Leakage of oil and gas pipe systems, water pipes and other pipe networks can cause serious environmental, health and economic problems. In order to minimise the damages brought to the environment, human health and the economic issues, rapid non-destructive detection of pipeline leakage is imperative. In recent works, number of non-destructive testing (NDT) methods was used in detecting this defect in pipeline systems such ultrasonic, magnetic particle inspection, pressure transient and acoustic wave methods. In this study, the acoustic wave method and a modal frequency technique are used to detect leakage in pipeline system. Finite element analysis (FEA) was employed to simulate acoustic wave propagation in fluid-filled pipes with leakage. Furthermore, experimental testing was conducted to validate some of the numerical results. The experiment performed consisted of the measurement of acoustic wave propagation in a straight fluid-filled pipe. The FEA analysis of fluidfilled pipe can be used to simulate the acoustic wave propagation and acoustic wave reflectometry of a fluid-filled pipe with leakage of different using the ACAX element in order for accurate predictions. Also, the measured signal of acoustic wave propagation in pipeline from the experiment can be decomposed and de-noised to identify and locate leakages of different sizes.
A magnetorheological (MR) damper can adapt its dynamic performance to the vibration environment by controlling the current applied. Compared to other types of dampers, the MR damper has a wider range of dynamic characteristics. Two different dampers: hydraulic, and MR dampers were tested under forced sinusoidal excitations of low to high frequencies. Also, different currents were applied on the MR damper to investigate its performance under varying electromagnetic fields. The results reveal that the two dampers have nonlinear dynamic characteristics and that characteristics of the hydraulic damper are different from those of the MR damper. The hydraulic damper provides slight nonlinear damping force whereas the MR damper shows a strong nonlinear property. In addition, the hydraulic damper is designed to provide an asymmetric damping force of rebound and compression whereas the MR damper provides a symmetric damping force. In the experiments conducted, the excitation frequency was varied from 3 Hz to 11 Hz and the amplitude from 2.5 mm to 12 mm. For the hydraulic damper, the lowest compression damping force only increases by about 0.54 kN while the rebound force increases by about 1.9 kN. In contrast, the variations of compression and rebound forces of the MR damper are 1.9 and 2.0 kN, respectively. Furthermore, the damping force of the MR damper increases as the current increases from 0 to 0.75 A.
Because the disturbances which govern the dynamical response of a structure cannot be precisely measured, and the system itself has many uncertainties, the development of control strategies that are implementable and that can accommodate uncertainties and imprecision are becoming a critical and challenging work. PID adaptive controller based on RBF Neural Networks Identifier is developed for structural control in this paper. The combined controller includes PID neural network controller and an identifier based on RBF neural networks. It was implemented on linear single degree of freedom system representation of structures subjected to external disturbances based on the El Centro (1940), Hachinohe (1988), Kobe (1995) and Northridge (1994) earthquake loadings. It is demonstrated that the neuro PID adaptive control method can effectively suppress the vibration of structures.
The work presented relates to the design and construction of an inexpensive distributive load sensor. It is to be used for impact tests on samples which are in the form of flexible or deformable beams of a considerable length. The sensor compromises of forty fingers made of steel, arranged next to each other and covering a total length of about a meter. Both ends of each finger are clamped. PZT ceramic patches, which are bonded to the bottom surfaces of each finger, are used to convert the impact response into an electrical signal. An amplifier and filter were designed for each finger. The frequency range over which each finger operates is extended by the use of a Butterworth filter. An amplifier box was built containing the charge amplifier circuitry and filter of each of the forty fingers comprising the distributive sensor. Tests are performed on the distributive sensor in order to show that this simple and inexpensive distributive sensor is effective. The results are presented and discussed.
This paper looks at the use of viscoelastic damping pockets in the suppression of structural vibration. These are in the form of cavities filled with a viscoelastic material. The benefits and uses of these designed-in damping treatments are highlighted. The vibration responses of viscoelastically-damped beams are predicted using the finite element method. A series of cantilevered beams are considered and the damping performance of several configurations of designed-in dampers are predicted and compared to that of a traditional CLD treatment. It is shown that the effectiveness of the damping pockets and sinks depends on their location and size with respect to the highly stressed regions of the beams. Although there is a practical limit on the sizes of the geometrical features that can be designed-in, it is shown that if located correctly the damping pockets and sinks can be more effective at suppressing structural vibration than traditional CLD treatments.
