Page 151 - Biaxial Multiaxial Fatigue and Fracture

136 R.€? KAUFMAN AND ZH. TOPPER

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07 I

amplitude planes combined with alternating shearing stresses on asperities, their height and the
fracture surface shape were examined with a stereographic microscope, a scanning electron
microscope and a confocal scanning laser microscope. Changes in the amount of interference
between the crack surfaces with varying magnitudes of the static mean stresses were deduced
from asperity height and fracture surface appearance. The length of the fatigue crack before the
onset of fast fracture was also determined. In the presence of a large tensile static mean stress,
the fracture surface profiles revealed minimal crushing or deformation of the asperity tips.
Consequently, all or almost all of the shearing force was transmitted directly to the crack tips,
and the crack growth was assumed to be essentially interference free. As static mean stresses
decreased from the interference free static mean stress level, there was an increase in the
deformation of the asperities tips and a reduction in asperity height. These features suggest that
there was a reduction in the shearing forces acting at the crack tips. This is consistent with the
observed increase in fatigue life with decreasing mean stress.
The fracture surfaces for the hard and as received steels listed in Table 4 were obtained from
specimens removed from the fatigue rig when a through thickness crack was detected. For the
tests conducted during this program, the number of cycles until the detection of a through
thickness crack was recorded as the fatigue life. The crack location was ascertained using a
stereographic microscope and a scanning electron microscope. The area encompassing the crack
was separated from the specimen by plasma cutting around the initiation point of the fatigue
crack and dipping the piece in liquid nitrogen. Pliers were then used to pull the fracture surfaces
apart ensuring that the morphology of the fracture surfaces was not damaged. Table 4 lists the