The influence of minor cycles on low cycle fatigue crack growth
Fatigue crack propagation rates have been measured for two titaniumbased
aeroengine disc alloys using compact tension test pieces. The
loading block employed simulates two features of the engine flight pattern.
A major stress cycle represents the start-stop operation which
leads to low cycle fatigue. In-flight vibrations, which may give rise
to high cycle fatigue, are represented by superimposed minor cycles of
high frequency. This combined loading is applied in a specially developed
test facility consisting of an electromagnetic vibrator mounted
above a servohydrau1ic actuator.
When the minor cycles are inactive the fractographic cracking processes
are those associated with major cycle crack growth. Once active, the
minor cycle growth may either generate extensive cyclic cleavage or increase
the separation of the fatigue striations associated with the
periodic major cycles. The contribution of the minor cycles to the
total growth rate is dependent on their relative number and size. In
gas turbine and compressor discs and blades, components which experience
large numbers of minor cycles per flight, the damage associated with
active minor cycles is dominant. Consequently, the onset of minor cycle
damage effectively determines the useful life of such components.
The threshold values associated with the minor cycles have been used to
predict the onset of minor cycle activity. Similarly the method of
linear superposition has been used to predict the subsequent fatigue
crack growth rates. These predictions are successful for Ti-6Al-4V,
whilst for Ti-5331S they are either found to be accurate or safe.
Although Ti-5331S displays a marginally greater resistance to the onset
of minor cycle crack growth, of greater significance is its reduced
crack growth rates prior to this event. As a consequence components
fabricated from Ti-5331S will exhibit longer fatigue crack propagation
lives when subjected to the conjoint action of high and low cycle