material & processes for ndt part 4
TRANSCRIPT
-
7/25/2019 Material & Processes for NDT part 4
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-
7/25/2019 Material & Processes for NDT part 4
2/15
16
Materials
and
Processes for
NDT Technology
facturing
processes
must
still
be defined
measured
by empirical
test
for each
material.
and
IRON
ALUMINUM
Figure
3-1
Atomic
structure
CLASSES OF PROPERTIES
The
application
to which a
material
is
put
deter-
mines
which
of its
properties
are most important.
Chemical
Properties.
The
chemical
properties
(reaction
with
other
materials)
are
of
interest
for
all
material
mainly
because of
the
almost universal
need
for
resistance
to conosion.
Although
aluminum
is
chemically
more active
than
iron,
in
most
atmos-
pheres
the
corrosion
byproducts
of
aluminum
form a
denser coating, which acts
as
a shield
to
further
corro-
sion, than
do
the corrosion
byproducts of iron.
While
the
atomic and
crystalline structure
of
all
metals
gives
them
high electrical
and
thermal
conduc-
tivity compared
to
nonmetals, individual metals
still
differ
considerably. Aluminum
is among
the best
electrical
conductors,
while
iron, although
much
more conductive than
nonmetals, is a
poor
conductor
compared
to
aluminum.
On
the
other
hand,
the
magnetic
properties
of iron make it much
more desir-
able
for
some electrical
uses
than
aluminum.
Physical
Properties. Physical
properties for each
material
are constants associated with
the
atomic
structure. These
properbies
include density
(weight
per
unit volume), crystalline
type,
atomic
spacing,
specific heat,
cohesive
strength
(theoretical),
and
melting
point.
Iron
has a much
higher melting
point
and density
than aluminum.
Iron
is allotropic,
mean-
ing
it
can
exist
in
several different
crystalline
struc-
tures as
opposed
to
aluminum,
which
always
exists
in
single
crystalline pattern. This
difference makes
pos-
sible,
for iron-based alloys,
methods
of
property con-
trol
by heat treatment
that
are
not
possible
for alumi-
num.
Some aluminum-based
alloys
may be
heat
treated
for
propertf
control, but
the
reaction
is entire'
ly
different.
Mechanical Properties.
Of
most
interest
to man-
ufacturing
are
the
mechanical
properties
of
hardness,
strength,
and others
that
are
of
prime
importance
in
design considerationg
for
determining
sizes and
shapes
necessary for
carrying loads.
These
qualities
will
also
determine
the work
loads
for
any deformation
type
of
manufacturing
process.
Neither
iron nor aluminum
in
the
pure
state
has many
applications in
manufac-
turing
because
their
strengths
are
low, but their
alloys,
particularly iron alloys,
are
the most
commou-
ly used
of
all metals.
Both
of
these materials
can be
strengthened over
their
weakest
forms by
factors
of
almost
ten by
suitable
alloying and
treatment, with
alloys of
iron
being approximately
five
times
as
strong
as
those
of
aluminum
on
a
volume
basis.
Processing
Properties.
As
pointed
out
at
the
beginning
of
the
chapter,
the
properties
that
have
been discussed
are
actually
dependent on the atomic
structure
of
a
material,
but in
practice
these
proper-
ties
must
be
separately
measured.
In
a
simi,lar way,
different
properties
that
are
related
to
hardness,
strength,
ductility,
and
other
physical
and mechanical
properties
and that
are frequently of
even
greater
importance to
manufacturing
must
in
practice
be
defined by
separate
tests.
These
include
tests
for
castability, weldability,
machinability,
and bending
that
describe
the
ability
of
the
material
to
be
proc-
essed
in definite
ways.
Tests
of this
type
may
be
developed
at
any
time
there
is need for determining
the
ability
of
the
material
to
meet critical
needs
of
processing,
and
they
are usually
performed
under
conditions
very
similar
to
those under
which the
process
is
performed.
SIGNIFICANCE
OF PROPERTIES
TO
DESIGN
A
designer
is
necessarily
interested
in
properties
because
he
must
know
material
strengths
before
he
can calculate sizes
and shapes
required
to
carry
loads,
chemical
properties
to
meet corrosive conditions, and
other
properties
to
satisfy
other
functional
require-
ments. Knowledge
of
processing
properties
is
likely
to
be
of
more
importance
to manufacturing
personnel
than
to
the
designer,
although even
he
must
be
able
to
choose material
that
can
be
manufactured
in
a
reasonably economical
manner. Many
manufacturing
problems
arise
from
choice
of materials
based
only
on
f
unctional
requirements without
considering
which
is the most
suitable for
lhe
processing
required.
