Published: Jan 2005
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THE COMPACT SPECIMENS OF FIG. 7 AND THE DISK SHAPED SPECIMEN of Fig. 8 are loaded using pin and clevis fixturing. The clevis design is shown in Fig. 10. For compact specimens, the load line is offset from the approximate specimen hinge point (located near mid-ligament) by about 3/4W. Hence, the specimen arms rotate open as the load is applied. The small flat surfaces at the bottoms of the clevis holes provide low friction sliding surfaces for the pins as the specimen arms rotate. The direction of sliding is forward, away from the crack tip, so that the starting position for the pins should be to the rear of the holes. Clevises should be made of ultra-high-strength steels such as maraging steel or high quality tool steels. Hardness of the clevis material should be 40 HRC or better. On the other hand, use of clevises made from ultra-high-strength steel is not recommended for pre-cracking, due to the sensitivity of ultra-high strength steels with respect to fatigue crack initiation.