New instructional tools for undergraduate mechanics: the purdue mechanics freeform classroom

Motivated by the need to address the broad spectrum of learning styles embraced by today’s engineering students, a desire to encourage active, peer-to-peer, and self-learning, and the goal of interacting with every pupil despite ever-expanding enrollments, the Purdue Mechanics Freeform Classroom represents a new approach to engineering mechanics education. This complete, yet evolving, course system combines the most successful elements of the traditional classroom, with new hybrid texts, extensive multimedia features, and web2.0 interactive technologies, rendering physical and virtual learning environments. This approach has been shown to produce favorable educational outcomes, yielding a far-superior educational experience for the students and, in many ways, a more positive experience for the course instructor. With continued development and expansion, the developers believe that the Purdue Mechanics Freeform Classroom can be adopted at colleges and universities across the globe, rendering a positive and uniform mechanics education experience for all

Thermomechanical response of HTPB-based composite beams subjected to near-resonant inertial excitation

At this time, there is a pressing need to develop new technologies capable of detecting, identifying, and potentially neutralizing energetic materials, preferably from a stand-off distance. To address this need, an improved understanding of the mechanics of energetic materials, prior to detonation or deflagration, must be developed. In light of this, the present effort seeks to characterize the thermomechanical response of a polymer-based composite material, which is a mechanical surrogate for a traditional composite explosive or propellant. This research focus is motivated by the fact that many polymer-based materials demonstrate significant self-heating when subjected to dynamic loading, due to the combination of appreciable internal dissipation and poor thermal diffusion. Such self-heating has the potential to enhance existing stand-off, vapor-based detection systems, due to the temperature sensitivity of vapor pressure attendant to many polymer-based energetic materials. In this effort, a thermomechanical model of a polymer-based composite beam is developed. The composite is modeled as a homogenized linear viscoelastic material and the mechanical response is determined using Euler–Bernoulli beam theory in conjunction with a harmonic base excitation. The system is excited near its first resonant frequency to elicit large mechanical responses and, thus, maximum heating. The heat generation resulting from the harmonic loading is derived using the hysteretic characteristics of the system. The Fourier Law of Conduction is then used in conjunction with the derived heat source, as well as numerical solvers, to obtain the thermal response. In addition to the aforementioned modeling efforts, experiments were conducted using a HTPB-based beam with embedded ammonium chloride (NH4Cl) crystals. The sample was subjected to harmonic base excitation and the thermal and mechanical responses were recorded using infrared thermography and scanning laser Doppler vibrometry, respectively. Direct comparisons of the results obtained through theory and experiments are presented for several distinct forcing levels. The acquired results show a strong dependence of the temperature distribution on the stress and strain fields produced by the mechanical loading. The effect of convection at the surfaces is also evident in the thermal response. Close agreement between the model predictions and experimental results is observed. In conclusion, by adopting a unified research approach, the authors hope to build upon recent research efforts related to explosives detection by bridging the substantial gap that exists between theory and experiments. The authors also hope that this effort will advance the worldwide research effort aimed at detecting and defeating hidden explosive materials

Union Station, Tacoma, Washington : a design study for a surplus rail site

Thesis (M. Arch.)--Massachusetts Institute of Technology, Dept. of Architecture, 1982.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ROTCHIncludes bibliographical references.Recent technological changes in railroads, mergers, major shifts in urban land use patterns, and declining rail passenger travel has resulted in a surplus of urban rail lands. These lands represent a significant resource for land poor cities. An unparalleled opportunity exists for major new intervention without the usual adverse effects of land assemblage and so called "urban renewal". This work is an urban design study for a 22 acre rail site and 15 acres of adjacent waterfront land in Tacoma, Washington. The site, including Union Station and its yards, is on the edge of Tacoma's central business district. Union Station represents the largest assembled parcel of developable land in the downtown area. An attempt is made to illustrate a possible site use scenario which reflects the divergent and often conflicting goals of various differing interests.by Jeffrey David Rhoads.M.Arch

