A pair of oppositely signed stress pulses are created at each crystal boundary in this case. If the acoustic impedances on either side of the crystal differ from that of the crystal, then multiple reflections occur at the interfaces, as described by Redwood (1963). Most piezoelectric transducers are reciprocal, so they act as receivers equally well. The spectral response of a transducer peaks at odd multiples of the fundamental frequency. When transducers resonate, they have a distinctive electrical impedance signature that can be measured electrically. In summary, the piezoelectric element sends out two acoustic signals for each electrical excitation in this simple model. Szabo, in Diagnostic Ultrasound Imaging: Inside Out (Second Edition), 2014 5.1.3 Summary Integration of FBG for SHM also considers important issues like temperature compensation, interrogation and fibre encapsulation techniques. To prolong the lifespan of these sensors, stringent practices are applied in manufacturing the fibre gratings while a protective coating is necessary for long-term reliability. The composite laminate construction also facilitates the embedment of these low-profile sensors where no strength degradation of the primary structure has been observed.
īecause of the advantages of FBG over strain gauges, these optical sensors have received significant interest for operational load monitoring in aerospace structures. Most studies observe the strain directly to monitor for damage or directly measure the light intensity and power in the reflection spectra from FBG. In addition, FBG can also replace thermocouples to measure thermal load profiles, as well as monitor the quality of cure and adhesive bondline of repair patches. multiple measurement locations along the fibre can be created by varying the grating spacing.įor damage detection, the FBG identifies this change as a deformation in shape and reduction in amplitude of the reflected spectrum due to the presence of non-uniform strains and loss in the structure’s load-carrying ability.
In addition, they have multiplexing capabilities, i.e. The advantages in SHM applications compared with conventional resistance strain gauges are that FBG are more durable, possess a lower profile and no electrical wiring is required. The optical sensor is thus effectively a strain measuring sensor. Strain in a structure is identified as a shift in the reflected signal from the FBG. Fundamental operating principle of fibre Bragg grating.