Long-length fiber Bragg grating (FBG) with the length of about 100 mm was embedded onto the surface of a carbon fiber reinforced plastics (CFRP) substrate and two CFRP adherends were joined by adhesive to form an adhesive bonded single-lap joint. The joint was subjected to 0.5 Hz cyclic tensile load and longitudinal strain distributions along FBG were measured at 5 Hz by the fiber-optic distributed sensing system based on optical frequency domain reflectometry (OFDR). We could successfully monitor the strain distributions accurately with high spatial resolution of around 1 mm.
In this research, longitudinal strain and peel stress in adhesive-bonded single-lap joint of carbon fiber reinforced plastics
(CFRP) were measured and estimated by embedded fiber Bragg grating (FBG) sensor. Two unidirectional CFRP
substrates were bonded by epoxy to form a single-lap configuration. The distributed strain measurement system is used.
It is based on optical frequency domain reflectometry (OFDR), which can provide measurement at an arbitrary position
along FBG sensors with the high spatial resolution. The longitudinal strain was measured based on Bragg grating effect
and the peel stress was estimated based on birefringence effect. Special manufacturing procedure was developed to
ensure the embedded location of FBG sensor. A portion of the FBG sensor was embedded into one of CFRP adherends
along fiber direction and another portion was kept free for temperature compensation. Photomicrograph of cross-section
of specimen was taken to verify the sensor was embedded into proper location after adherend curing. The residual strain
was monitored during specimen curing and adhesive joint bonding process. Tensile tests were carried out and
longitudinal strain and peel stress of the bondline are measured and estimated by the embedded FBG sensor. A
two-dimensional geometrically nonlinear finite element analysis was performed by ANSYS to evaluate the measurement
precision.
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