摘要:

The VAMAS (Versailles Agreement on Advanced Materials and Standards) Project, in providing a framework for international collaboration in pre-normative research, has enabled a unified approach to the measurement and interpretation of creep crack growth data. The concepts involved in the VAMAS initiative and the procedures adopted in defining agreed methods for determining creep crack growth rates are briefly described in this paper, which introduces a special issue of this journal devoted to creep crack growth.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689402

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

Fracture mechanics analyses are central in identifying the macroscopic, load and geometry-dependent fracture parameter that correlates cracking behaviour in a given material under given conditions. This paper focuses on the problem of correlating crack growth behaviour under creep conditions. A wide range of asymptotic analyses have been developed which, in many cases, were derived from close analogies with problems in elastic-plastic fracture. Nevertheless, these analogies generally involve the mathematical structure and do not directly lead to analogies involving the appropriate fracture parameter, particularly in the case of crack growth under small-scale creep. Important aspects of the latter problem are discussed in this paper.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689403

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

Several methods are available for estimating the creep fracture mechanics parameterC* used for describing creep crack growth in components and test specimens. A procedure is presented for determiningC*, from experimental data, which makes use of limit analysis concepts and requires little knowledge of the creep strain behaviour of materials. Transient effects, during the early stages of cracking due to stress redistribution and a gradual build-up of creep damage at the crack tip, are discussed. It is shown that improved correlations of creep crack growth data are achieved when these effects are taken into account.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689404

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

The results of a round-robin programme to evaluate crack-tip parameters such as K, C* and Ctfor characterizing creep crack growth behaviour are presented. The programme was sponsored by the E24 Committee on Fracture Mechanics Test Methods of the American Society for Testing and Materials (ASTM). The tests were made on a 1Cr-1Mo-0.25V turbine rotor steel (ASTM grade A470 class 8) and included specimens of two thicknesses from the compact type (CT) and the centre crack tension (CCT) geometries. The test temperatures were 538°C and 593°C (1000°F and 1100°F). All tests were found to be in the extensive creep regime where the C*-integral and the Ctparameter are identical. It was shown that all creep crack growth data from CT specimens correlated uniquely with the C*-integral or the Ctparameter and no unique correlation was found between creep crack growth rate and K. The creep crack growth rates in 6.3 mm (0.25 in) plane-sided CT specimens were about two to three times slower than those in 25.4 mm (1 in) thick specimens with 25% sidegrooves. These differences are most likely to be due to the differences in the state of stress at the crack tip, which can vary with thickness. Differences were found between the creep crack growth rates from the CT and CCT geometries at equal values of C* or Ct. These differences were attributed to the differences in the thicknesses of the specimens from the two geometries and also due to possible loss of dominance of the singular crack tip stress fields in the CCT specimens. The creep crack growth data obtained under this programme compared well with that from a Japanese programme that used a similar material. Several issues with regard to test-method standardization are also discussed in this paper.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689405

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

The creep crack growth behaviour of two high-temperature materials has been determined in 25 laboratories within the European Group on Fracture (EGF). This paper presents a summary of these round-robin tests on the‘measurement of crack growth under high temperature conditions'. It is shown that the most satisfactory correlations of creep crack growth rates are achieved with the creep fracture mechanics parameter C*. Little influence of specimen shape and size is observed and it is demonstrated that the specimen deformation and cracking behaviour occurs under plane stress conditions.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689406

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

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 parameterC*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/dfappears 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.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689407

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

The parameterQ* has been proposed for correlating or characterizing creep crack growth rate. The creep crack growth rate has been expressed in terms of an activation process equation, and the parameterQ* was defined as the exponent of the exponential in the thermal activation process equation for da/dt, in which the activation energy is expressed in terms of free energy. CrMoV steel was used for VAMAS round-robin tests, and other materials such as SUS 304 and the Nl-base alloy of Hastelloy-XR were used. For these materials good correlations are obtained. An analytical conclusion about the effect of thickness on crack growth is described.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689408

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

A number of methods for calculating the steady-state creep parameterC*are described. Emphasis is placed on simplified reference stress methods that enable application to complex engineering components and that allow the treatment of real material behaviour. It is shown that the reference stress method can be extended to describe the transient creep parameterC(t) for a variety of loading conditions. The application ofC*to component creep crack growth behaviour is discussed and illustrated by reference to plant applications within the electricity supply industry in the UK.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689409

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

Concepts relating to the initiation and early growth of creep cracks from pre-existing defects are reviewed and illustrated with original data for 1CrMoV steels. Creep crack initiation (CCI) occurs on the attainment of a critical local crack-tip strain, the magnitude of this strain depending upon the creep ductility of the material and the crack initiation criterion selected. A weakness with the existing CCI database is that the information has not been accumulated in a consistent manner and the adoption of a unified methodology is encouraged. An understanding of the process of CCI and early growth has been developed by specialists from Europe, the USA’and Japan to a level where high-temperature defect assessment can now be performed with some confidence. However, there is still a need for more effort to improve the extent of knowledge and the effectiveness of the methods employed to collect and apply experimental data to engineering structures.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689410

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

A creep-brittle titanium alloy Ti-6242, which is being used for aeroengine applications, was chosen to study the effects of loading conditions and microstructure on creep crack growth (CCG) behaviour at 500°C. Tests were made on compact tension (CT) specimens under both displacement rate control and under constant load. Crack growth data were correlated withC* (t), Ctand theJ−integral. The validity range of correlation of the various crack-tip parameters for a growing crack in creep-brittle materials is discussed.

ISSN:0960-3409    

DOI:10.1080/09603409.1992.11689411

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor