FATIGUE LIFE
7-3
individual tows, calculating the effects of load redistribution following each new kink
event. The code BINMOD is currently being enhanced to perform such calculations.
7.2 Tension-Tension Fatigue and Load Ratio Effects
Computational models relating fatigue life to textile architecture have not yet been
developed other than for compression-compression loading. Limited data suggest that
damage rates during a tensile cycle are much lower than during compression cycles (e.g.,
[7.4]). The most dangerous mode of damage is presumably shear plasticity leading to
kinking, which is a compression failure. Tensile failure requires fiber rupture, which is
brought about relatively slowly by fatigue.
Load ratio effects on the fatigue life of textiles remain a subject for research.
7.3 Delamination Crack Growth in Quasilaminar Textile Composites
The initiation of delamination fatigue cracks in a 3D quasilaminar composite, such
as a stitched laminate, will be similar to initiation in a conventional tape laminate. Initiation
sites will include delaminations caused by impact and points of stress singularity where
anisotropic plies meet a free edge. However, propagation following initiation will be very
different, with through-thickness reinforcement playing a strong and crucial role.
Delamination cracks may then propagate with various mode mixtures, from pure
Mode I under out-of-plane loads in certain symmetric specimens or parts to pure Mode II
under bending loads. Through-thickness reinforcement will supply bridging tractions
across the delamination crack, shielding the crack tip from the applied load. The mechanics
of bridged delamination cracks in polymer composites have been extensively studied in
recent years, especially in reference to stitched laminates [7.5-7.8]. If the bridging
reinforcement (stitching) remains intact, a steady state configuration will be reached when
the crack is sufficiently long in which the net crack tip stress intensity factor is independent
of the crack length (e.g., [7.9,7.10]). For cyclic loading, application of the J integral yields
an analytical relationship between the applied stress range,
∆σ
a
, and the range of the net tip
crack intensity factor,
∆
K
tip
. For example, for a delamination crack growing in the curved
stitched laminate shown in Fig. 4-8, where the bridging tractions follow a linear law, one
has [7.7]
∆
K
tip
= [E´h/F
s
E
s
]
1/2
∆σ
r
, . (7.3)
