The influence of the static I load mode and R ratio on mode III fatigue crack growth behaviour in mild steel

Abstract The influence of a superimposed static mode I load and of different mode III mean torques on mode III fatigue crack growth (antiplane shear mode) in cylindrical circumferentially notched AISI C1018 steel specimens has been studied. Increasing the static mode I crack opening by an axial load reduces the sliding crack closure influence (friction, abrasion and mutual support between the fracture surfaces). At high torsional stress levels it is possible to suppress the sliding crack closure almost completely in specimens with shallow cracks by applying an axial load; this is not the case for low torsional stresses. The crack growth rate may be correlated with an effective stress level from which the crack closure effect has been subtracted; this value is found by an extrapolation procedure. The crack growth rate is not influenced by small axial loads (a resulting KI value of 0–3 MPa m 1 2 ). However, if the axial load KI is increased to 4–9 MPa m 1 2 , the mode III crack growth rate increases. A further increase in the axial load changes the fracture mode from a macroscopically flat fracture surface to a “lamellar” fracture which influences the mode III crack growth correlations significantly. Mode III fatigue crack growth rates based on the effective load values are not influenced by changing the R values from −1 to −0.2. However, the sliding crack closure influence increases if R becomes greater than −0.2.