Christophe Paget

Autonomous Sensor Nodes for Aircraft Structural Health Monitoring

The current trend towards the use of novel materials and design concepts for aircraft structures may demand structural health monitoring (SHM) systems in the future. Wireless sensor networks (WSNs) could fulfill the required monitoring tasks concerning fatigue, damage or stress of structural parts. Autonomous sensor nodes (ASNs) are key elements for a WSN in order to allow a self-sufficient and maintenance free operation, without any complex wiring for power supply or communication purposes.

Passive impact localisation for the structural health monitoring of new airframe materials

This experimental work considers the use of permanently attached sensors for the detection and location of impacts to a carbon fibre reinforced plastic panel with stringers. Deterministic knowledge of the propagation of Lamb waves in the structure is not used. Instead a statistical measure of the signal is used to determine the arrival time of elastic waves propagating in the structure as a result of the impact. A comparison is made between a conventional method and the statistical method. The conventional method, which has been routinely used in industry for acoustic emission imaging, uses the timing of a peak in the recorded signal. The statistical method uses the Rayleigh maximum likelihood estimator. The statistical method is shown to provide both more precise and robust estimates of the elastic wave arrival time. An array of just four sensors is used to locate the impacts. The accuracy of the localisations is used to visualise the effectiveness of the two methods for the low sensor density used. Low sensor density is necessary for minimising system weight and cost. The equivalent net sensor density used in this experiment was five sensors per meter squared. Carbon fibre reinforced plastic is today used for both exterior surfaces and primary structure of airframes entering service. The industrial relevance of this work is to mitigate the diminishing role of visual inspection for evaluating the health of aerospace structures, where impact damage may not be visible.

Ribbon Tapes, Shape Sensors, and Hardware

For fiber optic sensors to be integrated in aerospace structures and for these sensors to provide data that can be used for structural health monitoring (SHM), development work has been performed in the SARISTU project. This particular chapter describes the inclusion of fiber optic sensors in a tape format, called ribbon tape, for damage detection and load monitoring with a secondary bonding procedure to install these in an aerospace structure. The ribbon tapes can be installed on composite structures either by co-bonding or secondary bonding techniques. Both procedures have been applied to composite coupons that have been tested in different environmental conditions. Moreover, a damaged tape repair procedure is developed. This chapter also describes the development of a fiber optic sensor to be used as a shape sensor inside a morphing aerospace structure to provide feedback on the structures shape. The hardware required to analyze and format the sensor readings for use in SHM is also provided in this chapter.

Multimodal location algorithm for Lamb waves propagating through anisotropic materials

Composite material use in aerospace structures has grown over the last two decades and more recently there has been an increase in the use of anisotropic composite layups. One of the most promising SHM techniques is Acoustic Emission (AE) using Lamb waves. Previous location algorithms, capable of locating damage such as cracks, delamination and debonding, have focused their application to either isotropic or quasi-isotropic structures. Previous work was dedicated to anisotropic structures based on single Lamb wave mode propagations. The scope of this work is to include different modes in the AE location algorithm to improve its location. There are cases where it is likely that different modes trigger different transducers for the same event. The transducer time-of-flight is dependent on the mode velocity, therefore an AE location calculated from single-modal algorithm would expect to have significant location inaccuracy. By considering the possibility of different Lamb wave modes triggering each sensor in the location algorithm, and using certain mathematical and physical assumptions, significant improvements of the AE location can be reached, reducing NDT burden. The multi-modal algorithm also includes the ability to locate AE in anisotropic material based on previous proven single-modal algorithm known as Elliptical algorithm. Such a multi-modal elliptical approach taken in the algorithm discussed in the work is expected to reduce significantly the AE location error for highly anisotropic material. Based on analytical equations, this algorithm processes large amounts of AE data in a condensed period of time, allowing live structural monitoring of large assets.

Guided wave interaction with aerospace aluminium stringer feet

Acoustic Emission has shown itself to be a valuable technology for reliably detecting damage initiation and growth in large structures. Monitoring of a structure, throughout its life, is possible with sparse sensor arrays. However aerospace structures can be geometrically complex and contain many structural features, the most common being stringers. Stringers are arranged in a way that they can span the length of the wings or fuselage, separated by less than 200mm in certain cases. Therefore it is almost inevitable that, for any reasonable sensor spacing, acoustic emission events propagating guided waves will interact with multiple stringers. A large aerospace aluminium panel is used to minimise the effects of edge reflections and to allow the two fundamental guided wave modes to separate before reception. It is shown that stringer foot height has a noticeable impact on guided wave propagation, for typical aerospace arrangements. A reduction in transmitted signal amplitude was noted as the stringer thickness was increased. However a local maximum was seen when the stringer foot thickness was equal to that of the plate thickness. This paper discusses quantitative analysis of stringer interaction with the fundamental guided wave modes. The effect of the stringer as a feature has been divided into three main interactions; stringer dimensions, coupling media and riveting. Stringer dimensions and coupling media interactions has been investigated here to quantify their effect on transmission and reflection of the fundamental guided wave modes.