Guided Lamb waves for identification of damage in composite structures: A review

An approach is proposed which combines simultaneously advantages both of pulse (PT) and modulated infrared thermography. In a nondestructive evaluation perspective, the specimen is pulse heated as in PT and the mix of frequencies of the thermal waves launched into the specimen is unscrambled by performing the Fourier transform of the temperature evolution over the field of view. Of interest is the maximum phase image with many attractive features: deeper probing, less influence of surface infrared and optical characteristics, rapid image recording (pulse heating, surface‐wide inspection), and the possibility to inspect high thermal conductivity specimens. Several results are presented and the theory is discussed as well.

Pulse phase infrared thermography

Stated in its most basic form, the objective of structural health monitoring is to ascertain if damage is present or not based on measured dynamic or static characteristics of a system to be monitored. In reality, structures are subject to changing environmental and operational conditions that affect measured signals, and these ambient variations of the system can often mask subtle changes in the system’s vibration signal caused by damage. Data normalization is a procedure to normalize datasets, so that signal changes caused by operational and environmental variations of the system can be separated from structural changes of interest, such as structural deterioration or degradation. This paper first reviews the effects of environmental and operational variations on real structures as reported in the literature. Then, this paper presents research progresses that have been made in the area of data normalization.

Effects of environmental and operational variability on structural health monitoring.

The paper provides a state of the art review of guided wave based structural health monitoring (SHM). First, the fundamental concepts of guided wave propagation and its implementation for SHM is explained. Following sections present the different modeling schemes adopted, developments in the area of transducers for generation, and sensing of wave, signal processing and imaging technique, statistical and machine learning schemes for feature extraction. Next, a section is presented on the recent advancements in nonlinear guided wave for SHM. This is followed by section on Rayleigh and SH waves. Next is a section on real-life implementation of guided wave for industrial problems. The paper, though briefly talks about the early development for completeness,. is primarily focussed on the recent progress made in the last decade. The paper ends by discussing and highlighting the future directions and open areas of research in guided wave based SHM.

https://iopscience.iop.org/article/10.1088/0964-1726/25/5/053001/pdf

Guided wave based structural health monitoring: A review

This paper presents an experimental and theoretical investigation of the applicability of the time-reversal concept to guided waves in plate-like structures, where the stress waves are dispersive and of multi-modes. It is shown that temporal and spatial focusing can be achieved through time reversal, although the dispersive behaviour of the flexural waves renders it impossible to exactly reconstruct the waveform of the original excitation. Based on the principle of the time-reversal concept, a digital imaging method suitable for distributed sensor/actuator networks has been developed. This new method, which overcomes the limitation of the conventional phased array method that operates under pulse-echo mode, provides an efficient imaging method for locating and approximate sizing of structural damages. In addition, it has been shown that signal strengths can be considerably enhanced by applying the present synthetic time-reversal method, thus reducing the number of sensors and actuators required to achieve a given signal-to-noise ratio.

https://iopscience.iop.org/article/10.1088/0964-1726/13/2/020/pdf

A synthetic time-reversal imaging method for structural health monitoring

A health monitoring system is presented composed of integrated disc-shaped, 100 µm thick and 5 mm diameter piezoelectric transducers (PZTs) working sequentially as Lamb wave emitters and receivers. The diagnostic is based on the analysis of Lamb wave signals recorded before and after damage. In the composite, delaminations are discontinuities producing mode conversion processes generating various outgoing modes. The multiresolution processing allows the isolation of various propagation modes and their extraction in order to measure, for various propagation paths, the time delay between the arrivals of the main burst and of a specific outgoing mode. This process permits, with good accuracy, the localization of damage and the estimation of its extent. The robustness and portability of this technique is demonstrated by the fact that, after validation in our laboratory, it was successfully applied to data coming from an experiment conducted in another laboratory using its own acousto-ultrasonic health monitoring hardware system.

https://iopscience.iop.org/article/10.1088/0964-1726/10/3/312/pdf

Structural health monitoring system based on diffracted Lamb wave analysis by multiresolution processing

After defining a good usage of pulse thermograms, in particular their ‘rectification’ thanks to log-log plot, and fitting by the TSR (thermographic signal reconstruction) technique, it is demonstrated that a simple and efficient normalisation is possible. The optimisation of pulse thermography is proposed based on two complementary approaches: the early detection of contrasts and the use of the TSR technique with logarithmic derivation. It is shown that in these conditions the quality of thermal images is greatly enhanced (reduction of blurring) and the accuracy of the identified defect parameters (depth and thermal resistance) is better. Based on experimental and simulated results it is shown that to optimise pulse stimulated thermography it is necessary to consider two parameters: the signal-to-noise ratio of the defect contrast and the precociousness of its exploitation. It is shown that this second parameter, which has been neglected by most of the users of pulse thermography who identified the defect...

Defense and illustration of time-resolved pulsed thermography for NDE

Introduction to Structural Health Monitoring

Pulsed thermography produces temperature rise images depending on the space variability of the optical properties of the
tested sample and of the fluence of the stimulating heat flux. It is demonstrated that a simple and efficient normalization of
the thermograms is possible and constitutes a drastic remedy to this weakness. This is illustrated by applications to realistic
defects and configurations. The way to optimize the normalization and to define a sound reference needed for defect
detection are also discussed and illustrated by the analysis of NDE experiments. After normalization, the amplitude based
pulsed thermograms present strong advantages compared to phase-based thermograms, in particular their large pass-band
allows the early detection/characterization of defects. The benefit of using the TSR technique is also shown, especially if
used in conjunction with the early detection approach.

Daniel L. Balageas

Papers

Structural health monitoring system based on diffracted Lamb wave analysis by multiresolution processing

Defense and illustration of time-resolved pulsed thermography for NDE

Introduction to Structural Health Monitoring

Defense and illustration of time-resolved pulsed thermography for NDE

EMIR: a photothermal tool for electromagnetic phenomena characterization