Harmonic excitation of bolted joints

Bolted joints provide one of the most common means of joining two structural components together. The joints themselves use friction to transmit force, torque and motion across a common interface from one component to another. In many cases a pretensioned bolt, running through a common hole at the joint interface, provides the clamping force. The friction force at a joint interface is highly nonlinear. This makes the analysis of dynamic systelTIS with joints unrealistic with conventional linear techniques. It has also been shown that the contact pressure at a joint interface is not necessarily uniform. A variable contact pressure results in a variable limiting friction load. Where the contact pressure can be shown to be smallest on an interface, local microslip can take place whilst the joint maintains its sticking contact elsewhere. Microslip is responsible for the dissipation of energy from within bolted joints that otherwise maintain their integrity. The level of energy dissipation caused by microslip can be significantly larger than that provided by other dissipative mechanisms within a structure. This provides an incentive to be able to describe and predict the energy losses and overall joint behaviour accurately. Difficulties arise when considering 3-Dimensional contact, changing contact conditions during dynamic loading and the nonlinear nature of friction phenomena. To investigate microslip behaviour in bolted joints a detailed finite element model of an isolated lap joint interface was constructed. The joint interface was subjected to a variety of preloads and applied torque. Output from the joint is in the form of hysteresis loops that reveal information about the energy dissipated and overall joint stiffness during a loading cycle. Representative models are presented that reduce the complexity of the joint, yet still maintain the defining characteristics of the hysteretic behaviour. The first representative model uses Jenkins elements that match the physical response of the joint at a number of discrete points during the loading cycle. Good agreement between the finite element model and the Jenkins element model is illustrated. The Jenkins element model is also capable of predicting the response of the finite element model when different magnitudes of preload and applied torque are applied. The second representative model is the Bouc-Wen representation of hysteresis. This model offers significant gains in efficiency when approximating the smooth transition from a fully sticking interface to the onset of joint failure. All of the hysteresis can be described using just four parameters, and matching with the finite element model is demonstrated. To demonstrate microslip behaviour physically an individual joint was experimentally analysed. A cantilever beam with a single lap joint near the clamped end is resonated to generate the dynamic joint hysteresis. The joint behaviour is monitored by local time domain measurements at a number of different preloads and excitation amplitudes. Microslip is demonstrated in the joint when the preload is reduced from a maximum "rigid" clamping value. Notably at low preloads the spectral content of the response reveals a large contribution from the superharmonics of the excitation frequency. Both the Jenkins element model and the Bouc-Wen model are successfully matched to the hysteresis output of the experimental joint

Stability and Retraction Force Verification of a New Retractor Design for Minimally Invasive Surgery

Minimally Invasive Surgery (MIS) needs continuous tool design innovation to support and facilitate the complex task executions of surgeons. In this article, an easily deployable magnetic structure design is presented, which is developed to retract the liver during MIS procedures. During the concept designing phase, a most critical research question, the stability of magnetic anchoring was investigated and analyzed through various experiments. The clinically relevant pulling forces have been applied to N52 neodymium magnets in different size, shape and arrangement to derive the maximum force certain retractor designs could withheld. The numeric results confirmed that the distributed load arrangement would be able to perform a stable human liver retraction. Magnetic encoring technology could have a significant future, encouraging other researchers to investigate the potential of magnetic tissue retraction in MIS procedures that could lead tothe development of specialized tools for human clinical deployment

Acyl-chain elongation drives ketosynthase substrate selectivity in trans-acyltransferase polyketide synthases

Type I modular polyketide synthases (PKSs), responsible for the biosynthesis of many biologically active agents, possess a ketosynthase (KS) domain within each module to catalyze chain elongation. Acylation of the KS active site Cys residue is followed by transfer to malonyl-acyl carrier protein, yielding an extended β-ketoacyl chain. To date, the precise contribution of KS selectivity in controlling product fidelity has been unclear. We submitted six KS domains from the trans-acyl transferase PKSs to a mass spectrometry-basedelongation assay, and identified higher substrat selectivity in the elongating step than in preceding acylation. A close correspondence between observed KS selectivity and that predicted by phylogenetic analysis was seen. Our findings provide insights into the mechanism of KS selectivity in this important group of PKSs, can serve as guidance for engineering, and show that targeted mutagenesis can be used to expand the repertoire of acceptable substrates

