Mechanistic Quantification of Microcracks from Dynamic Distributed Sensing of Strains

Abstract

Monitoring of distributed strains provides important information about the behavior of structural systems. However, detection and quantification of cracks while still at micro size levels will play a more-important role in structural-health monitoring. In addition to the early discovery of safety-related issues such as design flaws, detection of microcracks will allow for quick repairs, as opposed to later-stage infrastructure replacements and shut downs. This article focuses on the establishment of a theoretical approach for quantification of the crack-opening displacements from the measured dynamic distributed strains based on a Brillouin optical time-domain analysis (BOTDA) technique. The strain-averaging effect due to the spatial resolution, inherent in Brillouin systems, was also considered in the formulation of the method. The proposed approach takes into account the effect of cyclic loading and unloading on the accumulation of residual plastic strains in the optical-fiber coating. The experimental program involved employment of a BOTDA system in the dynamic mode for testing a 15-m length of a steel beam with two fabricated cracks. Experimental results indicated that it is possible to detect and quantify crack-opening displacements of 200 μm and larger.