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Finite Element Simulation of a Beam with Closed Crack and Breathing Crack Based on Subharmonic Detection Method



The occurrence of cracks in a structure which operate under loading conditions is a threat to structural security. Recent researches on damage detection commonly focused on breathing crack. However, most cracks in engineering structures are found to be closed crack. It is different from breathing crack that closed cracks is difficult to detect when they are closed for they can only be obviously observed when they are open. Subharmonics response is a competitive approach of detecting and evaluating closed crack but subharmonic responses may not always occur and are dependent on the input excitation conditions. The generation of subharmonic is under the effect of crack surface conditions, such as frictional force and the asperity. Two finite element models of a beam containing closed crack and breathing crack are developed, respectively. The model with closed crack make use of local node splitting and nonlinear spring elements at the crack surfaces, the modeling is based on the fact that the relationship of stress-strain is nonlinear at the crack region which varies with different crack surface conditions. And the model with breathing crack is established with contact pair which is treated as a frictional contact problem between the crack surfaces. Subharmonic in the model with closed crack will markedly increases when the conditions of excitation reaches a certain magnitude so-called threshold, while the model with breathing crack do not have such behavior at the same excitation. A comparison of the responses derived from two models using Fourier transforms for analysis is made to distinguish and investigate the conditions including frequency and amplitude on the specified excitation to the two models which will produce subharmonics.

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