This paper concerns the transient response of MR fluids and factors that can influence the response performance of the material. In this study, the MR fluid is subjected to a constant shear between two parallel discs, one of which is rotating at a constant speed and the other fixed. When a step current is applied, the closed-loop controller increases the torque delivered to the rotating disc in order to maintain the speed. By examining the transient response of this delivered torque, the relative response time and rise time for the MR fluid can be determined. Results showing the relative response time dependency on five variables, (i) applied current (from 0.25 A to 2.00 A), (ii) shear rate (from 50 rpm to 300 rpm), (iii) particle volumetric concentration (from 10% to 40%), and (iv) particle properties, are presented. It was found that the rate of shear does not have much influence on the relative response time, but the other four factors can affect the response characteristics quite significantly. For example, the relative response time increases with a decrease in concentration of the material, and using silicone oxide-coated magnetically active particle improves the response of the material.
The work described in this paper relates to the evaluation of some of the parameters determining the impact response properties of flexible beams made from a foam-filled fluid which is a blend of elastomeric capsules or beads in a matrix fluid. When these composites are impacted, the pressure that develops inside the beam plays a role in the shock absorbing properties of the composite. The beam composition could vary as to the volume of beads, the type of beads, the viscosity and the volume of the matrix fluid. For this work the same type of beads and fluid are used for all the tests. However, different levels of constraint are applied on the flexible and expandable material used for the skin of the beams. This is done by increasing the number of layers of skin material. In this way a higher pressure is allowed to develop inside the beam at different increments of constraint levels. It is shown that as the level of constraint is increased, so does the pressure that develops inside the beam, and so do the shock absorption characteristics of the composite.
The application of the principle of orthogonality of the vibration mode shapes of a structure to the design of shaped modal sensors, which detect the vibrational response of the structure for the mode for which they are designed, is presented. The principle was applied in the design of shaped polyvinylidene fluoride (PVDF) modal sensors for detecting the first and second modes of the bending vibration of a simply-supported beam. These sensors, which are designed as the mode 1 and the mode 2 sensors, respectively, were bonded to the same surface of a simply-supported steel beam. The beam was subjected to random vibration by an electromagnetic exciter connected to the opposite surface of the beam. The vibrational responses of the beam measured by the distributed mode 1 and mode 2 PVDF shaped sensors are compared with the vibrational responses measured using an accelerometer. It is shown that the distributed sensors produce maximum voltage output for modes 1 and 2, respectively, for which they were designed. Furthermore, is shown that by dividing the mode 1 and mode 2 sensors into two separate halves and adding or subtracting the output signals of these halves, the mode 1 sensor can be used to detect the second mode of vibration of the beam while the mode 2 sensor can be used to detect the first mode of vibration of the beam.
By means of the split Hopkinson pressure bar (SHPB) technique, the time domain dynamic mechanical properties of a polyisoprene elastomer were characterized over a range of temperatures. These properties include the dynamic stress- strain and compressional relaxation modulus characteristics of the elastomer. In the SHPB technique employed in the measurements, two identical long steel bars, which are known as the incident and transmitter pressure bars, were used as wave guides. Solid disc specimens of 10 mm diameter by 3 mm and 6 mm thickness of the polyisoprene rubber were sandwiched, in turn, between the bars. Strain pulses were generated in the incident pressure bar by the collinear impact of a hardened steel spherical ball, which was fired from a mechanical launcher, with the plane free end of the incident pressure bar via a small cylindrical anvil which was attached to the impacted end of the incident pressure bar. The strain pulses generated and propagated down the pressure bar were incident on, reflected from and transmitted through the polyisoprene specimen. These pulses were monitored by PZT sensors of dimensions 5 mm by 3 mm, which were bonded to the middle locations of the pressure bars, and were used to derive the dynamic properties of the specimens. It is shown that the stress and compressional relaxation modulus characteristics of this elastomer undergo larger variations and attain higher values at low temperatures than at high temperatures.
