5,662 research outputs found
Mesoscale modelling of the dynamic tensile strength enhancement of concrete in spalling tests using interface elements
The dynamic tensile strength of concrete has been experimentally reported to increase significantly with the increase of imposed strain rates. However, the intrinsic mechanisms accounting for the strength increase are not well understood so far. This paper presented numerical simulations based on the spalling technique to further explore mechanisms of the dynamic responses of concrete under impact loadings. Numerical results have been verified and validated against experimental evidence with various strain rates. The validity of utilizing the Novikov acoustic processing approximation for obtaining the spalling strength of concrete is identified and discussed. Results demonstrate that this indirect processing approach could overestimate the spalling strength because real material behavior tends to deviate from its basic assumption. Mechanisms accounting for the spalling strength increase from key aspects including the meso-structure, the strain rate-dependent material behaviour, the micro-crack inertia, and the structural inertial are also identified accordingly. Results demonstrate that the increment of concrete dynamic tensile strength in spalling tests is mainly caused by the strain rate-dependent material behaviour which should be incorporated in the material constitutive description. Besides that, the material heterogeneity also makes a considerable contribution to the increase of dynamic tensile strength in spalling tests and this contribution becomes increasingly prominent with the increase of the imposed strain rates. On the other hand, the structure inertial and the micro-crack inertial have little effect on the increase of spalling strength of concrete and thus may be ignored within the imposed strain rate range in spalling tests.</p
Semidiscrete Modeling of Systems of Wedge Disclinations and Edge Dislocations via the Airy Stress Function Method
We present a variational theory for lattice defects of rotational and translational
type. We focus on finite systems of planar wedge disclinations, disclination dipoles, and edge dislocations,
which we model as the solutions to minimum problems for isotropic elastic energies under
the constraint of kinematic incompatibility. Operating under the assumption of planar linearized
kinematics, we formulate the mechanical equilibrium problem in terms of the Airy stress function,
for which we introduce a rigorous analytical formulation in the context of incompatible elasticity.
Our main result entails the analysis of the energetic equivalence of systems of disclination dipoles and
edge dislocations in the asymptotics of their singular limit regimes. By adopting the regularization
approach via core radius, we show that, as the core radius vanishes, the asymptotic energy expansion
for disclination dipoles coincides with the energy of finite systems of edge dislocations. This proves
that Eshelby's kinematic characterization of an edge dislocation in terms of a disclination dipole is
exact also from the energetic standpoint
Temperature Reduction Technologies Meet Asphalt Pavement: Green and Sustainability
This Special Issue, "Temperature Reduction Technologies Meet Asphalt Pavement: Green and Sustainability", covers various subjects related to advanced temperature reduction technologies in bituminous materials. It can help civil engineers and material scientists better identify underlying views for sustainable pavement constructions
Enhancement of Cavitation Intensity in Co-Flow and Ultrasonic Cavitation Peening
Water cavitation peening is a surface treatment process used to generate beneficial compressive residual stresses while being environmentally sustainable. Compressive residual stresses generated by the collapse of the cavitation cloud at the workpiece surface result in enhanced high cycle fatigue and wear performance. Co-flow water cavitation peening, a variant of cavitation peening involves injection of a high-speed jet into a low-speed jet of water, which makes the process amenable to automation and imparts the variant with the ability to process large structural components. Ultrasonic cavitation peening, another variant of cavitation peening, is used for peening small areas. However, an increase in cavitation intensity is needed to reduce the processing time for practical applications and to enhance process capabilities for a wide range of materials in both these variants. An experimental investigation along with numerical modelling is presented to demonstrate cavitation intensity enhancement through suitable modifications to the inner jet nozzle design in co-flow water cavitation peening. Particularly, the effects of upstream inner jet organ pipe nozzle geometry, inner jetâŻnozzle orifice taper, and inner jetâŻnozzle orifice length are studied to show enhanced cavitation intensity, measured via extended mass loss tests, strip curvature and residual stress measurements, high-speed videography, and impulse pressure measurements.
It is found that the optimum inner jet organ pipe nozzle design, which generates enhanced pressure fluctuations through the introduction of a resonating chamber in the upstream section of the inner jet nozzle, generates 61% greater mass loss compared to the unexcited inner jet nozzle. Strip curvature, high speed imaging, and impulse pressure measurements support the mass loss results. Finally, residual stresses generated with the optimum organ pipe nozzle are shown to be deeper and more compressive than those generated with the unexcited nozzle design.
