259 research outputs found
Performance of the starâshaped flyer in the study of brittle materials: Three dimensional computer simulations and experimental observations
A three dimensional finite element computer simulation has been performed to assess the effects of release waves in normal impact softârecovery experiments when a starâshaped flyer plate is used. Their effects on the monitored velocityâtime profiles have been identified and their implications in the interpretation of wave spreading and spall signal events highlighted. The calculation shows that the starâshaped flyer plate indeed minimizes the magnitude of edge effects. The major perturbation to the oneâdimensional response within the central region of the target plate results from spherical waves emanating from the corners of the starâshaped plate. Experimental evidence of the development of a damage ring located in coincidence with the eight entrant corners of the flyer plate is reported. Microscopy studies performed in the intact recovered samples revealed that this damage ring eliminates undesired boundary release waves within the central region of the specimen. Consequently, the observed damage in compression and tension within this region can be attributed primarily to the conditions arising from a state of uniaxial strain.
Confluence Modulo Equivalence in Constraint Handling Rules
Previous results on proving confluence for Constraint Handling Rules are
extended in two ways in order to allow a larger and more realistic class of CHR
programs to be considered confluent. Firstly, we introduce the relaxed notion
of confluence modulo equivalence into the context of CHR: while confluence for
a terminating program means that all alternative derivations for a query lead
to the exact same final state, confluence modulo equivalence only requires the
final states to be equivalent with respect to an equivalence relation tailored
for the given program. Secondly, we allow non-logical built-in predicates such
as var/1 and incomplete ones such as is/2, that are ignored in previous work on
confluence.
To this end, a new operational semantics for CHR is developed which includes
such predicates. In addition, this semantics differs from earlier approaches by
its simplicity without loss of generality, and it may also be recommended for
future studies of CHR.
For the purely logical subset of CHR, proofs can be expressed in first-order
logic, that we show is not sufficient in the present case. We have introduced a
formal meta-language that allows reasoning about abstract states and
derivations with meta-level restrictions that reflect the non-logical and
incomplete predicates. This language represents subproofs as diagrams, which
facilitates a systematic enumeration of proof cases, pointing forward to a
mechanical support for such proofs
Performance of the starâshaped flyer in the study of brittle materials: Three dimensional computer simulations and experimental observations
A three dimensional finite element computer simulation has been performed to assess the effects of release waves in normal impact softârecovery experiments when a starâshaped flyer plate is used. Their effects on the monitored velocityâtime profiles have been identified and their implications in the interpretation of wave spreading and spall signal events highlighted. The calculation shows that the starâshaped flyer plate indeed minimizes the magnitude of edge effects. The major perturbation to the oneâdimensional response within the central region of the target plate results from spherical waves emanating from the corners of the starâshaped plate. Experimental evidence of the development of a damage ring located in coincidence with the eight entrant corners of the flyer plate is reported. Microscopy studies performed in the intact recovered samples revealed that this damage ring eliminates undesired boundary release waves within the central region of the specimen. Consequently, the observed damage in compression and tension within this region can be attributed primarily to the conditions arising from a state of uniaxial strain.
The annealing mechanism of AuGe/Ni/Au ohmic contacts to a two-dimensional electron gas in GaAs/AlGaAs heterostructures
Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/AlGaAs
heterostructures are often realized by annealing of AuGe/Ni/Au that is
deposited on its surface. We studied how the quality of this type of ohmic
contact depends on the annealing time and temperature, and how optimal
parameters depend on the depth of the 2DEG below the surface. Combined with
transmission electron microscopy and energy-dispersive X-ray spectrometry
studies of the annealed contacts, our results allow for identifying the
annealing mechanism and proposing a model that can predict optimal annealing
parameters for a certain heterostructure.Comment: 9 pages, 4 figure
Two-dimensional finite element simulation of fracture and fatigue behaviours of alumina microstructures for hip prosthesis
This paper describes a two-dimensional (2D) finite element simulation for
fracture and fatigue behaviours of pure alumina microstructures such as those
found at hip prostheses. Finite element models are developed using actual Al2O3
microstructures and a bilinear cohesive zone law. Simulation conditions are
similar to those found at a slip zone in a dry contact between a femoral head
and an acetabular cup of hip prosthesis. Contact stresses are imposed to
generate cracks in the models. Magnitudes of imposed stresses are higher than
those found at the microscopic scale. Effects of microstructures and contact
stresses are investigated in terms of crack formation. In addition, fatigue
behaviour of the microstructure is determined by performing simulations under
cyclic loading conditions. It is shown that crack density observed in a
microstructure increases with increasing magnitude of applied contact stress.
Moreover, crack density increases linearly with respect to the number of
fatigue cycles within a given contact stress range. Meanwhile, as applied
contact stress increases, number of cycles to failure decreases gradually.
Finally, this proposed finite element simulation offers an effective method for
identifying fracture and fatigue behaviours of a microstructure provided that
microstructure images are available
Growth aspirations and social capital:young firms in a post-conflict environment
This article explores the growth aspirations of owners and managers of young firms in a post-conflict economy by focusing on social capital. It treats social capital as a multidimensional, multilevel phenomenon, studying the effects of discussion network characteristics, trust in institutions, generalised trust in people and local ethnic pluralism. We argue that in a post-conflict country, ethnic pluralism is indicative of local norms of tolerance towards experimentation and risk taking which support growth aspirations. It also distinguishes between the aspirations of hired managers and owners-managers. The empirical counterpart and hypotheses testing rely on survey evidence drawn from young businesses in Bosnia and Herzegovina
Market-related reforms and increased energy efficiency in transition countries: empirical evidence
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