The measurement and prediction of the propagation of stress waves in impacted bars is the focus of this paper. Ultrasonic stress pulse propagation in bars is commonly monitored by means of stainless steel gauges. Because the outputs of these conventional gauges are very small and the frequency content of the stress pulses extend into the ultrasonic range, it is always necessary to employ amplifiers of a very high amplification and a very wide frequency bandwidth. On the other hand, PZT tiles of the same physical dimensions as the conventional gauge give voltage outputs which are factors of more than 10,000 greater than the conventional gauges. Consequently, the PZT tiles do not require any amplification system but produce signals which can be directly samples. In this paper PZT tiles of dimensions 5 by 3 mm, which are cut from standard PZT patches of dimensions 30 by 30 mm, are bonded to the surface of cylindrical bars and are used for monitoring the propagation of stress pulses induced in the bars by the collinear impact of spherical balls. The velocity of the spherical balls at impact is monitored by means of a pair of infra-red sensors. This velocity is used in conjunction with the Hertzian law of contact and the associated non-linear ordinary differential equation governing the impact of spherical balls against a rod to determine the force-time history of the impact. This force spectrum is used in a finite element model of the rod to predict the propagation of stress pulses in the rod. It is shown that the correlation between the measured and predicted stress pulses at different locations on the bars is very good.
Shock measurement accelerometers require protection from the high frequency components of input shock spectra which often cause irreversible damage to these transducers. The resonance frequencies of shock accelerometers are usually designed to be much greater than the highest frequency of their operating range. It is not unusual, however, for input shock waveforms to contain spectral components whose frequencies are much greater than the resonance frequencies of shock accelerometers. This is particularly true for shock waveforms of very short duration whose shape approach that of the classical Dirac delta function. Consequently, there is a need for mechanical filters which will isolate the accelerometers from the highest frequency components of shock loadings applied to structures. In this paper, the design of a mechanical filter comprised of metal discs, metal housing and viscoelastic elements is examined using the finite element method. The transformation of the frequency domain complex Young's modulus data to the time domain extensional relaxation function using collocation method is described. The procedures for the derivation of the Prony series coefficients from the time data for input into the finite element analysis code are presented. It is shown that effective mechanical filters can be designed using viscoelastic materials of optimal properties.
The use of viscoelastic spheres for damping the vibrations of square-section and rectangular- section hollow steel beams is presented. The transfer inertance frequency response functions of the beams in their vertical and horizontal orientations were measured under free-free boundary conditions. These frequency response characteristics were also measured when the beams were empty and when they were filled with low density viscoelastic spheres of 9 mm, 15 mm and 18 mm diameters. Using a nonlinear least squares curve fitting approach in conjunction with the modal analysis technique, the modal frequencies and the modal loss factors of the composite viscoelastic sphere-filled beams were determined from the measured transfer inertance frequency response functions. It is shown that the modal loss factors of the hollow steel beams when empty were in the range of 0.2% to 1.0%, whereas the modal loss factors of the hollow steel beams when filled with the viscoelastic spheres were in the range 2% to 31%. Thus, it is concluded that the modal loss factors of the hollow beams could be increased up to 40 times by filling them with the viscoelastic spheres.
In order to predict accurately the vibration characteristics of viscoelastic elements and viscoelastically damped structures, the use of frequency-dependent parameters such as complex modulus and Poisson's ratio is important. Several techniques have been developed for measuring the frequency-dependent complex modulus of viscoelastic materials. However, the accurate determination of Poisson's ratio of viscoelastic materials is much less developed. This quantity is important as its commonly quoted value of 0.5 can be very different when a viscoelastic material is in its transition or glassy region or if the material is compressible. In this paper, prismatic viscoelastic samples are employed to predict the value of Poisson's ratio using the finite element method (FEM). The transmissibility characteristics of these prismatic samples are established experimentally and FEM is used in conjunction with measured complex Young's modulus and iterated values of Poisson's ratio such that the predicted FEM results agree as well as possible with the experimental data. It is shown that the method suggested is able to predict accurately the Poisson's ratio of incompressible and compressible viscoelastic materials.
Analytical and empirical expressions that relate the complex dynamic stiffness of an elastomeric or viscoelastic anti-vibration mount to the complex Young's modulus of the constituent elastomeric or viscoelastic material of the mount are presented. These expressions incorporate the geometrical dimensions of the mount, the shape factor effects, and the non- linear deflection characteristics of the mount. It is shown that the use of these expressions, in conjunction with the complex Young's modulus of the constituent viscoelastic material, yields accurate predictions of the complex dynamic stiffness characteristics of an anti-vibration mount which was subjected to static prestrain levels of between 0% and 30% and was tested over a wide temperature range of -50 degree(s)C to 100 degree(s)C. Furthermore, it is shown that the master curves of complex dynamic stiffness predicted for the anti-vibration mount at static prestrain levels of 10% to 20% correlate fairly well with the master curves of complex dynamic stiffness derived from measured data using the temperature-frequency superposition principle.
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