George T. Flowers

Auburn University, Auburn, Alabama, United States

Are you George T. Flowers?

Claim your profile

Publications (115)30.95 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Electromagnetic interference (EMI) immune and light-weight, fiber-optic sensor based Structural Health Monitoring (SHM) will find increasing application in aerospace structures ranging from aircraft wings to jet engine vanes. Intelligent Fiber Optic Systems Corporation (IFOS) has been developing multi-functional fiber Bragg grating (FBG) sensor systems including parallel processing FBG interrogators combined with advanced signal processing for SHM, structural state sensing and load monitoring applications. This paper reports work with Auburn University on embedding and testing FBG sensor arrays in a quarter scale model of a T38 composite wing. The wing was designed and manufactured using fabric reinforced polymer matrix composites. FBG sensors were embedded under the top layer of the composite. Their positions were chosen based on strain maps determined by finite element analysis. Static and dynamic testing confirmed expected response from the FBGs. The demonstrated technology has the potential to be further developed into an autonomous onboard system to perform load monitoring, SHM and Non-Destructive Evaluation (NDE) of composite aerospace structures (wings and rotorcraft blades). This platform technology could also be applied to flight testing of morphing and aero-elastic control surfaces.
    03/2014;
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this work, our research team investigated the efficacy of using optical static and dynamic strain sensing with embedded Fiber Bragg Gratings (FBGs) in structural health monitoring (SHM) of a model composite airplane wing. A one-fourth scale model of a T38 airplane wing was designed and manufactured using fabric reinforced polymer matrix composites with FBG sensors embedded under the top layer of the composite. The accuracy and durability of the sensors were evaluated at the coupon and structural levels utilizing static and dynamic testing. Strain measurements using embedded FBGs with an optical interrogator were found to be in agreement with values measured using other strain measuring devices and with results obtained using finite element analysis (ANSYS®). Preferred locations for the FBG sensors were identified in accordance with contour maps of internal strain distributions resulting from critical load cases. Manufacturing techniques used to address handling, survivability and durability of the embedded sensors during and post manufacturing of the composites were evaluated and optimized.
    01/2014; 1581(1).
  • [Show abstract] [Hide abstract]
    ABSTRACT: Unlike traditional rotational gyroscopes, microelectromechanical systems (MEMS) gyroscopes use a vibrating proof mass rather than a rotational mass to sense changes in angular rate. They are also smaller and less expensive than traditional gyroscopes. MEMS gyroscopes are known to be susceptible to the effects of acoustic noise, in particular high frequency and high power acoustic noise. Most notably, this has been proven true in aerospace applications where the noise can reach levels in excess of 120 dB and the noise frequency can exceed 20 kHz. The typical resonant frequency for the proof mass of a MEMS gyroscope is between 3 and 20 kHz. High power, high frequency acoustic noise can disrupt the output signal of the gyroscope to the point that the output becomes unreliable. In recent years, considerable research has focused on the fascinating properties found in metamaterials. A metamaterial is an artificially fabricated device or structure that is engineered to produce desired material responses that can either mimic known behaviors or produce responses that do not occur naturally in materials found in nature. Acoustic metamaterials, in particular, have shown great promise in the field of sound attenuation. This paper proposes a method to mitigate the performance degradation of the MEMS gyroscope in the presence of high power, high frequency acoustic noise by using a new acoustic metamaterial in the form of a two-dimensional array of micromachined Helmholtz resonators. The Helmholtz resonators are fabricated in a silicon wafer using standard MEMS manufacturing techniques and are designed to attenuate sound at the resonant frequency of the gyroscope proof mass. The resonator arrays were diced from the silicon wafer in one inch squares and assembled into a box open on one end in a manner to attenuate sound on all sides of the gyroscope, and to seal the gyroscope inside the box. The resulting acoustic metamaterial device was evaluated in an acoustic chamber and was found to successfully attenuate sound as much as 18 dB. This attenuation is in the form of a notch filter at and around 14.5 kHz, which was the target frequency of attenuation. The notch filter attenuation occurred over a 700 Hz frequency band with 18 dB being the largest attenuation in the band.
