K. Nikbin

Prediction of Creep Crack Growth from Uniaxial Creep Data

In this paper uniaxial tensile creep data are used in conjunction with fracture mechanics concepts to predict creep crack growth rates in materials having a wide range of creep ductilities. A model is proposed of creep damage accumulation in a process zone ahead of the crack tip. The model allows all stages of creep to be incorporated in an approximate manner and creep ductility to be stress and stress-state sensitive. Good agreement is obtained with experimental crack growth data on a range of low alloy steels, a stainless steel, an aluminium alloy and a nickel-base superalloy. It is found that cracking rate is insensitive to the creep process zone size but inversely proportional to creep ductility. Crack growth rates under plane strain conditions are shown to be about fifty times those for plane stress loading.

Modes of failure under creep/fatigue loading of a nickel-based superalloy

Crack growth experiments have been carried out under combined creep and fatigue loading at 700° C on a hot isostatically pressed powder nickel alloy. A fractographic investigation has been undertaken of the modes of failure over a frequency range of 0.001 to 10 Hz. The observations indicate that under static loading and at low frequencies failure is intergranular and controlled by creep processes, whereas at high frequencies a transgranular fatigue fracture is obtained. The transition from creep to fatigue behaviour is found to be progressive, and to begin at a lower frequency the higher the ratio of cyclic to mean load. In the transition region a mixed intergranular and transgranular fracture surface is observed, which correlates well with the recorded proportion of creep to fatigue crack growth.

Influence of cyclic to mean load ratio on creep/fatigue crack growth

Crack growth data under combined creep and fatigue loading conditions are presented on a nickel base superalloy and a brittle and ductile low alloy steel. The main variables that have been examined are minimum to maximum load ratioR and frequency. It is shown that at high frequencies transgranular fatigue failure dominates and at low frequencies time dependent mechanisms govern. Where fatigue processes control, it is demonstrated that crack growth/cycle can be described by the Paris law and that the influence ofR ratio can be accounted for by crack closure caused by fracture surface roughness, oxidation, and creep and plastic strain developed at the crack tip. At the low frequencies where time dependent processes dominate, it is shown that crack growth can be characterized satisfactorily in terms of the creep fracture mechanics parameterC * using a model of crack extension based on ductility exhaustion in a creep damage zone at the crack tip. This model leads to enhanced resistance to creep/fatigue crack growth with increase in material creep ductility.

Influence of thermal boundary conditions on stress-strain distribution generated in blade-shaped samples

Abstract Cyclic thermal fatigue experiments have been performed on a single-edge wedge specimen of the polycrystalline nickel-based superalloy INI00 to simulate the conditions experienced by a turbine blade during operation. Induction heating and forced-air cooling of the wedge tip was used. The temperature at the leading edge was cycled between 200 and 1100 °C and the surface temperature distribution was measured by thermocouples throughout the thermal cycle. Thermal and stress analysis calculations have been carried out to determine the time-temperature-stress-strain histories generated in a specimen. It has been found that the magnitudes of the stresses and strains produced are sensitive to the precise surface temperature assumed. In particular, plastic deformation will generally be incurred during the heating and/or cooling phases of a cycle. This can have important consequences in making lifetime predictions from isothermal data. Failure is always predicted to initiate at the leading edge, in agreement with the experimental observations.

Influence of net section damage on creep crack growth

Abstract Fracture mechanics concepts for describing creep crack growth in terms of ductility exhaustion in a process zone at the crack tip are reviewed and extended to include damage accumulation in the ligament ahead of a crack. Applications are considered which show that net section damage has most influence for short cracks and plane stress conditions where significant damage can develop in the uncracked ligament. It is shown that, under plane strain loading, insufficient ligament damage occurs during the crack growth phase for it to have an appreciable effect on failure times. A method is also presented for accounting for the influence of an incubation period prior to the onset of cracking and for making residual life estimates.

