1,720,964 research outputs found
Ductile damage assessment of Ti6Al4V, 17-4PH and AlSi10Mg for additive manufacturing
No full textA great research effort has been spent during the latest years in the characterization of additive manufacturing (AM) alloys, mostly focused on the analysis of microstructure and on the assessment of mechanical strength, especially under high cycle fatigue loads. Post-process treatments have been also investigated, as methods to further improve the AM materials performance. On the contrary, still fewer data are available on AM materials ultimate static strength. The present paper is intended to present a comprehensive experimental and numerical static characterization of Ti6Al4V, 17-4PH, and AlSi10Mg alloys processed via selective laser melting. A dedicated set of specimen geometries was devised to induce desired multiaxial stress state, and experiments were carried out both on as built and machined AM samples. The results were employed to identify the constitutive behavior of the materials and to calibrate four different ductile damage models. The failure prediction capabilities of the tuned models were thoroughly analyzed and discussed. The overall mechanical properties and the ductility of the investigated alloys were estimated based on the experimental results and on the information provided by the tuned models. Additionally, a comparison with data collected on the corresponding wrought materials, performing the same experiments, was carried out. The results showed a limited reduction of yield and failure strength and a significant reduction in the ductility of AM materials with respect to their wrought counterparts. Moreover, for the less ductile alloys, a weaker dependence of the strain to fracture from the stress state was observed
Local formability and strength of TWIP-TRIP weldments for stamping tailor welded blanks (TWBs)
Full text linkThe assessment of the local formability and mechanical behavior of weldments in tailor welded blanks (TWBs) made of advanced high strength steels is of crucial importance in the automotive industry, both for the mechanical design of tailored components, and to set up accurate sheet forming numerical simulations. This work investigates the formability of TWBs made of a twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) steel, determining the local elastoplastic behavior of the different regions of the weldment. To this purpose, several micro-samples were extracted along the weld seam and heat affected zones and then subjected to tensile tests. Tensile tests were also performed on macro-samples to assess the overall mechanical behavior of the TWBs during plastic deformation. A digital image correlation (DIC) technique was used to measure the surface strain field of both micro- and macro-samples, to calculate the plastic strain ratio, through-thickness strain, and to point out possible inhomogeneities in the plastic behavior of the TWB weld beads. The TWB joint exhibited a significant formability of the fusion zone and of the TWIP heat affected zone, whereas it was rather limited at the TRIP side. Locally, an uneven plastic behavior within the fusion zone was observed, with a variable plastic strain ratio (i.e., thinning) in both axial and transverse direction. The other regions of the weld, instead, deformed more homogeneously. Preliminary considerations about the numerical modeling of TWIP/TRIP TWBs and their prospective use in sheet forming operations have been outlined
Global-local characterization and numerical modeling of TWB laser welded joints
No full textSheet forming of tailor welded blanks (TWBs) of advanced high strength steels is complex because of the notable differences in mechanical properties, and hence in formability, of base metals, heat affected zones and weld seam. In this work, an accurate characterization of the mechanical behavior of these regions in TWBs made of a DP and TRIP steel was carried out. Micro-samples, machined from base metals and fusion zone were employed to retrieve the local constitutive laws up to fracture. At the same time, macro-samples, extracted throughout the welded joint were tested to assess the weldment overall behavior. Along with global load-displacement data, strain and displacement fields of the joint were evaluated, using a Digital Image Correlation technique. An FE simulation of the entire joint was setup, using the previously identified material properties. In a comparison between the FE model and experiments, good results were obtained both at a global and local level, up to fractur
A study on the dynamic structural behavior of olympic sabres
