1,224 research outputs found
Influence of the intermediate material on the singular stress field in tri-material junctions
No full textAccording 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
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
No full textInterface 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
Fracture of solar-grade anisotropic polycrystalline Silicon: A combined phase field–cohesive zone model approach
No full textThis work presents a novel computational framework to simulate fracture events in brittle anisotropic polycrystalline materials at the microscopical level, with application to solar-grade polycrystalline Silicon. Quasi-static failure is modeled by combining the phase field approach of brittle fracture (for transgranular fracture) with the cohesive zone model for the grain boundaries (for intergranular fracture) through the generalization of the recent FE-based technique published in [M. Paggi, J. Reinoso, Comput. Methods Appl. Mech. Engrg., 31 (2017) 145–172] to deal with anisotropic polycrystalline microstructures. The proposed model, which accounts for any anisotropic constitutive tensor for the grains depending on their preferential orientation, as well as an orientation-dependent fracture toughness, allows to simulate intergranular and transgranular crack growths in an efficient manner, with or without initial defects. One of the advantages of the current variational method is the fact that complex crack patterns in such materials are triggered without any user-intervention, being possible to account for the competition between both dissipative phenomena. In addition, further aspects with regard to the model parameters identification are discussed in reference to solar cells images obtained from transmitted light source. A series of representative numerical simulations is carried out to highlight the interplay between the different types of fracture occurring in solar-grade polycrystalline Silicon, and to assess the role of anisotropy on the crack path and on the apparent tensile strength of the material
One, no one, and one hundred thousand crack propagation laws: a generalized Barenblatt and Botvina dimensional analysis approach to fatigue crack growth
No full textBarenblatt and Botvina with elegant dimensional analysis arguments have elucidated that Paris' power-law is a weak form of scaling, so that the Paris' parameters C and m should not be taken as material constants. On the contrary, they are expected to depend on all the dimensionless parameters of the problem, and are really ‘‘constants" only within some specific ranges of all these. In the present paper, the dimensional analysis approach by Barenblatt and Botvina is generalized to explore the functional dependencies of m and C on more dimensionless parameters than the original Barenblatt and Botvina, and experimental results are interpreted for a wider range of materials including both metals and concrete. In particular, we find that the size-scale dependencies of m and C and the resulting correlation between C and m are quite different for metals and for quasi-brittle materials, as it is already suggested from the fact the fatigue crack propagation processes lead to m=2-5 in metals and m=10-50 in quasi-brittle materials. Therefore, according to the concepts of complete and incomplete self-similarities, the experimentally observed breakdowns of the classical Paris' law are discussed and interpreted within a unified theoretical framework. Finally, we show that most attempts to address the deviations from the Paris' law or the empirical correlations between the constants can be explained with this approach. We also suggest that ‘‘incomplete similarity" corresponds to the difficulties encountered so far by the ‘‘damagetolerant" approach which, after nearly 50 years since the introduction of Paris' law, is still not a reliable calculation of damage, as Paris himself admits in a recent revie
The coefficient of proportionality k between real contact area and load, with new asperity models
No full textMost recent numerical works on fractal surfaces have simply compared the low load limit of the coefficient of proportionality κ of the relationship between real contact area and load. In particular, that provided by Persson's theory, and that obtained from the Bush, Gibson and Thomas (BGT-A) asperity contact theory, which is a generalized form of the Greenwood and Williamson (GW) one. The two theories differ only by a numerical constant κ = √8/π ≈ 1.6 vs κ = √2 π ≈ 2.5 , but neither of the two provide an accurate prediction, Persson's value being generally too low, and BGT-A's limit being only valid for extremely large separations. A detailed numerical comparison using a range of generated fractal surfaces permits to compare the existing models, finding for example that bandwidth is more important than Gaussianity of the surfaces. Then, we propose two new theoretical equations generalizing {GW} and {BGT} to take into account interaction effects in an approximate way (GW-I and BGT-I, respectively), which significantly improve the accuracy of original asperity models. Further, as a practical alternative to the tribologist, we suggest a new very simple discrete form of the {GW} model (called GW-RI) whose accuracy is similar to BGT-I, but with much lower computational cost, comparable to analytical solutions since the latter require the evaluation of the variance of the profile slopes, σ m 2 , with a surface defined at a given set of points. The GW-RI model additionally avoids an ambiguity over how to define numerically the variance of the profile slopes, σ m 2
Polyarthra platensis José, Paggi & Paggi, 2011, sp. nov.
