Civil Engineering Reference
In-Depth Information
static or dynamic in nature depending on the particular property being investigated.
Possibly the most common mechanical static tests are tensile and compressive tests
which are carried out on a wide range of materials. Ferrous and non-ferrous metals
are subjected to both forms of test, while compression tests are usually carried out on
many non-metallic materials, such as concrete, timber and brick, which are normally
used in compression. Other static tests include bending, shear and hardness tests,
while the toughness of a material, in other words its ability to withstand shock loads,
is determined by impact tests.
TENSILE TESTS
Tensile tests are normally carried out on metallic materials and, in addition, timber.
Test pieces are machined from a batch of material, their dimensions being specified
by Codes of Practice. They are commonly circular in cross section, although flat test
pieces having rectangular cross sections are used when the batch of material is in the
form of a plate. A typical test piece would have the dimensions specified in Fig. 8.1.
Usually the diameter of a central portion of the test piece is fractionally less than that
of the remainder to ensure that the test piece fractures between the gauge points.
Before the test begins, the mean diameter of the test piece is obtained by taking
measurements at several sections using a micrometer screw gauge. Gauge points are
punched at the required gauge length, the test piece is placed in the testing machine
and a suitable strain measuring device, usually an extensometer, is attached to the
test piece at the gauge points so that the extension is measured over the given gauge
length. Increments of load are applied and the corresponding extensions recorded.
This procedure continues until yield (see Section 8.3) occurs, when the extensometer
is removed as a precaution against the damage which would be caused if the test piece
fractured unexpectedly. Subsequent extensions are measured by dividers placed in
the gauge points until, ultimately, the test piece fractures. The final gauge length and
the diameter of the test piece in the region of the fracture are measured so that the
percentage elongation and percentage reduction in area may be calculated. The two
parameters give a measure of the ductility of the material.
A stress-strain curve is drawn (see Figs 8.8 and 8.12), the stress normally being calcu-
lated on the basis of the original cross-sectional area of the test piece, i.e. a
nominal
Fractionally
reduced
diameter
Diameter,
D
Gauge points
Gauge length (GL)
Radius,
R
F
IGURE
8.1
Standard
cylindrical test piece
Length,
L
