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Free vibrations of delaminated beam-type structures with crack bridging
No full textThe dynamic response of laminated beam-type structures presenting delaminations is characterized by stiffness degradation,
natural frequency shifts and, in special cases, the occurrence of opening modes during vibration. The paper investigates how
bridging mechanisms acting between the surfaces of the delaminations may affect these behaviors. Such mechanisms could be developed
by a through-thickness reinforcement (stitching, z-pins) applied to the laminate in order to improve its resistance and
tolerance against delamination fracture. A model based on the theory of bending of laminated plates is formulated where the
bridging mechanisms are modeled as a uniform distribution of linear elastic springs that oppose longitudinal and transversal relative
displacements between the surfaces of the delamination. The model is applied to predict natural frequencies and modal shapes of a
delaminated cantilever beam with a rectangular cross-section. A variety of solutions and transitions in the dynamic response is
found on varying the stiffness of the ligaments. A transition from damaged (unbridged) to intact beam values is predicted upon
increasing the stiffness of both longitudinal and transversal springs. An application of the model to a stitched carbon-epoxy laminate
for the aeronautical industry shows that low percentages of through-thickness reinforcement can substantially improve the dynamic
response of delaminated structure
Delamination and Dynamic response of Through-Thickness Reinforced Delaminated Composites
No full textRisposta dinamica di travi delaminate inflesse in presenza di rinforzi translaminari
No full textL’influenza di delaminazioni sul comportamento dinamico di compositi laminati si manifesta
con riduzioni di rigidezza, alterazioni delle frequenze proprie di vibrazione ed, in alcuni casi,
con modi di vibrazione caratterizzati da distacco dei lembi delaminati (modi di apertura). Nel
lavoro proposto viene investigata l’influenza di rinforzi translaminari sulle frequenze naturali
di una trave inflessa in presenza di una delaminazione parallela alla superficie della trave.
L’azione esercitata dai rinforzi è simulata mediante una distribuzione uniforme di molle
elastiche lineari che si oppongono a spostamenti relativi sia trasversali che longitudinali delle
superfici delaminate. In un’applicazione del modello in un caso in cui non si presentano modi
di apertura, si determinano le prime frequenze proprie di una mensola con delaminazioni di
differente lunghezza. Viene verificata l’efficacia dei rinforzi e la capacità del modello di
prevedere l’alterazione delle frequenze proprie indotte dalla presenza della delaminazione
Mode I Weight Functions for an Orthotropic Double Cantilever Beam
No full textApproximate weight functions are proposed and validated numerically for an orthotropic double cantilever
beam (DCB) loaded in mode I. They define the stress intensity factor at the crack tip due to a pair of
point forces acting on the crack surfaces and have been deduced from the corresponding isotropic result
using an orthotropy rescaling technique. The weight functions allow mode I large scale bridging problems
in beams and plates to be formulated as integral equations, in terms of stress intensity factors at the crack
tip, without the limitations imposed on accuracy by beam theory approximations. The proposed functions
are applied to investigate the influence of the orthotropy of the material on the fracture behavior of DCBs
in the presence of large scale bridging
Mode II weight functions for isotropic and orthotropic double cantilever beams
No full textApproximate weight functions are derived and validated numerically for isotropic and orthotropic double cantilever beams loaded in mode II. They define the stress intensity factor at the crack tip due to a pair of unit point forces acting tangentially to the crack surfaces and have been deduced using asymptotic matching through finite elements and an orthotropy rescaling technique. Along with the related mode I weight functions, which are recalled in the paper, the functions can be used to formulate mixed mode problems in beams and plates as integral equations so avoiding the limitations imposed on accuracy by approximations based on structural theories. An accurate method such as the weight function method becomes necessary when dealing with short cracks, crack initiation and large scale bridging delamination problems with crack wake mechanisms acting over several scales. The accuracy of mode I and mode II solutions based on beam theory approximations is shown to be strongly influenced by the length of Dugdale type cohesive regions, with short lengths giving rise to large errors in the fracture parameters. Stress intensity factors obtained using the proposed functions for special problems agree with solutions from the literature
Dynamic Response and Dynamic Fracture of Through-Thickness Reinforced Laminates
No full textA through-thickness reinforcement, in the form of stitching, short fibrous or metallic rods (z-pins), or woven or braided tows, offers a cost-effective approach to control and even suppress delamination fracture in composite laminates subject to static loading [1-9]. Recent experimental tests have demonstrated the effectiveness of a through-thickness reinforcement in improving the performance of laminates and layered systems against dynamic loading, low-velocity impact, ballistic impact and blast loading [10-16]. This paper deals with a theoretical investigation of the response of through-thickness reinforced structures subject to dynamic loading. The preliminary results of two studies aimed at investigating the effectiveness of a through-thickness reinforcement in reducing the delamination-induced degradation of the dynamic properties of composite structures and in improving the response of delaminated beams against dynamic fracture will be presented [17,18]. For the first problem, it will be shown that a through-thickness reinforcement can actually restore the dynamic properties, natural frequencies of vibration and modal shapes, of the intact beam. For the second problem it will be shown that for low to moderate crack velocities a through-thickness reinforcement can substantially improve the response of the structure against dynamic fracture
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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