    Journal of Applied Physics 01/2013; 113(2). · 2.21 Impact Factor
  • E.K. Snipes, G.T. Flowers, P. Lall, M.J. Bozack
    [Show abstract] [Hide abstract]
    ABSTRACT: (Part A) The structure zone (SZ) scheme allows for modification of the grain size of sputter deposited thin films by adjustment of the substrate temperature during deposition. Previous work by this group on whiskering has been done exclusively on room temperature sputter-deposited tin (Sn) films with a homologous temperature Ts/Tm = 0.6, where Ts is the substrate deposition temperature and Tm is the melting point of Sn (232°C). Such Zone T depositions result in a fine-grained Sn microstructure with an average grain size ~ 1 μm. In current experiments, we explore the impact on whisker growth of Sn films produced from other portions of SZ space by depositing films at a variety of (higher) substrate temperatures having Zone 2 and Zone 3 Sn microstructures, which have more columnar growth and average measured grain sizes of ~ 3 and 4.5 μm, respectively. (Part B) The role of incorporated hydrogen on the growth of tin (Sn) whiskers is currently unknown, despite suspicions for years that it plays a role in whisker incubation and growth processes. The presence of hydrogen is of particular concern for electrodeposited films, where the plating bath contains considerable amounts of hydrogen-containing molecules which are incorporated into the deposited thin film. In this work we have investigated the role of hydrogen in whiskering by implanting a roughly uniform distribution (~ 2 × 1019/cm3) of 550 keV protons into ~1.5 micron sputtered Sn films (on Si) by a particle accelerator. Prior to the implant, the Sn films were deposited under both tensile and compressive intrinsic stress conditions and each specimen had an implanted and (masked, control) non-implanted side. It is early in the incubation period, but results to date indicate no significant differences in whisker number densities between implanted and non-implanted film regions.
    Holm Conference on Electrical Contacts (HOLM) , 2013 IEEE 59th; 01/2013
  • Robert N. Dean, George T. Flowers
    [Show abstract] [Hide abstract]
    ABSTRACT: MEMS technology has greatly reduced the size and cost of gyroscopes. However, these sensors are also potentially susceptible to vibrational and acoustic excitation from their ambient operating environment. This mechanical noise can couple into the sensor and disrupt its performance in these harsh environments. No single solution will mitigate this problem for every application. Rather, a toolbox of techniques exists that each offers some degree of improvement in a MEMS gyroscope's performance in various environments. There are primarily two families of mitigation techniques. The first family involves altering the design of the sensor. For some applications, performance can be enhanced by increasing the resonating frequency of the sensor. Another technique is to embed the drive axis resonator in a feedback loop that maintains a constant amplitude of proof mass motion along the drive axis. The other family consists of techniques to enhance the device and system packaging. These techniques include passive and active vibration isolation between the sensor and the environment, mechanical noise absorbing materials and acoustic metamaterials to reduce mechanical noise transmission to the sensor. No one technique works for every sensor or every application. But a toolbox approach allows the engineer to select one of more techniques to improve gyroscope performance for a particular application.
    Reliability Science for Advanced Materials and Devices (RSAMD), 2013 IEEE Conference on; 01/2013
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ball grid array (BGA) packages have rapidly become the popular choice of manufacturers in the semiconductor industry over the past decade. The characteristics of BGA packages in a vibration environment have a deleterious effect after long term isothermal aging. The effectiveness of this characteristics was demonstrated with promising results through vibration testing of SAC 105 and 305 alloys. This experiment focuses on the vibration fatigue life of 10 mm, 15 mm and 19 mm BGA solder-joints at aging temperatures of 55°C for 6, 12 and 24 months. The deleterious effect of the on the characteristic fatigue lifetime is reported. The results show that the aging time has a direct impact on the total time to failure. The results show that the Time-To-Failure (TTF) of the solder joint decreases with aging. A step stress test approach was taken in selecting a vibration profile for the study in order to reduce the duration of the test with most failures and as to record the failure more precisely over aging across 6,12 and 24 months. The paper concludes with discussion of the deterioration intensity aging has on SAC alloys and the change in reliability over time. The results obtained in this work show that there is a significant deleterious effect on the mechanical strength of SAC alloy solder balls during elevated temperature aging. The data demonstrate that the low-silver-content alloy (SAC105) is more sensitive to elevated temperature aging.
    Advanced Packaging Materials (APM), 2013 IEEE International Symposium on; 01/2013
  • Source
    Jeff Miller, George Flowers, David Bevly
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents an approach for outdoor navigation of an autonomously guided canine using an embedded command module with vibration and tone generation capabilities and an embedded control suite comprised of a microprocessor, wireless radio, GPS receiver, and an Attitude and Heading Reference System. In order to determine the canine's motions, which inherently contain non-conventional noise characteristics, the sensor measurements were integrated using a specialized Extended Kalman Filter (EKF), equipped with a Fuzzy Logic controller for adaptive tuning of the Process Noise Covariance Matrix. This allowed for rejection of un-modelled canine motion characteristics which tend to corrupt accelerometer bias tracking in a standard EKF. The EKF solution provided an optimized estimate of the canine position and velocity and also proved to be effective in tracking the canine's position (within 7·5 m) and velocity (within 1·2 m/s) during simulated 10 second GPS outages.
