Showing papers in "International Journal of Mechanics and Materials in Design in 2018"

A 2D topology optimisation algorithm in NURBS framework with geometric constraints

Abstract: In this paper, the solid isotropic material with penalisation (SIMP) method for topology optimisation of 2D problems is reformulated in the non-uniform rational BSpline (NURBS) framework. This choice implies several advantages, such as the definition of an implicit filter zone and the possibility for the designer to get a geometric entity at the end of the optimisation process. Therefore, important facilities are provided in CAD postprocessing phases in order to retrieve a consistent and well connected final topology. The effect of the main NURBS parameters (degrees, control points, weights and knot-vector components) on the final optimum topology is investigated. Classic geometric constraints, as the minimum and maximum member size, have been integrated and reformulated according to the NURBS formalism. Furthermore, a new constraint on the local curvature radius has been developed thanks to the NURBS formalism and properties. The effectiveness and the robustness of the proposed method are tested and proven through some benchmarks taken from literature and the results are compared with those provided by the classical SIMP approach.

Phase field simulations of coupled microstructure solidification problems via the strong form particle difference method

Abstract: This paper presents the development of a strong form-based collocation method called the particle difference method (PDM), capable of predicting the spatiotemporal evolution of polycrystalline material solidification through coupling of multi-phase and temperature fields. Cross coupled phase field evolution and heat transfer equations are discretized via the PDM to obtain the interface kinematics of polycrystalline boundary during solidification. A distinct feature of the PDM is its ability to represent derivative operators via a moving least-square approximation of the Taylor expansion through point-wise computations at collocation points. The method discretizes directly the strong forms using the pre-computed derivative operators at each collocation point and elegantly overcomes the topological difficulty in modeling intricate moving interfaces. To verify the efficacy of the PDM, numerical results are compared with those obtained from the conventional finite difference method for uniform and irregular distributions of the collocation points. The scalability of the parallelized PDM is tested by measuring its efficiency with increasing the number of processors. We also provide a solidification simulation with two ellipsoidal inclusions to demonstrate the capability of the PDM in complex moving interface problems with high curvature.

Vibration characteristics of moving sigmoid functionally graded plates containing porosities

Abstract: This study investigates vibration characteristics of longitudinally moving sigmoid functionally graded material (S-FGM) plates containing porosities. Two types of porosity distribution, i.e., the even and uneven distributions, are taken into account. In accordance with the sigmoid distribution rule, the material properties of porous S-FGM plates vary smoothly along the plate thickness direction. The nonlinear geometrical relations are adopted by using the von Karman non-linear plate theory. Based on the d’Alembert’s principle, the nonlinear governing equation of the system is derived. Then, the governing equation is discretized to a set of ordinary differential equations via the Galerkin method. These discretized equations are subsequently solved by using the method of harmonic balance. Analytical solutions are verified with the aid of the adaptive step-size fourth-order Runge–Kutta method. By using the perturbation technique, the stability of the steady-state response is highlighted. Finally, both natural frequencies and nonlinear forced responses of moving porous S-FGM plates are examined. Results demonstrate that the moving porous S-FGM plates exhibit hardening spring characteristics in the nonlinear frequency response. Moreover, it is shown that the type of porosity distribution, moving speed, porosity volume fraction, constituent volume fraction and in-plane pretension all have significant influence on the nonlinear forced responses of moving porous S-FGM plates.

Influence of porosity on the flexural and vibration response of gradient plate using nonpolynomial higher-order shear and normal deformation theory

Abstract: The present article deals with flexural and vibration response of functionally graded plates with porosity. The basic formulation is based on the recently developed non-polynomial higher-order shear and normal deformation theory by the authors’. The present theory contains only four unknowns and also accommodate the thickness stretching effect. The effective material properties at each point are determined by two micromechanics models (Voigt and Mori–Tanaka scheme). The governing equations for FGM plates are derived using variational approach. Results have been obtained by employing a C0 continuous isoparametric Lagrangian finite element with eight degrees of freedom per node. Convergence and comparative study with the reported results in the literature, confirm the accuracy and efficiency of the present model and finite element formulation. The influence of the porosity, various boundary conditions, geometrical configuration and micromechanics models on the flexural and vibration behavior of FGM plates is examined.

