W.O. Wong

Sensitivity studies of parameters for damage detection of plate-like structures using static and dynamic approaches

Abstract In this paper, the sensitivities of static and dynamic parameters to damage occurring in plate-like structures are investigated systematically, and corresponding damage indices are proposed to analyse their identification capabilities. For static analyses, damage indices are formulated using the out-of-plane deflection, and its slope and curvature based on a finite element model; for dynamic analyses, two damage indices related to the curvature mode shape (CMS) and the strain frequency response function (SFRF) are proposed. Compared with the existing results, the effects of the numbers of the selected modes and defective areas on the CMS index and the influence of the “natural frequency shift” on the SFRF index are investigated. Numerical simulations and experimental tests are performed to verify the identification capability of the proposed indices, and guidance on selecting the proper parameters for damage detection is given.

Identification of damage locations for plate-like structures using damage sensitive indices: strain modal approach

Abstract This paper addresses the problem of identification of damage locations for plate-like structures using strain mode technique. Based on the Rayleigh–Ritz approach, the strain modal analysis of a damaged plate is performed and strain mode shapes are consequently obtained. In light of the continuity condition and the residual strain mode shape technique, two novel damage sensitive parameters are proposed to determine the locations of damage, and corresponding damage indices are constructed. Compared with the conventional indices, they are simple and intuitive, and easy to be used in practice. Numerical simulations and experiments are carried out. Results show a good consistency and a strong identification capability of the proposed indices.

Flow and Forced-Convection Characteristics of Turbulent Flow Through Parallel Plates with Periodic Transverse Ribs

ABSTRACT Two general turbulence models, the standard k–ϵ model and the Reynolds stress model (RSM), were used to predict the forced convection of a fully developed turbulent flow through an assembly of two horizontally oriented parallel plates in the Reynolds number range 22,000 < Re D < 94,000. The upper smooth plate was thermally insulated, whereas the bottom plate, attached with rectangular-cross-sectional ribs perpendicular to the mean air flow, was provided with a uniform heat flux. The ribs were uniformly spaced with the pitch-to-height ratio of p/e = 4, a height-to-hydraulic-diameter ratio of e/D = 0.25, and a width-to-height ratio of w/e = 2. The numerical approaches were based on the finite-volume technique. A second-order upwind scheme was applied in the calculation and a very fine mesh density was arranged in the regions near the wall boundaries. The SIMPLE algorithm was adopted to handle the pressure–velocity coupling in the calculation. Local Nusselt number distribution along the heated bottom ribbed surface was investigated, which was validated against corresponding experimental results conducted by Lorenz et al. [1]. It was found that in the simulation of the turbulent forced convection in this two-dimensional channel with a ribbed surface, the standard k–ϵ model had superiority over the Reynolds stress model. An anticlockwise vortex was found in the downstream region of a rib by using either of the two models; however, the length and relative strength of the vortex predicted by these two models were significantly different. Recirculating flow pattern was formed in the cavity between two adjacent ribs, while no reattachment of the mainstream flow was observed at the present pitch-to-height ratio of p/e = 4.

Simulation of Turbulent Flow and Forced Convection in a Triangular Duct with Internal Ribbed Surfaces

ABSTRACT A three-dimensional problem of fully developed turbulent flow through an equilateral triangular duct with internal ribbed surfaces has been simulated numerically through two two-dimensional approaches: turbulent forced convection between two parallel plates with ribbed bottom surface and that in an equilateral triangular duct with smooth internal surfaces. Effects of the complicated geometry on the turbulent forced convection, as well as the formation of the secondary flows around the ribs and in the triangular ducts corners, are analyzed in detail. Comparisons between the calculated and experimental results have been carried out and it has been found that the geometry effect from the attached uniformly spaced ribs on the triangular ducts thermal performance is more significant than that from the ducts corners.

