Process monitoring of aluminum-foam sandwich structures based on thermoplastic fibre–metal laminates using fibre Bragg gratings

Abstract This paper reports the use of fibre Bragg gratings (FBGs) as sensors for structural integrity assessment by monitoring the reduction in the flexural stiffness of a novel sandwich structure based on thermoplastic fibre-metal laminate (FML) skins and an aluminum-foam core. By monitoring the reduction in the structural flexural stiffness, the technique demonstrated its potential for detecting damage initiation in these hybrid systems. In addition, the method allows the estimation of the initial failure strain and the rate of subsequent damage development within the host material. Cyclic three-point bend tests were conducted to characterise the degree of damage in this material in which the maximum load in each cycle was increased progressively until failure occurred. The FBG sensors, which were initially used to monitor the fabrication process, were subsequently employed as damage sensors to monitor the development of damage in the host structure. The FBG sensors were either embedded between two plies of glass fibre reinforced polypropylene within the FML or bonded to its lower surface. The shift in the Bragg wavelength was recorded during the test cycle to monitor the strain response of the sandwich beam.