267 research outputs found

    Viscoplasticity: A thermodynamic formulation

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    A thermodynamic foundation using the concept of internal state variables is given for a general theory of viscoplasticity, as it applies to initially isotropic materials. Three fundamental internal state variables are admitted. They are: a tensor valued back stress for kinematic effects, and the scalar valued drag and yield strengths for isotropic effects. All three are considered to phenomenologically evolve according to competitive processes between strain hardening, strain induced dynamic recovery, and time induced static recovery. Within this phenomenological framework, a thermodynamically admissible set of evolution equations is put forth. This theory allows each of the three fundamental internal variables to be composed as a sum of independently evolving constituents

    Thermodynamics of Local State: Overall Aspects and Micromechanics Based Constitutive Relations

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    The thermodynamics of irreversible processes, based on a set of internal state variables, is revisited, paying attention on two complementary aspects:- The Generalized Standard Models are shown to introduce too stiff constraints, both for kinematic hardening and for damage modellings. A slightly less restrictive approach is then considered, based on several independent  potentials and several independent multipliers;- The micro-macro approach of elastoplasticity is formulated through the Transformation Field Analysis of Dvorak and the use of a correction method. Moreover, based on previous works of Suquet and Nguyen, the approach is generalized with energy considerations, incorporating continuous fields of eigenstrains

    On the thermodynamics of stress rate in the evolution of back stress in viscoplasticity

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    A thermodynamic foundation using the concept of internal state variables is presented for the kinematic description of a viscoplastic material. Three different evolution equations for the back stress are considered. The first is that of classical, nonlinear, kinematic hardening. The other two include a contribution that is linear in stress rate. Choosing an appropriate change in variables can remove this stress rate dependence. As a result, one of these two models is shown to be equivalent to the classical, kinematic hardening model; while the other is a new model, one which seems to have favorable characteristics for representing ratchetting behavior. All three models are thermodynamically admissible

    Finite element modelling of creep deformation in fibre-reinforced ceramic composites

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    The tensile creep and creep-recovery behaviour of a unidirectional SiC fibre-Si 3 N 4 matrix composite was analysed using finite element techniques. The analysis, based on the elastic and creep properties of each constituent, considered the influence of fibre-matrix bonding and processing-related residual stresses on creep and creep-recovery behaviour. Both two- and three-dimensional finite element models were used. Although both analyses predicted similar overall creep rates, three-dimensional stress analysis was required to obtain detailed information about the stress state in the vicinity of the fibre-matrix interface. The results of the analysis indicate that the tensile radial stress, which develops in the vicinity of the fibre-matrix interface after processing, rapidly decreases during the initial stages of creep. Both the predicted and experimental results for the composite show that 50% of the total creep strain which accumulated after 200 h at a stress of 200 MPa and temperature of 1200°C is recovered within 25 h of unloading.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44717/1/10853_2004_Article_BF00576283.pd
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