Enhanced Reference Stress Method Research Articles

Engineering estimates of crack opening displacement for non-idealized circumferential through-wall cracks in pipe

The present paper provides the engineering estimates of non-linear crack opening displacement (COD) for a circumferential non-idealized through-wall crack in a pipe based on both the GE/EPRI and the enhanced reference stress methods. For the GE/EPRI-type solution, the h2 values are newly calibrated based on the elastic–plastic finite element analyses. Also, elastic COD and the reference load solutions are proposed. Then, the proposed engineering estimates are validated against the FE results using actual tensile data of SA312 Type 316 stainless steel. From these comparisons, it is revealed that the enhanced reference stress method provides the best and overall satisfactory results.

Simplified estimates of the creep crack growth parameter C(t) under primary/secondary stresses using the enhanced reference stress method

This paper describes simplified methods to estimate the fracture mechanics parameter, C(t), related to creep crack growth rates in non-steady state creep conditions produced by primary or secondary stresses. The methods proposed incorporate effects from initial plasticity and redistribution during a short period after a loading dwell starts, in addition to the estimate of the steady state creep crack growth parameter C∗ by the enhanced reference stress method. The methods have been validated by performing finite element elastic–plastic creep analyses of a circumferentially cracked cylinder subjected to load-controlled tension or thermal loads.

Simplified estimate of elastic–plastic J-integral of cracked components subjected to secondary stresses by the enhanced reference stress method and elastic follow-up factors

This paper describes simplified methods to estimate the elastic–plastic J-integral, J, related to the crack growth rate in elastic–plastic situations. Estimating this parameter under general conditions entails costly detailed elastic–plastic FEA modelling of the cracked component concerned, and thus, some simplified methods that do not involve complex numerical calculations are required, particularly, for use in situations where plastic strains are produced by secondary stresses. For mechanical primary stresses, the reference stress method may provide reasonable estimates of J. The direct use of the reference stress method for secondary stresses, however, has not yet been fully established. The method presented in this paper is based on the enhanced reference stress method, which leads to more accurate estimates of J than the original method, and elastic follow-up factors for approximating the inelastic response of the component from the elastic FEA. The present method has been validated by performing detailed elastic–plastic FEA of cracked plates subjected to displacement-controlled loading and of a circumferentially cracked cylinder subjected to thermal loads.

반타원 표면균열 선단을 따른 참조응력 기반의 J-적분 예측

This paper discusses applicability of the enhanced reference stress method to estimate J-integral along the semi-elliptical surface crack front. It is found that angular variations of normalized J-integral are strongly dependent on the geometry, loading mode and loading magnitude. As application of the reference stress approach to semi-elliptical surface cracks implies proportional increases in the normalized J-integral, the present results pose a question in applicability of the reference stress approach. However, investigation of the error in the estimated J-integral in the present work suggests that the enhanced reference stress approach, recently proposed by authors, provides an effective engineering tool for estimating crack driving force along the semi-elliptical surface crack front.<br/>

광범위 크리프 조건에 대한 관통균열 배관의 크리프 파괴역학 해석

This paper compares engineering estimation schemes of C^* and creep COD for circumferential and axial through-wall cracked pipes at elevated temperatures with detailed 3-D elastic-creep finite element results. Engineering estimation schemes included the GE/EPRI method, the reference stress method where reference stress is defined based on the plastic limit load and the enhanced reference stress method where the reference stress is defined based on the optimized reference load. Systematic investigations are made not only on the effect of creep-deformation behaviour on C^* and creep COD, but also on effects of the crack location. the pipe geometry, the crack length and the loading mode. Comparison of the FE results with engineering estimations provides that for idealized power law creep. estimated C^* and COD rate results from the GE/EPRI method agree best with FE results. For general creep-deformation laws where eit her primary or tertiary creep is important and thus the GE/EPRI method is hard to apply, on the other hand, the enhanced reference stress method provides more accurate and robust estimations for C^* and COD rate than the reference stress method.

