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    ABSTRACT: Anti-windup systems are modified control structures, which are designed to compensate against the detrimental effects of saturations. This manuscript considers primarily the stability robustness of two well-known anti-windup structures. Sufficient conditions for the stability robustness of the anti-windup structures and optimal robustness against structured norm-bounded plant uncertainty are found. A saturated loop is said to be optimally robust if the constrained loop is as robust as its unconstrained counterpart. The robustness condition is shown to be less conservative than existing results on additive uncertainty. Although it is known that the conventional internal model control provides optimal robustness against additive unstructured uncertainty, this is not the case against the more general uncertainty structure. Copyright (c) 2013 John Wiley & Sons, Ltd.
    International Journal of Robust and Nonlinear Control 11/2014; 24(17). DOI:10.1002/rnc.3014
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    ABSTRACT: Part 1 of this paper described a specimen for the measurement of high strain rate flow and fracture properties of pipe material and for tuning a strain rate dependent damage model (SRDD). In part 2 the tuned SRDD model is used for the simulation of axial crack propagation and arrest in X100 natural gas pipelines. Linear pressure drop model was adopted behind the crack tip, and an exponential gas depressurisation model was used ahead of the crack tip. The model correctly predicted the crack initiation (burst) pressure, the crack speed and the crack arrest length. Strain rates between 1000s−1 and 3000s−1 immediately ahead of the crack tip are predicted, giving a strong indication that a strain rate material model is required for the structural integrity assessment of the natural gas pipelines. The models predict the stress triaxiality of about 0.65 for at least 1m ahead of the crack tip, gradually dropping to 0.5 at distances of about 5-7m ahead of the crack tip. Finally, the models predicted a linear drop in crack tip opening angle (CTOA) from about 11−12° at the onset of crack propagation down to 7−8° at crack arrest. Only the lower of these values agree with those reported in the literature for quasi-static measurements. This discrepancy might indicate substantial strain rate dependence in CTOA.
    International Journal of Pressure Vessels and Piping 10/2014; 122(1). DOI:10.1016/j.ijpvp.2014.07.001
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    ABSTRACT: A method is introduced by which the complete state of residual stress in an elastic body may be inferred from a limited set of experimental measurements. Two techniques for carrying out this reconstruction using finite element analysis are compared and it is shown that for exact reconstruction of the stress field via this method, the stress field must be measured over all eigenstrain-containing regions of the object. The effects of error and incompleteness in the measured part of the stress field on the subsequent analysis are investigated in a series of numerical experiments using synthetic measurement data based on the NeT TG1 round-robin weld specimen. It is hence shown that accurate residual stress field reconstruction is possible using measurement data of a quality achievable using current experimental techniques.
    International Journal of Solids and Structures 05/2014; 51(10):1980–1990. DOI:10.1016/j.ijsolstr.2014.02.008
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    ABSTRACT: This one-dimensional time-domain finite-element model achieves accurate quantitative modelling of ultrasonic wave propagation in multi-layered structures. First, a sinusoidal wave toneburst is sent into a single layer of material exhibiting inherent material nonlinearity characterised by the nonlinear parameter β and thick enough for the toneburst received in through transmission to be resolved. The signal processing protocol that yields the theoretically correct quantitative value of β involves measuring the received toneburst for several propagation distances as well as the use of scaling factors taking into account the fast Fourier transform implementation, input signal windowing and material damping. Using that model configuration, model parameters (element size, time step, frequency step, input pressure, etc.) are then optimised and chosen quantitatively to generate accurate results. Finally, these model parameters are used for cases of interest where the configuration is not such that the exact β value can be obtained – e.g. thinner sample, pulse-echo etc. but where confidence in the results remains. This quantitative model that can be used for multi-layered structures provides a tangible resource useful to NDE engineers: a new prediction tool expected to enable them to choose the experimental set-up, driving frequency and post-processing method that would optimise kissing bond detection capability.
    NDT & E International 01/2014; 61:45–52. DOI:10.1016/j.ndteint.2013.09.006
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    ABSTRACT: Structural integrity assessments of pressurised pipes include plastic collapse as a potential failure mode. This paper uses analytical and numerical models to explore the effect of the end conditions of the pipe on the collapse pressure. The pipe is open-ended and two bounding conditions are addressed: one where axial loading is applied to the ends of the pipe and the other where a fixed axial displacement is applied. The fixed axial displacement condition represents long-range or fit-up residual stress. It is common practice to treat long-range residual stress in the same way as axial loading, leading to the conclusion that such long-range residual stress reduces the collapse pressure. Pipes in a number of states are considered: pipes with no flaws, pipes with fully circumferential flaws and pipes with part circumferential flaws. The flaws consist of either a crack or a slot on the external surface of the pipe. For the axial load condition, the collapse pressure for a flawed pipe is reduced when higher magnitudes of tensile or compressive axial loads are applied. For the fixed displacement condition however, the magnitude of the displacement may have little or no effect on the collapse pressure. The results of the work indicate that substantially conservative assessments may be made of the collapse pressures of pipes containing flaws, when long-range residual stress is taken to be a form of axial loading.
