Abaqus UGENS subroutine for nonlinear analysis of periodic panels

This report describes an Abaqus UGENS subroutine for geometric and material nonlinear analysis of periodic panels using the first-order shear deformation theory. The structure is modelled with shell elements, as one layer of equivalent mechanical properties. The subroutine modifies the stiffness matrix of each shell element of the mesh separately based on its strain state. It relies on pre-computed stiffness curves that define the ABCD stiffness matrix of a unit cell. By looking at combinations of force and strain, the code interpolates the stiffness curves to calculate equivalent nonlinear stiffness. Complex stress states with different types of nonlinearity occurring simultaneously in the structure can be described. The examples show that the subroutine can deal with nonlinearities such as global buckling, local buckling and post-yield response with good accuracy and low computational cost compared to conventional FEM. The report includes the necessary information to set up the subroutine, including selection and compatibility of software and packages and input file preparation. Web-core sandwich panels are used as example throughout the report; the same principles are valid for any periodic structure. The full implementation is given in Appendix

Rationale, study design, and analysis plan of the Alveolar Recruitment for ARDS Trial (ART): Study protocol for a randomized controlled trial

Background: Acute respiratory distress syndrome (ARDS) is associated with high in-hospital mortality. Alveolar recruitment followed by ventilation at optimal titrated PEEP may reduce ventilator-induced lung injury and improve oxygenation in patients with ARDS, but the effects on mortality and other clinical outcomes remain unknown. This article reports the rationale, study design, and analysis plan of the Alveolar Recruitment for ARDS Trial (ART). Methods/Design: ART is a pragmatic, multicenter, randomized (concealed), controlled trial, which aims to determine if maximum stepwise alveolar recruitment associated with PEEP titration is able to increase 28-day survival in patients with ARDS compared to conventional treatment (ARDSNet strategy). We will enroll adult patients with ARDS of less than 72 h duration. The intervention group will receive an alveolar recruitment maneuver, with stepwise increases of PEEP achieving 45 cmH(2)O and peak pressure of 60 cmH2O, followed by ventilation with optimal PEEP titrated according to the static compliance of the respiratory system. In the control group, mechanical ventilation will follow a conventional protocol (ARDSNet). In both groups, we will use controlled volume mode with low tidal volumes (4 to 6 mL/kg of predicted body weight) and targeting plateau pressure <= 30 cmH2O. The primary outcome is 28-day survival, and the secondary outcomes are: length of ICU stay; length of hospital stay; pneumothorax requiring chest tube during first 7 days; barotrauma during first 7 days; mechanical ventilation-free days from days 1 to 28; ICU, in-hospital, and 6-month survival. ART is an event-guided trial planned to last until 520 events (deaths within 28 days) are observed. These events allow detection of a hazard ratio of 0.75, with 90% power and two-tailed type I error of 5%. All analysis will follow the intention-to-treat principle. Discussion: If the ART strategy with maximum recruitment and PEEP titration improves 28-day survival, this will represent a notable advance to the care of ARDS patients. Conversely, if the ART strategy is similar or inferior to the current evidence-based strategy (ARDSNet), this should also change current practice as many institutions routinely employ recruitment maneuvers and set PEEP levels according to some titration method.Hospital do Coracao (HCor) as part of the Program 'Hospitais de Excelencia a Servico do SUS (PROADI-SUS)'Brazilian Ministry of Healt

A nonlinear modeling approach for corrugated sandwich beams

Defence is held on 6th August 2021 at 12:00 Zoom link: https://aalto.zoom.us/j/64377696949Enhancements in structural efficiency are central in reducing the carbon footprint of the transport industry. With recent manufacturing advances, sandwich panels became viable to reduce the structural weight in large plated structures. Methodologies for their structural modeling and optimization have, however, limitations, as simplified models cannot predict scale interactions that arise in lightweight settings. This dissertation proposes a scale-dependent modeling approach to predict the geometrically nonlinear response of elastic sandwich beams. The approach can predict size effects and the influence of local elastic buckling in the global beam response. In this work, a computationally efficient multiscale approach is defined. A couple stress-based beam model is employed to describe the global behavior, with constitutive relations that represent unit-cell deformation modes. Scale transitions are embedded in the beam constitutive matrix. Terms associated with the axial behavior of the sandwich face sheets are progressively adapted according to the global strain-state and an associated local model. Stress recovery is pursued consistently with the averaging rules, including recovery of periodic terms. A finite element based on previous works is presented along with adapted nonlinear root-finding schemes to solve the equilibrium equations. Stiffness properties of selected sandwich cells are derived and presented in closed form. The results reveal that the modeling approach succeeds in predicting the nonlinear response of elastic sandwich panels under quasi-static loads. Accurate stress distributions were obtained for linear and moderately nonlinear responses. Bending and progressive buckling geometric failure paths were successfully traced, including the effect of progressive local face sheet buckling. The approach was tested against different combinations of structural parameters, revealing a wide applicability range. With the present approach, accurate elastic response predictions result with low modeling and computational costs. Reliable output is obtained, from beams with lightweight local-buckling-prone to denser size-effect-sensitive unit cell setups. The approach offers substantial improvements in relation to other low-complexity models available in the literature. Unlike Cauchy-based models, it is able to describe size-dependent behavior through the couple stress-related parameters. In relation to conventional single-layer models, it incorporates nonlinear local scale information to the average global continuum. In the linear scale-independent case and for an antiplane core, the model reduces to the textbook thick-face sandwich theory

