Papers by Author: C.A. Featherston

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Authors: Mark J. Eaton, Karen M. Holford, C.A. Featherston, Rhys Pullin
Abstract: The presence of impact damage in a carbon fibre composite can reduce its capacity to support an in-plane load, which can lead to an unexpected or premature failure. This paper reports on an investigation into two slender carbon/fibre epoxy panels, one un-damaged and one with an artificial delamination introduced using an embedded section of PTFE. The reported tests form part of a larger series of investigations using differing sizes of artificial delamination and real impact damage. An investigation of wave velocity propagation at varying angles to the composite lay up was completed to assist in source location. The specimens were loaded under, uniaxial in-plane loading and monitored using four resonant acoustic emission sensors. A full field optical measurement system was used to measure the global displacement of the specimens. Analysis of AE waveforms and AE hit rate were used to assess the buckling of the panel. The results compared favourably with the optical measurement results.
Authors: Mark J. Eaton, Rhys Pullin, C.A. Featherston, Karen M. Holford
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.
Authors: J.A. Badham, C.A. Featherston, J.A. Brandon
Authors: Timothy P. Bradshaw, Mark J. Eaton, Rhys Pullin, S.L. Evans, C.A. Featherston
Abstract: Eight composite plates 400x410x2.15mm were fabricated from carbon/epoxy in ((0,90)4)s lay-up. To ensure there was no damage in the plates prior to the impact investigations the plates were C-scanned. A drop test rig was used to apply a low velocity impact to the undamaged plates. A rebound mechanism was employed to prevent secondary impacts. AE sensors were selected for frequency and size due to the limited space on the test rig. Super glue was used both as a couplant and also to secure the sensors in position. During the impact wave streaming, time driven data and hit driven data were used to record the impact simultaneously. One test was conducted on each sample with two impacts completed at 5J, 6J, 8J and 10J (total of 8 samples). The impacts were designed to allow only slight visible impact damage to be formed. After completion of the impact investigation the plates were C-scanned to determine the area of debonded material. Analysis of the data showed the complete record of the impact event from the wave streaming, while the hit driven and time driven absolute energy data provided increasing correlations with the area of composite damaged.
Authors: C.A. Featherston, Mark J. Eaton, S.L. Evans, Karen M. Holford, Rhys Pullin, Matthew Cole
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.
Authors: M. Arnold, C.A. Featherston, Matthew R. Pearson, J. Lees, Aleksander Kural
Abstract: Autonomous structural health monitoring systems with independent power sources and wireless sensor nodes are increasingly seen as the best solution for monitoring a diverse range of machines and structures including pumps, bridges and aircraft. Powering these systems using harvested energy from ambient sources provides an attractive alternative to the use of batteries which may be either inaccessible for routine maintenance or unsuitable (for example in aerospace applications). A number of techniques are currently being considered including harvesting energy from vibration and thermal gradients. Harvesting energy can however lead to a highly variable power supply in opposition to the requirements of a wireless sensor node which requires continuous standby power with an additional capacity for power peaks during transmission of data. A power management system with embedded energy storage is therefore necessary in order to match supply and demand. Due to the low levels of power harvested in a number of applications, an important factor in the design of such a system is its efficiency to ensure sufficient power reaches the sensor node. Based on the requirements for a simple power management system for thermoelectric power harvesting consisting of a rectifier, a DC/DC convertors and a battery, this paper first examines the possibilities in terms of basic components with a number of commercially available units tested and characterised. Potential designs for a management system incorporating these components are then discussed and a blueprint for an optimal system is suggested.
Authors: C.A. Featherston, Karen M. Holford, Bea Greaves
Abstract: The concept of harvesting energy is not a new one: there has been an interest in this area for around 10 years. Devices typically use either vibration (rigid body motion) or thermal gradients and can harvest sufficient energy to power telemetry, small devices or to charge a battery or capacitance device. However, for the new generation of aircraft, (both fixed wing and rotating) there is now an urgent need to develop energy harvesting systems in order to provide localised power for sensors in structural health monitoring systems (SHM). By implementing SHM, aircraft manufacturers can benefit from improved safety, reduced maintenance and extended aircraft life. The work presented examines the feasibility of designing an energy harvesting system powered by the vibrations of aircraft panels generated in flight. PZT (lead zirconate titanate) harvesters are bonded to an aluminium alloy panel, representative of an aircraft wing panel which is vibrated across a range of amplitudes (up to + 0.2mm) and frequencies (up to 300Hz). By recording voltage and current outputs from each harvester, generated power is calculated which when normalised for area and mass indicates values of up to 7.0 Wm-2 and 2.5Wkg-1 respectively, representing mechanical to electrical energy conversion efficiencies of up to 35% dependant on frequency of vibration. From these values it is estimated that a harvester area of down to 71cm2 or mass of as little as 20g is necessary to meet the current minimum power requirements of SHM systems of 50mW. With predicted reductions in sensor power consumption indicating system power requirements in the order of 0.1-1mW, this work shows that piezoelectric energy harvesting has future potential for powering aerospace SHM systems.
Authors: C.A. Featherston, Karen M. Holford, C.A. Holt, D. Manning, A. Claisse
Authors: O.J. Wood, C.A. Featherston, D. Kennedy, Mark J. Eaton, Rhys Pullin
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.
Authors: C.A. Featherston, J. Mortimer, Mark J. Eaton, Richard L. Burguete, Rhiannon Johns
Abstract: For a structure subjected to an intermediate velocity impact in which the duration of loading is in the order of milliseconds and in excess of the period of it’s first free vibration mode there is a relationship between impact duration and buckling load. Although this relationship results in higher buckling loads for shorter duration impacts, the precise nature of the correlation depends on a number of other factors, one of which is geometry. Since the design of many lightweight structures subject to dynamic loading in this intermediate range is based on the use of a static buckling load to which a load factor is then applied, it is essential that this factor accurately represents the relationship between the two and takes of account of any variations. Failure to do so will at least result in an over designed structure and at worst in catastrophic failure. A series of finite element analyses (FEA) have been performed in order to determine the relationship between dynamic and static buckling loads for a range of stiffened panels with differing radii of curvature. This paper describes preliminary tests performed to determine the feasibility of using high speed digital image correlation (DIC) to study such an impact and hence provide validation of the earlier FEA analyses. These are performed on a longitudinally stiffened panel subject to uniaxial compression, clamped within a rig designed to provide built-in end conditions and allow motion of one end in the direction of loading only. The specimen is tested using an accelerated drop test rig. Impact load is monitored throughout using a load cell. Full field displacement contours are obtained using a high speed DIC system. Results are presented which demonstrate deflection contours during and after impact enabling the path of the shock wave through the specimens to be determined. An initial comparison is then made the FEA results.
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