Biomedical Engineering Reference
In-Depth Information
it very hard to grip them properly. in some cases a compression test can
provide the information needed. however, because materials can behave
very differently in compression compared with tension, it is necessary to fi nd
another way to test the tensile properties of materials that cannot reliably
undergo a uniaxial tensile test.
flexural strength
A fl exural or bend test can be performed on rod-shaped specimens with
either rectangular or circular cross-sections. A schematic of the setup for a
three-point bend test is shown in Fig. 5.2(b). this test can also be performed
as a four-point bend test, wherein the load is applied at two contact points
along the top, rather than just one, with a moderate distance between them.
When the load is applied, the ends of the specimen bend upwards, forcing
the mid-point lower. this places the top of the specimen in compression and
the bottom in tension. Since the tensile strength of most brittle ceramics is
approximately one-tenth that of the compressive strength, it can be assumed
that the specimen fails due to tension and thus represents a good value for
tensile strength. Care must be taken to ensure the specimen has broken as
close to the centre as possible, and always between the contact points rather
than towards the outside or at a contact point.
the formulae for calculating bending strength for a three-point bend are
shown below, both for rectangular and circular cross-sections:
3
2
fl
bd
[5.3]
fs
=
2
fl
R
[5.4]
=
fs
3
where
f
is the load,
l
is the distance between contact points,
b
and
d
are
the width and depth of a rectangular cross-section specimen, respectively,
and
R
is the radius of a circular cross-section specimen. the four-point bend
test may be generally preferred over the three-point bend test for brittle
ceramics. One cause for concern with the three-point test is that failure might
be initiated prematurely on the top face by a fl aw generated by the contact
point at the centre of the specimen. As the centre is also where the peak load
is, this is expected to be the failure location under normal conditions. in the
case of the four-point bend test, the bending moment is constant between
the top two contact points. in this case, however, premature failure caused
by a test-induced fl aw should be easily distinguishable from a correct result
where the specimen breaks between the two contact points. it should be
noted, as well, that four-point bend tests tend to give lower strength values
for ceramics. Since the load is spread over a wide area, there is more of a
