Crack width measurement with OFDR distributed fiber optic sensors considering strain redistribution after structure cracking

Crack monitoring is an important task in structural health monitoring. In this study, a procedure is developed to assess the crack width based on the strain curve of distributed fiber optic sensors (DFOS), taking into account of the strain redistribution of the structural substrate after cracking. Fifteen aluminum alloy plates with two or three pre-cut cracks spaced at varying intervals were installed with DFOS and subjected to a tensile test. During the test, the width of the cracks was measured using an optical microscope. The results revealed that cracks caused a peak value in the strain curve of DFOS, which is dependent on the spacing of the cracks. The peak strains overlap when the cracking spacing is less than 20 mm, as there is a significant strain interference between the two adjacent strain peaks. Depending on the number and location of cracks, thirteen scenarios are classified and a corresponding procedure is proposed to evaluate the crack width by considering the strain redistribution of the cracked substrate. Validation tests demonstrated that the proposed procedure reduced the relative measurement error to 6.64%. Therefore, the developed procedure improves the accuracy of crack width evaluation based on DFOS in practical engineering applications.