Penetrant Testing-Course Material.doc

7/24/2019 Penetrant Testing-Course Material.doc

1/59

Penetrant Testing

Course Material
http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/cc_pt_index.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/cc_pt_index.htm


2/59

troductio

Introduction and History of Penetrant Inspection

Liquid Penetrant inspection is a method that is used to reveal surface

breaking flaws by bleedout of a colored or fluorescent dye from the flaw.The technique is based on the ability of a liquid to be drawn into a "clean"

surface breaking flaw by capillary action. After a period of time called the"dwell," ecess surface penetrant is removed and a developer applied.

This acts as a blotter. !t draws the penetrant from the flaw to reveal its

presence. olored #contrast$ penetrants require good white light while

fluorescent penetrants need to be used in darkened conditions with anultraviolet "black light".

A very early surface inspection technique involved the rubbing of carbon black on gla%ed pottery, whereby

the carbon black would settle in surface cracks rendering them visible. Later, it became the practice in

railway workshops to eamine iron and steel components by the "oil and whiting" method. !n this method, aheavy oil commonly available in railway workshops was diluted with kerosene in large tanks so that

locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was

then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once thealcohol had evaporated. The ob&ect was then vibrated by being struck with a hammer, causing the residual

oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part

of the '(th century to approimately '()*, when the magnetic particle method was introduced and found tobe more sensitive for ferromagnetic iron and steels.

A different #though related$ method was introduced in the '()*+s. The surface under eamination was coated

with a lacquer, and after drying, the sample was caused to vibrate by the tap of a hammer. The vibration

causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer #stress coat$ hasbeen used primarily to show the distribution of stresses in a part and not for finding defects.

any of these early developments were carried out by agnaflu in hicago, !L, -A in association with

wit%er /ros., leveland, 01, -A. ore effective penetrating oils containing highly visible #usually red$

dyes were developed by agnaflu to enhance flaw detection capability. This method, known as the visibleor color contrast dye penetrant method, is still used quite etensively today. !n '()2, agnaflu introduced

the 3yglo system of penetrant inspection where fluorescentdyes were added to the liquid penetrant. These

dyes would then fluoresce when eposed to ultraviolet light #sometimes referred to as "black light"$rendering indications from cracks and other surface flaws more readily visible to inspectors.

Why a Penetrant Inspection Improves the Detectability of Flaws
http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt1.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt2.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt3.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt4.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt1.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt2.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt3.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/Keywords/pt4.htm


3/59

The advantage that a liquid penetrant inspection #LP!$ offers over an unaided visual inspection is that it

makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection processmakes flaws more easily seen. 4irst, LP! produces a flaw indication that is much larger and easier for the eye

to detect than the flaw itself. any flaws are so small or narrow that they are undetectable by the unaided

eye. 5ue to the physical features of the eye, there is a threshold below which ob&ects cannot be resolved. Thisthreshold of visual acuity is around *.**6 inch for a person with 2*72* vision.

The second way that LP! improves the detectability of a flaw is that it produces aflaw indication with a high level of contrast between the indication and the

background also helping to make the indication more easily seen. 8hen a visibledye penetrant inspection is performed, the penetrant materials are formulated

using a bright red dye that provides for a high level of contrast between the white

developer. !n other words, the developer serves as a high contrast background aswell as a blotter to pull the trapped penetrant from the flaw. 8hen a fluorescent

penetrant inspection is performed, the penetrant materials are formulated to glow

brightly and to give off light at a wavelength that the eye is most sensitive to

under dim lighting conditions.

Additional information on the human eye can be found as below.

isual !cuity of the Human "ye

The eye has a visual acuity threshold below which an ob&ect will go undetected. This threshold varies from

person to person, but as an eample, the case of a person with normal 2*72* vision can be considered. As

light enters the eye through the pupil, it passes through the lens and is pro&ected on the retina at the back ofthe eye. uscles called etraocular muscles move the eyeball in the orbits and allow the image to be focused

on the central retinal or fovea.

The retina is a mosaic of two basic types of photoreceptors9 rods, and cones. :ods are sensitive to blue;green

light with peak sensitivity at a wavelength of )(< nm, and are used for vision under dark or dim conditions.There are three types of cones that give us our basic color vision9 L;cones #red$ with a peak sensitivity of =>)

nm, ;cones #green$ with a peak sensitivity of =66 nm, and ;cones #blue$ with a peak sensitivity of )6?

nm.


