Introduction to Non-Destructive Tes

Introduction to Non
-
Destructive Testing Techniques
Liquid Penetrant Testing
Page
9
of
20

The radius of the flaw or the distance between the flaw walls.

The density or speci
fic gravity of the penetrant.

Microstructural properties of the penetrant.
The ideal dwell time is often determined by experimentation and is often very specif
ic
to a particular application. For example, the table shows
the dwell time requirements
for st
eel parts according to some of the
commonly used specifications.
Penetrant Removal Process
The penetrant removal procedure must effectively remove the penetrant from the
surface of the part without removing an appreciable amount of entrapped penetrant
from the
discontinuity
. If the removal process extracts penetrant from the flaw, the
flaw indication will be reduced by a proportional amount. If the penetrant is not
effectively removed from the part surface, the contrast between the indication and the
ba
ckground will be reduced.
Removal Method
As mentioned previously, p
enetrant systems are classified into four
types according to
the method used for
excess penetrant removal.
-
Method A: Water
-
Washable
-
Method B: Post
-
Emulsifiable, Lipophilic
-
Method C: Sol
vent Removable
Introduction to Non
-
Destructive Testing Techniques
Liquid Penetrant Testing
Page
10
of
20
-
Method D: Post
-
Emulsifiable, Hydrophilic
Method C, Solvent Removable, is used primarily for inspecting
small localized areas
.
This method requires hand wiping the surface with a
cloth moistened with the solvent
remover, and is, therefore,
too
labor intensive
for most production situations.
Method A, Water
-
Washable, is the
most economical
to apply
o
f the
different
methods
and it is easy to use
. Water
-
washable or self
-
emulsifiable penetrants contain an
emulsifier as an integral part of the
formulation. The excess penetrant may be
removed from the object sur
face with a simple water rinse.
When removal of the penetrant from the defect due to over
-
washing of the part is a
concern, a post
-
emulsifiable penetrant system can be used.
The p
ost
-
emuls
ifiable
methods are generally only used when very high sensitivity is needed
.
Post
-
emulsifiable penetrants
require a separate emulsifier
to breakdown the penetrant and
make it water washable. The part is usually
immersed
in the emulsifier but hydrophilic
e
mulsifiers may also be sprayed on the object. Brushing the emulsifier on to the part is
not recommended either because the bristles of the brush may force emulsifier into
discontinuities, causing the entrapped penetrant to be removed. The emulsifier is
all
owed
sufficient time to react with the penetrant on the surface
of the part but
not
given time to make its way into defects
to react with the trapped penetrant.
Controlling the reaction time is of essential importance when using a post
-
emulsifiable
system.
If the emulsification time is too short, an excessive amount of penetrant will
be left on the surface, leading to high background levels. If the emulsification time is
too long, the emulsifier will react with the penetrant entrapped in discontinuities,
ma
king it possible to deplete the amount needed to form an indication.
The hydrophilic post
-
emulsifiable method (
Method D
) is
more sensitive
than the
lipophilic post
-
emulsifiable method (
Method B
). The major
advantage
of hydrophilic
emulsifiers is that they
are
less sensitive to variation in the contact and removal time
.
When using an emulsifiable penetrant is used
, the penetrant
inspection
process
includes the following steps (
extra steps
are underli
n
ed
):
1
. pre
-
clean part,
2
. apply
penetrant and allow to
dwell,
3
.
pre
-
rinse to remove first layer of penetrant
,
4
.
apply
hydrophilic emulsifier and allow contact for specified time
,
5
. rinse to remove excess
penetrant,
6
. dry part,
7
. apply developer and allow part to develop, and
8
. inspect.
Rinse Method and
Time for Water
-
Washable Penetrants
The method used to rinse the excess penetrant from the object surface and the time of
the rinse should be controlled so as to prevent over
-
washing. It is generally
Introduction to Non
-
Destructive Testing Techniques
Liquid Penetrant Testing
Page
11
of
20
recommended that a coarse spray rinse or an air
-
agitated,
immersion wash tank
be
used. When a
spray
is being used, it should be directed at a 45° angle to the part
surface so as to not force water directly into any discontinuities that may be present.
The spray or immersion time should be kept to a minimum throu
gh frequent
inspections of the remaining background level.
Hand Wiping of Solvent Removable Penetrants
When a solvent removable penetrant is used, care must also be taken to carefully
remove the penetrant from the part surface while removing as little as
possible from
the flaw. The first step in this cleaning procedure is to
dry wipe
the surface of the part
