Fixation - Orthopaedic Biomaterials in Research and Practice - page 326

1000

Direct

100

Graded

10

?

Compliant

1

Implant Interface Cortical

bone

FIGUre 13.6 types of bonds between materials with different
moduli.

Direct . When two brittle materials of different moduli are to be
bonded, a layer of a very rigid material may be interposed. In some
cases, a low-shear-modulus adhesive will be used at one or the other
interface. The presence of this interposed stiff material, often called a
“backbone,” effectively prevents transmission of bending stress from one
brittle material to the other and thus preserves their integrity by limiting
their peak flexural strain. In engineering applications, this approach is
frequently used to bond glasses together.
Graded . Two materials that are miscible may be formed into a graded
“seal.” This consists of a continuously varying mixture across the
interface, with a concurrently varying elastic modulus. The effect is to
“smear” out the stress, greatly reducing the peak circum-interface stress
in either material for a given bending moment. An engineering example
is the seal between the metal base and the glass envelope of a television
tube. In this application, the graded seal also protects the brittle glass
envelope from stresses produced by unequal expansion and contraction
of base components during heating and cooling.
Compliant . Complete stress relief may be achieved by bonding a
very compliant layer, with an elastic modulus well below that of either
material, to both of them. Then, stress in one material, whether bend-
ing, longitudinal, or shear, is completely taken up within the compliant
layer without transmission to the other material. Compressive stresses
are still transmitted, as are tensile ones, up to the strength limits of the
interfacial layer. This is the design principle for vibration isolation pads
for machinery.