40,813 research outputs found
Quantitative non-destructive evaluation of composite materials based on ultrasonic wave propagation
The size, shape, and orientation of damage correlates well between the polar backscatter technique and the deply technique. There is good quantitative correlation between the areas of damage indicated by the two techniques. These results suggest that the polar backscatter technique is sensitive to specific orientations of damage. The polar backscatter technique provides a good qualitative image of the size and shape of the largest zone of damage in each of the principal orientations. A quantitative estimate of the extent of these largest damage zones is obtained from the polar backscatter technique. The selective sensitivity of polar backscatter provides a useful tool for further studies of the mechanisms of impact damage in graphite fiber reinforced composite laminates
Computer Tomograph Measurements in Shear and Gravity Particle Flows
The paper reports the recent results obtained on the applicability of cross-sectional digital
imaging method to study particle flow characteristics in 3D particle beds forced to move by
gravity or shear. X-ray CT imaging technique is widely used in medical diagnostics and, during
the last decades, its spatial and temporal resolution has been improved significantly. In this study,
an attempt was made to use this technique for engineering purposes. Two experimental set-ups
with different types of particle flows were investigated using Siemens Somatom Plus type CT
equipment. A series of trials were carried out in a small model hopper with flat bottom and
almost cylindrical side wall slightly deviating from verticality. Non steady-state flow was studied
during the outflow of particulate material from this vessel, through a central hole at the bottom.
Further investigation was fulfilled in a modified Cuette-type shearing device to study steady-state
shear flow. This equipment consisted of an almost cylindrical vessel identical to that used for
gravity flow measurements, and a smaller inner cylinder rotating within this vessel concentrically,
around its vertical axis. The surface of the inner cylinder was notched vertically, i.e.
perpendicularly to the direction of rotation to increase wall friction between the particles and the
cylinder. Almost spherical sucrose granules, also used for gravity flow measurements, were filled
into the gap between the rotating cylinder and the outer wall of the equipment. Movement of
particles took place due to shear, generated within the particle bed. By using X-ray CT technique,
cross-sectional digital images were obtained in every two seconds for both types of particle flows.
For this, the cross-sectional variation of the local Hounsfield density values were measured in a
matrix of 0.1x0.1x2.0 mm space elements. It was proved that the applied non-invasive crosssectional
imaging technique was suitable to distinguish the stationary and moving particle
regions, and by this, to estimate the location of the boundary zone between them
Two-dimensional flow of foam around a circular obstacle: local measurements of elasticity, plasticity and flow
We investigate the two-dimensional flow of a liquid foam around circular
obstacles by measuring all the local fields necessary to describe this flow:
velocity, pressure, bubble deformations and rearrangements. We show how our
experimental setup, a quasi-2D "liquid pool" system, is adapted to the
determination of these fields: the velocity and bubble deformations are easy to
measure from 2D movies, and the pressure can be measured by exploiting a
specific feature of this system, a 2D effective compressibility. To describe
accurately bubble rearrangements, we propose a new, tensorial descriptor. All
these quantities are evaluated via an averaging procedure that we justify
showing that the fluctuations of the fields are essentially random. The flow is
extensively studied in a reference experimental case; the velocity presents an
overshoot in the wake of the obstacle, the pressure is maximum at the leading
side and minimal at the trailing side. The study of the elastic deformations
and of the velocity gradients shows that the transition between plug flow and
yielded regions is smooth. Our tensorial description of T1s highlight their
correlation both with the bubble deformations and the velocity gradients. A
salient feature of the flow, notably on the velocity and T1 repartition, is a
marked asymmetry upstream/downstream, signature of the elastic behaviour of the
foam. We show that the results do not change qualitatively when various control
parameters vary, identifying a robust quasistatic regime. These results are
discussed in the frame of the actual foam rheology literature, and we argue
that they constitute a severe test for existing rheological models, since they
capture both the elastic, plastic and fluid behaviour of the foam.Comment: 41 pages, 25 figures, submitted to Journal of Fluid Mechanics (but
not in JFM style), short version of the abstrac
Experimental Investigation of Plastic Deformations Before Granular Avalanche
