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Omer Civalek - One of the best experts on this subject based on the ideXlab platform.
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vibrational characteristics of embedded Microbeams lying on a two parameter elastic foundation in thermal environment
Composites Part B-engineering, 2018Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In the present work, thermo-elastic vibrational behavior of thick Microbeams embedded in a two-parameter elastic foundation is studied. A Winkler-Pasternak type elastic foundation model is employed to simulate the interactions between microbeam and elastic medium. Size-dependent constitutive equations and associated boundary conditions are obtained by applying dynamic version of virtual work's principle based on modified couple stress and various beam theories. Several numerical examples are presented to examine the sensibility of various parameters associated with slenderness ratio, temperature rise, length scale, Winkler and shear layer parameters on the natural frequencies and critical temperature point of embedded Microbeams.
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Effects of thermal and shear deformation on vibration response of functionally graded thick composite Microbeams
Composites Part B: Engineering, 2017Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In this paper, thermal and shear deformation effects on the vibrational response of non-homogeneous Microbeams made of functionally graded (FG) materials are carried out. It is assumed that the temperature-dependent material properties of FG Microbeams change smoothly and gradually throughout the height according to the classical rule of mixture. The governing differential equations and related boundary conditions are derived by implementing Hamilton's principle on the basis of hyperbolic shear deformation beam and modified couple stress theories and they are analytically solved. The results are given together with other beam theories. A detailed parametric study is performed to indicate the influences of slenderness ratio, material length scale parameter, gradient index, shear correction factors and temperature rise on natural frequencies of FG Microbeams. It is revealed that the use of modified shear correction factor can provide more accurate and valid results for first-order shear deformable microbeam model.
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thermo mechanical buckling behavior of functionally graded Microbeams embedded in elastic medium
International Journal of Engineering Science, 2014Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract Thermo-mechanical size-dependent buckling analysis of embedded functionally graded (FG) Microbeams is performed based on sinusoidal shear deformation beam and modified couple stress theories. It is assumed that material properties vary smoothly and continuously throughout the thickness. Winkler elastic foundation model is used to simulate the interaction between FG microbeam and elastic medium. The governing equations and corresponding boundary conditions are obtained with the aid of minimum total potential energy principle. The buckling characteristics of simply supported embedded FG Microbeams in thermal environment are investigated. The obtained results are compared with the results of simple beam theory with no shear deformation effects and classical theory. Influences of thickness-to-material length scale parameter ratio, material property gradient index, slenderness ratio, temperature change and Winkler parameter on critical buckling loads of embedded FG Microbeams are discussed in detail.
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a new trigonometric beam model for buckling of strain gradient Microbeams
International Journal of Mechanical Sciences, 2014Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In this paper, a new microstructure-dependent sinusoidal beam model for buckling of Microbeams is presented using modified strain gradient theory. This microbeam model can take into consideration microstructural and shear deformation effects. The equilibrium equations and corresponding boundary conditions in buckling are derived with the minimum total potential energy principle. Buckling problem of a simply supported microbeam subjected to an axial compressive force is analytically solved by Navier solution procedure. Influences of thickness-to-length scale parameter and slenderness ratios on buckling behavior are discussed in detail. It is observed that the size dependency becomes more important when the thickness of the microbeam is closer to material length scale parameter. Also, it can be said that the effects of shear deformation are more considerable for short and thick beams with lower slenderness ratios.
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free vibration analysis of axially functionally graded tapered bernoulli euler Microbeams based on the modified couple stress theory
Composite Structures, 2013Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In the present study, vibration response of non-homogenous and non-uniform Microbeams is investigated in conjunction with Bernoulli–Euler beam and modified couple stress theory. The boundary conditions of the microbeam are considered as fixed at one end and free at the other end. It is taken into consideration that material properties and cross section of the microbeam vary continuously along the longitudinal direction. Rayleigh–Ritz solution method is utilized to obtain an approximate solution to the free transverse vibration problem. A detailed study is carried out to show the effects of material properties and taper ratios on natural frequencies of axially functionally graded tapered Microbeams. In order to demonstrate the validity and accuracy of the current analysis, some of present results are compared with previous results in the literature and an excellent agreement is observed between them.
