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Topology design of compliant mechanisms has gained wide popularity among the scientific community, and their use in the mechanical engineering field is being of upmost importance. In this paper, an isogeometric analysis (IGA) formulation is used to solve the topology optimization problem of compliant mechanisms. Stress constraints are introduced in the problem to guarantee the attainment of realistic solutions. For this purpose, an overweight constraint is considered for the design process, replacing the use of local stress constraints. The material distribution in the domain is modeled with quadratic B-splines and with a uniform relative density within each element of the mesh. These strategies to define the material layout are used to compare the IGA-based formulation with the finite element (FEM) formulation. The IGA formulation provides several advantages with respect to the classical FEM-based approaches that are shown and analyzed with an input-parameters sensitivity analysis. The sensitivity analysis and the assessment of the importance of introducing of stress constraints in the problem are developed by solving two benchmark problems. Regarding the sensitivity analysis of input parameters, the results show that the ratio between the material and the springs stiffnesses is the parameter with the largest influence on the solutions of the problem. Moreover, the advantages of the IGA formulations over FEM formulations are related with the computational time, the smoothness of the structural borders, and the non-appearance of the checkerboard patterns. With respect to the stress constraints, the results show that they have to be considered in order to avoid instability and structural integrity problems. [ABSTRACT FROM AUTHOR]

This study deals with an experimental investigation of the influence of friction stir welding parameters on the subsequent formability during incremental forming of aluminium alloy 6082-T6 blanks. Five welding conditions are considered, based on different combinations of rotational speed and feed rate. Truncated cones are chosen as benchmarks for single point incremental forming and tested up to their rupture limits. Formability is inferred measuring the cone height values, together with strain distributions, as recorded on the outer surface. The results clearly highlight the detrimental influence of a high feed rate of 100− 1at the friction stir welding stage, leading to a significantly reduced formability in incremental forming. Regarding other parameters, formability levels are similar to those of the unwelded blanks. Local mechanical properties are investigated by means of hardness measurements, to assess the evolution through the weld, and conventional tensile tests. Digital image correlation is used to properly capture the highly heterogeneous strain field and as a result, to estimate the local yield stress levels. The stress mismatch between the heat affected zone and the nugget zone presents the same trend as the formability. Finally, scanning electron microscopy observations of the fractured samples in tension highlight more differences between the base material and the welded specimen than between the different welded ones. This study therefore emphasises the importance and relevance of an integrated mechanical design approach, considering both welding and forming processes as a whole. [ABSTRACT FROM AUTHOR]

Nucleic acid-derived indices such as RNA/DNA ratios have been successfully applied as ecophysiological indicators to assess growth, nutritional condition and health status in marine organisms given that they provide a measure of tissue protein reserves, which is known to vary depending on changes in the environment. Yet, the use of these biochemical indices on highly mobile large predators is scarce. In this study, we tested the applicability of using nucleic acids to provide insights on the ecophysiological traits of two marine mammal species (common bottlenose dolphins and short-finned pilot whales) and explored potential related factors (species, sex, season, and residency pattern), using skin tissue (obtained from biopsy darts) of apparently healthy and adult free-ranging animals. Significantly higher RNA/DNA ratios were obtained for bottlenose dolphins (p < 0.001), and for visitor pilot whales when compared with resident pilot whales (p = 0.001). No significant changes were found between the sexes. Based on the percentile approach, the samples contain individuals in a general good condition (as the 10th percentile is not closer to the mean than the 75th percentile), suggesting that the studied region of Macaronesia may be considered an adequate habitat. The combination of this effective tool with genetic sexing and photographic-identification provided an overall picture of ecosystem health, and although with some limitations and still being a first approach, it has the applicability to be used in other top predators and ecosystems. [ABSTRACT FROM AUTHOR]

