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Thermoelastic and Fracture Responses of Periodic Materials: Theory and Applications to Laminates and Triply Periodic Minimal Surfaces
Full text linkIn order to provide better quality on a variety of equipment, services and
new technologies to the community, periodic materials as laminates and periodic structures as foams are constantly gaining more attention world-wide, due to the fact that these structures present suitable mechanical behaviours, enhanced physical properties and are yet low-cost. There-
fore, it is crucial to understand how these structures respond for different physical and mechanical problems. The present thesis exploits how cer-tain periodic structures behave and respond under thermo-mechanical loading and fracture phenomena.
In the first part of the thesis, a multi-scale variational-asymptotic ho-mogenization method for periodic microstructured materials for ther-moelastic problems with one relaxation time is exploited. The asymp-totic expansions of the micro-displacement and the micro-temperature fields are rewritten on the transformed Laplace space and expressed as power series of the microstructural length scale, leading to a set of re-cursive differential problems over the periodic unit cell.The solution of such cell problems leads to the perturbation functions. Up-scaling and
down-scaling relations are then defined, and the latter allow expressing the microscopic fields in terms of the macroscopic ones and their gra-dients. The variational-asymptotic scheme to establish an equivalence between the equations at macro-scale and micro-scale is developed. Av-
erage field equations of infinite order are also derived. The efficiency of the proposed technique was tested in relation to a bi-dimensional or-thotropic layered bodies with orthotropy axis parallel to the direction of the layers, where the mechanical and temperature constitutive prop-erties were well established. The dispersion curves of the homogenized medium, truncated at the first order are compared with the dispersion curves of the heterogeneous continuum obtained by the Floquet-Bloch theory. The results obtained with the two different approaches show a
very good agreement.
The second part of the thesis is focused on assessing the occurrence of fracture in Triply Periodic Minimal Surfaces (TPMS) foams subjected to compressive loading. TPMS, described by the mathematics commu-nity, may be exploited as a backbone for developing a new class of foams with open porosity for a wide range of engineering and biomedical appli-cations. Therefore, a comprehensive analysis of their fracture response is fundamental and is herein attempted. To this aim, a 3D phase field model is outlined and applied to TPMS foam structures under com-pression, with the goal to predict critical points for crack nucleation, potential crack paths, and the stiffness and maximum compressive stress of the unit cell, which can be related to the apparent Young’s modulus and apparent strength of a macro-scale composite made of such TPMS unit cells. A careful mesh sensitivity analysis was conducted on the specimens, to provide guidelines on how to identify the optimal finite el-
ement discretization consistent with the internal length scale parameter of the phase field approach to fracture. The major predicted mechanical properties for five different TPMS open foams, and for different levels of porosity, are summarized in Ashby plots. The predicted trends are in
agreement with previous results on TPMS taken from the literature and show that TPMS can outperform standard Aluminium open foams
Neurophysiological assessments of low-level and high-level interdependencies between auditory and visual systems in the human brain
Full text linkThis dissertation investigates the functional interplay between visual and auditory systems and its degree of experience-dependent plasticity. To function efficiently in everyday life, we must rely on our senses, building complex hierarchical representations about the environment. Early sensory deprivation, congenital (from birth) or within the first year of life, is a key model to study sensory experience and the degree of compensatory reorganizations (i.e., neuroplasticity). Neuroplasticity can be intramodal (within the sensory system) and crossmodal (the recruitment of deprived cortical areas for remaining senses). However,
the exact role of early sensory experience and the mechanisms guiding experience-driven plasticity need further investigation. To this aim, we performed three electroencephalographic studies, considering the aspects: 1) sensory modality (auditory/visual), 2) hierarchy of the brain functional organization (low-/high-level), and 3)sensory deprivation (deprived/non-deprived cortices). The first study explored how early auditory experience affects low-level visual processing, using time-frequency analysis on the data of early deaf individuals and their hearing counterparts. The second study investigated experience-
dependent plasticity in hierarchically organized face processing, applying fast periodic visual stimulation in congenitally deaf signers and their hearing controls. The third study assessed neural responses of blindfolded participants, using naturalistic stimuli together with
temporal response function, and evaluated neural tracking in
hierarchically organized speech processing when retinal input is absent, focusing on the role of the visual cortex. The results demonstrate the importance of atypical early sensory experience in shaping (via intra-and crossmodal changes) the brain organization at various hierarchical
