Andrea Vigliotti

Crashworthiness of helicopters on water: Test and simulation of a full-scale WG30 impacting on water

This paper deals with the test and simulation of a full-scale WG30 helicopter impacting on water with a vertical velocity of 8 m/s. The first part is devoted to the test preparation and experimental data measured at CIRA. The instrumentations of the test specimen and the test conditions are briefly discussed. After an overall description of the impact sequence, quantitative test results including acceleration on the floor and pressure time histories as well as measurements on an anthropomorphic test dummy are presented. Finally, pictures showing the main damage complete the description of the tested structure. The second part of the paper focuses on the first finite element simulation performed at DLR of the impact under the real test conditions. After a description of the FE model, it is shown that the calculated and observed deformations of the structure correlate well. Finally, calculated and measured time histories including accelerations and pressures are compared and the problem of predicting measured pressure peaks with FE tools is discussed.

Simulation of the cytoskeletal response of cells on grooved or patterned substrates

We analyse the response of osteoblasts on grooved substrates via a model that accounts for the cooperative feedback between intracellular signalling, focal adhesion development and stress fibre contractility. The grooved substrate is modelled as a pattern of alternating strips on which the cell can adhere and strips on which adhesion is inhibited. The coupled modelling scheme is shown to capture some key experimental observations including (i) the observation that osteoblasts orient themselves randomly on substrates with groove pitches less than about 150 nm but they align themselves with the direction of the grooves on substrates with larger pitches and (ii) actin fibres bridge over the grooves on substrates with groove pitches less than about 150 nm but form a network of fibres aligned with the ridges, with nearly no fibres across the grooves, for substrates with groove pitches greater than about 300 nm. Using the model, we demonstrate that the degree of bridging of the stress fibres across the grooves, and consequently the cell orientation, is governed by the diffusion of signalling proteins activated at the focal adhesion sites on the ridges. For large groove pitches, the signalling proteins are dephosphorylated before they can reach the regions of the cell above the grooves and hence stress fibres cannot form in those parts of the cell. On the other hand, the stress fibre activation signal diffuses to a reasonably spatially homogeneous level on substrates with small groove pitches and hence stable stress fibres develop across the grooves in these cases. The model thus rationalizes the responsiveness of osteoblasts to the topography of substrates based on the complex feedback involving focal adhesion formation on the ridges, the triggering of signalling pathways by these adhesions and the activation of stress fibre networks by these signals.

Analysis and design of lattice materials for large cord and curvature variations in skin panels of morphing wings

In the past few decades, several concepts for morphing wings have been proposed with the aim of improving the structural and aerodynamic performance of conventional aircraft wings. One of the most interesting challenges in the design of a morphing wing is represented by the skin, which needs to meet specific deformation requirements. In particular when morphing involves changes of cord or curvature, the skin is required to undergo large recoverable deformation in the actuation direction, while maintaining the desired shape and strength in the others. One promising material concept that can meet these specifications is represented by lattice materials. This paper examines the use of alternative planar lattices in the embodiment of a skin panel for cord and camber morphing of an aircraft wing. We use a structural homogenization scheme capable of capturing large geometric nonlinearity, to examine the structural performance of lattice skin concepts, as well as to tune their mechanical properties in desired directions.

Finite Element Implementation of a Multivariant Shape Memory Alloy Model

In this article a finite element implementation of a multivariant constitutive model of shape memory alloys for a mono-dimensional element is presented. The model has been implemented by writing the kinetic law and the equilibrium equation in the rate form and integrating this equation along the thermomechanical load path. The implemented model allows to simulate both the operation in isothermal condition, in full or partial pseudoelastic recover and the behavior of shape memory alloy’s wire in mixed thermal and mechanical load application such as in cyclic activations of a wire against a structure. The developed model is suitable for integration in structural finite element program allowing to simulate the action of SMA wires in integrated in complex structure. A number of numerical cases for the developed model are showed in order to verify the ability to reproduce expected typical SMA behaviors.

Prediction of Cell Alignment on Cyclically Strained Grooved Substrates

Cells respond to both mechanical and topographical stimuli by reorienting and reorganizing their cytoskeleton. Under certain conditions, such as for cells on cyclically stretched grooved substrates, the effects of these stimuli can be antagonistic. The biophysical processes that lead to the cellular reorientation resulting from such a competition are not clear yet. In this study, we hypothesized that mechanical cues and the diffusion of the intracellular signal produced by focal adhesions are determinants of the final cellular alignment. This hypothesis was investigated by means of a computational model, with the aim to simulate the (re)orientation of cells cultured on cyclically stretched grooved substrates. The computational results qualitatively agree with previous experimental studies, thereby supporting our hypothesis. Furthermore, cellular behavior resulting from experimental conditions different from the ones reported in the literature was simulated, which can contribute to the development of new experimental designs.

Structural Optimization of Lattice Materials

Lattice materials are characterized at the microscopic level by a regular pattern of voids confined by walls. Recent rapid prototyping techniques allow their manufacturing from a wide range of solid materials, ensuring high degrees of accuracy and limited costs. The microstructure of lattice material permits to obtain macroscopic properties and structural performance, such as very high stiffness to weight ratios, highly anisotropy, high specific energy dissipation capability and an extended elastic range, which cannot be attained by uniform materials. Among several applications, lattice materials are of special interest for the design of morphing structures, energy absorbing components and hard tissue scaffold for biomedical prostheses. Their macroscopic mechanical properties can be finely tuned by properly selecting the lattice topology and the material of the walls. Nevertheless, since the number of the design parameters involved is very high, and their correlation to the final macroscopic properties of the material is quite complex, reliable and robust multiscale mechanics analysis and design optimization tools are a necessary aid for their practical application. In this paper, the optimization of lattice materials parameters is illustrated with reference to the design of a bracket subjected to a point load. Given the geometric shape and the boundary conditions of the component, the parameters of four selected topologies have been optimized to concurrently maximize the component stiffness and minimize its mass.Copyright

