Alberto Pirrera

Shape morphing Kirigami mechanical metamaterials

Mechanical metamaterials exhibit unusual properties through the shape and movement of their engineered subunits. This work presents a new investigation of the Poisson’s ratios of a family of cellular metamaterials based on Kirigami design principles. Kirigami is the art of cutting and folding paper to obtain 3D shapes. This technique allows us to create cellular structures with engineered cuts and folds that produce large shape and volume changes, and with extremely directional, tuneable mechanical properties. We demonstrate how to produce these structures from flat sheets of composite materials. By a combination of analytical models and numerical simulations we show how these Kirigami cellular metamaterials can change their deformation characteristics. We also demonstrate the potential of using these classes of mechanical metamaterials for shape change applications like morphing structures.

Optimization of Wind Turbine Blade Spars

In order to be competitive in the energy market, the cost of wind power has to be driven down. Current blade designs exhibit a cubic relationship between length and mass. The aim of this project is to improve on this trend by exploring new structural concepts. Optimization techniques and high performance composites are used here as tools to devise novel ideas for the production of longer, more cost ecient blades that deliver cheaper kWh.

Adaptive compliant structures for flow regulation

This paper introduces conceptual design principles for a novel class of adaptive structures that provide both flow regulation and control. While of general applicability, these design principles, which revolve around the idea of using the instabilities and elastically nonlinear behaviour of post-buckled panels, are exemplified through a case study: the design of a shape-adaptive air inlet. The inlet comprises a deformable post-buckled member that changes shape depending on the pressure field applied by the surrounding fluid, thereby regulating the inlet aperture. By tailoring the stress field in the post-buckled state and the geometry of the initial, stress-free configuration, the deformable section can snap through to close or open the inlet completely. Owing to its inherent ability to change shape in response to external stimuli—i.e. the aerodynamic loads imposed by different operating conditions—the inlet does not have to rely on linkages and mechanisms for actuation, unlike conventional flow-controlling devices.

Geometrically Nonlinear First-Order Shear Deformation Theory for General Anisotropic Shells

A generalized first-order shear deformation theory for anisotropic multilayered shells is presented. It includes the effects of geometrically nonlinear deformations and general initial curvature. The field equations are expressed in orthogonal conjugate curvilinear coordinates in the shells middle surface. Hence, this formulation is particularly suitable for the analysis of monocoque structures formed using the increasingly exploited fiber-placement manufacturing techniques. A novel expression for the stiffness matrix is presented in which the relationship between the shell shape and the stiffness coefficients is highlighted. It is also shown that the stiffnesses herein obtained may lead to significantly different deformation fields from those based upon flat-plate expressions.

CARAPACE: A novel composite advanced robotic actuator powering assistive compliant exoskeleton preliminary design

A novel actuator is introduced that combines an elastically compliant composite structure with conventional electromechanical elements. The proposed design is analogous to that used in Series Elastic Actuators, its distinctive feature being that the compliant composite part offers different stable configurations. In other words, its elastic potential presents points of local minima that correspond to robust stable positions (multistability). This potential is known a priori as a function of the structural geometry, thus providing tremendous benefits in terms of control implementation. Such knowledge enables the complexities arising from the additional degrees of freedom associated with link deformations to be overcome and uncover challenges that extends beyond those posed by standard rigidlink robot dynamics. It is thought that integrating a multistable elastic element in a robotic transmission can provide new scenarios in the field of assistive robotics, as the system may help a subject to stand or carry a load without the need for an active control effort by the actuators.

A series elastic composite actuator for soft arm exosuits

The paper introduces a novel type of actuator for soft wearable exoskeletons providing assistance to the elbow joint motion. The mechanism consists of two DC motors, a multistable composite transmission which introduces series elastic properties, a high-efficiency non-backdrivable mechanism and a pair of Bowden cables to transmit the motion from the actuator to the joint. A test bench has been designed to experimentally characterize the performance of the proposed device. The control architecture is then introduced and described. The results of preliminary tests are shown and discussed. In conclusion, future developments and a embodiment of the envisioned application are introduced.