stages of sensory processing but also support the idea that some crossmodal effects emerge even with typical experience. This dissertation provides new insights into understanding the functional interplay between visual and auditory systems and the related mechanisms driving experience-dependent plasticity and may contribute to the development of sensory restoration tools and rehabilitation strategies for sensory-typical and sensory-deprived populations
A multi-site image-based data sharing initiative to assess structural brain changes in large cohorts of early and late blind individuals
No full textBlindness is a widely studied phenomena in order to under-
stand the neuroplasticity in case of sensory deprivation. Number of studies focusing on the structural plasticity during visual deprivation is fewer compared to the functional plasticity. In addition, samples and the results of those studies are heterogeneous. This dissertation utilized the largest dataset of MRI of blind subjects, that is gathered by the efforts of 10 different research labs. The aim was to overcome the previous challenges, and identify pre- and post-natal factors that are confounding the results. In the first chapter, an extensive
literature review revealing the current status of the field and identifying the problems is presented. The review is completed with a systematic meta-analysis. Structural plasticity in case of congenital blindness was examined in chapter 3. This chapter focused on between (congenital blind participants versus sighted controls) and within group (effect of the causes of blindness, residual light perception, Braille literacy) differences via utilizing voxel- and surface-based morphometry. In chapter 4, participants with late blindness was examined. In addition to the analyses conducted on the congenital blind subjects, effect of time spent as blind and blindness on-set on neuroplasticity was examined. The results indicate a
consistent atrophy on the visual pathway, and a critical pe-
riod for vulnerability. In addition, the effect of premature
birth, residual light perception, Braille literacy on the structural plasticity and limited sample sizes on the results were revealed
Cohesive and Variational Methods for Fracture Mechanics in Statics and Fatigue
Full text linkThe widespread use of material over the past century has dramatically changed the world today. The reliability of any machine depends on the multiple com-plex interactions in the system leading to a failure. Hence, it is necessary to
understand the failure mechanism of the structures with a multi-physics inter-action. The present thesis explores the role of complex multi-physics in failure tounderstand the overall structural performance, including an interface between different materials.
Specifcally, in Chapter-2, a phase feld (PF) approximation of fracture for func-tionally graded materials (FGM) using a diffusive crack approach incorporat-ing the characteristic length scale as a material parameter is herein proposed. A
rule of mixture is employed to estimate the material properties, according to the volume fractions of the constituent materials, which have been varied accord-
ing to given grading profles. Based on the ideas stemming from the study of size-scale effects, Γ-convergence for the proposed model is proved when the in-ternal length scale is either constant or a bounded function. Crack propagation
events in conjunction with the differences with respect to their homogeneous surrogates are discussed through several representative applications, providing equivalence relationships for size-scale effects and demonstrating the applica-bility of the current model for structural analysis of FGMs.
Failure processes in Laminated Fiber-Reinforced Composites (LFRCs) entail the development and progression of different physical mechanisms and, in particular, the interaction between inter-laminar and intra-laminar cracking. Reliable
modeling of such complex scenarios can be achieved by developing robust numerical predictive tools that allow for the interaction of both failure modes. In Chapter-3, a novel Multi Phase-Field (MPF) model relying on the Puck theory
of failure for intra-laminar failure at ply level is coupled with a Cohesive Zone Model (CZM) for inter-laminar cracking. The computational tool is applied to qualitatively predict delamination migration in long laminated fber-reinforced
polymers composites comprising 44 cross-ply laminates. The reliability of the current approach is examined via the correlation with experimental results. Finally, the present study is complemented with additional representative exam-
ples with the aim of providing further insight into the potential role of different aspects of the system in the delamination migration, including (i) the variation
of the ply angle in the migration zone, (ii) the load application point, and (iii) initial crack length.
The analysis of fracture phenomena of thin-walled structures has been a matter of intensive research in the last decades. These phenomena notably restrict the applicability of slender structures, especially under the infuence of temperature. The research in Chapter-4 is concerned with the development of a thermo dynamically consistent framework for the coupled thermo-mechanical phase-feld model for thin-walled structures using a fully-integrated fnite elements.
