Civil Engineering Reference
In-Depth Information
stress
as opposed to an
actual stress
(which is based on the actual area of cross section).
For ductile materials there is a marked difference in the latter stages of the test as
a considerable reduction in cross-sectional area occurs between yield and fracture.
From the stress-strain curve the ultimate stress, the yield stress and Young's modulus,
E
, are obtained (see Section 7.7).
There are a number of variations on the basic tensile test described above. Some of
these depend upon the amount of additional information required and some upon
the choice of equipment. Thus there is a wide range of strain measuring devices
to choose from, extending from different makes of mechanical extensometer, e.g.
Huggenberger, Lindley, Cambridge, to the electrical resistance strain gauge. The last
would normally be used on flat test pieces, one on each face to eliminate the effects
of possible bending. At the same time a strain gauge could be attached in a direction
perpendicular to the direction of loading so that lateral strains are measured. The
ratio lateral strain/longitudinal strain is Poisson's ratio,
ν
, (Section 7.8).
Testing machines are usually driven hydraulically. More sophisticated versions employ
load cells to record load and automatically plot load against extension or stress against
strain on a pen recorder as the test proceeds, an advantage when investigating the
distinctive behaviour of mild steel at yield.
COMPRESSION TESTS
A compression test is similar in operation to a tensile test, with the obvious differ-
ence that the load transmitted to the test piece is compressive rather than tensile.
This is achieved by placing the test piece between the platens of the testing machine
and reversing the direction of loading. Test pieces are normally cylindrical and are
limited in length to eliminate the possibility of failure being caused by instability (Chap-
ter 21). Again contractions are measured over a given gauge length by a suitable strain
measuring device.
Variations in test pieces occur when only the ultimate strength of the material in
compression is required. For this purpose concrete test pieces may take the form of
cubes having edges approximately 10 cm long, while mild steel test pieces are still
cylindrical in section but are of the order of 1 cm long.
BENDING TESTS
Many structural members are subjected primarily to bending moments. Bending tests
are therefore carried out on simple beams constructed from the different materials to
determine their behaviour under this type of load.
Two forms of loading are employed the choice depending upon the type specified in
Codes of Practice for the particular material. In the first a simply supported beam is
