Detection and monitoring of surface micro-cracks by PPP-BOTDA

Abstract

Appearance of micrometer size surface cracks is common in structural elements such as welded connections, beams, and gusset plates in bridges. Brillouin scattering-based sensors are capable of making distributed strain measurements. Pre-pump-pulse Brillouin optical time domain analysis (PPP-BOTDA) provides a centimeter-level spatial resolution, which facilitates detection and monitoring of the cracks. In the work described here, in addition to the shift in Brillouin frequency (distributed strains), change in the Brillouin gain spectrum (BGS) width is investigated for the detection and monitoring of surface micro-cracks. A theoretical analysis was undertaken in order to verify the rationality of the proposed method. The theoretical approach involved simulation of strain within a segment of the optical fiber traversing a crack and use of the simulated strain distribution in the opto-mechanical relations in order to numerically obtain the change in the BGS. Simulations revealed that the increase in crack opening displacements is associated with increase in BGS width and decrease in its peak power. Experimental results also indicated that the increases in crack opening displacements are accompanied with increases in BGS widths. However, it will be difficult to use the decrease in BGS power peak as another indicator due to practical difficulties in establishing generalized power amplitude in all the experiments. The study indicated that, in combination with the shift in Brillouin frequency, the increase in BGS width will provide a strong tool for detection and monitoring of surface micro-crack growths.

Distributed Fiber Optics Sensing and Coda Wave Interferometry Techniques for Damage Monitoring in Concrete Structures

“…Thus, as shown in Figure 1, direct damage detection and localization can be achieved contrary to short gauge and long gauge sensors where sophisticated algorithms are required [18]. Previous works [19,20,21,22,23] have already demonstrated the ability of DOFS to detect and localize cracks in concrete structures. However, the quantification of crack opening assessment from this kind of measurements remains a challenge that this paper aims to tackle.…”

“…Few efforts to apply this model for quantification of crack openings, either on steel [27], aluminum [23,28] or reinforced concrete structures [29], exist in the literature. However, the limitations of the used DFOS systems, in terms of spatial resolution and measurement rate, affected the accuracy of the results.…”

Surface Crack Detection in Prestressed Concrete Cylinder Pipes Using BOTDA Strain Sensors

“…When the critical stress is reached, cracking of the coating is initiated, resulting in strain singularity. The distributed FOSs investigated in [7][8][9] could detect the strain singularity induced by the cracking in a structure.…”

Distributed Fiber Optic Sensing and Data Processing of Axial Loaded Precast Piles

“…The distributed strain results provide a new insight into the characteristics of pile-soil interaction and the load transfer mechanisms [11,12]. The Pulse-Pre-Pump Brillouin Optical Time Domain Analysis (PPP-BOTDA) as a newly developed DFOS technology improves the spatial resolution to cm order, greatly increasing the sensing performance of DFOS for structural health monitoring (SHM) [13,14].…”