Bekir Akgoz - One of the best experts on this subject based on the ideXlab platform.
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vibrational characteristics of embedded Microbeams lying on a two parameter elastic foundation in thermal environment
Composites Part B-engineering, 2018Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In the present work, thermo-elastic vibrational behavior of thick Microbeams embedded in a two-parameter elastic foundation is studied. A Winkler-Pasternak type elastic foundation model is employed to simulate the interactions between microbeam and elastic medium. Size-dependent constitutive equations and associated boundary conditions are obtained by applying dynamic version of virtual work's principle based on modified couple stress and various beam theories. Several numerical examples are presented to examine the sensibility of various parameters associated with slenderness ratio, temperature rise, length scale, Winkler and shear layer parameters on the natural frequencies and critical temperature point of embedded Microbeams.
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Effects of thermal and shear deformation on vibration response of functionally graded thick composite Microbeams
Composites Part B: Engineering, 2017Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In this paper, thermal and shear deformation effects on the vibrational response of non-homogeneous Microbeams made of functionally graded (FG) materials are carried out. It is assumed that the temperature-dependent material properties of FG Microbeams change smoothly and gradually throughout the height according to the classical rule of mixture. The governing differential equations and related boundary conditions are derived by implementing Hamilton's principle on the basis of hyperbolic shear deformation beam and modified couple stress theories and they are analytically solved. The results are given together with other beam theories. A detailed parametric study is performed to indicate the influences of slenderness ratio, material length scale parameter, gradient index, shear correction factors and temperature rise on natural frequencies of FG Microbeams. It is revealed that the use of modified shear correction factor can provide more accurate and valid results for first-order shear deformable microbeam model.
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thermo mechanical buckling behavior of functionally graded Microbeams embedded in elastic medium
International Journal of Engineering Science, 2014Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract Thermo-mechanical size-dependent buckling analysis of embedded functionally graded (FG) Microbeams is performed based on sinusoidal shear deformation beam and modified couple stress theories. It is assumed that material properties vary smoothly and continuously throughout the thickness. Winkler elastic foundation model is used to simulate the interaction between FG microbeam and elastic medium. The governing equations and corresponding boundary conditions are obtained with the aid of minimum total potential energy principle. The buckling characteristics of simply supported embedded FG Microbeams in thermal environment are investigated. The obtained results are compared with the results of simple beam theory with no shear deformation effects and classical theory. Influences of thickness-to-material length scale parameter ratio, material property gradient index, slenderness ratio, temperature change and Winkler parameter on critical buckling loads of embedded FG Microbeams are discussed in detail.
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a new trigonometric beam model for buckling of strain gradient Microbeams
International Journal of Mechanical Sciences, 2014Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In this paper, a new microstructure-dependent sinusoidal beam model for buckling of Microbeams is presented using modified strain gradient theory. This microbeam model can take into consideration microstructural and shear deformation effects. The equilibrium equations and corresponding boundary conditions in buckling are derived with the minimum total potential energy principle. Buckling problem of a simply supported microbeam subjected to an axial compressive force is analytically solved by Navier solution procedure. Influences of thickness-to-length scale parameter and slenderness ratios on buckling behavior are discussed in detail. It is observed that the size dependency becomes more important when the thickness of the microbeam is closer to material length scale parameter. Also, it can be said that the effects of shear deformation are more considerable for short and thick beams with lower slenderness ratios.
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free vibration analysis of axially functionally graded tapered bernoulli euler Microbeams based on the modified couple stress theory
Composite Structures, 2013Co-Authors: Bekir Akgoz, Omer CivalekAbstract:Abstract In the present study, vibration response of non-homogenous and non-uniform Microbeams is investigated in conjunction with Bernoulli–Euler beam and modified couple stress theory. The boundary conditions of the microbeam are considered as fixed at one end and free at the other end. It is taken into consideration that material properties and cross section of the microbeam vary continuously along the longitudinal direction. Rayleigh–Ritz solution method is utilized to obtain an approximate solution to the free transverse vibration problem. A detailed study is carried out to show the effects of material properties and taper ratios on natural frequencies of axially functionally graded tapered Microbeams. In order to demonstrate the validity and accuracy of the current analysis, some of present results are compared with previous results in the literature and an excellent agreement is observed between them.