Size-dependent modelling of elastic sandwich beams with prismatic cores

Sandwich panel strips with prismatic cores are modelled using the modified couple stress theory and their elastic size-dependent bending behaviour investigated. Compatibility between the discrete sandwich and continuum beam kinematics is first discussed. A micromechanics-based framework to estimate effective mechanical properties is provided and unit cell models constructed with elementary beam elements to determine the stiffness parameters of various prismatic cores. Numerical studies show that the modified couple stress Timoshenko beam enhances the static deflection predictions of the classical Timoshenko model. A sensitivity measure based on structural ratios is proposed to estimate the influence of size effects in the global beam-level response. The parameters governing size effects in elastic sandwich beams are identified: Face-to-core thickness ratio, core density and topology, vertical corrugation order and set of load and boundary conditions. Size effects are shown more pronounced in low-density coresthat rely on corrugation bending as shear-carrying mechanism. Based on the external load, boundary conditions and sensitivity factor, one can assess whether size effects are non-negligible in a given engineering structure.Peer reviewe

A nonlinear couple stress model for periodic sandwich beams

| openaire: EC/H2020/745770/EU//SANDFECHA geometrically nonlinear model for periodic sandwich structures based on the modified couple stress Timoshenko beam theory with von Kármán kinematics is proposed. Constitutive relations for the couple stress beam are derived assuming an antiplane core and then extended for a generic periodic cell. A micromechanical approach based on the structural analysis of a unit cell is proposed and utilized to obtain the stiffness properties of selected periodic sandwich beams. Then, a localization scheme to predict the stress distribution over the faces of the selected beams is defined. The present model is shown to be equivalent to the thick-face sandwich theory for a linear elastic antiplane core cell. Numerical studies validate the present model against three-dimensional finite element models and the thick-face sandwich theory, and compare it with the conventional Timoshenko and couple stress Euler-Bernoulli beam theories. The present model is shown to predict deflections, stresses and buckling loads with good accuracy for different periodic cell setups. The model is able to describe elastic size effects in shear-flexible sandwich beams and the core stiffness influence on membrane and bending stress resultants.Peer reviewe

Stress magnification factor for angular misalignment between plates with welding-induced curvature

The construction of lightweight structures poses new design challenges as a result of the different mechanics of deformation experienced by thinner-plated structures. Because of a reduced bending stiffness, thin plates are particularly sensitive to welding-induced distortions, which include a curvature, in addition to the axial and global angular misalignment observed on thick plates. The curvature shape and amplitude determine a local angular misalignment at the welded joint, which causes non-negligible secondary bending effects. Therefore, the commonly used stress magnification factors km solution for flat plates need a further development to include the curvature effect. This study proposes new analytical formulations, which extend the applicability of the existing solutions to the assessment of the structural stress of a curved thin plate under an axial load. The improved formulations are consistent with the geometrical non-linear Finite Element Analysis under compression (up to 80% of the buckling limit) and tension (up to the yield strength). A sensitivity analysis is presented in order to show the dominant role of the curvature effect in the estimation of the km factor. Regardless of the load applied, the presence of the curvature causes a flat plate solution inaccuracy greater than 10% when the local angular misalignment is more than 1:25 times higher than the global angular misalignment in the case of a thin and slender structure.Peer reviewe

HOMOGENIZED AND NON-CLASSICAL BEAM THEORIES IN SHIP STRUCTURAL DESIGN - CHALLENGES AND OPPORTUNITIES

The paper gives an overview of the recent developments on the application of homogenized, non-classical beam theories used to predict the micro- and macrostructural stresses in the design of marine structures. These theories become important when ultralight-weight marine structures are developed and one needs to explore the regions where the length scales of beam openings are in the range of the characteristic lengths of the beams or when lattice/frame-type beams are used to reduce the weight of ship structures. The homogenized beam models are based on non-classical continuum mechanics that allow local bending inside the beams. This added feature allows the treatment of size effects with great accuracy. The resulting analytical and finite element models have special features in terms of shape functions and iterative solutions in non-linear problems. Non-classical beam models enable localization processes that recover the microstructural effects from homogenized solutions accurately and the models are able to handle limit states of serviceability and ultimate strength. The non-classical models are validated by experiments and 3D FE simulations of periodic beams and plates. The non-classical beam theories converge to the physically correct solutions for wider range of beam parameters than the classical beam theories do.Peer reviewe

A review on non-classical continuum mechanics with applications in marine engineering

| openaire: EC/H2020/745770/EU//SANDFECHMarine structures are advanced material and structural assemblies that span over different length scales. The classical structural design approach is to separate these length scales. The used structural models are based on classical continuum mechanics. There are multiple situations where the classical theory breaks down. Non-classical effects tend arise when the size of the smallest repeating unit of a periodic structure is of the same order as the full structure itself. The aim of the present paper is to discuss representative problems from different length scales of ship structural design.Peer reviewe