Creep Crack Initiation Time Research Articles

Numerical analysis of the creep crack constraint effects and the creep crack initiation for pressurized pipelines with circumferential surface cracks

The creep crack constraint effects using a load-independent creep constraint parameter Q* and the creep crack initiation (CCI) times were characterized by 3D finite element method for pipelines with circumferential surface cracks of different geometrical sizes. The results revealed that the distribution regulation of Q* along the crack front for circumferential internal surface cracks and external surface cracks was similar. The maximum constraint level occurred near the deepest crack front part for cracks with smaller a/c, while it occurred near the free surface for cracks with larger a/c. The constraint values at the same position (2Φ/π) increased with the increasing of the crack depth when a/c kept constant. The circumferential internal surface cracks of pipelines were proved more dangerous than the external surface cracks with the same geometrical size. Furthermore, the CCI times were decided by the peak values of constraint, or the CCI firstly occurred at the position where the constraint level was maximum. In addition, the empirical relationships between the CCI times and crack sizes were fitted, which was also verified effectively.

Effect of constraint on creep crack initiation time in test specimens in ASTM-E1457 standard

The creep crack initiation (CCI) time of six specimen geometries in ASTM E1457 standard has been predicted by using numerical simulations, and the constraint effect on CCI time was analyzed. The results show the specimens with different geometries, size ranges and side-grooves recommended by ASTM-E1457 produce different CCI time data due to different constraint levels. With increasing constraint levels in specimens, the CCI time is reduced. The quantitative correlation formulas of CCI time with creep constraint parameters have been established, and they may be applied in CCI life assessment of high-temperature components for accounting for constraint effect and improving accuracy.

Prediction of creep crack initiation in Cr–Mo–V steel specimens with different geometries

By using stress dependent creep ductility and strain rate model in a ductility exhaustion based damage model, the creep crack initiation (CCI) behaviour in Cr–Mo–V steel specimens with different geometries and dimensions (different constraints) over a wide range of C* has been predicted by finite element simulations. The predicted creep crack initiation time agree well with the existing experimental data. In low and transition C* regions, the constraint induced by specimen geometries and dimensions has obvious influence on CCI time. With increasing constraint level of specimens, the CCI time decreases due to the increase of stress triaxiality ahead of crack tip. Different CCI trends and constraint effects on CCI behaviour in a wide range of C* result from the interaction of crack-tip stress state and stress dependent creep ductility of the steel. It is suggested that in CCI life assessments of high temperature components, the long-term CCI time data at low C* region should be obtained and used, and the constraint effects need to be considered by using constraint dependent CCI data.

Determination of the creep crack initiation properties using pre-cracked small punch tests

The use of pre-cracked small punch creep test specimens (p-SPCT) is analyzed in this paper as an alternative method to obtain the Creep Crack Initiation (CCI) time in those cases where there is not enough material to use CT specimens. The material selected was an AZ31B magnesium alloy. The geometry of the specimens used was 10×10mm square with a thickness of 0.5mm. An initial crack-like notch was created in the SPT specimens by means of a laser micro-cutting technique. Conventional compact specimens (CT-20) were tested at T=150°C as a reference for CCI determination. The definition of an equivalent load for p-SPCT was seen to be a good methodology, and a λ parameter was defined to convert the p-SPCT to an equivalent C(T) load that produces the same CCI time. The influence of temperature and material on the λ parameter still remains to be investigated.

Simulation of rafting type and stress distribution in nickel-based superalloys with different volume fractions of γ′ phase

A representative volume element is introduced to represent the microstructure of γ/γ′ morphology with periodic boundary conditions to formulate the full mode of the micromechanical analysis. [0 0 1]-oriented alloys with γ′ volume fractions from 60 to 70% are simulated under tensile loading. A raft criterion is implemented into the user subroutine to predict the rafting type. The misfit stress is considered by different thermal expansion coefficients of the two phases. It is very high in γ phase and slightly decreases with the increasing of γ′ volume fraction. The stress distributions in the two phases change during creep deformation. The creep crack initiation time slightly increases with the increase of γ′ volume fraction. The stress components decrease with the increasing of γ′ volume fraction at the beginning and change due to the stress redistribution during creep loading.

