ANALYSIS OF THE ACCURACY OF FIBER-OPTIC STRAIN TRANSDUCERS INSTALLED BY USING COMPOSITE SMART PATCHES

The aim of the present work is to investigate the accuracy of fiber-optic strain transducer related to the incomplete load transmission that can occur on composite smart patches used as base support for the installation of such transducer, commonly used for structural monitoring of concrete elements. In detail, since the dimensions of the transducer are generally very small compared to those of the structural element being monitored, attention is given not to the global reinforcement effect, which occurs when the transducer alters the entire strain field of the transversal section of the monitored element, but rather to the so called local reinforcement effect, which is due to an incomplete load transmission from the analyzed structure to the transducer and results in a localized variation of the strain field, without affecting the global structural response of the monitored element. Theoretical studies carried out in this work have permitted the evaluation of a correction coefficient (C) which allows the user the correction of the measured strain or the proper transducer calibration, according to the elastic characteristics of the materials and the geometry of the structure-transducer system. The accuracy of the proposed theoretical model has been verified by FEM analysis, varying the main factors that influence the local load transmission. Successive experimental tests have corroborate the accuracy of the formula proposed for the correction of the measured strain. In summary, the results of this study showed that the use of thick patches made by high-modulus composite materials (CFRP etc.) or the use of short patches, can lead to coarse error on the strain measurement; on the contrary, for thin patches made by low-modulus composite materials (fiberglass etc.) and having sufficient base length, the error is less than 1%.