This enables the use of three-dimensional constitutive thermo-mechanical models for the materials. The proposed thermo-mechanical phase-feld models are equipped with the EAS and ANS leading ti locking free element. Moreover, the
same degradation function is used for both displacement feld and thermal feld.
The coupled equations are numerically solved with ad hoc effcient solution schemes for nonlinear problems. Several numerical examples with and without phase-feld (straight and curved shells) are provided to show the practicality
and reliability of the proposed modeling framework. Moreover, the model is extended to incorporate FGM and the corresponding numerical examples are explored.
Using the framework developed in the Chapter-4, the locking free solid shell element is extended to include the fatigue effects in Chapter-5. As a natural consequence of the developed model, SN curve and crack extension curves are
recovered for straight and curved shells
The troubled relationship between neuroscience and criminal responsibility: an overview and case study
Full text linkCriminal behavior and the assessment of legal insanity have always been topics of primary concern for the justice system and, at the same time, of great interest to mental health experts. Neuroscience has given mental health specialists an incredible tool to investigate the brain, but the use
of neuroimaging techniques in criminal trials has been received with a significant degree of resistance. This thesis illustrates an overview of the techniques involved in the study of the brain that gave birth to modern
neuroscience. Moreover, it highlights the philosophical consequences of some neuroscientific experiments that have alerted legal scholars to the problems of free will and criminal responsibility. This contribution goes
in-depth in analyzing the normative components of criminal
responsibility according to Italian law, and it presents some comparisons with Anglo-American law. Additionally, the present work focuses on the judgment of criminal responsibility in insanity defense cases and the
controversies around the mental conditions that can be relevant for a not guilty by reason of insanity verdict. Some real forensic cases are illustrated and discussed. They concern the role of neuroscientific evidence and a
debate on the relevance of a personality disorder for the insanity defense.
This thesis addresses the problems of translation of legal terms into scientific concepts and, the other way around, of scientific findings into pieces of evidence that are meaningful for legal doctrine. In conclusion,
the thesis aims to give an overall outlook on the relationship between law and contemporary forensic neuroscience
Public Perceptions of ‘the Other’s’ Heritage: Ottoman Heritage in Greece and Byzantine Heritage in Turkey
No full textThe relationship between cultural heritage and identity has long been acknowledged. Due to this relevance, utilizing past and its material remains for political purposes has been a common practice for the states; a phenomenon that has its roots in antiquity and is still actively used. Modern states have been active agents in this process by defining
what constitutes their ‘national identity’ and ‘national heritage’. This selection brings together the exclusion of the pasts and their remains that are ’dissonant’ with the national historiography, thus resulting in the deliberate and non-deliberate destruction of ‘the Other’s’ heritage.
Due to their attachment to public memory, historical monuments are the most affected subjects of this process.
This study investigates the historical and contemporary effects of ‘exclusion’ of cultural heritage from the national narratives by focusing on the Ottoman heritage in Greece and the Byzantine heritage in Turkey. The research focuses on the two particular aspects of ‘public’:
the state and the people. It critically analyses cultural heritage laws in Greece and Turkey and the states’ contemporary cultural policies with regards to ‘the Other’s’ heritage. People’s perceptions of heritage are investigated through public opinion surveys conducted in two case
studies from the two countries. The research deals with a topical subject, ‘heritage and identity’, which
is of high relevance to contemporary societies and the heritage literature. Its originality lies in its scope, ‘the Other’s’ heritages in Greece and Turkey, and the novel results it produces. In summary, the research shows that people’s and states' perceptions of ‘the Other’s’ heritage are interrelated and mostly governed by the states’ policies. The inclusion of the Ottoman heritage in the cultural heritage management in Greece has positively influenced the people’s perceptions. In Turkey, the ‘otherness’ of the Byzantine heritage is still felt by the people, but the potential benefits of their utilization enhance their protection by the public
Landscapes before the landscape in ancient Etruscan art
No full textThis thesis approaches the broad field of landscape study from a specialized standpoint, namely by investigating the different ways in which landscape is represented in Etruscan art.
In particular, this analysis aims to identify the significance of what we might nowadays refer to as “landscape elements” in the production-fruition systems of Etruscan visual culture. The iconographic analysis presented here shows that landscape features are working material which, employed in relation to other image elements, express specific meanings in the construction of the image.