Predicting mechanical properties of fuzzy fiber reinforced composites: radially grown carbon nanotubes on the carbon fiber

Abstract: It is intended to predict mechanical properties of fuzzy fiber reinforced polymer. An appropriate computational modeling is developed on the basis of bottom-up modeling covering all involved scales of nano, micro, meso and macro. The effective parameters of each scale are identified using top-down scanning approach and defining a representative volume element for each scale of analysis. At nano-scale, mechanical properties of isolated CNT are estimated. Then, at the upper scale of micro, the interaction between CNT and surrounding polymer is investigated considering non-bonded van der Waals interactions. Mechanical properties of the CNT/polymer nanocomposite with radial arrangement of CNT are derived at meso scale. Subsequently, mechanical properties of a single fuzzy fiber encompassing core carbon fiber and surrounding CNT/polymer are calculated. Finally, mechanical properties of the uni-directional and short fuzzy fiber reinforced composites are evaluated at the scale of macro. Treating CNT volume fraction and its arbitrary non-straight shapes as random parameters, developed modeling is conducted stochastically. The results imply on the importance of stochastic modeling, since deterministic modeling is led to a noticeable overestimation in predicted results. A very good agreement is reported between predicted results by stochastic modeling and published experimental data in literature.

Numerical study on load-bearing capabilities of beam-like lattice structures with three different unit cells

Abstract: The design and analysis of lattice structures manufactured using additive manufacturing technique is a new approach to create lightweight high-strength components. However, it is difficult for engineers to choose the proper unit cell for a certain function structure and loading case. In this paper, three beam-like lattice structures with triangular prism, square prism and hexagonal prism were designed, manufactured by SLM process using AlSi10Mg and tested. The mechanical performances of lattice structures with equal relative density, equal base area and height, and equal length for all unit cells were conducted by finite element analysis (FEA). It was found that effective Young’s modulus is proportional to relative density, but with different affecting levels. When the lattice structures are designed with the same relative density or the same side lengths, the effective Young’s modulus of lattice structure with triangular prism exhibits the maximum value for both cases. When the lattice structures are designed with the same base areas for all unit cells, the effective Young’s modulus of lattice structures with square prism presents the maximum. FEA results also show that the maximum stress of lattice structures with triangular prisms in each comparison is at the lowest level and the stiffness-to-mass ratio remains at the maximum value, showing the overwhelming advantages in terms of mechanical strength. The excellent agreements between numerical results and experimental tests reveal the validity of FEA methods applied. The results in this work provide an explicit guideline to fabricate beam-like lattice structures with the best tensile and bending capabilities.

On the nonlinear dynamics of a piezoelectrically tuned micro-resonator based on non-classical elasticity theories

Abstract: Size dependent static and dynamic behavior of a fully clamped micro beam under electrostatic and piezoelectric actuations is investigated. The microbeam is modeled under the assumptions of Euler–Bernoulli beam theory. Viscous damping and nonlinearities due to electrostatic actuation and mid-plane stretching are considered. Residual stress and fringing field effect are taken into account as well. Governing equation of motion is derived using Hamilton’s principle along with the strain gradient theory (SGT), which is a non-classical continuum theory capable of taking size effect of elastic materials into account. Reduced order model of the partial differential equations of the system is obtained using Galerkin method. Static deflection, pull-in voltage and the primary resonance of the microbeam are examined and the effect of piezoelectric voltage and its polarization on the size dependent static and dynamic response is studied. It is found that the piezoelectric voltage can effectively change the flexural rigidity of the system which in turn affects the pull-in instability regime. The effect of material length scale parameter is examined by comparing the results of the SGT with the modified couple stress (MCST) and classical theory (CT), both of which are special cases of the former. Comparison demonstrates that the CT underestimates the stiffness and consequently the pull-in voltage and overestimates the amplitude of periodic solutions. The difference between the results of classical and non-classical theories becomes more and more as the dimensions of the system gets close to the length scale parameter. Non-classical theories predict more realistic behaviors for the micro system. The results of this paper can be used in designing microbeam based MEMS devices.