Transition Effects in Creep-Brittle Materials

The definition for a creep-brittle material is considered in the light of material and geometric constraints that are imposed at the crack tip. It is assumed that creep-brittle fracture is achieved in plane strain and the damage at the crack tip is localized regardless of the creep ductility of the material. Data from four representative engineering alloys are considered. The state of stress local to the singularity is described in terms of the elastic and creep stresses present at initial loading. The rate of transition from the elastic field to the steady state creep field is dictated by the rate of stress redistribution that occurs at the crack tip but this transition time does not in itself explain the initial cracking rate. It has been found that the transient or stage one of crack growth in geometries, behaving in a creep-brittle manner, can be described in terms of a model based on C* incorporating the creep uniaxial ductility. The model assumes a process zone in which the transition time to accumulate sufficient crack tip creep strains dictate the initial cracking rate.

HIDA activity on P91 steel

The 9%-12% Cr-steels are strategic materials for new power plant and for component substitution for plant life extension. One of these steels, P 91 was included in the project BE-1702 (HIDA) to provide crack initiation and growth data for the improvement and validation of procedures for high-temperature defect assessment. The paper presents an outline of the testing programme and the initial results for P 91. In addition to uniaxial and static/cyclic creep crack growth tests on standard fracture mechanics geometries, feature tests are also included in the experimental programme. These consist of internally pressurised pipe welds, pipe bends and 4-point bend pipes, and C-shaped specimens. The majority of these tests are still ongoing. The static and cyclic loading conditions are being employed to consider the range of creep/fatigue interaction in this alloy. All tests are being conducted at 625°C.

SCATTER BANDS IN CREEP AND FATIGUE CRACK GROWTH RATES IN HIGH TEMPERATURE PLANT MATERIALS DATA

As a part of the European Commission supported project BE 1702: ‘HIDA’ Creep, creep-fatigue and high temperature fatigue crack growth data for five high temperature plant steels were accessed from a number of published and unpublished sources. These large sets of data were reviewed, and re-analysed where necessary, and plotted in terms of various crack growth rate correlating parameters. Thus limits of scatter bands and mean and upper 95% confidence limit creep and fatigue crack growth correlations are proposed. The present work covers a wide range of variables such as test specimen geometries, sizes, loading conditions and temperatures. Therefore, the correlations proposed are considered universal. However, it is envisaged that these correlations will be refined in future by enlarging the database and exploring the effect of the variables described above. The five materials studied are AISI 316 stainless, 2.25CrlMo, P91, 1CrMoV (forged), and 1CrMoV (cast) steel.

Fatigue assessment of the conductor jacket for the Next European Torus toroidal field and poloidal field coils

Abstract The Next European Torus (NET) superconducting toroidal field (TF) coils and the poloidal field (PF) coils, which are used to confine plasma, operate at cryogenic temperatures utilizing liquid helium as coolant. The jacket which is used to surround the coils contains the helium and provides structural strength. Candidate materials for the jackets are 316 LN stainless steel and a nickel-base alloy IN 908. It is essential that cracks introduced during fabrication by welding do not propagate by fatigue and cause a leak during the coils operational lifetime. Two defect assessment procedures BS PD6493 and R6 are applied to determine tolerable crack sizes for two extreme cases of thumbnail and circumferential flaws. It is shown, for the present materials, that leak before break can be demonstrated for some thumbnail flaws only. It is also demonstrated that the NET design limit of 2 × 105 cycles to failure can be achieved with an adequate margin of safety for thumbnail defects with current non-destructive inspection capabilities.

Prediction of Crack Growth Under Creep-Fatigue Loading Conditions

Crack growth data are presented on a brittle and a ductile low alloy steel and a nickel base superalloy which have been subjected to combined static and cyclic loading at elevated temperatures. the cyclic tests have been conducted between fixed load and fixed displacement ranges. Distinct regions of fatigue and creep dominated cracking have been identified. It is shown that crack growth eventually ceases because of load relaxation in the constant displacement amplitude tests. Fracture of a component made of the ductile steel is unlikely to occur under this type of low-cycle fatigue loading, but significant cracking is possible with the brittle steel. Where cyclic crack growth is creep controlled, it is shown that crack propagation rates can be predicted from static creep data using the creep fracture mechanics parameter C*.