Page 358 - Introduction to Marine Engineering

330 Engineering materials

stress reversals are counted and the machine is run until the specimen
breaks. The load and the number of reversals are noted and the
procedure repeated. The results will provide a limiting fatigue stress or
fatigue limit for the material.

Bend test
The bend test determines the ductility of a material. A piece of material
is bent through 180° around a former. No cracks should appear on the
material surface.

Non-destructive testing
A number of tests are available that do not damage the material under
test and can therefore be used on the finished item if required. These
tests are mainly examinations of the material to ensure that it is defect
free and they do not, as such, measure properties.
Various penetrant liquids can be used to detect surface cracks. The
penetrant liquid will be chosen for its ability to enter the smallest of
cracks and remain there. A means of detecting the penetrant is then
required which may be an ultra-violet light where a fluorescent
penetrant is used. Alternatively a red dye penetrant may be used and
after the surface is wiped clean, a white developer is applied.
Radiography, the use of X-rays or -y-rays to darken a photographic
plate, can be used to detect internal flaws in materials. The shadow
image produced will show any variations in material density, gas or solid
inclusions, etc.
Ultrasonic testing is the use of high-frequency sound waves which
reflect from the far side of the material. The reflected waves can be
displayed on a cathode ray oscilloscope. Any defects will also result in
reflected waves. The defect can be detected in size and location within
the material.

Iron and steel production

Iron and steel are the most widely used materials and a knowledge of
their manufacture and properties is very useful.
Making iron is the first stage in the production of steel. Iron ores are
first prepared by crushing, screening and roasting with limestone and
coke. The ore is thus concentrated and prepared for the blast furnace. A
mixture of ore, coke and limestone is used to fill the blast furnace.
Within the furnace an intense heat is generated as a result of the coke
burning. Blasts of air entering the furnace towards the base assist in this