4/59

ones are highly concentrated in a region near the center of the retina called the fovea region. The maimum

concentration of cones is roughly '


5/59

4or the case of normal visual acuity the angle Theta is '7>* of a degree. /y bisecting this angle we have a

right triangle with angle Theta72 that is '7'2* of a degree. -sing this right triangle it is easy to calculate thedistance B72 for a given distance d.

B72 C d #tan Theta72$

8hen visually inspecting an ob&ect for a defect such as a crack, the distance #d$ might be around '2 inches.

This would be a comfortable viewing distance. At '2 inches, the normal visual acuity of the human eye is*.**6)( inch. 8hat this means is that if you had alternating black and white lines that were all *.**6)( inch

wide, it would appear to most people as a mass of solid gray.

Contrast #ensitivity

8hen conducting a visible dye penetrant inspection, the contrast sensitivity of the eye is important. ontrast

sensitivity is a measure of how faded or washed out an image can be before it becomes indistinguishablefrom a uniform field. !t has been eperimentally determined that the minimum discernible difference in gray

scale level that the eye can detect is about 2D of full brightness. ontrast sensitivity is a function of the si%e

or spatial frequency of the features in the image. 1owever, this is not a direct relationship as larger ob&ects

are not always easier to see than smaller ob&ects #as demonstrated by the image below$.

!n the image below, the luminance of piels is varied sinusoidally in the hori%ontal direction. The spatial

frequency increases eponentially from left to right. The contrast also varies logarithmically from '**D at

the bottom to about *.=D at the top. The luminance of peaks and troughs remains constant along a givenhori%ontal path through the image. !f the detection of contrast was dictated solely by image contrast, the

alternating bright and dark bars should appear to have equal height everywhere in the image. 1owever, the

bars seem to be taller in the middle of the image.

ampbell, 4. 8. and :obson, E. F. #'(>


6/59

The Human "ye$s %esponse to &ight

The three curves in the figure above shows the normali%ed response of an average human eye to various

amounts of ambient light. The shift in sensitivity occurs because two types of photoreceptors called cones

and rods are responsible for the eye+s response to light. The curve on the right shows the eye+s response undernormal lighting conditions and this is called the photopic response. The cones respond to light under these

conditions.

As mentioned previously, cones are composed of three different photo pigments that enable color perception.

This curve peaks at === nanometers, which means that under normal lighting conditions, the eye is mostsensitive to a yellowish;green color. 8hen the light levels drop to near total darkness, the response of the eye

changes significantly as shown by the scotopic response curve on the left. At this level of light, the rods are

most active and the human eye is more sensitive to the light present, and less sensitive to the range of color.:ods are highly sensitive to light but are comprised of a single photo pigment, which accounts for the loss in

ability to discriminate color. At this very low light level, sensitivity to blue, violet, and ultraviolet is

increased, but sensitivity to yellow and red is reduced. The heavier curve in the middle represents the eye+sresponse at the ambient light level found in a typical inspection booth. This curve peaks at ==* nanometers,

which means the eye is most sensitive to yellowish;green color at this light level. 4luorescent penetrant

inspection materials are designed to fluoresce at around ==* nanometers to produce optimal sensitivity under

dim lighting conditions.

:eferences9

:obinson, . E. and chmidt, E. T., 4luorescent Penetrant ensitivity and :emovability ; 8hat the Hye an

ee, a 4luorometer an easure, aterials Hvaluation, Gol. )2, @o.


7/59

'asic Processing #teps of a &i(uid Penetrant Inspection

'. Surface Preparation: 0ne of the most critical steps of a liquid penetrant inspection is the surface

preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent

penetrant from entering flaws. The sample may also require etching if mechanical operations such as

machining, sanding, or grit blasting have been performed. These and other mechanical operations cansmear metal over the flaw opening and prevent the Penetrant from entering.

2. Penetrant Application:0nce the surface has been thoroughly cleaned and dried, the penetrantmaterial is applied by spraying, brushing, or

immersing the part in a penetrant bath

6. Penetrant Dwell: The Penetrant is left on the

surface for a sufficient time to allow as much

Penetrant as possible to be drawn from or to seepinto a defect. Penetrant dwell time is the total time

that the penetrant is in contact with the part surface.

5well times are usually recommended by the penetrant producers or required by the specification

being followed. The times vary depending on the application, penetrant materials used, the material,the form of the material being inspected, and the type of defect being inspected for. inimum dwell

times typically range from five to >* minutes. Fenerally, there is no harm in using a longer penetrantdwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by

eperimentation and may be very specific to a particular application.

