Page 372 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMEDICAL COMPOSITES 349

FIGURE 14.4 Failure mechanisms in a unidirectional fiber composite. (Adapted from Ref. 3.)

loads. Fiber pullout occurs because fibers do not all break at the crack plane but at random locations
away from this plane. If the pullout occurs against high frictional forces or shear stresses at the inter-
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face, there maybe a significant increase in fracture toughness. The same holds true for particulate
reinforcements, although here crack deflection is more common than bridging and pullout due to the
smaller aspect ratio. In an alumina-glass composite, as shown in Fig. 14.5, indentation cracks in
the glass matrix are deflected around the angular alumina granules in (a) but not as much in
(b), where the cracks propagate more through the granules, indicating a stronger interface but more
likely lower toughness. In laminate composites, delamination loads between the layers from in-plane
shear loads are a common failure mechanism, initiating at the free edge of the plate or a hole,
e.g., where screws would go in a carbon-reinforced bone plate.
Long-term durability of composites is a challenging and still developing area of study. Unlike
many metallic biomaterials, the static strength of most load-bearing composite materials does not
correlate well with their long-term performance, especially under cyclic loading. The reasons for this
include fatigue damage, matrix creep, and stress relaxation, as well as environmental effects at the
2
implant site. Fatigue damage and implant life are difficult to predict, unlike for metals, which have
a distinct endurance stress limit below which the material can be loaded an infinite number of times
without failure. Many composites do not exhibit an endurance limit. In addition, the loads on
implants are highly variable both in direction and magnitude and between patients, and the most
damaging loads may occur randomly as a result of accidents. Water absorption in polymer matrices
can cause swelling and have a plasticizing effect, which is problematic in dental composites and