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Effect of Surface Treatments on HCF Performance and FOD Tolerance of a Ti-6Al-4V Vane

From Lambda Technologies
 

 

Mechanical surface treatments including shot
peening (SP), laser shock peening (LSP) and low
plasticity burnishing (LPB) have been shown to
introduce compressive residual stresses that
improve high cycle fatigue (HCF) performance and
foreign object damage (FOD) tolerance. Shot
peening has been widely used to introduce a
shallow (<250 mm) beneficial compressive layer, at
the expense of roughening and heavily cold
working the surface. Minimizing cold work during
surface enhancement has been shown to improve
both thermal and mechanical stability of the
compressive layer. Compression deeper than
achievable by shot peening has been shown to
dramatically improve FOD tolerance in Ti-6Al-4V.
The effects of low plasticity burnishing (LPB) on the
HCF performance and FOD tolerance of a first
stage Ti-6Al-4V turbine engine vane are presented.
The loading conditions of the vanes were simulated
using a bending fatigue specimen with appropriate
geometry. Actual vanes from fielded engines with a
current 50 mm (0.002 in.) FOD limit were fatigue
tested in cantilever bending mode using specially
designed test fixtures. FOD was simulated with
controlled depth electrical discharge machined
(EDM) notches. Residual stress and cold work
distributions were measured using x-ray diffraction
techniques.
For both vanes and vane simulation specimens
with 500 mm (0.020 in.) deep FOD, the HCF
strength at 1x107 cycles after LPB is over 4 times
higher than the unprocessed counterparts. FOD up
to 2500 mm (0.10 in.) deep was tested. The HCF
performance was largely unaffected by FOD up to
750 mm (0.030 in.) deep. Even FOD up to 2500 mm
(0.10 in.) depth decreased the fatigue strength only
nominally. If traditional design criterion of Kt = 3 is
used, the LPB processed vane could be
considered tolerant of even 2500 mm (0.10 in) deep
FOD. The HCF life improvement and FOD
tolerance are shown by linear elastic fracture
mechanics modeling to be due to the deep
compressive layer produced by LPB.

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Topics of Interest
High cycle fatigue (HCF) strength and the resistance to foreign object damage (FOD) can be improved by the use of mechanical surface treatments like shot peening and low plasticity burnishing (LPB) to... (Read More)
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