No abstract

This article reviews the main mechanisms of stiffness variation typically found in nature. The temporal changes in stiffness may be fully or partially reversible, or completely irreversible, and can be very slow or fast in time depending on the strategy adopted to alter the mechanical properties. It is also possible to observe changes in the stiffness in order to recover the original mechanical properties in damaged natural materials by means of self-healing mechanisms. In addition to stiffness variations in time, natural materials can also exhibit stiffness changes in space. These variations can be represented by alterations in the spatial distribution of the microscopic constituents across multiple hierarchical scales, from very small physical scales to large macroscopic dimensions. In order to optimise the strength and multifunctionality of these materials, spatial changes can also occur over larger areas at one single scale. In addition, several examples are provided to illustrate how natural materials have been exploited further in order to develop new bio-inspired materials.

Corrugated skins provide a good solution to morphing wings due to their highly anisotropic behavior. If the low stiffness corrugation plane is aligned in the chordwise direction, the airfoil shape change is possible. In contrast to the traditional smooth skin of an airfoil, a corrugated skin influences both the local and global aerodynamics of a wing. The aim of this study is to investigate the effect of a corrugated skin on the global aerodynamics of an airfoil, particularly lift and drag characteristics. First, the aerodynamic analysis of a NACA 0012 airfoil with a smooth profile is conducted, both in the wind tunnel and by computational fluid dynamics, at different Reynolds numbers and compared to the data in the literature. The lift and drag coefficients at different angles of attack, between 210°and 10°, are considered. Next, two NACA 0012 airfoils with different sized corrugated profiles are investigated both experimentally and by numerical simulation. The effects of corrugation size and Reynolds number are analyzed and quantified relatively to the standard NACA0012 airfoil with a smooth skin. Preliminary numerical results are validated using the experimental data.

A comfort-based optimal method has been developed for designing floors using tuned inerter mass systems (TIMS) to reduce the vertical-vibration response of floors subjected to human-induced excitations. Acceleration response of the floor and output force of TIMS were first derived via stochastic analysis. Subsequently, variation patterns in acceleration-response and output-force ratios were studied and compared by changing values of TIMS parameters. Based on the results of this parametric investigation, a comfort-based optimal design method and the corresponding procedure to be followed were developed. In the proposed method, the weighted average of the required additional tuned mass and TIMS output force are expected to be minimized whilst realizing the expected level of satisfaction in terms of target comfort performance of the floor structure (i.e., target acceleration response ratio). For different design targets, optimized TIMS parameters can be obtained using the proposed design method, and such design cases have been demonstrated by performing time-history analysis under different external human-induced excitations. Results of the said analysis demonstrate that the proposed design method effectively satisfies comfort-performance objectives of a floor with optimal actual mass through use of an inerter system. In addition, the proposed method can be used to maintain a balance between the control cost, additional tuned mass, and structural comfort performance.