Analysis of smart damping of laminated composite beams using mesh free method

Abstract: This paper is concerned with the development of mesh free model for the performance analysis of active constrained layered damping (ACLD) treatments on smart laminated composite beams. The overall structure is composed of a substrate laminated composite beam integrated with a viscoelastic layer and a piezoelectric layer attached partially or fully at the top surface of the substrate beam. The piezoelectric layer acts as the active constraining layer of the smart beam and the viscoelastic layer acts as the constrained layer. A layer wise displacement theory has been used to derive the models. Both symmetric cross-ply and antisymmetric angle-ply laminated beams are considered for the numerical analysis. It is observed that ACLD treatment significantly improves the active damping properties of the substrate beam. The numerical results also reveal that the triangular ACLD treatment is more effective than the rectangular ACLD treatment of same thickness and volume for active damping of smart composite beams.

Planar Timoshenko-like model for multilayer non-prismatic beams

Abstract: This paper aims at proposing a Timoshenko-like model for planar multilayer (i.e., non-homogeneous) non-prismatic beams. The main peculiarity of multilayer non-prismatic beams is a non-trivial stress distribution within the cross-section that, therefore, needs a more careful treatment. In greater detail, the axial stress distribution is similar to the one of prismatic beams and can be determined through homogenization whereas the shear distribution is completely different from prismatic beams and depends on all the internal forces. The problem of the representation of the shear stress distribution is overcame by an accurate procedure that is devised on the basis of the Jourawsky theory. The paper demonstrates that the proposed representation of cross-section stress distribution and the rigorous procedure adopted for the derivation of constitutive, equilibrium, and compatibility equations lead to Ordinary Differential Equations that couple the axial and the shear bending problems, but allow practitioners to calculate both analytical and numerical solutions for almost arbitrary beam geometries. Specifically, the numerical examples demonstrate that the proposed beam model is able to predict displacements, internal forces, and stresses very accurately and with moderate computational costs. This is also valid for highly heterogeneous beams characterized by thin and extremely stiff layers.

Size-dependent internal resonances and modal interactions in nonlinear dynamics of microcantilevers

Abstract: The size-dependent internal energy transfer in the nonlinear dynamical behaviour of a microcantilever with an intermediate spring-support is investigated. A geometric size-dependent nonlinearity due to large changes in the curvature is taken into account in the longitudinal and transverse motions. Based on the modified couple stress theory, the potential energy of the system is developed; the kinetic energy is also constructed in term of the displacement field. The energy terms are balanced with the potential energy stored in the intermediate spring-support. The centreline-inextensibility assumption is applied leading to the continuous model of the system involving nonlinear inertial components as well as size-dependent nonlinear curvature components. Based on a weighted-residual technique, the continuous model is reduced and the resultant truncated model is solved via use of a continuation technique. The linear component of the truncated model is solved through an eigenvalue extraction method in order to verify the occurrence of internal energy transfer and modal interaction mechanisms. For the system tuned to internal resonances, the highly nonlinear dynamical response is obtained, taking into account both inertial and geometric (due to large rotations) nonlinearities. It is shown that taking into account the length-scale parameter changes the internal energy transfer mechanisms significantly.

Structural modelling of nanorods and nanobeams using doublet mechanics theory

Abstract: In this study, statics and dynamics of nanorods and nanobeams are investigated by using doublet mechanics. Classical rod theory and Euler–Bernoulli beam theory is used in the formulation. After deriving governing equations static deformation, buckling, vibration and wave propagation problems in nanorods and nanobeams are investigated in detail. The results obtained by using of doublet mechanics are compared to that of the classical elasticity theory. The importance of the size dependent mechanical behavior at the nano scale is shown in the considered problems. In doublet mechanics, bond length of atoms of the considered solid are used as an intrinsic length scale.

