Papers by Author: Rhys Pullin

Abstract: The ability of a Structural Health Monitoring (SHM) system to automatically identify damage in a composite structure is a vital requirement demanded by end-users of such systems. This paper presents the demonstration of a potential method. A composite fatigue specimen was manufactured and initially tested at 1Hz for 1000 cycles. Acoustic emission (AE) signals were recorded for complete fatigue cycles periodically in order to establish a base-line associated with undamaged specimens. The specimen was then subjected to impact damage to create barely-visible impact damage (BVID) and subjected to further fatigue cycles with acoustic emission recorded until failure. The data was subsequently analysed using a range of techniques including basic RMS signal levels and frequency-based analysis. At various stages during the test, C-scanning was used to validate the results obtained. Results demonstrated that AE is capable of detecting BVID in composite materials under fatigue loading. The proposed method has wide applicability to composite structures which are subjected to cyclic loading, such as wind turbine blades.

Abstract: Composite materials are increasingly being used in a wide range of structural applications in place of metallic materials. This presents a new range of challenges when considering the monitoring of damage and failure in complex components. This paper explores these challenges and presents a potential monitoring method using airborne acoustics which is both non-contact and easily implemented. A carbon composite panel was manufactured and statically loaded in tension until failure. During the test, Digital Image Correlation (DIC) was used to measure full field surface strain in the panel. An array of microphones, placed adjacent to the panel, was used to capture airborne acoustic signals between 400Hz and 20kHz during the test. The captured sound waves potentially contain signals originating from a range of sources, such as fibre failures and matrix cracking, but also contain background noise. A range of techniques have been used to examine the signals and determine the onset of failure, including Short-Time Fourier Transforms (STFT). The detection of failure using the airborne acoustic system has been validated using the strain data from the DIC measurements. The results presented demonstrate the applicability of the airborne system to monitoring of composite components.

Abstract: This paper investigates the use of acoustic emission (AE) to identify tensile cracks and shear movements in concrete structures. The analysis of AE signals detected during a concrete beam investigation mainly focused on the relationship between signal parameters namely rise time, amplitude, duration and counts. This method allowed crack classification and shear movements to be identified. A laboratory experiment test was completed on a reinforced concrete beam (150 x 250 x 1900 mm). During testing AE data was captured and analysed post-test. Results from the most severely damaged zone, based on visual inspection, was analysed. The results indicated that tensile cracks developed though out the area of analysis, as confirmed by visual observation and relatively little shear movement occurred as would be expected in a reinforced beam.

Abstract: Accurate knowledge regarding the ongoing condition of an aircraft’s structural condition together with future life predictions enable optimal use of material, hence reducing mass, cost and environmental effects. Previous work by the authors has demonstrated the potential for using energy harvested from vibrating aircraft panels to power a self contained health monitoring system based on the use of wireless sensor nodes for an aircraft structure. However the system proposed was far from optimal. Research is being undertaken to investigate the various factors affecting the power output of such a system, including the design of the harvesters used (length, width, number of layers), their positioning and their orientation. The work presented in this paper enables the determination of the optimised positions for a series of harvesters on a representative aircraft panel, based on the use of shape functions for the various modes of vibration over the expected frequency range, to derive a function related to power output which is then optimised. A series of recommendations are made.

Abstract: Composite patch repairs are being increasingly used throughout bridge structures in the UK. These patches offer a convenient and strong repair providing that the bond used to adhere the patch to the structure retains its integrity. Acoustic emission (AE), a passive approach and Acousto-Ultrasonic (AU), an active approach offer two methodologies for monitoring the structural bond and ensuring the patch repair remains effective. An experimental program was developed to assess the suitability of using AE and AU for monitoring the bond. Two concrete beams were manufactured and pre-cracked in three point bending prior to being repaired using a bonded composite patch. Two static tests were then completed to assess the performance of the two techniques for monitoring the bond. Results were compared with strain gauges adhered to the bonded patch and visual observation. For active monitoring a baseline of signals were captured at a known load and post damage a further series was captured at the same load. The signals sets were then compared using a cross correlation function technique. A simple accumulative acoustic energy analysis was then completed for the passive data. Results demonstrated that both techniques can be utilised to monitor the bonded structure. By comparing the results with those recorded by the strain gauges and visual inspection it was possible to demonstrate the successful effectiveness of the techniques for detecting global damage but specific debonding events would require further investigations.

