Kinetics of fatigue cracks in the rotor blades of the Mi-171 helicopter

Abstract

The rotor blades of the helicopter are operated on the principle of ensuring operability within the assigned resource, which is 2000 flight hours, after which they are subject to decommissioning. However, by the end of this period, as a rule, fatigue damage does not have time to propagate in the blade, so it can be operated on. Thus, the study of the process of nucleation and propagation of fatigue cracks in the blades will allow to determine the period during which the presence of cracks does not threaten the performance of the construction. This will make suggestions for a possible increase in the assigned resource of the blade, which, in turn, will lead to cost savings. The problem of the work is to study the propagation of fatigue cracks in the rotor blades of a helicopter to make recommendations on the possible increase in their assigned resource. Research objectives: development of methodology for full-scale testing of blades; determination of their endurance limit; development of a methodology for processing the results of full-scale tests (videos of the growth of fatigue cracks); assessment of the possibility of extending the assigned resource of the blades. Experimental methods of fracture mechanics and statistical methods for processing data obtained during experiments were used as research methods. As a result, it was found that the appearance and propagation of surface cracks in the blades with the test base of N = 1.6⋅107 cycles begin after the stresses exceed the level of 76.94 MPa. A fatigue crack in the blades propagates to failure within 150…170 hours, while the subcritical propagation of the crack lasts 130…150 hours. The period of stable slow propagation of cracks can be proposed for inclusion in the assigned resource of the blade.