Optimization of material composition to minimize the thermal stresses induced in FGM plates with temperature-dependent material properties

Abstract: A hybrid genetic algorithm with the complex method is developed for the optimization of the material composition of a multi-layered functionally graded material plate with temperature-dependent material properties in order to minimize the thermal stresses induced in the plate when it is subjected to steady-state thermal loads. In the formulation, the plate is artificially divided into an n l -layered plate, and a weak-form-based finite layer method is developed to obtain the displacement and stress components induced in the n l -layered plate using the Reissner mixed variational theorem. Two thermal conditions, namely the specified temperature and heat convection conditions, imposed on the top and bottom surfaces of the plate are considered. The through-thickness distributions of the volume fractions of the constituents are assumed as certain specific/non-specific function distributions, such as power-law, sigmoid, layerwise step and layerwise linear function distributions, and the effective material properties of the plate are estimated using the Mori–Tanaka scheme. Comparisons with regard to the minimization for the peak values of the stress ratios induced in the FGM plates with various optimal material compositions are conducted.

The role of thermal residual stress on the yielding behavior of carbon nanotube–aluminum nanocomposites

Abstract: The initial yield envelopes of aluminum (Al) nanocomposites reinforced with carbon nanotubes (CNTs) subjected to biaxial loading are predicted in the presence of thermal residual stress (TRS) arising from the manufacturing process. Micromechanical model based on the unit cell method is presented to generate the yielding surfaces. The formation of the interphase caused by the interfacial reaction between the CNT and Al matrix is taken into account in the analysis. The effects of several important parameters, i.e. the change of temperature, CNT volume fraction, interphase thickness and Al material properties on the yielding onset of the CNT/Al nanocomposite are explored extensively. The results clearly reveal that the initial yield surfaces of nanocomposite are dependent on the TRS. Also, the interphase has a significant influence on the yielding behavior of Al nanocomposite in the presence of TRS. The results demonstrate that the size of initial yield surfaces become minimum with considering the coupled effects of TRS and interphase. With increasing the temperature variation, interphase thickness, elastic modulus and coefficient of thermal expansion of Al matrix, the size of initial yield surfaces reduces. The present study is consequential for understanding the key role of TRS on the initial damage of CNT/Al nanocomposites.

Size-dependent behavior of slacked carbon nanotube actuator based on the higher-order strain gradient theory

Abstract: This paper proposes to investigate the nonlinear size dependent behavior of electrically actuated carbon nanotube (CNT) based nano-actuator while including the higher-order strain gradient deformation, the geometric nonlinearity due to the von Karman nonlinear strain as well as the slack effect, and the temperature gradient effects. The assumed non-classical beam model adopts some internal material size scale parameters related to the material nanostructures and is capable of interpreting the size effect that the classical continuum beam model is unable to pronounce. The higher-order governing equations of motion and boundary conditions are derived using the so-called extended Hamilton principle. A Galerkin based reduced-order model (ROM) modal decomposition is developed to prescribe the non-classical nanotube mode shape as well as its static behavior under any applied DC actuation load. Results of the static analysis is compared with those obtained by both classical elasticity continuum and strain gradient theories. A Jacobian method is utilized to determine the variation of the natural frequencies of the nanobeam with the DC load as well as the slack level. A thorough parametric study is conducted to study the influences of the size scale dependent parameters, the geometric nonlinearity, the initial curvature, the gate voltage, and the temperature gradient effect on structural behavior of the CNT-based nano-actuator. It is found that the size effect based on the strain gradient deformation has significant influence on the fundamental nanotube natural frequency dispersion. Also, varying this size effect have revealed the offering of numerous possibilities of modes veering and crossing, all shown to be dependent of the strain gradient parameters as well as the CNT slack level.

Hybrid molecular dynamics–finite element simulations of the elastic behavior of polycrystalline graphene

Abstract: In this paper, we develop an efficient multiscale molecular dynamics (MD)–finite element (FE) modeling scheme capable of determining the elastic and fracture properties of polycrystalline graphene. The local elastic properties of a grain boundary (GB) connecting two adjacent graphene grains, with different lattice orientations, were first determined using MD simulations. In a two-dimensional medium, randomly distributed grains connected with GBs were then created using the Voronoi tessellation method. The constructed Voronoi diagrams were used to create FE models of the polycrystalline graphene, where the GBs were represented by interphase regions with their local properties determined using MD. The grains were modeled as pristine graphene and the accuracy of the polycrystalline FE model was validated with MD simulations of a geometrically identical polycrystalline graphene. The results reveal good agreement between MD and FE simulations. They further show that the elastic and fracture properties of polycrystalline graphene are greatly influenced by the grain size and the misorientation angle. They also indicate that the predicted elastic properties are in agreement with earlier reported experimental and MD results. We believe that this newly proposed multiscale scheme could be easily integrated into current design software to model graphene based nano- and micro-devices.