    Journal of Navigation 07/2012; 65(03). · 0.62 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: MEMS gyroscopes are used in many applications including harsh environments such as high-power, high-frequency acoustic noise. If the latter is at the natural frequency of the gyroscope, the proof mass will be overexcited giving rise to a corrupted gyroscope output. To mitigate the effect of the high-power, high-frequency acoustic noise, it is proposed to use nickel microfibrous sheets as an acoustic damper. For this purpose, the characterization of vibration damping in Nickel microfibrous sheets was examined in the present research effort. The sheets were made from nickel fibers with cellulose as a binding agent using a wet-lay papermaking technique. Sintering was done at 1000 °C to remove all the cellulose giving rise to a porous material. Square sheets of 20 cm were made from three diameters of nickel fibers namely 4, 8, and 12 microns. The sheets were cut into smaller pieces to fit the requirements of a fixture specially designed for this study. The fixture was attached to a LDS V408 shaker with a mass resting on a stack of the microfibrous sheets to simulate transmitted vibration by base motion with the sheet stack acting as a damper. A series of experiments was conducted using these 3 fiber diameters, different number of layers of microfibrous sheets and varying the vibration amplitude. From the collected vibration data, the stiffness and damping ratio of the microfibrous material was characterized.
    Proc SPIE 03/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Relatively little is known about the fretting mechanism of high power connectors used in hybrid vehicles, even though the vehicles are widely being introduced to the market. This paper experimentally investigates the fretting mechanisms of silver-plated high power connectors caused by vibrations. In order to emulate operational and environmental effects, a test stand was designed that is capable of measuring electrical contact resistance (ECR), relative displacement and connector temperature. The experimental results show that the variation of ECR of connectors subject to vibration is primarily due to periodic changes of contact area caused by relative motion between the contact interfaces, rather than other fretting corrosions. This finding was reinforced by observing a good correlation between relative motion and the increase of ECR under vibration. When a vibration stops, the ECR decreased to a value that is slightly larger than the original value. A surface analysis shows no obvious corrosion until the coating is worn away. In addition, the effect of high current on the fretting mechanism is also investigated.
    IEEE Transactions on Components, Packaging, and Manufacturing Technology 01/2012; 2(2). · 1.26 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: High power connectors used in hybrid vehicles are exposed to vibrations that cause changes in connector resistance. When vibration starts, the connector resistance increases temporarily and oscillates. When vibration stops, the connector resistance returns to a value that is similar to the original state. In this paper, finite element models are developed to analyze this phenomenon and compared with experimental results. A two-dimensional finite element model was developed to predict the motions at any location of the connector system. A contact spring present between the female and male parts of the connector is modeled in three dimensions and used to analyze the time response. The analysis shows that the relative displacement is closely related to the changes of connector resistance during vibration, and the models can be used to improve connector design and ensure better performance and reliability.
    Mechanics Based Design of Structures and Machines - MECH BASED DES STRUCT MECH. 01/2012; 40(3):349-365.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Studies of tin (Sn) whisker growth from Sn solder alloys shows that SAC deposits can be whisker prone. In fact, even SnPb solders may produce whisker growth. The Sn alloy specimens consisted of 2400 A SAC305 (~3.0%Ag, 0.5%Cu and rest Sn), along with 750 and 1200 Å Sn-37Pb films, each sputtered under compressive stress conditions on electrochemically polished brass and incubated in ambient room temperature/humidity (RT/RH). After a month of incubation whisker growth is already observed on the SAC and 750 Å SnPb films. The thicker, 1200 Å, SnPb film did not whisker till after over a year of incubation with only 524 whiskers/cm2. At this time (~400 days) the 750 Å Sn-37Pb film has modest whisker numbers, while SAC has produced > 147,000 whiskers/cm2 after 590 days of incubation at RT/RH, making it apparent that whisker growth can occur on high Sn content alloy deposits. Comparing the 750 Å SnPb specimen to 750 A of pure Sn on brass after similar incubation periods, the Sn film produced ~ 1.75X the whisker density of the Sn-37Pb film, with > 5 X the average whisker length, highlighting the ability of incorporated Pb to suppress whiskers. This is in agreement with electroplated films studies, where the addition of Pb in Sn is observed to reduce film stress, mitigating whisker growth [e.g., E. Chason et. al., App. Phys. Letters, 92 (2008) 171901].
