Fracture source location in thin plates using the wavelet transform of dispersive waves

A new signal processing approach was presented for acoustic emission source location using the dispersive waves in a thin plate. For wave propagation in dispersive media, the accuracy of source location can be improved by using the arrival times of a single frequency component in the output signals at an array of sensors. The wavelet transform (WT) was used to resolve this problem. By utilizing the time-frequency data of the WT, the frequency-dependent arrival time traveling with the group velocity was shown to be easily determined. Experiments were performed using a lead break as the simulated fracture source on the surface of an aluminum plate. Two plate modes corresponding to the S(0) and A(0) Lamb waves were identified, and their group velocities were accurately measured. The source location results based on the WT method agreed well with the true locations. The WT method was also compared with the cross correlation technique, and both methods provide similar results.     

AE source location and orientation determination of tensile cracks from surface observation

A technique to determine the location and orientation of an acoustic emission (AE) source due to a tensile crack was studied. Theoretical AE waveforms from tensile cracks were synthesized on the basis of the generalized theory of AE. Numerical experiments were conducted using the waveforms. We considered a source location technique using only surface observations and took into account the effects of velocity anisotropy. The technique is based on the radiation pattern of P-waves and incorporates a procedure to determine the orientation of the AE source. The results accurately reproduced the location and orientation of simulated tensile cracks as well as the values of their moment tensor representation.     

Structural health monitoring of bridges with piezoelectric AE sensors

Monitoring the health of structures using acoustic emission technique offers many advantages such as early and quick detection of damage in real-time inexpensively. This paper is concerned with health monitoring of infrastructure hardware like highway bridges using this technique for timely intervention preventing catastrophic failure. Both experimental and numerical investigations were undertaken to determine the effectiveness and applicability of the method in the case of bridge superstructures. Three types of representative bridge girders of steel, reinforced concrete and prestressed concrete were designed, fabricated and monitored in the laboratory for structural integrity under cyclic loads with the help of an acoustic emission sensor system. After laboratory work, wavelet and Fourier transform techniques were applied to the recorded signals for de-noising and to diagnose the damaged state of the structure. Finally the locations of the cracks were determined by using the artificial neural network (ANN) approach.     

Water-wave scattering by two symmetric circular-arc-shaped thin plates

The problem of surface water-wave scattering by two symmetric circular-arc-shaped thin plates submerged in deep water is investigated in this paper assuming linear theory. The problem is formulated in terms of hypersingular integral equations which are solved approximately using finite series involving Chebyshev polynomials of the second kind. The coefficients of the finite series are obtained numerically by a collocation method. Very accurate numerical estimates for the reflection and the transmission coefficients are then obtained. The numerical results are depicted graphically against the wave number for different arc lengths of the plates, the depth of their submergence and the separation length. Known results for a circular cylinder and horizontal straight plate are recovered.     

Delamination detection in CFRP laminates with embedded small-diameter fiber Bragg grating sensors

Newly developed small-diameter fiber Bragg grating (FBG) sensors were applied for the detection of the delamination in carbon fiber reinforced plastic (CFRP) cross-ply laminates. Since the diameter of the FBG sensor was very small, the sensor was easily embedded into the 0° ply along the reinforcing carbon fibers. The reflection spectra from the FBG sensor were measured at various delamination lengths through four-point bending test. The form of the spectrum changed sensitively, as the delamination length increased. Moreover, the spectrum was calculated theoretically for confirmation of the measured spectrum. The calculated result reproduced the change in the measured spectrum very well. Then, we proposed the intensity ratio in the spectrum as an effective indicator for the prediction of the delamination length. This indicator has a potential to be applied to other laminate configurations.     

The viscosity of thin water films between two quartz glass plates

Summary An apparatus for the determination of the viscosity coefficient of water in the neighbourhood of a quartz glass surface is described. The law of Stefan-Reynolds is applied in a special modification. The relative viscosity coefficient of water between two quartz glass plates having a distance smaller than 100 ? lies beyond 10 for small shearing stresses. Increasing shearing stresses lead to smaller values of the relative viscosity coefficient of such water zone, so that a non-Newtonian behaviour of flowing water in very thin layers can be accepted.

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Résumé On décrit un appareil pour la détermination du coefficient de viscosité de l'eau au voisinage d'une surface de quartz transparent. On applique ici une variante de la loi de Stefan-Reynolds. Le coefficient de viscosité relatif de l'eau entre deux plaques de quartz transparent maintenues à seulement 100 ? l'une de l'autre est de plus de 10 pour les faibles contraintes de cisaillement. Des contraintes de cisaillement croissantes déterminent des valeurs plus faibles du coefficient de viscosité relative, et l'on peut donc admettre que l'eau, sous forme de ces couches très minces, se caractérise par un écoulement non newtonien.

     

Waves in charged thin plates

The spectrum of bending waves of a thin conducting plate bearing an electric charge is investigated. It is proved that there exists a critical value of the field upon exceeding which bending instability of the plate occurs.     

Elastic problem of an adhesion crack in T-junction of two orthotropic thin plates

In this study, the general solution is derived for stresses in a T-junction of two thin plates with an adhesion crack. The plates are orthotropic, and shear force is applied to the crack surface. The analysis is based on the supposition that the stresses in each plate can be approximated by the condition of plane stress. The results obtained are verified through numerical calculation using the finite element method. A singular stress field is obtained from the solution in the vicinity of a crack tip.     

