Optical fiber sensors for long- and short-term crack monitoring

This paper describes a fiber displacement sensor and analyzes its performance in evaluating the evolution of cracks for structural health monitoring applications, both over short and long time periods. The sensor is based on plastic optical fibers, a choice that allows having all the advantages typical of optical fibers, but with costs comparable to the more common electro-mechanical systems. Two sensor topologies have been realized and some experimental results about laboratory and in field tests are reported to assess the effectiveness of the proposed solution.

I. Introduction

Cracks in buildings are a natural phenomenon that has several causes, such as movements in the foundations, thermal expansions and structural deformations due to heavy loads, not to mention more disruptive phenomena such as earthquakes. In general, the simple presence of cracks cannot be considered as a symptom of serious structural damages, although only the analysis of their evolution in time can provide a definitive answer about the building structural health. Hence the increasing importance of crack monitoring systems as a tool to detect risky situations. In particular, long-term crack measurement campaigns allow evaluating the structural average deformation and they are typically carried out over a period ranging from few days to several months. On the contrary, short-term measurements of the crack changes are required when evaluating the building dynamic response to stresses, such as natural phenomena (strong winds, flooding, earthquakes, etc.) or occupant's activities (e.g. in industrial plants where heavy machines produce vibrations) [1].