offatigue
loading, viz. bending fatigue and impact fatigue.
Bending fatigue implies manageable problems for
the following reasons: easy to test in commercially
available fatigue testing machines, steels with high
bending fatigue limits are available and bending fatigue
failure may be avoided by designing the valves
so that the bending stresses are minimized.
Impact fatigue of valves is caused by repeated impacts
against the seat and characterized by the
tearing off of chips from the edges. Impact fatigue
is more difficult to control, for many reasons. The
phenomenon is not so well known as bending fatigue.
Impact fatigue testing machines are not commercially
available. To some extent, experimental work
may be carried out in compressors but the disadvantages
are obvious. It is difficult to measure the
impact intensity inside a compressor and the testing
is slow. There are only limited possibilities to
vary the impact intensity.