Fractured steel gear shaft from a slag car.
AISI/SAE 1038 medium carbon steel.
The service history of the shafting was unknown. However, it was known that the gear shaft was subject to abnormal bending and torsional loads due to gear misalignment.
Examination of the fracture surface revealed the cracking had initiated at the keyway in the shaft. Closer examination of the keyway revealed the presence of cracking at the 90° machined corner in the keyway. A sharp corner can act as a stress concentration site in the keyway causing localized stresses to be as much as ten times higher than the average nominal stress. The failure of this shaft could have been avoided if the design had specified:
- A properly machined keyway with a fillet radius to reduce the stress concentration at the keyway, and
- The use of a tougher steel (i.e. able to absorb energy and deform plastically before fracturing), such as an AISI/SAE 4340 (which has an appreciable nickel content to increase the shaft toughness coupled with a higher tensile strength).