THE CHOICE BETWEEN TEXTILES AND TAPE LAMINATES
insensitivity. Around any stress concentrator, such as a round hole or sharp notch, a
damage band forms in a ductile material, redistributing stress in such a way as to minimize
stress concentration. The size of the damage band dictates the notch sensitivity. It has a
characteristic length, l
ch
, given to order of magnitude by
l
ch
= E W
f
/
σ
c
2
(3.2)
where W
f
is the work of fracture and
σ
c
is the critical stress for the onset of damage.
When the notch size, c
o
, is much greater than l
ch
, the strength of the part falls as c
o
-1/2
.
When the notch size, c
o
, is much smaller than l
ch
, the material is notch insensitive: the
strength falls only as the net section stress rises. For tape laminates, l
ch
~ 1 mm. For 3D
interlock weaves, l
ch
~ 10-100 mm [3.16].
Two factors in these high measures of damage tolerance have already been
identified. First, the large tow diameter typical of most textiles favor crack deflection and
increase the pullout lengths of broken tows that continue to carry load across the primary
fracture plane. (It is fortuitous that large diameter tows also lower production costs.)
Second, the wrapping of through-thickness tows around in-plane tows clamps the latter
together even after they have failed, raising pullout stresses to unusually high values. These
mechanisms are discussed in more detail in Sect. 4.
High work of fracture and notch insensitivity will create many opportunities for
textile composites in applications demanding exceptional damage tolerance. These
opportunities have only begun to be explored.
References
3.1 M. S. Dadkhah, J. G. Flintoff, T. Kniveton, and B. N. Cox, "Simple Models for Triaxially
Braided Composites," Composites, 26, 91-102 (1995).
3.2 B. N. Cox and M. S. Dadkhah," The Macroscopic Elasticity of 3D Woven Composites," J.
Comp. Mater., 29[6], 785-819 (1995).
3.3 P. J. Minguet, M. J. Fedro, and C. K. Gunther, Test Methods for Textile Composites, NASA
Contractor Report 4609, Boeing Defense and Space Group, Philadelphia, 1994.
3.4 P.T. Curtis, and S.M. Bishop, “An Assessment of the Potential of Woven Carbon Fibre-
Reinforced Plastics for High Performance Applications,” Composites 15[4], 259-265 (1984).