). Excess Penetrant Removal:This is the most delicate part of the inspection procedure because theecess penetrant must be removed from the surface of the sample while removing as little penetrant

as possible from defects. 5epending on the penetrant system used, this step may involve cleaning

with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsingwith water.

=. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant

trapped in flaws back to the surface where it will be visible. 5evelopers come in a variety of forms

that may be applied by dusting #dry powdered$, dipping, or spraying #wet developers$.

>. Indication Development:The developer is allowed to stand on the part surface for a period of time

sufficient to permit the etraction of the trapped penetrant out of any surface flaws. This development


8/59

time is usually a minimum of '* minutes. ignificantly longer times may be necessary for tight

cracks.

?. Inspection:!nspection is then performed under appropriate lighting to detect indications from any

flaws which may be present.


9/59

Frinding cracks

0verload and impact fractures

Porosity

Laps

eams

Pin holes in welds

Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the ma&or limitations of a penetrant inspection is that flaws must be open to thesurface. To learn more about the advantages and disadvantages of LP!, proceed to the net page.

!dvantages and Disadvantages of Penetrant Testing

Like all nondestructive inspection methods, liquid penetrant inspection has both advantages and

disadvantages. The primary advantages and disadvantages when compared to other @5H methods aresummari%ed below.

Primary Advantages

The method has high sensitivity to small surface discontinuities.

The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic,

and conductive and nonconductive materials may be inspected.

Large areas and large volumes of parts7materials can be inspected rapidly and at low cost.

Parts with comple geometric shapes are routinely inspected.

!ndications are produced directly on the surface of the part and constitute a visual representation of

the flaw.

Aerosol spray cans make penetrant materials very portable.

Penetrant materials and associated equipment are relatively inepensive.

Primary Disadvantages

0nly surface breaking defects can be detected.

0nly materials with a relatively nonporous surface can be inspected.

Precleaning is critical since contaminants can mask defects.

etal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LP!.

The inspector must have direct access to the surface being inspected.

urface finish and roughness can affect inspection sensitivity.

ultiple process operations must be performed and controlled.

Post cleaning of acceptable parts or materials is required.

hemical handling and proper disposal is required.

Penetrant Testing Materials


10/59

The penetrant materials used today are much more sophisticated than the kerosene and whiting first used by

railroad inspectors near the turn of the 2*th century. Today+s penetrants are carefully formulated to producethe level of sensitivity desired by the inspector. To perform well, a penetrant must possess a number of

important characteristics. A penetrant must9

spread easily over the surface of the material being inspected to provide complete and even coverage.

be drawn into surface breaking defects by capillary action.

remain in the defect but remove easily from the surface of the part.

remain fluid so it can be drawn back to the surface of the part through the drying and developing

steps.

be highly visible or fluoresce brightly to produce easy to see indications.

not be harmful to the material being tested or the inspector.

All penetrant materials do not perform the same and are not

designed to perform the same. Penetrant manufactures have

developed different formulations to address a variety ofinspection applications. ome applications call for the detection

of the smallest defects possible and have smooth surfaces where

the penetrant is easy to remove. !n other applications, there&ectable defect si%e may be larger and a penetrant formulated to

find larger flaws can be used. The penetrants that are used to

detect the smallest defect will also produce the largest amount ofirrelevant indications.

Penetrant materials are classified in the various industry and

government specifications by their physical characteristics and

their performance. Aerospace aterial pecification #A$2>)), !nspection aterial, Penetrant, is now the primary specification used in the -A to control penetrant

materials. 1istorically, ilitary tandard 2='6=, !nspection aterials, Penetrants, has been the primary

document for specifying penetrants but this document is slowly being phased out and replaced by A2>)). 0ther specifications such as AT ')'?, tandard Practice for Liquid Penetrant Haminations, may

also contain information on the classification of penetrant materials but they are generally referred back to

!L;!;2='6= or A 2>)).

Penetrant materials come in two basic types. These types are listed below9

Type ' ; 4luorescent Penetrants

Type 2 ; Gisible Penetrants

4luorescent penetrants contain a dye or several dyes that

fluoresce when eposed to ultraviolet radiation. Gisible

penetrants contain a red dye that provides high contrast againstthe white developer background. 4luorescent penetrant systems

are more sensitive than visible penetrant systems because the

eye is drawn to the glow of the fluorescing indication.

1owever, visible penetrants do not require a darkened area andan ultraviolet light in order to make an inspection. Gisible

penetrants are also less vulnerable to contamination from things

such as cleaning fluid that can significantly reduce the strengthof a fluorescent indication.

