The impact strength of diamond under different rates of strain

Abstract
Diamond spheres were fractured by impacts of durations varying from 10 +3·5 to 10 –6 s. Except at the shortest time of contact, the spheres were loaded so that the maximum stress attained during a free impact of the prescribed duration was sufficient to give an even chance of breaking at least one sphere. The maximum ‘nominal’ stresses were calculated from the Hertz theory of impact. As the duration of impact decreased from 10 +3·5 to 10 –4 s, the nominal stress at fracture increased from 6·3 x 10 11 dyn/cm 2 to 1·3 x 10 12 dyn/cm 2 . At the shortest times of impact the nominal stress fell again, reaching 7·0 x 10 11 dyn/cm 2 at a duration of 10 -6 s. The results are explained by assuming that under impacts of duration more than 10 –4 s, fracture occurs by the slow formation of ring and cone cracks, followed by crystallographic fracture. The formation of the ring and cone cracks reduces the true maximum stress below the ‘nominal’ stress. In impacts of short duration there is no time for the rings and cones to form, and cones are never observed on the broken spheres. Crystallographic fracture then occurs directly under a stress which has not been reduced by the ring and cone cracks.

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