Fatigue Life Prediction Based On Crack Propagation With Application In Crane Metalic Structure

Crane are widely used in national economic construction as the very important equipment. In the factory floor, construction sites, ports, garages and others frequent load conditions, fatigue failures of crane metal structure are gradually highlighted. With the crane development towards the direction of the lightweight and renewable, high-strength structural steel is also widely used. Experimental studies have shown that the yield stress and ultimate stress of high-strength steel is relatively common structural steel has greatly improved, while the load varied impact of fatigue on the crane structure caused failure is greatly increased, people are concerned about the structural stiffness, strength, stability. Meanwhile, research on fatigue performance is relatively deficiency. To ensure reliable operation of cranes, not only for strength, stiffness and stability analysis, but also the need for a crane fatigue damage and remaining life analysis and forecasting. In this paper, crack in crane metal structure is the research object. Extended finite element method is adapted for determining the crack tip parameters. Fatigue crack propagation rate is studied under various factors. The fatigue life prediction of crane metal structure is discussed. The appropriate methods and strategies are also given. Crack propagation path is also simulated and the power spectrum density method is used for crane fatigue life prediction with consideration of load order.The stress intensity factor(SIF) is one of the important fracture mechanics parameters in fatigue life prediction. SIF can be obtained by the crack tip singular element in the conventional finite element method. With the expansion of the crack, the mesh in conventional finite element method must be changed with the re-division of the crack boundary and the crack propagation path must be along the mesh boundaries. The conventional finite element shows low efficiency and weakness for processing cracks and other discontinuities. Extended finite element method by adding additional nodes freedoms can deal with the problem of discontinuity with meshless. This paper presents an improved level set method for identifying and tracking the crack interface, combined with the numerical solution of crack tip stress field and the interaction integral is adapted for calculating the stress intensity factors. Although the extended finite element method has been integrated in Abaqus, there are still some shortcomings, the Matlab software is used for coding the extended finite element program for analysis the cracks and calculation of the stress intensity factor, which lays a foundation for subsequent research.The fatigue crack growth rate model must be first established for crane fatigue life prediction. Parameters of the different stress conditions in classic Paris crack growth rate formula requires number of experiments. In this paper, a new crack growth rate model is proposed with consideration of stress ratio, and also verified by variety of materials of commonly used for crane. Since the frequent starting and braking, repeated impact load caused the crane in poor working conditions, this paper studies the impact of overload effect on crack growth rate, and gives the corresponding Wheeler model parameters for life prediction using the crane industry. For the phenomenon of incomplete and irregular dimensionless similarity exists in Paris formula, based on Paris and Head formula, an exponential fatigue crack growth model is established that can degenerate to Paris and Head model. Life prediction using this model are compared with the experimental data obtained in the literature for verifing the validity of the model.For the life prediction of crane metal structure, the factors effect the life are analyed. The initial crack is a key factor to predict the life. There is no any uniform method to calculate the initial crack size in the crane industry. This paper using Kitagawa-Takahashi diagram and the threshold stress intensity factor gives a equivalent initial crack size with load-independent and its distribution. The stress intensity factors at the crack can not be expressed as crack growth rate is usually higher order function. To ensure the accuracy of solution, integration step need to be reduced, which needs a lot of numerical calculations for the integrand points. This paper introduces Kriging surrogate model to predict fatigue crack propagation life, and gives adaptive step-size algorithm based on Kriging surrogate model. The model is verified by using the materials of Q235 B and Q345 B. There is only a small part of stiffness matrix in extended finite element method needs to be updated with the new crack crack. A reanalysis method based on Cholesky matrix decomposition algorithm is given and examples are used to verify the high efficiency of the algorithm.Crane fatigue failure exists mostly in the connection parts. In order to obtain the fatigue load, the Equivalent member method is used to simplify the crane model. Several typical crane fatigue failure accidents with the actual load cases were used for simulation of crack propagation path by application of the model method presents in this paper. Crane life prediction method based on power spectral density function under impact load order is given and applied for box-girder crane fatigue life prediction, Consistent of predictions and experimental results in existing literature data verifies the effectiveness of proposed method.