Rayleigh-based crack monitoring with distributed fiber optic sensors: experimental study on the interaction of spatial resolution and sensor type

Cracks can negatively affect the durability of concrete structures, making effective crack monitoring crucial for maintenance. Utilizing coherent optical frequency domain reflectometry, it is possible to accurately localize cracks and measure their widths. However, long distributed fiber optic sensors (DFOS) lengths, required for structural health monitoring applications, are accompanied by a reduced spatial resolution due to technical limitations of the interrogator. This study investigates the impact of spatial resolution on signal quality in areas with large strain gradients and demonstrates its implications for crack monitoring. First, tensile tests were conducted on a calibration rig to determine the measurable strain gradient for different gage pitches (gps) and further influencing parameters. Subsequently, four types of DFOS – categorized as robust and filigree – were installed on a 4 m long reinforced concrete beam subjected to a 4-point bending test. Crack widths up to 0.5 mm were measured with varying gps to assess how spatial resolution affects crack width calculations. The results indicate that a reduced spatial resolution impairs the ability to accurately record regions with large strain gradients, leading to diminished signal quality and reduced measurable crack widths. While strain gradients of approximately \(1500\,\upmu \varepsilon\) $1500\mathrm{\mu }\epsilon$ /mm could be measured with the finest gp of 0.65 mm, this threshold dropped to about \(60\,\upmu \varepsilon\) $60\mathrm{\mu }\epsilon$ /mm for a gp of 5.20 mm. For the DFOS/adhesive combinations tested, and at the highest spatial resolution available for sensor lengths greater than 50 m (gp = 2.60 mm), reliable crack monitoring of crack widths larger than 0.2 mm was not possible. This limitation poses challenges for practical applications. A method for determining the required gp based on the expected crack width and the DFOS/adhesive combination used is proposed to ensure reliable crack monitoring.