The self-excitation damping ratio: A chatter criterion for time-domain milling simulations

Regenerative chatter is known to be a key factor that limits the productivity of high speed machining. Consequently, a great deal of research has focused on developing predictive models of milling dynamics, to aid engineers involved in both research and manufacturing practice. Time-domain models suffer from being computationally intensive, particularly when they are used to predict the boundary of chatter stability, when a large number of simulation runs are required under different milling conditions. Furthermore, to identify the boundary of stability each simulation must run for sufficient time for the chatter effect to manifest itself in the numerical data, and this is a major contributor to the inefficiency of the chatter prediction process. In the present article, a new chatter criterion is proposed for time-domain milling simulations, that aims to overcome this draw-back by considering the transient response of the modeled behavior, rather than the steady-state response. Using a series of numerical investigations, it is shown that in many cases the new criterion can enable the numerical prediction to be computed more than five times faster than was previously possible. In addition, the analysis yields greater detail concerning the nature of the chatter vibrations, and the degree of stability that is observed

Optimum design of sway frames

The work presented in this thesis consists mainly of three design methods for sway frames based on the minimum-weight concept. For multistorey frames the aim has been to produce quick, simple and inexpensive design methods that can be used without recourse to a large computer. The proposed direct design procedure for multistorey sway frames to limiting deflections at working load is suitable for hand calculation and the discrete properties of standard rolled sections can be taken into consideration. Design charts have been presented to reduce the amount of calculation. For the design of multistorey frames to strength, stability and deflection requirements, an approximate elastic-plastic analysis procedure has been developed using substitute frames. Design by this method is based on the frame behaviour up to collapse while satisfying the permissible deflections at working load as specified by the codes. For small frames, hand calculations can be used, whereas simple programmes suitable for desk-top computers enable rapid design of large frames. A computer analysis procedure for members with varying cross- sections has been developed using the matrix displacement method. This has been used to evolve an optimum design method for single-bay pitched-roof tapered portal frames to strength and deflection requirements. A linear programming technique has been used to solve the non-linear functions of the optimisation problem by following a multi-linear path

Efficient modelling of RC walls for accurate simulations under earthquake loading

Remarkable advances have been achieved in earthquake engineering in the past decades, given the growing awareness and concern regarding the global seismic risk. In the wake of the damage wrought by the recent earthquakes, reinforced concrete (RC) walls are commonly employed as effective seismic resisting components in new building structures or retrofitting solutions to enhance the seismic performance of existing sub-standard frame buildings. Current codes of practice recommend using nonlinear dynamic analysis as the most accurate technique for the seismic evaluation of RC buildings under earthquake loading. This necessitates the development of reliable numerical strategies for accurate simulation of RC walls under cyclic loading conditions representing seismic actions. This research starts with a critical appraisal of currently available modelling strategies for RC walls associated with different levels of sophistication. They include: (i) the wide column approach with 1D beam elements, (ii) 2D FE models with nonlinear shell elements and (iii) detailed 3D FE descriptions with solid elements and embedded bar elements. Numerical simulations have been performed considering experimental slender and short wall specimens subjected to cyclic loading. Numerical-experimental comparisons highlight some drawbacks of existing modelling strategies as their inability to represent the actual degradation of strength and stiffness and the pinching characteristics of the cyclic behaviour, especially in the case of wall samples whose response is governed by flexure-shear interaction. In view of these limitations and to achieve more accurate response predictions, an efficient and practical 2D macro-element representation for RC walls is proposed in the second part of the research. It incorporates a biaxial concrete model based on the rotating crack approach to account for the nonlinear response under cyclic loading conditions. Accuracy and efficiency of the macro-element model have been demonstrated by validation studies, focusing on RC walls with different aspect ratios and an RC coupled wall system. The ability of the proposed model to predict the main cyclic response characteristics of RC walls, including stiffness and strength degradation, energy dissipation capacity, and pinched shapes of the hysteresis loops, has been confirmed by a favourable agreement between the numerical predictions and experimental findings. The final part of this research proceeds with an application study on seismic analysis of a realistic four-storey RC frame-wall building. The developed macro-element model accounting for shear deformability and potential shear damage and failure provides a more realistic representation for RC walls than the wide column approach widely used in practice. Moreover, the macro-element modelling strategy requires a comparable computational cost to the wide column approach, which renders it suitable for nonlinear dynamic analysis of large scale structures and realistic seismic assessment of RC buildings with shear walls.Open Acces

Modeling operational risk data reported above a time-varying threshold

Typically, operational risk losses are reported above a threshold. Fitting data reported above a constant threshold is a well known and studied problem. However, in practice, the losses are scaled for business and other factors before the fitting and thus the threshold is varying across the scaled data sample. A reporting level may also change when a bank changes its reporting policy. We present both the maximum likelihood and Bayesian Markov chain Monte Carlo approaches to fitting the frequency and severity loss distributions using data in the case of a time varying threshold. Estimation of the annual loss distribution accounting for parameter uncertainty is also presented

