Fatigue Crack Initiation and Growth in Gray Cast Iron
by
James W. Fash
Department of Mechanical and Industrial Engineering
Abstract
Fatigue properties of gray cast irons are related to the morphology of the free graphite phase. Deformation resulting in the initiation of fatigue cracks occurs at low stress levels due to the local stress concentration of the graphite flakes. Formation and growth of fatigue cracks during totally reversed strain control fatigue tests of a pearlitic gray iron has been monitored using a surface replicating procedure. Cracks initiate within the first 10% of specimen fatigue lives and the development and growth of multiple crack systems has been observed. A continually decreasing peak tensile load throughout fatigue tests is shown to relate closely to the developing crack systems and is not the result of cyclic softening. Results of fatigue tests of two ferritic gray irons with differing eutectic cell sizes are also reported. Observation of decreasing peak tensile loads infers the accumulation of damage as the development of cracks. The rate of load drop is found to increase with increasing eutectic cell diameter.
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