Uniform TitleCreep resistance and strain-rate sensitivity of nanocrystalline materials
NameBarai, Pallab (author), Weng, George (chair), Baruh, Haim (internal member), Bottega, William (internal member), Rutgers University, Graduate School - New Brunswick,
SubjectMechanical and Aerospace Engineering,
DescriptionA micromechanics-based continuum model is developed to determine the creep resistance and strain-rate sensitivity of the nanocrystalline materials. The solid is idealized as a two (or three) phase composite, where the grains were treated as spherical inclusions, the grain boundary as the matrix and the pores/voids as the third phase (if present in the solid) of the composite. The strain of an individual phase is taken to be the sum of elastic and creep/viscoplastic components. Within the elastic context the homogenization scheme is developed based on the Eshelby-Mori-Tanaka approach. The Laplace transform was used to convert the linear elastic homogenization
method to a linear viscoelastic one, and then to convert the viscoelastic response to viscoplastic one, during which the Maxwell viscosity of the viscoelastic phases is replaced by the secant viscosity of the viscoplastic phases. A nonlinear-rate dependent constitutive equation is assumed for both the grain interior and grain boundary to calculate the secant viscosity of the individual phase at a given stage of deformation. The drag stress of the grain interior is assumed to follow the Hall-Petch effect, but that of the grain boundary phase is taken to be size-independent. By using the field-fluctuation method, the effective stress (or effective strain rate) of the constitutive phase is derived in terms of the applied stress (or applied strain rate). The change in porosity under different loading conditions is also incorporated within the model.
The validity of the model was verified by comparing the predicted stress-strain results with the experimental data of Sanders et al. , Wang et al.  and Wang et al.  for the creep response, and Khan et al.  and Khan and Zhang  for the constant strain-rate loading. The model is capable of capturing both hardening and softening of material as grain size decreases from coarse grain to the nanometer range. The latter characteristic is also known as the inverse Hall-Petch effect and this occurs in both creep and constant strain-rate response. As a result, the critical grain size at which the solid has maximum strength can be estimated using this method. With the presence of porosity, the developed model is also able to capture the nonlinearity in the stress-strain plot under hydrostatic loading.
NoteIncludes bibliographical references (p. 94-98).
CollectionGraduate School - New Brunswick Electronic Theses and Dissertations
Organization NameRutgers, The State University of New Jersey
RightsThe author owns the copyright to this work.