1,721,056 research outputs found
CDM modeling of ductile failure in ferritic steels: Assessment of the geometry transferability of model parameters
No full textOne of the most important features of micromechanical models, with respect to other global approaches to fracture, is that progressive damage and failure can be described only by parameters characteristic of the material and not of the geometry. In spite of the large number of papers on micromechanical modeling presented in literature, a detailed assessment the geometry transferability of model parameters has been addressed in a limited number of works. In most of the cases, the model parameters transferability is analyzed only for one geometrical configuration other than that - usually a uniaxial tensile bar - on which the parameters set has been identified. In this paper, the continuum damage mechanics approach, as proposed by [Bonora, N., 1997. A non-linear CDM model for ductile failure. Engineering Fracture Mechanics 58, 11-28] is used to model ductile damage processes in ferritic steels. The geometry transferability of the damage parameters is demonstrated both in the range of low and high stress triaxiality. The possibility to accurately predict constraint effects on material crack resistance curve is demonstrated
Modello di previsione per l'area di leakage in tubazioni con cricche passanti comunque orientate
No full textPrimary Creep Modeling Based on the Dependence of the Activation Energy on the Internal Stress
No full textIn high temperature design, the accumulation of creep strain during the primary stage has to be considered since most of the allowable design strain occurs in this stage. In this work, assuming that the creep rate in the transient regime can be given as a fraction of the steady state creep rate and function of the internal stress, a mechanism based model for primary creep has been derived. Taking into account that the apparent activation energy varies with the internal stress, which evolves with creep strain, an exponential form of the creep rate versus creep strain has been obtained. The proposed model for primary creep requires the identification of two material parameters only which are shown to be function of the applied stress and independent of temperature. The proposed model has been validated for high chromium steel P91
Time-independent formulation for creep damage modeling in metals based on void and crack evolution
No full textAn advanced creep modeling, based on dislocation mechanics and incorporating damage effects, is developed at continuum scale. In the proposed formulation, creep damage does not depend on time (time-independent damage formulation) but on the accumulated creep strain. Thus, the tertiary creep stage can be predicted as the evolution of the secondary stage in which the current stress is increased by damage effects, and possible other microstructural instability processes, in addition to geometry modifications. The proposed formulation extends the initial continuum damage mechanics approach proposed by Kachanov in order to have a more explicit correlation between material creep response, damage mechanics and material microstructure. The possibility to account for possible microstructure modifications that may occur as a result of solid-solution kinetics, by means of the identification of the evolution law of damage parameters is discussed. An example of the applicability of the proposed model to IMI834 titanium alloy is given
Mechanism Based Creep Model Incorporating Damage
No full textThe increasing demand of reliable creep design for very long lives (exceeding 100.000 h), as those for high stress-low temperatures and high temperature-low stress regimes, requires a model formulation capable to account for the nonlinearity in the stress dependence of the logarithm of the creep rate as a result of the combination of both diffusional and dislocation type creeps. In this paper, a creep model, where the effect of mechanism change has been accounted for through an explicit dependence of the creep exponent n on stress, has been proposed. The model has been also extended, incorporating damage processes and characteristics of tertiary creep stage, adopting a time independent damage formulation proposed by the authors. An application example of the proposed approach to high purity aluminum is given
A model for steady stage III creep regime at low-high stress/temperature range
No full textAlthough diffusional flow creep is often considered out of practical engineering applications, the need for a model capable to account for the resulting action of both diffusional and dislocation type creep is justified by the increasing demands of reliable creep design for very long lives (exceeding lOO.OOOh), high stress-low temperatures and high temperature-low stress regimes. In this paper, a creep model formulation, in which the change of the creep mechanism has been accounted for through an explicit dependence of the creep exponent "n" on stress and temperature, has been proposed. An application example of the proposed approach to high purity aluminum is given
Creep modelling of 316H stainless steel over a wide range of stress
No full textInvestigation of material creep behaviour in the diffusion controlled creep regime is often unfeasible because of the long duration associated with low stress levels. On the other side, extrapolation from higher creep rates usually provides inaccurate results because of the sharp change in the data trend as a result of the change in the governing deformation mechanism from dislocation to diffusion type controlled creep. Similarly, extrapolation based on creep models, which have been formulated and validated mainly for dislocation type creep (such power law creep with the creep exponent ranging from 6 to 9), underestimates the actual creep rate significantly. Recently, Bonora and Esposito (2010) developed a mechanism-based model (BE model) capable to account for deformation and damage mechanism occurring in creep. In this work the BE model was applied to AISI 316H stainless steel for which considerable creep data in both dislocation and diffusion temperature/stress controlled regime were available. Using the same data set, the predictive capabilities of several models were compared
Transient creep modeling based on the dependence of the activation energy on the internal stress
No full textAlthough primary stage is often neglected in creep design, in several alloys for high temperature applications, most of the allowable design strain can occurs before reaching the minimum creep rate. The kinetic of the primary creep stage is important since it determines the microstuctural conditions for the subsequent creep regime. Under rapid stress change creep transient occurs and, according to the developed dislocation substructure, a stress dependence of the creep rate is observed. In order to account for these features in the material response, an appropriate modelling is required. Assuming that the current creep rate in the transient regime can be expressed as a function of the steady state creep rate, a model based on the evolution of the internal stress is derived. The proposed model does not rely on any particular formulation for the steady state creep and therefore is of general applicability. In the present paper, a mechanism based secondary creep stage model, with an explicit dependence of the creep exponent n on stress, as proposed by the authors, has been used1). An application to polycrystalline copper is presente
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