13,234 research outputs found
Crack propagation in honeycomb cellular materials: a computational approach
No full textComputational models based on the finite element method and linear or nonlinear fracture mechanics are herein proposed to study the mechanical response of functionally designed cellular components. It is demonstrated that, via a suitable tailoring of the properties of interfaces present in the meso- and micro-structures, the tensile strength can be substantially increased as compared to that of a standard polycrystalline material. Moreover, numerical examples regarding the structural response of these components when subjected to loading conditions typical of cutting operations are provided. As a general trend, the occurrence of tortuous crack paths is highly favorable: stable crack propagation can be achieved in case of critical crack growth, whereas an increased fatigue life can be obtained for a sub-critical crack propagation
Structural integrity of hierarchical composites
Full text linkInterface mechanical problems are of paramount importance in engineering and materials science.
Traditionally, due to the complexity of modelling their mechanical behaviour, interfaces are often treated as
defects and their features are not explored. In this study, a different approach is illustrated, where the interfaces
play an active role in the design of innovative hierarchical composites and are fundamental for their structural
integrity. Numerical examples regarding cutting tools made of hierarchical cellular polycrystalline materials are
proposed, showing that tailoring of interface properties at the different scales is the way to achieve superior
mechanical responses that cannot be obtained using standard material
Influence of the intermediate material on the singular stress field in tri-material junctions
Full text linkAccording to the mathematical formalism of the eigenfunction expansion method, the problem of stress-singularities arising from multi-material junctions is addressed. The wedges are composed of isotropic homogeneous materials and are in a condition of plane stress or strain. The order of the stress-singularity is provided for tri-material junctions, paying special attention to the role played by Mode-I and Mode-II deformation. The effect of cracks inside either the softer or the stiffer material is also investigated. Numerical results can be profitably used for establishing optimum material configurations
Modelling fatigue in quasi-brittle materials with incomplete self-similarity concepts
No full textIn this study, a generalized Barenblatt and Botvina dimensional analysis approach to fatigue crack growth is proposed in order to highlight and explain the deviations from the classical power–law equations used to characterize the fatigue behaviour of quasi-brittle materials. According to this theoretical approach, the microstructural-size (related to the volumetric content of fibers in fiber-reinforced concrete), the crack-size, and the size-scale effects on the Paris’ law and on the Wöhler equation are presented within a unified mathematical framework. Relevant experimental results taken from the literature are used to confirm the theoretical trends and to determine the values of the incomplete self-similarity exponents. All this information is expected to be useful for the design of experiments, since the role of the different dimensionless numbers governing the phenomenon of fatigue is herein elucidated. Finally, a numerical model based on damage mechanics and nonlinear fracture mechanics is proposed for the prediction of uniaxial S–N curves, showing how to efficiently use the information gained from dimensional analysis and how the shape of the S–N curves is influenced by the parameters of the damage model
Interface tailoring as an active approach to materials design
Full text linkInvited paper published in the Newsletter of the European Structural Integrity Society, No. 47, 201
Interface mechanical problems in heterogeneous materials
No full textThe simplified assumption of material homogeneity underlying many problems of structural
mechanics is very often far from the complex reality we have to cope with. Junctions and
interfaces between di®erent materials must typically sustain mechanical and thermo-elastic
stresses without failure. Consequently, they exert an important and sometimes controlling
influence on the overall performance of the material. Therefore, current attempts in materials
engineering to increase the strength and ductility of materials require a full appreciation of
material interfaces, their properties and characterization.
Since interface problems are one of the main concern in civil, mechanical and electronic
engineering, as well as in biomechanics and in materials science, this research field is characterized
by multidisciplinary aspects. New concepts in engineering the material microstructure
mark the beginning of a paradigm shift in the way we think about materials and structures.
Due to recent advances in material processing, material and structural design considerations
are moving toward a full integration.
With this respect, it is evident that a proper modeling of the mechanical behavior of
interfaces at di®erent length scales is an outstanding point. The possibility of controlling the
mechanical behavior of the material over all the scales by tailoring interfaces clearly emerges
as one of the new challenges of the scientific community.
