1,721,002 research outputs found
Stability of a Four Story Steel Frame Building under Seismic Loading
Get PDFProgressive collapse and seismic resistance are separate topics that have many examples in the literature. However, because both of these events occur rarely, there are not many accurate research examples that have been conducted. Life threatening earthquakes occur once in every 50 years in high risk earthquake zones. Inelastic behavior of steel using a simple and reliable approach is an ongoing process. There are some conclusions about using lateral bracing and shear wall but this makes the design and the cost of the structure inaccurate for the contractor as well as limits the architectural designs.
In this project, a time history analysis of a four story moment resisting steel frame will be conducted. For the distribution of the energy released from the ground motion, a strong column-weak beam approach will be used. The structural system and every element in the system will be compact to resist flexure and lateral torsion that occur during the acceleration. Specific columns from the first floor will be removed and the structure will be accelerated under specific earthquake examples.
As a result of this project, an ideal four story steel frame resisting collapse under seismic loading will be obtained. Pros and cons of this method will be explained. This will influence further research development on the related topic. Postponing collapse events or limiting the local failure will save many lives and keep the economy stable
Modeling of Tool Wear and Tool Fracture in Micromilling
Get PDFMicromachining is the next generation of precision material removal at the micro scale level due to the increase in miniaturization of commercial products. The applications of this technology extend anywhere from electronics to micro scale medical implants. Micromilling has the potential to be the most cost effective and efficient material removal process due to ease of use and accessibility of the tools.
This research analyzes vibration of a high speed spindle and then studies micromilling of aluminum and titanium. Finite element analysis and tool modeling compliment experimental data. Cumulative tool wear based on Taylor model shows decreasing tool life with increasing feed rate and increasing cutting speed on aluminum. Inconsistent results are seen when micromilling titanium due to premature chipping of tool noses. A significant nose wear plastically deforms a micromilled subsurface and is verified with microstructure study and microhardness measurements
Analysis of Oil Lubricated Thrust Collars for Application in Integrally Geared Compressors
Get PDFIntegrally geared compressors (IGCs) comprise of single stage impellers installed on the ends of pinion shafts, all driven by a main bull gear (BG) and shaft system. When compared to single shaft multistage centrifugal compressors, the benefits of IGCs include better thermal efficiency, reduced footprint and simple foundation, dispensing with a high speed coupling, as well as better access for maintenance and overhauls. In IGCs the compression of the process gas induces axial loads on the pinion shafts that are transmitted via thrust collars (TCs) to the main drive shaft and balanced by a single thrust bearing. The TCs, located on either side of pinion gears, slightly overlap with the BG outer diameter to form lentil-shaped lubricant-wetted regions.
A numerical model, based on classical thin film lubrication theory, predicts the force response of lubricated thrust collars for use in integrally geared compressors. The predictive model determines performance parameters such as lubricant flow rate, mechanical power loss, peak pressure, lubricant temperature rise, as well as rotordynamic stiffness and damping coefficients for a lubricated TC and bull gear pair. A Newton-Raphson based iterative procedure determines an equilibrium operating position for a given set of TC and BG operating conditions and geometry. Periodic, dynamic displacements from the equilibrium position renders complex dynamic stiffnesses (H = K+i��C), from which the fluid film force and moment stiffness and damping coefficients are determined.
In a lubricated thrust collar and bull gear, a hydrodynamic pressure builds in the lower half of the lubricated zone and lubricant cavitation occurs in the upper half. The minimum film thickness and peak pressure in the lubricated zone shift location due to the difference in taper angles between the TC and BG surfaces.
For a given applied load, a study on the differences in taper angle between the TC and BG surfaces reveals that current angular tolerances of ��0.1 produce TC/BG pairs with similar performance parameters (power loss, lubricant temperature rise, etc.) and dynamic force and moment stiffness and damping coefficients. Increasing the taper angles of both the TC and BG decreases mechanical power loss and lubricant temperature rise, but also decreases the fluid film axial stiffness and damping coefficients. In addition, static angular misalignments of the TC and BG about the horizontal (x) axis joining the BG and TC centers alters the shape and extent of the lubricant cavitation region. This change alters the load carrying capacity and mechanical power losses of the lubricated element.