Similar
results
can occur
when
inspectability
has
not
been
given
proper
consideration
in design.
Material
Choice
a
Compromise.
Most
products
can
be manufactured
from
a number
of different
pos-
sible materials
that will
satisfy
the functional require-
ments. However, some are
more desirable
from
the
product
standpoint
than
others, and one
particular
material
may
have
the
best
possible
combination
of
properties.
Likewise, all
materials
can
be
manu-
factured
by
some means,
although costs
of
manufac-
turing
u'ill
vary,
and there
will
likely be one
single
material from which a usable
product
could be manu-
factured
at
lowest cost.
Seldom
can
a
material
be
chosen that
has
optimunr
properties
for both the
-
7/25/2019 Material & Processes for NDT part 4
3/15
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7/25/2019 Material & Processes for NDT part 4
4/15
18
Materials
and
Processes
for NDT
Technology
have
been developed
to
the
same
magnitude
and
in
the
same
way
but
opposite
in direction
and
combined
with
compressive
stress
instead
of
tensile
stress.
Shear
stress
exists
alone
only
in
a
bar
subjected
to
pure
torsion,
that
is, a
bar
being
twisted
with
no
tension,
compression,
or
bending
present.
Shear
stresses
ate
important
to
our
manufacturing
processes because
these
are
the
forces
that
cause
material
to
shift
in
plastic
flow
and
permit shape
changing
by
deforma-
tion
processes.
Bending.
Bending
loads
create
a
combination
of
stresses.
The
concave
side
of
a
bent
body
will
be
in
compression
and the
convex
side
in
tension
with
transverse
shear
occurring
along
the
axis
between
them.
The
maximum
unit
stress
will
be
in
the
outer
fibers of
the
bent
body
and
is
represented
by
the
formula
56
=
Mc/I
where.
M
equals
bending
moment,
c equals
distance
from
neutral
axis,
and
.f
equals
moment
of
inertia
of
the
bodY.
Effects
of
Stresses.
The
principal
point
to
be
made
in
this
discussion
of
forces and
stresses
is
that
structural
designs
must
be
of
suitable
size
and
shape
and
must
be
made
of material
with
proper
strength
values
to
withstand
the
loads imposed
upon
them'
When
a
structural
member
(almost any
object)
is
physically loaded
by
weight,
by
pressure
from
mechanical,
hydraulic,
or
pneumatic
sources,
by
thermal
expansion
or contraction,
or
by other
means'
intemal
stresses
are
set
up
in the
member.
The
size,
direction,
and
kind
of
stresses
are
dependent
upon
the
loading
system.
The
magnitude
of
the
unit
stresses will
be dependent
not
only
upon
the
applied
force
but
also
upon
the
area
of
material
resisting
the
stresses.
As
loads
ate
increased,
unit
stresses
will
increase
to
the
point
where,
in
some
direction,
one
or
more
reach
critical
values
in
relation
to
the
material'
Failure
by
plastic
flow
or
by
fracture
can
then
be
expected,
depending
upon which
critical
values
are
reached
first.
In
nearly
all
cases
of
fracture
failure,
the
separation
of
material
is
preceded
by
at
least
a
small
amount
of
plastic flow.
In
those
cases
in
which
plastic
flow
occurs
to
a
Iarge degree,
fracture
failure
will
finally
result.
TESTING
Testing
of
material
is
essential
to
gain
practical
knowledge
of
how materials
react
under
various
situa-
tions.
The
ultimate
goal
of
any
test
is to
enable
the
making
of
decisions
that
provide the
best
economic
results.
In
practice,
two
general
methods
of
testing
are
used.
Direct
Testing.
The
only
test
that
supplies
abso-
lute
information
about
a workpiece
or
a material
is
a
test
of
the
particular
property of
interest
conducted
on
that
part
itself.
In this
method
of
direct
testing,
an
attempt
is made
to
use
the
materials
under
the
exact
conditions
of
practical use' and
the
test
may
be
con-
cerned
with
a
product,
a
process'
or
both.
Direct
test-
ing
is usually
time-consuming,
and,
for
the
results
to
ha-ve
statistical
significance,
often
requires
compila-
tion
of
data
from
many
test
samples.
The
procedure
is necessary,
however,
for
those
cases
in which
simp-
ler
methods
are
not
available
and
in
which
sufficient
historical
information
has
not
been
accumulated
to
permit
corelation
between
the
attribute
about
which
information
is
desired
and
some
other
measurable
factor.
