Strain Measurement of Carbon Composites Using Advanced Sensor Technology

This paper reports the sensor integration method into carbon composites for the development of Structural Health Monitoring (SHM). Early detection and long term monitoring of the specific properties of composites will improve the security and prolong the time of service 1 . The core function of SHM is the damage detection and identification at elevated temperatures, over loads and corrosion. Various sensing technologies are currently available which can be integrated to monitor the composites. These include optical fiber sensors, piezoelectric sensors, strain gage, carbon fibre, nanotube, MEMS (Micro-Electro-Mechanical Systems), eddy current sensor arrays and shape memory alloys. In this study, optical FBG (Fibre Bragg Grating) sensors and strain gauges were used for the strain and temperature measurement of carbon composites. They are often used for highly stressed composite materials and can be either surface mounted and imbedded into the composites 2-5 . The Quickstep process is a recently developed technique for rapid processing of composites using a Heat Transfer Fluid (HTF) instead of metal mould. The tool is supported between two flexible membranes in the pressure chamber. With Quickstep, fast heating and cooling are possible, which lowers the initial viscosity of the resin. The prepreg used in this study was stitched carbon epoxy prepreg, 5 harness satin weave, MTM 44-1 supplied by ACG. 8 plies were laminated at 0 o direction using conventional lay-up tool, sealed in a vacuum bag. First, optical fibres were embedded in different plies of prepreg as shown in Fig.1 (a), to optimise the position of FBG sensors. Array of four FBG strain sensors (at a Bragg wavelength of 1525, 1530, 1535 and 1545 nm, respectively) and one temperature sensor (at 1550 nm) were then located at appropriate position during lay-up. The laminate specimens were cured under light pressure using the Quickstep process at 130 o C for 75 mins and followed by 180 o C for further 120 mins. The average thickness of manufactured specimens was 2 mm. On other hand, Strain gauges (supplied by VISHAY) were patched on the surface of laminate as shown in Fig. 1 (b). Fig. 1: Lay-up of carbon epoxy prepreg and surface mounted Strain Gauge Tensile test results of carbon epoxy laminate at 0 o direction and optical fibre embedded specimen (25 x 300 x 2 mm 2 ) are summarized in Table 1. Tensile property of optical fibre embedded MTM 44-1 carbon epoxy shows minimum change. Morphological property of sensor embedded carbon laminate was characterised using Scanning Electron Microscope (SEM). Ballistic impact testing on sensor embedded laminate was performed, and mechanical and failure behaviour specimen were observed. C-scan analysis on sensor imbedded laminate was also carried out after impact test. Table 1. Tensile property comparison of optical fibre embedded carbon epoxy laminates Location of optical fibre in 8 plies of carbon epoxy laminate Peak stress (MPa) Stain at Break (%) Modulus (GPa) None 606.6 0.010 59.43 Between plies 3 and 4 605.1 0.012 51.51 Between plies 7 and 8 687.4 0.012 54.73