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As the machine pulls the specimen,
it stretches – not a great deal, but enough to register on the strain gauge.
If we gradually
increase the pulling force, the amount of strain will also increase. A force of
20 kilonewtons (4500 lb.) will cause
twice the strain produced by a force of 10 kilonewtons (2250 lb.). The steel is
elastic. Until the elastic
limit of the
specimen has been reached, the amount of strain will be directly proportional
to the amount of pull, and the specimen
will always return to its original length if the pulling force is released.
If we continue to apply pulling force
at a gradually increasing rate, watching both the force and strain gauge hands
closely, we reach a point where the
strain gauge hand continues to move while the force gauge hand remains
stationary, or even drops a bit. We
have now reached the elastic limit of the steel. If at this point, the pulling
force is released,
the specimen will not return to its original length. It has undergone permanent
deformation. The force required
to produce a slight amount of permanent deformation, expressed as megapascals
(MPa) or pounds per square
inch (psi) of specimen cross-section, is termed yield point or yield strength.
If, instead of releasing the pulling
force when the yield point has been reached, we continue to increase that force,
the test specimen will stretch at a
more rapid rate until the pulling force reaches a maximum point. Then it will
begin to ”neck down” or grow
visibly narrower at some point; the force gauge hand will start to drop, while
the hand on the
strain gauge will continue to climb. Then the specimen will break, after ”necking
down” substantially. The value
established by the highest reading registered on the force gauge is termed the
tensile strength or ultimate tensile
strength of the steel. To be more specific: The ultimate tensile strength is the
maximum force registered on Fig.
8-1. Tensile strength is expressed in terms of the directly-applied pull required
to break apart.
Hard-
Surfacing,
Building
Fusion
Welding
Carbon
Welding
Non-Ferrous
Metals
Heating
& Heat
Treating
Braze
Welding
Welding
Cast Iron
Welding
Ferrous
Metals
Brazing
&
Soldering
Equipment
Set-Up
Operation
Equipment
For
OXY-Acet
Structure
of
Steel
Mechanical
Properties
of Metals
Oxygen
&
Acetylene
OXY-Acet
Flame
Physical
Properties
of Metals
How Steels
Are
Classified
Expansion
&
Contraction
Prep
For
Welding
OXY-Acet
Welding
& Cutting
Safety
Practices
Manual
Cutting
Oxygen
Cutting By
Machine
Appendices
Testing
&
Inspecting
