The object of this work was to investigate the relation between fatigue strength and number of pin-holes of metal specimens (the latter depended upon the gas content of the molten metal). The specimens were prepared from molten Lo-Ex having various contents of gas, which had been determined by reduced pressure solidification test. The number of pin-holes was counted by improved color check test. In the fatigue test, the number of cycles before rupture under ±12kg/mm
2 of repeated tension and compression stress was counted with Vibrophore type machine.
The results obtained were summarized as follows.
(1) When the gas content in the molten alloy increased, the number of pin holes on the surface of the specimen increased and the fatigue strength remarkably decreased; whereas, the tensile strength was scarcely affected by the gas content.
(2) The fatigue strength was high when the specimen had only a few (not more than 3-4/cm
2) pin-holes and the majority (more than 1/2) of such specimens represented endurance limit of over 10
7 cycles.
(3) The fatigue strength remarkably decreased when the specimen had many (not less than about 4/cm
2) pin-holes and very few of such specimens represented endurance limit of over 10
7 cycles.
(4) When the endurance limit was not over 10
7 cycles, the relation between the endurance limit and the number of pin-holes was approximately expressed by a logarithmic linear function.
(5) In the practical work of melting and casting, it was found that fluxes containing moisture reduced fatigue strength owing to the increase of gas content in the molten metal.
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