Simultaneous measurement of temperature and strain in glass fiber/epoxy composites by embedded fiber optic sensors. Part II – Post-cure testing

In this paper the use of fiber optic sensors embedded into GFRP laminates for structural health monitoring of thermo-mechanical loads is presented. The proposed sensing system, consisting of two coupled fiber Bragg grating (FBG) sensors, allows the simultaneous measurement of both temperature and strain by decoupling the change in reflected wavelength induced by temperature variations from that induced by either mechanical or apparent strain, with significant advantages either for monitoring the composite cure kinetics, as described in the accompanying paper (Part I), or for assessing the structural response to thermo-mechanical loads. Both thermal ramp and mechanical static/dynamic bending tests were performed in the laboratory on instrumented GFRP samples with 12, 18 and 24 layers. It was shown that, by combining the wavelength shift outputs of the two fiber Bragg gratings, the longitudinal material CTE coefficient can be evaluated with ±3% accuracy. This parameter can be used to discriminate mechanical and apparent strains from the FBG reflected spectrum. Static four-point bending tests showed good agreement between values measured by the embedded strain grating and those predicted by the classical lamination theory of composite materials, being the maximum deviation within 2%. Dynamic tests performed with both sinusoidal and square wave inputs have proved the effectiveness of the proposed solution to track the strain field accurately up to 10 Hz.