A well-known challenge in vehicle dynamics is to design a vehicle that will not only keep the occupants comfortable, but will also ensure safe and stable operation during various manoeuvres over multiple driving surfaces. A soft and compliant suspension is generally required for good ride comfort, while a stiff suspension with a low centre of mass is required for improved handling. These contradicting factors in the design process is commonly referred to as the ride comfort versus handling compromise. A newly developed semi-active hydropneumatic suspension system is proposed to reduce or negate this compromise by being able to change its characteristics according to the dynamic state of the vehicle. The unit is equipped with two proportional solenoid valves that can provide continuously variable damping. In addition, the valves are able to completely close off flow to compressible gas volumes to provide four discrete stiffness characteristics. This suspension system is based on a previously developed suspension that had only two state (open or closed) valves, which provided discrete damping characteristics. A thorough investigation of the older system proved that the system was capable of addressing the ride comfort versus handling compromise. The purpose of this study was to investigate whether the updated design could deliver improved performance and to recommend focus areas for future research initiatives. The suspension system’s characteristics were determined experimentally by actuating the unit on a test bench. Results indicated that the unit produced the desired stiffness, low damping and response time characteristics. A mathematical model of the suspension unit was developed and validated against experimental data. The model was used in single degree of freedom simulations to investigate both passive and semi-active controlled performance. Results indicated that the suspension could be semi-actively controlled for improve ride comfort. However, the magnitude of improvements with semi-active control, which includes a suitable response time, proved to be rather insignificant compared to the optimum passive suspension. / Dissertation (MEng)--University of Pretora, 2019. / Mechanical and Aeronautical Engineering / MEng (Mechanical) / Unrestricted

This dissertation proposes engineering principles for stress wave dissipation found in woodpeckers. From the experimental study of a woodpecker’s beaks via electron microscopy and mechanical testing, the three main design factors were pointed out. First, a woodpecker’s beak has wavy lines inside of the beak for local shearing. The waviness of wavy lines found in the woodpecker’s beaks was 1 while chicken’s was 0.3, and toucan’s was 0.05. Second, the woodpecker showed elongated the keratin scales to the pecking direction with a dimension ratio of 3.67 (width/height) while chicken’s and toucan’s were 3 and 1, respectively. Third, a woodpecker’s beak bone was less porous for structural strength. The porosity of a woodpecker’s beak bone was about 9.9 % while chicken’s and toucan’s were 42.3 % and 61.5 %, respectively. Also, by using computational simulations, unique geometries including hyoid apparatus and suture interfaces found in woodpeckers were investigated to assess their damping capabilities. Surrounding a woodpecker’s head, the hyoid apparatus composed of core cartilage and muscle encasing a core cartilage. The spiral and thinning geometry of the hyoid apparatus converted the normal waves into shear waves. Then shear waves generated lateral displacement of the hyoid bone, and lateral displacement brought strain energy into surrounding muscle, in which energy loss occurred by viscoelastic behavior of the muscle. Quantitatively, as the stress wave traveled from the anterior to the posterior end of the hyoid apparatus, its pressure decreased 75 % and the impulse decreased 84 %. Suture interfaces, which is another unique feature observed from woodpecker’s beak, was investigated for their geometrical effects on the dynamic impact mitigation. A sinusoidal pattern of suture interfaces induced wave scattering at its boundary causing conversion of longitudinal waves into shear waves. The suture gap also brought pressure decay by storing strain energy in its viscoelastic material. As a result, a bar with a suture interface attenuated stress waves about 37 % more than a bar with a flat interface. Based on the results and ideas presented herein, one can develop bio-inspired material for energy absorbing.

No description available.