Seeing the body distorts tactile size perception

Vision of the body modulates somatosensation, even when entirely non-informative about stimulation. For example, seeing the body increases tactile spatial acuity, but reduces acute pain. While previous results demonstrate that vision of the body modulates somatosensory sensitivity, it is unknown whether vision also affects metric properties of touch, and if so how. This study investigated how non-informative vision of the body modulates tactile size perception. We used the mirror box illusion to induce the illusion that participants were directly seeing their stimulated left hand, though they actually saw their reflected right hand. We manipulated whether participants: (a) had the illusion of directly seeing their stimulated left hand, (b) had the illusion of seeing a non-body object at the same location, or (c) looked directly at their non-stimulated right-hand. Participants made verbal estimates of the perceived distance between two tactile stimuli presented simultaneously to the dorsum of the left hand, either 20, 30, or 40 mm apart. Vision of the body significantly reduced the perceived size of touch, compared to vision of the object or of the contralateral hand. In contrast, no apparent changes of perceived hand size were found. These results show that seeing the body distorts tactile size perception

Remembering 'zeal' but not 'thing':reverse frequency effects as a consequence of deregulated semantic processing

More efficient processing of high frequency (HF) words is a ubiquitous finding in healthy individuals, yet frequency effects are often small or absent in stroke aphasia. We propose that some patients fail to show the expected frequency effect because processing of HF words places strong demands on semantic control and regulation processes, counteracting the usual effect. This may occur because HF words appear in a wide range of linguistic contexts, each associated with distinct semantic information. This theory predicts that in extreme circumstances, patients with impaired semantic control should show an outright reversal of the normal frequency effect. To test this prediction, we tested two patients with impaired semantic control with a delayed repetition task that emphasised activation of semantic representations. By alternating HF and low frequency (LF) trials, we demonstrated a significant repetition advantage for LF words, principally because of perseverative errors in which patients produced the previous LF response in place of the HF target. These errors indicated that HF words were more weakly activated than LF words. We suggest that when presented with no contextual information, patients generate a weak and unstable pattern of semantic activation for HF words because information relating to many possible contexts and interpretations is activated. In contrast, LF words tend are associated with more stable patterns of activation because similar semantic information is activated whenever they are encountered

Non-Drude THz conductivity of graphene due to structural distortions

The remarkable electrical, optical and mechanical properties of graphene make it a desirable material for electronics, optoelectronics and quantum applications. A fundamental understanding of the electrical conductivity of graphene across a wide frequency range is required for the development of such technologies. In this study, we use terahertz (THz) time-domain spectroscopy to measure the complex dynamic conductivity of electrostatically gated graphene, in a broad $\sim$0.1 - 7 THz frequency range. The conductivity of doped graphene follows the conventional Drude model, and is predominantly governed by intraband processes. In contrast, undoped charge-neutral graphene exhibits a THz conductivity that significantly deviates from Drude-type models. Via quantum kinetic equations and density matrix theory, we show that this discrepancy can be explained by additional interband processes, that can be exacerbated by electron backscattering. We propose a mechanism where such backscattering -- which involves flipping of the electron pseudo-spin -- is mediated by the substantial vector scattering potentials that are associated with structural deformations of graphene. Our findings highlight the significant impact that structural distortions and resulting electrostatic vector scattering potentials can have on the THz conductivity of charge-neutral graphene. Our results emphasise the importance of the planar morphology of graphene for its broadband THz electronic response.Comment: 74 pages, 21 figure

