Creep Crack Growth Behavior Research Articles

Investigation of residual stress effects on creep crack initiation and growth using local out-of-plane compression

The effects of residual stress on creep crack initiation and growth behavior were investigated. It has been shown that higher compressive residual stress ahead of crack tips delays creep crack initiation and increases creep initiation life. The mechanism of residual stress effect on the creep crack initiation behavior was analyzed by distributions of equivalent stress and stress triaxiality ahead of crack tips. The residual stress essentially has no effect on creep crack growth rate. In creep life assessments of high temperature components, the effect of residual stress on creep crack initiation should be mainly considered.

Quantifying the constraint effect induced by specimen geometry on creep crack growth behavior in P92 steel

In this paper, the effect of the specimen geometry on the creep crack growth behavior in P92 steel was quantified and six different types of cracked specimens (including C-ring in tension CS(T), compact tension C(T), single notch tension SEN(T), single notch bend SEN(B), middle tension M(T), and double edge notch bend tension DEN(T)) were employed. Results revealed that the creep crack growth rate against C⁎ relations varied with changing specimen geometry. For a given C⁎ value, C(T) and CS(T) showed the highest crack growth rates, which were three times as the lowest crack growth rates in M(T). This revealed that distinctions in specimen geometry influenced the in-plane constraint level ahead of crack tip. Moreover, constraint parameter Q was introduced to quantify the in-plane constraint. During the creep crack propagation, the distributions of Q varied as specimen geometry changed. The specimen order in terms of Q values from high to low was CS(T), C(T), DEN(T), SEN(B), SEN(T) and M(T), which basically represented the constraint level in these specimens.

Effects of Residual Stress on Creep Crack Initiation and Growth of Cr-Mo-V Steel in Cracked C(T) Specimen

The effects of residual stress on the creep crack initiation and growth behavior were investigated. It has been shown that the higher compressive residual stress ahead of crack tips delays creep crack initiation and increases creep initiation life, while the higher tensile residual stress promotes creep crack initiation and decreases the initiation life. The residual stress essentially has no effect on creep crack growth rate. In creep life assessments of high temperature components, the effect of residual stress on the creep crack initiation should be mainly considered.

J0310105 数値解析による一方向凝固Ni基超合金のクリープき裂成長挙動におよぼす結晶方位の効果 第2報

J0310105 数値解析による一方向凝固Ni基超合金のクリープき裂成長挙動におよぼす結晶方位の効果 第2報

In-plane and out-of-plane constraint effects on creep crack growth rate in Cr–Mo–V steel for wide range of C*

In this work, the stress dependent creep ductility and strain rate model have been implemented in a ductility exhaustion based damage model and the creep crack growth (CCG) rates of a Cr–Mo–V steel in compact tension (C(T)) and middle tension (M(T)) specimens with different thicknesses and crack depths have been simulated over a wide range of C*. The effects of in-plane and out-of-plane constraints on CCG rates are examined. The results show that the in-plane and out-of-plane constraint effects on CCG rate are more pronounced for the high constraint specimen geometry (C(T)), while such effects are less significant for low constraint specimen geometry (M(T)). The constraint effects on CCG rates mainly occur in low and transition C* regions and the CCG rate increases with increasing in-plane and out-of-plane constraints. There exists interaction between in-plane and out-of-plane constraint in terms of their effects on CCG rate. The higher in-plane constraint strengthens the out-of-plane constraint effect on CCG rate and higher out-of-plane constraint also strengthens the in-plane constraint effect on CCG rate. The constraint effects on creep crack growth behaviour for a wide range of C* mainly arise from the interaction of crack-tip stress states and stress dependent creep ductility of the steel in different C* levels.

Prediction of creep crack growth behavior in Cr–Mo–V steel specimens with different constraints for a wide range of C∗

The stress dependent creep ductility and strain rate model were implemented in a ductility exhaustion based damage model, and the creep crack growth rates in Cr–Mo–V steel specimens with different constraints over a wide range of C∗-integral have been predicted by finite element analyses. The predicted creep crack growth rates agree well with the existing experimental data. The creep crack growth rate increases with increasing specimen constraint, and the specimen constraint has effect on the C∗-integral values of the slope turning points on the crack growth rate curves. The mechanical mechanism of specimen constraint effects on creep crack growth behavior was analyzed and discussed. The results show that the constraint effects on the creep crack growth behavior for a wide range of C∗-integral mainly arise from interaction of crack-tip stress states and stress dependent creep ductility of the steel. The constraint dependent creep crack growth rate data in a wide range of C∗-integral should be obtained and used in creep life assessments of components.