Resonance frequency detuning (RFD) is a piezoelectric-based vibration reduction approach that applies to systems experiencing transient excitation through the system*s resonance—for example, turbomachinery experiencing changes in rotation speed, such as on spool-up and spool-down. This technique relies on the inclusion of piezoelectric material and manipulation of its electrical boundary conditions, which control the stiffness of the piezoelectric material. Resonance frequency detuning exploits this effect by intelligently switching between the open-circuit (high stiffness) and short-circuit (low stiffness) conditions as the excitation approaches resonance, subsequently shifting the natural frequency to avoid this resonance crossing and limit the response. The peak response dynamics are then determined by the system*s sweep rate, modal damping ratio, electromechanical coupling coefficient, and, most importantly, the trigger (represented here in terms of excitation frequency) that initiates the stiffness state switch. This thesis identifies the optimal frequency-based switch trigger over a range of sweep rates, damping ratios, and electromechanical coupling coefficients. With perfect knowledge of the system, the optimal frequency-based switch trigger decreases approximately linearly with the square of the coupling coefficient. Furthermore, phase of vibration at the time of the switch has a very small effect; switching on peak strain energy is marginally optimal. In practice, perfect knowledge is unrealistic and an alternate switch trigger based on an easily measurable parameter is necessary. As such, this thesis also investigates potential methods using the open-circuit piezoelectric voltage response envelope and its derivatives. The optimal switch triggers collapse to a near linear trend when measured against the response envelope derivatives and, subsequently, an empirical control law is extracted. This control law agrees well with and produces a comparable response to that of the optimal control determined using perfect and complete knowledge of the system.

This thesis investigated the application of seismic analysis methods and the response of idealized shear frames subjected to seismic loading. To complete this research, a Design Basis Earthquake (DBE) for a project site in San Luis Obispo, CA, and five past earthquake records were considered. The DBE was produced per the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-10) and used for application of the Equivalent Lateral Force Procedure (ELFP) and Response Spectrum Analysis (RSA). When applying RSA, the modal peak responses were combined using the Absolute Sum (ABS), Square-Root-of-the-Sum-of-Squares (SRSS), and Complete Quadratic Combination (CQC) method. MATLAB scripts were developed to produce several displacement, velocity, and acceleration spectrums for each earthquake. Moreover, MATLAB scripts were written to yield both analytical and numerical solutions for each system through application of Linear Time History Analysis (THA). To obtain analytical solutions, two implicit forms of the Newmark-beta Method were employed: the Average Acceleration Method and the Linear Acceleration Method. To generate a comparison, the ELFP, RSA, and THA methods were applied to shear frames up to ten stories in height. The system parameters that impacted the accuracy of each method and the response of the systems were analyzed, including the effects of classical damping and nonclassical damping models. In addition to varying levels of Rayleigh damping, non-linear hysteric friction spring dampers (FSDs) were implemented into the systems. The design of the FSDs was based on target stiffness values, which were defined as portions of the system’s lateral stiffness. To perform the required Nonlinear Time History Analysis (NTHA), a SAP2000 model was developed. The efficiencies of the FSDs at each target stiffness, with and without the addition of low levels of viscous modal damping are analyzed. It was concluded that the ELFP should be supplemented by RSA when performing seismic response analysis. Regardless of system parameters, the ELFP yielded system responses 30% to 50% higher than RSA when combing responses with the SRSS or CQC method. When applying RSA, the ABS method produced inconsistent and inaccurate results, whereas the SRSS and CQC results were similar for regular, symmetric systems. Generally, the SRSS and CQC results were within 5% of the analytical solution yielded through THA. On the contrary, for irregular structures, the SRSS method significantly underestimated the response, and the CQC method was four to five times more accurate. Additionally, both the Average Acceleration Method and Linear Acceleration Method yielded numerical solutions with errors typically below 1% when compared with the analytical solution. When implemented into the systems, the FSDs proved to be most efficient when designed to have stiffnesses that were 50% of the lateral stiffness of each story. The addition of 1% modal damping to the FSDs resulted in quicker energy dissipation without significantly reducing the peak response of the system. At a stiffness of 50%, the FSDs reduced the displacement response by 40% to 60% when compared with 5% modal damping. Additionally, the FSDs at low stiffnesses exhibited the effects of negative lateral stiffness due to P-delta effects when the earthquake ground motions were too weak to induce sliding in the ring assemblies.

