Specimens having fractured due to impact fatigue in
SIFT have been examined by light microscopy and
scanning electron microscopy. It·has been found
that initiation of impact fatigue fracture occurs
close to the surface of the SIFT -specimen. Micro
cracks appear primarily on the impact side of the
SIFT -specimen at a distance of 0. 1-0. 3 mm from
the rounded edge. The location of the initiation
varies at random around the tip of the specimen.
Micro cracks from an early stage of the fatigue
process are always radial. Examples of typical impact
fatigue fractures created in SIFT are shown in
figures 8-9. Also square specimens have been
tested in SIFT, figure 10. The appearance of the
fatigue fractures created in SIFT are very similar
to impact fatigue fracture of compressor valves.
Impact fatigue testing with SIFT should therefore be
considered relevant. Moreover, the effect of design
of specimen and seat on the fatigue resistance
can be examined in SIFT.