©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A
Elong-
ation
Reduction
of Area
Fracture
Yield
Strength
Ultimate
Strength
1.6.2
Yield
Strength - When a metal is placed in tension,
it acts somewhat like a rubberband.
When a load of limited magnitude is applied, the metal stretches, and when
the load is released, the metal returns
to its original shape. This is the elastic characteristic
of metal and is represented by letter
A in Figure 5. As a greater load is applied, the metal
will reach a point where it will no
longer return to its original shape but will continue to
stretch. Yield strength is the
point where the metal reaches the limit of its elastic character-
istic and will no longer return to
its original shape. 1.6.3
Ultimate
Tensile Strength - Once a metal has exceeded its
yield point, it will continue
to stretch or deform, and if the load is suddenly released, the metal will not
return to its
original shape, but will remain in its elongated form. This is called the
plastic region of the
metal and is represented by the letter B in Figure 5. As this plastic deformation
in- creases, the
metal strains against
further elongation, and an
increased load must be applied
to stretch the metal. As the
load is increased, the metal will
finally reach a point where it no
longer resists and any fur- ther
load applied will rapidly cause
the metal to break. That point
at which the metal has withstood
or resisted the maximum applied load is its ultimate tensile strength. This
infor- mation
is usually recorded in pounds per square inch (psi). 1.6.4
Percentage
of Elongation - Before a tensile specimen is placed
in the tensile tester,
two marks at a measured distance are placed on the opposing ends of the circular
shaft. After the specimen is
fractured, the distance between the marks is measured and
recorded as a percentage of the original
distance. See Figure 5. This is the percentage of
elongation and it gives an indication
of the ductility of the metal at room temperature. 1.6.5
Reduction
of Area - A tensile specimen is machined to exact
dimensions. The area
of its midpoint cross-section is a known figure. As the specimen is loaded
to the point of
fracture, the area where it breaks is reduced in size. See Figure 5. This
reduced area is calculated
and recorded as a percentage of the original cross-sectional area. This
informa- tion
reflects the relative ductility or brittleness of the metal.
1.6.6
Charpy
Impacts - Metal that is normally strong and ductile
at room temperature may
become very brittle at much lower temperatures, and thus, is susceptible to fracture
if FIGURE 5
STRAIN - INCHES
A B
C
NOMINAL STRESS - STRAIN CURVE
