A special testing device has been developed - Sandvik
Impact Fatigue Tester, SIFT. The specimen is
similar to a reed valve and is set in motion by short
duration compressed air pulses. The specimen
thickness generally is 0. 381 mm (. 015") which is a
standard thickness for valve steel. The shape of
the specimen and seat is presented in figure 1. The
specimen is clamped in the specimen holder. The
cantilever part of the specimen is 50 mm.
The specimens are blanked out longitudinally in
relation to the strip and are ground to the correct
dimensions and shape. Finally their edges are hand
polished, especially at the semicircular end, so as
to give an evenly rounded edge and to eliminate any
grinding marks. Normally no preparation of the
surfaces of the specimens is done.
In order to generate short duration compressed air
pulses a rapid valve is needed. In an ordinary valve
the speed is limited by the inertial forces of the mechanical
components. The rapidity can be improved
fractured when the fatigue Test is delibera;:e~y d:.scontinued
and are marked w:.th open symbols. The
result has been treated statistically and S-N
curves corresponding to 10 and 50% probabili~y of
fracture have been calculated.
Considering the general shape of the S-N curves it
is evident that the curves level off at approximately
107 cycles. It can be concluded that tnese sceels
show fatigue limits when they are subjected to repeated
impacts. It can also be concluded that l 0 7
cycles is a proper criterion o£ runout for this type
of loading.
Comparing SANDVIK ZOC and 7C27Mo2 with reference
to impact fatigue strength it is evide:J.t that
SANDVIK 7CZ7MoZ is superior to ZOC. This is in
accordance with p~·actical experience for these
valve steels.