Effect of nonlocal elasticity on the performance of a flexoelectric layer as a distributed actuator of nanobeams

Abstract: The paper is concerned with the development of finite element model for the static analysis of smart nanobeams integrated with a flexoelectric layer on its top surface, using nonlocal elastic theory. The flexoelectric layer acts as a distributed actuator of the nanobeam. A layerwise displacement theory has been used to derive the element stiffness matrices from variational principles incorporating nonlocal effects. The finite element model for nonlocal response of the beams has been validated with the exact solution for the case of a simply supported standalone flexoelectric layer. Also, the finite element model of the simply supported smart beam has been validated with exact solutions and numerical models for the local elastic case. The performance of the flexoelectric actuator has been compared for different values of nonlocal parameters and different combinations of nonlocal and local elastic substrate and flexoelectric layer. Further, the model developed has been utlized for investigating the performance of the active flexoelectric layer in case of cantilever beam, for which the exact solutions are not available.

Preparations and tribological properties of soft-metal/DLC composite coatings by RF magnetron sputter using composite targets

Abstract: This work reports the characteristics and tribological properties of both Ag/DLC nanocomposite coatings (RF-Ag-DLC) and Cu/DLC nanocomposite coatings (RF-Cu-DLC) with hydrogen-free DLC matrix deposited by RF magnetron sputtering using a concentric composite target (CCT). The CCT consisted of a C base target and metal tablet, and the tablet was located on the center of the base target concentrically where the etching rate by Ar ions is extremely low. By changing the diameter of Ag or Cu tablets in CCT, RF-Ag-DLC with an Ag concentration ranging from 6 to 65 at.% and RF-Cu-DLC with Cu concentration ranging from 7 to 75 at.% can be prepared. These coatings show a granular structure having Ag or Cu nano-crystals with a diameter ranging from 5 to 10 nm dispersed homogeneously in the hydrogen-free DLC matrix. The friction coefficient of DLC varied depending on the species and content of metal. The transition of the friction coefficient became stable when metal-rich tribofilms formed on the counterfaces.

Buckling characteristics of nanocrystalline nano-beams

Abstract: The buckling and the postbuckling characteristics of nanocrystalline nano-beams with/without surface stress residuals are investigated. A hybrid model is proposed where a non-classical beam model is incorporated with a size-dependent micromechanical model. The micromechanical model has the merit of accounting for the beam material structure effects, i.e. the grain size and the grain boundary effects. To account for the beam size effects, the couple stress theory is implemented where some measures are added to capture the grain rigid rotation effects. The proposed hybrid model is harnessed to derive the governing equations of a nano-beam subjected to an axial compressive load accounting for the mid-plane stretching according to von-Karman kinematics and the surface stress residuals. Analytical solutions for the prebuckling and postbuckling configurations and natural frequencies as functions of the applied compressive axial load are derived. The effects of the beam material structure and the beam size on the beam’s prebuckling characteristics and the postbuckling configurations and natural frequencies are studied. The obtained results reveal that both the size and the material structure of nanobeams have great impacts on their buckling characteristics.

Effect of uncertainty in material properties on wave propagation characteristics of nanorod embedded in elastic medium

Abstract: The effect of uncertainty in material properties on wave propagation characteristics of nanorod embedded in an elastic medium is investigated by developing a nonlocal nanorod model with uncertainties. Considering limited experimental data, uncertain-but-bounded variables are employed to quantify the uncertain material properties in this paper. According to the nonlocal elasticity theory, the governing equations are derived by applying the Hamilton’s principle. An iterative algorithm based interval analysis method is presented to evaluate the lower and upper bounds of the wave dispersion curves. Simultaneously, the presented method is verified by comparing with Monte-Carlo simulation. Furthermore, combined effects of material uncertainties and various parameters such as nonlocal scale, elastic medium and lateral inertia on wave dispersion characteristics of nanorod are studied in detail. Numerical results not only make further understanding of wave propagation characteristics of nanostructures with uncertain material properties, but also provide significant guidance for the reliability and robust design of the next generation of nanodevices.