    Electronics Packaging Technology Conference (EPTC), 2012 IEEE 14th; 01/2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: Some mechanistic models of tin (Sn) whisker growth presume that surface oxidation plays an important role in whisker formation. The notion is that Sn whiskers grow through weak spots in the Sn oxide, which is cracked during whisker growth and propagation. It is often implied that a surface oxide is necessary for whisker growth. However, the premise of a surface oxide requirement for whiskering is clearly questionable. Not only have there been anecdotal reports of Au whiskers, but studies by Moon, Handwerker, et al. [31] and our laboratory show that Sn whiskers can grow from atomically clean surfaces and/or surfaces having extremely low amounts of oxygen. This paper reports a definitive test of the surface oxidation hypothesis by attempting to grow whiskers from surfaces having no native oxide (Au). The results show that high aspect ratio Au whiskers can be grown within a few weeks from intrinsically (compressively) stressed thin films of sputtered Au on silicon.
    Electrical Contacts (Holm), 2012 IEEE 58th Holm Conference on; 01/2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: Application of conformal coatings has shown promise as a practical scheme for preventing whisker growth in electronic assemblies. Coatings have been proposed utilizing elastomeric, ceramic, and metallic materials. The ability to prevent whiskers is particularly important in assemblies containing Pb-free materials and finishes. Further, many otherwise desirable COTS (commercial off the shelf) electronic components and assemblies are available only with Pb-free tin as the termination finish or Pb-free SAC solder balls under BGAs. In this work we have investigated a variety of proposed whisker-impenetrable hard metal caps through use of Auger electron spectroscopy (AES) and Rutherford backscattering spectroscopy (RBS) and discuss plausible reasons why only certain metals are effective in blocking Sn whiskers.
    Electrical Contacts (Holm), 2012 IEEE 58th Holm Conference on; 01/2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: Many microelectromechanical systems (MEMS) devices possess charged capacitor structures where the suspension system allows relative electrode motion due to internal or external stimuli. When such a device is subjected to external mechanical vibrations present in a harsh operating environment, unwanted movement between the capacitor plates can generate a noise current which is injected into the connected circuitry. This paper analyzes this phenomenon and presents a model for the dynamics of a MEMS device with capacitive plates experiencing relative motion due to external stimuli. A Fourier series expansion of the current is developed to characterize the frequency content of the signal in closed form for a given vibration frequency, and simulation and experimental results are presented.
    IEEE Transactions on Industrial Electronics 08/2011; · 6.50 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Microelectromechanical systems (MEMS) gyroscopes are typically smaller and less expensive than their macroscale counterparts. For this reason, they are being used in many new applications, including in harsh environments. It has been well documented that the performance of unprotected MEMS gyroscopes can be deleteriously affected by exposure to mechanical shock or high-frequency vibrations. The results of this investigation experimentally demonstrate that MEMS gyroscopes are also susceptible to high-power high-frequency acoustic noise when acoustic energy frequency components are close to the resonating frequency of the gyroscope's proof mass. Additionally, due to microfabrication tolerances and the resulting differences between otherwise identical devices, there can be significant differences in the acoustically sensitive bandwidth between otherwise identical MEMS gyroscopes. This phenomenon is characterized for the ADXRS300 MEMS gyroscope.
    IEEE Transactions on Industrial Electronics 08/2011; · 6.50 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The response of a MEMS device that is exposed to a harsh environment may range from an increased noise floor to a completely erroneous output to temporary or even permanent device failure. One such harsh environment is high power acoustic energy possessing high frequency components. This type of environment sometimes occurs in small aerospace vehicles. In this type of operating environment, high frequency acoustic energy can be transferred to a MEMS gyroscope die through the device packaging. If the acoustic noise possesses a sufficiently strong component at the resonant frequency of the gyroscope, it will overexcite the motion of the proof mass, resulting in the deleterious effect of corrupted angular rate measurement. Therefore if the device or system packaging can be improved to sufficiently isolate the gyroscope die from environmental acoustic energy, the sensor may find new applications in this type of harsh environment. This research effort explored the use of microfibrous metallic cloth for isolating the gyroscope die from environmental acoustic excitation. Microfibrous cloth is a composite of fused, intermingled metal fibers and has a variety of typical uses involving chemical processing applications and filtering. Specifically, this research consisted of experimental evaluations of multiple layers of packed microfibrous cloth composed of sintered nickel material. The packed cloth was used to provide acoustic isolation for a test MEMS gyroscope, the Analog Devices ADXRS300. The results of this investigation revealed that the intermingling of the various fibers of the metallic cloth provided a significant contact area between the fiber strands and voids, which enhanced the acoustic damping of the material. As a result, the nickel cloth was discovered to be an effective acoustic isolation material for this particular MEMS gyroscope.
