Design and test of an adjustable quasi-zero stiffness device and its use to suspend masses on a multi-modal structure

In some applications, such as ground vibration testing in the aerospace industry, it is of interest to observe the modal behaviour of a slender structure while it is statically loaded. One way of statically loading such a structure is to suspend masses using very soft springs. If the springs are linear, then this results in an extremely large static deflection of the springs. This problem could be overcome by dynamically isolating the masses using quasi-zero stiffness (QZS) springs. This paper describes the design, construction and experimental testing of a device that can exhibit QZS. A novel design is proposed that allows the stiffness and the symmetry of the device to be adjusted independently using separate adjustment mechanisms. Quasi-static and dynamic testing of the device show that it can be adjusted to have an extremely low stiffness within the limits of measurement. The main trend of the force-displacement curve shows that it has a cubic stiffness characteristic, and that friction is responsible for its hysteretic behaviour. Dynamic testing shows that the device locks-up due to friction at low frequencies, but at high frequencies the device acts as an efficient linear isolator. An experiment was also performed where a mass was suspended on a multi-modal beam structure via the QZS device. It was shown that a static load could be applied to the beam without the attached mass appreciably affecting the dynamic response of the beam, even though the suspended mass was about 12% of that of the host structure

An investigation into model extrapolation and stability in the system identification of a nonlinear structure

Estimating a nonlinear model from experimental measurements of a vibrating structure remains a challenge, despite huge progress in recent years. A major issue is that the dynamical behaviour of a nonlinear structure strongly depends on the magnitude of the displacement response. Thus, the validity of an identified model is generally limited to a certain range of motion. Also, outside this range, the stability of the solutions predicted by the model are not guaranteed. This raises the question as to how a nonlinear model derived using data from relatively low amplitude excitation can be used to predict the dynamical behaviour for higher amplitude excitation. This paper focuses on this problem, investigating the extrapolation capabilities of data-driven nonlinear state-space models based on a subspace approach. The experimental vibrating structure consists of a cantilever beam in which magnets are used to generate strong geometric nonlinearity. The beam is driven by an electrodynamic shaker using several levels of broadband random noise. Acceleration data from the beam tip are used to derive nonlinear state-space models for the structure. It is shown that model predictions errors generally tend to increase when extrapolating towards higher excitation levels. Furthermore, the validity of the estimated nonlinear models become poor for very strong nonlinear behaviour. Linearised models are also estimated to have a complete view of the performance of each candidate model

Brexit budget or business as usual? Unpicking the 2016 Autumn statement

The 2016 Autumn Statement has provided the first substantive indication of the fiscal direction of Theresa May’s new government as Brexit negotiations loom on the horizon. Kate Alexander Shaw analyzes the key announcements and checks the political small print

Will Labour’s ‘six tests’ hold the government to account on the UK’s Brexit deal?

With Article 50 triggered, Kate Alexander Shaw analyses the Labour Party’s ‘six tests for Brexit’, arguing that they may let the government off the hook rather than holding them to account over the UK’s final EU deal

Why austerity may be making a post-COVID-19 comeback in Britain

Is austerity coming back? Or has our thinking changed since the aftermath of the global financial crisis? Kate Alexander Shaw identifies key narratives which suggest that austerity still exerts a powerful pull on policy discourse in the UK

The British Intelligence Community in Singapore, 1946-1959: Local Security, Regional Co-ordination and the Cold War in the Far East

