CHAPTER 3
METALS AND PLASTICS
CHAPTER LEARNING OBJECTIVES
Upon completing this chapter, you should be able to do the following:
Explain the methods used to select metals for various applications.
Explain the methods used to select plastics for various applications.
An MR repairs broken parts and manufactures
PROPERTIES OF METAL
replacements according to samples and blueprints. To
choose the metals and plastics best suited for that
The physical properties of a metal determine its
purpose, you must know the physical and mechanical
behavior under stress, heat, and exposure to
properties of materials. You also must know how to
chemically active substances. In practical application,
identify materials that are not clearly marked. For
the behavior of a metal under these conditions
instance, stainless steel and nickel-copper are quite
determines its mechanical properties; indentation and
similar in appearance, but they are completely
rusting. The mechanical properties of a metal are
different in their mechanical properties and cannot be
important considerations in selecting material for a
used interchangeably. Some of the properties of
specific job.
materials that an MR must know are presented in this
chapter.
Stress
As with any shop equipment you must observe all
posted safety precautions. Review your equipment
Stress in a metal is its internal resistance to a
operators manual for safety precautions and any
change in shape when an external load or force is
chapters of Navy Occupational Safety and Health
applied to it. There are three different forms of stress.
(NAVOSH) Program Manual for Forces Afloat,
Tensile stress pulls a metal apart. Compression stress
OPNAV Instruction 5100.19B, that pertain to the
squeezes the metal. Shear stress is forces from
equipment you will be operating.
opposite directions that work to separate the metal.
When a piece of metal is bent, both tensile and
compression stresses are applied. The side of the
Many alloy metals are toxic, and can be
metal where the force is applied undergoes tensile
potentially carcinogenic. The dust or fumes given off
stress as the metal is stretched, while the opposite side
by these alloys could be inhaled and cause serious
is squeezed under compression stress. When a metal
health problems. Respiratory protection may be
is subjected to torque, such as a pump shaft driven by
required when machining, sanding or melting alloy
an electric motor, all three forms of stress are applied
metals.
to some degree.
Strain
METALS
There are hundreds of metals and alloys that you
Strain is the deformation or change in shape of a
may machine in a machine shop. You need not
metal when a stress or load is applied. When the load
is removed, the metal is no longer under a strain. The
understand all metals and alloys, but you must have a
working knowledge of them and know the common
type of deformation will be similar to the form of
terms associated with them.
stress applied.
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