Viscoelastic Effects in Metal-Polymer Laminate Inflatable Structures

A 1 m long inflatable-rigidizable mast was developed as a payload for InflateSail: a 3U CubeSat technology demonstration mission. The thin-walled cylindrical mast consists of an aluminum-polymer laminate, and long-term structural performance is ensured through strain-rigidization: the packaging creases are removed through plastic deformation of the aluminum plies. During ground tests it was observed that after rigidization the internal pressure dropped more rapidly than could be accounted for by leakage of inflation gas alone. It was hypothesized that viscoelastic behaviour of the laminate material causes a further, time-dependent (order of seconds), increase in cylinder diameter, with a corresponding drop in internal pressure. Additional experiments revealed an increase in diameter, including large visco-elastic shear in the adhesive of the lap joint. This was not found to be sufficient to fully account for the observed reduction in pressure. An increase in temperature of the gas during inflation, with subsequent cooling down to ambient is thought to cause the additional pressure drop

Virtual Testing of Experimental Continuation

We present a critical advance in experimental testing of nonlinear structures. Traditional quasi-static experimental methods control the displacement or force at one or more load-introduction points on a structure. This approach is unable to traverse limit points in the control parameter, as the immediate equilibrium beyond these points is statically unstable, causing the structure to snap to another equilibrium. As a result, unstable equilibria---observed numerically---are yet to be verified experimentally. Based on previous experimental work, and a virtual testing environment developed herein, we propose a new experimental continuation method that can path-follow along unstable equilibria and traverse limit points. To support these developments, we provide insightful analogies between a fundamental building block of our technique---shape control---and analysis concepts such as the principle of virtual work and Galerkin's method. The proposed testing method will enable the validation of an emerging class of nonlinear structures that exploit instabilities for novel functionality

How to Set up a CubeSat Project - Preliminary Survey Results

CubeSats have been developed by many different institutions since they were introduced by California Polytechnic State University and Stanford University in 1999. A number of papers give lessons learned for individual satellites, some from a technical perspective and other from an educational point of view. However, there is no existing overview of how to set up a CubeSat project. The aim of this paper is to fill this gap, in order to offer those wishing to start a CubeSat programme some ideas of where to start, what equipment is needed and some lessons learned in terms of management. This information was gathered via a survey which was publicised via conferences, mailing lists and LinkedIn groups. At time of writing, 40 groups have completed the survey, including universities, agencies and companies. The respondents came from the US, Europe, Canada, Taiwan, Korea, China, Africa, Australia and South America. The majority of the groups were building 1U or 3U CubeSats with Technology Demonstrator or Science Experiment payloads. The groups were asked a series of questions relating to the characteristics of their projects, including the duration of the project, costs and on what they spent their money. They were also asked what first steps they took in setting up their programme, what equipment and facilities were necessary and how they managed and scheduled the project across multiple cohorts of students. This was identified as challenging by many groups and a variety of ideas and solutions were proposed. Lessons learned covered many aspects of the project with some common themes emerging: planning, learning from other groups, student continuity, documentation, integrating the project within the curriculum, mentoring, software development, simplicity and testing. The groups were asked for their advice to future programme leaders and this is summarised in the paper

Interview with Rita Rupp

An interview with Rita Rupp regarding her experiences in a one-room school house.https://scholars.fhsu.edu/ors/1026/thumbnail.jp

Experimental Path-Following of Equilibria Using Newton’s Method, Part I:Theory, Modelling, Experiments

Modern numerical path-following techniques provide a comprehensive suite of computational tools to study the bifurcation behaviour of engineering structures. In contrast, experimental testing of load-bearing nonlinear structures is still performed using simple force control (dead loading) or displacement control (rigid loading). This means that established experimental methods cannot trace equilibrium manifolds in their entirety because structures snap to alternative equilibria at limit points in the forcing parameter and because branch switching to alternative equilibria cannot be controlled and performed reliably. To extend current testing methods, in Part I of this paper, we implement an experimental path-following method that uses tangent quantities (stiffness and residual forces) and Newton's method to continue along stable and unstable equilibrium paths and traverse limit points. In addition to enforcing the displacement at primary load-introduction points, the overall shape of the structure is controlled via secondary actuators and sensors. Small perturbations of the structure using the secondary actuators allow an experimental tangent stiffness to be computed, which is then used in a control algorithm. As a pertinent test case, the experimental method is applied to a transversely loaded shallow {circular} arch. Due to the complexity of the test setup, the experiment is first designed using a virtual testing environment based on a surrogate finite element model. Experimental results demonstrate the robustness of the proposed experimental method and the usefulness of virtual testing as a surrogate, but also highlight that experimental efficiency and the effects of noise and sensor uncertainty is of particular concern. In Part II, we present perspectives on future research directions and novel testing capabilities that are enabled by extending the methodology to pinpointing of critical points, tracing of critical boundaries, and branch switching