In recent years, ultrasonic guided waves gained attention for reliable testing and characterization of metals and composites. Guided wave modes are excited and detected by PZT (Lead Zirconate Titanate) transducers either in transmission or reflection mode. In this study guided waves are excited and detected in the transmission mode and the phase change of the propagating wave modes are recorded. In most of the other studies reported in the literature, the change in the received signal strength (amplitude) is investigated with varying degrees of damage while in this study the change in phase is correlated with the extent of damage. Feature extraction techniques are used for extracting phase and time-frequency information. The main advantage of this approach is that the bonding condition between the transducer and the specimen does not affect the phase while it can affect the strength of recorded signal. Therefore, if the specimen is not damaged but the transducer-specimen bonding is deteriorated then the received signal strength is altered but the phase remains same and thus false positive predictions for damage can be avoided.
Applicability of specific Lamb wave modes for delamination detection and quantification in a laminated aluminum plate is investigated. The Lamb modes were generated in the plate using a broadband piezoelectric transducer structured with a rigid electrode. Appropriate excitation frequencies and modes for inspection were selected from theoretical dispersion curves. Sensitivity of antisymmetric and symmetric modes for delamination detection and quantification has been investigated using the Hilbert–Huang transform. The mode conversion phenomenon of Lamb waves during progressive delamination is observed. The antisymmetric mode is found to be more reliable for delamination detection and quantification. In this investigation, the changes in the phase of guided Lamb wave modes are related to the degree of delamination, unlike other studies, where mostly the attenuation of the propagating waves has been related to the extent of the internal damage, such as cracks and corrosions. Appropriate features for delamination detection and quantification are extracted from the experimental data.
The newly-developed StifPipe® is an effective technology for repair and strengthening of existing pipes and culverts. The wall of this pipe consists of a lightweight honeycomb core with carbon or glass fiber reinforced polymer (FRP) applied to the skin. The presence of the hollow honeycomb introduces challenges in the nondestructive testing (NDT) of this pipe. In this study, it is investigated if guided waves, excited by PZT (Lead ZirconateTitanate) transducer can detect
damages in the honeycomb layer of the StifPipe®. Multiple signal processing techniques are used for in-depth study and understanding of the recorded signals. The experimental technique for damage detection in StifPipe® material is described and the obtained results are presented in this paper.
Investigations with the aid of longitudinal guided waves in cylindrical structures have been regularly carried out for nondestructive
evaluation (NDE) and structural health monitoring (SHM). While earlier works concentrated on the
amplitude reduction of the propagating waves due to structural anomalies in this work the change in time-of-flight is
investigated. Longitudinal (axisymmetric) modes are excited by a PZT (Lead Zirconate Titanate) transducer for detection
of any fluctuation or change in the surface of a steel pipe. Propagating waves are analyzed after proper signal processing.
To observe the small change in TOF due to lamination on the surface of a steel pipe, cross-correlation technique is used
to attain a higher temporal resolution. The experimental technique is discussed and the obtained results are presented in
Damage in the form of cracks near rivet holes in a steel channel section can be characterized by inspecting ultrasonic
signals containing valuable information about these anomalies.
Time-frequency representation (TFR) of time-history
signal is an effective way to extract damage features out of an ultrasonic signal scattered from cracks. Several techniques
are available to obtain Time-frequency representation and out of which feature extraction can be performed. However,
every technique has its own advantages and disadvantage which makes it cumbersome to ascertain which specific
technique is suitable to which specific problem. In present study, six TFR techniques e.g. Short Time Fourier Transform,
Continuous Wavelet Transform, Wigner-Ville Spectrum, Hilbert-Huang Transform, Williams-Choi Transform and Stransform
have been used to extract feature out of time-history signal obtained from finite element based wave scattering
simulation of a plate with and without cracks near the rivet holes. Extracted damage features have been used to quantify
the damage as a unique value by defining damage index formulation. Further, a comparison study has been carried out to
assess these six techniques for their ability to give effective, reliable and consistent information about the cracks. Matlab
codes have been developed to perform feature extraction and damage index calculation.
Several techniques are used to diagnose structural damages. In the ultrasonic technique structures are tested by analyzing
ultrasonic signals scattered by damages. The interpretation of these signals requires a good understanding of the
interaction between ultrasonic waves and structures. Therefore, researchers need analytical or numerical techniques to
have a clear understanding of the interaction between ultrasonic waves and structural damage. However, modeling of
wave scattering phenomenon by conventional numerical techniques such as finite element method requires very fine
mesh at high frequencies necessitating heavy computational power. Distributed point source method (DPSM) is a newly
developed robust mesh free technique to simulate ultrasonic, electrostatic and electromagnetic fields. In most of the
previous studies the DPSM technique has been applied to model two dimensional surface geometries and simple three
dimensional scatterer geometries. It was difficult to perform the analysis for complex three dimensional geometries. This
technique has been extended to model wave scattering in an arbitrary geometry. In this paper a channel section idealized
as a thin solid plate with several rivet holes is formulated. The simulation has been carried out with and without cracks
near the rivet holes. Further, a comparison study has been also carried out to characterize the crack. A computer code has
been developed in C for modeling the ultrasonic field in a solid plate with and without cracks near the rivet holes.