Indirect
Testing.
Indirect
testing
involves
the
use
of
such
a correlation,
such
that
accurate
knowledge
of
the
relationship
between
the
two
factors
must
exist. The
ability
of
grinding
wheels
to
resist
the
centrifugal
forces
imposed
in
use
is
directly
tested
by
rotating
them
at
higher
speeds
than
those
of actual
use.
Such
a test
indicates
that
the
wheel
strength
is
sufficient
for normal
use
with
some
safety
margin.
An
indirect
test
that
is
sometimes
used
for
the
same
purpose
can
be
performed
by
rapping
a
suspended
wheel
to
cause
mechanical
vibrations
in
the
sonic
range.
A clear
tone
indicates
no cracks.
A
danger
of
indirect
testing
is that
the
conclusions
depend
on
the
assumption
that
the
correlation
between
the
meas-
ured
factor
and the
critical
factor
exists
under
all
conditions.
The
rapping
test
for
grinding
wheels
does
not
give
any
real
indication
of
strength,
unless
knowl-
edge of
the
wheel's
history
permits the
assumption
that
with
no
cracks
it has
sufficient
strength
for use.
Destructive
Testing.
A
large
number
of
direct
tests
are destructive.
These
also are
dangerous
because
the
assumption
must
be
made
that
those
materials
not
tested
are
like
the
ones
for
which test
informa-
tion
has
been
obtained.
A
portion
of weld
bead
may
be
examined
for
quality by
sectioning
it to
look
for
voids,
inclusions,
penetration,
bond,
and
metallurgical
structure
by
visual
examination.
By
this
cperation,
this
portion
of
the
bead
has
been
destroyed;
regard-
less of
the
quality that
was
found,
the
only
knowl-
edge
acquired
about
the
remaining
portion
of
the
weld
comes
from
an assumption
that
it is
similar
to
that examined
because
it was
made
under
the
same
conditions.
Nondestructive
Testing.
In
addition to
the
nondes-
tructive
feature,
these
tests
almost
entirely
are
indi-
rect
tests
that
require
first,
correlation
with
the
de'
fects
that are
being
sought,
and
second,
expert
evalua-
tion
or
interpretation
of the
evidence
that
is
gathered'
Nondestructive
tests
may
be
for
faults
and
discontin-
unities
located
on either
the
surface
or
internally
and
may
be
performed before,
during,
and
after
the
manufacturing
process.
These
testJJre
performed by
(1)
exposing
the
prod-
uct
material
to some
kind
of
probing
medium
(radia'
tion
energy,
sonic
energ:y,
magnetic
and
electrical
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F
F
z
20 Materials
and
Processes
for NDT
Technology
mations in the elastic
range,
nd as
long
as
the
load
at
B
is
not exceeded, the material
will
resume
its original
position
and
shape
after
removal of
the
load.
B is the
elastic
limit for
this
particular
material,
and
loads
above
that
limit will cause
permanent
deformation
(plastic
flow) that
cannot
be recovered
by removal
of
the
load.
At
the
load represented
by the
point
at
C,
plastic
flow
is occurring at
such
a
rate
that
stresses
are
being
relieved faster
than
they
are
formed,
and strain
increases
with
no additional,
or
even
with
a
reduction
of,
stress.
The
unit
stress
at
C
is
known
as
the
yield
point.
Figure
3-6
Stress-strain
diagram
Plastic
flow
occurring
at normal
temperature
is
called
cold working,
regardless
of
the
kind
of
loading
system
under which
it is
accomplished.
As
plastic
flow takes
place,
the crystals and
atoms
of the
materi-
al rearrange
internally
to
take stronger
positions
resisting
further change. The
material
becomes
stronger and harder
and is said
to
be
work
hardened'
At
the
point
D
in Figure 3-6,
the curve
suddenly
turns
upward, indicating
that
the
material
has
become
stronger
because of work
hardening and
that
higher
loads
are
required
to
continue deformation.
Tlhe
deformation
rate, however,
increases
until at
point E
the ultimate
strength is
indicated.
Ultimate
and
Breaking
Strengths.
The
ultimate
tensile
strength of
a material
is defined
as
being
the
highest strength
in
pounds
per
square
inch,
based
on
the
original
cross-sectional
area. By this
definition,
ductile
materials that
elongate appreciably
and
neck
down
with
considerable
reduction
of cross-sectional
area,
rupture at a
load lower than that
passed
through
previous
to
fracture.
