Deformation - Orthopaedic Biomaterials in Research and Practice - page 27

Undeformed

- F F

To rsional deformation

FIGUre 2.6 torsional deformation.

bar is a measure of the amount of bending deformation. In the case of
Figure 2.5, the deformation is 30°.
Figure 2.6 shows torsional deformation of a rod, the result of oppos-
ing moments acting around its longitudinal axis. Again, the ghost rect-
angles are distorted, but even more so than in shear or bending, with the
long edges becoming longer, slanting with respect to the rod axis, and
taking on a distinct curvature, whereas the short edges are unchanged in
length, orientation, or shape. This shape change suggests that torsional
deformation includes shear deformation. Marks at the end of a line on
the side of the cylinder provide a way of measuring the amount of tor-
sional deformation. In the case of Figure 2.6, the deformation is 60°.

elastic
deformation

Each of these load-deformation curves has two characteristic parts or
regions. At low force or moment magnitude, the curve is straight and
then begins to curve gently. The deformations recorded are reversible;
prompt removal of the force or moment will permit the object to return
to its original size and shape. Deformations in this region are called elas-
tic deformations and are said to be fully recoverable. There is a point of
maximum force or moment that is just enough to produce a permanent
deformation after unloading; the level force or moment just below this
value is called the elastic limit . The word elastic comes from a Greek
root elaunein meaning “to drive,” reflecting the power with which such
materials “snap back” when released.
The elastic limit is a useful point to identify. It predicts the maximum
force or moment that a designer would wish to permit a device, such as
an intramedullary (IM) rod, to sustain. On a moment-angulation curve,
this point corresponds to the highest value of force attained, the peak of
the curve. Unfortunately, this point is difficult to determine experimen-
tally. More importantly, if a bending moment-angulation diagram were
to be made for a group of IM rods with different diameters, the elastic
limit would occur at different values of moment for each device. This is