Penetrants are then classified by the method used to remove the ecess penetrant from the part. The four

methods are listed below9


11/59

ethod A ; 8ater 8ashable

ethod / ; Post;Hmulsifiable, Lipophilic

ethod ; olvent :emovable

ethod 5 ; Post;Hmulsifiable, 1ydrophilic

8ater washable #ethod A$ penetrants can be removed from the part by rinsing with water alone. These

penetrants contain an emulsifying agent #detergent$ that makes it possible to wash the penetrant from the partsurface with water alone. 8ater washable penetrants are sometimes referred to as self;emulsifying systems.

Post;emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. !n post;emulsifiers, lipophilicsystems #ethod /$, the penetrant is oil soluble and interacts with the oil;based emulsifier to make removal

possible. Post;emulsifiable, hydrophilic systems #ethod 5$, use an emulsifier that is a water soluble

detergent which lifts the ecess penetrant from the surface of the part with a water wash. olvent removablepenetrants require the use of a solvent to remove the penetrant from the part.

Penetrants are then classified based on the strength or detectability of the indication that is produced for a

number of very small and tight fatigue cracks. The five sensitivity levels are shown below9

Level K ; -ltra Low ensitivity Level ' ; Low ensitivity

Level 2 ; edium ensitivity

Level 6 ; 1igh ensitivity

Level ) ; -ltra;1igh ensitivity

The ma&or - government and industry specifications currently rely on the - Air 4orce aterials

Laboratory at 8right;Patterson Air 4orce /ase to classify penetrants into one of the five sensitivity levels.This procedure uses titanium and !nconel specimens with small surface cracks produced in low cycle fatigue

bending to classify penetrant systems. The brightness of the indication produced is measured using a

photometer. The sensitivity levels and the test procedure used can be found in ilitary pecification !L;!;2='6= and Aerospace aterial pecification 2>)), Penetrant !nspection aterials.

An interesting note about the sensitivity levels is that only four levels were originally planned. 1owever,

when some penetrants were &udged to have sensitivities significantly less than most others in the level '

category, the K level was created. An ecellent historical summary of the development of test specimens forevaluating the performance of penetrant materials can be found in the following reference.

:eference9

4laherty, E. E., 1istory of Penetrants9 The 4irst 2* ears, '()';>', aterials Hvaluation, Gol. )), @o. '2,

@ovember '(, pp. '6?';'6?), '6?>, '6?


12/59

capability, viscosity, color, brightness, ultraviolet stability, thermal stability, water tolerance, and

removability.

ore information on how some of these properties can affect penetrant testing can be found by followingthese links.


13/59

#urface "nergy *#urface Wetting Capability+

As previously mentioned, one of the important characteristics of a liquid penetrant material is its ability to

freely wet the surface of the ob&ect being inspected. At the liquid;solid surface interface, if the molecules of

the liquid have a stronger attraction to the molecules of the solid surface than to each other #the adhesiveforces are stronger than the cohesive forces$, wetting of the surface occurs. Alternately, if the liquid

molecules are more strongly attracted to each other than the molecules of the solid surface #the cohesiveforces are stronger than the adhesive forces$, the liquid beads;up and does not wet the surface of the part.

0ne way to quantify a liquid+s surface wetting characteristics is to measure the contact angle of a drop of

liquid placed on the surface of an ob&ect. The contact angle is the angle formed by the solid7liquid interface

and the liquid7vapor interface measured from the side of the liquid. #ee the figure below.$ Liquids wet

surfaces when the contact angle is less than (* degrees. 4or a penetrant material to be effective, the contactangle should be as small as possible. !n fact, the contact angle for most liquid penetrants is very close to %ero

degrees.

8etting ability of a liquid is a function of the surface energies of the solid;gas interface, the liquid;gas

interface, and the solid;liquid interface. The surface energy across an interface or the surface tension at the

interface is a measure of the energy required to form a unit area of new surface at the interface. Theintermolecular bonds or cohesive forces between the molecules of a liquid cause surface tension. 8hen the

liquid encounters another substance, there is usually an attraction between the two materials. The adhesive

forces between the liquid and the second substance will compete against the cohesive forces of the liquid.Liquids with weak cohesive bonds and a strong attraction to another material #or the desire to create adhesive

bonds$ will tend to spread over the material. Liquids with strong cohesive bonds and weaker adhesive forces

will tend to bead;up or form a droplet when in contact with another material.