Alberto Bravin - One of the best experts on this subject based on the ideXlab platform.
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Rat sensorimotor cortex tolerance to parallel transections induced by synchrotron-generated X-ray Microbeams
Scientific Reports, 2017Co-Authors: Erminia Fardone, Alberto Bravin, Elke Bräuer-krisch, Alfredo Conti, Herwig Requardt, Domenico Bucci, Giuseppe Battaglia, Pantaleo RomanelliAbstract:Microbeam radiation therapy is a novel preclinical technique, which uses synchrotron-generated X-rays for the treatment of brain tumours and drug-resistant epilepsies. In order to safely translate this approach to humans, a more in-depth knowledge of the long-term radiobiology of Microbeams in healthy tissues is required. We report here the result of the characterization of the rat sensorimotor cortex tolerance to microradiosurgical parallel transections. Healthy adult male Wistar rats underwent irradiation with arrays of parallel Microbeams. Beam thickness, spacing and incident dose were 100 or 600 µm, 400 or 1200 µm and 360 or 150 Gy, respectively. Motor performance was carried over a 3-month period. Three months after irradiation rats were sacrificed to evaluate the effects of irradiation on brain tissues by histology and immunohistochemistry. Microbeam irradiation of sensorimotor cortex did not affect weight gain and motor performance. No gross signs of paralysis or paresis were also observed. The cortical architecture was not altered, despite the presence of cell death along the irradiation path. Reactive gliosis was evident in the microbeam path of rats irradiated with 150 Gy, whereas no increase was observed in rats irradiated with 360 Gy.
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Response of the rat spinal cord to X-ray Microbeams.
Radiotherapy and Oncology, 2013Co-Authors: Jean Laissue, Alberto Bravin, Stefan Bartzsch, Elke Bräuer-krisch, Valentin Djonov, Hans Blattmann, Dominique Dalléry, Albert Hanson, John Hopewell, Barbara Kaser-hotzAbstract:BACKGROUND AND PURPOSE: To quantify the late dose-related responses of the rat cervical spinal cord to X-ray irradiations by an array of Microbeams or by a single millimeter beam. MATERIALS AND METHODS: Necks of anesthetized rats were irradiated transversely by an 11 mm wide array of 52 parallel, 35 μm wide, vertical X-ray Microbeams, separated by 210 μm intervals between centers. Comparison was made with rats irradiated with a 1.35 mm wide single beam of similar X-rays. Rats were killed when paresis developed, or up to 383 days post irradiation (dpi). RESULTS: Microbeam peak/valley doses of ≈357/12.7 Gy to 715/25.4 Gy to an 11 mm long segment of the spinal cord, or single beam doses of ≈146-454 Gy to a 1.35 mm long segment caused foreleg paresis and histopathologically verified spinal cord damage; rats exposed to peak/valley doses up to 253/9 Gy were paresis-free at 383 dpi. CONCLUSIONS: Whereas microbeam radiation therapy [MRT] for malignant gliomas implanted in rat brains can be safe, palliative or curative, the high tolerance of normal rat spinal cords to similar microbeam exposures justifies testing MRT for autochthonous malignancies in the central nervous system of larger animals with a view to subsequent clinical applications.
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Microbeam radiosurgery using synchrotron-generated submillimetric beams: a new tool for the treatment of brain disorders
Neurosurgical Review, 2011Co-Authors: David J. Anschel, Alberto Bravin, Pantaleo RomanelliAbstract:Since its advent during the mid-twentieth century, radiosurgery has undergone a steady evolution. Gamma Knife and linear accelerator based systems using rigid frames preceded the development of frameless devices. The present report describes the development of microbeam radiosurgery, a technique which uses submillimetric beams of radiation to treat disease. Typically, the technique is employed using parallel arrays of beams delivered via a high-fluence synchrotron source. Beam widths between 20 and 950 μm have been used with the majority of studies utilizing beam widths less than 100 μm. In addition to its high precision, the technique allows users to take advantage of two unique properties of Microbeams. The first is a remarkable tolerance of healthy tissue to Microbeams delivered at doses up to several hundred grays, while at the same time, tumors are highly susceptible to the lethal effects of Microbeams. Together, these findings allow for a “preferential tumoricidal effect” beyond the typical dose–volume relationship. Although only used in animal experiments so far, we explore the hypothetical clinical role of microbeam radiosurgery which may be feasible in the near future. In addition to the treatment of traditional radiosurgery targets such as malignancies and vascular malformations, Microbeams may allow the non-invasive treatment of functional disease such as movement disorders, epilepsy, and mental illness.
