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.

http://eig.stanford.edu/publications/hyunwoo_park/jsv_04_park.pdf

Time reversal active sensing for health monitoring of a composite plate

Group velocity and characteristic wave curves of Lamb waves in composites: Modeling and experiments

In this paper we review the state of the art in an emerging new technology: embedded ultrasonic non-destructive evaluation (NDE). Embedded ultrasonic NDE permits active structural health monitoring, i.e. the on-demand interrogation of the structure to determine its current state of structural health. The enabling element of embedded ultrasonic NDE is the piezoelectric wafer active sensor (PWAS). We begin by reviewing the guided wave theory in plate, tube, and shell structures, with special attention to Lamb waves. The mechanisms of Lamb wave excitation and detection with embeddable PWAS transducers is presented. It is shown analytically and verified experimentally that Lamb wave mode tuning can be achieved by the judicious combination of PWAS dimensions, frequency value, and Lamb mode characteristics. Subsequently, we address in turn the use of pitch-catch, pulse-echo, and phased array ultrasonic methods for Lambwave damage detection. In each case, the conventional ultrasonic NDE results are contrasted with embedded NDE results. Detection of cracks, disbonds, delaminations, and diffuse damage in metallic and composite structures are exemplified. Other techniques, such as the time reversal method and the migration technique, are also presented. The paper ends with conclusions and suggestions for further work.

/pdf/embedded-non-destructive-evaluation-for-structural-health-jldh78vfv5.pdf

Embedded non-destructive evaluation for structural health monitoring, damage detection, and failure prevention

Impact damage, excessive loading, and corrosion have been identified as critical and long-term problems that constantly threaten the integrity and reliability of structural systems (e.g., civil infrastructures, aircrafts, and naval vessels). While a variety of sensing transducers have been proposed for structural health monitoring, most sensors only offer measurement of structural behavior at discrete structural locations. Here, a conformable carbon nanotube-polyelectrolyte sensing skin fabricated via the layer-by-layer technique is proposed to monitor strain and impact damage over spatial areas. Specifically, electrical impedance tomographical (EIT) conductivity mapping techniques are employed to offer two-dimensional damage maps from which damage location and severity can be easily and accurately quantified. This study deposits carbon nanotube-based sensing skins upon metallic structural plates with electrodes installed along the plate boundary. Based on boundary electrical measurements, EIT mapping captures both strain in the underlying substrate as well as damage (e.g., permanent deformation and cracking) introduced using an impact apparatus.

/pdf/carbon-nanotube-sensing-skins-for-spatial-strain-and-impact-2z3g39i0io.pdf

Carbon nanotube sensing skins for spatial strain and impact damage identification

In this paper a signal processing technique is developed to detect delamination on composite structures. In particular, a wavelet-based signal processing technique is developed and combined with an active sensing system to produce a near-real-time, online monitoring system for composite structures. A layer of piezoelectric patches is used to generate an input signal with a specific wavelet waveform and to measure response signals. Then, the response signals are processed by a wavelet transform to extract damage-sensitive features from the original signals. The applicability of the proposed method to delamination identification has been demonstrated by experimental studies of a composite plate under varying temperature and boundary conditions.

https://iopscience.iop.org/article/10.1088/0964-1726/13/1/017/pdf

Nathan P. Limback

Papers

Wavelet-based active sensing for delamination detection in composite structures