Use of Fiber-optic Strain Sensors and Holder Exponents for Detecting and Localizing Damage in an Experimental Plate Structure

The vibration-based structural health monitoring paradigm is predicated on the practitioner's ability to acquire accurate structural response data and then to use that information to infer something about the structure's health. Here the authors combine advances in both sensing and signal analysis and demonstrate the ability to detect damage in a simple experimental structure. A distributed network of nine fiber-optic strain sensors is used to acquire time series data from a rectangular steel plate where damage is considered as a cut of varying lengths. Both the sensors and the associated optical hardware are described. A new feature, Holder continuity, is then introduced as a means of identifying the presence and location of the cut length. This particular metric is derived from the field of nonlinear dynamics and is based on a phase space description of a structure's dynamic response. Specifically, the authors compute the Holder exponent which quantifies the differentiability of the functional relationship between an ‘undamaged’ and a ‘damaged’ structural response. As damage is incurred, this relationship is expected to degrade. Both univariate and multivariate applications of the method are presented. The metric shows sensitivity to damage comparable to that exhibited by the plate's modal frequencies, a traditionally used feature in health monitoring applications.