Geometry and Material Constraint Effects on Creep Crack Growth Behavior in Welded Joints

Abstract In this work, the geometry and material constraint effects on creep crack growth (CCG) and behavior in welded joints were investigated. The CCG paths and rates of two kinds of specimen geometry (C(T) and M(T)) with initial cracks located at soft HAZ (heat-affected zone with lower creep strength) and different material mismatches were simulated. The effect of constraint on creep crack initiation (CCI) time was discussed. The results show that there exists interaction between geometry and material constraints in terms of their effects on CCG rate and CCI time of welded joints. Under the condition of low geometry constraint, the effect of material constraint on CCG rate and CCI time becomes more obvious. Higher material constraint can promote CCG due to the formation of higher stress triaxiality around crack tip. Higher geometry constraint can increase CCG rate and reduce CCI time of welded joints. Both geometry and material constraints should be considered in creep life assessment and design for high-temperature welded components.

Prediction of creep crack initiation and growth for P91 at 600°C using MOD-NSW model

In this paper, predictions of creep crack initiation times and growth rates using the NSW-MOD model are compared with experimental data for the parent and weld metals of P91 at 600°C. For the weld metal, creep crack growth rates are found to be bounded by upper and lower bounds of the predictions, but creep crack initiation times are close to the upper bound. For the parent material, creep crack growth rates are bounded by upper and lower bounds of the predictions using the minimum creep rates, but creep crack initiation times are bounded by those using the average creep rates.

Creep damage and crack initiation in P92–BNi2 brazed joint

Nickel-based brazed joints are widely used at high temperatures, and it is very important to ensure the creep strength. Using three-dimensional finite element method, this paper presents a study on the creep damage and creep crack initiation (CCI) in P92–BNi2 brazed compact tension (CT) joint specimen, and the effect of brazed residual stress on CCI has been investigated. Firstly, the as-brazed residual stress is simulated. Then the effect of residual stress on creep and damage is calculated by an uncoupled Norton equation and the ductility exhaustion approach. The results show that the CCI is a function of residual stress, load level, joint thickness and notch radius. The brazed residual stress has a great effect on creep damage, and it can lead to the initiation of creep crack lonely without any mechanical load. The combination of applied load not only accelerates the creep damage and decreases the CCI time, but also determines the creep crack propagation rate. The CCI time decreases with the increase of applied load and joint thickness. As the notch radius increases, the CCI time increases.

Influences of ellipse cooling holes on creep crack initiation and surface morphology of nickel based superalloys

A creep damage model is developed with the consideration of cavitation and material microstructure degradation as damage factors. Five models are studied with different ratios between the length of ellipse hole axis parallel (denoted by y) and perpendicular to the loading direction (denoted by x), where y maintains constant. Two loading conditions are taken into account: constant external load and constant net section stress. The creep crack initiation time increases with increasing y/x ratio at both loading conditions. The stress distribution morphologies are quite similar for all the models at the start of creep where there are high stress strips between holes in the adjacent rows and low stress strips between holes in the same rows. During the creep, stress relaxation and redistribution induce some differences in the morphologies for different models at the crack initiation time. The damage distributions provide important information on crack initiation and propagation path.

Effects of residual stress on creep damage and crack initiation in notched CT specimens of a Cr–Mo–V steel

In this work, the effects of residual stress on creep damage and crack initiation were investigated by numerical analyses. Two cases of residual stress alone and residual stress combined with primary load were analyzed. The results show that larger residual stress relaxation occurs at initial creep stage, and creep strain and damage are accumulated ahead of notches during residual stress relaxation. It has been demonstrated that the effects of residual stress on creep crack initiation time are related to the creep ductility of materials and primary load levels. This was analyzed by the change in residual stress relaxation time in materials.

Evaluation of constraint effects on creep crack growth by experimental investigation and numerical simulation

Effects of constraint induced by crack depth on creep crack behavior of P92 steel was investigated by experimental investigation and numerical simulation. Creep crack growth tests were carried out on compact tension specimens with different crack depths. It revealed that the high constraint affected creep crack initiation time and steady creep crack growth rate. Then, the crack-opening stress distribution at crack tip during creep crack growth process was obtained by finite element method (FEM) analysis. The corresponding variation of constraint parameter Q was also obtained. Finally, a C*–Q approach was employed to characterize the creep crack growth rate.