Elastic–plastic fracture mechanics assessment of test data for circumferential cracked pipes

This paper presents experimental validation of two reference stress based methods for circumferential cracked pipes. One is the R6 method where the reference stress is defined by the plastic limit load. The other is the enhanced reference stress method, recently proposed by the authors, where the reference stress is defined by the optimised reference load. Using 38 published pipe test data, the predicted maximum instability loads according to both methods are compared with the experimental ones for pipes with circumferential through-thickness cracks and with part circumferential surface cracks. It is found that the R6 method gives conservative estimates of the maximum loads for all cases. Ratios of the experimental maximum load to the predicted load range from 0.54 to 0.98. On the other hand, the proposed method gives overall closer maximum loads than R6, compared to the experimental data. However, for part through-thickness surface cracks, the estimated loads were slightly non-conservative for four cases, and possible reasons are fully discussed.

참조응력 개념을 이용한 원주방향 관통균열 배관의 파괴 역학 해석 - 실배관 실험 데이터와의 비교 -

This paper presents experimental validation of the enhanced reference stress based J estimates for circumferential through-wall cracked pipes, recently proposed by authors. Using the pipe test data for circumferential through-wall cracked pipes, the predicted fracture initiation and maximum moments according to the proposed enhanced reference stress method are compared with experimental ones as well as predictions from the R6 method. The results show that both the R6 method and the proposed method give conservative estimates of initiation and maximum moments for circumferential through-wall cracked pipes, compared to experimental data. For longer cracks, the proposed method reduces conservatism embedded in estimated J according to the R6 method, and the resulting predictions are less conservative, compared to those from the R6 method. For shorter cracks, on the other hand, the proposed method reduces possible non-conservatism embedded in estimated J according to the R6 method. and the resulting predictions are slightly more conservative.

내압이 작용하는 원주방향 관통균열 배관의 비선형 파괴역학 해석법

This paper provides engineering J-integral and crack opening displacement(COD) estimation equations for circumferential through-wall cracked pipes under internal pressure and under combined internal pressure and bending. Based on selected 3-D finite element calculations for the circumferential through-wall cracked pipes under internal pressure using the idealized power law materials, the elastic and plastic influence functions for fully plastic J-integral and COD solutions are found as a function of the normalized crack length and the mean radius-to-thickness ratio. These developed GE/EPRI-type solutions are then re-formulated based on the enhanced reference stress method. Such re-formulation not only provides simpler equations for J-integral and COD estimations, but also can be easily extended to combined internal pressure and bending. The proposed equations are compared with elastic-plastic finite element results using actual stress-strain data, which shows overall excellent agreement.

Effect of Lüders strain on engineering crack opening displacement estimations for leak‐before‐break analysis: finite element study

As a companion paper to our previous paper [Fatigue Fract. Engng Mater. Struct. 24, 243–254], this note provides further validation on the engineering crack opening displacement (COD) estimation equation based on the enhanced reference stress method against three‐dimensional, elastic–plastic finite element (FE) results using actual tensile data of three typical ferritic steels exhibiting a wide range of Lüders strain. Furthermore, the resulting FE results are also compared with the GE/EPRI COD predictions. It is found that the proposed enhanced reference stress method gives overall more accurate and robust COD results than the GE/EPRI method, compared with the FE results.

Application of enhanced reference stress method to nuclear piping LBB analysis : finite element validation

Three-dimensional, elastic-plastic finite element analyses for circumferential through-wall cracked pipes are performed using actual tensile data of stainless steels, for two purposes. The first one is to validate the recently-proposed enhanced reference stress (ERS) method to estimate the J-integral and COD for circumferential through-wall cracked pipes. The second one is to compare those results with the GE/EPRI estimations. It is found that the J-integral and COD estimations according to the GE/EPRI method can be very sensitive to how the stress-strain data are fitted using the Ramberg-Osgood relation. Moreover, no tendency can be found regarding the most appropriate fitting range for the Ramberg-Osgood fit. On the contrary, the J-integral and COD estimations based on the ERS method give more accurate results than the GE/EPRI estimation. The present results provide confidence in applying the proposed method to the Leak-Before-Break(LBB) analysis.