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.
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.
The objective was to determine design changes that reduce the risk of damage in an embedded piezoceramic transducer. Finite element analyses were performed to calculate stresses in embedded piezoceramic transducers. The model consisted of elements representing the piezoceramic, the interconnectors and the conductive adhesive, and also included the cross-ply laminate. The parameters chosen in the study were the thickness of the interconnector, the material properties, as well as the length and thickness of the conductive adhesive. The stress state in the transducer was determined for different parameter combinations to find a design with low damage risk. For the parameters studied, the lowest risk for damage initiation was obtained for a transducer with a compliant adhesive was a small thickness, and an adhesive that covered the entire piezoceramic element from edge to edge. The strain at failure in the transducer was estimated, and the position for damage initiation in the transducer was determined. The findings from the finite element analysis were supported by the experimental results.
The concept of a built-in structural health monitoring system has attracted great interest as such systems may allow reduced maintenance cost and increased safety. One attractive technique to realize such a system in composites is to use embedded transducers to generate Lamb waves. The damage in the structure is detected by determining the change in the character of the Lamb waves as an effect of damage. In this paper, the performance of embedded piezoceramic transducers used as Lamb-wave generators was investigated. The composite specimens with a piezoceramic transducer embedded in the mid- plane were subjected to tensile and compressive static loading as well as fatigue loading. A surface-attached acoustic emission sensor further detected the Lamb waves. Measurements of the impedance were also performed during static loading to evaluate the electrical conduction of the piezoceramic transducer. During static loading, the embedded piezoceramic transducers functioned near to the final failure of the composite without significant changes in the generated Lamb waves, although the microscopic examination indicated damage. Debonding between the surfaces of the piezoceramic element and the interconnectors as well as failure in the piezoceramic element had occurred. In fatigue, for a stress ratio of -1, the piezoceramic transducers also showed a large working range. The amplitude and frequency of the Lamb waves, generated by the piezoceramic transducers, were not significantly changed after a large number of cycles, around 50,000 - 100,000 cycles at strain levels of ± 0.20%. The changes in terms of amplitude and frequency of the Lamb waves, occurring after a large number of cycles, were associated with increasing matrix cracks in the specimen.
The objective of this paper is to determine the strength reduction due to the embedment of a piezoelectric ceramic transducer in a composite. The composite was made from carbon/epoxy prepreg with a cross-ply lay-up. The transducer was embedded in the mid-plane of the composite material. The specimens were tested in tensile and compressive static loading. It was found that the embedded piezoelectric ceramic element with its interconnectors did not reduce the strength of the composite. In tensile and compressive static tests, the final failure did not coincide with the embedded piezoelectric ceramic transducer location in the composite.
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