The sensing principle of a new type of crack sensor based on linear macro-bending loss of an optical fiber and its experimental investigation

For traditional distributed optical fiber sensors, the sensing principle is based on a complex relationship between the crack opening displacement (COD) and the macro-bending loss of the fiber, which introduces many difficulties in its application in practical engineering projects. To solve this problem, a simple linear relationship between the macro-bending loss of the fiber and the COD is established in this work by controlling the fiber winding shaft to ensure the bending radius remains constant. Based on this linear relationship, a crack sensing principle of optical fiber is proposed. To verify the feasibility of this crack sensing principle, a validation experiment was performed using three groups of fiber winding shafts with different diameters and three groups of working light sources with different wavelengths. The experimental results show that the macro-bending loss is linearly related to the COD. Based on the proposed crack sensing principle, and a new type of optical fiber crack sensor to monitor the COD of pre-existing cracks and contraction joints of structures is designed, and the performance is verified by an experiment.