For the specific TC/BG pair investigated herein, there exists a maximum difference between the two taper angles for which the mechanical element can support an imposed thrust load. Increasing the speed of the BG (and proportionally the TC speed) increases the mechanical power loss and lubricant temperature rise and decreases the fluid film axial stiffness and damping coefficients. As with most fluid film bearings, increasing the applied load increases the power loss, lubricant temperature rise, and axial stiffness and damping coefficients.
The thesis delivers a predictive tool, yet to be benchmarked against experimental data, that provides insight to both the static and dynamic force performance of a lubricated thrust collar, not currently in the archived literature
Development of Control Setup for a Material Testing Machine and Experimental Analysis of the Effect of Void Volume Fraction on the Strength of Recycled High-Density Polyethylene Material
Get PDFThis study is part of a larger research effort to investigate the mechanical response of
composite beams to cyclic loading caused by waves striking beams imbedded in a low-cost sea
wall. In the course of the research, it has been determined that material characterization of the
beams is needed, and toward this end an MTS (Materials Test Systems) testing machine has been
purchased by the TAMU Ocean Engineering Department. The testing machine was used for the
purpose of testing recycled high-density polyethylene glass fiber composite specimens in order to
characterize the stress-strain response and to measure the effect of void volume fraction (VVF)
within the high-density polyethylene beams. Samples taken from different areas of the beams with
varying VVF were tested and the effect of these voids was investigated for the purpose of
determining the efficacy of deploying these structural components within low-cost sea walls. The
study includes developing testing protocols, calibrating, and proof testing the machine in order to
perform the experiments. Since the lab���s testing machine does not have a controller, a control setup
is developed herein to control the MTS Testing Machine with signal processing via a National
Instrument USB-6346 Data Acquisition Device and LabVIEW software.
Results display the decrease in the stiffness as the VVF increases as well as decreases in the
ultimate stress and ultimate strain. Increasing strain rates displayed higher stiffness, ultimate stress, and ultimate strain values. The results are discussed and invalid tests are described
in detail wherein large voids locally misrepresent the general behavior and are incongruent with
the valid tests
Bayesian Estimation of Material Properties in Case of Correlated and Insufficient Data
No full textIdentification of material properties has been highly discussed in recent times thanks to better technology availability and its application to the field of experimental mechanics. Bayesian approaches as Markov-chain Monte Carlo (MCMC) methods demonstrated to be reliable and suitable tools to process data, describing probability distributions and uncertainty bounds for investigated parameters in absence of explicit inverse analytical expressions. Though it is necessary to repeat experiments multiple times for good estimations, this might be not always feasible due to possible incurring limitations: the thesis addresses the problem of material properties estimation in presence of correlated and insufficient data, resulting in multivariate error modeling and high sample covariance matrix instability. To recover from the lack of information about the true covariance we analyze two different methodologies: first the hierarchical covariance modeling is investigated, then a method based on covariance shrinkage is employed. A numerical study comparing both approaches and employing finite element analysis within MCMC iterations will be presented, showing how the method based on covariance shrinkage is more suitable to post-process data for the range of problems under investigation
Microdrilling of Nitinol
Get PDFNitinol, a shape memory alloy, has seen increased use in a variety of industries, especially the medical industry due to its biocompatibility. The growing demand for product miniaturization combined with these newfound uses has accelerated the need for scientific research into microdrilling. Conventional microdrilling in particular shows promise for its ability to produce accurate, high quality holes in a cost efficient manner when compared to other technologies.
This research investigated the minimization of tool wear when drilling with ��127��m uncoated tungsten carbide tools, as well as with AlTiN/Si3N4 and AlTiN coated tools in minimum quantity lubrication condition. The proper and optimal coatings protected the tool surface and reduced tool wear at higher cutting speeds but were ineffective at lower cutting speeds. Additionally, control of built up edge formation proved critical in decreasing wear, reducing drill wandering and improving hole quality.
