Detection and Location of Nonlinearities using Reciprocity Breakdown

Damage in a structure often results in local stiffness nonlinearities and detecting these nonlinearities can be used to monitor the health of the structure. It is well-known that nonlinearities in structures lead to a breakdown in reciprocity, where the frequency response function between two points on the structure depends upon the forcing location. This paper proposes a measure to quantify the level of non-reciprocity in a structure and investigates the effect of the location and form of nonlinearity on this non-reciprocity measure. A simulated discrete mass-spring system was used to determine the effect of the excitation and response locations on the ability to detect the nonlinearity. Stepped-sine testing is commonly used to characterise a nonlinear system since harmonic excitation emphasises nonlinear phenomena and can, for example, allow the system to exhibit multiple solutions. Thus, a simulation of a stepped sine test was used as the benchmark to highlight reciprocity breakdown in the most favourable case. However, impact excitation is much easier and faster to implement in practice and consequently the effect of the type of excitation on the detection and location of nonlinearities was considered. Finally, the prospects for using a measure of reciprocity in a structural health monitoring system are discussed

Seismic assessment of the Matera cathedral

This paper presents the seismic assessment of the Cathedral of Matera, in southern Italy, to determine the capacity of the structure when subjected to earthquakes. This church dates back to the 13th century and is one of the most representative monuments of the Apulian Romanesque architecture. Within the context of the evaluation of the seismic response of the cathedral, modal identification tests were performed in order identify and characterize the main dynamic properties of the structure. The results of these tests were used to develop a representative finite element model, which is able to provide the response to seismic actions. A pushover analysis was performed to characterize the seismic behavior of the structure. The results of the seismic analyses on the cathedral show that its vulnerability is high, being the transversal direction the less stiff and resistant. Elements as the nave and the façade, along with the bell tower, might be the most vulnerable to seismic actions. Additionally, it was observed that components as the trusses of the central nave strongly modify the seismic response and capacity of the structure. Apparently, the structure might not be able to withstand a strong earthquake from the region or might present several damage after one. Hence, it is recommendable to perform further studies about the seismic behavior, especially of the most vulnerable elements.The authors would like to acknowledge the University of Minho for supporting the experimental campaign. Thanks is also extended to Dr. Nuno Mendes, University of Minho, for his guidance and help for performing the in-situ tests on the cathedral. The authors would also like to thank to the ELARCH project number 552129-EM-1-2014-1-IT-ERASMUS MUNDUS-EMA 21 for funding the graduate studies of the first author

Conceptual-level evaluation of a variable stiffness skin for a morphing wing leading edge

A morphing leading edge produces a continuous aerodynamic surface that has no gaps between the moving and fixed parts. The continuous seamless shape has the potential to reduce drag, compared to conventional devices, such as slats that produce a discrete aerofoil shape change. However, the morphing leading edge has to achieve the required target shape by deforming from the baseline shape under the aerodynamic loads. In this paper, a conceptual-level method is proposed to evaluate the morphing leading edge structure. The feasibility of the skin design is validated by checking the failure index of the composite when the morphing leading edge undergoes the shape change. The stiffness of the morphing leading edge skin is spatially varied using variable lamina angles, and comparisons to the skin with constant stiffness are made to highlight its potential to reduce the actuation forces. The structural analysis is performed using a two-level structural optimisation scheme. The first level optimisation is applied to find the optimised structural proper- ties of the leading edge skin and the associated actuation forces. The structural properties of the skin are given as a stiffness distribution, which is controlled by a B spline interpolation function. In the second level, the design solution of the skin is investigated. The skin is assumed to be made of variable stiffness composite. The stack sequence of the composite is optimised element-by-element to match the target stiffness. A failure criterion is employed to obtain the failure index when the leading edge is actuated from the baseline shape to the target shape. Test cases are given to demonstrate that the optimisation scheme is able to provide the stiffness distribution of the leading edge skin and the actuation forces can be reduced by using a spatially variable stiffness skin

Excess baggage for birds: inappropriate placement of tags on gannets changes flight patterns.

Published onlineJournal ArticleResearch Support, Non-U.S. Gov'tThis is the final version of the article. Available from Public Library of Science via the DOI in this record.Devices attached to flying birds can hugely enhance our understanding of their behavioural ecology for periods when they cannot be observed directly. For this, scientists routinely attach units to either birds' backs or their tails. However, inappropriate payload distribution is critical in aircraft and, since birds and planes are subject to the same laws of physics during flight, we considered aircraft aerodynamic constraints to explain flight patterns displayed by northern gannets Sula bassana equipped with (small ca. 14 g) tail- and back-mounted accelerometers and (larger ca. 30 g) tail-mounted GPS units. Tail-mounted GPS-fitted birds showed significantly higher cumulative numbers of flap-glide cycles and a higher pitch angle of the tail than accelerometer-equipped birds, indicating problems with balancing inappropriately placed weights with knock-on consequences relating to energy expenditure. These problems can be addressed by carefully choosing where to place tags on birds according to the mass of the tags and the lifestyle of the subject species.This study would have not been carried out without the financial support from the California Department of Fish and Game's Oil Spill Response Trust Fund (through the Oiled Wildlife Care Network at the Wildlife Health Center, School of Veterinary Medicine, University of California, Davis) and the Royal Society for Prevention of Cruelty to Animals (RSPCA, Wilberforce Way, Southwater, Horsham, West Sussex, RH13 9RS, United Kingdom). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Automatic two-plane balancing for rigid rotors

Abstract We present an analysis of a two-plane automatic balancing device for rigid rotors. Ball bearings, which are free to travel around a race, are used to eliminate imbalance due to shaft eccentricity or misalignment. The rotating frame is used to derive autonomous equations of motion and the symmetry breaking bifurcations of this system are investigated. Stability diagrams in various parameter planes show the coexistence of a stable balanced state with other less desirable dynamics