Shock Wave Boundary Layer Interaction Measurement Assembly on the EXPERT Capsule

In order to improve the level of confidence in the design of one of the most critical technology for re-entry space vehicles, a dedicated flight experiment on the shock wave boundary layer interaction phenomenon to be flown on the EXPERT capsule has been conceived. As matter of fact, the common approach to design a space vehicle is based on ground experimental tests, computational predictions and ground-to-flight extrapolation methodologies even though a modern approach foresees to improve such design tools by validating them with respect to flight experiments. At the moment the lack of hypersonic flight data that can serve as a point of reference for this validation process makes it impossible, especially for some of the most challenging hypersonic problems, as the shock wave boundary layer interaction phenomena that could take place in proximity of a deflected control surface. In the frame of the EXPERT program of the European Space Agency, focused to collect in-flight data on relevant aerothermodynamic phenomena, a certain number of experiments/payloads has been conceived in order to collect flight data for further analysis and comparison with numerical and experimental results. Among the others, the shock wave boundary layer interaction in proximity of two opposite deflected flaps will be analyzed through a dedicated payload in order to consolidate the knowledge on this particular phenomenon. The study of flow separation ahead of the open flap induced by a shock wave boundary layer interaction constitutes payload PL07, currently under investigation: two-symmetrically opposite faces, ahead the 20deg fixed open flaps, will be instrumented with temperature, pressure and heat flux sensors in order to analyze the interaction effects and to characterize the boundary layer approaching the flap. Measurements will be performed along the entire trajectory of representative surface properties to be used to assess the understanding about three dimensional viscous interactions and real gas effects on shock wave boundary layer interaction phenomena in high enthalpy re-entry flow conditions.

Role of boundary conditions in determining cell alignment in response to stretch

Significance Alignment of cells in response to mechanical cues plays an important role in a wide range of physiologic processes. Multiple cell types orient perpendicular to applied cyclic stretch in 2D culture but parallel to stretch in 3D culture, and the mechanisms underlying this behavior remain elusive. We tested a promising hypothesized mechanism called strain avoidance and showed that it cannot explain cell alignment across the conditions that we examined. By contrast, a computational model of stress fiber kinetics incorporating the influence of traction boundary conditions and altered strain transmission in soft gels reproduced all of our experimental results as well as published 2D stretch experiments. These findings could improve understanding, modeling, and therapeutic modulation of tissue development, regeneration, and repair. The ability of cells to orient in response to mechanical stimuli is essential to embryonic development, cell migration, mechanotransduction, and other critical physiologic functions in a range of organs. Endothelial cells, fibroblasts, mesenchymal stem cells, and osteoblasts all orient perpendicular to an applied cyclic stretch when plated on stretchable elastic substrates, suggesting a common underlying mechanism. However, many of these same cells orient parallel to stretch in vivo and in 3D culture, and a compelling explanation for the different orientation responses in 2D and 3D has remained elusive. Here, we conducted a series of experiments designed specifically to test the hypothesis that differences in strains transverse to the primary loading direction give rise to the different alignment patterns observed in 2D and 3D cyclic stretch experiments (“strain avoidance”). We found that, in static or low-frequency stretch conditions, cell alignment in fibroblast-populated collagen gels correlated with the presence or absence of a restraining boundary condition rather than with compaction strains. Cyclic stretch could induce perpendicular alignment in 3D culture but only at frequencies an order of magnitude greater than reported to induce perpendicular alignment in 2D. We modified a published model of stress fiber dynamics and were able to reproduce our experimental findings across all conditions tested as well as published data from 2D cyclic stretch experiments. These experimental and model results suggest an explanation for the apparently contradictory alignment responses of cells subjected to cyclic stretch on 2D membranes and in 3D gels.

The homeostatic ensemble for cells

Cells are quintessential examples of out-of-equilibrium systems, but they maintain a homeostatic state over a timescale of hours to days. As a consequence, the statistics of all observables is remarkably consistent. Here, we develop a statistical mechanics framework for living cells by including the homeostatic constraint that exists over the interphase period of the cell cycle. The consequence is the introduction of the concept of a homeostatic ensemble and an associated homeostatic temperature, along with a formalism for the (dynamic) homeostatic equilibrium that intervenes to allow living cells to evade thermodynamic decay. As a first application, the framework is shown to accurately predict the observed effect of the mechanical environment on the in vitro response of smooth muscle cells. This includes predictions that both the mean values and diversity/variability in the measured values of observables such as cell area, shape and tractions decrease with decreasing stiffness of the environment. Thus, we argue that the observed variabilities are inherent to the entropic nature of the homeostatic equilibrium of cells and not a result of in vitro experimental errors.

1D SMA Models

In this chapter two approaches for the 1D modelling of Shape Memory Alloys are presented. The first approach is based on a phenomenological description of the Austenite-Martensite transformation, assuming that the two configurations are always at equilibrium and the fractions of Martensite and Austenite are a cosine function of the temperature and the stress, which matches experimental measurement. The approach is extended to account for both the stress and the thermal variant of the Martensite and can handle incomplete transformations. The second model is able to reproduce the pseudoelastic and shape memory behavior of SMA and to take into account the asymmetric response in tension and in compression, the different elastic properties of austenite and martensite and the reorientation process. Furthermore, the model is able to simulate the cyclic behavior, including training and two way memory effects. This approach is based on the time integration of the transformation kinetic equations that are function of the strain and the temperature. The prediction of the models for SMA elements subject to different loading histories have been compared to experimental results and have shown very good agreement.