Full text linkSabres used in Olympic fencing are subject to severe elastic deformations during matches and training sessions. Even though strict rules for their manufacturing are prescribed by the international fencing federation, with requirements in terms of geometrical constraints and material (steel) properties, nonetheless frequent unexpected ruptures are observed. These may cause injuries to the fencers, and involve the replacement of the blade. In this study an experimental-numerical approach is adopted to investigate the underlying failure mechanisms. To this purpose, several attacks, "bouts" in fencing, were live filmed during actual practice with digital cameras and a trajectory tracking analysis was performed on the most critical of them, taking advantage of markers fixed on the blades at different positions. The post-processed data were subsequently used as boundary conditions of a 3D finite element model of the blade. Running a non-linear transient analysis, global and local quantities such as maximum stored elastic energy, stress and strain states, strain rates and possible permanent plastic strains were evaluated. A validation of the FE model with experiments was also carried out. From the critical analysis of experimental and numerical results it was possible to speculate about the influence of materials and dynamic related effects on the structural behavior of the blade. Eventually, hypotheses on fracture mechanisms were formulated
Calibration and prediction assessment of different ductile damage models on Ti6Al4V and 17-4PH additive manufactured alloys
Full text linkNowadays, metal additive manufacturing is becoming always more popular, being able to deliver complex shaped high quality products. Though many studies have been conducted on the high cycle fatigue behavior of these materials, yet ductile failure has still not been completely investigated, to identify the failure limits under static complex stress states. In the present study, the calibration of three ductile damage models on two popular additive manufactured alloys was carried out. The selected alloys were Ti6Al4V, processed via Electron Beam Melting, and 17-4PH fabricated with Selective Laser Melting technology; both broadly used in actual industrial applications. For each material a set of samples, was fabricated to perform a thorough static mechanical characterization, involving tensile tests on round smooth bars, notched bars, tests under plane strain conditions and torsion tests. The stress state in the critical points was retrieved relying on FEM simulations, and the data collected via the hybrid experimental-numerical procedure subsequently used to tune the damage models. Specifically, the selected models are the Rice and Tracey, the Modified Mohr-Coulomb by Wierzbicki and the one proposed by Coppola and Cortese. While the former does not take into account the effect of Lode parameter, the latter two consider its influence on fracture onset. A minimization algorithm was used for their calibration, and different optimization strategies were adopted to check the robustness of identified parameters. The resulting strains to fracture as a function of damage parameters were plotted for each formulation. The failure prediction accuracy of all models was assessed and compared to the others
A mixed FEM and lumped-parameter dynamic model for evaluating the modal properties of planetary gearboxes
No full textPlanetary gearboxes are extensively used in mechatronic applications due to their compactness and high reduction ratios. The complex kinematic of such systems and the requirement of low-vibration-gears suggest studying these gearboxes with appropriate dynamic models. To this purpose, a planar lumped parameter model with 18 degrees of freedom was implemented. Model parameters such as the stiffness of the bearings and the gear meshes were calculated using dedicated FEM simulations. Two different gearbox configurations were simulated and experimentally tested. The compared results showed a good agreement. The main goal of the research was to assess the validity of such methodology for the prediction of the system eigenfrequencies. Additionally, from a kinematic analysis of the gearbox it was possible to individuate the self-excitation frequencies. The overlapping of these self-excited frequencies with the eigenfrequencies of the system was individuated as the major cause of the gearbox noise
Effects of Temperature and Strain Rate on the Ductility of an API X65 Grade Steel
No full textIn the last few decades, great effort has been spent on advanced material testing and the development of damage models intended to estimate the ductility and fracture of ductile metals. While most studies focused on static testing are applied at room temperatures only, in this paper, multiaxial tests have been executed to investigate the effects of dynamic action and temperature on the mechanical and fracture behavior of an API X65 steel. To this end, a Split Hopkinson Bar (SHB) facility for dynamic tests, and a uniaxial testing machine equipped with a high-temperature furnace, were used. Numerical simulations of the experiments were setup for calibration and validation purposes. Based on the experimental results, the Johnson–Cook and Zerilli–Armstrong plasticity models were first tuned, resulting in a good experimental–numerical match. Secondly, the triaxiality and Lode angle dependent damage models proposed by Bai–Wierzbicki and Coppola–Cortese were also calibrated. The comparison of the fracture surfaces predicted by the damage models under different loading conditions showed, as expected, an overall significant increase in ductility with temperature; an appreciable increase in ductility was also observed with the increase in strain rate, in the range of low and moderate triaxialities
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
- …