Enhanced Acoustic Transmission Into Dissipative Solid Materials Through The Use Of Inhomogeneous Plane Waves

A number of applications, for instance ultrasonic imaging and nondestructive testing, involve the transmission of acoustic energy across fluid–solid interfaces into dissipative solids. However, such transmission is generally hindered by the large impedance mismatch at the interface. In order to address this problem, inhomogeneous plane waves were investigated in this work for the purpose of improving the acoustic energy transmission. To this end, under the assumption of linear hysteretic damping, models for fluid–structure interaction were developed that allow for both homogeneous and inhomogeneous incident waves. For low-loss solids, the results reveal that, at the Rayleigh angle, a unique value of the wave inhomogeneity can be found which minimizes the reflection coefficient, and consequently maximizes the transmission. The results also reveal that with sufficient dissipation levels in the solid material, homogeneous incident waves yield lower reflection values than inhomogeneous waves, due to the large degrees of inhomogeneity inherent in the transmitted waves. Analytical conditions have also been derived which predict the dependence of the optimal incident wave type on the dissipation level and wave speeds in the solid medium. Finally, implications related to the use of acoustic beams of limited spatial extent are discussed

Use of Evanescent Plane Waves for Low-Frequency Energy Transmission Across Material Interfaces

The transmission of sound across high-impedance difference interfaces, such as an air-water interface, is of significant interest for a number of applications. Sonic booms, for instance, may affect marine life, if incident on the ocean surface, or impact the integrity of existing structures, if incident on the ground surface. Reflection and refraction at the material interface, and the critical angle criteria, generally limit energy transmission into higher-impedance materials. However, in contrast with classical propagating waves, spatially decaying incident waves may transmit energy beyond the critical angle. The inclusion of a decaying component in the incident trace wavenumber yields a nonzero propagating component of the transmitted surface normal wavenumber, so energy propagates below the interface for all oblique incident angles. With the goal of investigating energy transmission using incident evanescent waves, a model for transmission across fluid-fluid and fluid-solid interfaces has been developed. Numerical results are shown for the air-water interface and for common air-solid interfaces. The effects of the incident wave parameters and interface material properties are also considered. For the air-solid interfaces, conditions can be found such that no reflected wave is generated, due to impedance matching among the incident and transmitted waves, which yields significant transmission increases over classical incident waves

Developing an Electromechanical Carbon Dioxide Sensor for Occupancy Monitoring

The Energy Information Administration reported in 2012 that heating and cooling processes consume nearly 35% of the total energy used by commercial buildings. In an effort to limit the amount of energy wasted in conditioning empty buildings and rooms, various occupancy detection techniques have been developed that can be paired with a smart heating, ventilation, and air conditioning (HVAC) control system. This work focused on the development of a novel carbon dioxide detector that is sensitive enough to accurately determine if, and when, a room is occupied. To test the new sensor design, a customized chamber with gas inlets was used to isolate the sensors in a controlled environment. The sensors were tested in this chamber alongside various commercial-off-the-shelf options for the purpose of both validating the developed sensors and observing if they exhibited increased sensitivity and selectivity over previous designs. Following these tests, the overall performance of the sensors was compared. The results of this comparison were subsequently used to assess the capabilities of the sensor designs and to identify areas for further improvement

The Influence of Macroscale Stress Concentrations on the Near-Resonant Thermomechanics of Mock Energetic Materials

The characterization of particulate composite energetic materials, both with and without stress concentration, is currently of great interest to the defense community. This work seeks to further characterize the self-heating effect of composite energetic plates, particularly around regions of high stress, when subjected to harmonic excitation near resonance. Mock energetic plates with macroscale stress concentrations are prepared in various compositions based on the PBXN-109 formulation, and are tested near the first resonant frequency using an electrodynamic shaker. The resulting mechanical and thermal responses are recorded using a laser Doppler vibrometer and an infrared camera, respectively. Upon comparison between the regions of heightened strain and stress, a strong correlation was found between the respective areas of heightened response. Additionally, the effect of the type of stress concentration on the resulting levels of stress and strain is discussed. This characterization will aid the defense community in their mission to better understand particulate composite energetic materials

