F. van Keulen

Characterizing size-dependent effective elastic modulus of silicon nanocantilevers using electrostatic pull-in instability

This letter presents the application of electrostatic pull-in instability to study the size-dependent effective Young’s Modulus ? ( ~170–70?GPa) of [110] silicon nanocantilevers (thickness ~1019–40?nm). The presented approach shows substantial advantages over the previous methods used for characterization of nanoelectromechanical systems behaviors. The ? is retrieved from the pull-in voltage of the structure via the electromechanical coupled equation, with a typical error of ? 12%, much less than previous work in the field. Measurement results show a strong size-dependence of ?. The approach is simple and reproducible for various dimensions and can be extended to the characterization of nanobeams and nanowires.

Development and experimental validation of a three-dimensional finite element model of the human scapula:

Abstract A new modelling approach, using a combination of shell and solid elements, has been adopted to develop a realistic three-dimensional finite element (FE) model of the human scapula. Shell elements were used to represent a part of the compact bone layer (i.e. the outer cortical layer) and the very thin and rather flat part of the scapula—infraspinous fossa and supraspinous fossa respectively. Solid elements were used to model the remaining part of the compact bone and the trabecular bone. The FE model results in proper element shapes without distortion. The geometry, material properties and thickness were taken from quantitative computed tomography (CT) data. A thorough experimental set-up for strain gauge measurement on a fresh bone serves as a reference to assess the accuracy of FE predictions. A fresh cadaveric scapula with 18 strain gauges fixed at various locations and orientations was loaded in a mechanical testing machine and supported at three locations by linkage mechanisms interconnected by ball joints. This new experimental set-up was developed to impose bending and deflection of the scapula in all directions unambiguously, in response to applied loads at various locations. The measured strains (experimental) were compared to numerical (FE) strains, corresponding to several load cases, to validate the proposed FE modelling approach. Linear regression analysis was used to assess the accuracy of the results. The percentage error in the regression slope varies between 9 and 23 per cent. It appears, as a whole, that the two variables (measured and calculated strains) strongly depend on each other with a confidence level of more than 95 per cent. Considering the complicated testing procedure on a fresh sample of scapula, the high correlation coefficients (0.89-0.97), the low standard errors (29-105 μeP) and percentage errors in the regression slope, as compared to other studies, strongly suggest that the strains calculated by the FE model can be used as a valid predictor of the actual measured strain. The model is therefore an alternative to a rigorous three-dimensional model based on solid elements only, which might often be too expensive in terms of computing time.

Generalized Continuum Theories: Application to Stress Analysis in Bone

Bone is a heterogeneous material with microstructural features. Continuum models of bone on the basis of classical elasticity ignore microstructure-related scale effects on the macroscopic mechanical properties. Consequently, these models do not provide a complete description of the mechanical behavior when the microstructural size of bone approaches the macroscopic length scale. Such effects are most pronounced near bone–implant interfaces and in areas of high strain gradients. This issue is investigated here by studying generalized continuum mechanics theories which account for the influence of microstructure-related scale effects on the macroscopic properties of bone. Furthermore, a two-dimensional finite element on the basis of the micropolar continuum theory is summarized. As a simple illustrative example, a bone–prosthesis configuration is analyzed. Results show that the stress concentrations in bone near the bone–prosthesis interface are substantially smaller with micropolar theory compared to classical theory of elasticity.

Refined semi-analytical design sensitivities

Efficient structural optimization routines require availability of gradient information. Semi-analytical (SA) design sensitivities are rather popular, as they combine ease of implementation with computational efficiency. Their main drawback however, is their well-known inaccuracy problem for shape design sensitivities. It was found that the inaccuracies are especially unacceptable for slender structures and become more pronounced when relatively large rigid body motions can be identified for individual finite elements. Based on these observations, the authors recently developed a refined SA method taking full advantage of analytical differentiation of rigid body modes. The present article presents a sound and unified formulation of refined semi-analytical (RSA) design sensitivities for linear, linearized buckling, geometrically nonlinear and limit point analyses. Numerical results are presented in order to demonstrate the efficiency of the proposed method. It is concluded that the refined SA method possesses the advantages of the traditional SA method, whereas it does not exhibit its unacceptable inaccuracies.

Topology optimization using a topology description function

The topology description function (TDF) approach is a method for describing geometries in a discrete fashion, i.e. without intermediate densities. Hence, the TDF approach may be used to carry out topology optimization, i.e. to solve the material distribution problem. However, the material distribution problem may be ill-posed. This ill-posedness can be avoided by limiting the complexity of the design, which is accomplished automatically by limiting the number of design parameters used for the TDF. An important feature is that the TDF design description is entirely decoupled from a finite element (FE) model. The basic idea of the TDF approach is as follows. In the TDF approach, the design variables are parameters that determine a function on the so-called reference domain. Using a cut-off level, this function unambiguously determines a geometry. Then, the performance of this geometry is determined by a FE analysis. Several optimization techniques are applied to the TDF approach to carry out topology optimization. First, a genetic algorithm is applied, with (too) large computational costs. The TDF approach is shown to work using a heuristic iterative adaptation of the design parameters. For more effective and sound optimization methods, design sensitivities are required. The first results on design sensitivity analysis are presented, and their accuracy is studied. Numerical examples are provided for illustration.