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Dosimetry of intensive synchrotron Microbeams
Radiation Measurements, 2011Co-Authors: Michael L F Lerch, Alberto Bravin, Elke Bräuer-krisch, Herwig Requardt, Marco Petasecca, A P Cullen, A Hamad, Vladimir Perevertaylo, Anatoly B RosenfeldAbstract:Abstract Intensive synchrotron X-ray Microbeams form an integral part of microbeam radiation therapy (MRT). MRT is a novel radiation medicine modality being developed for inoperable and otherwise untreatable brain tumours. The extremely high dose rate (∼20 kGy/s), laterally fractionated radiation field and steep dose gradients utilized in this therapy make real-time dosimetry a significant challenge. In order for this treatment to advance to the clinical trial stage of development real-time dosimetry systems must be developed. This paper demonstrates the capabilities of a new dosimetry system based on an epitaxial silicon detector. The system combines high spatial resolution and real-time readout and we have measured the lateral dose profile of the MRT radiation field which incorporates 59 X-ray Microbeams. All microbeam peaks and valley regions between two Microbeams are clearly resolved. The measured detector response at any point is reproducible to within 0.5% after scaling for the known synchrotron storage ring beam current lifetime. The variation of the lateral dose profile at different depths in a PMMA phantom has been measured with the results compared to those from Penelope Monte Carlo simulations. The trend in the measured response with depth agrees with the simulation data (within the experimental variation of the central five Microbeams peaks and valleys measured). However the measured peak-to-valley ratio response is a factor of 4.5 ± 0.1 times lower than that expected. The disagreement was further investigated and shown to be contributed to by charge recombination effects at the low bias voltages used.
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Determination of dosimetrical quantities used in microbeam radiation therapy (MRT) with Monte Carlo simulations
Medical Physics, 2006Co-Authors: E A Siegbahn, E. Brauer-krisen, Jan Stepanek, Alberto BravinAbstract:Microbeam radiation therapy (MRT) is being performed by using an array of narrow rectangular x-ray beams (typical beam sizes 25 microm X 1 cm), positioned close to each other (typically 200 microm separation), to irradiate a target tissue. The ratio of peak-to-valley doses (PVDR's) in the composite dose distribution has been found to be strongly correlated with the normal tissue tolerance and the therapeutic effect of MRT. In this work a Monte Carlo (MC) study of the depth- and lateral-dose profiles in water for single x-ray Microbeams of different shapes and energies has been performed with the MC code PENELOPE. The contributions to the dose deposition from different interaction types have been determined at different distances from the center of the microbeam. The dependence of the peak dose, in a water phantom, on the microbeam field size used in the preclinical trials, has been demonstrated. Composite dose distributions for an array of Microbeams were obtained using superposition algorithms and PVDR's were determined and compared with literature results obtained with other Monte Carlo codes. The dependence of the PVDR's on microbeam width, x-ray energy used, and on the separation between adjacent Microbeams has been studied in detail.
Henry Guckel - One of the best experts on this subject based on the ideXlab platform.