Analysis of historical cartography, wind direction data, orientation of Volturno River mouth and shoreline changes through time point out a series of morphological changes due both to natural and man-induced processes, which occurred along the Domitia coast (Litorale Domitio) since the second half of the 50s. Meteorological time series analysis showed evidence of a change in wind frequency and direction before and after the second half of the 70s, with almost complete disappearance of wind calm periods. Particularly, after 1975, the angle of wave attack had changed and intensity and frequency increased, thus accelerating coastal erosion in some areas. The morphological response to these environmental changes, likely due to global warming and short climatic crises interacting with anthropic modifications of landscape, resulted in different orientation of the Volturno River mouth which rotated from NW (between the 50s and 70s) to SW (from the mid-70s until the present). Moreover, the NW-SE littoral drift caused a clockwise rotation of the entire coastal segment with fulcrum pole close to the river mouth, since the 50s. These findings may allow the assessment of land-use scenarios tuned with evolution trends in order to properly address planning of these reactive and dynamic landscapes. [ABSTRACT FROM AUTHOR]

Single point incremental forming (SPIF) is an emerging application in sheet metal prototyping and small batch production, which enables dieless production of sheet metal parts. This research area has grown in the last years, both experimentally and numerically. However, numerical investigations into SPIF process need further improvement to predict the formed shape correctly and faster than current approaches. The current work aims the use of an adaptive remeshing technique, originally developed for shell and later extended to 3D 'brick' elements, leading to a Reduced Enhanced Solid-Shell formulation. The CPU time reduction is a demanded request to perform the numerical simulations. A two-slope pyramid shape is used to carry out the numerical simulation and modelling. Its geometric difficulty on the numerical shape prediction and the through thickness stress behaviour are the main analysis targets in the present work. This work confirmed a significant CPU time reduction and an acceptable shape prediction accuracy using an adaptive remeshing method combined with the selected solid-shell element. The stress distribution in thickness direction revealed the occurrence of bending/unbending plus stretching and plastic deformation in regions far from the local deformation in the tool vicinity. [ABSTRACT FROM AUTHOR]

This paper focus on the simulation of plastic anisotropic effects, regarding both initial yielding and hardening, in sheet metal forming processes. A constitutive model for a general stress state, suitable for any yield criteria based on an anisotropic stress potential, is presented. In order to predict anisotropic hardening effects, such as the Bauschinger's effect, the model also accounts for a mixed isotropic-nonlinear kinematic hardening rule. Aiming to the application of this model to the Finite Element Method, two generalized stress integration algorithms are derived. The first is formulated using the forward-Euler approach and uses the sub-incrementation technique. This technique allows larger time steps without losing convergence and improving the quality of the results. The second one is based on the backward-Euler approach and includes a multi-stage methodology in order to help the convergence of the return mapping procedure. The model was implemented in the commercial finite element code Abaqus by means of a 'user-defined' material subroutine for Yld91 and Yld2004-18p yield criteria. The model and the algorithms were assessed in different sheet metal forming simulations: U-draw bending test and cylindrical cup drawing test. The obtained results were in good agreement with the experimental data available in the literature, and the comparison of springback results and earing profiles obtained with both algorithms are presented, along with the corresponding computational costs. [ABSTRACT FROM AUTHOR]

The efficiency and safety of vehicles represent today one of the most important lines of developing in the automotive industry, for example by the introduction of new materials. In fact, the investment in advanced materials represents one of the most important strategies to reduce injury among vehicle occupants in traffic accidents. Associated with the development of safety systems, there is also the possibility of improving efficiency by the introduction of materials that lead to weight reduction, having a direct impact on fuel consumption and carbon dioxide emissions. Metallic foams are one of these materials, due to the excellent ratio between mechanical properties and density. The main goal of this investigation is to study the mechanical behaviour of aluminium sandwich structures, composed by a metallic foam core with two outer layers of metallic sheets. With this work, the authors intend to contribute to a better understanding and consequently to provide design guidelines for the plastic forming of these composites. In order to correctly characterize the mechanical behaviour of the sandwich structure, the foam core and sheets were tested separately. For the aluminium sheet a series of tensile tests were performed, using samples obtained along three different angles to the rolling direction. For the metal foam core, uniaxial compression tests were used. Finally, with the numerical model defined considering isotropic and anisotropic constitutive models, a set of numerical and experimental bulge tests were performed to evaluate the capacity of forming of these panels, using hydroforming processes. [ABSTRACT FROM AUTHOR]

