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B F Dyson - One of the best experts on this subject based on the ideXlab platform.

  • breakdown of the Skeletal stress technique for lifetime prediction of notched tension bars due to creep crack growth
    Engineering Fracture Mechanics, 1994
    Co-Authors: D R Hayhurst, B F Dyson, Jianguo Lin
    Abstract:

    Abstract The paper defines the range of applicability of the Skeletal stress technique as a means of predicting the creep lifetime of a circumferentially notched tension bar subjected to steady load. The Skeletal stress approach involves the quantification of the effective stress Σ c and its ratio with the maximum principal tension stress Σ l at a Point, known as the Skeletal Point, at the throat of the notch. The stress Σ c and stress state Σ l /Σ e at the Skeletal Point are assumed to remain constant and to determine the notched bar lifetime, which is determined by direct integration of the constitutive equations. This paper assesses the range of validity of this technique using Continuum Damage Mechanics (CDM) calculations, based on finite element analysis techniques, to predict the exact notch behaviour for a range of material conditions appropriate to nickel superalloys. It has been shown that by reducing the ductility parameter in the constitutive equations associated with the creep constrained cavitation damage variable ω 2 , the CDM computations are capable of predicting two extreme types of behaviour. Firstly, widespread CDM behaviour, and secondly CDM growth on a highly localised-planar scale which models the phenomenon of Creep Crack Growth (CCG). The reference stress method is shown to be appropriate for widespread CDM behaviour, and to progressively become valid as the material ductility is reduced. This trend is accompanied by a shift from modest notch weakening to severe notch weakening. It is also shown that the stress level applied to the bar, and the strength of the dislocation softening damage mechanism, denoted by a second damage variable ω 1 , also influences the above behaviour. The paper defines bounds of applicability for the Skeletal stress approach to lifetime prediction, and recommends the use of complete CDM finite clement analysis for those situations where breakdown occurs.

  • continuum damage mechanics modelling of circumferentially notched tension bars undergoing tertiary creep with physically based constitutive equations
    Acta Metallurgica Et Materialia, 1994
    Co-Authors: A M Othman, B F Dyson, D R Hayhurst
    Abstract:

    Abstract The paper reports the result of finite element computations, based on Continuum Damage Mechanics (CDM), carried out on circumferentially notched tension bars undergoing tertiary creep and failure. The material constitutive equations are physically based and generalised from thos applicable to polycrystalline nickel-based superalloys. The equations describe the stress level dependence of creep rate using a sinh function and two damage state parameters to model tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. The paper presents values of the computed lifetimes, normalised with respect to plain bar lifetimes at the same average applied stress across the notch throat, and determines their sensitivity to a wide range of material parameters. The parameters include those which determine: the relative strength of the two damage rate mechanisms; the power v, to which the stress-state sensitivity parameter (Σ1/Σe) is raised for damage evolution due to grain boundary cavitation; and, their dependence upon the applied stress levels. The effects of the material parameters on the predicted transition from notch strengthening to notch weakening are rationalised analytically using a stress state modified Skeletal Point stress methodology. The predictive capability of this methodology was found to be extremely good in most circumstances but broke down seriously when the uniaxial ductility was reduced to approximately 1%.

  • Skeletal Point stresses in circumferentially notched tension bars undergoing tertiary creep modelled with physically based constitutive equations
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 1993
    Co-Authors: A M Othman, D R Hayhurst, B F Dyson
    Abstract:

    Constitutive equations are proposed in which the stress level dependence of creep rate is described by a sinh function, and two damage state-parameters are used to model the tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. These constitutive equations are applicable to polycrystalline nickel-base superalloys and are used together with a continuum damage mechanics finite element based solver, DAMAGE XX, to study the behaviour of axisymmetrically notched tension bars and simulate the complex stress states that may be encountered at geometrical stress-raisers in high temperature components. Numerical studies of such bars show that their behaviour can be accurately represented in terms of a ‘Skeletal effective stress’ located at a Point within the notch throat, and the stress state at this Point. It is shown that this conclusion is valid not only for those materials that fail by grain boundary cavitation alone, but also for materials such as superalloys where grain boundary cavitation is accompanied by mobile dislocation multiplication.

K Laha - One of the best experts on this subject based on the ideXlab platform.