Singapore was the stronghold of British intelligence in the Far East during the Cold War. The small city-colony played host to a diverse range of British intelligence organisations including regional outposts of MI5 and the Secret Intelligence Service (SIS), specialist technical intelligence centres, open source reporting centres and the police Special Branch. These intelligence outfits operated across three levels: the local, the regional and the national. This thesis investigates the British intelligence milieu in Singapore, focusing upon its organisation and status; its working culture and operations; and its impact or influence. In so doing, the thesis interrogates to what extent we can speak of a definable British intelligence ‘community’ in Singapore during the early Cold War. It concludes that there were instead two distinct communities: a local intelligence community, and a regional-national one. Nevertheless, there were two core similarities. Security intelligence was at the forefront of both communities as the most appropriate response to the nature of the Cold War both within Singapore and the Southeast Asian region. Secondly, both intelligence communities played a significant role not just in shaping official perceptions but as avenues for covert policy implementation. At the regional level, intelligence activities enabled Britain to fight the Cold War through clandestine measures, fulfilling the key policy goal of providing containment without (extensive) commitments. Locally, security intelligence was a major driving force in the engagement between the Singapore government, communist ‘terrorists’ and anti-colonial nationalists. This thesis is not just about British intelligence in the Cold War. It also provides original insight into Singapore’s transition to self-government between 1946 and 1959 by focusing on the crucial role played by Special Branch. Intelligence services were vital in ensuring that Singapore was rendered ‘safe’ for decolonisation, and their activities indicate continuity between colonial and post-colonial government in Singapore

Why austerity may be making a post-COVID comeback – in Britain, at least

Is austerity coming back? Or has our thinking changed since the aftermath of the global financial crisis? Kate Alexander Shaw (LSE) identifies key narratives which suggest that austerity still exerts a powerful pull on policy discourse in the UK

Chemical underpinning of the tea bag index: An examination of the decomposition of tea leaves

Decomposition is a key flux of terrestrial carbon to the atmosphere. Therefore, gaining a better understanding of how plant litter decomposes in soil, and what governs this process, is vital for global climate models. The Tea Bag Index (TBI) was introduced by Keuskamp et al. (2013) as a novel method for measuring litter decomposition rate and stabilisation. The TBI uses two types of tea bags representing fast (green tea) and slow (rooibos tea) decomposition substrates as standardised litter bags. To date, the TBI method has been used in over 2000 locations across the globe. However, before now, there has been no information on how the composition of the tea leaves change during incubation. These data are crucial in determining the validity of the use of the TBI method globally, to ensure the tea leaves decompose in a way that is representative of so-called “native” litters. To provide chemical underpinning of the TBI method, a laboratory incubation of the tea bags was conducted with destructive sampling at 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, and 91 d. The incubated tea was analysed for total C and N. In addition, C was characterised as alkyl, O-alkyl, aromatic, or carbonyl C using solid-state 13C nuclear magnetic resonance spectroscopy with cross-polarization and magic angle spinning (CPMAS NMR). The results suggest that changes in carbon in both tea types are comparable to other litter studies, with a net decrease in total C and relative proportion of O-alkyl C fraction, which contains carbohydrates and cellulose. We conclude that the decomposition of tea leaves in the bags used in the TBI is representative of other litters

Fluctuations and luck in droplet growth by coalescence

After the initial rapid growth by condensation, further growth of a cloud droplet is punctuated by coalescence events. Such a growth process is essentially stochastic. Yet, computational approaches to this problem dominate and transparent quantitative theory remains elusive. The stochastic coalescence problem is revisited and it is shown, via simple back-of-the-envelope results, that regardless of the initial size, the fastest one-in-a-million droplets, required for warm rain initiation, grow about 10 times faster than the average droplet. While approximate, the development presented herein is based on a realistic expression for the rate of coalescence. The results place a lower bound on the relative velocity of neighboring droplets, necessary for warm rain initiation. Such velocity differences may arise from a variety of physical mechanisms. As an example, turbulent shear is considered and it is argued that even in the most pessimistic case of a cloud composed of single-sized droplets, rain can still form in 30 min under realistic conditions. More importantly, this conclusion is reached without having to appeal to giant nuclei or droplet clustering, only occasional “fast eddies.” This is so because, combined with the factor of 10 accelerated growth of the one-in-a-million fastest droplets, the traditional turbulent energy cascade provides sufficient microshear at interdroplet scales to initiate warm rain in cumulus clouds within the observed times of about 30 min. The simple arguments presented here are readily generalized for a variety of time scales, drizzle production, and other coagulation processes