1,720,980 research outputs found
Crack kinking in isotropic and orthotropic micropolar peridynamic solids
No full textA micropolar peridynamic model is presented for characterizing crack propagation in isotropic and orthotropic brittle materials. The analytical formulation of the two-dimensional model is based on the definition of a micropotential energy function that accounts for the four independent elastic constants that define orthotropy and that in the limit can be reduced to isotropy. A distinctive feature of the model is that the bonds’ elastic parameters are continuous functions of orientation with respect to principal material axes. By defining three deformation parameters that quantify bond stretch, bond shear deformation and particles relative rotation, the first continuum bond-based peridynamic model is obtained for two-dimensional Cauchy orthotropic materials characterized by four independent material moduli that is suitable for describing fracture as well as homogeneous and non-homogeneous deformations. The accuracy of the computational model as applied to crack-tip analyses is assessed by comparing the displacement and stress fields within the boundary layer that develops in the immediate vicinity of a crack with the analytical asymptotic results for an orthotropic continuum. The extension of such cracks when they are subjected to mixed-mode loading is simulated under the assumption of illustrative crack extension criteria, and the predictions are compared to those of the maximum hoop stress intensity factor criterion (HSIF-criterion) and the maximum energy release rate criterion (G-criterion)
Are configurational forces real forces?
No full textConfigurational forces are fundamental concepts in the description of the motion of dislocations, cracks and other defects, possibly introducing singularities within the solid state. They are defined by considering variations in energies associated with the movement of such defects, and are therefore different from the classical forces that enter the balance laws of Newtonian mechanics. We show how a configurational force can be viewed as the resultant of the (Newtonian) contact forces acting on the perturbed shape of an object of substance equivalent to the defect, and evaluated in the limit of the shape being restored to the primitive configuration. The object of substance is, for the case of dislocations, the inclusion that provokes the same stress field. For cohesive cracks, it is the bundle of cohesive ligaments forming the process zone, reducing to a material point coincident with the crack tip in the small-scalebridging limit. The expressions for the configurational forces obtained in the paradigmatic examples are in agreement with those determined using classical variational arguments. As a further step towards a physical interpretation of the contour integral of the energy momentum tensor, we propose to extend the classical J-integral approach for a propagating crack by prolonging the contour path inside the crack gap. This extension implicitly accounts for any source of dissipation associated with material separation such as cohesive forces, establishing an energetic balance à la Griffith in the limit of small scale bridging. The method finds a direct application to phase-field models of fracture mechanics where no neat material separation occurs, because it allows using closed contour paths that traverse the thin band where damage accumulates. A generalized energy momentum tensor permits to calculate the energy release rate associated with a propagating band, also when a residual elasticity is supposed to remain in the completely damaged material. These findings may open a new prospective in the use of configurational forces by permitting their physical and intuitive visualization as classical "real" forces, as well as by providing an enhanced tool for their calculation when the contours of the defect is not clearly identifiab
Pullout Capacity of Headed Anchors in Prestressed Concrete
No full textA combined experimental and computational study shows that the pullout capacity of anchors embedded at small depths in prestressed concrete is associated with the strongest possible (linear elastic fracture mechanics) size effect. A design formula is proposed that reflects the effects of embedment depth and the nondimensional parameters that quantify the level of prestressing and the characteristic length of the matrix
Influence of Compressive Stresses on the Concrete Cone Breakout Capacity of Cast‐in‐Place Headed Anchors
No full texthttp://www.northeastern.edu/emi2011/
Geometry of sliding lamellae dictates the constitutive properties of nacre-like hierarchical materials
No full textThe mechanical properties of nacre (mother-of-pearl), which are superior to those of its constituent materials, derive from a micro-architecture formed by the arrangement into parallel laminae of individual aragonite tiles with surface asperities, bonded by organic interlayers. A mechanical model is developed to determine how the contact profile that defines the geometry of the tile surface affects the constitutive response of hierarchical metamaterials of this type when they are strained in the lamellar plane. We consider the structured character of the deformation, consisting of elastic distortion of the tiles and the inelastic contribution from the disarrangements produced by relative sliding of the lamellar interfaces, which is formulated within the thermodynamic framework for irreversible processes in generalized materials. The resistance to sliding associated with constrained shear-induced dilatation of the tiles and the bridging of the organic interlayers are represented by a generalized cohesive-frictional law, effective at the level of averaged separation plane, providing a phenomenological approach that by-passes the difficulty in analytically solving the contact problem at asperities. The nominal stress–strain curves of the nacreous structure are shown to be highly dependent, qualitatively and quantitatively, on the geometry of the contact profiles. The model not only produces results that are in excellent agreement with experiments on nacre, but it can also describe complex stress–strain curves with pinched hysteresis loops having branches of various shapes, as well as serrated inelastic regimes. The versatility of this theoretical approach offers promise as a design tool for innovative metamaterials
(INVITED) Flexible photonics for biomedical applications: A review
Full text linkFlexible photonics is a powerful tool that has emerged in last few decades to solve footprint and shape related limitations in photonic devices. Indeed, flexible photonic devices offer several key features: ability to adjust to complex shape surfaces, small footprint, high resilience to mechanical damage, immunity to electromagnetic interference. Thanks to these characteristics, these devices are very attractive for applications in the biomedical field. For instance, the flexibility allows optimal adhesion to the skin, or the reduced size facilitates the realization of wearable devices. In this paper, we will first present the characteristic of flexible photonic devices and discuss about the so far impact of flexible photonics in the biomedical field. Then, we will analyze the currently existing devices and the main used components, with a focus on the applications
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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