Application of an Advanced Creep–Fatigue Procedure for Flexible Design of Steam Turbine Rotors Based on Fracture Mechanics Methods

The demand for steam turbine components is driven not only by high efficiency but also by high plant operational flexibility. Steam turbine rotors are therefore exposed to increased temperatures and increased number of stress cycles. These aspects should be considered for life-time prediction. Fracture mechanics methods are usually applied when crack like defects are detected not only for new rotors but also for rotor components in service. Based on the findings, a decision has to be made with respect to acceptability considering high temperature effects as well as the expected future operating regime. For defect analysis in the high temperature range, crack initiation and crack propagation under combined creep and fatigue loading need to be taken into account. Based on fracture mechanics methods and long-term testing data, an advanced creep–fatigue procedure for the evaluation of crack initiation and crack growth has been developed within the German Creep Group W14 for creep crack growth (CCG) behavior. Furthermore, recent studies show that the crack size for creep crack initiation (CCI) depends on material ductility and creep strain in the ligament. This paper demonstrates the industrial application of the abovementioned method for steam turbine rotor assessment, which has a focus on crack initiation and crack growth under creep–fatigue conditions. For crack initiation, a simplified approach based on defect size and material ductility is compared to a standard approach—two-criteria-diagram (2CD). For the advanced evaluation concept, the CCI criterion is combined for analysis with a creep–fatigue crack growth (CFCG) procedure. The benefit of the method especially for ductile material will be highlighted.

Effect of stress dependent creep ductility on creep crack growth behaviour of steels for wide range of C*

In this work, the effect of stress dependent creep ductility on the creep crack growth (CCG) behaviour of steels has been investigated by finite element simulations based on ductility exhaustion damage model. The relationship between the transition region of creep ductility and the transition behaviour of CCG rate on da/dt-C* curves has been examined and the CCG life assessments of components and CCG resistance of materials for a wide range of C* were discussed. The results show that with increasing the transition region size of creep ductility, the transition C* region size on da/dt-C* curves increases. With moving transition region position of creep ductility to high stress region (increasing transition stress levels), the transition C* region on the da/dt-C* curves also moves to high C* region. Decreasing transition stress levels and transition region sizes of creep ductility and increasing the lower shelf and upper shelf creep ductility values can improve the CCG resistance of materials. If the extrapolation CCG rate data from the high C* region or from the transition C* region are used in life assessments of the components at low C* region, the non-conservative or excessive conservative results may be produced. Therefore, the CCG rate data should be obtained for a wide range of C* by long term laboratory tests or numerical predictions using the stress dependent creep ductility and model.

The influence of stress-regime dependent creep model and ductility in the prediction of creep crack growth rate in Cr–Mo–V steel

In this paper, the stress-regime dependent creep model and ductility have been implemented in a ductility exhaustion based damage model, and their influence on creep crack growth (CCG) behavior of materials have been analyzed. By using the stress-regime dependent creep model and ductility, the CCG rate in a Cr–Mo–V steel over a wide range of C* has been predicted by finite element analyses. The predicted CCG rates agree with the available experimental data in the literature. The analysis results show that with increasing C*, the creep model and ductility for determining crack-tip creep damage accumulation change from the low-stress regime model and ductility through a combination of low- and high-stress regime model and ductility to high-stress regime model and ductility. These changes lead to the line segments of the da/dt−C* curves. In CCG life analyses and designs of high temperature components, the stress-regime dependent creep model and ductility need to be used.

Mismatch effect in creep properties on creep crack growth behavior in welded joints

The finite element method based on ductility exhaustion model was used to systematically investigate the mismatch effect in creep properties on creep crack growth (CCG) behavior in welded joints. The crack-tip damage, stress states, CCG paths, CCG rate and rupture life were calculated for different configurations of creep properties between weldment constituents under the same load level, and the creep life assessment and design for welded joints were discussed. The results show that when the zone containing the crack is softer than at least one of the other two surrounding materials or both, the creep crack propagates straight along the initial crack plane. Otherwise, it will form a second crack in the soft material near interface. These simulation results were confirmed by the experimental observations in the literature, and the mechanism was analyzed. The harder surrounding materials can lead to higher CCG rate and shorter rupture life due to the higher constraint given from them. The early initiation and propagation of the second cracks increase CCG rate and reduce rupture life, and the incubation time of the second cracks in soft materials near interfaces should be accurately determined in the creep life assessment and design for the welded joints. A proper mismatch design with harder material containing crack and softer surrounding material can improve CCG properties of welded joints (decreasing CCG rate and prolong rupture life).