This dissertation presents the results of a study performed to investigate methods for increasing the damping capacity of concrete. A variety of additives, both particle and latex based, were added to standard concrete mixtures by replacing up to 20% of the fine aggregate to measure their effects on strength, stiffness, damping, and air content. The additives included rubber particles from recycled tires, calcium carbonate particles, styrene butadiene rubber (SBR) latex, and a commercially available product named ConcreDamp which contains vegetable gum suspended in styrene butadiene latex. An initial investigation resulted in the observation that all of the additives with the exception of the SBR latex would both increase air content and decrease compressive strength. As a result, combinations of additives were investigated to see if both the mechanical and dynamic properties could be improved. The addition of steel fibers to mixtures with ground rubber were found to significantly increase air content which offset any gains in compressive strength. The combination of ground rubber and latex was shown to improve both increase compressive strength and reduce air content. The study advanced to investigate the effects of rubber size on air content, strength, and damping. It was found that for the same volume of rubber, a larger rubber particle would decrease air content, decrease compressive strength, and improve damping. The results of this study show that the best performing additive was the vegetable gum latex which improved the concrete damping by a factor of 2 when added as 15% of the fine aggregate. Additionally, an equation is presented for calculating a strength reduction factor for concrete containing rubber particles of different sizes. Finally, two full scale footbridge laboratory specimens were tested to investigate the effect of increased material damping at the structural level. One footbridge was constructed using a base concrete mixture without damping admixtures. The second was constructed with a concrete mixture that contained a replacement of 15% of the fine aggregate with ground rubber. The results were used to create a finite element model in SAP2000 that was used to predict the effects that high damping concretes would have on the footbridge specimen. / Ph. D.

This study uses Additive Manufacturing (AM) processes to fabricate 316L stainless steel beams with pockets of unfused powder for increased damping. Modal testing was completed to compare damping factors of beams with varying pocket geometries as well as number of pockets and pocket location. For the first three bending modes that were tested, an initial damping increase was observed when pocket height is greater than powder diameter. Following the initial increase there is a height threshold, which is mode dependent, that is required to achieve a statistically significant increase in damping.

This thesis explores the effects of two assumptions commonly used in mathematical models related to a piezoelectric damping method known as State Switching. The technique relies on changing the stiffness state of a piezoelectric patch through control of the electrical boundary conditions. The transition between stiffness states is assumed to occur instantaneously and in concurrence with the switch event. In actuality, the transition will occur over a finite time and will trail behind the switch event by a finite time. For these assumptions to be valid, the effects of switch duration and delay on the performance of the State Switching method must be examined. The vibration reduction for various switch duration/delay values was calculated using a numerical solver; the results of the simulations were used to provide a range in which the two aforementioned assumptions produce negligible error, defined here as a 10% decrease in method performance. Switch durations of more than 3% of the forcing period lead to significant performance decrease, for most values of damping and coupling coefficient. Results of the switch delay simulations were counter-intuitive and require further examination and validation.

No description available.

We investigate the relative equilibria of a gyrostat with a spring-mass-dashpot damper to gain new insights into the dynamics of spin-stabilized satellites. The equations of motion are developed using a Newton-Euler approach, resulting in equations in terms of system momenta and damper variables. Linear and nonlinear stability methods produce stability conditions for simple spins about the nominal principal axes. We use analytical and numerical methods to explore system equilibria, including the bifurcations that occur for varying system parameters for varying rotor momentum and damper parameters. The equations and bifurcations for zero rotor absolute angular momentum are identical to those for a rigid body with an identical damper. For the more general case of non-zero rotor momentum, the bifurcations are complex structures that are perturbations of the zero rotor momentum case. We examine the effects of spring stiffness, damper position, and inertia properties on the global equilibria. Stable equilibria exist for many different spin axes, including some that do not lie in the nominally principal planes. Some bifurcations identify regions where a jump phenomenon is possible. We use Liapunov-Schmidt reduction to determine an analytic relationship between parameters to determine if the jump phenomenon occurs. Bifurcations of the nominal gyrostat spin are characterized in parameter space using two-parameter continuation and the Liapunov-Schmidt reduction technique. We quantify the effects of rotor or damper alignment errors by adding small displacements to the alignment vectors, resulting in perturbations of the bifurcations for the standard model. We apply the global bifurcation results to several practical applications. We relate the general set of all possible equilibria to specific equilibria for dual-spin satellites with typical parameters. For systems with tuned dampers, where the natural frequency of the spring-mass-damper matches the gyrostat precession frequency, we show numerically and analytically that the existence of certain equilibria are related to the damper tuning condition. Finally, the global equilibria and bifurcations for varying rotor momentum provide a unique perspective on the dynamics of simple rotor spin-up maneuvers. / Ph. D.