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Sealed-cavity resonant microbeam accelerometer
Sensors and Actuators A: Physical, 1996Co-Authors: D.w Burns, William R. Herb, R. D. Horning, Jonathan D Zook, Henry GuckelAbstract:Resonant microbeam strain-sensing elements have been combined with a highly symmetric multiwafer silicon microstructure to form the first micromachined accelerometer with a polysilicon resonant microbeam pickoff. The MARIMBA (micromachined resonant integrated microbeam accelerometer) uniquely combines bulk-micromachining with advanced surface-micromachining techniques and a multiple-wafer structure to produce an all-silicon acceleration sensor capsule with a direct acceleration-to-digital frequency output. The Microbeams are fabricated from thin films of fine-grained polysilicon with integral polysilicon vacuum encapsulation, electrostatic drive and piezoresistive sense. The silicon microstructure features a dual open-web suspension system formed monolithically with the silicon proof mass and frame. Silicon caps provide squeeze-film gas damping, overrange protection, and environmental protection. Initial devices have been fabricated with scale factors greater than 700 Hz g-1 on +/-20g devices with base microbeam frequencies of 500-700 kHz. Root Allan variances below 0.1 Hz and seven-day stability measurements less than 2 ppm of the base frequency on test Microbeams indicate that milli-g stability and micro-g sensitivities are attainable.
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Optically excited self-resonant Microbeams
Sensors and Actuators A: Physical, 1996Co-Authors: J. David Zook, William R. Herb, Henry Guckel, David W. Burns, Joon-won Kang, Yongchul AhnAbstract:Abstract Optically excited self-resonance of polysilicon Microbeams sealed in a cofabricated vacuum enclosure has been achieved. Unmodulated low-power laser diodes from 650 to 840 nm have been used to excite resonances ranging from 65 to 750 kHz on Microbeams ranging from 0.79 to 2.38 μm in thickness. The photovoltaic excitation mechanism uses a p-n junction photodiode underneath the microbeam. The structure forms an effective optomechanical modulator at the microbeam resonance frequency, and the resonance can be readily detected with the reflected laser light, which is modulated at levels that can approach 100%. Analysis of the conditions for self-resonance gives predictions of minimum Q -values for self-resonance. Observed Q -values (20 000 to 130 000) are well in excess of the required values. Thicknesses of the microbeam and vacuum gaps above and below it are critical for achieving low oscillation thresholds, which may be as low as 1 μW of optical power. The clamped—clamped Microbeams are sensitive strain transducers with high gage factors, low temperature sensitivity, and wide dynamic range. These are the first optically powered active devices to achieve gain by interchanging optical, electrical, and mechanical energy in a merged structure. They uniquely combine silicon microfabrication technology with optoelectronic technology and can form the basis for a new class of fiber-optic sensors for pressure, temperature, acceleration, and other variables that can be converted to a strain using an appropriate silicon microstructure.
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sealed cavity resonant microbeam pressure sensor
Sensors and Actuators A-physical, 1995Co-Authors: David W. Burns, William R. Herb, J D Zook, Robert D Horning, Henry GuckelAbstract:A quasi-digital pressure sensor based on polysilicon resonant Microbeams has been demonstrated. Pressure sensitivities of nearly 4000 counts per second per psi have been attained on a 10 psi device with a base frequency of 233 000 Hz. Short-term stability as low as 0.01 ppm of the base frequency is typical. The Microbeams are fabricated with their own integral vacuum cavities, allowing high-Q operation in the differential pressure mode or in contact with liquids such as silicone oil. Design considerations include the effects of internal strain and lead to a push-pull layout configuration independent of microbeam strain or diaphragm thickness. Fabrication technology incorporates fine-grained polysilicon, surface micromachining, bulk micromachining, and reactive sealing. Packaging into precision avionics headers is being used for preliminary testing. Testing results indicate suitability for precision avionics, industrial, and commercial applications. Optical methods have been used to test resonant microbeam pressure sensors and verify the push-pull design methodology. Testing methods developed under this effort include electrostatic drive/piezoresistive sensing, optical drive/optical sensing, substrate piezoelectric drive/optical sensing, and electrostatic drive/laser vibrometer sensing. Wafer-level testing of 200 μm×46 μm×1.9 μm Microbeams shows an average fundamental frequency of 553 150 and first overtone of 1 332 550 Hz. The standard deviations across the wafer are 0.15 and 0.10%, respectively. The internal strain and effective thickness can be determined with high resolution. Laser vibrometer measurements through the microbeam shell verify the fundamental frequency and reveal at least ten overtones up to 25 MHz.