Thermally Driven Morphing and Snap-Through Behavior of Hybrid Laminate Shells

Analytical and experimental results are presented regarding the nonlinear temperature-curvature relationship displayed by composite bimorph shells. Snap-through action, driven solely by temperature change, is demonstrated using fiber–metal hybrid laminates. These laminates exploit the high coefficient of thermal expansion mismatch between composites and metals to yield thermal bimorphs with tailorable properties. To predict the potentially nonlinear response of these laminates, an energy-based multistability model is developed and made available online. The model utilizes experimentally measured one-dimensional thermally induced curvatures as input parameters to predict a corresponding shell’s two-dimensional flexural behavior. Initial curvature is found to be a critical component in enabling snap-through behavior, especially when partnered with highly orthotropic internal moments. Interestingly, the [0n/90n] class of unsymmetric laminates popular in the study of thermally induced bistability are shown to...

Morphing structures: non-linear composite shells with irregular planforms

The concept of morphing structures refers to devices that exhibit large scale shape changes, whilst maintaining load bearing capability, in response to distinctive operating conditions. This behavior comes from combinations of non-linear material or kinematic responses. Here, we limit our interests to shell type structures made from low strain, elastic materials that exhibit highly nonlinear kinematic behavior. Often, but not necessarily, the response can be bistable. Models predicting the multistability of shells often present a compromise between computational efficiency and accuracy of results. Moreover, they deal mainly with regular domains, such as rectangular or elliptical planforms. Few studies have been done to investigate the performance and the possible advantages of exploiting multistable structures with more general domains. In the present work, the multistability of thin shallow composite shells with irregular domains is investigated. An accurate and computationally efficient energy-based model is developed, in which the membrane and the bending components of the total strain energy are decoupled using the semi-inverse formulation of the constitutive equations. Transverse displacements are approximated using Legendre polynomials and the membrane problem is solved in isolation by combining compatibility conditions and equilibrium equations. The result is the total potential energy as a function of curvatures only. Stable shapes are recovered by minimizing the total energy with respect to curvature. The accurate evaluation of the membrane energy is a key step in order to accurately capture the bifurcation points. Here the membrane problem is solved using the Differential Quadrature Method (DQM), which provides accuracy at a relatively small computational cost. However, DQM is limited to rectangular domains. For this reason, blending functions are used to map the irregular physical domain into a regular computational domain. This approach allows multistable shells with arbitrary convex planforms to be described without affecting the computational efficiency and the accuracy of the proposed model.

Structural efficiency measures for sections under asymmetric bending

Shape factors evaluate the efficiency of material usage in a structure. Previously, they have been developed for simple bending but, in practice, beams often have a more complicated bending response. Therefore, shape factors that account for asymmetric bending are introduced. The shape factors are applied to six example beam sections to demonstrate the effect of shape and load on structural efficiency. The shape factors are also enhanced for inclusion in a more general measure of structural efficiency, the performance index, comprising elements of both geometry and material. Next, a study is performed to show how the asymmetry of a beam section affects structural efficiency. The shape factors can quantitatively evaluate the structural efficiency of beam sections, demonstrating the effect of asymmetric bending on the structural response. Therefore, these shape factors can be used for concept selection and to provide insight into optimal structural design.

Thermally driven morphing with hybrid laminates and metal matrix composites

Analytical and experimental results are presented regarding thermally-driven morphing laminated shells. Owing to exploitation of the geometric nonlinearity of thin shells, they can demonstrate highly nonlinear displacement response to thermal loading, including multistability and snap-through behavior. In order to predict this behavior, an energy-based multistability model is proposed which utilizes experimentally-measured 1D thermally-induced curvatures as input parameters to determine the shell’s 2D flexural behavior. Experiments are conducted to measure the 1D curvatures in both hybrid CFRP-metal laminates, as well as high-temperature capable SiC/Ti metal matrix composites. Data from these experiments is used to predict the geometrically nonlinear response of thermally loaded shells, and results are compared with experiment using 3D digital image correlation. The potential impact of this research is the realization of thermal and fluid control devices capable of operating autonomously in extreme environments such as gas turbine engine cores.