Predicting creep crack initiation in austenitic and ferritic steels using the creep toughness parameter and time‐dependent failure assessment diagram

AbstractMethods for evaluating the creep toughness parameter, are reviewed and data are determined for a ferritic P22 steel from creep crack growth tests on compact tension, C(T), specimens of homogenous parent material (PM) and heterogeneous specimen weldments at 565 °C and compared to similar tests on austenitic type 316H stainless steel at 550 °C. Appropriate relations describing the time dependency of are determined accounting for data scatter. Considerable differences are observed in the form of the data and the time‐dependent failure assessment diagrams (TDFADs) for both the 316H and P22 steel. The TDFAD for P22 shows a strong time dependency, but is insensitive to time for 316H. Creep crack initiation (CCI) time predictions are obtained using the TDFAD approach and compared to experimental results from C(T) specimens and feature components. The TDFAD based on parent material properties can be used to obtain conservative predictions of CCI on weldments. Conservative predictions are almost always obtained when lower bound values are employed. Long‐term test are generally more relevant to industrial component lifetimes. The different trends between long‐ and short‐term CCI time and growth data indicate that additional long‐term test are required to further validate the procedure to predict the lifetimes of high temperature components.

Predicting creep crack initiation in austenitic and ferritic steels using the creep toughness parameter and time‐dependent failure assessment diagram

ABSTRACTMethods for evaluating the creep toughness parameter, Kmatc, are reviewed and Kmatc data are determined for a ferritic P22 steel from creep crack growth tests on compact tension, C(T), specimens of homogenous parent material (PM) and heterogeneous specimen weldments at 565 °C and compared to similar tests on austenitic type 316H stainless steel at 550 °C. Appropriate relations describing the time dependency of Kmatc are determined accounting for data scatter. Considerable differences are observed in the form of the Kmatc data and the time‐dependent failure assessment diagrams (TDFADs) for both the 316H and P22 steel. The TDFAD for P22 shows a strong time dependency, but is insensitive to time for 316H. Creep crack initiation (CCI) time predictions are obtained using the TDFAD approach and compared to experimental results from C(T) specimens and feature components. The TDFAD based on parent material properties can be used to obtain conservative predictions of CCI on weldments. Conservative predictions are almost always obtained when lower bound Kmatc values are employed. Long‐term test are generally more relevant to industrial component lifetimes. The different trends between long‐ and short‐term CCI time and growth data indicate that additional long‐term test are required to further validate the procedure to predict the lifetimes of high temperature components.

The influence of test duration and geometry on the creep crack initiation and growth behaviour of 316H steel

Short-term creep crack growth (CCG) tests have often been performed so that they may be incorporated into research projects with a relatively narrow time frame. Therefore tests are generally performed at relatively high loads, leading to high values of the crack-tip driving-force parameter C*, to reduce the test duration. For the case of ductile materials with relatively low yield strengths, this can lead to significant plastic strains being generated in the specimen on loading. Recently, long-term CCG tests have been performed on the low constraint double edge notch tension and high constraint compact tension specimen geometries of 316H stainless steel at 550 °C. The CCG test data is examined in terms of the experimentally determined C* parameter and compared to data available from a wide range of specimen geometries over a range of C* values. At high C* values similar CCG behaviour is observed for the various specimen geometries. A difference in CCG behaviour is observed between long and short-term tests on the compact tension geometry which may be explained by the effects of constraint loss due to plasticity effects. However, similar trends are observed in the CCG rate data of the alternative, relatively lower constrained geometries, at both high and low C* values. The influence of test duration and specimen geometry on creep crack initiation times is also studied and experimental results are compared with predictions from analytical models.

Creep plastic zone and crack initiation in double edge notched specimens of 304 stainless steel at 650 °C

Creep tests of 304 stainless steel using double edge notched specimens (6 mm thickness, 8 mm net width, 0.3 mm notch width, notch depth a = 0.5, 1 and 2 mm) were carried out in air at 650 °C at a net stress of 88 MPa till crack initiation on the notch root surface. Creep crack initiation time: the time when ∼0.5 mm long crack is formed on the notch root surface, increased with increasing notch depth, i.e. from ∼3200 h for a = 0.5 mm, to ∼4000 h for a = 1 mm and to ∼5000 h for a = 2 mm. The creep crack initiation site was the notch root of near mid-thickness for a = 2 mm, while it was the notch root of near surface thickness for a = 0.5 mm. Development of the creep plastic zone was observed by the subsequent recrystallization thermal etching technique using multiple specimens which were crept to various times up to crack initiation. It is shown that the difference of the crack initiation site is related whether creep plastic zone is subjected to gross yielding or not.