In isogeometric analysis (IGA), the functions used to describe the CAD geometry (such as NURBS) are also employed, in an isoparametric fashion, for the approximation of the unknown fields, leading to an exact geometry representation. Since the introduction of IGA, it has been shown that the high regularity properties of the employed functions lead in many cases to superior accuracy per degree of freedom with respect to standard FEM. However, as in Lagrangian elements, NURBS-based formulations can be negatively affected by the appearance of non-physical phenomena that 'lock' the solution when constrained problems are considered. In order to alleviate such locking behaviors, the Assumed Natural Strain (ANS) method proposed for Lagrangian formulations is extended to NURBS-based elements in the present work, within the context of solid-shell formulations. The performance of the proposed methodology is assessed by means of a set of numerical examples. The results allow to conclude that the employment of the ANS method to quadratic NURBS-based elements successfully alleviates non-physical phenomena such as shear and membrane locking, significantly improving the element performance. [ABSTRACT FROM AUTHOR]

In this work, three distinct return mapping algorithms are presented and analyzed in detail: (i) a semi-explicit algorithm that accounts for the sub-incrementation technique, which reduces to (ii) a fully-explicit algorithm and, finally, (iii) a semi-implicit algorithm,. In order to describe the complex anisotropic behaviour of some metals, such as aluminium alloys, two non-quadratic anisotropic yield criteria were implemented: the Yld91 and Yld2004-18p. The performance of the developed algorithms is inferred in a series of sheet metal forming benchmarks and the quality of the results is assessed when compared to experimental results presented in the literature. The numerical simulations show that the semi-implicit algorithm is quite efficient with the von Mises yield criterion. However, when anisotropy is taken into account, the algorithm requires several iterations to return the stresses to the yield surface, particularly when the stresses are located at corner regions of that surface. The semi-explicit algorithm proved to be the most robust and efficient algorithm with anisotropic yield criteria. The good agreement between the experimental data and the obtained numerical results demonstrate the high efficiency of the presented algorithms and the ability of the anisotropic criteria to predict the material's complex anisotropic behaviour. [ABSTRACT FROM AUTHOR]

This paper presents three optimal low-order fully integrated geometrically nonlinear solid-shell elements based on the enhanced assumed strain (EAS) method and the assumed natural strain method for different types of structural analyses, e.g. analysis of thin homogeneous isotropic and multilayer anisotropic composite shell-like structures and the analysis of (near) incompressible materials. The proposed solid-shell elements possess eight nodes with only displacement degrees of freedom and a few internal EAS parameters. Due to the 3D geometric description of the proposed elements, 3D constitutive laws can directly be employed in these formulations. The present formulations are based on the well-known Fraeijs de Veubeke-Hu-Washizu multifield variational principle. In terms of accuracy as well as efficiency point of view, the choice of the optimal EAS parameters plays a very critical role in the EAS method, therefore a systematic numerical study has been carried out to find out the optimal EAS parameters to alleviate different locking phenomena for the proposed solid-shell formulations. To assess the accuracy of the proposed solid-shell elements, a variety of popular numerical benchmark examples related to element convergence, mesh distortions, element aspect ratios and different locking phenomena are investigated and the results are compared with the well-known solid-shell formulations available in the literature. The results of our numerical assessment show that the proposed solid-shell formulations provide very accurate results, without showing any numerical problems, for a variety of geometrically linear and nonlinear structural problems. [ABSTRACT FROM AUTHOR]

