抄録
The corrosion fatigue properties of cast irons with various graphite structures were investigated by using a rotary-bending fatigue machine. The graphite structures of the specimens were spheroidal, quasi-nodular and flaky. Distilled water, artificial sea water and 1% aquous solution of sulphuric acid were used as the corrosive solutions. These solutions were conducted to the surface of rotating specimen to make a liquid film on its surface. From these experiments it was observed that the fatigue life in the corrosive circumstances was longer than that in the air at any stress levels higher than the endurance limit of the specimen. On the contrary, the former was shorter than the latter at stress levels lower than that limit. Consequently, any defined endurance limit could not be determined in the corrosive circumstances. The fatigue lives in the air were measured at various periods of corrosion fatigue tests. A large prolongation of life was observed with specimens subjected to corrosion for a short time, for example about one hour. The favourable effect of corrosion might be attributed to the reduction of the notch effect of graphite tips and fatigue cracks made by corroding the matrix around them. Reversely, the fatigue life decreased after a long period of corrosion because of promoting fatigue cracks by the corrosion. The change in fatigue limit occurred in the sequence of fatigue test at stress levels higher than the endurance limit of the specimen was measured. These values of cycles detemined at various stress levels were connected to each other to give a fatigue failure curve of the specimen. The formation of fatigue cracks and their propagation were microscopically observed on the surface of specimens during fatigue test. The fatigue cracks were formed at the peripheries of graphite particles and slowly propagated into the matrix and neighbouring graphites until a fatigue damage was developed. The rate of the crack propagation rapidly increased after the fatigue damage had once appeared in the specimen. The length of the cracks on the specimen surface was about two millimeters at the initial stage of fatigue damage. These cracks before the occurence of the damage were called ”not propagating crack” and had no effect upon the fatigue limit of the specimens. Here, the static tensile strength also was not weakened until the fatigue life was nearly consumed. From this fact it was considered that the depth of the fatigue cracks to be initiated was limited only to a shallow range under the surface of specimen.
