The safe service life of the casting crane is related to the daily lifting load, load operation track, lifting load times, as well as daily spot inspection, regular maintenance and repair of track defects. According to the service experience of casting cranes in domestic iron and steel enterprises at present, the service life of domestic A7 casting cranes with the largest service amount of equipment is more than 20 years under medium busy conditions; Under the busy condition, the service life of the equipment is less than 20 years; Under very busy conditions, the service life of the equipment is about 15 years.
In the last 2-3 years of the safe use period, the strength index and stiffness index of the main load-bearing structural members of the equipment tend to weaken, the stability index changes very little, and the structural members’ weak strength, stress concentration, and sudden changes in cross section successively appear cracks. Therefore, the technology of risk identification, safety evaluation and life prediction in the structural health monitoring of casting cranes is closely related to the safe operation and maintenance of equipment throughout its life cycle.
The purpose and significance of structural health monitoring of casting crane is to evaluate the safety of in-service load-bearing structures, and to evaluate the safety degree or health status of the metal structure as a whole and its key parts based on on-site inspection tests, online or offline analysis of monitoring data and damage detection methods. Therefore, it is a work of high engineering value to define the health indicators, change characteristics and their correlation with the late symptoms of structural fatigue life. It is not only instructive to the risk assessment of the accumulated strength damage history and the scrapping time point of the casting crane, but also directly verifies the accuracy of the crane fatigue life estimation technology.
According to the above engineering examples, the static stiffness, dynamic stiffness and stress intensity of the structure have obvious and interrelated changes in the middle and late stages of fatigue life estimation. The camber of the main beam decreases gradually, and the structural elasticity declines gradually. The static rigidity of the main beam is increased and the rigidity is weakened. The dynamic stiffness of the whole machine is obviously reduced, and the structure is gradually flexible. In terms of strength index, the strength weak points and stress concentration points of structural members show obvious stress growth and strength attenuation according to the order of sensitivity to external load.
In the middle and late stages of fatigue life estimation, regular and paired cracking failure occurs in the weld or base metal at the mutation of the section at the stress concentration nearest to the moving load. Especially in the late fatigue life, this kind of damage has the characteristics of not easy to check, not easy to repair, fast expansion, and recursive location.