ANALYTICAL METHODS FOR TEXTILE COMPOSITES
7-4
where
∆σ
r
=
η∆σ
a
is the cyclic amplitude of the through-thickness stress corresponding to
∆σ
a
and f
s
and E
s
are the spatially averaged volume fraction and axial modulus of the
stitching fibers. Since
∆Κ
tip
is independent of crack length, the delamination crack growth
rate should take a constant value, v
ss
, in this limit. Furthermore, provided there is no
delamination notch or region of failed stitches, v
ss
will be an upper bound to the growth rate
for all crack lengths. Thus Eq. (7.3) provides the basis for simple, rigorous bounding
estimates to fatigue life. Similar relations can be demonstrated for Mode II and mixed mode
fatigue crack growth.
A fatigue life strategy based on Eq. (7.3) is currently being developed.
References
7.1. N. A. Fleck and W. S. Slaughter, "Compressive Fatigue of Fibre Composites," J. Mech. Phys.
Solids 41[8], 1265-84 (1993).
7.2. M. S. Dadkhah, W. L. Morris, and B. N. Cox, "Compression-Compression Fatigue in 3D Woven
Composites", Acta Metall. Mater. 43[12], 4235-45 (1995).
7.3. B. N. Cox, M. S. Dadkhah, J. Flintoff, R. V. Inman, M. R. Mitchell, and W. L. Morris, Design
and Reliability Guide for Triaxially Braided Composites," Rockwell Science Center, Final Report
to Plastic Products Division, December, 1993.
7.4 B. N. Cox, M. S. Dadkhah, and W. L. Morris, Failure Models for Textile Composites, NASA
CR-4686 (NASA, 1995).
7.5 L. K. Jain and Y.-W. Mai, “On the Effect of Stitching on Mode I Delamination Toughness of
Laminated Composites,” Composites Sci. Tech. 51, 331-45 (1994).
7.6 T.-J. Lu and J. W. Hutchinson, “Role of Fiber Stitching in Eliminating Transverse Fracture in
Cross-Ply Ceramic Composites," J. Amer. Ceram. Soc. 78[1], 251-3 (1995).
7.7 B. N. Cox, R. Massabò, and K. Kedward, "Suppression of Delamination in Curved Parts by
Stitching," Composites, in press.
7.8 R. Massabò and B. N. Cox, “Concepts for Bridged Mode II Delamination Cracks," submitted to
Mech. Materials.
7.9 R. M. McMeeking and A. G. Evans, “Matrix Fatigue Cracking in Fiber composites," Mech.
Materials 9, 217-27 (1990).
7.10 B. N. Cox and D. B. Marshall, "Concepts for Bridged Cracks in Fracture and Fatigue," Overview
No. 111, Acta Metall. Mater., 42 (1994) 341-63.