  • creep life prediction of 9cr 1mo steel under multiaxial state of stress
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014
    Co-Authors: Sunil Goyal, K Laha
    Abstract:

    Abstract Creep rupture life of 9Cr–1Mo steel under multiaxial state of stress has been assessed. Stress multiaxiality in cylindrical specimens during creep tests has been introduced by incorporating circumferential U-notches of different notch root radii. Creep tests were carried out on both smooth and notched specimens of the steel at 873 K over the net applied stresses in the range of 110–210 MPa. The creep rupture life of the steel was found to be higher in the presence of notch than that of smooth specimen indicating ‘notch strengthening’ behavior under multiaxial state of stress. The extent of strengthening tends to saturate for relatively sharper notches. Finite element analysis of stress and strain distributions across the notch was carried out to assess the notch strengthening behavior observed in the steel. The reduction in the von-Mises stress, extent of which increased and tends towards saturation with increase in notch root radii, resulted in increase in rupture life under multiaxial state of stress. Estimation of the creep rupture life under multiaxial state of stress has been assessed based on the different models, invoking the concept of Skeletal Point for calculating the representative stress. It has been observed that the creep rupture behavior of the steel under multiaxial state of stress is predominantly governed by the von-Mises stress. The creep rupture life has been predicted using finite element analysis coupled with continuum damage mechanics.

  • creep life prediction of modified 9cr 1mo steel under multiaxial state of stress
    Procedia Engineering, 2014
    Co-Authors: Sunil Goyal, K Laha, M.d. Mathew
    Abstract:

    Abstract Creep life prediction of modified 9Cr-1Mo steel under multiaxial state of stress has been carried out in the present investigation. Creep tests were carried out on smooth and notched specimens of modified 9Cr-1Mo steel in the stress range 150 - 230 MPa at 873 K. The creep rupture life was found to be higher in the presence of notch than that of smooth specimens indicating ‘notch strengthening’ behaviour. The extent of strengthening tends to saturate for relatively sharper notches. Finite element analysis was used to estimate the variation of stresses across the notch throat plane. As the stresses varied across the notch throat plane, the stresses at the Skeletal Point were used for estimating the representative stress. The models proposed by Cane, Hayhurst and Nix were examined for creep life prediction under multiaxial state of stress. The von-Mises stress was found to predominantly govern the creep rupture life of the steel. The creep rupture life has been predicted based on Cane's model using finite element analysis and continuum damage mechanics.

  • finite element analysis of effect of triaxial state of stress on creep cavitation and rupture behaviour of 2 25cr 1mo steel
    International Journal of Mechanical Sciences, 2013
    Co-Authors: Sunil Goyal, K Laha, S Panneerselvi, M.d. Mathew
    Abstract:

    Abstract Effect of triaxial state of stress on creep rupture behaviour of 2.25Cr–1Mo steel has been investigated by performing creep tests on plain and circumferential U-notch specimens of different notch acuity ratios ranging from 1 to 20. The creep rupture life of the material was found to increase in the presence of notch and the strengthening effect tends to saturate at higher notch acuity ratio. Fractographic observations revealed the plasticity induced intragranular ductile failure with dimple appearance for relatively shallow notches (low notch acuity ratio) and creep cavitation induced intergranular brittle failure for relatively sharp notches (high notch acuity ratio). The creep damage initiated at the centre of notch plane for shallow notches and at the notch root for sharper notches. In order to understand the effect of notch sharpness on stress distribution and damage across the notch plane, finite element (FE) analysis was carried out. The peak value of maximum principal and hydrostatic stresses was found to increase and shift from the centre of the notch plane to close to notch root with the increase in notch acuity ratio. On the other hand, the von-Mises stress decreased with increase in notch acuity ratio but remained nearly uniform across the notch plane for shallow notches and high at the notch root for sharp notches. The difference in creep fracture mode of the notched specimen with notch acuity ratio has been explained based on the FE analysis. The Skeletal Point stresses obtained from FE analysis were used for predicting the relative contribution of each stresses on governing the rupture life of notched specimens.

