Staff members at Los Alamos National Laboratory (LANL) produced a summary of the structural health monitoring literature in 1995. This presentation will summarize the outcome of an updated review covering the years 1996 2001. The updated review follows the LANL statistical pattern recognition paradigm for SHM, which addresses four topics: 1. Operational Evaluation; 2. Data Acquisition and Cleansing; 3. Feature Extraction; and 4. Statistical Modeling for Feature Discrimination. The literature has been reviewed based on how a particular study addresses these four topics. A significant observation from this review is that although there are many more SHM studies being reported, the investigators, in general, have not yet fully embraced the well-developed tools from statistical pattern recognition. As such, the discrimination procedures employed are often lacking the appropriate rigor necessary for this technology to evolve beyond demonstration problems carried out in laboratory setting.

A review of structural health monitoring literature 1996-2001

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.

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

An active diagnostic technique for identifying impact damage in composites is presented. A network of piezoelectric actuators and sensors were built into composites to generate and receive ultrasonic stress waves in the structures. Scattered waves were produced from the damage and were detected by sensors. A signal-processing scheme composed of smoothing filtering a joint time-frequency analysis was applied to minimize noise interference and convert sensor measurements into time-frequency spectrograms for interpretation. An identification method based on time-of- flight information obtained from the spectrograms was developed. Experiments were performed on composites with different configurations. It was demonstrated that the method was able to detect the presence of the damage and characterize the damage to a satisfactory precision. Identification results were verified with X-ray images of the structures.

Diagnosis of impact damage in composite structures with built-in piezoelectrics network

Data pre-processing forms an important element of pattern recognition procedures for mechanical and structural damage detection. This paper discusses feature extraction and selection procedures. The discussion is focused on time-frequency and time-scale analysis. Case studies of damage detection exercises are presented for illustration.

Advanced data pre-processing for damage identification based on pattern recognition

A smart structure is one that senses its internal state and external environment, and based on the information gained responds in a manner that fulfils its functional requirements. The primary advantage of moving towards smart structures technology is the potential cost benefit of condition-based maintenance strategies and the prospective life extension that may be achieved through in situ health monitoring. The monitoring of operational health and performance, and diagnosis of any faults as they occur, is a relatively new concept that is being developed globally to provide advantages of safer , more reliable and affordable structures. Health monitoring can be achieved by positioning (embedded or surface mounted) sensor systems on a structure to measure those physical parameters that are informative with respect to the state of the structural health. Information relating to the severity and location of damage, as well as to the nature of the loading is of obvious importance to this endeavour. In the aerospace industry bonded composite patches are increasingly being used to extend the operational life of ageing aircraft. The application of bonded composite patches to repair or reinforce defective metallic structures is widely acknowledged as an effective and versatile procedure. Such patches have been successfully applied to the repair of cracked structures, to the reinforcement of components subject to material loss due to corrosion damage and as a general means of stress reduction through the provision of a supplementary load path. However, certification requirements mandate the need for a methodology for monitoring the damage state of both the defective underlying structure and of the repair. In this case the concept of smart structures can be used to detect damage in the repair itself as well as monitor damage growth in the parent structure. This paper reports on the development of a `perceptive repair' or `smart' system which will provide information on the in-service performance of the repair and the associated structure. In this respect, this paper focuses on the detection of disbond in the adhesive layer between adhered and the metallic parent structure. One of the criteria of this smart system is that it must be economical, reliable and, preferably, self-powered. To this end, it was proposed that piezoceramic of piezoelectric material be utilized because of their ease of application. These materials were chosen because they can be used both as a actuator and as a sensor. This paper presents a set of numerical investigations performed to highlight the viability of using this material system, and the associated signal analysis that can be employed to detect the presence or the development of disbonds in the adhesive in a bonded repair situation.

http://iopscience.iop.org/article/10.1088/0964-1726/9/4/309/pdf

Damage Detection in Bonded Repairs using Piezoceramics

The purpose of this paper is to provide a concise introduction to the developments and recent findings of a BRITE-EURAM program of work (BRE2.CT94-0990 , Structurally Integrated System for the comprehensive evaluation of COmposites). The aim of the program has been to develop an acoustic/ultrasonic based structural monitoring system for composite structures using material compatible sensors. Since plate-like structures have been investigated, it has been a requirement to utilise the propagation of ultrasonic Lamb waves through the sample materials. Preliminary investigations utilised conventional piezo-electric sources coupled to the sample via perspex wedges. The Lamb waves generated by these sources were monitored using either a fully embedded or surface mounted optical fibre sensors. The system was tested with a variety of different carbon and glass fibre reinforced panels, and the interaction of the Lamb waves with different defects in these materials was monitored. Conventional signal processing allowed the location of defects such as impact damage sites, delaminations and holes. Subsequent investigations have endeavoured to refine the system. This paper reports the development of advanced wavelet based signal processing techniques to enhance defect visibility, the optical connectorisation of composite panels, and the development of flexible low profile acoustic sources for efficient Lamb wave generation.

I. R. James

Papers

Ultrasonic condition monitoring of composite structures using a low-profile acoustic source and an embedded optical fiber sensor