Abstract: A method of measuring structural defects of a structure including the steps of: a) attaching to the structure at least one structural defect sensor each including a fibre optic cable in turn including a plurality of longitudinally extending optical fibres mounted in a predetermined spaced apart substantially parallel co-planar array, selected fibres having Bragg Gratings located along their length in their surfaces, one side of the array being adapted for attachment to a surface of a structure to be tested, b) coupling to one end of the fibre optic cable of each sensor a light source, and an optical sensor having time domain measuring and multiplexing capabilities, and, c) monitoring a variation of a light parameter measured by the optical detector using time domain measurement to locate the position of a crack along a fibre and using multiplexing for determining from which fibre the light source is being analyzed, and d) monitoring point strains by measuring a characteristic of light reflected from the Bragg