We present an experimental study of the deformation inside a granular
material that is progressively tilted. We investigate the deformation before
the avalanche with a spatially resolved Diffusive Wave Spectroscopy setup. At
the beginning of the inclination process, we first observe localized and
isolated events in the bulk, with a density which decreases with the depth. As
the angle of inclination increases, series of micro-failures occur periodically
in the bulk, and finally a granular avalanche takes place. The micro-failures
are observed only when the tilt angles are larger than a threshold angle much
smaller than the granular avalanche angle. We have characterized the density of
reorganizations and the localization of micro-failures. We have also explored
the effect of the nature of the grains, the relative humidity conditions and
the packing fraction of the sample. We discuss those observations in the
framework of the plasticity of granular matter. Micro-failures may then be
viewed as the result of the accumulation of numerous plastic events
Computerized Analysis of Magnetic Resonance Images to Study Cerebral Anatomy in Developing Neonates
The study of cerebral anatomy in developing neonates is of great importance for
the understanding of brain development during the early period of life. This
dissertation therefore focuses on three challenges in the modelling of cerebral
anatomy in neonates during brain development. The methods that have been
developed all use Magnetic Resonance Images (MRI) as source data.
To facilitate study of vascular development in the neonatal period, a set of image
analysis algorithms are developed to automatically extract and model cerebral
vessel trees. The whole process consists of cerebral vessel tracking from
automatically placed seed points, vessel tree generation, and vasculature
registration and matching. These algorithms have been tested on clinical Time-of-
Flight (TOF) MR angiographic datasets.
To facilitate study of the neonatal cortex a complete cerebral cortex segmentation
and reconstruction pipeline has been developed. Segmentation of the neonatal
cortex is not effectively done by existing algorithms designed for the adult brain
because the contrast between grey and white matter is reversed. This causes pixels
containing tissue mixtures to be incorrectly labelled by conventional methods. The
neonatal cortical segmentation method that has been developed is based on a novel
expectation-maximization (EM) method with explicit correction for mislabelled
partial volume voxels. Based on the resulting cortical segmentation, an implicit
surface evolution technique is adopted for the reconstruction of the cortex in
neonates. The performance of the method is investigated by performing a detailed
landmark study.
To facilitate study of cortical development, a cortical surface registration algorithm
for aligning the cortical surface is developed. The method first inflates extracted
cortical surfaces and then performs a non-rigid surface registration using free-form
deformations (FFDs) to remove residual alignment. Validation experiments using
data labelled by an expert observer demonstrate that the method can capture local
changes and follow the growth of specific sulcus
Internet and gaming addiction: a systematic literature review of neuroimaging studies
In the past decade, research has accumulated suggesting that excessive Internet use can lead to the development of a behavioral addiction. Internet addiction has been considered as a serious threat to mental health and the excessive use of the Internet has been linked to a variety of negative psychosocial consequences. The aim of this review is to identify all empirical studies to date that used neuroimaging techniques to shed light upon the emerging mental health problem of Internet and gaming addiction from a neuroscientific perspective. Neuroimaging studies offer an advantage over traditional survey and behavioral research because with this method, it is possible to distinguish particular brain areas that are involved in the development and maintenance of addiction. A systematic literature search was conducted, identifying 18 studies. These studies provide compelling evidence for the similarities between different types of addictions, notably substance-related addictions and Internet and gaming addiction, on a variety of levels. On the molecular level, Internet addiction is characterized by an overall reward deficiency that entails decreased dopaminergic activity. On the level of neural circuitry, Internet and gaming addiction led to neuroadaptation and structural changes that occur as a consequence of prolonged increased activity in brain areas associated with addiction. On a behavioral level, Internet and gaming addicts appear to be constricted with regards to their cognitive functioning in various domains. The paper shows that understanding the neuronal correlates associated with the development of Internet and gaming addiction will promote future research and will pave the way for the development of addiction treatment approaches
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