Fatigue Life Analysis and Tensile Overload Effects with High Strength Steel Notched Specimens

Fatigue crack growth results are presented for a series of tests of high strength steel notched-bending specimens. Eight values of stress concentration factor from 1.5 to 4 were represented in the tests, as well as six forging procedures with yield strengths from about 1000 to 1200 MPa. The cyclic lives of the specimens, ranging from about 2000 to 100,000 cycles, were analyzed, using fatigue stress range calculated from stress concentration factor and from a fracture mechanics method. A statistical comparison of the two methods was performed. Photoelastic and finite element methods were used to obtain some of the notch root stresses. The effects of a single prior tensile overload on fatigue life were considered for many of the tests. There appeared to be critical ratio about unity of cyclic stress range at the notch root relative to yield strength, below which a tensile overload extended fatigue life and above which a tensile overload shortened life. The effects of prior thermal overload on fatigue life were also investigated in five tests of two specimen geometries. Rapid cooling of the outer diameter of a hollow disk segment of a cylinder extended the fatigue life in subsequent cyclic bending testing of the segment.