保証試験の研究

“等価使用時間” で表された除荷時間

抄録

Proof test is useful for rejecting ceramics of undesirable short life time. But the crack in the proof test survivors propagates in the unloading period of the proof stress. So the shortest life time of the survivors may be shorter than the needed life time. In this paper, the influence of unloading time on the shortening of the life time of the proof test survivors was studied on the basis of fracture mechanics of ceramics. Some formulae are derived for expressing the shortened life time of the survivor during unloading, the ratio of the shortened life time to the needed one, and the longest unloading time allowable to make the shortest life time of the survivors longer than the needed one. According to these formulae, the unloading time gives substantial influence on the life time of the survivors from the point of view of the shortening effect for the life time. The formulae expressing the ratio of shortened life time to the needed one tells that the unloading time must be of the order of 10μs to make the shortest life time negligibly small compared with the needed one, when the proof stress is selected so as to be the value which corresponds to the needed life time. In an actual application of proof test to ceramics, some safety factor is multiplied to the proof stress, so the safety factor is studied in correlation with the unloading time. According to the study, the unloading time can be increased in proportion to Δp/(1+Δp)², where (1+Δp) is the safety factor. Then, the unloading time reaches the maximum at the value of 2, in an approximation. But even when 2 is selected as the safety factor, the unloading time must be of the order of 5-10μs to make the shortest life time of the survivors equal to or longer than the needed value. Such a short unloading time seems to be inapplicable to the test. Therefore, it can be concluded that the complete elimination of the ceramics having undesirable short life time needs another complementary method such as AE monitoring for proof test.