Stress and fatigue analysis of SVI-tested camshaft lobes

Abstract

Nondestructive evaluation techniques were employed to fully characterize three 2.3L camshafts tested in an engine simulator for an equivalent of 100,000 miles. Optical microscopy, acoustic microscopy (SAM), and profilometry were used to characterize wear and fatigue, crack depth, and surface roughness, respectively. Results show cracking to occur mainly in the opening ramp of the most abusively ground cam lobes. No clear evidence was found for subsurface cracking at depths as great as 200 μm from the lobe's surface. Profilometry results show no evidence of any major tribological effect due to the sliding friction of the follower.

Fractography studies show a difference between fracture surfaces among the cracks examined; straight cracks exhibit features resembling fatigue propagation, while fracture surfaces from pitted cracks show a more brittle behavior. Small grinding cracks (approximately 300 μm in length) were found in the opening ramps of the most abusively ground lobes prior to testing. Knoop and Nanoindenter microhardness indicate a near-surface rehardening for the most abusively ground lobe (confirmed by metallography), and temper burn for the remaining lobes. X-ray residual stress results made in the opening ramp of the tested lobes show evidence of residual stress relaxation. X-ray line width data as a function of depth does not correlate with residual stress.