1,720,979 research outputs found
Model calibration of 3D printed lattice structures
No full textNowadays cellular materials are receiving great attention for their excellent mechanical properties, being applied in energy absorbers or in structural components having optimized mass distribution. In this paper stretch-dominated lattice structures have been considered. A 3D periodic lattice structure of different cell size, TPMS (triply periodic minimal surface), made of epoxy resin by DLP technology was studied. Compression tests at different strain rate (10-3 to 103 1/s) have been performed and a constitutive model to assess the experimental findings has been calibrated
Influence of Core Cellular Structures on Collapse Mechanisms Maps of Sandwich Beams
No full textThe main advantage of 3D printing is manufacturing complex and innovative shapes which guarantee high mechanical properties. Therefore, it is necessary easily figure out the most suitable structure for the required design requirements. The well-known strategy to design sandwich panels is evaluating collapse maps as they determine the panel performances based on their geometrical features. The aim of this study is to update the traditional collapse maps by showing how the core shape can improve the sandwich beam performance. The collapse maps proposed are based on advanced analytical models than the traditional Gibson theories. The analytical modelling of the indentation phenomenon is based on Vlasov’s model. The analytical modelling of the bending phenomenon is based on the First Shear Order Theory. The overall panel stress and strain maps are computed superposing both effects. A composite sandwich panels with Gyroid core based are analyzed to verify the proposed model consistency. A core failure criterion is chosen by experimental testing evidence on the representative core structure. Once the computed stress state overtakes failure criterion ones, the critical load is defined. In the end, the model is exploited to compare the performances of four sandwich panels with cores based on different lattice structure
Modelling of low-velocity impacts on composite beams in large displacement
No full textThe paper provides an evaluation of the nonlinear dynamic response of a cantilever beam made of composite material subjected to low-velocity impacts. The structure is assumed to respond in a quasi-static manner and modelled by a continuous beam in large displacement with a lumped mass attached. First, an analytical model was developed to study the free vibrations of a beam, taking into account the nonlinearities due to large displacements and inertia. Then, the analytical findings were compared with experimental test data. The vibration of a real composite beam has been acquired through high-speed imaging technique. The displacements of the beam were extracted by digital image analysis; then, the nonlinear parameters of the analytical model were determined by the Fitting Time History technique. The results obtained by the analytical model and the experimental test are compared with numerical analysis. The validated analytical model was adapted to study a low-velocity impact; the lumped mass was associated with a rigid projectile, whose initial speed represents the impact velocity
High Strain Rate Tests by a 90 m Long Tension-Torsion Hopkinson Bar
No full textThis work describes the design, construction, and first experimental results of an innovative device of the Hopkinson bar type with a length of 90 m for performing high strain rate tests on metals in a combined tension-torsion state. Analogously to the classic split Hopkinson bar technique, the system configuration consists of three bars: a pre-stressed bar, an input bar, and an output bar; the measurement is also based on the classical three-wave method, where the incident, transmitted, and reflected waves are measured. The length of the bars is designed so that the tensile wave reaches the sample from the output bar side at the same time as the torsion wave comes from the input bar. A successful test has been conducted on a hollow aluminum sample; it has been possible to measure the tension-torsion stress-strain curves; in addition, the dynamic equivalent stress-equivalent strain curves have been evaluated
Effect of temperature and strain rate on the formation of shear bands in polymers under quasi-static and dynamic compressive loadings: Proposed constitutive model and numerical validation
Full text linkThe mechanical behaviour of polymers is strongly influenced by temperature and strain rate conditions. Their effects are clearly observed in the Young modulus and Yield strength assessment. In some cases, a change in material failure mode can be noted, which can be both ductile and brittle. For polymers exhibiting shear bands during compression loading, test conditions may enhance or diminish this effect. In this study, isotactic Polypropylene has been considered due to its strong sensitivity to test conditions; with some combinations of strain rate and temperature, the material is able to undergo large compression with nearly uniform strain distribution, whereas significant shear bands can be observed with other strain rate and temperature combinations. This paper proposes a constitutive law for the mechanical characterization of polymers that show shear banding in compression loading. The proposed law extends a visco-plastic model present in literature, allowing to predict the onset or not of micro-shear banding compression instability. The dependence of constitutive parameters with test conditions is determined by applying the Ree-Eyring equation. The model parameters have been calibrated analytically according to experimental data obtained from compression tests performed in a wide range of strain rates (10−3 to 103 1/s) and temperatures (253–313 K). In addition, the constitutive model has been implemented in a commercial FEM code by means of a user-defined subroutine. Results show a good correspondence between experimental observations and numerical calculations, regarding both stress-strain relationship and shear band formation
High strain rate compression behaviour of 3D printed Carbon-PA
Full text linkAbstract: In the last few years, Fused Filament Fabrication is growing in the industrial field for the manufacture of final products by using new materials with high mechanical performances. Among those, one of the strongest is Carbon-PA. This is a composite material made by Nylon thermoplastic matrix filled with short carbon fibers reinforces. The aim of this work is to investigate its mechanical properties in static and dynamic conditions. Cylindrical specimens were produced by extruding the material in the three main printing directions. Then, uniaxial quasi-static and dynamic compression tests have been performed to evaluate its strain rate sensitivity. Dynamic tests have been carried out through a direct Split Hopkinson Bar setup with a pulse-shaping technique. The results show a compression behaviour dependent on the printing direction and strain rate. The behaviour of Carbon-PA was different between static and dynamic condition, passing from ductile to brittle. Moreover, a tomography analysis was carried out on the samples to evaluate the voids distribution
Nonlinear vibrations of a composite beam in large displacements: Analytical, numerical, and experimental approaches
Full text linkThis paper investigates the nonlinear dynamic behavior of a cantilever beam made of composite material without and with lumped mass fixed along its length. The analysis compares the results coming from analytical and numerical modeling with experimental observations. The first part focuses on the analytical model. The model takes into account the nonlinearity derived from large amplitude vibration and inertia. The second part deals with the experimental test, where the specimen and the data acquisition are defined. Then, the nonlinearity of the acquired data is determined by the fitting time history (FTH) technique. The third part deals with the finite element model. Finally, the results obtained by the analytical method, the experimental method, and the numerical method are compared between each other
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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