Papers by Author: Brian R. Mace

Abstract: A method is presented for locating discontinuities in a uniform waveguide from two or more point frequency response functions (FRFs). The phase of the FRF exhibits modulation when plotted against wavenumber because of interference of waves reflected from the discontinuity, and this is related to the distance of the excitation point from the discontinuity. Such discontinuities might be known boundaries or unknown damage sites. An inverse Fourier transform is used to transform from the wavenumber domain to the spatial domain in order to extract the locations of the discontinuities. The use of the transform relies upon knowledge of the dispersion relation for the waveguide. Experimental results are presented for several uniform isotropic beams which were damaged to differing extents by sawing transverse slots in them. The results show the method to be successful in locating the slots.

Abstract: The wave reflection coefficients of damage such as cracks, notches and slots in otherwise uniform beams depend on frequency and on the size of the damage. Experimental results are presented for the wave power reflection coefficients of transverse slots of various depths sawn into a number of beam specimens. These results are compared with a conventional spring model to estimate the depth of the slot. The method appears to work well for larger slot depths (greater than about 30% of the thickness of the beam) and at higher frequencies, allowing their existence to be inferred and their size to be estimated. This is due to the fact that the reflection coefficients are larger in these regimes. For smaller slots or at low frequencies, noise and experimental errors, such as miscalibration errors and ill-conditioning, become more significant.

Abstract: This paper concerns flexural and axial wave motion in a cracked beam. A combined finite element (FE) and spectral element (SE) model of a cracked beam is presented. A portion of the beam, which contains the crack, is modelled using FE analysis and combined with semi-infinite SEs. From the combined model the reflection and transmission coefficients of the crack are estimated. To determine the accuracy of this approach, a beam with a mass discontinuity is considered in the first instance. The reflection coefficients are estimated numerically and compared with experimental results. Secondly, a slot-type transverse crack is cut along the width of the beam. The experimental results are compared with both an FE model and a conventional lumpedparameter spring model. The purpose of this work is to investigate further the use of audiofrequency wave propagation as a basis for crack assessment and provide a valid model to use in the development of an assessment procedure.