    International Journal of Pressure Vessels and Piping 11/2013; s 111–112:54–62. DOI:10.1016/j.ijpvp.2013.04.030
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    ABSTRACT: Conventional SHM systems typically rely on permanently attached sensor networks glued on to a structure. These add complexity and weight through either a wiring requirement or the use of wireless sensing nodes. This paper reports on an alternative approach whereby the wire between transducer and ultrasonic equipment is replaced by an inductive coupling. This allows a passive small sensor unit to be embedded into a composite component. Here a model is presented to describe the performance and optimization of such a coupling. The resulting sensors are embedded in a CFRP component and shown to exhibit excellent performance. Signal processing to account for the effects of misalignment is described. Finally, the ability of such a system to detect typical impact damage is demonstrated.
    The Journal of the Acoustical Society of America 11/2013; 134(5):4131. DOI:10.1121/1.4831175
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    ABSTRACT: Harmonic generation measurements typically make use of the plane wave result when extracting values for the nonlinearity parameter, β, from experimental measurements. This approach, however, ignores the effects of diffraction, attenuation, and receiver integration which are common features in a typical experiment. Our aim is to determine the importance of these effects when making measurements of β over different sample dimensions, or using different input frequencies. We describe a three-dimensional numerical model designed to accurately predict the results of a typical experiment, based on a quasi-linear assumption. An experiment is designed to measure the axial variation of the fundamental and second harmonic amplitude components in an ultrasonic beam, and the results are compared with those predicted by the model. The absolute β values are then extracted from the experimental data using both the simulation and the standard plane wave result. A difference is observed between the values returned by the two methods, which varies with axial range and input frequency.
    Ultrasonics 06/2013; 54(2). DOI:10.1016/j.ultras.2013.05.012
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    ABSTRACT: A novel specimen for the measurement of strain rate and triaxiality dependent fracture properties of metals is described. The specimen is used in a conventional tensile split Hopkinson pressure bar test. The specimen is a flat 10 mm wide and 1 mm thick notched bar. Notch lengths between 2 mm and 8 mm were used. Several tests were performed on specimens cut from an X100 pipe at room temperature and strain rates up to 2000 s−1. Finite element modelling of the stress–strain fields in the specimen immediately prior to crack propagation across the ligament was used to extract the flow properties, the damage initiation strain and the crack propagation energy as functions of stress triaxiality and strain rate. This data is used for tuning the strain rate dependent damage model (SRDD). The SRDD model was validated against the experiments and a good agreement was observed. In part 2 of this paper the tuned SRDD model is used for the simulation of an axial crack propagation and arrest in pressurised natural gas pipelines. Very good agreement with the burst tests is observed, thus validating the utility of the proposed specimen design and of the SRDD model.
    International Journal of Pressure Vessels and Piping 05/2013; s 105–106:60–68. DOI:10.1016/j.ijpvp.2013.03.003
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    DP Stoten · J-Y Tu · G Li ·
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    ABSTRACT: The experimental technique for testing engineering systems via the method of dynamic substructuring is receiving significant global interest, for example in the fields of large-scale structural, aerospace, and automotive system testing. Dynamically substructured systems (DSSs) enable full-size, critical components of a complete system to be physically tested in real-time, within a laboratory environment, while the remainder of the system is modelled numerically. The intention is that the combined physical-numerical DSS behaves as if it were the complete (or emulated) system. In an ideal mechanical DSS, for example, perfect synchronization of displacements and forces at the interfaces between the numerical and physical components (or substructures) is required. Hence, a key design feature of successful DSS systems is the high fidelity of the control action. Equally, a DSS controller must be able to cope with non-linear, time-varying, and uncertain parameters within the physical substructure dynamics. The main purpose of this paper is to present a generalized DSS framework, together with associated linear and adaptive control strategies, that are specifically tailored to achieve high synchronization performance. The initial studies of this problem, as described in an earlier paper by Stoten and Hyde, are therefore continued by generalizing both the DSS dynamics and the control strategies to include (a) a number of newly defined modes of operation and (b) multivariable dynamics. In addition, comparative implementation and simulation studies are included, based upon the DSS testing of a mechanical system (a planar quasi-motorcycle rig), which was specifically designed to highlight the main features of this research. The comparative studies show that excellent DSS control can be achieved, especially with the addition of an adaptive component to the controller, despite significant changes to the physical substructure dynamics.
    Proceedings of the Institution of Mechanical Engineers Part I Journal of Systems and Control Engineering 04/2013; DOI:10.1243/09596518JSCE635
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    ABSTRACT: The paper examines the transient melting rates of ice slurries being pumped through pipes whose walls are initially warmer than the freezing temperature of the slurry. The application for this work is the new innovative pipe cleaning method known as ice pigging. Ice pigging consists of introducing a length of ice slurry consisting of small ice crystals in a fluid matrix of water and a freezing point depressant into a pipe. The ice slurry forms a semi-solid plug whose temperature is at the freezing temperature of ice at the prevailing operating conditions. The plug cools the pipe walls, which in turn results in some phase change within the slurry. The plug is propelled along the pipe by a pressurised fluid introduced behind the plug. This results in the pipe walls being exposed to large time-varying temperature changes, with the front of the plug receiving the greatest heat flux. The purpose of undertaking ice pigging is to clean the pipe walls with the high shear at between the walls and the semi-solid slurry.Analytical transient conduction theory is adopted to estimate the energy removed from the pipe wall as a result of the passing ice pig. This is used to develop equations predicting the amount of phase change occurring in the pig and hence estimate the distance it can travel before it has melted. The resultant model enables ice pig users to estimate how much ice slurry is required to undertake successfully a specific ice pigging task.
    Applied Thermal Engineering 01/2013; 50(1):743–748. DOI:10.1016/j.applthermaleng.2012.07.038
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