The
breaking
strength,
or
rup-
ture strength,
for this
material
is shown
at
F,
consid-
erably below the
ultimate
strength. This
is typical
of
ductile
materials, but
as
materials
become
less
ductile,
the
ultimate strength
and
the breaking
strength
get
closer
and closer
together
until
there
is
no detectable
difference.
Yield Point
and
Yield
Strength.
Many materials
do
not
have a
well-defined
or
reproducible
yield
point.
Plotting
of
tensile
stress-strain
values
produces
a
curve
of the type
shown
in
Figure
3-7.
For these
materials,
an artificial
value
similar
to
the
yield point,
called
yield
strength,
may be calculated'
The
yield
strength
is
defined
as
the amount
of
stress
required
to
produce
a
predetermined
amount
of
permanent
strain.
A commonly
used
strain
or
deformation
is
0.002
inch
per
inch,
or
0.27o
offset, which must be
necessarily
indicated with
the
yield
strength
value.
The
yield
strength
is
the
stress
value
indicated
by
the
intersection
point
between
the
stress-strain
cuwe
and
the offset
line drawn
parallel
to the
straight
portion
of
the curve.
Modulus
of
Elasticity.
In
the stress
range
below
the
elastic
limit,
the
ratio
of unit
stress
to
unit defor-
mation,
or
the slope
of
the
curve,
is
referred
to
as
the
Figure
3-7
Yield strength
modulus
of
elasl,icity,
or
Youug's modulus,
and
is
represented
by
E. E, therefore,
equals s divided
by 6.
F'ollowing
are
listed the
values
of
Z'
for
some
of
the
more
common structural
materials:
TABLE
3.1
Aluminum alloys
.
' '
'10
million
psi
(6'9
X 10'
Pa)
Copper
alloys
.
.....
.14
to
19 million
psi
Grav
iron
..
.12to'l
9 milliott
Psi
Steel
and
high-strength
irons
.28
to
30
million
psi
Cemented
carbides
.approx
50
million
psi
The
gross
values
of
the
modulus
of
elasticity
are
important to
the
design
of
members
when
deflection
o
o
r
tt)
F
z
tr
UNIT
DEFORMATION
UNIT
DEFORMATION
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-
7/25/2019 Material & Processes for NDT part 4
8/15
22
Materials
and
Processes
for
NDT
Technology
TRANSVERSE
RUPTURE
TESTING
Limitations
of
Tensile
Tests
fot
Btittle
Materials'
In
a
number
of
cases
a
substitute
for
the
standard
tensile
test
is necessary.
With
some
materials
that
are
difficult
to
shape
or
very
brittle
in
nature,
it
is im-
practical
to
produce
a specimen
for tension
testing.
This
condition
occurs
particularly
with
ceramics.
With most
materials
that
are
very
brittle
in
character,
even
though
a
tensile
specimen
might
be
produced,
the results
from the
standard
tensile
test
would
have
only
limited
significance.
It
is
almost
impossible
to
insure
in
the
tension
test that
the
applied
load
will
be
precisely
centered
in the
specimen
and
will
be exactly
parallel
to
the
axis
of
the specimen.
If this
is
not
the
case,
bending
moments
are
introduced
in
the
speci-
men.
With
a
ductile
material,
small
amounts
of
plastic
flow
take
place
in
the
specimen,
particularly where
the
load
is applied;
the
specimen
aligns
itself
properly
with
the
load; and
the
stresses
are uniform
across
the
tested area.
With
a brittle
material
in which
this
align-
ment
cannot take place,
the
bending moments result
in
higher
stresses on
one
side
of
the
specimen
than
on
the
other. The
specimen
fails when
the
highest
stress
reaches
some
critical
value,
but the
observed
stress
at
this time,
based
on
the assumption
of
uniformity,
is
somewhat
lower. As
a consequence,
the
results
from
testing
a
number
of
similar
brittle
specimens
exhibit
wide
variations
and
are
not
representative
of
the
true
strength
of
the material.
The Tlansverse Rupture
Test.
The transverse
rup-
ture
test, while
it
gives
less complete
information
than
the tension test,
is a fast and
simple
test,
making
use
of more easily
prepared
specimens,
and
is espe6i-
ally
well
suited
to brittle
materials.
In
many
instances
the
specimen can
be an actual workpiece.
The
test
is
particularly
well
suited for those materials
that
are
to
be
used
in
beam applications.
It
is
really
the
only
meaningful
type of
strength test
for reinforced
con-
crete.