The main goal of the present paper is the numerical study of the factors influencing the onset and propagation of wrinkling defects in sheet metal forming, by means of the Finite Element Method (FEM). Doing so, the authors focused the analyzes on two particular points: a) the influence of both a given finite element formulation and numerical integration choice on the onset of wrinkling in the walls and flange zones of cup drawing formed parts; and b) the influence of the chosen anisotropic constitutive model and corresponding parameters in the correct prediction and propagation of wrinkling deformation modes during forming operations. [ABSTRACT FROM AUTHOR]

This work deals with the identification of constitutive parameters by inverse methodology. Two different approaches are presented and analysed: the single-point and FE analysis. The use of these two different methodologies for the evaluation of objective functions in the identification process is still an open question and the interest in this field has been increasing among the metal forming community. To discuss this issue, two different constitutive models suitable for metals were used, i.e. an elastoplastic hardening model and an elastoplastic model with isotropic and kinematic hardening. The determined material parameters for the two models, the respective objective function values and the CPU time required to perform the simulations are presented and discussed. [ABSTRACT FROM AUTHOR]

Reinforced structures for aircraft fuselages are conventionally composed by base (skin) aluminium plates and reinforcement elements (stringers), joined by riveting operations. During the last decade more effective approaches for reinforced fuselage and wings, such as the Integrally Stiffened Panels (ISP), have appeared. These homogeneous reinforced structures are obtained in an integral form by extrusion, allowing for lower manufacturing costs. During service conditions, these structures can be subjected to extreme compressive loading conditions and, due to their slenderness and low weight, ISP design must account for a reliable determination of buckling loads. However, complexities of the cross-sectional geometrical shapes, together with the occurrence of elastoplastic non-linear effects prior or after buckling, completely impair the use of analytical tools, being the analysis by the Finite Element Method (FEM) imperative in a reliable design process. In the present work, the structural performance of ISP structures is assessed, accounting for buckling in the elastoplastic range, by means of numerical simulation with the Finite Element Method. Also, the buckling load-carrying capacity of multiple sets of reinforced structures, composed by a finite number of ISP and joined by friction stir welding (FSW) operations, is also studied. In doing so, it is possible to numerically infer about the influence of the presence of FSW zones in the overall stiffness and mechanical behaviour of ISP structures with complex cross-section geometries. [ABSTRACT FROM AUTHOR]

Traffic-related accidents are a major threat to life in the European Union. In spite of the significant improvements in vehicle safety over the past 25 years, the current number of deaths (42000) and injuries (1.6E6), plus all the associated social and economic costs, remain unacceptable. It is generally considered that these numbers are an unacceptable high burden for Europe´s society and economy. World-wide vehicles safety experts agree that significant further reductions in fatalities and injuries can be achieved as a result of, for instance and among other factors, the use of new energy absorbing materials. In this field, passive safety systems still have great potential to reduce fatalities and injuries, as in the case of using new lightweight energy-absorbing materials. From the environmental view, the use of materials optimized in terms of greater absorption of energy by impact, in the weight, has a direct impact on thermal efficiency of engines, from the standpoint of a higher yield of fuel and emits less greenhouse gases. Metal foams can provide high resistance ratios and stiffness in relation to its mass or weight. These high specific structural properties make the metal foams highly attractive from the point of view of structural applications “ultra-light” [, ]. [ABSTRACT FROM AUTHOR]