Prediction of creep crack growth behaviour in 316H stainless steel for a range of specimen geometries

The specimen geometry and constraint effects on the creep crack growth behaviour of Type 316H stainless steel at 550 °C have been examined over a wide range of load levels using finite element simulations. Creep crack growth predictions are performed on a range of specimen geometries by employing stress dependent creep ductility and strain rate trends in creep damage calculations. The predicted creep crack growth rates are characterised using the C* fracture mechanics parameter and validated through comparison with the existing experimental data. Comparisons have been made between the predicted short term and long term creep crack growth behaviour in different specimens and the results are discussed in terms of the specimen constraint effects on the crack growth behaviour of the material. Two material states including as-received and pre-compressed conditions have been considered and their predicted creep crack growth results are compared in each of the specimen geometries examined.

Experimental investigation of specimen size effect on creep crack growth behavior in P92 steel welded joint

In order to clarify the effect of constraint induced by specimen size on creep crack growth behavior of P92 steel welded joint, creep crack tests were carried out on the compact tension specimens with thick thickness and thin thickness, the crack tip of which were located at different distinct zones of welded joint. Tested results revealed that even in thin thickness specimens, fine grained heat affected zone specimens exhibited a fast creep crack growth rate compared with other micro-zones specimens due to a low creep crack resistance and a high multistress state. The fractographies of these specimens exhibited an accelerated number of spherical particles that were caused by the coalescence of creep voids. Furthermore, the correlation of C* with creep crack growth rate was dependent on specimen thickness. As the specimen thickness increased from 10 to 30mm, the creep crack growth rate increased. This was due to the increase in constraint level ahead of crack tip during creep crack propagation.

Creep crack initiation and growth behavior in weldments of high Cr steels

Creep crack initiation and growth behavior in weldments of high Cr steels

Creep Crack Growth Behavior of a P91 Steel Weld

Modified 9Cr-1Mo steel (P91) weld joints operating at elevated temperatures are well known to be prone to premature failure due to cracking in the heat affected zone because of the gradients in microstructure, popularly referred to as Type IV cracking. A campaign was undertaken to study the creep crack growth behaviour of modified 9Cr-1Mo weld joints. Creep crack growth (CCG) tests were carried out on compact tension (CT) specimens machined from P91 weld joints prepared using multipass shielded manual metal arc welding procedure. Specimens with two notch locations have been employed, (i) within the weld metal, between the centreline and the fusion line and (ii) in the heat affected zone. Constant load CCG tests were carried out at different applied loads at 798 and 898 K. The C* - da/dt correlations (da/dt =A C*m) were established for both notch locations. At 898 K, a higher A (0.064) and lower m (0.533) for the case with notch placed in the HAZ, compared to those for the case of notch in the weld (0.0399 and 0.75 respectively) were observed indicating the higher creep crack growth in HAZ, confirming type IV cracking. The difference increases at lower C* levels which correspond to long term behaviour. Heavy creep damage was observed in the HAZ region even for the sample with notch in the weld, whereas the weld metal regions showed relatively less damage. As the crack grew, a change in its course to follow the HAZ region was observed.

GS12 微視クリープ損傷力学解析に基づくクリープき裂成長試験におけるC(T)試験片サイドグループ形状の特定

Creep crack growth tests using C(T) specimens are standardized by ASTM E1457. To realize uniform creep crack growth, C(T) specimens with side-groove are used for the tests. However, due to difference of creep ductility and creep crack growth behavior for each material, the optimum depth of side-groove has not been yet defined strictly. In this paper, to characterize the optimum depth of the side-groove, diffusion analysis of vacancy around a notch tip was conducted based on micro damage mechanics.

Numerical Prediction of the Long Term Creep Crack Growth Behaviour in Type 316H Stainless Steel

Life assessment of Type 316H stainless steel (SS) is of great concern to industry due to the widespread use of this material in power plant components that often operate at a temperature of around 550 °C. An important issue to be understood and considered in the life assessment of 316H components is the creep deformation and crack growth behaviour of this material in short term and long term operation times. Due to the limited long term test data available in the literature (i.e. [1, ), a new technique have been recently developed to estimate uniaxial creep ductility trends and subsequently creep crack growth (CCG) behaviour of the material at low load levels [. In this new method the creep ductility trends have been estimated as a function of the applied stress normalised by the temperature dependent 0.2% proof stress of the material, σ/σ0.2, to include the plasticity effects on the creep deformation behaviour of the material. These trends have been implemented in finite (FE) simulations to predict the CCG behaviour of 316H in intermediate and long term tests at 550 °C and provisional results are presented in [.

