Cavity Growth Model Research Articles

The effect of hydrostatic pressure on the uniaxial creep life of a 2 ¼ % Cr 1% Mo steel

The effect of a superimposed hydrostatic pressure on the ductility, the creep life and the failure mechanism of a 2 ¼ % Cr 1 % Mo steel, with an over-aged upper bainite microstructure, subject to different uniaxial stresses is described. Creep tests have been made at 923 K with uniaxial stresses in the range 55-80 MPa and superimposed hydrostatic pressures up to 35MPa. Optical and electron optical microscopy have been used to assess the accumulation of grain boundary damage arising from creep deformation. When failure is controlled by intergranular cavitation, increasing the hydrostatic pressure causes an increase in the creep ductility and a decrease in cavitation, and thus an increase in time to failure. In addition, increasing pressure effects a change in failure mode from one controlled by the nucleation and growth of intergranular cavities to one controlled by plastic flow. The results for the creep of this 2¼ % Cr 1 % Mo steel are discussed in terms of a diffusional cavity growth model which includes continuous nucleation. Moreover, these results are compared with data previously obtained for single phase materials tested with a superimposed hydrostatic pressure. The relative contributions of the principal and equivalent stresses to the creep fracture of this low alloy steel are also examined. The estimation of realistic long-term creep life from the results of short-term creep tests is also discussed.

Growth of intergranular creep cavities

The current status of theory and experiment concerning the growth of intergranular cavities during high -temperature creep is reviewed. The proposed models for cavity growth are presented and their mathematical analysis is outlined. The predictions of these models are then compared. Experimental techniques for determining cavity growth rates or testing the predictions of the growth models (density, time to fracture, and metallographic measurements) are discussed. The results of recent experiments which purport to test cavity growth models are discussed in some detail and recommendations are made for further experiments. From the available evidence it would appear that all of the following modes of cavity growth can occur: (i) simple diffusional growth controlled by grain -boundary diffusion; (ii) deformation -enhanced diffusional growth; (iii) continuum plastic hole growth; (iv) crack -like diffusional growth controlled by surface diffusion; (v) unstable finger-like diffusional growth; and (vi) constrained diffusional growth. The relative ranges of operation of these mechanisms are indicated.

Crack initiation and stable crack growth resistance in A508 steels in relation to inclusion distribution

Several heats of A508 steel containing different volume fraction of MnS inclusions and different inclusion size have been investigated. Fracture toughness tests have been performed at 100°C using CT specimens cut in different orientations in order to determine the quantities relevant to the macroscopic characterization of crack initiation and stable crack growth, i.e. COD, J 1C and dJ da . The microscopic parameters characterizing inclusion distribution were determined using quantitative metallography. Large variations in the resistance to crack initiation and stable crack growth are found. These differences are empirically related to the inclusion distribution. A modified cavity growth model is utilized to describe the variations observed at crack initiation. This model is partly based on the results obtained on axisymmetric notched specimens which were used to evaluate the critical cavity growth at rupture. The results of numerical and analytical studies published in the literature are equally used to calculate cavity growth in front of a blunted crack. Finally a statistical approach incorporating the distribution of inclusions located in the vicinity of a crack tip is proposed.

High temperature failure mechanisms in ceramics

A statistical model of high temperature failure in ceramics, by the viscous or diffusive growth of cavities, has been developed. General expressions for the creep strain associated with cavitation and for the failure time have been derived, and shown to be in good agreement with available data. Specific models of cavity growth along grain facets, by diffusive and viscous mechanisms, have then been generated, and coupled with the statistical model: to provide specific predictions of the creep strain and the failure time. The predictions are in reasonable accord with experimental results.

Microscopy Of Voiding During Creep Of Alpha‐Brass

SUMMARYMetallographic and fractographic methods have been used to study void formation during high‐temperature creep of alpha‐brass. The appearances of the creep fracture surfaces were found to differ markedly at high and low stress levels. Cavity development was therefore monitored using samples tested to various fractions of their creep lives which were then fractured in an intergranular manner by stress‐corrosion at room temperature. The different modes of void formation observed can be rationalized in terms of a model for cavity growth based on the movement of lattice dislocations in the plane of the grain boundary.

The kinetics of cavity growth and creep fracture in silver containing implanted grain boundary cavities

An experimental technique has been developed for implanting water vapor bubbles (H 2O (v) of fixed size and spacing along the grain boundaries of polycrystalline silver. The effects of this microstructure on the creep fracture behavior have been studied by conducting constant stress creep tests over the temperature range 200 to 550°C. The stress and temperature dependence of creep fracture has been determined for samples containing 1 μm diameter gas bubbles spaced 10 μm apart. The time to rupture varies with the applied stress as: t r ~ σ −3.7. The activation energy for the creep fracture is found to be 85.5 kJ/mol, which is in close agreement with the activation energy for surface self diffusion. Scanning electron microscopy of the creep fracture surfaces indicates a brittle intergranular failure process. The grain boundary facets on the fracture surface exhibit a dimpled structure. The spacing of these dimples corresponds to the initial gas bubble spacing, indicating that fracture occurs solely by the growth of the pre-existing gas bubbles. It is found that the stress, temperature and microstructural dependence of the rupture time, as well as the absolute rupture time itself, can be described accurately by a model of lenticular cavity growth and creep fracture proposed by Chuang and Rice. In this treatment, self diffusion along the cavity surface is the rate limiting step and the rupture time is predicted to vary as σ −3, in close agreement with the experimental data.

On the mechanisms of hydrogen attack

A highly sensitive dilatometer was designed for the use of studying the rate of hydrogen attack on carbon steels, measured by the swelling rate, for specimens exposed to various hydrogen pressures and temperatures. The size and distribution of cavities were determined after exposures using standard metallographic techniques. An analytical cavity-growth model which semiquantitatively describes the observed dependence of swelling rate on hydrogen pressure and temperature has been developed. Ferrovac 1020 specimens were exposed to hydrogen pressure of 900 psi at 475/sup 0/C until the substance attained a volume change (..delta..V/V) between 10/sup -3/ and 10/sup -2/. An incubation time, arbitrarily defined as the time required to attain a ..delta..V/V = 10/sup -4/ (usually 40h), and a set of swelling rates were obtained from measured versus time curves. Transverse and longitudinal sections of samples cooled in circulating helium were polished mechanically or electrolytically and examined in either unetched or etched condition by optical or scanning-electron microscopy. The measurements of the rate of hydrogen attack and the methane cavity growth model prove that expansion by creep and surface-reaction rates for methane formation are the primary processes which determine the rate of attack after incubation. A complete quantitative description requires additional study.