Creep crack initiation and growth in AISI 316 (N) weld – FEM and experiments

There are two aspects of the creep crack growth behaviour, namely, the crack initiation and the crack propagation. An incubation period is often observed prior to the onset of creep crack growth. In this study, creep crack initiation and propagation in pre-cracked compact tension (CT) specimens of a 316 (N) stainless steel weld at T = 550 and 625°C under static loading is investigated. Both the crack initiation time and the crack growth rate are measured as a function of fracture parameter C*. It is shown that it is possible to correlate the creep crack initiation time with the C* parameter. It is also shown that the creep crack growth rate can be correlated with the C* integral. Additionally, finite element analyses by using the ANSYS software have been performed at one test condition (T=625°C) in order to estimate numerically the crack mouth opening displacement rate history for a propagating crack using the node release technique. When the FEM results are compared with the experimental data, the results show a very satisfactory prediction capability.

An analytical and computational study of crack initiation under transient creep conditions

An analytical method has been developed to predict creep crack initiation (CCI), based on the accumulation of a critical level of damage at a critical distance. The method accounts for the re-distribution of stress from the elastic or elastic–plastic field, experienced on initial loading, to a steady state creep stress distribution, via a transient creep region. The method has been applied to predict CCI times in a fracture specimen of type 316H stainless steel at 550 °C. The failure model has been also been implemented into a finite element (FE) framework. Reasonable and conservative predictions of CCI time can be obtained from the analytical solution relative to FE solutions. Conservative predictions of experimental CCI times are obtained when stress redistribution is taking into account. However, CCI times predicted from a steady state creep model are found to be non-conservative.

Analysis of Creep Crack Initiation and Growth in Different Geometries for 316H and Carbon Manganese Steels

Abstract Results obtained from a European collaborative project aimed at producing a draft code of practice for creep crack growth testing are presented. The tests were performed on an Austenitic Type 316H Stainless Steel at 550°C and a carbon manganese steel at 400°C. A range of specimen test geometries has been examined. These are the single edge notched specimen in tension and bending, the double edge notched specimen in tension, the middle cracked specimen in tension, and the C-shaped specimen in tension. The results obtained are compared to those obtained using the standard compact tension specimen following the ASTM Standard Test Method for Measurement of Creep Crack Growth Rates in Metals (E 1457). Creep crack creep growth rates and initiation times have been correlated with the fracture mechanics parameter C*. The applicability of the stress intensity factor K to describe the creep cracking behavior is also assessed. Recommendations are made to support the use of different geometries in creep crack growth and/or creep crack initiation testing.

Comparison of methods for obtaining crack-tip stress distributions in an elastic-plastic material

Under linear elastic and elastic-plastic conditions the K field and the HRR (Hutchinson-Rice-Rosengren) field respectively are expected to provide an accurate representation of the stress field close to the crack tip in an elastic-plastic material. It has recently been proposed in French and UK defect assessment procedures that the Neuber method, originally developed for sharply curved notches, provides an alternative approach to estimate both notch and crack-tip stress fields, for use in conjunction with the sigma- d (σd) method to predict creep crack initiation times. In this work, the crack-tip stress fields under plane strain conditions, predicted from the Neuber approach, are compared with the HRR and K fields as well as those obtained from full-field finite element calculations. A compact tension and a single edge notched tension specimen have been examined; the material model used is the Ramberg-Osgood, power law plasticity model. As expected, the K field and HRR field have been found to provide a good representation of the near-tip fields at low and high loads respectively. Satisfactory solutions have also been obtained through the use of the reference stress to estimate the amplitude of the crack-tip stress in conjunction with the HRR field. The Neuber approach provides a good estimate of the equivalent (von Mises) stresses over the full range of load levels. However, but the use of the Neuber approach directly to predict the maximum principal stress in the plane of the crack provides a non-conservative prediction. A modified Neuber method, using an appropriate scaling function, has been proposed to determine the maximum principal stress in the plane of the crack from the equivalent (von Mises) stress predicted by the Neuber approach. Using the proposed method, a significantly improved estimate of the crack-tip stresses is obtained.

Creep crack initiation and creep crack growth assessments in welded structures

In this study, creep crack initiation and creep crack growth in welded structure are analysed. An interaction phenomenon between base metal and weld metal in a cross weld plate is highlighted with a finite element model. A simplified method based on the reference stress approach is proposed to evaluate C* in welded structures. This simplified method is applied for creep crack initiation and creep crack growth assessments in the case of a double edge notched tension cross-welded plate. Correlations between crack initiation time and C* on the one hand, and between creep crack growth rate and C* on the other hand are used. Creep crack initiation time estimation for the full size welded plate is very conservative when crack initiation properties of CT specimen are used. Concerning creep crack growth evaluation, simplified estimation is in good agreement with experimental results when CT specimen crack propagation properties are used.