Brian F. Davis

Measurement and Prediction of the Thermomechanical Response of Shape Memory Alloy Hybrid Composite Beams

An experimental and numerical investigation into the static and dynamic responses of shape memory alloy hybrid composite (SMAHC) beams is performed to provide quantitative validation of a recently commercialized numerical analysis/design tool for SMAHC structures. The SMAHC beam specimens consist of a composite matrix with embedded pre-strained SMA actuators, which act against the mechanical boundaries of the structure when thermally activated to adaptively stiffen the structure. Numerical results are produced from the numerical model as implemented into the commercial finite element code ABAQUS. A rigorous experimental investigation is undertaken to acquire high fidelity measurements including infrared thermography and projection moiré interferometry for full-field temperature and displacement measurements, respectively. High fidelity numerical results are also obtained from the numerical model and include measured parameters, such as geometric imperfection and thermal load. Excellent agreement is achieved between the predicted and measured results of the static and dynamic thermomechanical response, thereby providing quantitative validation of the numerical tool.

Measurement and prediction of the thermomechanical response of shape memory alloy hybrid composite beams

Previous work at NASA Langley Research Center (LaRC) involved fabrication and testing of composite beams with embedded, pre-strained shape memory alloy (SMA) ribbons within the beam structures. That study also provided comparison of experimental results with numerical predictions from a research code making use of a new thermoelastic model for shape memory alloy hybrid composite (SMAHC) structures. The previous work showed qualitative validation of the numerical model. However, deficiencies in the experimental-numerical correlation were noted and hypotheses for the discrepancies were given for further investigation. The goal of this work is to refine the experimental measurement and numerical modeling approaches in order to better understand the discrepancies, improve the correlation between prediction and measurement, and provide rigorous quantitative validation of the numerical analysis/design tool. The experimental investigation is refined by a more thorough test procedure and incorporation of higher fidelity measurements such as infrared thermography and projection moire interferometry. The numerical results are produced by a recently commercialized version of the constitutive model as implemented in ABAQUS and are refined by incorporation of additional measured parameters such as geometric imperfection. Thermal buckling, post-buckling, and random responses to thermal and inertial (base acceleration) loads are studied. The results demonstrate the effectiveness of SMAHC structures in controlling static and dynamic responses by adaptive stiffening. Excellent agreement is achieved between the predicted and measured results of the static and dynamic thermomechanical response, thereby providing quantitative validation of the numerical tool.

Pseudoelastic SMA radius size effects on the damping of structural vibrations

The design of pseudoelastic shape memory alloy (SMA) passive damping devices for structural vibration is dependent on the geometry of the SMA. By changing the effective radius size of an attached SMA element, one simultaneously changes the nonlinear stiffness and damping contributed to the system by the SMA. In order to identify the coupled nonlinear dynamic behavior, this work focuses on the steady state frequency response functions of a simple SDOF system with an attached SMA element under base excitation. An equivalent linearization method is used to produce a qualitative representation of the frequency response of the structure for multiple radius sizes and excitation amplitudes. These results are then compared to corresponding frequency response functions produced from the Seelecke, Muller, and Achenbach SMA model. These results give insight into jump phenomenon, hysteretic damping effects, and identify the stable branches of the nonlinear frequency response. Additionally, optimal radius sizes are presented for a range of harmonic excitation amplitudes and frequencies. These results lead to an initial investigation into the physical mechanisms responsible for choosing optimal radius sizes for an arbitrary excitation.

Hole-Projection Method for Calculating Feshbach Resonances and Inner-Shell Vacancies

Inner-shell vacancy states play an important role in understanding quantum systems. Experimentally, they appear in the form of a discrete spectrum that is embedded in the continuum. In most cases, they are coupled to the continuum via the Coulomb interaction, although this coupling may be very weak, so that the energy level and wave function can be obtained approximately by using square-integrable basis functions. This coupling can cause considerable difficulty in the theoretical treatment of these states. These vacancy states can be formed in atomic, molecular, nuclear, and solid systems. They arise in various physical processes; for example, in resonant or nonresonant collisions, photoabsorption, nuclear decay, or elementary particle capture, etc. Once formed, they may decay by inner-shell or outer-shell autoionization, Coster-Kronig transition, x-ray emission, etc.(1) The application of these effects covers a wide range of disciplines in physics and electronics as well as medical and geophysical sciences.