# Prediction of Remaining Fatigue Life of In-Service Bridge Cranes
## 1 Introduction  
A bridge crane is an important tool and equipment for the realization of mechanization and automation of production processes in hosting transportation and modern industrial production. Bridge cranes play an important role in the production activities in China. With the increasingly extensive application of bridge cranes in various production departments, their safety hazards are becoming increasingly prominent. The sudden fracture caused by fatigue cracks will often cause major property loss and personal injury following the resulting bridge crane accident. Therefore, it is very important to estimate the remaining life of the bridge crane in service. The general methods to obtain a stress spectrum include the field test method [1] and simulation method [2]. A stress spectrum with high reliability can be obtained by the field test method for long-time test measurement, but the field working condition is complex, the measurement time is long, the test cost is high, and it is difficult to realize in a short time. Although the simulation method is low in cost and short in simulation time, it is difficult to align with the actual working conditions. To obtain a stress spectrum with high reliability, this paper uses the LHS sampling method in a Monte Carlo numerical simulation [3, 4] to randomly sample the measured characteristic parameters [3] affecting the stress spectrum of the in-service bridge crane. The finite element analysis software is used to build the dynamic simulation model of the bridge crane under the cycliccondition, then the stress spectrum is obtained by the rainflow counting method, the field test data is compared, and then the residual fatigue life calculation is carried out to predict the residual life. Due to the complex factors affecting the fatigue of metal structures, it is difficult to accurately estimate the work cycle of the residual fatigue life of the metal structure of bridge cranes. Currently, the fatigue life estimation methods commonly used domestically and abroad mainly include the following: damage mechanics method [5], nominal stress method [6], local stress and strain method [7; 8], fracture mechanics method [9; 10], etc. Among them, the fracture mechanics method can be used globally for fatigue crack extension life prediction [11] with cracks already present. The metal structure of the bridge crane bears less stress and requires a long service life (generally 30 years, ranging from 15 years to 50 years). The main girder of the bridge crane is of welded construction, with initial defects such as air holes and slag inclusions. Based on the damage tolerance design theory [12; 13; 14], the initial defects of the metal structure can be measured using non-destructive testing [15; 16; 17]. Fatigue testing of the crane's model beams revealed that the formation of visible cracks was very short-lived and began with the initial defects described above. Therefore, the initial defects can be considered as the initial cracks, and its life is considered as the life of the crack growth. The method of fracture mechanics is used to estimate the residual fatigue life of bridge cranes with a stress spectrum. [PERSON] [4] et al. used the linear elastic fracture mechanics method to predict the service life of an in-service bridge crane. Due to the presence of plastic deformation in the region of the crack tip produced by the bridge crane, linear elastic fracture mechanics are not fully applicable, and such cannot be calculated using only linear elastic fracture mechanics. [PERSON] [18] et al. calculated the remaining life of the bridge crane in service by using the damage tolerance design method for the bridge crane with the finite element analysis software but did not verify by comparing it with field test data, which has its limitations. [PERSON] [19] et al. calculated the service life of the crane with the root-mean-square method, but the root-mean-square method is an average algorithm.