Smeared crack models of RC beams with externally bonded CFRP plates

Concrete, a complex mix of variously sized aggregates, sand, water, additives and cement binder, is one of the more common engineering materials used for the design and construction of structures and bridges. Concrete is characterised by good compressive strength properties but it demands the use of internal reinforcement, generally in the form of round steel bars, to carry tensile stresses. The strength of the resulting element is dependent on the amount and distribution of steel reinforcement included during construction. It is not however possible to include additional internal reinforcement after construction in the event of the applied loading being increased and therefore consideration must be given to strengthening the structure externally, demolishing it, or confining it to specific usage, for example a maximum weight restriction on a bridge. In circumstances where restricted usage is not practicable structural strengthening is generally more favourable than demolition and replacement. Research in the area of strengthening of existing bridge beams is currently topical in the European Union given recent EU directives, aimed at encouraging free trade and movement of goods and services, which require all bridges to take 40 tonne vehicles. This paper describes the numerical modelling procedures employed, using smeared crack models available in ANSYS V5.4, to capture the load-deformation response and modes of failure, of reinforced concrete beams which have been strengthened, using carbon fibre reinforced polymer (CFRP) composite material plates. Experimental verifications of these simulations have also been performed and are discussed in the present paper. Received 12 December 1999     

Fatigue strength of CFRP strengthened welded joints with corrugated steel plates

This paper presents an experimental and numerical research on the carbon fibre reinforced polymer (CFRP) strengthened welded joints with corrugated plates. The effectiveness of the strengthening in the improvement of fatigue strength has been examined experimentally on the test joints through varying the number and the layout of the CFRP laminates. The test results show that the joints with transition curvature region reinforcement and single side reinforcement produce slightly lower rigidity but longer fatigue life in contrast to those with full width reinforcement on the double side of the main plate. Furthermore, a simplified two dimensional analytical model which allows for the geometric characteristics of the joint has been proposed to investigate the stress intensity factor of mode I. The proposed analytical model has been simulated by finite element technique and its solution result is compared with previously reported theoretical calculation. Parametric studies have been performed to investigate the effects of the number of CFRP layers and the moduli of carbon fibre & adhesive on the stress intensity factor. The combined influence of the corrugation angle and crack depth has also been considered. It has been found that these effects on the stress intensity factor are more significant for the joints with smaller corrugation angle.     

Temperature stresses in thin viscoelastic plates with heat transfer

The solution is obtained of the quasistatic problem of thermoviscoelasticity for an unbounded plate heated by a concentrated heat source and the solution of the dynamic problem of thermoviscoelasticity for a semibounded plate heated along the boundary.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 17, No. 5, pp. 892–899, November, 1969.     

Two-dimensional equations for electroelastic plates with relatively large shear deformations

A set of two-dimensional, nonlinear equations for electroelastic plates in moderately large thickness-shear deformations is obtained from the variational formulation of the three-dimensional equations of nonlinear electroelasticity by expanding the mechanical displacement vector and the electric potential into power series in the plate thickness coordinate. As an example, the equations are used to study nonlinear thickness-shear vibrations of a quartz plate driven by an electrical voltage. Nonlinear electrical current amplitude-frequency behavior near resonance is obtained. The equations and results are useful in the study and design of piezoelectric crystal resonators and the measurement of nonlinear material constants of electroelastic materials.     

Flexural response predictions of reinforced concrete beams strengthened with prestressed CFRP plates

Existing experimental studies showed that the reinforced concrete (RC) beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates had three possible flexural failure modes (including the compression failure, tension failure and debonding failure) according to the CFRP reinforcement ratio. Theoretical formulas based on the compatibility of strains and equilibrium of forces were presented to predict the nominal flexural strength of strengthened beams under the three failure modes, respectively, and a limitation on the tensile strain level developed in the prestressed CFRP plate was proposed as the debonding failure occurred. In addition, the calculation methods for cracking moment, crack width and deflection of strengthened beams were provided with taking into account the contribution of prestressed CFRP plates. Experimental studies on five RC beams strengthened with prestressed CFRP plates and a nonlinear finite element parametric analysis were carried out to verify the proposed theoretical formulas. The available test results conducted by other researchers were also compared with the predicted values.     

氧化镍薄膜气体传感器性能提升

本文的目的在于优化甲醛气体传感器.微型甲醛气体传感器设计是以石英玻璃当作基材,白金(Pt)被当作微型加热器电阻来加热感测层,并以氧化镍(NiO)薄膜作为感测层.当环境内有甲醛气体存在时,NiO薄膜层上导电度会增加,因而导致感测层电阻值降低.此微传感器,膜厚为0.34μm,在300℃反应时间只需6秒,灵敏度可达13.5 kΩ/ppb,最低侦测限度可以量测到40 ppb. 而本研究中针对不同的甲醛气体浓度,分别添加金当其催化剂、玻璃基材上共溅镀氧化镍与氧化铝、并比较有无指叉电极、改变基材温度…等,以提升其氧化镍薄膜感测性能.     

Two-dimensional analysis of magnetoelectric effects in multiferroic laminated plates

Two-dimensional equations for coupled extensional, flexural, and thickness-shear motions of laminated plates of piezoelectromagnetic layers are obtained from 3-dimensional equations. The equations are used to analyze magnetoelectric couplings in the extensional deformation of a laminated plate of piezoelectric and piezomagnetic layers. Magnetoelectric effects in 4 specific configurations of laminates are calculated and examined.     

Size effects in thin CFRP panels subjected to impact

The static indentation and impact behaviour of three- and nine-ply CFRP panels circularly clamped with rings of 100 and 300 mm diameter have been evaluated. The maximum static force is similar for small and large panels for both three- and nine-ply laminates. Increasing the panel size of the three-ply laminates subjected to impact only appears to affect the threshold of perforation. For the nine-ply laminates an increase in panel size produced a reduction in delamination area and backface cracking. Interpretation of energy maps is suggested as a means of identifying when the peak impact force has occurred and the threshold of perforation.