Conjoint bending torsion fatigue — fractography

The fractography of conjoint bending torsion fatigue (CBTF) has been reported in this paper for a superalloy (Nimonic 75A) material used in gas turbine engines. The fracture surface features were characteristic of bending fatigue (BF), a distinct area of CBTF, and a region between the BF and CBTF, overloading (OL). Striations were documented in the two former regions, whereas ductile dimples were noted in the latter. The CBTF fracture comprised predominately 80% or more of the total fracture surface area. However, between these two cracked areas (BF and CBTF), another region experiences overloading, resulting in the failure of flange bolts. Fatigue striations were counted for the CBTF mode and crack growth rate as a function of crack length, and crack length is presented as a function of cumulative striations.