, plane stress and strain conditions could also be determined from the predicted CCGR curve. It was observed that the experimental CCGR curve was located near the plane strain condition where no deformation occurred in the lateral direction.

Accounting for crack tip cyclic plasticity and creep reversal in estimating (Ct)avg during creep‐fatigue crack growth

AbstractCreep‐fatigue crack growth (C‐FCG) rates in a P91 steel at 625°C were correlated as the average time rate of crack growth during hold time, (da/dt)avg, with (Ct)avg. At 60‐second hold time, the rates were lower than for 600‐second hold time. At 600‐second hold time, the crack growth rates converged on to the creep crack growth rate (CCGR) trend. Thus, the CCGR trend represents the upper bound for time‐dependent crack growth rates in P91 materials. The analytical expressions based on considering just the elastic and secondary creep deformation rates overestimated the magnitudes of (Ct)avg by as much as a factor of 10 for the 600‐second hold time tests. After accounting for the effects of cyclic plasticity during unloading, and accounting for only partial reversal of creep strains accumulated during hold time, the estimates of (Ct)avg compared well with the measured values. CR represents the extent of crack tip creep strain reversal, and tpl is the time required for the crack tip creep zone during the hold time to become equivalent in size to the cyclic plastic zone in terms of stress carried by that region. Together, these parameters accurately account for the effects of crack tip cyclic plasticity on the magnitude of (Ct)avg.Both tpl and CR depend on material properties, and the latter also depends on the hold time. A parameter ϕ is introduced that is dependent only on material properties and from which CR can be estimated for a given hold time. tpl and ϕ can be reported as part of the test results from C‐FCG testing.

The influence of plasticity-induced crack closure on creep-fatigue crack growth in two heat-resistant steels

Finite element simulations are performed to predict plasticity-induced crack closure and creep-fatigue crack growth rates in two heat-resistant steels, the austenitic 20Cr-25Ni (Alloy 709) and martensitic 9Cr-1Mo (modified 1Cr-9Mo) steels. In a creep-fatigue cycle, total crack growth rate is usually computed by the addition of the fatigue crack growth rate during cyclic loading and creep crack growth rate during hold time. Two-dimensional finite element analyses of compact tension specimens are performed to simulate crack growth under cyclic and time-dependent loading conditions, with the consideration of elastic-plastic and creep deformations of the material at the crack tip. The simulations quantify the effect of hold time on crack opening load induced by the combined action of plasticity-induced crack closure and creep-induced stress relaxation at the crack tip. It is shown that increasing hold time during a creep-fatigue cycle results in a decrease of crack-tip opening load, thus increasing crack growth rate during the next cyclic loading. Experimental results of creep-fatigue crack growth rates in Alloy 709 are presented. The finite element simulations predicted crack growth rates under creep-fatigue loading in agreement with experimental values for both Alloy 709 and modified 9Cr-1Mo steels.

A Numerical Study of Creep Crack Growth in an Aero-engine Turbine Disc using XFEM

In this paper, a turbine disc of an aero-engine having a macro crack under creep loading is studied to predict the creep crack growth. Extended finite element method (XFEM) is used as it does not require conformal meshing and remeshing due to change in the topology during crack growth. The elasto-plastic behavior of the material is modelled by Ramberg-Osgood model and von-Mises yield criterion. The creep behavior is modelled by coupling of spatial and temporal dimensions to capture the effect of stress relaxation and redistribution due to creep strain. The creep crack growth rate is computed by the C(t)-integral which includes small-scale creep, transition creep and extensive creep. The crack growth direction is estimated by the maximum principal stress criterion using mode-I and mode-II stress intensity factors (SIFs). The interaction integral approach is implemented for the evaluation of stress intensity factors of different modes. The history fields of plasticity and creep are transferred properly from the old configuration to new configuration after the crack growth which is the main challenge in this analysis. This proposed numerical scheme is then utilized to obtain the creep crack growth in the component made of elasto-plastic-creeping material. The creep crack growth variation with time is estimated by the XFEM to evaluate the life of an aero-engine turbine disc under creep conditions.