"On the Spot": travelling artists and Abolitionism, 1770-1830

Until recently the visual culture of Atlantic slavery has rarely been critically scrutinised. Yet in the first decades of the nineteenth century slavery was frequently represented by European travelling artists, often in the most graphic, sometimes voyeuristic, detail. This paper examines the work of several itinerant artists, in particular Augustus Earle (1793-1838) and Agostino Brunias (1730–1796), whose very mobility along the edges of empire was part of a much larger circulatory system of exchange (people, goods and ideas) and diplomacy that characterised Europe’s Age of Expansion. It focuses on the role of the travelling artist, and visual culture more generally, in the development of British abolitionism between 1770 and 1830. It discusses the broad circulation of slave imagery within European culture and argues for greater recognition of the role of such imagery in the abolitionist debates that divided Britain. Furthermore, it suggests that the epistemological authority conferred on the travelling artist—the quintessential eyewitness—was key to the rhetorical power of his (rarely her) images. Artists such as Earle viewed the New World as a boundless source of fresh material that could potentially propel them to fame and fortune. Johann Moritz Rugendas (1802-1858), on the other hand, was conscious of contributing to a global scientific mission, a Humboldtian imperative that by the 1820s propelled him and others to travel beyond the traditional itinerary of the Grand Tour. Some artists were implicated in the very fabric of slavery itself, particularly those in the British West Indies such as William Clark (working 1820s) and Richard Bridgens (1785-1846); others, particularly those in Brazil, expressed strong abolitionist sentiments. Fuelled by evangelical zeal to record all aspects of the New World, these artists recognised the importance of representing the harsh realities of slave life. Unlike those in the metropole who depicted slavery (most often in caustic satirical drawings), many travelling artists believed strongly in the evidential value of their images, a value attributed to their global mobility. The paper examines the varied and complex means by which visual culture played a significant and often overlooked role in the political struggles that beset the period

What is embodiment? a psychometric approach

What is it like to have a body? The present study takes a psychometric approach to this question. We collected structured introspective reports of the rubber hand illusion, to systematically investigate the structure of bodily self-consciousness. Participants observed a rubber hand that was stroked either synchronously or asynchronously with their own hand and then made proprioceptive judgments of the location of their own hand and used Likert scales to rate their agreement or disagreement with 27 statements relating to their subjective experience of the illusion. Principal components analysis of this data revealed four major components of the experience across conditions, which we interpret as: embodiment of rubber hand, loss of own hand, movement, and affect. In the asynchronous condition, an additional fifth component, deafference, was found. Secondary analysis of the embodiment of runner hand component revealed three subcomponents in both conditions: ownership, location, and agency. The ownership and location components were independent significant predictors of proprioceptive biases induced by the illusion. These results suggest that psychometric tools may provide a rich method for studying the structure of conscious experience, and point the way towards an empirically rigorous phenomenology

Deciphering the complex three-way interaction between the non-integrin laminin receptor, galectin-3 and Neisseria meningitidis

The non-integrin laminin receptor (LAMR1/RPSA) and galectin-3 (Gal-3) are multi-functional host molecules with roles in diverse pathological processes, particularly of infectious or oncogenic origins. Using bimolecular fluorescence complementation and confocal imaging, we demonstrate that the two proteins homo- and heterodimerize, and that each isotype forms a distinct cell surface population. We present evidence that the 37 kDa form of LAMR1 (37LRP) is the precursor of the previously described 67 kDa laminin receptor (67LR), whereas the heterodimer represents an entity that is distinct from this molecule. Site-directed mutagenesis confirmed that the single cysteine (C173) of Gal-3 or lysine (K166) of LAMR1 are critical for heterodimerization. Recombinant Gal-3, expressed in normally Gal-3-deficient N2a cells, dimerized with endogenous LAMR1 and led to a significantly increased number of internalized bacteria (Neisseria meningitidis), confirming the role of Gal-3 in bacterial invasion. Contact-dependent cross-linking determined that, in common with LAMR1, Gal-3 binds the meningococcal secretin PilQ, in addition to the major pilin PilE. This study adds significant new mechanistic insights into the bacterial–host cell interaction by clarifying the nature, role and bacterial ligands of LAMR1 and Gal-3 isotypes during colonization