Steve Galea

Health monitoring of composite repairs and joints using optical fibres

With the increasing use of composite bonded repairs to ageing aircraft there is a growing need to monitor their in-service health. One approach is to incorporate various in situ health monitoring systems within the repair. This paper summarises a collaborative research program between the Australian Defence Science and Technology Organisation (DSTO) and the DSTO Centre of Expertise in Structural Mechanics at Monash University into the use of optical fibre sensors to monitor the structural health of composite repairs and composite joints.

Optical fibre sensors for health monitoring of bonded repair systems

Abstract In the aircraft industry the use of externally bonded composite repairs has become an accepted way of repairing fatigue, or corrosion, damaged metallic structural components. However, current NDI and damage assessment techniques for composite repairs are passive and generally performed on ground. The challenge is to develop new techniques utilising recent analytical and experimental tools. This report examines the use of optical fibre sensors. Optical fibres offer a means of monitoring the load transfer process in these repairs, and can therefore be used to provide an indication of the integrity of the repair. This paper describes the use of an array of fibre Bragg grating strain sensors (FBGs) for the in situ monitoring of bonded repairs to aircraft structures and, in particular, the monitoring of crack propagation beneath a repair. In this work the FBGs have been multiplexed using a combination of wavelength and spatial techniques employing a tunable Fabry–Perot (FP) filter to track individual gratings. The multiplexed FBGs were then surface-mounted on a boron–epoxy unidirectional composite patch bonded to an aluminium component. The sensors were located so as to monitor the changing stress field associated with the propagation of a crack beneath the patch. The ability of relating experimental results to sensor readings is then confirmed using both a thermo-elastic scan of the patch and 3D finite element analysis.

A low profile vibro-impacting energy harvester with symmetrical stops

This paper reports on an investigation into the use of a vibro-impact approach to construct a relatively broadband kinetic energy harvester. Potentially, the vibro-impacting process may be exploited as an autotuning mechanism for energy harvesting in an environment where the source vibration spectrum varies in time, such as an aircraft in flight. The energy harvester examined in this paper is based on a vibro-impacting oscillator with double-sided, symmetrical, piezoelectric bimorph-stops. The energy harvester operates in the frequency range of 100–113 Hz and has a (non-optimized) maximum energy of 5.3 mW from an rms host vibration of 450 mG.

Smart structure application in bonded repairs

Abstract In the aerospace industries bonded composite patches are being increasingly used to extend the operational life of aging 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 will report 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 will focus on the detection of disbond in the adhesive layer between adherend and the metallic parent structure. Since this is a relatively new area, a series of numerical studies were initially performed to reveal the salient features of the signals expected. These numerical findings were subsequently confirmed experimentally.

Use of piezoelectric films in detecting and monitoring damage in composites

The increasing emphasis over the past few years on intelligent material systems and structures has resulted in a significant research effort in the areas of embedded and bonded sensors and actuators. Of the many sensing materials available, piezoelectric sensors offer a number of ad vantages. The sensor output, proportional to changes in surface displacement over a large area, can be used to interpret variations in structural and material properties, e.g., the compliance of the material. This type of sensor has also been used as an ultrasonic transducer. It offers the advantage of having a low structural impedance, thus giving an accurate measurement of the change in area, and consequently can only be used as an actuator on flexible structural systems. The aim of this article is to demonstrate the use of piezoelectric film sensors as a structural integrity monitoring device for composite materials. Tests to date have been aimed at detecting and monitoring impact damage in composite materials, as well as detecting damage in composite-to-metal mechanically fastened joints.

A broadband vibro-impacting power harvester with symmetrical piezoelectric bimorph-stops

The certification of retrofitted structural health monitoring (SHM) systems for use on aircraft raises a number of challenges. One critical issue is determining the optimal means of supplying power to these systems, given that access to the existing aircraft power system is often problematic. Previously, the Australian Defence Science and Technology Organisation has shown that a structural strain-based energy harvesting approach can be used to power a device for SHM of aircraft structure. Acceleration-based power harvesting from airframes can be more demanding than a strain-based approach because the vibration spectrum of an aircraft structure can vary dynamically with flight conditions. A vibration spectrum with varying frequency may severely limit the energy harvested by a single-degree-of-freedom resonance-based device, and hence a frequency agile or (relatively) broadband device is often required to maximize the energy harvested. This paper reports on an investigation into the use of a vibro-impact approach to construct a piezoelectric-based kinetic power harvester that can operate in the approximate frequency range of 29?63?Hz.

Smart structure for composite repair

This paper reports on an experimental study aimed at assessing the feasibility of detecting damage in a bonded composite structure by means of surface mounted PZT sensor/actuator elements. A boron/epoxy composite patch repair is artificially degraded through the introduction of teflon inserts at the bond line. PZT elements attached to the patch and host structure are then excited and sensed in both transfer function and self-impedance configurations and the measurements examined to assess whether bond degradation is detectable. These measurements indicate that both methods offer good prospect of detecting debonds however, some limitations have been identified.

An investigation of thin PVDF films as fluctuating-strain-measuring and damage-monitoring devices

A study of piezoelectric films as sensors for detecting fluctuating area-averaged strain has been conducted. Both theoretical and experimental results on the characterization of piezoelectricity are presented for KYNAR Piezo Films. The theoretical formula relating the in-plane area-averaged strain to the electric output of a piezo film through a charge amplifier was derived, and the relevant constants were determined experimentally. The present results show that if the films are prepared and adhered properly, they can be used satisfactorily for fluctuating strain measurement. The present work has also investigated the possibility of using PVDF films as sensors for monitoring crack growth on a metallic specimen.

Wideband vibro-impacting vibration energy harvesting using magnetoelectric transduction

This article reports on proof-of-concept experimental work carried out to demonstrate a wideband vibro-impacting energy harvesting approach based on a magnet/bearing arrangement coupled with a magnetoelectric transducer. The harvesting arrangement uses a Terfenol-D/Pz27 laminate transducer (disc with radius 5 mm) positioned between an oscillating spherical chrome-steel bearing and a rare earth magnet. The oscillating bearing steers magnetic field through the magnetoelectric transducer, generating an oscillating charge that can be harvested. A vibro-impacting arrangement between the oscillating bearing (radius 12.7 mm) and a pair of aluminium mechanical stops is designed to produce a wideband frequency response. For a 434 mG host acceleration, the vibro-impact mechanism produced a bandwidth of ∼7.2 Hz (between 6 and ∼13.2 Hz). The issue of damage to the mechanical stops caused by the vibro-impacting process is also explored and was demonstrated experimentally and theoretically to be inconsequential. This non-optimized wideband harvesting approach has demonstrated a generated power of 3.3 µW from a root mean square host acceleration of 180 m-g at 8.0 Hz.

Detection of Disbond Growth in a Cyclically Loaded Bonded Composite Repair Patch Using Surface-mounted Piezoceramic Elements

This paper reports on an experimental study in which an array of surface-mounted lead zirconate titanate elements (PZT) are used for the in situ detection of disbond growth in a bonded composite repair patch. Two techniques are used to track the evolution of disbond growth: the transfer function method and the electromechanical impedance method. Both techniques were found to provide a reliable and robust basis for the detection of disbond growth. The results also demonstrate the importance of transducer placement relative to the disbond location as a factor in the sensitivity to disbond growth.