A. Trochidis

Crack identification in beams using wavelet analysis

In this paper a simple method for crack identification in beam structures based on wavelet analysis is presented. The fundamental vibration mode of a cracked cantilever beam is analyzed using continuous wavelet transform and both the location and size of the crack are estimated. The position of the crack is located by the sudden change in the spatial variation of the transformed response. To estimate the size of the crack, an intensity factor is defined which relates the size of the crack to the coefficients of the wavelet transform. An intensity factor law is established which allows accurate prediction of crack size. The viability of the proposed method is investigated both analytically and experimentally in case of a cantilever beam containing a transverse surface crack. In the light of the results obtained, the advantages and limitations of the proposed method as well as suggestions for future work are presented and discussed.

Crack identification in plates using wavelet analysis

In this paper, a method for crack identification in plates based on wavelet analysis is presented. The case of an all-over part-through crack parallel to one edge of the plate is considered. The vibration modes of the plate are analyzed using the continuous wavelet transform and both the location and depth of the crack are estimated. The position of the crack is determined by the sudden change in the spatial variation of the transformed displacement response. To estimate the depth of the crack, an intensity factor is defined which relates the depth of the crack to the coefficients of the wavelet transform. An intensity factor law is established which allows accurate prediction of crack depth. The viability of the proposed approach is demonstrated using simulation examples. In view of the obtained results, the advantages and limitations of the proposed approach as well as suggestions for future work are presented and discussed.

Microdamage evaluation in human trabecular bone based on nonlinear ultrasound vibro-modulation (NUVM).

The primary aim of this work is to investigate the potential of nonlinear ultrasound for microdamage detection in human bone. Microdamage evaluation in human bone is of great importance, because it is considered a significant parameter for characterizing fracture risk. Experiments employing nonlinear acoustic vibro-modulation were carried out in human femoral trabecular specimens removed during surgery. A frequency mixing (inter-modulation) was observed between an ultrasound wave, propagating in the bone, and a low-frequency vibration applied directly to the bone specimens. The appearance of side frequencies, which are related to the vibrational excitation, around the fundamental ultrasound frequency manifests the modulation nonlinear phenomenon. Instead of inducing microdamage by mechanical fatigue loading, specimens with different degree of osteoporosis were used. The experiments demonstrated that osteoporotic bone exhibits stronger nonlinearity compared to healthy bone presenting significant increase of the modulation amplitude with increasing degree of osteoporosis. The obtained results indicate that, in contrast to conventional hysteretic nonlinearity, dissipative acoustic nonlinearity can be of significance in the generation of nonlinear modulation effects. In the proposed technique the size and the shape of samples are not crucial compared to nonlinear resonant ultrasound spectroscopy (NRUS). Furthermore, the method is sensitive to the presence of microdamage, non-invasive, easy to implement and most important, it can be proved valuable tool for in vivo bone damage characterization.

Analysis of Vibroacoustic Modulations for Crack Detection: A Time-Frequency Approach Based on Zhao-Atlas-Marks Distribution

The vibro-acoustic modulation (VAM) technique is probably the most widely used nonlinear method for crack detection. The VAM method is based on the effect of modulation of high-frequency acoustic waves by a low-frequency vibration. The intensity of the modulation is related to the severity of the damage and has been used so far as a damage index. The damage index simply based on the amplitude of the first side bands in the spectral domain often leads to controversial results about the severity of the damage. In this work, the nonlinear characteristics of the vibro-modulation were systematically investigated by employing time-frequency analysis based on the Zhao-Atlas-Marks (ZAM) distribution. The results of the analysis show that the amplitude of the sideband components is modulated by the low frequency vibration and the modulation amplitude depends on the size of the crack. Based on the obtained results, a new damage index was defined in relation to the strength of the modulation. The new damage index is more sensitive and robust and correlates better with crack size compared to the index based on the amplitude of the sidebands.

Insertion loss measurement of panels by time delay spectrometry

This paper describes a new method for measuring the insertion loss of panels or partitions based on time delay spectrometry (TDS). Using a TEF 12 analyzer the insertion loss of single leaf steel panels were measured. The results are shown to be in reasonable agreement with existing measurements giving confidence to the use of the method for design purposes. Based on several measurements, the performance and the limitations of the method proposed are analyzed and discussed.

Design and evaluation of novel barriers

This work presents the results of a project for the design of novel noise barriers with improved performance. The aim of the project was twofold. First, to optimize the performance of barriers by appropriately modifying both the shape and the acoustical conditions at the edge to suppress the sound pressure. Second, to quantify the acoustic performance of the novel barriers both mathematically and experimentally in a standardized, well controlled process. The insertion loss of different types of barriers with modified edge shapes and acoustical conditions were investigated systematically in comparison with conventional ones using a method based on BEM. The comparison highlighted the influence of shape and edge configuration on barrier efficiency and allowed the design of novel barriers with optimal performance. The novel barriers were tested and their efficiency was quantified experimentally using the Maximum Length Sequence (MLS) technique, which offers the advantage of in‐situ measurements in the presenc...