1,721,534 research outputs found
Post Conflict Reconstruction
No full textViolence and conflict are two of the greatest challenges the world will face in this millennium. Indeed, since the turn of the century, it is estimated that approximately four million people have died as a result of armed conflict. Ending these seemingly intractable conflicts is a priority for global stability. However, the signing of the peace accord or the ending of formal hostilities does not automatically bring a return to normality in these fractured societies. In practice, it is more likely that these fractured societies will face a period in the twilight between war and peace, a time when the world turns its attention to new problems and seemingly more pressing matters, leaving the country to struggle towards peace and a new social order. The books contributors deal with the challenges faced in creating the foundations for the development of a positive peace from a variety of multi-disciplinary perspectives, such as development studies, politics, psychoanalysis, psychology, sports studies and neuroscience. This breadth of perspectives offers innovative insights into the grey space between war and peace, which is home to millions of people across the globe and explores interventions which aim to create the conditions for positive post-conflict reconstruction
Principal component analysis of the cross-axis apparent mass nonlinearity during whole-body vibration
No full textDuring whole-body vibration (WBV), dynamic forces measured at the excitation-subject interface in directions other than the excitation axis, i.e. cross-axis response, are analysed using the principal component analysis (PCA) and virtual coherence techniques. The study applied these operations to the inline and cross-axis forces measured with twelve semisupine human subjects exposed to longitudinal horizontal nominally random vibration between 0.25 and 20 Hz at root mean square acceleration levels of 0.125 ms-2 and 1.0 ms-2. The source identification is realised by a reversed path, aiming to identify relative contributions and correlations between the forces in response to a single axis excitation. The inline longitudinal and the cross-axis vertical forces were found to be correlated to each other from a low (e.g. 1 to 3 Hz) to a medium frequency range (e.g. 10 to 15 Hz). Above this range, where the forces were much reduced, the two forces tended to be independent in their contribution to the overall response. The singular vectors and virtual coherences were able to establish the degree of correlation in each of the frequency band identified. A signal processing framework is then proposed to take into account cross-axis responses for human vibration
Experimental investigation of the variability in the dynamics of connected structures
No full textHydraulic pipes and cable bundles attached to host structures are widely found in engineering. This paper explores how variability in the connection points between structures affects the coupled dynamics. One at a time, two different one-dimensional waveguides are attached to a thin plate through a different set of point connections. Measurements considering randomly spaced connections were made and the experimental results are presented and compared to previously developed models. When multiples attachments are considered, the structure accommodates standing-like waves between the attachments, amplifying its response. It was possible to see the variability due the random spacing and, in a frequency-averaged sense, good agreement between the experimental data and the models were obtained. A comparison of the spatial response of the experiment and the infinite system is also presented
The dynamics of coupled structures possessing inhomogeneous attachments
No full textStructures with hydraulic pipes and cable bundles attached to them are commonly found in engineering, therefore it is of interest to determine the effects of such attachments on the dynamics of the host structures and quantify them. Moreover, manufacturing processes often lead to deviations from the original design. One way to take these deviations into account is to consider small variations in the mechanical properties and/or geometry of the structures. This paper models the attachments as slowly varying inhomogeneous beams and uses the WKB (Wentzel, Kramers and Brillouin) approximation and wave propagation to derive the input and transfer mobilities of said beams. The properties of the beam are described as a random field and are expressed using the Karhunen-Loève expansion. Once the mobilities are known, a mobility approach can be used to couple the attachments to a host structure. Rigid connections are considered along with variations of the spacing between the attachments
The effect of isolation in controlling the variability of point connected structures with uncertain structural attachments
No full textAs the demand for the transmission of electric power and communication signals in automotive and aerospace vehicles increases, so does the number of structures comprising simplified one-dimensional attachments, such as electrical cabling, affixed to a host plate like primary structure. These attachments are typically uncertain in their geometric or material properties, potentially affecting the response of the built-up structure. Difficulties then arise in the prediction of the response of the assembly. This study shows how the variability, due to the uncertain attachments, might be reduced by considering flexible connections. A mobility analysis compares systems connected with either rigid links or elastic springs. A frequency is identified at which the assembly dynamically uncouples; the effect on the host response variability due to the uncertain attachment decreases above this frequency with a reduction of the order of 60 dB in the coefficient of variation. This uncoupling or effective isolation frequency can be simply estimated from the mobility of the elastic connection and the properties of the nominal structural attachment. For design purposes, this frequency can be adjusted to achieve a more predictable response above a given frequency
A friction control strategy for shock isolation
No full textA control strategy is presented incorporating friction which can be adapted within a cycle of vibration. During base shock input, the friction is switched on and off based on specified response parameters. The predicted response of a semi active system is compared with that of a passive isolation system. The strategy is shown to produce an improved displacement reduction and a smaller maximum displacement compared to the base input; a result which cannot be obtained with a typical passive system. The models are then validated using an experimental rig, representing a two degree of freedom system, having an electromagnet to switch on and off friction via the control logic. Good agreement is obtained in addition to identifying optimum parameter choices
A two stage Polynomial Chaos Expansion application for bound estimation of uncertain FRFs
No full textPolynomial Chaos Expansion (PCE) is a method for analysing uncertain vibratory structures with lower computational effort. It may simply be described as a curve fitting method with orthogonal basis terms, where the polynomial type, dimension and order are predefined for the uncertain responses. However, the polynomial order in PCE must be very high to accurately estimate statistical moments of the frequency response function in resonance regions of lightly damped and uncertain structures. To solve this issue different transformation techniques are reported in the literature, where implementations of PCE produce higher accuracy with a lower order polynomial. However, these transformations lose the attraction for using PCE, since they require some additional mathematical operations and, mostly, they present high accuracy if the higher orders of polynomials are again of interest. In this study, an efficient approach is presented for the upper bound estimation of the uncertain frequency response functions (FRFs) via PCE with lower order terms without performing any transformation. Rather than one-stage application of PCE for the desired response of an uncertain problem, the approach comprises a two-stage application of the classical PCE, i.e. first for the natural frequencies and then for the FRF calculations. As an example application of the approach, a thin beam for two different uncertainty cases is considered, namely local and global uncertainty. The local and global input uncertainties are generated by variability of lumped masses added at the boundary and Young’s modulus, respectively. The FRF bounds are compared with extensive experimental and numerical Monte Carlo simulations, showing that low order polynomials are sufficient to calculate the bounds accurately with the technique described
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