Numerical modeling of strain localization in engineering ductile materials combining cohesive models and X-FEM

Abstract: The present work aims at numerically predicting the current residual strength of large engineering structures made of ductile metals against accidental failure. With this aim in view, the challenge consists in reproducing within a unified finite element-based methodology the successive steps of micro-voiding-induced damage, strain localization and crack propagation, if any. A key ingredient for a predictive ductile fracture model is the proper numerical treatment of the critical transition phase of damage-induced strain localization inside a narrow band. For this purpose, the strong discontinuity cohesive model and the eXtended Finite Element Method are combined. A propagation algorithm is proposed and studied in the context of ductile materials. Physics-motivated criteria to pass from the phase of more or less diffuse damage to strain localization and from strain localization to crack propoagation are proposed. Finally, a 2D numerical example is shown to study the performance of the failure analysis model when implemented into an engineering finite element computation code, namely Abaqus.

Magneto-thermo-mechanical dynamic buckling analysis of a FG-CNTs-reinforced curved microbeam with different boundary conditions using strain gradient theory

Abstract: In this article, dynamic buckling analysis of an embedded curved microbeam reinforced by functionally graded carbon nanotubes is carried out The structure is subjected to thermal, magnetic and harmonic mechanical loads Timoshenko beam theory is employed to simulate the structure Furthermore, the temperature-dependent surrounding elastic foundation is modeled by normal springs and a shear layer Using strain gradient theory, the small scale effects are taken into account The extended rule of mixture is employed to estimate the equivalent properties of the composite material The governing equations and different boundary conditions are derived based on the energy method and Hamilton’s principle Dynamic stability regions of the system are obtained using differential quadrature method The aim of this paper is to investigate the influence of different parameters such as small scale effect, boundary conditions, elastic foundation, volume fraction and distribution types of carbon nanotubes, magnetic field, temperature and central angle of the curved microbeam on the dynamic stability region of the system The results indicate that by increasing the volume fraction of CNTs, the frequency of the system increases and thus the dynamic stability region occurs at higher frequencies

Scavenging phenomenon and improved electrical and mechanical properties of polyaniline–divinylbenzene composite in presence of MWCNT

Abstract: A semi-doped polyaniline (PANI)–dodecylbenzenesulfonic acid (DBSA) complex is added with a suspension of multiwall carbon nanotubes (MWCNT)–divinylbenzene (DVB) to prepare PANI–MWCNT based thermosetting conductive resin system. Firstly, unreinforced nanocomposites with various loading of MWCNT are prepared. Continuous improvement in the electrical conductivity is observed with increasing MWCNT loading in the composite, while improvement in the mechanical properties is observed only up to 0.2 wt% MWCNT loading. On further MWCNT loading, the decrease in mechanical properties is observed. Flexural strength increased by 18% with 0.2 wt% of MWCNT in the unreinforced nanocomposite while electrical conductivity increased continuously to 0.68 S/cm (at 0.5 wt% of MWCNT loading) from 0.25 S/cm (neat sample). DSC and TGA analysis show that MWCNT effectively contributed to enhance the scavenging effect of PANI, affecting degree of DVB polymerization at higher loading of MWCNT. Samples were characterized by FTIR analysis. DMA analysis is also performed to understand the mechanical behavior of the cured unreinforced nanocomposite under dynamic loading. SEM observation has been employed to understand the dispersion behavior of MWCNT into the matrix. PANI-wrapping behavior on MWCNT is observed from the SEM images. Wrapping of PANI on MWCNT increased doping state and surface area of PANI which subsequently contribute to the increased scavenging behavior of PANI at higher MWCNT loading. A structural thermosetting nanocomposite with electrical conductivity of 0.68 S/cm, flexural modulus of 1.87 GPa and flexural strength up to 35 MPa is prepared. In addition, PANI–DBSA/DVB matrix with MWCNT is also used to impregnate carbon fabrics to prepare highly conductive CFRPs. A CFRP with 1.67 S/cm electrical conductivity in through-thickness direction and 328 MPa flexural strength is obtained with the addition of 0.2 wt% MWCNT into the resin system.

