: In grinding- that a rotating grinding wheel of high speed is used as a tool, the heaviest hazard results from a bursting accident of the rotating grinding wheel.
    Kinetic energy of a rotating grinding wheel is proportion to the mass and a square of rotating speed of the wheel.
    And most of grinding wheels in practice are used in very high kinetic energy state, since the operating speed and size of the grinding wheels are increasing recently. Therefore when the above grinding wheel bursts away in use, it is assumed that damage of the hazard from that will expand greater for men and facilities. In order to prevent like that hazard, one of the most important and fundamental subjects is to investigate on strength of grinding wheels scientifically.
    Recently in Japan, an out put of resinoid wheel is almost twice of it of vitrified wheel. However, it is regretful that, in spite of the large number of reports on strength of vitrified wheel, only a few studies have been done on strength of resinoid wheel. Especially, research reports on strength behavior of the resinoid wheel under repeating load have apparently not been published to date.
    And then we investigated about one case of the strength behavior of resinoid wheel under repeating load.
    In this study, diametral compression loading method as shown in Fig.4 is used in order to apply repeating load on specimens. Specimens used in this experiment are resinoid grinding wheels of grain A, grain-size 20, grade P and some vitrified wheels of grain WA, grain-size 60, grade K as shown in Fig.5.
    Experimental result of static loading test of the diametral compression method is shown in Fig.8, and it of repeating loading test of the same method is shown in Fig.9 and Fig.10.
    It is confirmed experimentally that the result of repeating test of the resinoid wheel dose not depend only on effect of time-dependence (delayed fracture) based on our experimental result of static fatigue test that dead load applies on the resinoid specimen for 48 hours.
    We conclude from the experiment described above that the strength of resinoid wheel under repeating load depends upon the cycle numbers of repeating load up to fracture and the resinoid wheel has sharper fatigue behavior than it of vitrified wheel within the limits of this experiment.