Fatigue behaviour of BS 2L65 aluminium alloy pin - aerade

22
~ETHOD OF LIFE ESTIMATION ALLOWING FOR RESIDUAL STRESSES
Section 4.2 described the construction of Figs 26 and 27 which present the
local stress histories in bushed and unbushed lugs under constant amplitude
loading. This, in combination with the relevant S-N curves enabled the local
mean-local alternating stress diagram of Fig 43 to be drawn for bushed lugs. It
was decided to construct the diagram for the case of passivated bushes, since
this is likely to be the situation met in practice. Fatigue results were avail-
able at only the low mean stress for passivated bushes but at both low and high
mean stress for non-passivated bushes. However it was found, in section 4.1 that
zt the lower mean stress passivation had little effect except at long lives where
~t reduced the fatigue strength. As the higher mean stress reduced the fatigue
strength at long lives to a very low value, see Fig 11, passivation could have
had very little further effect and thus it was assumed that the S-N curves for
both passivated and unpassivated bushes at the higher mean stress were identical.
It will be noted that a Goodman-type straight line relationship was assumed on
the mean-alternating stress diagram.
Having plotted the mean-alternating stress diagram new S-N curves could be
constructed at any new local mean stress. The local mean stresses for the random
spectra tested were obtained from the local stress measurements, see ~igs 29-33,
and new S-N curves were constructed for each random spectra case, which were used
with the appropriate spectrum measurement to calculate the fatigue life allowing
for residual stresses.