Design assessment of a multiple passenger vehicle component using load transfer index ( $$ {\text{U}}^{*} $$ U ∗ ) method

Abstract: In recent years, the study of the load transfer in the structure has achieved a growing attention from mechanical engineers, specifically in the vehicle industry. To further develop this relatively new branch of structural analysis and in particular the $$ {\text{U}}^{*} $$ index theory for load transfer analysis, it seems necessary to apply this method to different vehicle components. Therefore, in this study, one of the main load carrying components of a multiple passengers carrying vehicle was chosen for load transfer analysis. This choice has significant importance due to the focus of previous $$ {\text{U}}^{*} $$ index studies on small passenger vehicles, which have completely different structure and load paths. Another important feature of this study is the application of a sophisticated and detailed approach for choosing the loading and boundary condition. To address an actual working condition, a full model of the vehicle was analyzed under different working loads and the most severe loading condition was selected for this study. Then, a detailed $$ {\text{U}}^{*} $$ index analysis was performed on the structure to evaluate the load transfer for both loading and reaction forces. Based on the results of this analysis, parts with questionable stiffness were located and a design modification was proposed to improve the structural behavior. In addition, to verify the computer model and conclusions of the $$ {\text{U}}^{*} $$ index analysis, the structure was tested physically under same loading condition. Finally, the proposed modified design of the structure was analyzed with $$ {\text{U}}^{*} $$ index theory, and using the design criteria suggested in the theory, it was shown that the new design has great potential for better performance and more efficient load transfer.

Damage effects of adhesives in modern glass façades: a micro-mechanically motivated volumetric damage model for poro-hyperelastic materials

Abstract: This paper presents a micro-mechanically motivated volumetric damage model accounting for cavitation effects in modern glass connections, e.g. laminated glass connections. The volumetric part of an arbitrary Helmholtz free energy function is equipped with an isotropic damage formulation. To develop a micro-mechanical damage model, the porous micro-structure of a transparent structural silicone adhesive is analyzed numerically applying hydrostatic loading conditions. Based on the structural responses of different types of cubic representative volume elements incorporating an initial void fraction, three damage parameters are fitted utilizing the Levenberg–Marquard algorithm. The present volumetric damage model is implemented into ANSYS FE Code using a UserMat subroutine, where the algorithmic setting is described in detail in the present paper. To compare the structural responses of cubic equivalent homogeneous materials with representative volume elements, benchmark tests under hydrostatic loading are performed. The results indicate that the novel damage model accounts adequately for volumetric damage due to the cavitation effect. A special form of the pancake test is described briefly. The test allows for visualizing the cavitation effect during experimental testing. The experimental results of the pancake test are compared with numerical results, where the pancake test is simulated incorporating the micro-mechanical damage model. The micro-mechanically motivated scalar, internal damage variable is equipped with the obtained damage parameters from the structural response of the representative volume elements. The results show an adequate approximation of the experiment through the simulation. However, to optimize the results of the simulation, an optimization study on the damage parameters is conducted utilizing the Downhill-Simplex algorithm. Using the optimized damage parameters, the simulation of the pancake tests is further improved. Hence, it is shown that the novel micro-mechanically motivated volumetric damage model is excellently suited to represent the cavitation effect in poro-hyperelastic materials.

A material point method model and ballistic limit equation for hyper velocity impact of multi-layer fabric coated aluminum plate

Abstract: A multi-layer fabric coated aluminum plate is usually used in the hard upper torso of space suit to protect astronauts from getting hurt by space dust. In this paper, the protective performance of the multi-layer fabric coated aluminum plate is investigated. To establish its ballistic limit equation, thirteen hyper velocity impact tests with different impact velocities (maximum velocity is 6.19 km/s) and projectile diameters have been conducted. To provide data for impact velocity higher than 6.2 km/s which is hard to be obtained by tests due to the limitations of test equipment capacity, a material point method (MPM) model is established for the multi-layer fabric coated aluminum plate and validated/corrected using the test results. The numerical results obtained using the corrected MPM model for impact velocity higher than 6.2 km/s are used together with the test results to develop the ballistic limit equation. The corrected MPM model and the ballistic limit equation developed for the multi-layer fabric coated aluminum plate provide an effective tool for the space suit design.