  • effect of multiaxial state of stress on creep rupture behaviour of 2 25cr 1mo steel
    Procedia Engineering, 2013
    Co-Authors: Sunil Goyal, K Laha, V D Vijayanand, Panneer S Selvi, M.d. Mathew
    Abstract:

    Abstract Creep tests on both plain and notched specimens having ratio of notch throat radius to notch root radius ranging from 1 to 20 keeping the specimen diameter to notch throat diameter ratio fixed at 1.67. The creep rupture life of the material was found to increase in presence of notch at all the applied stresses. The extent of increase in rupture life was more at higher applied stresses. Creep rupture life was found to increase with notch acuity and tend to saturate at larger notch acuity. SEM fractography investigation revealed the presence of both plasticity induced dimpled intragranular ductile failure and creep cavitation induced intergranular brittle failure. The relative proportion of plasticity induced ductile failure to creep cavitation induced brittle failure decreased with increase in notch acuity and decrease in applied stress. Finite element analysis was carried out to study the effect of notch acuity on the stress distribution across the notch during creep exposure and its effect on creep rupture life. The variation of stresses at the Skeletal Point with notch acuity was used to characterize the state of stress in the notched specimen. The reduction in von-Mises stress with notch acuity at the Skeletal Point has been considered for the increase in rupture life of the relatively ductile 2.25Cr-1Mo steel.

Sunil Goyal - One of the best experts on this subject based on the ideXlab platform.

  • creep life prediction of 9cr 1mo steel under multiaxial state of stress
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014
    Co-Authors: Sunil Goyal, K Laha
    Abstract:

    Abstract Creep rupture life of 9Cr–1Mo steel under multiaxial state of stress has been assessed. Stress multiaxiality in cylindrical specimens during creep tests has been introduced by incorporating circumferential U-notches of different notch root radii. Creep tests were carried out on both smooth and notched specimens of the steel at 873 K over the net applied stresses in the range of 110–210 MPa. The creep rupture life of the steel was found to be higher in the presence of notch than that of smooth specimen indicating ‘notch strengthening’ behavior under multiaxial state of stress. The extent of strengthening tends to saturate for relatively sharper notches. Finite element analysis of stress and strain distributions across the notch was carried out to assess the notch strengthening behavior observed in the steel. The reduction in the von-Mises stress, extent of which increased and tends towards saturation with increase in notch root radii, resulted in increase in rupture life under multiaxial state of stress. Estimation of the creep rupture life under multiaxial state of stress has been assessed based on the different models, invoking the concept of Skeletal Point for calculating the representative stress. It has been observed that the creep rupture behavior of the steel under multiaxial state of stress is predominantly governed by the von-Mises stress. The creep rupture life has been predicted using finite element analysis coupled with continuum damage mechanics.

  • creep life prediction of modified 9cr 1mo steel under multiaxial state of stress
    Procedia Engineering, 2014
    Co-Authors: Sunil Goyal, K Laha, M.d. Mathew
    Abstract:

    Abstract Creep life prediction of modified 9Cr-1Mo steel under multiaxial state of stress has been carried out in the present investigation. Creep tests were carried out on smooth and notched specimens of modified 9Cr-1Mo steel in the stress range 150 - 230 MPa at 873 K. The creep rupture life was found to be higher in the presence of notch than that of smooth specimens indicating ‘notch strengthening’ behaviour. The extent of strengthening tends to saturate for relatively sharper notches. Finite element analysis was used to estimate the variation of stresses across the notch throat plane. As the stresses varied across the notch throat plane, the stresses at the Skeletal Point were used for estimating the representative stress. The models proposed by Cane, Hayhurst and Nix were examined for creep life prediction under multiaxial state of stress. The von-Mises stress was found to predominantly govern the creep rupture life of the steel. The creep rupture life has been predicted based on Cane's model using finite element analysis and continuum damage mechanics.

  • finite element analysis of effect of triaxial state of stress on creep cavitation and rupture behaviour of 2 25cr 1mo steel
    International Journal of Mechanical Sciences, 2013
    Co-Authors: Sunil Goyal, K Laha, S Panneerselvi, M.d. Mathew
    Abstract:

