Health monitoring for propagating crack faults

There are lots of crack faults in structures of aircrafts and other machinery, which mostly caused by fatigue, impact loads, cyclic loads etc. Effective monitoring and identification of cracks are useful to reduce or avoid the losses. To monitor the crack faults, its propagation and its effect on the structure must be investigated. There are several crack propagation models, such as the unique crack growth rate curve model, the residual strength model, the crack propagation life model, the random fatigue crack growth model and so on. To quantify the crack propagation, the natural frequency is chosen as the characterization parameter, which can indicate the crack depth and its location. Based on fatigue crack growth model, the natural frequency of the beam can be calculated with finite element methods. Finally, through numerical simulation, the free oscillation equation of a cracked beam has been solved and the numerical results of natural frequency of the cracked beam with different crack depths and locations have been obtained. Some conclusions can be drawn based on the results. (1) The depth of crack is increasing with time. The crack propagation rate is affected by the loads and other conditions. (2) Cracks can lead to the decrease of natural frequency and the natural frequency will keep decreasing when the depth of crack is increasing. (3) The deep cracks closed to fixed end will lead to the faster rate of descent of the first order natural frequency. These numerical results can be compared with other effective ones to test the accuracy and efficiency of this method. These results can be used to analyze the crack faults.