Akio Fuji

Estimation of creep crack growth rate in IN-100 based on the Q* parameter concept

Since the high-strength Ni-based superalloy, cast IN-100, is considered to be brittle at high temperatures, the stable creep crack growth region is limited. Therefore, technically, it is very difficult to perform creep tests and there are few experimental results on the creep crack growth behaviour of this material. We performed creep crack growth tests using Ni-based superalloy, cast IN-100, and derived the Q* parameter for this material, which characterizes the creep crack growth rate. Using this Q* parameter, we derived a law for the creep rupture life of this material.

Results of a Japanese round robin on creep crack growth evaluation methods for Ni-base superalloys

Abstract Creep crack growth (CCG) tests on Ni-base superalloys were carried out in the Japanese VAMAS group as part of a round-robin program in order to assist the standardization of the CCG test method for creep-brittle alloys. The effect of temperature, load, specimen thickness and material microstructure on CCG behavior was investigated. The applicable range of fracture mechanical parameters to evaluate the CCG rate was evaluated. The CCG rate was characterized by the C∗ parameter independent of testing conditions in the range where the CCG rate accelerated. However, the acceleration stage occupied only a small portion of life time for the creep-brittle superalloys. In the range where the CCG rate was constant, the CCG rate and the fracture life could be predicted approximately by the Q∗ method based on the thermally activated process.

Evaluation of creep crack growth properties using circular notched specimens

It is important to evaluate the effect of multiaxial stress conditions on initiation and growth of creep cracks, when the laboratory data are subsequently applied to structural components under the same or similar stress state. The round robin tests of creep crack growth using circular notched specimens of 1CrMoV steel at 538 and 594 °C and 12CrWCoB steel at 650 °C were conducted by the Japanese VAMAS TWA25 group. The effect of notch depth and specimen size, i.e. stress multiaxiality on crack growth properties was investigated. The test procedure including criteria for crack length measurement by electric potential drop was established. The circular notched specimens fractured intergranularly and showed different crack growth behaviour from that of a CT specimen due to the multiaxial stress field. The creep crack growth rate for the same C* value increased as the ratio of the notch depth to specimen diameter, i.e. stress multiaxiality increased. The Q* evaluation method based on the thermally activated process can also be applied to the circular notched specimen.

Results of an intercomparison of creep crack growth tests made in Japan

This paper concerns the results and analysis of Japanese round-robin tests on the VAMAS TWA 11 Creep Crack Growth Project. A high-temperature ductile material, CrMoV steel, was used. For high strain rates, creep crack growth rate is correlated by the parameter C* within less than one order of magnitude of scatter in the temperature range between 538 and 594°C and in the load range tested. However, at lower strain rates the scatter becomes larger (i.e. greater than one order of magnitude). In the early stages of crack growth in particular, it was found that a dual value of da/df appears with respect to the same value of C*. This peculiarity has been explained by the relation between the characteristics of creep crack length and creep strain with respect to time. By comparison, if the parameter Q* is used such a peculiar pattern does not appear at an early stage.

The proposal of Q* parameter and derivation of the law of creep crack growth life for a round bar specimen with a circular notch for Cr–Mo–V steel

Abstract Using a round bar circular notched specimen for Cr–Mo–V steel and the proposed equation predicting crack length on the basis of electric potential drop method, the creep crack growth tests were previously conducted to investigate the effect of multi-axial stress on creep crack growth rate(CCGR). In this paper, more detailed experiments and analyses on the creep crack growth were conducted and the Q* parameter which characterizes CCGR for this circular notched specimen was derived. Furthermore, using the Q* parameter, the prediction law of creep crack growth life was also derived.

Characterization of Structural Embrittlement of Creep Crack Growth for W-Added 12%Cr Ferritic Heat-Resistant Steel Related to the Multiaxial Stress

In components under static creep loading condition, the multiaxial stress fields appear due to the plastic constraint and they produce a more brittle type cracking behavior. From a practical standpoint, the characterizations of creep crack growth rates under the multiaxial stress field are important to improve the methods for creep life extension. In this paper, creep crack growth tests were conducted using round bar specimens with sharp circular notches for tungsten-added 12%Cr ferritic heat-resistant steel (W12%Cr steel), and the effect of multiaxiality on creep ductility and creep crack growth rate were investigated. Furthermore, three-dimensional elastic-plastic creep finite element analyses were conducted to clarify the effect of multiaxiality on creep crack growth.

Influence of notch shape and geometry during creep crack growth testing of TiAl intermetallic compounds

Abstract In order to establish the optimum pre-crack shape and geometry for creep crack growth (CCG) testing of high-temperature brittle materials, several shapes of starter notches were evaluated experimentally on TiAl intermetallic compound. Fatigue pre-cracks are frequently inappropriate in these materials because they tend to promote out-of-plane crack growth in the lamellar structured TiAl materials. Based on the experimental results with several notch shapes and also an analytic assessment of stress distribution around notch-tip, a V-notched specimen is proposed as being optimum for generating creep crack growth data in lamellar structured TiAl. A procedure for machining the notches to minimize its effect on the CCG behavior is also outlined.

A case study and damage assessment of some heavy section components operating for more than 100 000 h in fossil-fired power plants

Abstract A main steam pipe T-piece, a main steam stop valve and a reheater steam piping Y-piece have been taken out of three different power boiler plants that have seen each an accumulated service life of over 100 000 h, involving more than 500 start-and-stop operations, and their states of degradation examined. The degradation was assessed in terms of the changes found in the mechanical properties and the metallographic structure, the flaws actually detected in them, and analyses of the distributions of temperature and stress for start-and-stop operations as well as steady state were calculated on the basis of the service histories. Detailed stress analysis afforded information to indicate the portions that sustained large damage, and on which damage assessment calculations were conducted. The remaining service lifetime calculation was conducted on the basis of analyses of propagation of the defects and showed that the component had sufficient allowances for continued service under severe operating conditions.

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