!n liquid penetrant testing, there are usually three surface interfaces involved, the solid;gas interface, theliquid;gas interface, and the solid;liquid interface. 4or a liquid to spread over the surface of a part, two

conditions must be met. 4irst, the surface energy of the solid;gas interface must be greater than the combined

surface energies of the liquid;gas and the solid;liquid interfaces. econd, the surface energy of the solid;gasinterface must eceed the surface energy of the solid;liquid interface.

A penetrant+s wetting characteristics are also largely responsible for its ability to fill a void. Penetrant

materials are often pulled into surface breaking defects by capillary action. The capillary force driving the

penetrant into the crack is a function of the surface tension of the liquid;gas interface, the contact angle, andthe si%e of the defect opening. The driving force for the capillary action can be epressed as the following

formula9

Force ! r"#cos

8here9


14/59

r C radius of the crack opening #2pr is the line of contact between the liquid and the solid tubular surface.$

LFC liquid;gas surface tension

C contact angle

ince pressure is the force over a given area, it can be written that the pressure developed, called thecapillary pressure, is

Capillary Pressure $!

"#cos

%& r

The above equations are for a cylindrical defect but the relationships of the variables are the same for a flaw

with a noncircular cross section. apillary pressure equations only apply when there is simultaneous contact

of the penetrant along the entire length of the crack opening and a liquid front forms that is an equidistantfrom the surface. A liquid penetrant surface could take;on a comple shape as a consequence of the various

deviations from flat parallel walls that an actual crack could have. !n this case, the epression for pressure is

Capillary Pressure !$S#' s S"%&r ! &r

8here9

FC the surface energy at the solid;gas interface.

LC the surface energy at the solid;liquid interface.

r C the radius of the opening.

C the adhesion tension #F; L$.

Therefore, at times, it is the adhesion tension that is primarily responsible for a penetrant+s movement into a

flaw and not the surface energy of the liquid;gas interface. Adhesion tension is the force acting on a unit

length of the wetting line from the direction of the solid. The wetting performance of the penetrant isdegraded when adhesion tension is the primary driving force.

!t can be seen from the equations in this section that the surface wetting characteristics #defined by the

surface energies$ are important in order for a penetrant to fill a void. A liquid penetrant will continue to fill

the void until an opposing force balances the capillary pressure. This force is usually the pressure of trappedgas in a void, as most flaws are open only at the surface of the part. ince the gas originally in a flaw volume

cannot escape through the layer of penetrant, the gas is compressed near the closed end of a void.

ince the contact angle for penetrants is very close to %ero, other methods have been devised to make relative

comparisons of the wetting characteristics of these liquids. 0ne method is to measure the height that a liquidreaches in a capillary tube. 1owever, the solid interface in this method is usually glass and may not

accurately represent the surface that the penetrant inspection will be performed on. Another method of

comparative evaluation is to measure the radius, the diameter, or the area of a spot formed when a drop ofpenetrant is placed on the test surface and allowed to stand undisturbed for a specific amount of time.

1owever, using this method, other factors are also acting in the comparison. These methods include the

density, viscosity, and volatility of the liquid, which do not enter into the capillarity equations, but may havean effect on the inspection as discussed in the related pages.

:eferences9

art%, L., @ondestructive Testing, A !nternational, aterials Park, 01, '((=, pp. '6=;'6>.

Tugrul, A. /., apillarity Hffect Analysis for Alternative Liquid Penetrant hemicals, @5T M H

!nternational, Golume 6* @umber ', Published by Hlsevier cience Ltd., 0ford Hngland, 4ebruary '((?, pp.'(;26.


15/59

Density or #pecific ,ravity

The density or specific gravity of a penetrant material has a slight to negligible effect on the performance of

a penetrant. The gravitational force acting on the penetrant liquid can be working either with or against the

capillary force depending on the orientation of the flaw during the dwell cycle. 8hen the gravitational pull isworking against the capillary rise, the strength of the force is given by the following equation9

Force r!(pg

8here9

r C radius of the crack opening

h C height of penetrant above its free surfacep C density of the penetrant

g C acceleration due to gravity

8hen the direction of capillary flow is in the same direction as the force of gravity, the added force driving

the penetrant into the flaw is given by the formula shown below9

Force (Ap

8here9

h C height of penetrant above its free surface

A C cross;sectional area of the openingP C density of the penetrant

!ncreasing the specific gravity by decreasing the percent of solvent #by volume$ in the solution will increase

the penetration speed.

