Use of fibre-optic (FBG) sensors in the structural health monitoring of a battlefield helicopter rotor blade

As the use of fibre composite materials and components become more widely accepted, so does the inherent risks of sudden and possibly catastrophic failure. This creates a distinct need for sound, structural health monitoring (SHM) methods to be employed to both warn of, and prevent impending failure. \n \nFor aviation related fibre composite components this is of paramount importance; however, a secondary but equally important consideration is that of service life. Any extension of a components service life is of great financial and operational benefit to both civil and military operators of aviation assets. This is particularly true of military helicopters which use fibre reinforced composite rotor blades, such as the Boeing CH-47 Chinook. Experience has shown that these highly exposed components are frequently damaged during combat operations and rapidly come into short supply as a result of often minor damage. This minor damage may necessitate blade replacement prior to the aircraft being authorised for further flight. \n \n \nThis project seeks to use finite element analysis (FEA) methods and physical blade testing via the use of optical fibre Bragg grating (FBG) sensors to evaluate typical battlefield, ballistic penetration damage by small arms fire projectiles to a composite Boeing CH-47 Chinook rotor blade test section. \n \n \nAbaqus FEA software was used to create both a flat plate simulation and a Boeing-Vertol VR-7 Aerofoil assembly model. Physical testing was conducted on a blade by applying incremental load increases as well as incremental levels of simulated damage. Both FBG and strain gauge systems were used to assess the micro-strain levels at predetermined, critical locations. The data response from these systems was then validated as far as possible by FEA methods, with correlations able to be drawn between the strain systems and the FEA results. \n \n \nThis research demonstrated that the use of FBG sensors on the surface of a complex composite component is an appropriate method for determining strains in the vicinity of damage, which was validated in specific areas by FEA methods. It also concluded that FEA methods alone are very difficult to use in a practical sense when assessing the significant size, type and random nature of ballistic damage to a complex composite structure. \n \n \nWith further future development the possibility of the embedding FBG sensor systems at manufacture into a composite rotor blade for real time SHM or lifing assessment exists. This may in turn lead to enhanced service life management of such components by moving to an on-condition based lifing approach.