Residual strain development in an AS4/PPS thermoplastic composite measured using fibre Bragg grating sensors

This paper demonstrates the use of fibre Bragg grating (FBG) sensors for the measurement of residual strain development during the consolidation of a thermoplastic composite. During the processing of the carbon fibre-reinforced polyphenylene sulphide (AS4/PPS) laminate, FBG sensors respond to changes in material state, for example the glass-rubber transition and solid–liquid transition. The sensors also permit the observation of wavelength shifts and spectral form changes induced by the contraction of the composite during cooling. The experimental data are compared to a generalized plane strain, thermoelastic numerical model with temperature dependent matrix dominated properties. The model provides solutions for two limiting cases: one where the specimen contracts freely, and one where the specimen is restricted by perfect contact with the mould. Strain values calculated in each case are inserted into optomechanical equations, which convert the strain state in the FBG to a corresponding change in wavelength. In this way, the modelled cases are compared to FBG wavelength shifts during consolidation.