The inner jet nozzle variants with diverging, zero and converging tapers are investigated experimentally and numerically to understand their influence on cavitation intensity. It is shown that the converging taper nozzle generates greater cavitation intensity, measured via mass loss and strip curvature measurements, than the zero and diverging taper nozzles. Impulse pressure measurements show the greater frequency of high-intensity events generated by the converging taper nozzle compared to the zero and diverging taper nozzles. Computational fluid dynamics (CFD) simulations help explain the experimental findings.
Four nozzle variants with varying inner jet nozzle orifice length to orifice diameter ratios of 1,2,5 and 10 are investigated experimentally and numerically. The inner jet nozzle with an orifice length to orifice diameter ratio of 2 is shown to generate greater cavitation intensity than the other inner jet nozzles.
A PEO aqueous solution (cavitation media) with 1000 parts per million by weight (wppm) polymer concentration is shown to enhance cavitation intensity by 69% over cavitation media with only water. High speed videography, impulse force, and surface roughness measurements confirm the greater cavitation activity in the 1000 wppm PEO aqueous solution. This demonstrates that suitable modifications can be engineered in the cavitation media to enhance cavitation intensity in ultrasonic cavitation peening.
Thus, this thesis presents experimental and numerical investigations leading to superior inner jet nozzle design in co-flow cavitation peening and an experimental investigation of the role of polymer additives for suitable modification of cavitation media to enhance cavitation intensity in ultrasonic cavitation peening.Ph.D
An Optimized, Easy-to-use, Open-source GPU Solver for Large-scale Inverse Homogenization Problems
We propose a high-performance GPU solver for inverse homogenization problems
to design high-resolution 3D microstructures. Central to our solver is a
favorable combination of data structures and algorithms, making full use of the
parallel computation power of today's GPUs through a software-level design
space exploration. This solver is demonstrated to optimize homogenized
stiffness tensors, such as bulk modulus, shear modulus, and Poisson's ratio,
under the constraint of bounded material volume. Practical high-resolution
examples with 512^3(134.2 million) finite elements run in less than 32 seconds
per iteration with a peak memory of 21 GB. Besides, our GPU implementation is
equipped with an easy-to-use framework with less than 20 lines of code to
support various objective functions defined by the homogenized stiffness
tensors. Our open-source high-performance implementation is publicly accessible
at https://github.com/lavenklau/homo3d
The anisotropic grain size effect on the mechanical response of polycrystals: The role of columnar grain morphology in additively manufactured metals
Additively manufactured (AM) metals exhibit highly complex microstructures,
particularly with respect to grain morphology which typically features
heterogeneous grain size distribution, anomalous and anisotropic grain shapes,
and the so-called columnar grains. In general, the conventional morphological
descriptors are not suitable to represent complex and anisotropic grain
morphology of AM microstructures. The principal aspect of microstructural grain
morphology is the state of grain boundary spacing or grain size whose effect on
the mechanical response is known to be crucial. In this paper, we formally
introduce the notions of axial grain size and grain size anisotropy as robust
morphological descriptors which can concisely represent highly complex grain
morphologies. We instantiated a discrete sample of polycrystalline aggregate as
a representative volume element (RVE) which has random crystallographic
orientation and misorientation distributions. However, the instantiated RVE
incorporates the typical morphological features of AM microstructures including
distinctive grain size heterogeneity and anisotropic grain size owing to its
pronounced columnar grain morphology. We ensured that any anisotropy arising in
the macroscopic mechanical response of the instantiated sample is mainly
associated with its underlying anisotropic grain size. The RVE was then used
for meso-scale full-field crystal plasticity simulations corresponding to
uniaxial tensile deformation along different axes via a spectral solver and a
physics-based crystal plasticity constitutive model. Through the numerical
analyses, we were able to isolate the contribution of anisotropic grain size to
the anisotropy in the mechanical response of polycrystalline aggregates,
particularly those with the characteristic complex grain morphology of AM
metals. Such a contribution can be described by an inverse square relation
Mixed formulation for the computation of Miura surfaces with Dirichlet boundary conditions
Miura surfaces are the solutions of a constrained nonlinear elliptic system
of equations. This system is derived by homogenization from the Miura fold,
which is a type of origami fold with multiple applications in engineering. A
previous inquiry, gave suboptimal conditions for existence of solutions and
proposed an -conformal finite element method to approximate them. In this
paper, the existence of Miura surfaces is studied using a mixed formulation. It
is also proved that the constraints propagate from the boundary to the interior
of the domain for well-chosen boundary conditions. Then, a numerical method
based on a least-squares formulation, Taylor--Hood finite elements and a Newton