No full textPolyarthra platensis sp. nov. Type locality. Shallow lake in Reserva Ecológica U.N.L., Santa Fe (floodplain of Paraná River), Santa Fe Province, Argentina (31 ° 38 ’ 16 ” S 60 ° 40 ’ 21 ” W; temperature 27 ºC, conductivity 1065 µS.cm - 1, pH 8, dissolved oxygen 9 mg.l - 1). February 11 th, 2009. Type material. Holotype: 1 parthenogenetic Ψ, complete, mounted on a slide, Accession Number MACN-In 38202. Paratypes: two parthenogenetic Ψ, dissected and mounted on two slides, from type locality, Accession numbers MACN-In 38203–38204; two parthenogenetic Ψ, complete, mounted on two slides, all from type locality and on the same date. Accession number MFA/Z 8 – 9. Additional material. Numerous specimens in liquid samples, preserved in formalin deposited in INALI. Shallow lake in Reserva Ecológica U.N.L., Santa Fe (type locality), February 24 th, 2009. Laguna Gonzalez, near Santa Fe (floodplain of Paraná River), Santa Fe Province, Argentina (31 ° 40 ’ 28 ” S 60 ° 34 ’ 11 ” W), November 5 th, 2009. Pond situated one Km south of Cuay Grande River, Corrientes Province, Argentina (28 ° 41 ’ 41 ” S 56 ° 14 ’ 30 ” W), November 16 th, 2004. Swamp at Garrucho, Corrientes Province, Argentina (28 ° 7 ’ 50 ” S 55 ° 42 ’ 30 ” W), November 16 th, 2004. Pond in Saenz Peña, Formosa Province (26 º 54 ’ 26 ” S 60 º 27 ’ 37 ” W), December 1, 2007. The environmental conditions at all five listed sites combined were: temperature 25–27.5 ºC, conductivity 292–1065 µS.cm - 1, pH 7.5–9, dissolved oxygen 4.26–6.90 mg.l - 1. S hort diagnosis. Polyarthra with four bundles of well developed paddles, all similar in length, but shorter than body. Each bundle with two different types of paddles. Ventral accessory paddles present. Vitellarium with 8 nuclei. Unci with single tooth. Manubria gently curved in anterior half, with no projection or lamella present on its surface. Rami with single large tooth on inner margin. Distal part of rami, anterior to large tooth, round with one apical lobe. Description of parthenogenetic female. Shape of body, in dorsoventral view, subrectangular or roughly subpentagonal (Figs. 1–2), more rarely, with lateral flanks progressively widening posteriorly. Maximum width about 60-70 % of total body length. Posterior end of body frequently produced into a wide, convex depressed process. Along the dorsal wall of the body, on both sides, and posterior to each bundle of paddles, there is an integumental expansion or ridge. This ridge is somewhat variable in shape, but could generally be described as being at an obtuse angle with the apex, located midway between the insertion of paddles and lateral antenna (Figs 2, 3, 5–7). On occasion this ridge is accompanied by a small expansions near end of body (Fig. 7). In lateral view, body progressively tapers to posteriorly, dorsal expansions do not reach outline of maximum thickness. None of the studied specimens exhibited a “ proloba- like” ventral protuberance. Vitellarium with 8 nuclei. However, an additional nucleus was observed in just two specimens collected from two different localities. Corona with relatively long cilia, one wide and low ciliate protuberance in central part, a pair of short, inconspicuous apical antennae close to outer limit of corona and two short protuberances positioned close to each antenna (Figs. 1–3). Eyespot conspicuous and relatively large (6.5–7 % of body length), dark-red in colour and hemispherical in shape, with a posterolateral inclination of its plane (Figs. 1–2). Dorsal antenna, easily visible due to a thickening that partially surrounds the pore, located midway between dorsal bundles of paddles (Figs. 2–3). Lateral antennae, not so conspicuous, located close to distal insertion of paddle muscles (Fig. 2). Four bundles of major paddles located close to limit between head and rest of body. Each bundle with three paddles similar in length extending beyond the posterior end of the body by about 20–25 % of paddles length. Paddle bundles comprise two lanceolate and one ensiform paddle, all 10–20 % shorter than body length, with a welldefined central longitudinal vein. Lanceolate paddles with maximum width, 13–15 % of length, at distal fourth fifth; with 14–17 short teeth on each side, those on distal third as end of a sort pattern of oblique veins or pleats (Fig. 8). Ensiform paddle, slightly longer than lanceolate paddles, with maximum width, 7–9 % of length, at middle; margins coarsely serrated, with 11–14 well defined larger teeth and surface of blade smooth (Fig. 9). Accessory pair of ventral paddles, ligulate and extremely slender, basal width 2.5 –3.0 % of length, with smooth margins and as long as or slightly shorter than half of body length, inserted midway between ventral bundles (Figs. 1, 3, 4 and 10). Throphi virgate, about half as long as body, somewhat poorly sclerotized (Fig. 11). In ventral view, fulcrum seems