Modelling techniques for biological systems

The objective of this investigation has been to develop and evaluate techniques which are appropriate to the modelling and simulation of biological reaction system behaviour. The model used as the basis for analysis of modelling and simulation techniques is a reduced version of the biological model proposed by the IAWPRC Task Group for mathematical modell ing in wastewater treatment design. This limited model has the advantage of being easily manageable in terms of analysis and presentation of the simulation techniQues whilst at the same time incorporating a range of features encountered with biological growth applications in general. Because a model may incorporate a number of different components and large number of biological conversion processes, a convenient method of presentation was found to be a matrix format. The matrix representation ensures clarity as to what compounds, processes and react ion terms are to be incorporated and allows easy comparison of different models. In addition, it facilitates transforming the model into a computer program. Simulation of the system response first involves specifying the reactor configuration and flow patterns. With this information fixed, mass balances for each compound in each reactor can be completed. These mass balances constitute a set of simultaneous non-linear differential and algebraic eQuations which, when solved, characterise the system behaviour

Mass-spring modelling of vault springboard contact

Vaulting is a discipline in Men's and Women's Artistic Gymnastics. While the springboard contact is not judged, the success of the rest of the vault is underpinned by it. The purpose of this study was to develop an understanding of the mechanics of the springboard contact phase of gymnastic vaulting. An analysis of hopping in place, forward hopping and running jumps on a force platform showed that the force-mass centre displacement relationship during ground contact approximated that of a mass rebounding on a linear spring. Subsequently, two mass-spring models were developed using a symbolic mathematics package. Both models represented the gymnast as a rigid cylinder, with personalized linear and angular inertia characteristics, connected at its mass centre to a linear spring. A one spring model combined the springiness of the gymnast and the springboard in a single linear spring, while a two spring model treated them as separate linear springs. Handspring vaults performed by an elite male gymnast at a range of approach speeds and springboard settings were analysed to provide model inputs. Springboard properties were empirically determined and revealed that the springboard stiffness varied appreciably depending upon feet contact position. Given the touchdown kinematics and takeoff angle of the gymnast, the models estimated spring stiffness and linear and angular takeoff velocities, the spring stiffness and takeoff vertical velocity estimates showing some sensitivity to spring angle at touchdown. Simulations in which the touchdown kinematics and spring stiffnesses were systematically adjusted, identified their influence on takeoff kinematics and provided an insight into the mechanics of springboard. contact. Estimated (leg) spring stiffnesses were consistent with those reported in the literature for other activities and'simulation results showed that simple rebounds accounted for the majority of the takeoff velocities. Spring angle at touchdown was found to be most effective at modifying each of the takeoff variables, however to produce a selective effect on takeoff required a combination of adjustments to the touchdown. In proposing strategies for gymnasts, their ability to control each of the touchdown variables has to be considered

Development of an event-based simulator for analysing excluded volume effects in a Brownian gas

Il presente lavoro si pone come scopo lo sviluppo di un simulatore in C++ di dinamica molecolare utilizzando un approccio event-based, in grado di simulare la dinamica newtoniana semplice di molecole bidimensionali di forma arbitraria. Abbiamo utilizzato il simulatore NOCS per imbastire un primo tentativo di ricerca e di analisi degli effetti di volume escluso sul moto Browniano di molecole. In particolare si vogliono ricercare violazioni locali di isotropia nel moto Browniano. Nella parte teorica dell'elaborato, si analizzano gli strumenti matematici e statistici fondamentali della Kinetic Theory (teoria cinetica dei gas) ed i principali modelli della depletion force, uno dei fenomeni causati da potenziale di volume escluso

A Cluster-Based Opposition Differential Evolution Algorithm Boosted by a Local Search for ECG Signal Classification

Electrocardiogram (ECG) signals, which capture the heart's electrical activity, are used to diagnose and monitor cardiac problems. The accurate classification of ECG signals, particularly for distinguishing among various types of arrhythmias and myocardial infarctions, is crucial for the early detection and treatment of heart-related diseases. This paper proposes a novel approach based on an improved differential evolution (DE) algorithm for ECG signal classification for enhancing the performance. In the initial stages of our approach, the preprocessing step is followed by the extraction of several significant features from the ECG signals. These extracted features are then provided as inputs to an enhanced multi-layer perceptron (MLP). While MLPs are still widely used for ECG signal classification, using gradient-based training methods, the most widely used algorithm for the training process, has significant disadvantages, such as the possibility of being stuck in local optimums. This paper employs an enhanced differential evolution (DE) algorithm for the training process as one of the most effective population-based algorithms. To this end, we improved DE based on a clustering-based strategy, opposition-based learning, and a local search. Clustering-based strategies can act as crossover operators, while the goal of the opposition operator is to improve the exploration of the DE algorithm. The weights and biases found by the improved DE algorithm are then fed into six gradient-based local search algorithms. In other words, the weights found by the DE are employed as an initialization point. Therefore, we introduced six different algorithms for the training process (in terms of different local search algorithms). In an extensive set of experiments, we showed that our proposed training algorithm could provide better results than the conventional training algorithms.Comment: 44 pages, 9 figure

Ultimate load behaviour of steel box girders and their components

Imperial Users onl