This thesis begins with an historiographical overview of the ways landscape representation has been investigated over time, including by visualizing the relationships and influences between the different disciplines that have approached landscape as a research topic. The core section then considers all kinds of representations of landscape elements (landscape elements that appear isolated within images as well as more structured and coherent ensembles of landscape features) in Etruscan art, from its earliest period to the threshold of Hellenism. The analysis and presentation of images is based on a ‘situational’ categorization (theme/context-based categories) designed to enable transversal readings. Dealing with the multiple ‘landscapes’ that existed before the landscape (aesthetically appreciable as a pictorial theme), entails moving along two different but interconnected paths. On the one hand, we can see landscape features performing different functions in different visual occurrences. On the other hand, from the perspective of an unfolding elaboration of figurative structures, these features can be analyzed as individual signifying structures that were only organized into broader uniform configurations over time
Models and applications for the Bitcoin ecosystem
Full text linkCryptocurrencies are widely known and used principally as a means of investment and payment by more and more users outside the restricted circle of technologists and computer scientists. However, like fiat money, they can also be used as a means for illegal activities, exploiting their pseudo-anonymity and easiness/speed in moving capitals. This thesis aims to provide a suite of tools and models to better analyze and understand several aspect of the Bitcoin blockchain.
In particular, we developed a visual tool that highlights transaction islands, i.e., the sub-graphs disconnected from the super-graph, which represents the whole blockchain. We also show the distributions of Bitcoin transactions types and define new classes of nonstandard transactions. We analyze the addresses reuse in Bitcoin, showing that it corresponds to malicious activities in the Bitcoin
ecosystem. Then we investigate whether solids or weak forms of arbitrage strategies are possible by trading across different Bitcoin Exchanges. We found that Bitcoin price/exchange rate is influenced by future and past events. Finally, we present a Stochastic Model to quantitative analyze different consensus protocols. In particular,
the probabilistic analysis of the Bitcoin model highlights how forks happen and how they depend on specific parameters of the protocol
Delamination of thin layers promoted by local buckling
Full text linkThis dissertation investigates the combined phenomena of
buckling and fracture, which occur in thin superficial layers subjected to compressive forces. As a representative case, the four-point bending test on laminated specimens with midspan, through-the-width delaminations, is taken into consideration: a mechanical model of the test was developed and experimental tests on carbon fiber laminates were conducted. The thesis is subdivided into six Chapters. Chapter 1 presents a general introduction to the investigated problem. Chapter 2 illustrates the state of the art on delamination in composite materials with focus on buckling-driven delamination and four-point bending tests. Chapter 3 illustrates the experimental Mode II fracture toughness characterization of the carbon fiber laminates investigated in the thesis. In Chapter 4, the experimental campaign with four-point bending tests is illustrated. Chapter 5 presents the analytical model to simulate and interpret the four-point bending tests. Chapter 6 shows a comparison between the analytical prediction and the experimental evidence. In Chapter 7, results are summarized and future developments are outlined
Bit-precise Verification of Numerical Properties in Fixed-point Programs
Full text linkNumerical software is prone to inaccuracies due to the finite representation of numbers. These inaccuracies propagate, possibly non-linearly, throughout the statements of a program, making it hard to predict the accumulated errors. Moreover, in programs that contain control structures, numerical errors can affect the control flow. As a result of these inaccuracies, reachability, and thus safety, may be altered with respect to the intended infinite-precision computation. This thesis considers programs that use fixed-point arithmetic to compute over non-integer quantities in finite precision. We first define a semantics of fixed-point operations in terms of operations over bit-vectors. The proposed semantics generalizes current attempts to a standardization of fixedpoint arithmetic. We then consider the problem of bit-precise numerical accuracy certification of fixed-point programs with control structures and arithmetic over variables of arbitrary,
mixed precision and possibly non-deterministic value.
By applying a set of parametrized transformation rules based
on computable expressions for the errors incurred by single program statements, we reduce the problem of assessing whether a fixed-point program can exceed a given error bound to a reachability problem in a bit-vector program. We present an experimental evaluation of the certification technique, implemented in a prototype analyzer in a
bounded model checking-based verification workflow. Our
experiments on a set of fixed-point arithmetic routines commonly used in the industry show that the proposed technique can successfully certify numerical errors and can do so bitprecisely, making it the only such verification technique