Kinematically admissible folding mechanisms for the progressive collapse of foam filled conical frusta

Abstract: In this paper, the progressive collapse of foam filled conical frusta is investigated analytically using four different kinematically admissible folding mechanisms with varied straight folds. Comparisons are made between these four kinematically admissible mechanisms; specifically, pure inward folding, pure outward folding, first inward followed by outward folding, and first outward followed by inward folding. The instantaneous force as well as the mean crushing force was derived based on the principle of energy conversation, and the crushing energy was absorbed by the plastic deformation of the shell, the crushing of foam filler and the foam/shell interaction. The resulted upper bound solution of the four different mechanisms is compared with the finite element predictions of the same system. Our parametric study reveals that first outward then inward folding mechanism is generally energy favorable except for cases involving greater foam resistance, thin shell thickness, and/or large taper angle in which the pure outward folding mechanism may be preferable.

An augmented incompressible material point method for modeling liquid sloshing problems

Abstract: The incompressible material point method was proposed for modeling the free surface flow problems based on the operator splitting technique which decouples the solution of the velocity and the pressure in our previous work. To further model the coupling problems between the incompressible fluid and the moving irregular solid bodies, an augmented incompressible material point method is proposed in this paper based on the energy minimization form of operator splitting technique. The interaction between the fluid and the solid is taken into account via the work done by the fluid pressure on the solid bodies. By minimizing the total work done by the fluid pressure, volume-weighted pressure Poisson equations are obtained. The proposed method is validated with liquid sloshing in a rectangular tank subjected to various base-excitations, and is then used to study the optimal height of baffles mounted on the bottom of the tank to mitigate the sloshing wave.

Stochastic analysis of dynamic characteristics and pull-in instability of FGM micro-switches with uncertain parameters in thermal environments

Abstract: In this paper, the stochastic vibration characteristics of a functionally graded material micro-switch with random material properties near the pull-in instability are investigated. The uncertainties of the material properties and randomness of the composition of constituents due to the fabrication process are considered in this study. The micro-switch is modeled as a micro-beam and is under the effects of electrostatic and Casimir forces. The properties of the constituent materials are temperature dependent, and the system undergoes a change in temperature. The governing equations of motion of the Euler–Bernoulli micro-beam are derived based on modified couple stress theory and are solved using the state space form finite difference method. The statistics of the dynamic characteristics are obtained based on Monte Carlo simulation method. The effects of Casimir force, the length scale parameter, temperature dependencies of the material properties, the temperature change, the applied voltage and the volume fraction index on stochastic properties of the first natural frequency, the second natural frequency, the pull-in gap, and the pull-in voltage are studied in detail.

Multicriteria materials selection for extreme operating conditions based on a multiobjective analysis of irradiation embrittlement and hot cracking prediction models

Abstract: A methodology for evaluating different combinations of materials specifications for extreme environment applications is presented. This new approach addresses the materials selection problem using a multicriteria stringency level methodology that defines several thresholds obtained by analyzing different prediction models of irradiation embrittlement and hot cracking. To solve the conflicts among thresholds as provided by the different prediction models, a multiobjective approach is carried out. Materials for reactor pressure vessels have been considered as case study. It has been concluded that the best option to manufacture a pressure vessel for a pressurized water modern reactor is the selection of German manufacturing standards. Finally, a sensitivity analysis of the proposed methodology has been performed to evaluate the divergences between the single stringency level methodology and the new proposal including multicriteria decision making aspects.

Damage identification in gravity dams using dynamic coupled hydro-mechanical XFEM

Abstract: In order to maximize efficiency and reduce the risk of failure in operational dams, an effective and efficient method which employs the inverse analysis of a dynamic coupled hydro-mechanical problem is proposed. The numerical model is based on the extended finite element model. The proposed method is able to identify cracks which may be detrimental to structural performance and reliability. An attempt is made using both deterministic and heuristic based strategies to solve the ill-posed inverse problem and identify the location, dimension and orientation of the crack. More so, the influence of the search space and conditioning of the cost function in identifying crack parameters are investigated. The proposed method shows promising results in the identification of cracks in a fully operational dam.