Determination of engineering strain distribution in a rotor blade with fibre Bragg grating array and a rotary optic coupler

Structural damage occurs in materials subjected to repeated or fluctuating stresses that must be minimized by design to increase fatigue life, and strains distributed in a blade need to be experimentally determined for evaluating stress levels. In order to measure local strains in a rotating blade, efficient signal transmission between a sensor installed on a blade and a stationary interrogator is significantly important. In this paper a strain-independent fibre Bragg grating (FBG) sensor and a rotary optic coupler were used as means of compensating light intensity loss change arising from blade rotation. Working principles of a FBG sensor and a rotary optic coupler were detailed, and procedures for measuring local strains were also described. Eventually, strain distributions were obtained in terms of mean strain and strain amplitude. Measured strains were then directly compared with analytical ones. Experimental procedures and results offer an improved insight into a strain measurement technique for a rotating mechanical system.