to be as long as rami, bacilliform with a small rounded expansion at distal end; in lateral view, shorter than rami, subrectangular, with dorsal margin straight and ventral margin concave, expanded at distal end (middle height = 16–17 % of length and distal height = 28–29 % of length) (Figs. 11–12). In lateral view, it can be seen that rami are longer than fulcrum and curved from coronal plane to transverse plane (Fig. 12). Basal apophyses half as long as fulcrum and perpendicular to longitudinal axis of trophi, with distal part ending in a rounded, posteriorly expanded protuberance. Outer half of each ramus subhemicircular, with thin lamella whose anterior limit is barely visible. Inner part of each ramus with one conspicuously stout conical tooth at beginning of distal fourth; anterior to this tooth there is a rounded structure with one lobule at tip; posterior to stout conical tooth there is a sort of smooth flap followed by a smooth, gently concave margin (Figs. 13–14). Manubria rod-like, slender and curved, with distal part straight or slightly recurved in dorsoventral view; approximately as long as fulcrum; smooth surface without lamella (Figs. 15–16). Unci approximately as long as one third of manubria, tapered into one relatively blunt tip (Figs. 15–16). Dimensions (in µm, mean ± standard deviation; n = 20). Body: length = 86.4 ± 7.0, width = 58.8 ± 4.7, height = 53.0 ± 2.4. Paddles: major lanceolate, length = 75.7 ± 5.3, width = 10.7 ±1.0; major enciform, length = 77.2 ± 3.4, width = 6.7 ± 1.1; accessory ventral, length = 38.2 ± 10, width = 1.1 ± 0.1. Trophi: total length = 41.3 ± 2.4 [48.5 ± 2.7]; fulcrum length = 20.9 ± 0.8, rami: length = 19.7 ± 9.8 [26.0 ± 0.9], width 10.8 ± 1.5; manubria length = 23.4 ± 1.5; unci length = 6.7 ± 0.5. Due to the rami being curved in a sagittal plane two measurements are given for total length and rami length: 1) in ventral view, with coverslip supported on plasticine so as not to compress the trophi, 2) between square brackets, in lateral view or in ventral view but with rami straightened by crushing with a coverslip. Etymology. The specific name refers to the origin of the studied material, which is water bodies located within the La Plata River Basin. La Plata Basin or Cuenca del Plata is the largest water system in South America. Differential diagnosis and discussion. Polyarthra platensis sp. nov. should be included in the “ P. vulgaris group” due to the presence of ventral accessory paddles and taking into account certain features of the trophi. The species may be considered phylogenetically close to P. dolichoptera Idelson, 1925. However, there are several consistent differences between these two species. First, there are two characters that seem to be autapomorphic to P. platensis; specifically 1) the presence of a pair of lateral, normally angled expansions of dorsal integument and 2) the peculiar morphological heterogeneity of the main paddles. The unusual presence of the lateral expansion of dorsal integument has not been reported before in any of the other species in the genus and may be interpreted as a hydrodynamic adaptation associated to the rapid skipping motion used to avoid predators, which is typical of the species of this genus. Jersabek et al. (2003) show a photograph of a specimen, in lateral view, questionably identified as “ P. l u m i - nosa? Kutikova, 1962 ” (Catalog Number: ANSP 749), with a morphological feature that resembles this integumental expansion. However, it is not discernable in the dorsal view photographs of the same specimen. In comparison, the lateral expansions of integument are readily visible in dorsal view in P. platensis, even at low magnification. With respect to the morphological differences between paddles within a bundle, at least two other cases are known, in P. luminosa Kutikova, 1962 and P. minor Voigt, 1904. However, in both species this morphological heterogeneity is constrained to either the dorsal bundles, and only to the left dorsal bundle in P. minor. Of additional note, P. minor does not belong to “ P. vulgaris- group” because it lacks the accessory ventral paddles. In P. platensis sp. nov., all bundles have the same pattern of heterogeneity, while the distinct paddle in P. platensis sp. nov. is ensiform, in P. luminosa and P minor are dagger-shaped (i.e. leaf-shaped but widest in the proximal part). Apart from these autapomorphies, P. platensis sp. nov. and P. dolichoptera may be differentiated by the following features: 1) in P. dolichoptera the general body shape tapers to posteriorly, while in P. p l a t e n s i s sp. nov. the posterior part is not tapered; 2) the main paddles in P. dolichoptera are as long as, or longer than the body, while in P. platensis sp. nov. the paddles are shorter than the body; 3) the ventral accessory paddles of P. dolichoptera have serrated edges and are relatively