Fan Blade Fatigue Fractures in CFM56-7B Engines

The National Transportation Safety Board investigated two accidents involving fan blade fatigue fractures in CFM International CFM56-7B engines. Fatigue cracks developed in the Ti-6Al-4V blades in an area of high contact stresses in the dovetail. Additional cracked blades were found using nondestructive inspections developed after the first accident. Although the blades had been shot peened, many of the blades had relaxed compressive residual stress profiles at the surface, making them more susceptible to fatigue crack initiation under the combined effects of loading/friction changes over time, coating condition, surface condition, and engine operational differences. Flight cycle estimates based on striation counting on the fracture surfaces revealed that blades with normal residual stress profiles had been overhauled after cracks had initiated, masking the original surface condition. As a result of investigation findings, new procedures were developed, including more frequent dovetail lubrications, improved visual inspection procedures, and new ultrasonic testing and eddy current procedures, which were implemented to reduce contact stresses in the dovetail and to detect any potential cracks that might develop in the dovetail before blade fracture.