An Intensity-based Plastic Optical Fiber Sensor For Fatigue Damage

Fatigue is one of the most important causes of structural failure in various failure modes,also it is a major consideration in structural reliability and safety.So,fatigue monitoring of structures is of great significance.Many scholars focus on fatigue monitoring of structures and components,however,problems do exist.Developing a new type of fatigue monitoring sensor to detect the fatigue damage of structures or components or on-line long-term real-time monitoring has certain academic and engineering significance.The research background and significance of large-scale structures with long-term real-time monitoring was introduced in this paper.The research of the existing fatigue monitoring technology theory and the existing problems and ongoing challenges in fatigue monitoring were summarized.Based on the introduction of various types of fiber optic sensors,the plastic optical fiber with its good flexibility,durability and stability was used as sensor element to develop a transmissive plastic optical fiber featured and influenced by its luminous flux change.Combined with the RMS-based envelope extraction method and Hilbert band-pass filter,the fatigue life of steel specimens is monitored.In this system,luminous flux change in the plastic optical fiber serves as the sensing principle,the amount of luminous flux change is converted into an electric signal by a photoelectric converter and the real-time monitoring of fatigue crack detection of the steel structure is achieved through measurement of the electric signal and corresponding signal processing algorithm.The main research work and conclusions of the dissertation are as follows:1.The theory of optical fiber transmission and fiber loss were Introduced,and a plastic optical fiber fatigue sensor was designed and manufactured based on the principle of luminous flux variation.Thermal cutting method was used on the fiber to ensure the flatness of the optical fiber cutting end face.To protect and position the optical fiber,the processed PTFE tube was used as a housing.To realize sensing,fiber was placed in accordance with certain rules.2.The testing experiment was carried out to test the accuracy and range of the sensor.The results showed that the fatigue sensor can be applied to the fatigue crack monitoring which had a good linearity in the 20mm spacing range.Moreover,by analyzing the sensor structure parameters,it was found that the sensitivity of the sensor is in inverse proportion to the core diameter of the plastic optical fiber.It is also proved that the sensor has good adaptability and stability under different temperature conditions by measuring the distance tests.3.Through the fatigue test of steel components,the optical fiber fatigue sensor is verified,the plastic optical fiber sensor is installed on the surface of the test piece,the crack initiation detection and subsequent monitoring of crack propagation was achieved combining signal processing and a comparative test was conducted by using the measuring instrument of the existing acoustic emission(AE)sensors and crack opening displacements(COD).The results showed remarkable resemblance in terms of crack initiation and propagation identification exhibited by all three types of sensors,which demonstrated the availability of the plastic optical fiber fatigue sensor.