Failure cause investigation of flange bolts from the bi-axial shaft fatigue test rig was carried out and crack growth rates in combined bending and torsion conditions were determined. Flange bolts are used in gas turbine engines that are pre-torqued exerting bending and torsion fatigue situation arising from high centrifugal stresses. The fracture surface features were characteristic of bending fatigue (BF), where striations were observed, a distinct area within which conjoint bending and torsion fatigue (CBTF) features were documented, and a region between the BF and CBTF, a region of overloading (OL), where ductile dimples were observed. Striations dominated within BF and CBTF areas. The CBTF fracture comprised predominantly 80 percent or more of the total fracture surface area. However, as these two modes progress from opposite sides, the region between them experiences overloading and fails as a result. These features were documented for each case. Fatigue striations were counted for CBTF mode and crack growth rate as a function of crack length and crack length as a function of cumulative striations were presented. Since the failure occurred with the application of 60,000 minor cycles, cumulative striation counts were 1/5 of this life. The fatigue crack nucleation stage was affected since the part contained burr marks.
(1999). Flange Bolt Failure Analysis. Journal of the Mechanical Behavior of Materials, 10 (4), 205-214.