shorter: ca. one fourth of the main paddle length, while in P. p l a t e n s i s sp. nov. these paddles have smooth margins and are somewhat more than half as long as the major paddles; 4) in P. dolichoptera the manubria have well-developed lamellae, while in P. platensis sp. nov. lamellae are absent; and 5) in P. dolichoptera the basal apophyses of rami have a knob-like recurved (anteriorly directed) termination, while in P. platensis sp. nov. the protuberance is directed to posteriorly. Other differences also exist, which are not easy to circumscribe in terms of shape or size, but are clearly identifiable when specimens are compared. Examples include, the shape of the tips and inner margin of the rami, the size of the teeth, and the basal structure of the rami. P. platensis sp. nov. also shares several features with P. l u m i n o s a. However, these two species are clearly distinguished by the following features: 1) in P. l u m i n o s a the shape of the accessory paddles is broad, lanceolate and with serrated edges, while in P. platensis sp. nov. these paddles are narrow, ligulate, and with smooth edges; 2) in P. luminosa the manubria have a lamella, while in P. platensis sp. nov. the manubria are subcyclindrical and bare; 3) in P. l u m i n o s a the unci are almost as long as the manubria, while in P. platensis sp. nov. the unci are definitely shorter, measuring about one third of the length of the manubria; 4) in P. luminosa the outer side of rami are bare, while they have a wide lamella in P. platensis sp. nov.; and 5) in P. l u m i n o s a the basal apophyses are bifurcated terminally, while in P. p l a t e n s i s sp. nov. they end in a knob. In P. platensis sp. nov., the anterior end of the rami (after the major tooth) resembles that of P. i n d i c a, described by Segers and Babu (1999). However, P. indica has four alternating teeth that precede the major tooth, while the manubria have a well-developed lamella, and the paddles are homogeneous in shape. Specimens of Polyarthra from the Broa reservoir in Brazil were named ‘ Polyarthra sp. near vulgaris Carlin’ by Segers and Dumont (1995). Because this species is also a member of the vulgaris -group, similarities exist with P. platensis sp. nov. However, the former species has a number of traits that differ from P. platensis sp. nov., such as, the shape of accessory ventral paddles, the absence of a pair of dorso-lateral expansions of the integument, the homogenous shape of the main paddles, and several details of trophi structure. The discovery of this new species, which, at first glance, is easily assignable to the P. vulgaris- group, suggests that a revision of the genus in the Neotropical region is pressingly needed.Published as part of José, Susana B., Paggi, De & Paggi, Juan C., 2011, A new species of Polyarthra Ehrenberg, 1834 belonging to the vulgaris- group (Rotifera: Monogononta: Synchaetidae) from Argentina, with a key to the identification of species in the Neotropical Region, pp. 51-57 in Zootaxa 2828 on pages 52-56, DOI: 10.5281/zenodo.20376
Contact mechanics of functionally graded rough surfaces
No full textIn this paper the Greenwood and Williamson contact theory for microscopically
rough surfaces is generalized by considering a grading on the elastic modulus of the asperities.
This situation can be representative of surfaces receiving superficial treatments or characterized by
a chemical degradation. The effect of an exponential grading on the applied normal load, on the
relationship between the real contact area and the load, on the plasticity index, as well as on the
contact conductance is illustrated with detailed numerical examples
Nonlinear fracture dynamics of laminates with finite thickness adhesives
No full textFinite thickness interfaces, such as structural adhesives, are often simplified from the modeling point of view by introducing ideal cohesive zone models that do not take into account the finite thickness properties in the evaluation of the interface stiffness and inertia. In the present work, the nonlinear dynamic response of those layered systems is numerically investigated according to the finite element method. The weak form of the dynamic equilibrium is written by including not only the contribution of cohesive interfaces related to the virtual work exerted by the cohesive tractions for the corresponding relative displacements, but also considering the work done by the dynamic forces of the finite thickness interfaces resulting from their inertia properties. A fully implicit solution scheme both in space and in time is exploited and the numerical results for the double cantilever beam test show that the role of finite thickness properties is not negligible as far as the crack growth kinetics and the dynamic strength increase factor are concerned
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