Ho-Jun Lee's research while affiliated with NASA and other places

Analytical formulations are presented which account for the coupled mechanical, electrical, and thermal response of piezoelectric composite shell structures. A new mixed multi-field laminate theory is developed which combines "single layer" assumptions for the displacements along with layerwise fields for the electric potential and temperature. This laminate theory is formulated using curvilinear coordinates and is based on the principles of linear thermopiezoelectricity. The mechanics have the inherent capability to explicitly model both the active and sensory responses of piezoelectric composite shells in thermal environment. Finite element equations are derived and implemented for an eight-noded shell element. Numerical studies are conducted to investigate both the sensory and active responses of piezoelectric composite shell structures subjected to thermal loads. Results for a cantilevered plate with an attached piezoelectric layer are com- pared with corresponding results from a commercial finite element code and a previously developed program. Additional studies are conducted on a cylindrical shell with an attached piezoelectric layer to demonstrate capabilities to achieve thermal shape control on curved piezoelectric structures.

Analytical formulations for thermopiezoelectric composite materials are extended to account for thermal effects arising from temperature dependent material properties. The updated mechanics also has the inherent capability to capture thermal effects which arise due to coefficient of thermal expansion mismatch and pyroelectric phenomena, providing a comprehensive thermal analysis capability. The thermal effects are represented at the material level through the thermopiezoelectric constitutive equations. These equations are incorporated into a layerwise laminate theory to provide a unified representation of the coupled mechanical, electrical, and thermal response of piezoelectric composite materials. Corresponding finite element equations are derived and implemented for beams and plates to model the active and sensory response of piezoelectric composite laminates. Applications demonstrate the significance of incorporating temperature dependent material properties on the response of piezoelectric composite beams and plates in both active and sensory configurations.

Previously developed analytical formulations for piezoelectric composite plates are extended to account for the nonlinear effects of temperature on material properties. The temperature dependence of the composite and piezoelectric properties are represented at the material level through the thermopiezoelectric constitutive equations. In addition to capturing thermal effects from temperature dependent material properties, this formulation also accounts for thermal effects arising from: (1) coefficient of thermal expansion mismatch between the various composite and piezoelectric plies and (2) pyroelectric effects on the piezoelectric material. The constitutive equations are incorporated into a layerwise laminate theory to provide a unified representation of the coupled mechanical, electrical, and thermal behavior of smart structures. Corresponding finite element equations are derived and implemented for a bilinear plate element with the inherent capability to model both the active and sensory response of piezoelectric composite laminates. Numerical studies are conducted on a simply supported composite plate with attached piezoceramic patches under thermal gradients to investigate the nonlinear effects of material property temperature dependence on the displacements, sensory voltages, active voltages required to minimize thermal deflections, and the resultant stress states.

Previously developed mechanics for piezoelectric composite materials are extended to account for thermal effects. The updated mechanics accounts for thermal effects which arise due to: (1) coefficient of thermal expansion mismatch between the various composite and piezoelectric layers, (2) pyroelectric effects on the piezoelectric plies, and (3) the temperature dependence of the composite and piezoelectric properties. The coupled mechanical, electrical, and thermal response of piezoelectric composite materials is captured at the material level. A layerwise laminate theory is formulated with the inherent capability to model both the active and sensory response of piezoelectric composite materials. Finite element equations are developed and implemented for a bilinear plate element. Numerical studies are conducted on a simply supported graphite/epoxy plate with attached piezoceramic patches to investigate thermal shape control applications and to study the resultant stress state.

Analytical formulations are presented which account for the coupled mechanical, electrical, and thermal response of piezoelectric composite laminates and plate structures. A robust layerwise theory is formulated with the inherent capability to explicitly model the active and sensory response of piezoelectric composite plates having general laminations in thermal environments. Finite element equations are derived and implemented for a bilinear 4-noded plate element. Applications demonstrate the capability to manage thermally induced bending and twisting deformations in symmetric and antisymmetric composite plates with piezoelectric actuators and attain thermal stability. The resultant stresses in the thermal piezoelectric composite laminates are also investigated.

Previously developed analytical formulations for piezoelectric composite plates are extended to account for the nonlinear effects of material property temperature dependence. Using manufacturer's experimental data, the temperature variation of the different piezoelectric properties are incorporated at the material level into the thermopiezoelectric constitutive equations. These constitutive equations are used in turn to formulate a layerwise laminate theory and to derive finite element equations, which are implemented into a bilinear four noded plate element. This unified representation leads to an inherent capability to model both the active and sensory response, as well as accounting for the coupled mechanical, electrical, and thermal behavior of piezoelectric composite plates. Numerical studies demonstrate the nonlinear effects introduced by the temperature dependence of material properties on the displacements, sensory voltages, and stresses of a simply supported composite plate with attached piezoceramic patches.

Analytical formulations are presented which account for the coupled mechanical, electrical, and thermal response of piezoelectric composite laminates and plate structures. A layerwise theory is formulated with the inherent capability to explicitly model the active and sensory response of piezoelectric composite plates having arbitrary laminate configurations in thermal environments. Finite element equations are derived and implemented for a bilinear 4-noded plate element. Application cases demonstrate the capability to manage thermally induced bending and twisting deformations in symmetric and antisymmetric composite plates with piezoelectric actuators, and show the corresponding electrical response of distributed piezoelectric sensors. Finally, the resultant stresses in the thermal piezoelectric composite laminates are investigated.

Previously developed discrete layer mechanics are extended to incorporate thermal effects to account for the complete coupled mechanical, electrical, and thermal response of piezoelectric composite beams. Thermal effects in both the elastic and piezoelectric media are captured at the material level. This unified representation leads to an inherent capability to model both the sensory and active responses of piezoelectric composite beams in thermal environments. Finite element equations are developed and implemented for a beam element with linear shape functions. Results from the current formulation are compared with results from a conventional thermoelastic finite element analysis and classical beam theory. Additional numerical studies demonstrate capabilities of the current formulation to predict the thermal deformation of composite beams, as well as the active compensation of these thermal deformations using piezoelectric structures. The corresponding sensory response and the resultant stress state in the piezoelectric composite beam are also presented.

Although there has been extensive development of analytical methods for modeling the behavior of piezoelectric structures, only a limited amount of research has been performed concerning the implications of thermal effects on both the active and sensory response of smart structures. Thermal effects become important when the piezoelectric structure has to operate in either extremely hot or cold temperature environments. Consequently, the purpose of this paper is to extend the previously developed discrete layer formulation of Saravanos and Heyliger to account for the coupled mechanical, electrical, and thermal response in modern smart composite beams. The mechanics accounts for thermal effects which may arise in the elastic and piezoelectric media at the material level through the constitutive equations. The displacements, electric potentials, and temperatures are introduced as state variables, allowing them to be modeled as variable fields through the laminate thickness. This unified representation leads to an inherent capability to model both the active compensation of thermal distortions in smart structures and the resultant sensory voltage when thermal loads are applied. The corresponding finite element formulation is developed and numerical results demonstrate the ability to model both the active and sensory modes of composite beams with heterogeneous plies with attached piezoelectric layers under thermal loadings.