2,237 research outputs found
Time domain analysis of the nonlinear transient response of simple structures to shock excitation
No full textThe response of structures to blast wave excitation is a rather complex area of research within the field of structural dynamics. It involves numerous aspects of nonlinear behaviour including large deformation, multimodal response, material nonlinearities, etc. This paper is aimed at providing an insight into the physical nonlinear response process by analysing the dynamics of a single-degree-of-freedom (SDOF) mass-spring system in the time domain. Different types of non-linear restoring forces have been included to predict the system response when subjected to step and impulse excitation and free vibration. Numerically obtained data using adaptive Runge-Kutta time stepping algorithm is compared with analytically derived original expressions for simplified and special cases. The results and comparisons are also represented graphically in a non-dimensional form, and indicate the relative importance or otherwise of features of the input and the parameters of the simple dynamic system
A hybrid methodology for the frequency response function variability due to joint uncertainty
No full textIn vibroacoustic engineering, the most probable cause of dynamic uncertainty are the joints since it is not easy to strictly control the properties of manufactured joints. Although uncertainty in joints is localized in a complex structure, it may affect the dynamic response of the whole structure especially at higher frequencies. Generally, uncertain industrial structures are modelled numerically by FE whereas the uncertainty is modelled by performing Monte Carlo Simulations (MCS). These combined approaches are named FE-MCS. Application of FE-MCS to analyse local uncertainty in a complex structure is computationally slow, as FE and MCS requires a high number of elements and sampling respectively. A possible solution is to introduce a combined hybrid Wave Finite Element and FE (shortly hybrid WFE) model, treating the uniform structures as waveguides joined by a local FE joint representation. Then, Polynomial Chaos Expansion (PCE) can be applied to introduce and model the uncertainty. The methodology is developed herein and tested on two right angled beams forming a L-shaped joint. The joint thickness is assumed to have a uniform distribution as an uncertain parameter. The scattering coefficients and frequency response function for both beams, are selected as the resulting uncertain variables. The results are subsequently verified with FE-MCS simulations using 200 samples and a limited number of experiments. It is clearly shown that the methodology introduced is an efficient tool for the structures possessing local uncertainty in terms of computational load as well as producing good frequency response function predictions when compared to both FE-MCS simulations and experimental validation
Uncertainty propagation in locally damped dynamic systems
No full textIn the field of stochastic structural dynamics, perturbation methods are widely used to estimate the response statistics of uncertain systems. When large built up systems are to be modelled in the mid-frequency range,perturbation methods are often combined with finite element model reduction techniques in order to considerably reduce the computation time of the response. Existing methods based on Component Mode Synthesis(CMS) allow the uncertainties in the system parameters to be treated independently in each of the substructures and the perturbation in the local parameters to be propagated to the full system global parameters. However, local treatment of damping uncertainty is usually avoided by assuming proportional damping. Here, a perturbation method that includes local modal damping uncertainty and its propagation to the global response is proposed. Local damping is accounted for in the CMS model by use of complex modes. A perturbation relationship between local and global modal properties is stated for non-classically damped systems. In this way, the response statistics for uncertain systems with localized damping can be computed at a lowcomputational cost. A numerical example is presented to illustrate the performance of the proposed method
Ferguson-Smith, Malcolm: transcript of a video interview (06-Jun-2015)
No full textInterview with Professor Malcolm Ferguson-Smith, conducted by Ms Emma M. Jones, for the History of Modern Biomedicine Research Group, 06 June 2015, in Glasgow. Transcribed by Mrs Debra Gee, and edited by Professor Tilli Tansey and Mr Alan Yabsley. The project management was undertaken by Mr Adam Wilkinson. Professor Malcolm Ferguson-Smith (b. 1931) is Emeritus Professor of Pathology, University of Cambridge. He graduated in medicine at Glasgow University in 1955 and, while undertaking postgraduate training there in pathology, was introduced to research on sex chromatin under Bernard Lennox. An interest in Klinefelter’s syndrome in 1957 to 1958 led to his appointment as Fellow in Medicine at Johns Hopkins University, Baltimore, in 1959, where he established the first chromosome diagnostic service in the USA, and undertook cytogenetic research into Turner syndrome. Research interests include molecular cytogenetics, karyotype evolution, vertebrate sex determination and comparative genomics. He is joint author of 'Essential Medical Genetics'.The History of Modern Biomedicine Research Group is funded by the Wellcome Trust, which is a registered charity (no. 210183). The current interview has been funded by the Wellcome Trust Strategic Award entitled “Makers of modern biomedicine: testimonies and legacy” (2012-2017; awarded to Professor Tilli Tansey)
Eigenvalue sensitivity minimisation for robust pole placement by the receptance method
No full textThe problem of robust pole placement in active structural vibration control by the method of receptance is considered in this paper. Expressions are derived for the eigenvalue sensitivities to parametric perturbations, which are subsequently minimised to improve performance robustness of the control of a dynamical system. The described approach has application to a vibrating system where variations are present due to manufacturing and material tolerances, damages and environment variabilities. The closed-loop eigenvalue sensitivities are expressed as a linear function of the velocity and displacement feedback gains, allowing their minimisation with carefully calculated feedback gains. The proposed algorithm involves curve fitting perturbed frequency response functions, FRFs, using the rational fraction polynomial method and implementation of a polynomial fit to the individual estimated rational fraction coefficients. This allows the eigenvalue sensitivity to be obtained entirely from structural FRFs, which is consistent with the receptance method. This avoids the need to evaluate the M,C,K matrices which are typically obtained through finite element modelling, that produces modelling uncertainty. It is also demonstrated that the sensitivity minimisation technique can work in conjunction with the pole placement and partial pole placement technique using the receptance method. To illustrate the working of the proposed algorithm, the controller is first implemented numerically and then experimentally.</p
