The usage of ultrasonic guided Lamb wave approach in structural health monitoring has been prevalent and proven to be an effective method. During flight, aircraft or spacecraft structures sometimes experience rapid temperature changes. The propagation of guided Lamb waves can be affected by these abrupt changes. In this paper, the effects of rapid temperature variation, due to which a sharp temperature gradient is achieved, on the propagation of guided Lamb waves through aluminum and composite beams are compared. The heating and cooling cycles for gradual temperature changes are firstly obtained for comparison. An abrupt change in the temperature is brought out by heating the beam to an elevated temperature and rapidly cooling it using liquid nitrogen. The design guidelines for the experimental setup used in the research are provided. The effects of rapid change in the temperature on the piezoelectric wafer active sensors (PWAS) are measured. Two different adhesives between the PWAS sensors and the beams are tested and the results obtained from the experiments are discussed.

A novel technique to grow carbon nanotubes (CNTs) on the surface of carbon fibers in a controlled fashion using simple lab set up is developed. Growing CNTs on the surface of carbon fibers will eliminate the problem of dispersion of CNTs in polymeric matrices. The employed synthesis technique retains the attractive feature of uniform distribution of the grown CNTs, low temperature of CNTs’ formation, i.e. 550 °C, via cheap and safe synthesis setup and catalysts. A protective thermal shield of thin ceramic layer and subsequently nickel catalytic particles are deposited on the surface of the carbon fiber yarns using magnetron sputtering. A simple tube furnace setup utilizing nitrogen, hydrogen and ethylene (C 2 H 4 ) were used to grow CNTs on the carbon fiber yarns. Scanning electron microscopy revealed a uniform areal growth over the carbon fibers where the catalytic particles had been sputtered. The structure of the grown multiwall carbon nanotubes was characterized with the aid of transmission electron microscopy (TEM). Dynamical mechanical analysis (DMA) was employed to measure the loss and storage moduli of the hybrid composite together with the reference raw carbon fiber composite and the composite for which only ceramic and nickel substrates had been deposited on. The DMA tests were conducted over a frequency range of 1–40 Hz. Although the storage modulus remained almost unchanged over the frequency range for all samples, the loss modulus showed a frequency dependent behavior. The hybrid composite obtained the highest loss modulus among other samples with an average increase of approximately 25% and 55% compared to composites of the raw and ceramic/nickel coated carbon fibers, respectively. This improvement occurred while the average storage modulus of the hybrid composite declined by almost 9% and 15% compared to the composites of reference and ceramic/nickel coated samples, respectively. The ultimate strength and elastic moduli of the samples were measured using standard ASTM tensile test. Results of this study show that while the addition of the ceramic layer protects the fibers from mechanical degradation it abolishes the mechanisms by which the composite dissipates energy. On the other hand, with almost no compromise in weight, the hybrid composites are good potential candidate for damping applications. Furthermore, the addition of CNTs could contribute to improving other mechanical, electrical and thermal properties of the hybrid composite.

As the field of Tissue Engineering advances to its ultimate goal of engineering a fully functional organ, there’s an increase need for enabling technologies and integrated system. Important roles in scaffold guided tissue engineering are the fabrication of extra-cellular matrices (ECM) that have the capabilities to maintain cell growth, cell attachment, and ability to form new tissues. Three-dimensional scaffolds often address multiple mechanical, biological and geometrical design constraints. With advances of technologies in the recent decades, Computer Aided Tissue Engineering (CATE) has much development in solid freeform fabrication (SFF) process, which includes but not limited to the fabrication of tissue scaffolds with precision control. Drexel University patented Precision Extrusion Deposition (PED) device uses computer aided motion and extrusion to precisely fabricate the internal and external architecture, porosity, pore size, and interconnectivity within the scaffold. The high printing resolution, precision, and controllability of the PED allows for closer mimicry of tissues and organs. Literatures have shown that some cells prefer scaffolds built from stiff material; stiff materials typically have a high melting point. Biopolymers with high melting points are difficult to manipulate to fabricate 3D scaffold. With the use of the PED and an integrated Assisting Cooling (AC) device; high melting points of biopolymer should no longer limit the fabrication of 3D scaffold. The AC device is mounted at the nozzle of the PED where the heat from the material delivery chamber of the PED has no influence on the AC fluid temperature. The AC has four cooling points, located north, south, east, and west; this allows for cooling in each direction of motion on a XY plane. AC uses but not limited to nitrogen, compressed air, and water to cool polymer filaments as it is extruded from the PED and builds scaffolds. Scaffolds fabricated from high melting point polymers that use this new integrated component to the PED should illustrate good mechanical properties, structural integrity, and precision of pore sizes and interconnectivity.

A magnetorheological shock absorber (MRSA) system is designed and tested to integrate semi-active shock and vibration mitigating technology into the existing EFV (Expeditionary Fighting Vehicle) forward seating positions. Based on the operational requirements of the vehicle, the MRSA is designed so that it can not only isolate occupants from harmful whole body vibration (WBV) during normal operations but also reduce injury risk during extreme events such as a “rogue” wave or ballistic/UNDEX shock event. The MRSA consists of a piston with a circular flow-mode valve, a magnetorheological (MR) fluid cylinder, and a nitrogen accumulator. Piston motion forces MR fluids enclosed in the fluid cylinder to flow through the valve where it is activated by a magnetic field in the valve. Based on the Bingham-plastic constitutive relation and a steady state fluid motion model, the valve parameters are determined using a magnetic circuit analysis tool and are validated by electromagnetic finite element analysis (FEA). The high-speed field-off viscous force of the MRSA is predicted using computational fluid dynamic analysis. To experimentally evaluate the damping performance of the MRSA and validate the design, the MRSA is tested under single frequency sinusoidal displacement excitation on a material dynamic testing machine for low piston velocities (up to 0.9 m/s) performance evaluation. For performance evaluation at high piston velocities (up to 2.2 m/s), the MRSA is tested under impact loading on a rail-guided mass-drop test stand. Equivalent viscous damping is used to characterize the controllable damping behavior of the MRSA. To describe the time response of the MRSA, a dynamic model is developed based on geometrical parameters and MR fluid properties.