Powerful polymeric thermal microactuator with embedded silicon microstructure

A powerful and effective design of a polymeric thermal microactuator is presented. The design has SU-8 epoxy layers filled and bonded in a meandering silicon (Si) microstructure. The silicon microstructure reinforces the SU-8 layers by lateral restraint. It also improves the transverse thermal expansion coefficient and heat transfer for the bonded SU-8 layers. A theoretical model shows that the proposed SU-8/Si composite can deliver an actuation stress of 1.30?MPa/K, which is approximately 2.7 times higher than the unconstrained SU-8 layer, while delivering an approximately equal thermal strain.

REFINED CONSISTENT FORMULATION OF A CURVED TRIANGULAR FINITE ROTATION SHELL ELEMENT

The present paper considers a finite rotation formulation for curved shell elements with rotations about the element sides as nodal degrees of freedom. Attention is mainly on the derivation of a consistent finite rotation formulation. Significant simplifications of the governing equations are presented. These simplifications lead to more efficient finite element implementations. Numerical examples demonstrate the differences between the present consistent and previous approximate formulations.

Design Overview of a Resonant Wing Actuation Mechanism for Application in Flapping Wing MAVs

This paper shows the design and analysis of the actuation mechanism for a four winged flapping wing MAV. The design is set up to exploit resonant properties, as exhibited by flying insects, to reduce the energy expenditure and to provide amplitude amplification. In order to achieve resonance a significantly flexible structure has to be incorporated into the design. The elastic structure used for the body of the MAV is a ring type structure. The ring is coupled to the wings by a compliant amplification mechanism which transforms and amplifies the ring deflection into the large wing root rotation. After initial sizing, the structures are analyzed by finite elements (eigenvalue and transient analysis). Based on the initial analysis, the structures are realized to be tested later. The wings are first analyzed independent of the structure in order to tune wing hinge stiffness to efficiently generate lift, exploiting passive wing pitching. The wings are tuned by using a quasi-steady aerodynamic model. The tuned wings are tested to judge if manufactured wings reflect the predicted performance. The ring-shaped thorax structure is combined with the wings to test resonant performance of the assembled structure. A test setup is built to quantify lift production. Lift is tested by suspending the prototype on a flexible beam and measuring changes in deflection when the model is actuated. Significant lift is produced using the current prototype. Kinematic patterns present during resonant actuation show correct timing of wing rotation.

Polymeric Thermal Microactuator With Embedded Silicon Skeleton: Part I—Design and Analysis

This paper presents the modeling of a new design of a polymeric thermal microactuator with an embedded meander-shaped silicon skeleton. The design has a skeleton embedded in a polymer block. The embedded skeleton improves heat transfer to the polymer and reinforces it. In addition, the skeleton laterally constrains the polymer to direct the volumetric thermal expansion of the polymer in the actuation direction. The complex geometry and multiple-material composition of the actuator make its modeling very involved. In this paper, the main focus is on the development of approximate electrothermal and thermoelastic models to capture the essence of the actuator behavior. The approximate models are validated with a fully coupled multiphysics finite element model and with experimental testing. The approximate models can be useful as an inexpensive tool for subsequent design optimization. Evaluation, using the analytical and numerical models, shows that the polymer actuator with the embedded skeleton outperforms its counterpart without a skeleton, which is in terms of heat transfer and, thus, response time, actuation stress, and planarity.

3-D geometric modeling of a draped woven fabric

One of the components required for design and optimisation of fabric reinforced products is a realistic and accurate 3-D geometry model for a repeating element of the reinforcement. Such a model, which reflects the deformations that result from weaving the fibre bundles and forming (draping) the woven fabric over a product mould, is proposed in the present paper. A fibre bundle architecture, which exhibits undulation and variable cross-section dimensions, is introduced to this effect. Every bundle is described by its in-plane centreline path, its double curved horizontal midplane and the thickness distribution of the cross-sections. These parameters are, in turn, defined by invariant shape functions and variable fibre bundle dimensions. The selected geometry description enables straightforward determination of individual fibre paths. In order to verify the model experimentally, cross-sections are cut out from undeformed laminates with plain-weave reinforcements, and laminates of which the plain-weaves have been subjected to stretching or shear deformations during draping. The laminate cross-sections are made along and perpendicular to the mean directions of the impregnated fibre bundles (yarns). All yarns exhibit out-of-plane undulation, and curvature and twist of the midplane. Correlation between experiment and the proposed modeling scheme is good. Draping results in significant fibre reorientations and variations between the individual fibre paths, which are not reflected by existing modeling schemes. These geometry deviations may significantly affect the stress distribution, and should be taken into account in order to predict material properties accurately.