Page 31 - Mechanics Analysis Composite Materials

16 Mechanics and analysis of composite materials

t T t

I 4

(a) (b) (c)
Fig. 1.10. Testing of a straight tow (a), tows with a loop (b), and tow with a knot (c).

Table 1.3
Normalized strength of carbon tows.

Ultimate strain, E (%) Normalized strength
Straight tow Tow with a loop Tow with a knot

0.75 1 0.25 0.15
1.80 1 0.53 0.18

I .2.2. Matrix materials

To utilize high strength and stiffness of fibers in a monolithic composite material
suitable for engineering applications, fibers are bound with a matrix material whose
strength and stiffness are, naturally, much lower than those of fibers (otherwise,
no fibers would be necessary). Matrix materials provide the final shape of the
composite structure and govern the parameters of the manufacturing process.
Optimal combination of fiber and matrix properties should satisfy a set of
operational and manufacturing requirements that sometimes are of a contradictory
nature and have not been completely met yet in existing composites.
First of all, the stiffness of the matrix should correspond to the stiffness of the
fibers and be sufficient to provide uniform loading of fibers. The fibers are usually
characterized with relatively high scatter of strength that could be increasing due to
the damage of the fibers caused by the processing equipment. Naturally, fracture of
the weakest or damaged fiber should not result in material failure. Instead, the
matrix should evenly redistribute the load from the broken fiber to the adjacent ones
and then load the broken fiber at a distance from the cross-section at which it failed.
The higher is the matrix stiffness, the smaller is this distance, and the less is
the influence of damaged fibers on material strength and stiffness (which should be
the case). Moreover, the matrix should provide the proper stress diffusion (this is the