Development of sensor technology to facilitate in-situ measurement of damage in composite materials for spacecraft applications

摘要

CFRP is used in a variety of applications where its high strength to weight ratio and high specific modulus is advantageous. Impact damage can however significantly reduce the strength and the modulus of the material through the creation of areas of delamination, matrix cracking and fibre failure. Such impact damage is often hard to locate and its severity hard to assess. For applications in remote or inaccessible locations traditional methods of damage assessment are not feasible and therefore a method of assessment in-situ and in service is often required to predict the optimal reparation or replacement period. Such a technology is particularly relevant to spacecraft applications where reparation and replacement costs are prohibitive and where impacts are often sustained from micrometeoroid and space debris impingement. Optic fibre Bragg gratings reflect light at a characteristic wavelength which is a function of sensor characteristics, strain and temperature. Changes in strain (or temperature), result in a wavelength shift of the order of 1.2 nm/µ epsilon and 10nm/°C. Due to their small size (typically 9µm in diameter) optical fibres containing Bragg grating sensors can be included within the matrix of fibre reinforced plastics with little detriment to their physical properties. Each optical fibre line can contain a number of discrete grating based sensors with as little as 1mm separating each grating. The limit to sensor length is sub-millimetre. These sensors are well suited to CFRP condition monitoring. At present the most significant barrier to the widespread application of this technology is the difficulty in accurately interrogating each grating based sensor to discern its characteristic wavelength at any time. Modifications made to an existing interrogation system to enable impact damage identification are detailed in this paper. A Bragg grating based sensor array was placed in the CFRP structure to enable accurate strain profiling of the material pre-impact and post-impact. This strain information is related to the observed damage. Dynamic strain information (during impact events) has also been recorded. The importance of this data for the development of the sensor technology is discussed.