7-1
FATIGUE LIFE
None of the codes reported in this handbook provides fatigue life predictions.
Nevertheless, some of the principles by which such a prediction could be made have been
discussed in the literature; and the output of codes that calculate the distribution of stresses
in different tows could be combined with test data to make predictions at least for the flat
panel composites that are the focus of this first edition.
7.1 Kink Formation in Compression-Compression Fatigue
As discussed in Section 4, in a well-designed 3D composite delamination and Euler
buckling are suppressed in compression and failure under aligned loads occurs by kink
band formation. This is true for both monotonic and cyclic loads. The kink bands form at
locations where tows (or plies) are most severely misaligned because the topology of the
textile architecture necessitates misalignment or some irregularity has been introduced
during manufacture. Damage apparently accumulates through nonlinear processes inside
the tow in proportion to the magnitude of the axial shear stress in the misaligned segment.
It appears to be weakly correlated with features of the reinforcement architecture other than
the local misalignment angle.
Two mechanisms have been proposed by which damage may accumulate [7.1,7.2].
Cyclic axial shear may exhibit ratchetting, causing the tow to rotate to greater misalignment
angles; or accumulating plastic damage in the resin within tows may lower the effective
flow stress,
c
. In 3D interlock weaves, the absence of observed rotations before kinking
supports the latter mechanism, but this remains a topic of research. In either case, the
elapsed cycles to kink band formation can be modeled by steps analogous to those
commonly used for low cycle fatigue. For example, a law for the rate of accumulation of
damage consisting of changes in
τ
c
in some misaligned tow segment can be written [7.2]
(7.1)
for some material constants
and m, with σ
s
the local axial stress. For a fixed local cyclic
stress amplitude, ∆σ
s
, this leads to an expression for the total cycles, N
k
, to kink band
formation:
( )
. (7.2)