method is introduced to approximate Miura surfaces. The numerical method is
proved to converge at order one in space and numerical tests are performed to
demonstrate its robustness
Homogenization of elastomers filled with liquid inclusions: The small-deformation limit
This paper presents the derivation of the homogenized equations that describe
the macroscopic mechanical response of elastomers filled with liquid inclusions
in the setting of small quasistatic deformations. The derivation is carried out
for materials with periodic microstructure by means of a two-scale asymptotic
analysis. The focus is on the non-dissipative case when the elastomer is an
elastic solid, the liquid making up the inclusions is an elastic fluid, the
interfaces separating the solid elastomer from the liquid inclusions are
elastic interfaces featuring an initial surface tension, and the inclusions are
initially -spherical () in shape. Remarkably, in spite of the
presence of local residual stresses within the inclusions due to an initial
surface tension at the interfaces, the macroscopic response of such filled
elastomers turns out to be that of a linear elastic solid that is free of
residual stresses and hence one that is simply characterized by an effective
modulus of elasticity . What is more, in spite of the fact
that the local moduli of elasticity in the bulk and the interfaces do not
possess minor symmetries (due to the presence of residual stresses and the
initial surface tension at the interfaces), the resulting effective modulus of
elasticity does possess the standard minor symmetries of a
conventional linear elastic solid, that is,
. As a first application,
numerical results are worked out and analyzed for the effective modulus of
elasticity of isotropic suspensions of incompressible liquid -spherical
inclusions of monodisperse size embedded in an isotropic incompressible
elastomer
Innovative unidirectional recycled carbon fiber tape structure for high performance thermoplastic composites: technological developments, technology-structure-property relationship and modeling of composite tensile properties
The rapidly growing demand for carbon fiber reinforced plastics in high-tech industries, such as aerospace, defense, automotive, wind turbine engineering, building and sports, resulted in a high amount of waste in the form of dry waste (e.g., production off-cuts), wet waste (e.g., out-of-date prepreg) and end-of-life components waste (e.g., aircraft components). Furthermore, the production of carbon fibers is cost and energy-intensive. Therefore, technological developments for the gentle processing of recycled carbon fiber and its integration into high-performance composites with promising tensile properties have gained considerable attention. Consequently, injection molding, nonwovens and hybrid yarn technologies were developed in recent years to integrate recycled carbon fiber into the high-performance thermoplastic composite. It is unfortunate that these technologies develop composites with a lack of unidirectional fiber orientation; therefore, the potential of recycled carbon fiber in high-performance composites is not thoroughly exhausted.
This thesis primarily addresses the development of an innovative structure with a unidirectional fiber orientation termed âunidirectional recycled carbon fiber tape structureâ for high-performance thermoplastics composites. The technological concept of the unidirectional structure comprises fiber opening, carding, drawing and a novel tape-forming process. In this concept, fiber opening, carding, and drawing processes were utilized to develop homogeneous, uniform, and highly oriented hybrid slivers. In the next step, these hybrid slivers were converted into a unidirectional recycled carbon fiber tape structure through a novel tape-forming process. To implement this concept, technological developments (investigations, modifications, optimization and further developments), were carried out in fiber opening, carding and drawing processes to develop a hybrid sliver with improved uniformity, homogeneity and unidirectional orientation. In the second phase, conception, design, technological developments, construction and prototype development were implemented to develop a novel tape-forming process. The result confirms that tape development technology comprising fiber opening, carding, drawing and prototype tape forming processes is an innovative, eco-friendly and sustainable technology compared to existing technologies.
Furthermore, the consolidation process transformed the unidirectional tape structure into high-performance thermoplastic composites. Subsequently, technology-structure-property relationships were established to develop composites with tailor-made properties. The analysis reveals that selecting optimum technological, consolidation and structural parameters develop tape and composite structures with unidirectional fiber orientation. As a result, experimental results of a high-performance composite developed from a unidirectional recycled carbon fiber tape structure show a very high tensile strength of 1350 ± 28 MPa and an E-module of 84.7 ± 2.3 GPa. This analysis confirms that unidirectional fibers configuration in composites brings a revolution toward developing cost-efficient, high-performance composites for load-bearing structural applications. Finally, theoretical and finite element modeling of tensile properties of high-performance composites reveals that modified models show good agreement with composite tensile properties
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