This paper investigated the feasibility of the real-time monitoring of fatigue damage of steel structures and composite structrues under fatigue loading using intensity-based optical fiber(IBOF) sensors. Prior to the tests for the fatigue damage monitoring using the optical fiber sensors, the effects of the embedded optical fiber on the mechanical characteristics of the composite laminates were examined. Stiffness, strength, Poisson's ratio and matrix crack density under static tension and compression and fatigue life of composite laminates were measured experimentally. Stress and strain fields around the embedded optical fiber and the resin rich area were obtained by finite element method. The reason for the reduction of fatigue was analyzed by observing the fracture mechanism and obtaining the stress field by finite element method. From the experiments and analyses, it can be said that, generally speaking, the effects of the embedded optical fibers on the static properties such as stiffness, strength and Poisson's ratio and on the initiation and growth of the matrix cracks were not significant. However, some previous studies had reported the degradation of the strength under static compression when embedded perpendicular to the reinforcing fiber and/or the loading direction. The fatigue life was not affected so significantly when the optical fibers were embedded parallel to the reinforcing fibers, while it might be reduced when the optical fibers were embedded perpendicular to the reinforcing fibers. Therefore, it can be concluded that to minimize any adverse influence of embedded optical fiber sensors on the mechanical characteristics of the composite structures, optical fibers should be embedded parallel to the reinforcing fibers and the loading direction if possible and the loading direction should be applied in the direction parallel to the embedded optical fiber sensors if controllable.
The IBOF sensor system used in this study is relatively simple, easy to construct and utilizes relatively inexpensive instrumentation such as white light source and photodiode detector. It can also do real-time monitoring under high loading frequency because it does not require high data sampling rate. Although it has relatively low resolution in contrast to interferometric sensors, the stiffness measurement does not require precise measurement of strain but measurement of the slope of strain change with respect to loading. In contrast to other optical fiber sensors, the IBOF sensor system used in this study is a very attractive sensor when considered cost and simplicity in installation and maintenance.
The feasibility of real-time monitoring of the fatigue damage of composite laminates under fatigue loading using the embedded IBOF sensors was evaluated. The performance of the IBOF sensors was compared with a surface-mounted extensometer. The process of conversion of the detected IBOF sensor signal into the stiffness using the chain rule was proposed. Experimental results showed that the sensing response of embedded IBOF sensors showed good agreement with that of the surface-mounted extensometer. The IBOF sensor system showed good potential as a health monitoring system that can give composite structures the capability of fatigue damage monitoring by detecting the stiffness change of the composite structures under the fatigue loading. The IBOF sensors showed relatively good durability under fatigue loading compared with commercial electrical strain gages.
We evaluated the potentiality of the application of IBOF sensors to the monitoring of crack growth behavior of steel structures subjected to fatigue loading. The monitoring was based on the fracture mechanics theory of cracked structures that the growth of fatigue crack redistributes the stress field around the crack, which results in changes of compliance at a point around the crack tip. The evaluation was performed with center-cracked-tension steel specimens and a model bridge by comparing with a surface-attached strain gauge. Experimental results showed that the attached IBOF sensor could detect the compliance changes caused by the crack growth of steel structures. We also confirmed the characteristic advantages of the IBOF sensor, such as durability to fatigue and immunity to electromagnetic interference, compared to the strain gage.