Reconstruction of fan blade dynamic loading prior to its fracture

To maintain the functionality of fan blades, it is important to know the distribution of dynamic stresses in the blade, their amplitude and vibration frequency. Understanding of the dynamic loading pattern will allow us to determine under what conditions the engine was operated, to identify and prevent emergencies that could lead to the blade fracture. The purpose of this work is to understand the cause of the fan blade fracture that occurred during the engine ground start. Due to fractographic analysis of blade fragments it was revealed that the fracture occurred due to the initiation of fatigue cracks in blades. The place of crack initiation and parameters of crack growth were established, spectral analysis of the fracture was carried out. To establish the reason of fatigue crack initiation it was necessary to determine the dynamic state of the blades during their destruction. The sections of the second stage of stable crack growth, during which fatigue striations are formed, were determined using the fractographic method. Using the spacing of the fatigue striations and Paris's law, the crack stress intensity range was determined. Modeling of crack propagation in the blade was carried out to define the stress state. The ability to determine the stress intensity factor at each step of crack growth and its comparison with research data made this work possible. The simulation showed under what conditions manifold increase in stresses occurs and made it possible to obtain the expected value of vibration amplitude. Additional modal analysis showed a resonant form that caused the fatigue nature of crack propagation. The demonstrated approach established crack growth conditions and revealed the cause of blade fracture.