Control
In applications where the applied load rather than the applied strain is constrained,
fatigue failure follows the formation of just a few kink bands. Test specimens of 3D
interlock weaves containing 200-300 mm
3
of material fail when two or three kink bands
have formed in separate aligned tows [7.2]. In stitched laminates, the first kink band to
form is usually catastrophic. The fatigue life of load controlled textile structures that fail by
kinking can accordingly be estimated by substituting a value for the misalignment angle,
φ
,
in Eq. (7.2) that is representative of the extremes of the distribution of misalignment
angles.
Equation (7.2) leads to fatigue load-life curves that are not far from straight on log-
log plots, with slope ≈ m. Typical values of m for carbon/epoxy systems are 30 [7.2]; for
glass/urethane systems, 15 [7.3]. Comparison of predicted and measured load-life curves
in carbon/epoxy 3D interlock weaves and glass/urethane triaxial braids supports the
adequacy of the power law of Eq. (7.1) and the low cycle fatigue approach.
If the material constants A,
φ
, and m are known, fatigue life thus follows from
prediction of the local axial stress, σ
s
. As long as the applied loads are sufficiently small
that widespread plasticity is not induced, which will be a design condition in an airframe,
the required prediction of local stresses can be made by one of the computer codes
presented in Section 5.
In practice, neither the constants A and m nor the distribution of misalignment
angles will be known a priori. They will be evaluated for a particular material by calibrating
tests. The life prediction method outlined above would then be used to assess the effects of
variations in load or structural geometry, provided the latter are confined to operations such
as cutting out holes that would not affect the distribution of misalignments.
In applications where strain rather than load is constrained, structural failure in 3D
kink bands. Under strain control, these materials exhibit exceptional damage tolerance in
distribution of misalignment angles into Eq. (7.2) would then be unduly conservative.
predictions should account for the gradual evolution of the population of kink bands in