Creep crack growth behavior of P91 steel weldments

The steels operating at elevated temperatures are well known to be exposed to premature failure due to cracking caused by constant thermal stress, i. e. secondary creep process. Therefore, creep crack growth tests were carried out on compact tension specimens machined from P91 weld joint at 600°C to determine its behavior in realistic conditions. At the same time, numerical method for predicting the creep crack growth in compact tension specimens by a series of incremental steady-state finite element analysis were performed using Norton’s law to represent creep behavior. Verification of the finite element predictions were obtained for weld metal and heat affected zone by comparison with experimental results, indicating at the same time that creep crack growth rates are significantly higher for weld metal than for base metal.

Modeling the creep damage effect on the creep crack growth behavior of rotor steel

AbstractTo evaluate the fracture life of steam turbine HP-IP rotors more accurately, an understanding of the creep damage effect on creep cracking performance of service-exposed rotor steel, and a proper creep crack growth (CCG) model considering the creep damage effect, is essential. In the present work comparative CCG tests were carried out on virgin and rotor steel that had been in service for 16 years, to examine the differences in creep crack microstructure and CCG rate between these two specimen types. Test results showed that the CCG rate of service-exposed steel is accelerated by creep damage due to metallurgical deterioration. Based on the CCG model derived by Webster, an improved CCG model is proposed by replacing the constant exponent with a creep-damage-related variable. To validate the improved model, a comparison of da/dt vs. C* between experiment data and the CCG models was conducted. The predictive result of the improved model is in better agreement with the experiment results than the classical CCG and Webster models for the service-exposed rotor steel. With decrement of C* value within the improved CCG model, the remaining life of HP-IP rotors may be predicted more accurately.

Experimental and numerical simulation analysis on creep crack growth behavior of CLAM steel

The experiment and numerical simulation were performed to analyze the creep crack growth behavior of China Low Activation Martensitic (CLAM) steel under temperatures ranging from 500 °C to 600 °C with a step of 50 °C. The creep crack growth micro-mechanism under multiaxial state of stress was investigated with the compact tension (CT) specimens. The creep crack growth rates were simulated based on the Nikbin-Smith-Webster (NSW) model. The difference of the crack propagation behavior is correlated with the evolution of two types of precipitates: Cr-rich M23C6 and W-rich M6C phases. The coarsening of M23C6 carbides and the precipitation of M6C phase particles lead to dissolving of the solid solution elements and deteriorating the creep properties during the testing processes.

Study of creep crack growth behaviour of a type 316(N) stainless steel weld and its mechanism

As the high temperature structural integrity of the welded components is often decided by the extent of crack growth in the weld itself, the evaluation of the creep crack growth (CCG) rate and its micromechanical characterization in the welds are of importance to the designers. This study presents the CCG characterization of a type 316(N) stainless steel (SS) weld containing about 5 FN (ferrite number) of delta-ferrite. Constant load CCG tests using compact tension (CT) specimens have been carried out in the temperature range of 823–923 K as per ASTM E1457-15. The creep crack growth rate (ȧ) vs. C* [ȧ-C*] plots, commonly expressed as ȧ=D(C*)φ, were established and compared in the temperature range of 823–923 K. The SEM/ EBSD analysis, carried out in the vicinity of the crack propagation path, has revealed inter-granular nature of creep crack propagation. This is attributed to the grain boundary σ-phase precipitations, which were found to be coarser as compared to the σ-phase precipitations in the grain interiors, leading to easier damage accumulation and microcrack formation in their vicinity.

Mixed mode crack growth in elasto-plastic-creeping solids using XFEM

In this work, an extended finite element based fully implicit algorithm is proposed to predict the creep deformation and creep crack growth in elasto-plastic-creeping solids. The effect of stress relaxation and redistribution due to creep strain is captured through elasto-plastic-creep analysis. In creep crack growth analysis, mostly C∗ is used to compute the creep crack growth which includes only extensive creep whereas C(t)-integral includes small-scale creep, transition creep and extensive creep. Thus, in the present study, creep crack characterization parameter C(t)-integral is evaluated using XFEM to compute creep crack growth rate. The mixed mode stress intensity factors are evaluated using domain based interaction integral approach to estimate the crack growth direction. Various numerical issues related to history dependency of plasticity and creep are properly addressed. To validate the present approach, various numerical problems are solved and the predicted creep deformation and creep crack growth results are compared with the experimental observations.