Fatigue Fracture of a 1040 Steel Splined Shaft

The splined shaft (1040 steel, heat treated to a hardness of 44 to 46 HRC and a tensile strength of approximately 1448 MPa, or 210 ksi) from a front-end loader used in a salt-handling area broke after being in service approximately two weeks while operating at temperatures near -18 deg C (0 deg F). During the summer, similar shafts had a service life of 5 to eight months. Examination of the fracture surface showed brittle fatigue cracks, and visual examination of the splines disclosed heavy chatter marks at the root of the spline, with burrs and tears at the fillet area. Evidence found supports the conclusion that the shaft failed as the result of stress in the sharp fillets and rough surfaces at the root of the splines. Cold weather failure occurred sooner than in hot weather because ductile-to-brittle transition temperature of the 1040 steel shaft was too high. Recommendations include redesign of the fillet radius to a minimum of 1.6 mm (0.06 in.) and a maximum surface finish in the spline area of 0.8 microns. Material for the shafts should be modified to a nickel alloy steel, heat treated to a hardness of 28 to 32 HRC before machining.