in one direction
using a white, lint
-
free, cotton rag. One dry pass in one direction is all
that should be used to remove as much penetrant as possibl
e. Next, the surface should
be wiped with
one pass in one direction with a rag moistened with cleaner
.
One dry
pass followed by one damp pass
is all that is recommended. Additional wiping may
sometimes be necessary; but keep in mind that with every additio
nal wipe, some of
the entrapped penetrant will be removed and inspection sensitivity will be reduced.
Use and Selection of a Developer
The use of developer is almost always recommended.
T
he output from a fluorescent
penetrant
is improved significantly
wh
en a suitable powder developer
is
used.
Also,
the
use of developer can have a dramatic effect on the probability of detection
of an
inspection.
Nonaqueous developers are generally recognized as the
most sensitive
when properly
applied
.
However, if the thic
kness of the coating becomes too great, defects can be
masked. The relative sensitivities of developers and application techniques as ranked
in
Volume II of the Nondestructive Testing Handbook
are shown in the table below.
Ranking
1
2
3
4
5
6
7
8
9
10
Dev
eloper Form
Nonaqueous, Wet Solvent
Plastic Film
Water
-
Soluble
Water
-
Suspendable
Water
-
Soluble
Water
-
Suspendable
Dry
Dry
Dry
Dry
Method of Application
Spray
Spray
Spray
Spray
Immersion
Immersion
Dust Cloud (Electrostatic)
Fluidized Bed
Dust Cloud (Air Agit
ation)
Immersion (Dip)
Introduction to Non
-
Destructive Testing Techniques
Liquid Penetrant Testing
Page
12
of
20
The following table lists the main advantages and disadvantages of the various
developer types.
Developer
Advantages
Disadvantages
Dry
Indications tend to remain
brighter and more distinct
over time
Easily to apply
Does not form
contrast
background so cannot be
used with visible systems
Difficult to assure entire part
surface has been coated
Soluble
Ease of coating entire part
White coating for good
contrast can be produced
which work well for both
visible and fluorescent
system
s
Coating is translucent and
provides poor contrast (
not
recommended for visual
systems
)
Indications for water
washable systems are dim
and blurred
Suspendable
Ease of coating entire part
Indications are bright and
sharp
White coating for good
contrast
can be produced
which work well for both
visible and fluorescent
systems
Indications weaken and
become diffused after time
Nonaqueous
Very portable
Easy to apply to readily
accessible surfaces
White coating for good
contrast can be produced
which work w
ell for both
visible and fluorescent
systems
Indications show
-
up rapidly
and are well defined
Provides highest sensitivity
Difficult to apply evenly to
all surfaces
More difficult to clean part
after inspection
Introduction to Non
-
Destructive Testing Techniques
Liquid Penetrant Testing
Page
13
of
20
Quality
&
Process
Control
Quality control of the penetrant inspection process
is
essential
to get
good and
consistent
results
.
Since several steps and materials are involved in the
inspection
process
, there are quality control procedures for each of them.
Temperature Co
ntrol
The temperature of the penetrant materials and the part being inspected can have an
effect on the results. Temperatures from 27 to 49
°
C are reported in the literature to
produce
optimal results
. Many specifications
allow testing in the range
of 4 to
52
°
C.
Raising the temperature
beyond this level will significantly
raise the speed of
evaporation of penetrants
causing them to dry out quickly.
Since the surface tension of most materials decrease as the temperature increases,
raising the temperature of t
he penetrant will increase the wetting of the surface and
the capillary forces. Of course, the opposite is also true,
so
lowering the temperature
will have a negative effe
ct on the flow characteristics.
Penetrant Quality
Control
The quality of a penetrant
inspection is highly dependent on the quality of the
penetrant materials used. Only products meeting the requirements of an industry
specification, such as AMS 2644, should be used. Deterioration of new penetrants
primarily results from aging and contamin
ation. Virtually all organic dyes deteriorate
over time, resulting in a loss of color or fluorescent response, but deterioration can be
slowed with proper storage. When possible, keep the materials in a closed container
and protect from freezing and expos
ure to high heat.
Contamination can
occur during storage and use.
Of course, open tank systems are
much more susceptible to contami
nation than are spray systems.
Regular checks must
be performed to ensure that the material performance has not degraded.
When the
penetrant is first received from the manufacturer, a sample of the fresh solution
should be collected and stored as a standard for future comparison. The standard
specimen should be stored in a sealed, opaque glass or metal contain