    Abstract Effect of triaxial state of stress on creep rupture behaviour of 2.25Cr–1Mo steel has been investigated by performing creep tests on plain and circumferential U-notch specimens of different notch acuity ratios ranging from 1 to 20. The creep rupture life of the material was found to increase in the presence of notch and the strengthening effect tends to saturate at higher notch acuity ratio. Fractographic observations revealed the plasticity induced intragranular ductile failure with dimple appearance for relatively shallow notches (low notch acuity ratio) and creep cavitation induced intergranular brittle failure for relatively sharp notches (high notch acuity ratio). The creep damage initiated at the centre of notch plane for shallow notches and at the notch root for sharper notches. In order to understand the effect of notch sharpness on stress distribution and damage across the notch plane, finite element (FE) analysis was carried out. The peak value of maximum principal and hydrostatic stresses was found to increase and shift from the centre of the notch plane to close to notch root with the increase in notch acuity ratio. On the other hand, the von-Mises stress decreased with increase in notch acuity ratio but remained nearly uniform across the notch plane for shallow notches and high at the notch root for sharp notches. The difference in creep fracture mode of the notched specimen with notch acuity ratio has been explained based on the FE analysis. The Skeletal Point stresses obtained from FE analysis were used for predicting the relative contribution of each stresses on governing the rupture life of notched specimens.

  • effect of multiaxial state of stress on creep rupture behaviour of 2 25cr 1mo steel
    Procedia Engineering, 2013
    Co-Authors: Sunil Goyal, K Laha, V D Vijayanand, Panneer S Selvi, M.d. Mathew
    Abstract:

    Abstract Creep tests on both plain and notched specimens having ratio of notch throat radius to notch root radius ranging from 1 to 20 keeping the specimen diameter to notch throat diameter ratio fixed at 1.67. The creep rupture life of the material was found to increase in presence of notch at all the applied stresses. The extent of increase in rupture life was more at higher applied stresses. Creep rupture life was found to increase with notch acuity and tend to saturate at larger notch acuity. SEM fractography investigation revealed the presence of both plasticity induced dimpled intragranular ductile failure and creep cavitation induced intergranular brittle failure. The relative proportion of plasticity induced ductile failure to creep cavitation induced brittle failure decreased with increase in notch acuity and decrease in applied stress. Finite element analysis was carried out to study the effect of notch acuity on the stress distribution across the notch during creep exposure and its effect on creep rupture life. The variation of stresses at the Skeletal Point with notch acuity was used to characterize the state of stress in the notched specimen. The reduction in von-Mises stress with notch acuity at the Skeletal Point has been considered for the increase in rupture life of the relatively ductile 2.25Cr-1Mo steel.

D R Hayhurst - One of the best experts on this subject based on the ideXlab platform.

  • breakdown of the Skeletal stress technique for lifetime prediction of notched tension bars due to creep crack growth
    Engineering Fracture Mechanics, 1994
    Co-Authors: D R Hayhurst, B F Dyson, Jianguo Lin
    Abstract:

    Abstract The paper defines the range of applicability of the Skeletal stress technique as a means of predicting the creep lifetime of a circumferentially notched tension bar subjected to steady load. The Skeletal stress approach involves the quantification of the effective stress Σ c and its ratio with the maximum principal tension stress Σ l at a Point, known as the Skeletal Point, at the throat of the notch. The stress Σ c and stress state Σ l /Σ e at the Skeletal Point are assumed to remain constant and to determine the notched bar lifetime, which is determined by direct integration of the constitutive equations. This paper assesses the range of validity of this technique using Continuum Damage Mechanics (CDM) calculations, based on finite element analysis techniques, to predict the exact notch behaviour for a range of material conditions appropriate to nickel superalloys. It has been shown that by reducing the ductility parameter in the constitutive equations associated with the creep constrained cavitation damage variable ω 2 , the CDM computations are capable of predicting two extreme types of behaviour. Firstly, widespread CDM behaviour, and secondly CDM growth on a highly localised-planar scale which models the phenomenon of Creep Crack Growth (CCG). The reference stress method is shown to be appropriate for widespread CDM behaviour, and to progressively become valid as the material ductility is reduced. This trend is accompanied by a shift from modest notch weakening to severe notch weakening. It is also shown that the stress level applied to the bar, and the strength of the dislocation softening damage mechanism, denoted by a second damage variable ω 1 , also influences the above behaviour. The paper defines bounds of applicability for the Skeletal stress approach to lifetime prediction, and recommends the use of complete CDM finite clement analysis for those situations where breakdown occurs.