Lessons Learned from Ferguson: Ending Abusive Collection of Criminal Justice Debt
No full textOn March 4, 2015, the Department of Justice released its scathing report of the Ferguson Police Department calling for “an entire reorientation of law enforcement in Ferguson” and demanding that Ferguson “replace revenue-driven policing with a system grounded in the principles of community policing and police legitimacy, in which people are equally protected and treated with compassion, regardless of race.” Unfortunately, abusive collection of criminal justice debt is not limited to Ferguson. This Article, prepared for a discussion group at the Southeastern Association of Law Schools conference in July 2015, identifies the key findings in the Department of Justice’s report and discusses the major points to be learned from the allegations in Ferguson. The lessons learned from Ferguson should be a guide to other municipalities that are or may be on the brink of developing similar abusive collection practices
Graphical comparisons of relative disease burden across multiple risk factors
No full textBackground Population attributable fractions (PAF) measure the proportion of disease prevalence that would be avoided in a hypothetical population, similar to the population of interest, but where a particular risk factor is eliminated. They are extensively used in epidemiology to quantify and compare disease burden due to various risk factors, and directly influence public policy regarding possible health interventions. In contrast to individual specific metrics such as relative risks and odds ratios, attributable fractions depend jointly on both risk factor prevalence and relative risk. The relative contributions of these two components is important, and usually needs to be presented in summary tables that are presented together with the attributable fraction calculation. However, representing PAF in an accessible graphical format, that captures both prevalence and relative risk, may assist interpretation. Methods Taylor-series approximations to PAF in terms of risk factor prevalence and log-odds ratio are derived that facilitate simultaneous representation of PAF, risk factor prevalence and risk-factor/disease log-odds ratios on a single co-ordinate axis. Methods are developed for binary, multi-category and continuous exposure variables. Results The methods are demonstrated using INTERSTROKE, a large international case control dataset focused on risk factors for stroke. Conclusions The described methods could be used as a complement to tables summarizing prevalence, odds ratios and PAF, and may convey the same information in a more intuitive and visually appealing manner. The suggested nomogram can also be used to visually estimate the effects of health interventions which only partially reduce risk factor prevalence. Finally, in the binary risk factor case, the approximations can also be used to quickly convert logistic regression coefficients for a risk factor into approximate PAFs
Can Technology Prevent Another Ferguson?
No full textIn my home town of St. Louis, a petition has gained traction calling for police to wear body cameras that capture everything in front of them while they are doing their jobs. This would be just a step beyond the idea of putting dashboard cameras on all police cars, which some police agencies around the country have started doing. The reasoning goes that with these cameras, we might deter police misconduct and get better answers to disputed questions like what happened in the Michael Brown shooting in Ferguson
Structural response of corroded concrete columns with different rebar confinements under cyclic compressive loading
No full textThe new codes have recently introduced seismic detailing for new structures, but there are still older reinforced concrete structures without proper ductile detailing for earthquake resistance. These structures are further impacted by the corrosion of their embedded rebars which further reduces the strength and ductility under loading. This paper presents summary of the results of an experimental investigation performed on short RC columns, with different confinement configurations subject to varying degrees of corrosion, to investigate the structural responses to cyclic loading. The experiment was conducted on 20 short RC columns (square and circular) with two levels of confinements and steel reinforcement corrosion loss ranging from 0% to to ~30% subjected to cyclic compressive load. The test results show that corrosion and rebar confinements significantly impact the structural responses of corroded columns
Understanding how kurtosis is transferred from input acceleration to stress response and it's influence on fatigue life
No full textHigh cycle fatigue of metals typically occurs through long term exposure to time varying loads which, although modest in amplitude, give rise to microscopic cracks that can ultimately propagate to failure. The fatigue life of a component is primarily dependent on the stress amplitude response at critical failure locations. For most vibration tests, it is common to assume a Gaussian distribution of both the input acceleration and stress response. In real life, however, it is common to experience non-Gaussian acceleration input, and this can cause the response to be non-Gaussian. Examples of non-Gaussian loads include road irregularities such as potholes in the automotive world or turbulent boundary layer pressure fluctuations for the aerospace sector or more generally wind, wave or high amplitude acoustic loads. The paper first reviews some of the methods used to generate non-Gaussian excitation signals with a given power spectral density and kurtosis. The kurtosis of the response is examined once the signal is passed through a linear time invariant system. Finally an algorithm is presented that determines the output kurtosis based upon the input kurtosis, the input power spectral density and the frequency response function of the system. The algorithm is validated using numerical simulations. Direct applications of these results include improved fatigue life estimations and a method to accelerate shaker tests by generating high kurtosis, non-Gaussian drive signals
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