Creep Behavior of P92 and P92 Welds at 675 °C

The results of an accelerated creep test programme on the leading new build high temperature power plant pipework ferritic steel P92 and two P92 welds are presented. Tests were performed at 675 °C, which is above the service operating temperature range recommended for P92, but allowed for more realistic operating stresses to be used. Comparison with similar tests of P92 at lower temperatures has shown that testing at 675 °C produces the same general creep behavior, and can therefore be used for component life assessments in the service operating temperature range. Axially loaded parent P92 material uniaxial round bar and notched bar specimens, given a heat treatment equivalent to that given postwelding, are compared with weld metal (WM) specimens and cross-weld (C-W) specimens extracted from welds made using both a similar P92 consumable and a dissimilar IN625 nickel alloy consumable. Both welds exhibited typical premature ferritic steel weld failure at the heat-affected zone (HAZ) and parent material (PM) interface, known as type IV. The creep crack growth (CCG) behavior of parent material, weld metal, and the HAZ was studied using compact tension (CT) specimens. Impression testing was performed to determine the relative creep behavior of the HAZ. The HAZ was found to have the highest minimum creep strain rates and creep crack growth rates, indicating the relative weakness of this region.

The influence of pre-compression on the creep deformation and failure behaviour of Type 316H stainless steel

The influence of pre-compression to 8% plastic strain at room temperature has been examined on the tensile properties, uniaxial creep deformation, creep crack initiation and growth behaviour of Type 316H stainless steel at 550°C. Uniaxial creep and crack growth tests have been performed on the pre-compressed (PC) material and the results compared to existing long term (>15,000h) and short term test data on as-received (AR) (i.e. uncompressed) material. Pre-compression has been found to increase the materials subsequent yield stress in tension. Therefore the extent of non-linearity observed on the load–displacement curves of uniaxial creep rupture and crack growth tests on PC material is limited compared to AR material. In addition pre-compression causes a significant reduction in creep ductility and rupture time, although similar average and minimum creep strain rates are found in PC and AR materials. The creep crack growth (CCG) data on PC and AR materials have been characterised using the steady state creep C* parameter employing appropriate validity criteria and geometry dependent fracture mechanics parameter solutions. The CCG results are compared to the creep crack growth prediction models. Based on the creep properties, creep ductility and metallurgical observations of the fracture behaviour of the AR and PC materials, it has been shown that short term creep crack growth tests on PC material may be used to predict long term creep crack initiation (CCI) and CCG behaviour of AR material at 550°C.

Effect of residual stress on creep crack growth behavior in ASME P92 steel

Effect of residual stress on creep crack growth behavior in ASME P92 steel was investigated using compact tension (CT) specimen. Tensile residual stresses were generated ahead of crack tip by loading in compression beyond yield and then unloading. The maximum region of residual stress was obtained by numerical simulations which calculated the models with various notch radiuses and penetration loads. After pre-compression, creep crack growth tests were carried out on pre-compressed and original CT specimens to study the effect of residual stress. Furthermore, the corresponding approach on predicting creep crack growth behavior for components with residual stress was also investigated.

Difficulties in interpreting data from creep crack growth tests on type 316H weldments

Experimental creep crack growth data are generally obtained by following standard methods such as ASTM E1457-07 and subsequently characterised using the C* parameter. These data are then used in assessment procedures, such as R5, together with reference stress estimates of C* in the component, to predict creep crack growth behaviour. Some modifications to the ASTM E1457 creep crack growth testing and analysis methods have already been proposed following a previous analysis of data from long term creep crack growth tests on type 316H parent material. This paper reports the results of creep crack growth tests on type 316H heat affected zone material at 550°C using compact tension (CT) specimens manufactured from non-stress relieved thick section butt welds. It is shown that interpretation of the data from these weldment tests is complicated by both the discontinuous nature of the cracking process and the presence of significant residual stresses in the CT specimens. Further modifications to creep crack growth testing and analysis methods are proposed to address difficulties arising from the discontinuous nature of the cracking process, and further work is identified to investigate the influence of the residual stresses present in the specimens on the observed crack growth behaviour.