Abstract
In this thesis, CFM56-7B turbofan engine recent accidents were researched, as well as possible defects of the fabricated front fan blades via conventional and optimized manufacturing methods, with special focus on the effect of mechanical fatigue on the front fan blade as a possible failure cause. Assuming that the material of the fan blades was Ti-6Al-4V (Grade 5) and the fan rotational speed was 4000 rpm, computer aided design (CAD), theoretical calculations, stress and fatigue simulated analyses, laboratory testing, and microscopic examination were conducted on an un-notched and notched blades for understanding the effect of dominant stresses on microstructural dislocation growth in the blade and the failure mechanisms that may lead to catastrophic blade failure while in-service. Simulation solution of the modeled blades yielded that the maximum total deformation occurred at the cross-section tip of the blade leading edge. It was 70.0 mm for the un-notched blade and 69.6 mm for the notched blade (average). The notch location had trivial effect on the blade total deformation value. Also, for all blade models that were simulated in this study, the maximum equivalent alternating stress was below Ti-6Al-4V (Grade 5) yield strength. It was found to be 694.0 MPa for the un-notched blade and 662.7 MPa for the notched blade (average). It occurred at the blade edge close to the tip by the leading edge. The maximum normal stress for all blades that were simulated was 420.7 MPa for the un-notched fan blade, and 437.0 MPa for the notched fan blade (average). It occurred at the edge of the blade between the blade root and the propeller disk. The highest normal stress value was for the notched blade-scenario1. For the notched blade scenarios that were simulated, notched blade-scenario1 also had highest maximum values of equivalent (Von Mises) and equivalent alternating stresses on the blade, which an indication that the notch proximity to the root of the blade directly influences these stress values on the blade, and accordingly increases the blade susceptibility for fatigue failure which may shorten its life. From simulations outcome it has been concluded that, except for the points of singularities where the equivalent stress is at maximum, the blade has an infinite life at 4000 rpm rotational speed. Laboratory testing outputted an infinite life for the unnotched Ti-6Al-4V specimens, as they did not experience fatigue fracture when tested to number of cycles to failure greater than 1×108 cycles. In the other hand, the notched specimens broke at 3×106 cycles, approximately. Microscopic examination showed brittle-appearing fracture with clear fatigue striations marks, voids, and cracks in the microstructure of the fractured notched specimens.