Finite element analysis was used with the orthogonal cutting model to predict tool fracture as a function of cutting speed and chip load. The method provided reasonable estimates of cutting and axial forces involved in the microdrilling process, but failed to predict the formation of built up edge and its effect
Massively-Parallel Direct Numerical Simulation of Gas Turbine Endwall Film-Cooling Conjugate Heat Transfer
Get PDFImprovements to gas turbine efficiency depend closely on cooling technologies,
as efficiency increases with turbine inlet temperature. To aid in this process, simulations that consider real engine conditions need to be considered. The first step
towards this goal is a benchmark study using direct numerical simulations to consider
a single periodic film cooling hole that characterizes the error in adiabatic boundary
conditions, a common numerical simpliflication. Two cases are considered: an adiabatic case and a conjugate case. The adiabatic case is for validation to previous work
conducted by Pietrzyk and Peet. The conjugate case considers heat transfer in the
solid endwall in addition to the
fluid, eliminating any simplified boundary conditions.
It also includes an impinging jet and plenum, typical of actual endwall configurations.
The numerical solver is NEK5000 and the two cases were run at 504 and 128 processors for the adiabatic and conjugate cases respectively. The approximate combined
time is 100,000 CPU hours. In the adiabatic case, the results show good agreement
for average velocity profiles but over prediction of the film cooling effectiveness. A
convergence study suggests that there may be an area of unresolved flow, and the film cooling momentum flux may be too high. Preliminary conjugate results show
agreement with velocity profiles, and significant differences in cooling effectiveness.
Both cases will need to be refined near the cooling hole exit, and another convergence
study done. The results from this study will be used in a larger case that considers
an actual turbine vane and film cooling hole arrangement with real engine conditions
Predictive Faulting Models in Jointed Concrete Pavement
Get PDFFaulting is a major and commonplace distress in Jointed Concrete Pavement (JCP), which can directly cause pavement roughness and adversely influence the ride quality of a vehicle. Faulting also plays an essential role in concrete pavement design. Notwithstanding the importance of faulting, the accuracy and reasonability of the existing faulting prediction models are still controversial. To enhance the concrete pavement design, this research proposes the novel models to estimate faulting depth at joints in the wheel path in JCP, including mechanistic-empirical models, machine learning model, and probabilistic model.
The mechanistic-empirical models are explicit mathematical equations that are easily used for the concrete pavement design and pavement maintenance. The mechanistic-empirical models were proposed in this research by a comprehensive exploration of the full process of faulting in which two faulting stages are found and an inflection point as a critical faulting depth is discerned to differentiate the two stages of faulting. This research proposes two mechanistic-empirical models to characterize the jointed faulting over the entire service life. 1) One is the complete model which can be used to characterize the faulting depth over time in the full progress of faulting that contains two stages faulting (i.e., the prior-inflection point and posterior-inflection point); 2) the other is the load-related model to determine the axle load distribution on faulting initiation at the stage of prior-inflection point.
Machine learning is believed to be powerful method to explore the knowledge in the LTPP faulting data. Using the LTPP data, a list of the popular machine learning models were trained and validated with the LTPP data. The best model from the machine learning model candidates was selected for the faulting prediction. Through a test of the cross validation, the random forest model was chosen for faulting prediction and proved to be compelling and reliable as it holds the satisfactory accuracy of prediction.