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Optically resonant Microbeams
Micro-Optics Micromechanics and Laser Scanning and Shaping, 1995Co-Authors: J. David Zook, David W. Burns, Jeffrey N. Schoess, Henry GuckelAbstract:Polysilicon Microbeams in integral vacuum enclosures on silicon substrates have optical and mechanical properties that provide excellent opportunities for fiber-optic sensors. The microbeam, shell, and silicon substrate form a structure with Fabry-Perot-like properties that functions as an optomechanical modulator. When the beam vibrates incident light is modulated and reflected light is used to sense vibration of the beam. Thus, the structure can be used as a mechanical vibration or acoustic emission sensor. Microbeams attached to the substrate at both ends are highly strain sensitive and form the basis of a variety of sensors, including pressure sensors, accelerometers, strain, vibration, and temperature sensors. Excitation at the wafer level by a polymer film piezoelectric transducer provides a simple non-contact optical method for testing the Microbeams before the water is cut into sensor die. Modulated light from a laser diode can also be use to excite the Microbeams into resonance. The test results suggest that optically resonant Microbeams can be used for low-cost precision fiber-optic sensors. Fiber-optic sensors are especially attractive for aerospace applications because optical fibers provide wide-bandwidth communication capability while eliminating electromagnetic interference (EMI), ground loops, and shielding requirements.
Jean A Laissue - One of the best experts on this subject based on the ideXlab platform.
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mosfet dosimetry for microbeam radiation therapy at the european synchrotron radiation facility
Medical Physics, 2003Co-Authors: Elke Brauerkrisch, Alberto Bravin, Michael L F Lerch, Anatoly B Rosenfeld, J Stepanek, M Di Michiel, Jean A LaissueAbstract:Preclinical experiments are carried out with ∼20–30 μm wide, ∼10 mm high parallel Microbeams of hard, broad-“white”-spectrum x rays (∼50–600 keV) to investigate microbeam radiation therapy(MRT) of braintumors in infants for whom other kinds of radiotherapy are inadequate and/or unsafe. Novel physical microdosimetry (implemented with MOSFET chips in the “edge-on” mode) and Monte Carlo computer-simulated dosimetry are described here for selected points in the peak and valley regions of a microbeam-irradiated tissue-equivalent phantom. Such microbeam irradiation causes minimal damage to normal tissues, possible because of rapid repair of their microscopic lesions. Radiation damage from an array of parallel Microbeams tends to correlate with the range of peak-valley dose ratios (PVDR). This paper summarizes comparisons of our dosimetricMOSFET measurements with Monte Carlo calculations. Peak doses at depths 22 mm and valley doses at all depths investigated (2 mm–62 mm) are within 2–13 % of the Monte Carlo values. These results lend credence to the use of MOSFET detector systems in edge-on mode for microplanar irradiation dosimetry.
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mosfet dosimetry for microbeam radiation therapy at the european synchrotron radiation facility
Medical Physics, 2003Co-Authors: Elke Brauerkrisch, Alberto Bravin, Michael L F Lerch, Anatoly B Rosenfeld, J Stepanek, M Di Michiel, Jean A LaissueAbstract:Preclinical experiments are carried out with approximately 20-30 microm wide, approximately 10 mm high parallel Microbeams of hard, broad-"white"-spectrum x rays (approximately 50-600 keV) to investigate microbeam radiation therapy (MRT) of brain tumors in infants for whom other kinds of radiotherapy are inadequate and/or unsafe. Novel physical microdosimetry (implemented with MOSFET chips in the "edge-on" mode) and Monte Carlo computer-simulated dosimetry are described here for selected points in the peak and valley regions of a microbeam-irradiated tissue-equivalent phantom. Such microbeam irradiation causes minimal damage to normal tissues, possible because of rapid repair of their microscopic lesions. Radiation damage from an array of parallel Microbeams tends to correlate with the range of peak-valley dose ratios (PVDR). This paper summarizes comparisons of our dosimetric MOSFET measurements with Monte Carlo calculations. Peak doses at depths 22 mm and valley doses at all depths investigated (2 mm-62 mm) are within 2-13% of the Monte Carlo values. These results lend credence to the use of MOSFET detector systems in edge-on mode for microplanar irradiation dosimetry.