Aircraft fuselage panels are conventionally differential reinforced structures, with aluminum skin and stringers being conventionally joined by riveting operations. As a consequence, these structures can present a number of distinct components, making assembly operations time consuming and expensive. Recently, developments in aluminum extrusion have opened up possibilities for new design procedures for aircraft applications. Particularly, complex-section panels known as Integrally Stiffened Panels (ISP), which are reinforced structures integrally obtained from extrusion, are a good example. In structural terms, ISP may be considered as thin-walled structures in which the stability for compressive loads is strongly dependent on the buckling strength of the cross section itself. Besides the difficulties in the project for buckling deformation modes within the elasto-plastic range, in aeronautical design it is mandatory to find the best compromise between weight and structural performance. In this sense, the present work presents an optimization algorithm based on a Hybrid Differential Evolution and Particle Swarm Optimization (HDEPSO) formulations, applied in finding the optimal cross section shape and dimensions for general ISP structures, under compressive buckling loads and accounting for elasto-plastic behaviors. The results and design guidelines are obtained by a set of parameterized nonlinear finite element simulations in ABAQUS, accounting for buckling carrying load levels as the principal design criteria. [ABSTRACT FROM AUTHOR]

Incremental sheet forming technology is nowadays a promising process to produce small batch components, with a growing potential application into the field of rapid prototyping, part’s customization or biomechanics. Doing so, its major features are the low cost, relative simplicity and flexible implementation for complex deep-drowned parts, when compared to well-established sheet metal forming technologies. Nevertheless, although many experimental and numerical studies have being carried out recently, the low speed of production and low dimensional precision are still the main obstacles in the application of this technique. The present work concerns and is focused on a preliminary numerical study of some of the issues raised in the numerical simulation of single point incremental forming processes (SPIF) by means of the Finite Element Method. To do so, simulation of well-known SPIF benchmark is carried out, based on solid and solid-shell finite element formulations, along with implicit integration schemes. Comparisons are made between several types of three-dimensional finite elements formulations as well as analyzing the influence of the number of through-thickness layers adopted. Furthermore, solid-shell finite element formulations based on the enhanced assumed strain method are also taken into account, allowing for models with just one layer of finite elements but including a variable number of integration points along the thickness direction. The obtained results are compared with experimental values. [ABSTRACT FROM AUTHOR]

Despite their high specific stiffness and strength, laminated composite materials, e.g. fibre-reinforced plastic plies stacked at different fibre orientations, are susceptible to damage. Damage can be divided into interalaminar damage and interlaminar damage. Delamination is a typical kind of interlaminar damage which occurs in laminated composite materials, often accompanied with intralaminar damage, and may lead to a catastrophic structural collapse. The first and most crucial step in the prediction of failure of Laminated Composite Structures (LCS) is to accurately determine the stresses, particularly the three transverse stress components, also called the interlaminar stresses. It is proposed in the present paper that the integration of a displacement based solid-shell formulation and partial-hybrid stress formulation will lead to an accurate and robust solid-shell element, suitable for the efficient and detailed interlaminar stress calculation. [ABSTRACT FROM AUTHOR]

The application of layerwise theories to correctly model the displacement field of sandwich structures or laminates with high modulus ratios, usually employs plate or facet shell finite element formulations to compute the element stiffness and mass matrices for each layer. In this work, a different approach is proposed, using a high performance hexahedral finite element to represent the individual layer mass and stiffness. This 8-node hexahedral finite element is formulated based on the application of the enhanced assumed strain method (EAS) to resolve several locking pathologies coming from the high aspect ratios of the finite element and the usual incompressibility condition of the core materials. The solid-shell finite element formulation is introduced in the layerwise theory through the definition of a projection operator, which is based on the finite element variables transformation matrix. The new finite element is tested and the implemented numerical remedies are verified. The results for a soft core sandwich plate are hereby presented to demonstrate the proposed finite element applicability and robustness. [ABSTRACT FROM AUTHOR]