Probabilistic model of the faulting was modeled as a stochastic process. Markov Chain, which is the one of the widely used stochastic models, was adopted to characterize the uncertainty in faulting predictions across the chain. The transition probability matrix in the Markov Chain model was constructed to connect a sequence of Markov chains. The development of Markov chain model requires a sufficient and extensive sample of data. The LTPP data seem inadequate for the model development such that the Monte Carlo simulation was adopted to generate a large amount of data. In a progressive manner, the development of Markov chain model includes 1) preparation of data by conducting the Monte Carlo simulation 2) the repeated construction of the transition probability matrices across chains by counting the occurrences of each state of faulting
Measurements of the Static and Dynamic Load Performance of a Water Lubricated Hybrid Thrust Bearing
Get PDFAfter a successful static load testing campaign in 2015, the thrust bearing test rig has been reconfigured for dynamic load tests to determine force coefficients (stiffness, damping, and added mass). Continuous lubrication flow is supplied by a new, closed-loop water supply system capable of a wide range of operating conditions and long duration testing with little waste. In addition, a fully-instrumented loading system enables simultaneous static and dynamic (impact) loading. Cumulatively, these changes enable reliable dynamic load testing while maintaining full static load test functionality. The test rig is equipped to produce experimental results to validate bearing force coefficient model predictions. Static force vs. displacement measurements are taken to validate test rig performance after modification to accommodate the new loading and water supply systems. Overall static load results match well with predictions as well as prior experimental results. Axial clearance (C0) increases as the TTB lubrication pressure (Ps) increases. With a constant supply pressure (Ps), however, the test TB axial clearance (C0) decreases as the applied axial load (W/A) increases. Large uncertainties, due in part to the allowable run out of the thrust collar (��10��m), continue to plague axial displacement measurements. Dynamic load tests are then performed for moderate shaft speed (up to 6krpm) and quantified with force coefficients derived from the system complex dynamic stiffness function. Measurements indicate that the test thrust bearing shows no appreciable change in dynamic force coefficients (axial stiffness, damping and inertia) for the range of rotor speeds examined herein. This behavior proves the bearing behaves in a mainly hydrostatic mode as it lacks hydrodynamic features that capitalize on bearing surface speed for performance. In contrast, with an increase in applied static load at constant supply pressure the test bearing stiffness and damping increase while added mass does not show appreciable change. Finally, an increase in supply pressure under constant applied load yields an increase in stiffness and damping coefficients with no change to added mass. As with static load measurements, inherent uncertainty associated clearance measurements are large and combine with an additional error due to repeatability of the impact tests. A recommendation for future work is to fabricate a new rotor with tighter tolerances for the critical run out dimension. Lower run out tolerance will greatly reduce the clearance uncertainty, instilling better confidence in both static and dynamic experimental results
Analytical Study on Adhesively Bonded Joints Using Peeling Test and Symmetric Composite Models Based on Bernoulli-Euler and Timoshenko Beam Theories for Elastic and Viscoelastic Materials
Get PDFAdhesively bonded joints have been investigated for several decades. In most analytical studies, the Bernoulli-Euler beam theory is employed to describe the behaviour of adherends. In the current work, three analytical models are developed for adhesively bonded joints using the Timoshenko beam theory for elastic material and a Bernoulli-Euler beam model for viscoelastic materials.
One model is for the peeling test of an adhesively bonded joint, which is described using a Timoshenko beam on an elastic foundation. The adherend is considered as a Timoshenko beam, while the adhesive is taken to be a linearly elastic foundation. Three cases are considered: (1) only the normal stress is acting (mode I); (2) only the transverse shear stress is present (mode II); and (3) the normal and shear stresses co-exist (mode III) in the adhesive. The governing equations are derived in terms of the displacement and rotational angle of the adherend in each case. Analytical solutions are obtained for the displacements, rotational angle, and stresses. Numerical results are presented to show the trends of the displacements and rotational angle changing with geometrical and loading conditions.
In the second model, the peeling test of an adhesively bonded joint is represented using a viscoelastic Bernoulli-Euler beam on an elastic foundation. The adherend is considered as a viscoelastic Bernoulli-Euler beam, while the adhesive is taken to be a linearly elastic foundation. Two cases under different stress history are considered: (1) only the normal stress is acting (mode I); and (2) only the transverse shear stress is present (mode II). The governing equations are derived in terms of the displacements. Analytical solutions are obtained for the displacements. The numerical results show that the deflection increases as time and temperature increase.
The third model is developed using a symmetric composite adhesively bonded joint. The constitutive and kinematic relations of the adherends are derived based on the Timoshenko beam theory, and the governing equations are obtained for the normal and shear stresses in the adhesive layer. The numerical results are presented to reveal the normal and shear stresses in the adhesive
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