Masataka Yatomi

Numerical Prediction of Constraint Effect of Creep Crack Growth

This paper presents the effect of constraint on creep crack growth (CCG) using FE analysis based on the stress and strain rate state at the crack tip. The comparison is made by modelling C(T) specimen tests under plane stress and plane strain conditions using creep properties of three different steels (C-Mn steel, P91 steel, and 316H austenitic steel). In addition, in order to examine the constraint effect on CCG due to geometry single edge notch specimen (SENT), centre cracked tension specimen (CCT) and three point bending (3PB) specimen have also been analysed. In all cases it is found that when the reference stress under plane strain conditions is higher than the yield stress, there is little difference between CCG rates under plane stress and plane strain.Copyright

Difference of creep crack growth behaviour for base, heat-affected zone and welds of modified 9Cr–1Mo steel

AbstractThis paper presents creep crack growth behaviour for P91 base material, simulated HAZ material, and welded joint. Creep crack growth tests were carried out using C(T) specimens at 600°C and the same loading condition. The initial crack for P91 welds was inserted into the fine grained HAZ region and therefore Type IV cracking was observed for the P91 weld specimen. The comparison was carried out using time versus crack growth. The CCG life for fine-grained heat affected zones and welded joint is about 45% and 20% of that for base metal, respectively.In addition, FE analysis was conducted using the material parameter. The C* parameter for welded joint is obtained from a numerical line integral and creep load line displacement rate in FE analysis. Under the steady state, the C* value obtained from numerical line integral is little different to that obtained from the load line displacement rate. Therefore ASTM E 1457 provides a suitable estimation for the C* parameter for Type IV cracking such as weld...

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-N 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,

Creep Crack Growth of P92 Welds

This paper represents creep properties and creep crack growth properties for P92 welds. The CCG tests were carried out using cross-welded compact tension (C (T) specimens at several temperatures. The crack front was located at HAZ region to simulate Type IV crack. Finite element analysis was conducted to simulate multiaxiality in welded joints and compare the experimental results. The constitutive behaviour for these materials is described by a power law creep model.Copyright

Evaluation of creep crack growth properties of Gr. 92 steel weldment

High Cr ferritic heat resistant steels are used for boiler components in ultra-super critical thermal power plants. In weld components of these steels, Type-IV creep damages formed in the fine-grained heat-affected zone (HAZ), causing their creep strength to decrease at high temperatures. To assist the standardization of the testing method for creep crack growth in weld components, we conducted round robin tests using ASME Grade 92 steel as part of the VAMAS TWA31 collaboration. The CCG tests were carried out using the CT specimen and the circumferentially notched round bar specimen for both the base metal and weld joint of Gr. 92 steel. The effects of specimen configuration, temperature, load and stress triaxiality conditions on the crack initiation and growth properties were investigated.

The Equivalency-Based Linear Regression Method for Statistical Analysis of Creep/Fatigue Data

Materials scientists and mechanical engineers working on structural integrity are making increasing use of statistical analysis in interpreting creep/fatigue data as they contain an inherent scatter which cannot be substantially reduced even under controlled testing conditions. In practice, in most cases the uniaxial failure or cracking data can be reasonably approximated by a straight line on log-log coordinates, indicating that there is a linear log relationship with the appropriate correlating parameter. Linear regression is the most used method in statistical data analysis and is being recommended in American Society for Testing and Materials (ASTM), British Standards (BS), Det Norske Veritas (DNV) and many other engineering standards. Recently, the current practice on linear regression as adopted by the engineering standards has been critically reviewed, and the shortcomings of these procedures have been clearly demonstrated. A new statistical method based on the equivalency between all variables involved has been proposed for S-N curve analysis. In this paper, a large amount of creep and fatigue data of engineering materials collected from several well known data bases generated in US, Europe, and Japan are systematically analyzed with conventional standard and the new equivalency method. The results are compared and discussed. Finally, a recommendation to improve the fitting parameters taking into account of the scatter in both axes is presented.Copyright

An Approach for Cost Effective Assessment in Risk-Based Maintenance as a Life-Cycle Maintenance (LCM) Model

For aged power plants in Japan, the life extension with retaining the safety and cost-effective beyond the original design lifetime is proposed. Therefore it is important to minimise the risk and maintenance cost to keep operating the plants. Life-Cycle Maintenance (LCM) is proposed for optimising maintenance plan with reliability in the life of the plants. Risk Based Maintenance (RBM) is included in the LCM to assess the risk of components in the plants. LCC and the investment assessment may be also conducted to decide the most cost effective maintenance strategy, if several maintenance strategies are proposed in RBM. In this paper, concept and an application of the LCM are described to optimise maintenance plan in the lifetime of a plant. It was found that the LCM is quite useful method to plan the most cost effective maintenance strategies in the lifetime of the plant.Copyright

Evaluation of damage and fracture of high Cr steel welds at elevated temperatures

High Cr ferritic steels have been used in the 600°C class ultra-super critical (USC) thermal power plants. More than a decade has passed from the application of high Cr steels to USC power plants in Japan, and type-IV creep damages in the welded components become concerns. In the present paper, long-term creep tests for the welded joints of the high Cr steels (Gr.91 and Gr.122 steels) were conducted. Microstructures and creep damages in the heat affected zone (HAZ) were investigated using the thick plate specimens. Remaining life assessment methods of high Cr steel welds were discussed based on the experimental results. Further, to assist the standardization of the testing method for creep crack growth in welded joints, we have conducted the round robin tests using the high Cr steels (Gr.91 and Gr.92 steels) and their welds as part of the VAMAS TWA31 collaboration.

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

W added high Cr ferritic heat-resistant steels have been developed as a boiler material. Most of boiler component structures are mainly fabricated by welding which are likely to the regions of crack initiation and propagation. However, the method of predicting the life of creep crack initiation and growth have not been clearly established for weldments of high Cr ferritic heat resistant material due to many factors such as the variation in micro-structures and the residual stress caused by welding and thermal cycles. In the present study, the experiments of creep crack growth using a circular notched round bar specimen with variation of notch location in HAZ were conducted and the characteristics of creep crack growth rate and creep crack initiation life were summarized in terms of Q ∗ parameter, which has been proposed as fracture mechanics parameter to describe creep crack growth rate.

Reference Stress Based Assessment for Creep-Fatigue Life Under Multiaxial Condition

The reference stress method has been developed to assess the service life for many components under creep condition. However, in order to use the reference stress as a representative stress to assess the creep fatigue life, some modifications are needed. The σd approach based on reference stress, used in RCC-MR, is one of the most common method to assess creep fatigue crack incubation. The aim of this paper is to develop a novel model based on reference stress and the σd approach to estimate the life subjected to creep-fatigue interaction under multiaxial condition for defect free structures. Three kinds of circumferentially notched bar specimens were used in order to examine the creep-fatigue interaction under multiaxial conditions. Finite element analyses were conducted to develop a simplified life assessment method and to calculate the creep fatigue damage in the test specimens. The predicted creep fatigue lives are in good agreement with the experimental results using notched bar specimens of 316H stainless steel.Copyright