Strain Transfer Of Optical Fiber Under Damage Conditions And Its Application In Multi-layered Pavements

For the multimedia of component materials and the complex of interfacial contacting conditions, the behavior and damage evolution of pavement structure is difficult to be accurately reflected by mechanical theory and finite element simulation, and the lab tests are confined to modal scale and simplification of loads. Therefore, it is important to employ optical fiber sensing technology to accurately monitor the inner stress and damage state of pavement and effectively assess the structural behavior of pavement in field, which can instruct the maintenance and design of pavement. To improve the capacity of durability of optical fiber based sensors in the long-term detection of engineering structures. the packaging technique is indispensable. For the existence of middle layers, such as the protective layer and adhesive layer, the strain measured by the sensor cannot entirely indicate the actual strain of host material. Strain transfer theory is thus developed to establish the quantitative relationship of strains between the sensor and host material, which aims to eliminate the strain transfer error and increase the testing precision of the industrialized sensors so as to reflect the practical strain of structures. In this paper, theoretical analysis on strain transfer of the optical fiber based sensors applied to common structures and multi-layered pavements with the consideration of various damages considered has been conducted. Optimization design methods and superiority indexes have been put forward based on the proposed strain transfer theory, and then the applicable optical fiber based sensors have been developed to satisfy the testing requirements of complicated pavement structures. Furthermore, distributed variable gauge-length based testing method has been presented to realize the full-scale smart monitoring of long-span pavement. Finally, the application of these smart sensors and monitoring methods in urban asphalt pavements has been carried out and testing data in much long time has been collected to assess the performance of the pavements. The main content of this paper includes,(1) According to the difference of host materials, damage and loading loads, theoretical analysis on strain transfer relationship of optical fiber sensors embedded in or bonded on the surface of structures has been conducted, and the cases that elastic-plastic and plastic characteristic of host material and the model with cracks and interface debonding have been considered. The corresponding strain transfer error modification functions have been provided. Based on the theory at elastic stages under static and dynamic conditions, the sensitivity analysis of parameters that influences the strain transfer coefficient has been carried out.(2) For the special construction technique and damage characteristics of asphalt pavement, theoretical analysis on strain transfer of quadrate-packaged FBG sensor and asphalt mastic packaged optical fiber sensor has been conducted. The cases that viscoelastic and creep effect of host material and crack in host material on the strain transfer mechanism of the embedded distributed sensor has been investigated, with the corresponding strain transfer equations for error modification provided. Based on the derived strain transfer theory, the interfacial debonding between two-layered structure has been detected indirectly.(3) According to the different paths for transferring deformation, the embedded and surface-bonded sensors have been classified systematically and the principle of mechanical action has been explained. Based on the strain transfer analysis, the optimization design method of industrialized optical fiber sensors has been proposed, with superiority indexes and suggested. Furthermore, various novel FBG and distributed optical fiber sensors have been developed to meet the complicated inner stress detection of multi-layered pavement structure, and the applicable of these sensors in engineering has been verified by basic tests.(4) Utilizing the developed sensors and considering the multi-scale evolution feature of damage of long-span pavements, the variable gauge-length based testing method for damage detection in the whole process has been put forward, and the multi-scale sensing technology based on the combined optical fiber and FBG sensors have been adopted for the simultaneous monitoring of the integral and local structural behavior. The availability and feasibility of the proposed testing techniques have been validated by laboratory tests.(5) Based on the theoretical analysis, method descriptions and experimental validations mentioned above, the developed optical fiber sensors have been embedded in multi-layered asphalt pavement to implement the much long-term detection. Performance assessment of asphalt pavement has been carried out based on the collected effective data in field and some suggestions on the optimization design of pavement structure have been given.