Susheel Kumar Yadav

On sequencing the feature extraction techniques for online damage characterization

The current state of the health-monitoring technology lacks a generalized and definitive approach to the identification and localization of mechanical damage in structural materials. In past decades, several signal-processing tools have been used for solving different health-monitoring problems but the commutability of the tools between different problems has been restricted. The fundamental reasons for this shortcoming have never been investigated in detail. A thorough study is presented in this article employing almost all promising feature extraction tools on a representative problem—a plate with rivet holes. The cracks around rivet holes in a joint panel of a steel truss bridge are very difficult to detect. Although well established, Lamb wave–based nondestructive evaluation techniques are revisited and new tools are developed to address this issue. The simulation of scattered ultrasonic wave field is carried out using the finite element method. This ultrasonic wave field is further analyzed to evaluate the integrity of the structure using various feature extraction techniques. The joint time–frequency–energy representation is obtained from ultrasonic signals recorded at various locations on the plate (joint panel) and used to extract damage-sensitive features. Those features were then used to formulate a new damage parameter for better visualization of the crack. The results are shown to demonstrate the comparative effectiveness of these techniques. It is concluded that any particular feature extraction technique cannot detect all possible sizes and orientations of the crack. It is suggested that the statistical occurrence and pattern of the crack must be visualized through few selective feature extraction techniques in a sequence.

Detection and quantification of pipe damage from change in time of flight and phase

The use of ultrasonic guided waves for damage detection in pipes is continuously increasing. Generally longitudinal (axial symmetric) modes are excited and detected by PZT (Lead Zirconate Titanate) transducers in transmission mode for this purpose. In most studies the change in the received signal strength with the extent of damage has been investigated while in this study the change in the phase and the time-of-flight (TOF) of the propagating wave modes with the damage size is investigated. The cross-correlation technique is used to record the small changes in the TOF as the damage size varies in steel pipes. Dispersion curves are calculated to carefully identify the propagating wave modes. Differential TOF is recorded and compared for different propagating wave modes. Feature extraction techniques are used for extracting phase and time-frequency information. The main advantage of this approach is that unlike the recorded signal strength the TOF and the phase are not affected by the bonding condition between the transducer and the pipe. Therefore, if the pipe is not damaged but the transducer-pipe bonding is deteriorated then although the received signal strength is altered the TOF and phase remain same avoiding the false positive alarms of damage.

Detection and quantification of diameter reduction due to corrosion in reinforcing steel bars

Guided wave–based techniques are becoming popular for damage detection in pipes, rods, and plates. For monitoring reinforced concrete beams, the longitudinal guided wave is excited and recorded after its transmission through the reinforcing steel bar for estimating its corrosion level. Recorded signal amplitude is affected by the corrosion level. Thus, the corrosion level is estimated from the transmitted wave amplitude. Instead of investigating the amplitude of the transmitted guided waves, the differential time-of-flight of the propagating wave modes is recorded in this article. The differential time-of-flight is obtained from the time–frequency representations of the recorded transient signals and from the high temporal resolution using the cross-correlation technique. It is observed that the corrosion level can be quantified from the change in time-of-flight of the L(0,1) mode. The guided wave modes are experimentally generated, recorded, and compared with the theoretical dispersion curves to identify different modes and select the most efficient mode for quantifying the corrosion level. Unlike the recorded signal strength, the time-of-flight is not influenced by the bonding condition between the sensors and the specimen; therefore, the time-of-flight-based corrosion-monitoring technique is less influenced by the sensor bonding condition. This investigation is necessary because most investigators have studied the effect of corrosion on the recorded signal strength instead of its time-of-flight.

On suitability of feature extraction techniques for local damage detection

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.

Advanced DPSM approach for modeling ultrasonic wave scattering in an arbitrary geometry

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.

Reinforcement Effect on the Static Analysis of Circular Footing Resting over Winkler Elastic Foundation

This paper pertains to the development of a lumped parameter model for predicting the flexural response of a circular footing resting on an engineered reinforced soil bed. The bed is constructed by placing a smooth circular geo-mat over the natural loose sand deposit on which a compacted sand fill is placed. The dense and the loose sand strata are idealized with Winkler springs of different stiffness values. Considering the problem to be axi-symmetric the resulting governing differential equations have been derived and solved for appropriate boundary and continuity conditions by using finite difference technique. Effects of the different parameters like the ratio of flexural rigidity and the dimensions of the footing and the reinforcing element, the ratio of stiffness of the upper and the lower sand layers, and the placement depth of the reinforcement on the settlement and flexural response of the footing have been presented.

Advanced signal processing technique for damage detection in steel tubes

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.

Guided wave technique for non-destructive testing of StifPipe®

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.

Change in time-of-flight of longitudinal (axisymmetric) wave modes due to lamination in steel pipes

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 this paper.