Fatigue Strength Restoration in Corrosion Pitted 4340 Alloy Steel via Low Plasticity Burnishing

Low plasticity burnishing (LPB) is a surface
enhancement process with significant economic
and physical attributes that make it attractive for
component repair/refurbishment applications in
aging aircraft. The current work addresses the
efficacy of fatigue strength restoration by
applying LPB directly to a corroded surface
without first removing damaged layers.
Compressive residual stresses of the order of
material yield strength in quenched and
tempered, 38 HRC 4340 steel were achieved via
LPB on as-corroded surfaces and sub-surface
layers. The total depth of compression was about
1.25 mm (0.05 in.).
Corrosion damage from 100 and 500-hour salt
fog exposures reduced the 107-cycle fatigue
strength respectively by about 25 and 50 percent
relative to the as machined uncorroded fatigue
strength. LPB applied to the corroded surfaces
after superficial cleaning to remove loose
corrosion product restored the fatigue strength of
the 100-hour exposed material to 110 percent of
the as-machined, uncorroded level. Fatigue
strength restoration was 85 percent in 500-hour
exposed material. Similar degrees of fatigue
strength restoration were achieved in the finite
life regime as well. Fractography revealed that
fatigue failures of salt fog-exposed specimens
initiated at corrosion pits. Fatigue failures in
LPB treated corroded specimens also initiated at
corrosion pits. Nonetheless, fatigue strengths
were greatly improved by such treatment.