Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 59

“backbones” linked together with a few covalent bonds (“cross-
links”) and many van der Waals bonds. Thus, they are generally
weak and ductile but may be very tough, especially if the mol-
ecules are very long (of high molecular weight) and are tangled
together or covalently cross-linked.

relationship of
interatomic bonds
to properties of
materials

Elastic behavior depends very strongly on the nature of the most com-
mon bond in a material. Thus, ionically bonded materials (ceramics)
are generally stiffer than metallically bonded ones (metals) and they in
turn are stiffer than covalently and van der Waals-bonded ones (poly-
mers). Furthermore, the stiffness of a material is very poorly related to
details of its chemical composition. Thus, all types of stainless steels
(see Chapter 7) have essentially the same Young's modulus, despite wide
variations in the alloying elements.
Ductile behavior is much more difficult to predict. Metals are gener-
ally ductile since layers or planes of metallically bonded atoms can move
over each other. Polymers are also ductile, resulting from the relative ease
with which van der Waals bonds can be broken and remade and cova-
lent bonds may rotate about their axes. Ceramics, on the other hand, are
brittle, since one atomic spacing deformation between adjacent planes of
atoms produces repulsive rather than attractive forces. However, details
of structure, including inhomogeneities in metals and physical arrange-
ment (intertwining) and cross-linking in polymers, can radically affect
ductility. (These topics will be discussed at greater length in Chapters 6
t h rough 8.)
However, the problem of strength is more general. Like elasticity, it
should depend directly on the nature of the most common interatomic
bonds in any solid. However, in the vast majority of cases measured,
strengths are one-tenth to one-hundredth those that are expected, on the
basis of the properties of the interatomic bonds involved. This disparity
between theory and experience has two principal origins.

1. At an atomic level, well-ordered solids, such as metals, may pos-
sess “defects” in the planes of atoms. These may be missing atoms,
extra atoms, folds, steps, and so on. Although some of these defects
interfere with slip, others, such as extra atoms or extra partial
planes of atoms, make slip easier than predicted and the materials
will fail when the strength of their weakest portion is exceeded.
This is a common effect in metals. However, polymers are gener-
ally not sufficiently ordered for slip to be an important deformation
mechanism.

2. At a molecular level, and on a somewhat larger scale, real mate-
rials usually are not ideal, containing cracks, voids, and so on.
As discussed in Chapter 2, these discontinuities produce signifi-
cant stress concentrations, causing apparently premature failure
resulting from local stresses far exceeding those determined from
external loading and dimensions. At the atomic level, cracks may
be extremely “sharp,” producing stress concentration factors of
as great as 100. This effect is very obvious in brittle materials,