Page 94 - Mechanics Analysis Composite Materials

Chapter 3. Mechanics of a unidirectional ply 19

3.2.4.Fracture toughness
Fracture toughness is a very important characteristic of a structural material
indicating resistance of a material to cracks and governed by the work needed to
destroy a material (work of fracture). As known, there exist brittle and ductile metal
alloys whose typical stress-strain diagrams are shown in Fig. 3.25. Comparing
alloys with one and the same basic metal (e.g., steel alloys) we can see that while
brittle alloys have higher strength, 5, ductile alloys have higher ultimate elongation,
E, and, as a result, higher work of fracture which is proportional to the area under
the stress-strain diagram. Though brittle materials have, in general, higher strength,
they are sensitive to cracks which propagating can cause material failure under
stress that is much lower than the static strength. That is why designers usually
prefer ductile materials with lower strength but higher fracture toughness. A typical
dependence of fracture toughness on static strength for metals is shown in Fig. 3.26
(line I). For composites, this dependence is entirely different (line 2) - higher static
strength corresponds usually to higher fracture toughness (Mileiko, 1982). This
phenomenon is demonstrated for a unidirectional boron-aluminum composite in
Fig. 3.27 (Mileiko, 1982). As can be seen, an increase in fiber volume fraction, of,

Fig. 3.26. Typical relations between fracture toughncss (K) and strength (3)for metals (1) and
composites (2).

0
0 0.1 0.2 0.3 0.4 OB
Fig. 3.27. Dependence of static strength (I), work of fracture (2),and fatigue strength (3) on fiber volume
fraction for a boron -aluminum composite material.