Fatigue Crack Propagation Near Threshold under Combined Torsional and Axial Cyclic Loading.

Abstract

Near-threshold fatigue crack propagation tests were performed on circumferentially precracked round bars of a medium carbon steel under torsional loading or tensile-torsional loading. The applied stress ranges expressed in terms of the stress intensity ranges of mode I, ΔKI, and mode III, ΔKIII. were divided into three levels : no-crack growth, crack arrest, and fracture. The threshold condition of crack extension from a pre-crack is expressed by the following elliptic function : (ΔKI/ΔKI th)2+(ΔKIII/ΔKIII th)2=1 where ΔKI th and ΔKIII th are the threshold stress intensity ranges of crack propagation for mode I and III, respectively. The threshold condition for fracture is also expressed by another elliptic function. At the intermediate stress ranges, the propagation rate of initiated cracks decreased with crack extension because of the contact of the crack faces. Scanning electron microscopic observation showed the flat fatigue fracture facet made coplanar to the precrack plane at stress level just above the crack extension threshold value for any mode ratio. When the stress level was higher than threshold value and when ΔKIII component was predominant, the fatigue fracture surface turned to be factory-roof type after a small flat extension of a crack.