Abstract: At present, the safety of structures, such as aeroplane et al, is mainly achieved through fatigue design and damage tolerance design. But the service life is hard to predict precisely in the design stage because of the difference between the loading spectrum in design and the loading spectrum in service. For the safety of the structures in service, the three-dimensional fracture theory based on-line damage monitoring system is developed. The loading spectrum, real-time monitored by sensors, is analyzed with three-dimensional fracture theory and the damage propagation is computed. Field bus and Ethernet are used in the system. The field bus is suitable for less data transmission and high real-time requirement, while Ethernet is suitable for more data transmission and relatively low real-time requirement. According to the requirement of multipoint monitoring for large scale structures, two distributed monitoring systems respectively based on field bus and Ethernet are proposed. The working principle and technology feasibility are verified by monitoring the fatigue crack propagation of standard specimens under constant amplitude loading.

Key words: Ethernet, Fatigue crack propagation, Field bus, On-line monitoring system, Three dimensional fracture theory