Adaptive structures made of composite materials that can respond and adapt to various external stimuli are appealing for the development of multi-tasking intelligent systems. This study focuses on characterizing nonlinear properties and understanding performance of adaptive structures subjected to elevated temperatures, mechanical loading, and high electric field. The adaptive composite structures to be studied consist of ferroelectric ceramic and metal constituents whose compositions and micro-structural arrangements vary continuously through the thickness - such systems are known as functionally graded materials or FGMs. The nonlinearity is due to thermo-electro-mechanical coupling effects. For example, ferroelectric ceramics can experience polarization switching under high compressive stresses and hysteresis electric fields. The objectives of this investigation are to manufacture adaptive functionally graded composites using a powder metallurgy method; test the composite samples at different thermo-electro-mechanical histories, including quasi-static, creep-relaxation, and hysteresis loading; and establish an analytical and computational framework for predicting nonlinear response and simulating shape changes in adaptive structures in response to various external stimuli.
An investigation of the thermo-electro-mechanical coupling effects will open an opportunity to further explore long-term material degradation due to oxidation and aging, and fatigue failure mechanisms in intelligent structures. These research activities will contribute to a graduate course development in multifunctional materials and structures, creating visualization and animation of shape changes in adaptive structures, and involving undergraduate and graduate students as well as high school teachers in scientific research. The analysis tools and characterization methods can benefit many industries that manufacture and use devices made from adaptive composite materials, by reducing cost and effort in material characterization requirements.
Adaptive structures that can respond and adapt to various external stimuli are appealing for the development of multi-tasking intelligent systems, which have applications in structural components for extreme environments, energy harvesting and harnessing devices, actuators, sensors, and many others. This study focuses on understanding nonlinear properties and performance of composites, comprised of ceramics and metallic constituents, subjected to elevated temperatures, mechanical loading, and high electric field. Two composite systems were studied, which are aluminum-alumina and barium titanate-silver composites. The composites were manufactured with compositions and micro-structural arrangements of the constituents vary 1) uniformly throughout the composites and 2) discretely through the thickness, known as functionally graded systems. The manufactured composite specimens were characterized for their physical, thermal, mechanical, and electrical properties at various temperatures. The specimens were also tested under different thermo-mechanical and electro-mechanical histories, such as ramp loadings at different rates and cyclic loadings at various frequencies. To further understand the behaviors of the above composites, micromechanical models that consider detailed microstructural morphologies of the composites were generated and used to obtain the overall properties and responses of the composites. Finally, several microstructure-dependent nonlocal nonlinear beam theories were developed and implemented in a finite element (FE) framework. This framework is useful to simulate shape changes in adaptive structures in responses to various external stimuli. The above research activities also contribute to a graduate course development in multifunctional materials and structures at Texas A&M University (TAMU), creating visualization and animation of shape changes in adaptive structures, and involving undergraduate and graduate students, and international students in scientific research. Part of the research findings were also presented during summer camps at TAMU on June 10 and 17 2013, in which the campers (K-12 students) learned more about mechanical engineering major in general and the use and development of composite materials and structures. This project has also involved two exchange students from Germany, in which research activities and findings from this NSF project were disseminated internationally. Highlights of research findings are summarized below: 1) The effect of processing conditions (compacting pressures, sintering temperatures and time, amounts of binders and ethanol) and size of the constituents on the overall density, elastic moduli, and thermal expansion of aluminum and alumina composites were examined. The sizes of the constituents, and compacting pressures and ethanol contains during the processing strongly influence the overall mechanical and physical properties of these composites, illustrated in Image 1. 2) Micromechanics models were generated from scanning electron microscopy images of composite samples. The micrograph image is divided into several regions, each region is considered as a representative microstructure of composites, and is implemented in finite element (FE). The effect of sizes and microstructural arrangements on the mechanical behaviors were studied. The micromechanics models were capable in capturing the thermo-elastic properties and stress-strain response of the composite (see Image 2). Micromechanics models were also used to study degradation in the mechanical properties of functionally graded composites during several heating and cooling cycles. 3) Experimental tests and micromechanics studies were also performed to understand the thermo-electro-mechanical responses of active composites. The dielectric constant, thermal expansion, heat capacity, thermal conductivity, elastic modulus, and piezoelectric constant strongly depend on temperatures (see example in Image 3). Temperature changes induce phase changes in the composites, affecting their properties. Micromechanics models were also used to obtain the effective properties and study the effect of different microstructural morphologies on the performance of active composites. 4) A phenomenological constitutive model that includes time-temperature dependent and polarization reversal behaviors has been formulated for piezoelectric ceramics subjected to various histories of applied electric fields (see Image 4). The model is also capable of predicting hysteretic response during polarization switching at various temperatures. The nonlinear constitutive models was integrated to micromechanics and FE in order to study shape changes responses in active composite and functionally graded beams under cyclic electric fields. 5) The effect of geometric nonlinearity, temperature, material gradation associated with functionally graded material, and microstructure-dependent nonlocal parameter on the overall responses of functionally graded composites was studied (Image 5). The nonlocal scale parameter has the effect of stiffening the beams. The numerical results also show that the deflection of the simply supported beam predicted by the new model is smaller than those by the classical beam model. The smaller the beam thickness, the larger the differences in the deflection values predicted by the two models. However, the differences diminish with the increase of the beam thickness. For the free vibration problem, it is found that the natural frequency predicted by the current nonlocal model is higher than that by the classical model, and the difference between the two sets of predictions is significant only for very thin beams. These predicted trends confirm the size effect at the micron scale observed in experiments.
