Fatigue Strength Restoration in Corrosion Pitted 4340 Alloy Steel Via Low Plasticity Burnishing
LAMBDA RESEARCH CINCINNATI OH
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Low plasticity burnishing LPB is a surface enhancement process with significant economic and physical attributes that make it attractive for component repairrefurbishment 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.