The Construction of Acoustic Waveforms from Plane Wave Components to Enhance Energy Transmission into Solid Media

The transmission of acoustic energy into solid materials is of interest in a wide range of applications, including ultrasonic imaging and nondestructive testing. However, the large impedance mismatch at the solid interface generally limits the transmission of incident acoustic energy. With the goal of improving the fraction of the energy transmitted into solid materials, the use of various bounded spatial profiles, including commonly-employed forms, such as Gaussian distributions, as well as newly-constructed profiles, has been investigated. The spatial profile is specified as the pressure amplitude distribution of the incident wave. Bounded acoustic beams are represented here as sums of harmonic plane waves, and results obtained for the optimal parameters of incident plane wave components are used to inform the construction of bounded wave profiles. The effect of the form of the spatial profile is investigated, with the total energy carried by the incident wave held constant as the profile is varied, and the relationship with the plane wave components which superimpose to form the bounded wave is discussed. Direct comparisons are made for the efficiency of the energy transmission of different profiles. The results reveal that, by tuning the form of the profile, substantial improvements in the total energy transmission can be achieved as compared to Gaussian and square waveforms

On The Use Of Evanescent Plane Waves For Low-Frequency Energy Transmission Across Material Interfaces

The transmission of airborne sound into high-impedance media is of interest in several applications. For example, sonic booms in the atmosphere may impact marine life when incident on the ocean surface, or affect the integrity of existing structures when incident on the ground. Transmission across high impedance-difference interfaces is generally limited by reflection and refraction at the surface, and by the critical angle criterion. However, spatially decaying incident waves, i.e., inhomogeneous or evanescent plane waves, may transmit energy above the critical angle, unlike homogeneous plane waves. The introduction of a decaying component to the incident trace wavenumber creates a nonzero propagating component of the transmitted normal wavenumber, so energy can be transmitted across the interface. A model of evanescent plane waves and their transmission across fluid-fluid and fluid-solid interfaces is developed here. Results are presented for both air-water and air-solid interfaces. The effects of the incident wave parameters (including the frequency, decay rate, and incidence angle) and the interfacial properties are investigated. Conditions for which there is no reflection at the air-solid interface, due to impedance matching between the incident and transmitted waves, are also considered and are found to yield substantial transmission increases over homogeneous incident waves

The Influence of Thermal Conditions on the Thermomechanics of Particulate-Composite, Mock Explosive Samples under Near-Resonant Excitation

Vapor detection is one of the most effective ways to find hidden plastic-bonded explosives in the field today. In recent years, it has been demonstrated that providing near-resonant vibratory excitation to explosives dramatically increases their vapor pressure, allowing for easier detection. Unfortunately, there currently exists a limited understanding of the thermomechanics of energetic material. This study seeks to help fill this technical void by exploring the thermomechanics of mock plastic-bonded explosives using direct mechanical excitation with varying thermal conditions. Using two different ambient thermal boundary conditions (insulated geometric boundaries and boundaries with free convection), a 7 by 10 by 0.5 HTPB/Ammonium Chloride particulate-composite plate was tested by fixing it to an electrodynamic shaker and vibrating the sample at low frequencies (under 1000 Hz). Vibratory and thermal data was collected using a Polytec scanning laser Doppler vibrometer and a FLIR infrared camera. It was determined that insulating boundary conditions, allow the mock energetic material temperature to increase significantly as compared to the convective boundaries under near-resonant excitation. Future work will investigate alternate thermal boundary and initial conditions, as well as alternate mock energetic materials