ばね鋼の疲労強度に影響を及ぼす諸因子の定量的評価 : 介在物, ショットピーニング, 脱炭層, 微小表面ピットの影響

概要

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Effects of nonmetallic inclusions, shot peening, the decarburized layer and a small surface pit on automobile suspension spring steels are investigated. The effects of all factors can be evaluated from the viewpoint of Murakami et al.'s fatigue strength prediction equation which predicts the fatigue strength with the Vickers hardness Hv of the matrix, the square root of the projection area of an inclusion √<area>, and the stress ratio R. Shot peening influences the fatigue strength through the residual stress, i.e., through the stress ratio and the surface roughness. The decarburized layer has low hardness and its hardness hardly changes with shot peening. A surface pit lowers the fatigue strength through its size √<area>. There are two possible fatigue fracture origins due to the hardness variation and residual stress distribution. One is at the surface and the other is at the interior, 300〜400 μm below the surface. The lower bound of a scatter of the fatigue strength of a definite number of specimens can be estimated from the prediction equation with the maximum size of inclusion √<area>_<max> which is predicted by the statistics of the extremes of inclusions.
1991-10-25