Design of a non-traditional dynamic vibration absorber

A non-traditional dynamic vibration absorber is proposed for the minimization of maximum vibration velocity response of a vibrating structure. Unlike the traditional damped absorber configuration, the proposed absorber has a linear viscous damper connecting the absorber mass directly to the ground instead of the main mass. Optimum parameters of the proposed absorber are derived based on the fixed-point theory for minimizing the maximum vibration velocity response of a single-degree-of-freedom system under harmonic excitation. The extent of reduction in maximum vibration velocity response of the primary system when using the traditional dynamic absorber is compared with that using the proposed one. Under the optimum tuning condition of the absorbers, it is proved analytically that the proposed absorber provides a greater reduction in maximum vibration velocity response of the primary system than the traditional absorber.

Quantitative vibration amplitude measurement with time-averaged digital speckle pattern interferometry

Abstract A new method of using time-averaged digital speckle pattern interferometry for the quantitative measurement of vibration amplitude was developed. Signal processing techniques especially the Hilbert transformation for quantitative evaluation of the Bessel fringes obtained in time-averaged digital speckle pattern interferometry were explored. The quadrature signal after Hilbert transformation is equivalent to a 90° phase-shifted interferogram for a monotonically increasing or decreasing phase function. An algorithm was developed for Bessel fringe contrast enhancement and phase extraction. The techniques were tested numerically and experimentally. Sub-fringe quantification of the time-averaged vibration fringes is realised with the proposed method. Compared with the commonly used phase shift method which requires a minimum of two images for image processing, this method requires only one fringe pattern for data extraction.

Contrast and sensitivity of the vibration fringes in time-averaged electronic speckle pattern interferometry : effect of variations of force level

Abstract The time averaged frame subtraction technique is improved by subtracting two Bessel fringe patterns at two different force levels. The technique enables significant enhancement of fringe contrast and increased measurement sensitivity. The contrast and sensitivity of the fringes obtained at different force ratios are investigated. Both theoretical and experimental results are presented.

Identification of antinodes and zero-surface-strain contours of flexural vibration with time-averaged speckle pattern shearing interferometry.

A new time-averaged frame subtraction technique is introduced for vibration analysis by digital speckle shearing interferometry. The technique permits the enhancement of fringes by subtracting two Bessel fringe patterns at different forcing levels. Compared with the phase-shift method, this method is more efficient and easier to implement for qualitative vibration measurement, providing a means for fast inspection of plate vibration behavior. It is also capable of tracing contours of zero strain and locating antinodes on vibrating plates.

Minimization of the mean square velocity response of dynamic structures using an active-passive dynamic vibration absorber

An optimal design of a hybrid vibration absorber (HVA) with a displacement and a velocity feedback for minimizing the velocity response of the structure based on the H(2) optimization criterion is proposed. The objective of the optimal design is to reduce the total vibration energy of the vibrating structure under wideband excitation, i.e., the total area under the velocity response spectrum is minimized in this criterion. One of the inherent limitations of the traditional passive vibration absorber is that its vibration suppression is low if the mass ratio between the absorber mass and the mass of the primary structure is low. The active element of the proposed HVA helps further reduce the vibration of the controlled structure, and it can provide very good vibration absorption performance even at a low mass ratio. Both the passive and active elements are optimized together for the minimization of the mean square velocity of the primary system as well as the active force required in the HVA. The proposed HVA was tested on single degree-of-freedom (SDOF) and continuous vibrating structures and compared to the traditional passive vibration absorber.

Vibration analysis by laser speckle correlation

Time-averaged laser speckle correlation is used for flexural vibration-studies. The proposed method employs a diverging laser beam to illuminate the test object and records the object image at a defocused plane with a CCD camera. The speckle correlation coefficient is a function of the out-of-plane tilt of the object. The time-averaged speckle pattern recorded with the object vibrating at a natural mode is subtracted from the one recorded in a static condition. The anti-nodes of the vibrating object can then be identified easily on the subtracted image. A brief analysis of the technique and some experimental results on a cantilever beam and a vibrating plate are presented.