Abstract: A project investigating the possibility to transmit electrical power (several mW) along the structure of an aircraft by using an intermediate medium of ultrasonic Lamb waves is being carried out at Cardiff University in conjunction with Airbus. This power supply method is aimed at wireless, surface-bonded sensor packages, primarily for the aircraft structural health monitoring (SHM) applications. It is expected to replace conventional batteries or energy harvesting devices. This paper presents methods of piezoelectric transducer characterisation, electric power measurement and electric circuit simulation that were developed in support of the ultrasonic power transmission project. The unique combination of low power and a high AC frequency range (up to 200 kHz) precluded the use of conventional power measurement instruments and called for a tailored system and software to be developed. Two approaches were developed: one relying on the measurement of the ultrasonic transducers impedance characteristics and their subsequent use in a circuit simulation; and another relying on the direct measurement of voltage waveforms in the power transmission setup. The two methods were found to be capable of producing closely matching results up to 300 kHz. Results of early power transmission trials are also presented. The optimum approach resulted in 1 mW of power transmission over a distance of 74 cm in a 1.5 mm thick aluminium plate.

Abstract: Damage detection and location in aerospace composites is currently of great interest in the research community and is being driven by the need to reduce weight of commercial aircrafts and hence make substantial environmental improvements. The increased use of composites as safety critical components has led to the need for development of structural health monitoring (SHM) systems. Acoustic Emission (AE) offers an excellent potential for delivering the necessary information of damage detection to maintenance engineers in terms of location however there are currently no methodologies that can use AE signals to characterise damage sources. This paper explores a methodology for damage characterisation based on measuring the amplitude ratio (MAR) of the two primary plate wave modes, to allow identification of in-plane (matrix cracking) and out-of-plane sources (delamination). Results from a large-scale buckling test show good correlation between signal characterization and observed damage mechanisms.

Abstract: Acoustic Emission (AE) is a passive form of non-destructive testing that relies on the detection and analysis of stress waves released during crack propagation. AE techniques are successfully employed number of industries there remains some scepticism in aerospace engineering. The reported investigation details a single four point bend test specimen undergoing fatigue loading. This test is part of a much larger programme designed to demonstrate a technology readiness level (TRL) of five of the use of AE to detect crack initiation and growth in landing gear structures. The completed test required that crack growth had to be monitored to allow a comparison with the detected and located AE signals. The method of crack monitoring had to be non-contact so as not to produce frictional sources of AE in the crack region, preventing the use of crack mouth opening displacement gauges. Furthermore adhesives on the specimen surface had to be avoided to eliminate the possibility that the detected AE was from adhesive cracking, thus the use of strain gauges or foil crack gauges was not possible. A method using Digital Image Correlation (DIC) to monitor crack growth was investigated. The test was stopped during fatigue loading at 1000 cycle intervals and a DIC image captured at peak load. The displacement due to crack growth was observed throughout the investigation and the results compared with the detected AE signals. Results showed a clear correlation between AE and crack growth and added further evidence of TRL5 for detecting fractures in landing gears using AE.

Abstract: The effect of lightning attachment to structures and vehicles is a cause of major concern to a number of different industries, in particular the aerospace industry, where the consequences of such an event can be catastrophic. In 1963, a Boeing 707 was brought down in Maryland killing 81 people on board, triggering the improvement of lightning protection standards. However, commercial jets are still struck on average once every 10,000 hours of flight time and between 1963 and 1989 forty lightning related accidents were recorded within the U.S.A alone. The rapid increase in the use of composite materials in aircraft design and the consequent increase in complexity when determining the effects of a lightning strike, has led to new challenges in aircraft protection and the requirement for improved understanding and standardisation.

Abstract: The detection of damage in gear teeth is paramount to any condition monitoring or structural health monitoring (SHM) tool for aerospace power transmissions such as those used in helicopters. Current inspection techniques include vibration analysis and time-inefficient visual inspection. Acoustic Emission (AE) is a very sensitive detection tool that has been successfully used in many SHM systems. Successful application of AE for damage detection in gear teeth will enable the optimisation of gear box design (and hence weight saving) in addition to safety improvements. This paper details a small aspect of a larger project designed to demonstrate automatic detection and location of common gear tooth defects. A novel test rig was designed to allow the fatigue loading of an individual gear tooth which was monitored using AE. The gear tooth was static in order to exclude the detection of AE signals arising from rotation; this allows initial development of the methodology prior to investigating rotating gears. Digital Image Correlation was used to determine the onset of cracking for comparison with the detected AE. Preliminary results of the investigation show that the developed methodology is appropriate for developing an automated gear health monitoring system and that future work should concentrate on the development of sensors and data acquisition methods associated with obtaining signals from rotating machinery.