Developments in the numerical simulation of the hydroforming process of tubular metallic components, tailor-welded before forming, are presented. Both technologies, tailor-welded joining operations and hydroforming processes, are well known in industry, although most commonly separately used. Tailor-welded joining operations are usually encountered in plain sheets, subsequently formed by stamping. Tube hydroforming processing, on the other hand, is frequently associated with parts consisting of uniform thickness, material properties, and dimensions. The present analysis focuses on the influence of process parameters, such as the position of the weld-line and initial thickness values, in the innovative process of combining tailor-welded tubes (with distinct thickness values) and hydroforming. Particular attention is posed on the relation of imposed axial displacement vs. imposed hydraulic pressure into the tube, forming parameters that are not known a-priori in the manufacturing of a new part. Another point of practical interest is the numerical simulation of the weld-line movement, after forming is complete. The finite element method is directly employed in the numerical simulation by means of innovative solid elements suited for incompressibility applications, and included by the authors into the commercial program ABAQUS as user-elements. The obtained results can then lead to a better understanding, along with design tools, for the process of hydroforming of tailor-welded tubular parts, accounting for dissimilar thickness of the basic components. [ABSTRACT FROM AUTHOR]

This study describes the development and full validation of a stability-indicating HPLC method to quantify ritonavir (RTV) and lopinavir (LPV) in soft gelatin capsules. The method uses a LiChrospher® 100 RP-18 (250 mm × 4.6 mm, 5 µm, Merck) column and isocratic elution. The mobile phase consisted of a mixture of acetonitrile-water-methanol (53:37:10, v/v/v), pumped at a flow-rate of 1.0 mL min−1 and UV detection at 210 nm using a photodiode array detector. LPV and RTV were exposed to thermal, photolytic, hydrolytic and oxidative stress conditions, and the stressed samples were analyzed by the proposed method. The response was linear over a range of 40-360 µg mL−1 for LPV and 10–90 µg mL−1 for RTV ( r > 0,999 for both drugs). The mean recoveries were 99.46 and 100.81% for LPV and RTV, respectively. The RSD values for intra- and inter-day precision studies were < 0.70% for both drugs. Degradation studies showed that lopinavir is stable in thermal, alkaline and oxidative conditions, while ritonavir degraded under these conditions. The method was found to be stability-indicating and can be used for the routine analysis of the association LPV/RTV in soft gelatin capsules. [ABSTRACT FROM AUTHOR]

In this work a previously proposed solid-shell finite element, entirely based on the Enhanced Assumed Strain (EAS) formulation, is extended in order to account for large deformation elastoplastic thin-shell problems. An optimal number of 12 enhanced (internal) variables is employed, leading to a computationally efficient performance when compared to other 3D or solid-shell enhanced elements. This low number of enhanced variables is sufficient to (directly) eliminate either volumetric and transverse shear lockings, the first one arising, for instance, in the fully plastic range, whilst the last appears for small thickness’ values. The enhanced formulation comprises an additive split of the Green-Lagrange material strain tensor, turning the inclusion of nonlinear kinematics a straightforward task. Finally, some shell-type numerical benchmarks are carried out with the present formulation, and good results are obtained, compared to well-established formulations in the literature. [ABSTRACT FROM AUTHOR]

This work aims to assess the influence of different finite element formulations in the performance and quality of solution obtained by numerical simulation in the analysis of tailor-welded hydroformed tubular parts. Tube hydroforming represents a cost effective forming process for high-strength, low weight products on, as an example, automotive and airspace applications. On the other side, the use of tailor-welding in order to obtain custom-made combinations of thicknesses and materials - leading to a wide variety of user-defined products - can be introduced into conventional tubular hydroforming processes in order to further improve the applicability range of the later process. The main goal of the present work is to describe the state-of-the-art in the field, focusing on distinct finite element formulations and providing guidelines for the simulation of tubular hydroforming process combined with tailor-welded joining techniques. Hexahedral solid and solid-shell enhanced assumed strain elements, either with reduced and full numerical integration procedures, are analyzed in order to infer about the potentialities of the combined forming technology. Material characterization of the heat affected zone is included and the influence of finite element modeling on defects onset and prediction during forming is considered. [ABSTRACT FROM AUTHOR]

An unusual case of bilateral aneurysms of the internal cervical carotid arteries in a 38 year old man with right hemiplegia and aphasia is described. [ABSTRACT FROM AUTHOR]