Page 68 - Ship Construction.DJ Eyres 6Ed

Ch07-H8070.fm Page 57 Wednesday, October 18, 2006 6:53 AM
Testing of Materials 57
straight portion of the curve. The inter-section of this line with the actual
stress/strain curve marks the proof stress.
It is worth noting at this stage that the ship’s structure is designed for
working stresses which are within the elastic range and much lower than
the ultimate tensile strength of the material to allow a reasonable factor
of safety.

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Classification Society Tests for Hull Materials
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Both mild steel and higher tensile steel plates and sections built into a ship
are to be produced at works approved by the appropriate classification society.
During production an analysis of the material is required and so are pre-
scribed tests of the rolled metal. Similar analyses and tests are required by
the classification societies for steel forgings and steel castings, in order to
maintain an approved quality.
Destructive tests are made on specimens obtained from the same product
as the finished material in accordance with the societies’ requirements
which may be found in the appropriate rules. These tests usually take the
form of a tensile test, and impact test.
ึ
TENSILE TEST The basic principle of this test has already been described,
a specimen of given dimensions being subject to an axial pull and a minimum
specified yield stress, ultimate tensile stress, and elongation must be obtained.
In order to make comparisons between the elongation of tensile test pieces
of the same material the test pieces must have the same proportions of
sectional area and gauge length. Therefore a standard gauge length equal
to 5.65 times the square root of the cross-sectional area, which is equivalent
to a gauge length of five times the diameter is adopted by the major classifi-
cation societies.

IMPACT TESTS There are several forms of impact test, but the Charpy V
notch test or Charpy U notch test is commonly specified and therefore
described in this text. The object of the impact test is to determine the tough-
ness of the material, that is its ability to withstand fracture under shock loading.
In Figure 7.2 the principle of the Charpy test machine is illustrated as
also is the standard test specimen for a Charpy V notch test. This specimen
is placed on an anvil and the pendulum is allowed to swing so that the
striker hits the specimen opposite the notch and fractures it. Energy
absorbed in fracturing the specimen is automatically recorded by the
machine. Basically, making allowances for friction, the energy absorbed in
fracturing the specimen is the difference between the potential energy the
pendulum possesses before being released, and that which it attains in swing-
ing past the verical after fracturing the specimen. A specified average impact