    Proc SPIE 03/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: An experimental investigation of the damping effects in several different gas chemistries on a vibrating micromachined structure was conducted. A corresponding numerical study using an analytical model was also performed and the results correlated with the experimental observations. The analytical model is based on the compressible Reynolds equation. The theoretical and experimental quality factor show reasonable agreement for different gases. As expected, the quality factor decreases as the gas pressure increases. Both experiments and numerical predictions suggest that these various gases may not affect the mechanical behavior significantly, and could be used for their unique electrical properties, such as permittivities.Research Highlights►An experimental investigation of the damping effects in several different gas chemistries on a vibrating micromachined structure was conducted. ►A corresponding numerical study using an analytical model was also performed and the results correlated with the experimental observations. ►The analytical model is based on the compressible Reynolds equation. ►The theoretical and experimental quality factor show reasonable agreement for different gases. ►As expected, the quality factor decreases as the gas pressure increases. ►Both experiments and numerical predictions suggest that these various gases may not affect the mechanical behavior significantly, and could be used for their unique electrical properties, such as permittivities.
    Tribology International 02/2011; 44(2):125-133. · 2.17 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper explores the notion that the nucleation and growth of Sn whiskers is motivated by net compressive intrinsic thin film stresses. In this view, a threshold level of stress should exist at which Sn whiskers nucleate; furthermore, whiskering will relieve the compressive stress by a measurable amount. We examine the threshold stress for whisker nucleation and measure the amount of stress relieved during Sn whisker growth on brass substrates. The stress evolution has been evaluated by traditional bent beam analysis via novel machine vision techniques. Whisker nucleation and growth of the Cu-Sn intermetallic layer was observed by FIB sectioning, EDX mapping, and electron microscopy. Results show that the measured stress evolution shows little correlation to whisker nucleation and intermetallic growth. Further, we observe whisker population densities under both compressive and near neutral thin film stress conditions.
    01/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Studies of Sn whiskers under controlled, calibrated humidity conditions shows that the highest whisker densities occur for ~ 85% RH. The whisker specimens were 1500 Å Sn films sputtered under compressive stress conditions on silicon and electrochemically polished brass. Subsequently, the samples were exposed to a series of saturated aqueous salt solutions (which generated calibrated relative humidity values of 33, 43, 70, 76, 85, 98% RH) for ~140 days at room temperature. The Sn on brass case at 85% RH produced 6X greater whisker densities than Sn on brass exposed to pure O2, which in turn produced 9X greater whisker densities than Sn on brass exposed to ambient room temperature/humidity. The longest average whisker lengths (6.1 µm for Sn on brass and 9.3 µm for Sn on Si) occurred for 70% RH on both substrates. Corrosion features were observed on all samples, but the 98% RH samples experienced excessive corrosion. Generally, we find a dramatic increase in whisker density at > 60% RH and especially around 85% RH, in agreement with batch processed whisker experiments involving humidity (H. Reynolds et. al., IEEE Trans. Electron. Packag. Manuf. 33 (2010) and P. Oberndorff et. al., IEEE Electronic Comp. and Tech. Conference (2005) 429).
    01/2011;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Unlike their macroscale counterparts, MEMS gyroscopes use a vibrating proof mass rather than a rotational mass to sense changes in angular rate. They are also smaller and less expensive than traditional gyroscopes. For this reason, MEMS gyroscopes are being used in many new applications, some of which include operation in harsh environments. There has been much research on the negative effects of the performance of MEMS gyroscopes in environments that experience mechanical shock, high frequency vibration, and high frequency acoustic noise in air. However, MEMS gyroscopes are beginning to be used in underwater applications such as autonomous underwater vehicles, digital compasses, and torpedo guidance systems. The results of this experiment demonstrate that MEMS gyroscopes submerged in water are susceptible to high power, high frequency acoustic noise at and near the resonant frequency of the proof mass. These effects are demonstrated using the ADXRS300 MEMS gyroscope.
    ASME 2011 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference; 01/2011