The
test
consists of
loading a
simple beam
as
illus-
trated
in Figure 3-9.
While
some
standards
have been
set
for
particular
materials,
there
are
no
univeral
standards
for
specimen
sizes and
shapes
as there
are
for
the
tension
test.
The
modulus
of rupture,
or
beam
strength,
is cal-
culated
by the
formula
^
gPL
sr
=
2bd,,
Limitations
of
Ttansverse
Rupture
Testing.
While
this
formula
is the
formula
that
is
used
to
calculate
the
maximum
actual
stress
in
the
outer
fibers
in
a
beam,
it is
based
on the
assumption
that
stress
re-
mains
proportional
to
strain. This
is
not
the
case
for
most
materials
when
highly
loaded,
with
the
result
that the
calculated
"stress"
is higher
than
the
actual
stress
in
the
outer
fibres at
rupture,
and
direct
com-
parison
cannot
be made
with
ultimate
tensile
strength
values
taken
from
a tension
test,
nor
can
the
values
of
modulus of
rupture
be
used
as design
tensile
strength
values.
The
values
are useful
for comparing
materials,
and they are
useful
in design
when
the material
is to
be
used
as
a beam.
Figure
3-9
Transverse
ruPture test
SHEAR
TESTING
In
the
section
dealing
with
material
failure,
it
was
pointed
out
that when
a
bar
is subjected
to
a
tension
load as
in the
tension
test,
the
value
of
shear
stress
existing
in
the bar
at
failure
can
be calculated
from
the
load
and
the dimensions
of the
bar
(Figure
3'10).
Figure 3-10
Heads
of
a torsion
testing
machine.
Torsion
is the simplest
way
of obtaining
pure
shear
stress.
Results
are
useful
for
evaluating
cold-working
properties
of
metals
-
7/25/2019 Material & Processes for NDT part 4
9/15
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-
7/25/2019 Material & Processes for NDT part 4
10/15
24 Materials
and Processes
for NDT
Technology
has
flattened
out, and
stressing
at
this
level could
be
continued
indefinitely without
failure.
Endurance
limits
correlate
fairly
closely
with
tensile
strength
and
for
most materials are
from about one-third
to
one-
half the stress
required to
break a
tensile
specimen.
Fatigue
Strength.
For some
materials
the curve
does not
flatten
even
after several
hundred
million
cycles.
When
the
endurance
limit cannot
be
de-
termined,
or
it
is
impractical
to
carry
on
a
test
long
enough
for
this determination,
it
is common
practice
to use another value,
fatigue strength,
to
evaluate
the
ability
of a
material
to
resist fatigue
failure.
Fatigue
strength
is
the
stress
that can be applied
for
some
arbitrary
number of
cycles
without
failure.
The
num-
ber of cycles for which
a fatigue strength
is valid
must
always
be specified because
the
operating
stress
chosen
may
be
at a
level
where
the
S-N curve
still
slopes,
and indefinite
cyclic operation
could
cause
fatigue failure.
CREEP
TESTING
The term
creep is used
to
describe
the continuous
deformation
of a
material under
constant
load,
producing
unit stresses
below
those
of the
elastic
limit.
At normal temperature,
the effect
of
creep
is
very
small and can
be
neglected.
As
operating
temper-
atures
increase,
however, this deformation
by
slow
plastic
flow becomes very
important in the
design
and
use of material. Recognition
of
this
phenomenon
is
most important
for
the
higher
strength
materials
that
are to
be
used
at elevated temperatures.
Creep
tests are conducted
by
applying
a constant
load to a material specimen
held at the desired
temp-
eratule and
measured
periodically
for
deformation
over a
long
period
of time.
The
results may
be
plotted
on
a
graph
of elongation against
time, as
in Figure
3-13,
with
an
indication
of
the
maintained tempera-
ture
and
stress level under
which
the test
was
con-
ducted. Most
creep tests are carried
on
for
periods
of
at
least 1,000
hours,
so
this
is
a time-consuming
test.
The
creep strength
of
a material
is the
stress
required
to
produce
some
predetermined
creep
rate
(the
slope
of
the
straight
portion
of
a
curve)
for
a
prolonged
period
of
time.
Commonly,
the
stress
required
to
produce
a creep
rate
of
1% in
10,000
hours
is used
as
creep strength.
Sfress rupture
strength
is
defined
as
the
stress
required
to
produce
failure
at
prescribed
values
of
time
and temperature.
