Kamyar Ghandi

Hybrid finite element model for phase transitions in nonlinear electromechanically coupled material

A finite element approach for modeling phase transitions in electro-mechanically coupled material is presented. The approach is applicable to modeling a broad range of material behavior, including repolarizations in ferroelectrics (PZTs) as well as ferroelectric-antiferroelectric phase transitions in electroceramics such as lead lanthanum zirconate stannate titanate. A 3D 4 node hybrid element has been formulated. In addition to nodal displacement and voltage degrees of freedom used in conventional coupled elements, the hybrid element also utilizes internal electric displacement degrees of freedom, resulting in improved numerical efficiency. The elements utilize energy based nonlinear constitutive relations for more accurate representation of material response at high electric fields. The phase/polarization state of each element is represented by internal variable,s which are updated at each simulation step based on a phenomenological mode. The material model has been roughly fitted to response of PZT-5H under free strain conditions. The model reproduces strain and electric displacement hysteresis loops observed in the material. The hybrid finite element model results are demonstrated for a complex geometry with non-uniform fields.

Nonlinear finite element modeling of phase transitions in electromechanically coupled material

A finite element approach has been used to model phase transitions in electro-mechanically coupled material. The approach is applicable to modeling a broad range of material behavior, including repolarizations in ferroelectrics as well as ferroelectric-antiferroelectric phase transitions in electroceramics such as lead lanthanum zirconate stannate titanate. A 3D 8 node element with nodal displacement and voltage degrees of freedom has been formulated using standard isoparametric shape functions. The elements utilize nonlinear constitutive relations for more accurate representation of material response at high electric fields. The phase/polarization state of the material is represented by internal variables in each element, which are updated at each simulation step based on phenomenological model. The model reproduces strain and electric displacement hysteresis loops observed in the material. The approach allows modeling of complex actuator geometries subject to non-uniform electric fields. An a sample application, the response of a piezoelectric wafer with interdigitated electrodes is analyzed. Such a geometry leads to stresses arising from non-uniform poling in the sample which can be computed using the finite element model.

Shape memory ceramic actuation of adaptive structures

Field-induced phase transitions in electroceramics with the advantages of large strain and shape memory are investigated as a mechanism for structural actuation. Several lanthanum and niobium doped lead zirconate stannate titanate compositions were manufactured and studied. The dependence of the material response on various factors relevant to structural actuation has been examined. Factors such as induced stress, actuation frequency, and temperature play an important role in the response of the material. A prototype adaptive structure using shape memory ceramic active materials has been constructed, tested, and used to validate model predictions.

Mixed-domain traveling-wave motor model with lossy (complex) material properties

A piezoelectric traveling-wave motor model has been developed with parameters entirely related to physical properties. The approach is well-rooted in the formulation suggested earlier by Hagood and McFarland, but several model improvements have been integrated in an effort to realize an accurate model suited for automated design optimization. Additional model considerations include a flexible rotor model and a hysteretic stick-slip friction contact model which replace the previous assumptions of a rigid rotor and pure slip. The most notable contribution has been the use of lossy (complex) material properties to account for inherent material losses, supplanting the use of non-physical damping coefficients. The model is partly formulated in the frequency domain, and by representing the modal states and forces as Fourier series expansions and retaining higher harmonic terms, it has been generalized to account for non-ideal traveling-wave excitation. Needing to simulate the hysteretic contact model in the time domain, a mixed-domain solution procedure has been implemented to maintain some of the computational efficiency of frequency domain analysis. A preliminary validation study has demonstrated excellent correlation between simulation results and experimental data for a commercial motor.