This thesis aims at giving a reasonably complete overview of the most relevant mathematical
and numerical techniques that can be applied to solve interface mechanical problems
in heterogeneous materials. With this objective in mind, the connections between the wide
literature of Linear Elastic and nonlinear Fracture Mechanics and Contact Mechanics are
deeply investigated and emphasized.
Novel features of this work include: (1) theoretical and numerical characterization of
stress-singularities arising at multi-material interfaces in 2D linear elastic problems; (2) numerical
and experimental study of brittle and fatigue crack growth in multi-layered materials;
(3) definition of a unified interface constitutive law for the study of decohesion and contact
problems at bi-material interfaces; and (4) interpretation of size-scale e®ects in new advanced
composite materials, such as the prediction of the critical grain sizes for the activation of
the superplastic behavior in fine grained composites and for the inversion of the Hall-Petch
relationship at the nano-scale
A dimensional analysis approach to fatigue in quasi-brittle materials
Full text linkRIASSUNTO. Nel presente lavoro si propone uno studio di analisi dimensionale del fenomeno della fatica nei
materiali quasi-fragili. Esso costituisce una generalizzazione della metodologia pionieristica proposta da
Barenblatt e Botvina e si prefigge di interpretare le deviazioni dalle leggi di potenza classiche usate per
caratterizzare il comportamento a fatica dei materiali. In base a questo approccio teorico, gli effetti dovuti alla
dimensione microstrutturale (correlabile al contenuto volumetrico di fibre nei calcestruzzi fibrorinforzati), alla
dimensione delle fessure e alla scala strutturale sulla legge di Paris e sulle curve di Wöhler sono discussi in un
contesto matematico unificato. Il modello teorico è confermato dal confronto con rilevanti risultati sperimentali
disponibili in letteratura, usati per determinare i valori degli esponenti di autosimilarità incompleta. Le
informazioni fornite da questa teoria possono essere particolarmente utili per guidare la progettazione di nuovi
esperimenti, dal momento che viene chiarito il ruolo delle diverse variabili adimensionalizzate che governano il
fenomeno della fatica.
ABSTRACT. In this study, a generalized Barenblatt and Botvina dimensional analysis approach to fatigue crack
growth is proposed in order to highlight and explain the deviations from the classical power-law equations used
to characterize the fatigue behaviour of quasi-brittle materials. According to this theoretical approach, the
microstructural-size (related to the volumetric content of fibres in fibre-reinforced concrete), the crack-size, and
the size-scale effects on the Paris’ law and the Wöhler equation are presented within a unified mathematical
framework. Relevant experimental results taken from the literature are used to confirm the theoretical trends
and to determine the values of the incomplete self-similarity exponents. All these information are expected to
be useful for the design of experiments, since the role of the different dimensionless numbers governing the
phenomenon of fatigue is herein elucidated
Singular, hypersingular and singular free electromagnetic fields at wedge tips in metamaterials
No full textThe engineering response of metamaterials has a dramatic impact on the physics, optics and engineering communities, because they offer electromagnetic properties that are difficult or impossible to achieve with conventional materials. In this paper, an asymptotic analysis of the electromagnetic fields at multi-material wedges composed of metamaterials is proposed. This is made possible by removing the assumption of positive electric permittivities and magnetic permeabilities, an hypothesis which usually applies to conventional materials. Exploring the whole range of variability of these electromagnetic properties, it is shown that, in addition to the classical real eigenvalues 0 ⩽ λ < 1 leading to power-law singularities of the type O(rλ−1) as r → 0, it is also possible to find imaginary eigenvalues leading to hypersingular solutions, as well as nonsingular configurations for a suitable choice of the negative electric permittivities and magnetic permeabilities of the media. Moreover, to fully characterize the asymptotic fields, the analysis is not only restricted to the determination of the lowest real and complex eigenvalues, but is also extended to the evaluation of the higher-order nonsingular ones. The obtained analytical results collected in synthetic diagrams are expected to have impact on the design of micro- and nano-electro-mechanical systems
Integrity and durability of photovoltaic modules: An overview of mechanical failure modes
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