The influence of high R ratio on unnotched fatigue behavior of 1045 steel with three different heat treatments

Abstract High tensile mean stress fatigue tests for R ratios of 0.8 and 0.9 were conducted on unnotched SAE 1045 steel uniaxial fatigue specimens with hardness levels of Rc=10, 37 and 50. S–N f curves for all three hardness levels for both R=0.8 and 0.9 were very flat except Rc=50 specimens with R=0.8. At high R ratios, the fatigue strength based on S max was higher than that of for low R ratios but were lower when based on S a . Acquired ϵ-N applied curves showed that cyclic creep/ratcheting was present for all three hardness levels, however, Rc=10, and 37 had significantly more cyclic creep/ratcheting than Rc=50. Scanning Electron Microscopy, SEM, analysis showed that both Rc=10 and 37 monotonic and cyclic specimens fractured in a ductile manner with cup-cone shaped final fracture, where internal microvoid coalescence (ductile dimples) was dominant with small amounts of micro cleavage. Rc=50 cyclic specimens fractured in a brittle manner involving less cyclic creep/ratcheting. Rc 50 short life (N f 3 ) specimens were very similar to tensile and cyclic final fracture surfaces of Rc 10 and 37. The failure of these specimens was caused by cyclic creep/ratcheting rather than fatigue cracking. Rc=50 specimens with an N f >3x10 3 were the only specimens that failed due to thumbnail surface fatigue cracking. Fatigue life calculations, using S–N f and ϵ-N f models, were made to compare actual fatigue or cyclic lives with calculated lives. Both S–N f and ϵ-N f models were unsuccessful in accurately calculating the fatigue lives for these high R ratios. The unnotched fatigue resistance was less than that of mild notched (K t =1.65) specimens in 5 of 6 cases.