Deformation - Orthopaedic Biomaterials in Research and Practice - page 28

Force

(N)

1000

F /2 F /2

θ

500

4 10 20

Angulation, θ (˚)

FIGUre 2.7 im rod in four-point bending (Problem 2.1).

because the elastic limit, as determined in this manner, is an extrinsic
property, dependent both on the size and shape of each device, as well as
on properties of the material from which it is made.
Load-deformation curves have a valuable use; they may be used to
determine general aspects of deformation under loading of devices. Of
particular value is the fact that deformation above the elastic limit for
a metallic device is unrecoverable, although the relationship between
force and deformation is essentially unchanged.

Plastic
deformation

Loading beyond the elastic limit results in deformations with increas-
ingly large degrees of unrecoverability after load release. This is called
plastic deformation. * The Greek root plassein means “to shape or
mold,” nicely reflecting the observation that plastic or unrecoverable
deformation changes the dimensions or shape of a material in a perma-
nent way, as already seen in necking during tensile deformation. The
amount of plastic deformation can be determined by locating the point
of interest on a load-displacement curve and constructing a line paral-
lel to the elastic portion of the curve. The intersection of this line with
the displacement/angulation axis gives the quantity of permanent plastic
deformation (see Problem 2.1).

PROBLEM 2.1
An IM rod tested in vitro has the load-angulation diagram in Figure 2.7.
If a removed rod of this size were found to have a 4° bend (although it
had been inserted non-pre-bent), what is the maximum angulation that it
would have undergone in vivo ?

* The word plastic has unfortunately come to describe a class of materials more properly
known as polymers (see Chapter 6). The use of the term plastic to describe unrecov-
erable deformation is the more general and appropriate use; polymers have come to
be called plastics in common usage because most display plastic deformation at room
temperature.