Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 71

On the superior face or side of internal fixation plates used as tension
bands, at a screw hole where the plate cross section is a minimum
(Figure 3.9)
At abrupt changes in cross-sectional area or shape (see Figure 3.11)
At scratches or nicks produced in prostheses during installation
(Figure 3.12)
At bends or knots in cerclage wire (Figure 3.12)

It may be difficult to determine the point of maximum local stress
on a device from the usual external evidence of anteroposterior and lat-
eral radiographs and a general knowledge of anatomic biomechanics.
The picture is complicated by the variable stress-concentrating features
of cracks (depending on their shape) and by the presence of internal
stresses, produced by processes or, in some cases, by deformation dur-
ing device insertion.

Impact resistance The toughness of a material determines its impact resistance. External

devices and implants often undergo impact loading during use owing to
patient falls, blows, or other traumatic events.
The toughness is defined as the area under the stress-strain curve to
ε u , as stated earlier. However, since the ability to produce single-cycle
failure is dependent on stress concentration factors including inhomo-
geneities and surface finish, toughness is usually evaluated directly by a
specialized test.
In a typical arrangement, a sample of the material is machined to
standard fixed dimensions with a carefully prepared notch. It is placed in
a clamp, and a swinging hammer is allowed to strike it on one side of the
notch. The kinetic energy of the hammer is calculated before and after
impact; the difference represents the work of fracture of the specimen.
Unfortunately, the results depend to some degree on the shape of the
notch and the configuration of the test device, so that the value obtained
is best used in a comparative manner than in an absolute manner. The
units for impact strength are those of work (N*m or ft*lb), and the most
common test arrangement is called the Charpy test, so the result of such
a test is frequently reported as a Charpy (impact) toughness.

Hardness The hardness of a material is its ability to resist local plastic deformation,

especially at its surface. Hardness is an important design parameter in
many applications, including impact surfaces, cutting edges, and articu-
lating interfaces. It should be possible to infer it directly from the knowl-
edge of E and σ y , for any material. However, most materials have different
mechanical properties at their surfaces than in their interiors, even if delib-
erate steps are not taken to alter surface properties. (See Chapter 7 for some
examples of surface treatments of metals that affect hardness.)
Hardness is usually measured directly with one or another varia-
tion of a basic technique. This consists of pressing an indentor into the
surface with a known load and measuring some aspect of the resulting
impression. Such tests produce large impressions in soft materials and