NOTCHED
BAR
TESTING
Materials
are
often
used
in
situations
in
which
dynamic
loads are
suddenly
applied
to
produce shock
that increases
the
effective
load
far above
that
which
would
be expected
from
gradual
application
of
the
same
load
or
a
similar
static
load.
Tests
designed
to
check
the
ability
of a
material
to
withstand
this
kind
of
loading are energy
absorption
tests
that
seldom
can
____J
TIME
+
Figure
3-13
Creep test
be
used
to
give
information
that
can
be used
directly
in
design,
but
primarily
provide
data for
com-
parison
of
different
materials.
While such tests
are
frequently
called
impact
tests,
the energy required
to
cause
failure
does not
differ
greatly
from
that
re-
quired
if
the
load
were
applied
slowly.
Tlue
impact
failure,
in
which the
energy-absorbing
capacity
of
a
material is
greatly
reduced,
occurs
only at much
high-
er
speeds.
Charpy Test.
The
most
commonly
conducted
tests are bending impact
tests, using
one of two
kinds
of
notched
speciments
(Figure
3'14).
The Charpy
i
2
9
F
(,
z
o
J
lrJ
'e
specimen is supported
at
both
ends
by
a standard
Figure 3-14
lmpact
specimens
r-----
CONSTANT
LOAD
-
7/25/2019 Material & Processes for NDT part 4
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-
7/25/2019 Material & Processes for NDT part 4
12/15
26
Materials and Processes
for
NDT
Technology
number
1
as the softest.
the
standard
Mohs
scale
is
as
follows:
1...
Talc
2
...
Gypsum
3...
Calcite
4... Fluorite
5...
Apatite
6...
7...
8...
9...
10...
Orthoclase
(Feldspar)
Quartz
Topaz
Corundum
Diamond
If
a
material
can be noticeably
scratched
by
the
mineral
topaz
(number
8)
but
cannot be
scratched
by
quartz
(number
7), it
would
have
a hardness
value
between 7 and
8
on the Mohs
scale. The
Mohs
scale
of
hardness
has little value
for
hardness
testing
of
metals but
is
still
widely
used
in
the
field of
minera-
logy.
File Test. Another
abrasion
or
scratch
method
of
measuring
hardness that
does
have
some
practical
use
in
metal
working
is the
file
test. Standard
test
files
can be used
to
gage
quickly
the
approximate
hardness
of a material
and,
although
not
very
accurate,
can
be
used
in
many shop
situations
with
satisfactory
results.
Experience
and
comparison
with
standard
test
blocks
will
permit
a fair
degree
of accuracy
to
be
attained.
Brinell Test.
In
1900
Johan
August
Brinell,
a
Swedish engineer,
introduced
a new
universal
system
for
hardness measurement.
The
method
involves
impressing,
with
a definite load, a
hardened
steel
ball
into
the material to be
tested and
calculating
a Brinell
hardness number
from
the
impression size
(Figure
3-15).
A
wide
range of hardnesses
can
be tested
by
varying the size of the ball and
the
loads imposed,
but
in
the hardness range
most
frequently
tested,
a ball
10
millimeters
in
diameter is
impressed
into
the
ma-
terial under a load
of
3,000
kilogtams for
10
seconds
to
check steel and under a
load
of
500 kilograms
for
30 seconds
to
check
nonferrous materials.
The
numerical value
of
the
Brinell
hardness number
is
obtained
by
dividing
the
load
in
kilograms
by the
area
of the
spherical
impression
in
millimeters.
In
practice,
the
average
diameter
of
the
impression
is
usually
read
with
a measuring
microscope
and
the Brinell
hardness
number
determined
directly
from
a
table.
Advantage
and
Limitation
of Brinell
Tests.
The
Brinell
hardness
method
has the
advantage,
as com-
pared
to
most other
measuring methods,
of
determin-
ing
a hardness
value over
a
relatively
large area,
thus
reducing the
inconsistencies
caused
by
flaws, imper-
fections, and
nonhomogeneity
in the
material,
likely
to
be introduced
with
small
area
measurement
that
includes
only
a
few metallic
grains.
With
plain
carbon
and
low
alloy
steels,
the
relation
between
tensile
strength and
Brinell
hardness is
so consistent
in
the
medium
hardness
range
that
the
tensile
strength of
the steel
can be
closely approximated
by multiplying
the Brinell
hardness
number
(BHN)
by
500.
The
principal
disadvantages
of the Brinell
method
are
that
the
machine
to
supply
the
load
for
impressing
the
ball
into
the
material
is often
cumbersome and
cannot
always
produce
the