  • continuum damage mechanics modelling of circumferentially notched tension bars undergoing tertiary creep with physically based constitutive equations
    Acta Metallurgica Et Materialia, 1994
    Co-Authors: A M Othman, B F Dyson, D R Hayhurst
    Abstract:

    Abstract The paper reports the result of finite element computations, based on Continuum Damage Mechanics (CDM), carried out on circumferentially notched tension bars undergoing tertiary creep and failure. The material constitutive equations are physically based and generalised from thos applicable to polycrystalline nickel-based superalloys. The equations describe the stress level dependence of creep rate using a sinh function and two damage state parameters to model tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. The paper presents values of the computed lifetimes, normalised with respect to plain bar lifetimes at the same average applied stress across the notch throat, and determines their sensitivity to a wide range of material parameters. The parameters include those which determine: the relative strength of the two damage rate mechanisms; the power v, to which the stress-state sensitivity parameter (Σ1/Σe) is raised for damage evolution due to grain boundary cavitation; and, their dependence upon the applied stress levels. The effects of the material parameters on the predicted transition from notch strengthening to notch weakening are rationalised analytically using a stress state modified Skeletal Point stress methodology. The predictive capability of this methodology was found to be extremely good in most circumstances but broke down seriously when the uniaxial ductility was reduced to approximately 1%.

  • Skeletal Point stresses in circumferentially notched tension bars undergoing tertiary creep modelled with physically based constitutive equations
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 1993
    Co-Authors: A M Othman, D R Hayhurst, B F Dyson
    Abstract:

    Constitutive equations are proposed in which the stress level dependence of creep rate is described by a sinh function, and two damage state-parameters are used to model the tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. These constitutive equations are applicable to polycrystalline nickel-base superalloys and are used together with a continuum damage mechanics finite element based solver, DAMAGE XX, to study the behaviour of axisymmetrically notched tension bars and simulate the complex stress states that may be encountered at geometrical stress-raisers in high temperature components. Numerical studies of such bars show that their behaviour can be accurately represented in terms of a ‘Skeletal effective stress’ located at a Point within the notch throat, and the stress state at this Point. It is shown that this conclusion is valid not only for those materials that fail by grain boundary cavitation alone, but also for materials such as superalloys where grain boundary cavitation is accompanied by mobile dislocation multiplication.

A M Othman - One of the best experts on this subject based on the ideXlab platform.

  • continuum damage mechanics modelling of circumferentially notched tension bars undergoing tertiary creep with physically based constitutive equations
    Acta Metallurgica Et Materialia, 1994
    Co-Authors: A M Othman, B F Dyson, D R Hayhurst
    Abstract:

    Abstract The paper reports the result of finite element computations, based on Continuum Damage Mechanics (CDM), carried out on circumferentially notched tension bars undergoing tertiary creep and failure. The material constitutive equations are physically based and generalised from thos applicable to polycrystalline nickel-based superalloys. The equations describe the stress level dependence of creep rate using a sinh function and two damage state parameters to model tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. The paper presents values of the computed lifetimes, normalised with respect to plain bar lifetimes at the same average applied stress across the notch throat, and determines their sensitivity to a wide range of material parameters. The parameters include those which determine: the relative strength of the two damage rate mechanisms; the power v, to which the stress-state sensitivity parameter (Σ1/Σe) is raised for damage evolution due to grain boundary cavitation; and, their dependence upon the applied stress levels. The effects of the material parameters on the predicted transition from notch strengthening to notch weakening are rationalised analytically using a stress state modified Skeletal Point stress methodology. The predictive capability of this methodology was found to be extremely good in most circumstances but broke down seriously when the uniaxial ductility was reduced to approximately 1%.

  • Skeletal Point stresses in circumferentially notched tension bars undergoing tertiary creep modelled with physically based constitutive equations
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 1993
    Co-Authors: A M Othman, D R Hayhurst, B F Dyson
    Abstract:

    Constitutive equations are proposed in which the stress level dependence of creep rate is described by a sinh function, and two damage state-parameters are used to model the tertiary softening caused by: (i) grain boundary cavity nucleation and growth, and (ii) the multiplication of mobile dislocations. These constitutive equations are applicable to polycrystalline nickel-base superalloys and are used together with a continuum damage mechanics finite element based solver, DAMAGE XX, to study the behaviour of axisymmetrically notched tension bars and simulate the complex stress states that may be encountered at geometrical stress-raisers in high temperature components. Numerical studies of such bars show that their behaviour can be accurately represented in terms of a ‘Skeletal effective stress’ located at a Point within the notch throat, and the stress state at this Point. It is shown that this conclusion is valid not only for those materials that fail by grain boundary cavitation alone, but also for materials such as superalloys where grain boundary cavitation is accompanied by mobile dislocation multiplication.