:eference9

Tugrul, A. /., apillarity Hffect Analysis for Alternative Liquid Penetrant hemicals, @5T M H!nternational, Golume 6* @umber ', Published by Hlsevier cience Ltd., 0ford Hngland, 4ebruary '((?, pp.

'(;26.

iscosity

Giscosity describes a fluid+s resistance to flow. Liquids such as water that flow easily, have a lower viscosity

than do liquids such as ketchup. Giscosity has little effect on the ability of a penetrant material to enter adefect but it does have an effect on the speed at which the penetrant fills a defect. The equations for the fill

times of a cylindrical void and an elliptical void are shown below9

ylindrical Goid

Fill time $!l!%& rcos"#

Hlliptical Goid

Fill time )$!l!% & "#cos* + )a!,-!& $a,-%a-*

8here9 lC defect depth

C viscosity

rC radius of the crack opening

LFC liquid;gas surface tension


16/59

C contact angle

a C flaw widthb C flaw length

4rom these equations, it can be seen that fill time is directly proportional to penetrant viscosity. 8hile it has

no real bearing on this discussion, it should be noted that the two equations do not take into account

entrapped gas that could be present in a closed end capillary.

:eference9

5eutsch, . A, Preliminary tudy of the 4luid echanics of Liquid Penetrant Testing, Eournal of :esearch of

the @ational /ureau of tandards, Gol.


17/59

Why Things Fluoresce

4luorescent Penetrant materials usually contain several dye compounds that are especially suited for the

production of fluorescence. 4luorescence is the process wherein a molecule absorbs a photon of radiantenergy at a particular wavelength and then quickly re;emits the energy at a slightly longer wavelength. !t is

the rapid and short;term re;remittance of energy that distinguishes fluorescence from phosphorescence.

Phosphorescence is usually the result of a chemical reaction which sustains the release of energy for a

significant period of time. 4luorescence was first described in the siteenth century and was probablyobserved long before that time since a large number of plant and animal products fluoresce.

The phenomenon of fluorescence requires a short lesson in quantum mechanics which eplains why

fluorescence was not understood until the twentieth century. !n the nineteenth century, 1uygen+s wave theoryof light had replaced @ewton+s concept of the particulate nature of light and fluorescence was one of the

embarrassing phenomena which simply could not be eplained by use of the wave theory. The wave theory,

as with most classical physics, generally assumes change to be a continuous process with no abrupt changes.@ear the beginning of the twentieth century, a Planck suggested that energy changes might occur in a

stepwise manner. This concept forms the basis of quantum mechanics and Hinstein applied the quantum

concept of energy to light and revived the idea of the particulate nature of light. Planck formali%ed the

relationship with the equation shown below9

E (n

8here9

H C energy

h C a constant

nC the frequency of light

This equations shows that the si%e of the energy steps change with the

frequency or wavelength of the light. Hinstein introduced the term photonto describe the smallest increments of light.

!n today+s current model of the atom, protons and neutrons are found in the nucleus and electrons are found

spinning around outside the nucleus. Hlectrons spin and rotate around the nucleus billions of times a second.

According to modern theory, electrons are arranged in energy levels as they rotate around the nucleus. 8henelectrons gain or lose energy, they &ump between energy levels as they are rotating around the nucleus. As

electrons gain energy, they move to the third, or outer level and as they lose energy, they move to the inner or

first energy level. ince the energy of the system is restricted to certain energy values, the atom is said to be

quanti%ed. !n the animated image below, it can be seen that the electrons move to a different energy stateonly when a specific amount of energy is added to or removed from the system.


18/59

Another way of illustrating this point is with an energy diagram as presented below. This diagram shows the

quanti%ed energy levels for an atom. Hach energy level corresponds to a quantum state of the atom. Thelowest energy state is called the ground state and is the H* line in the diagram. !f energy is added to the

system, an electron or electrons will &ump to a higher level and the atom is said to be at an ecited state. The

upward arrow in the illustration represents a quantum &ump of the atom from the ground state to the secondecited state. 5epending on the amount of energy input into the atom, the energy &ump could have been to

any of the levels. 1owever, the &ump must be to one of the levels shown, as the atom cannot have an

intermediate value of energy. Atoms will generally be in their ground state.

8hen considering fluorescence, energy must be considered at a molecular level. 8hen molecules form, twoor more atoms form an association where the energy of the molecule is lower than that of the constituent

atoms when they were separate. The total energy of the molecule is the sum of the energies holding the

nuclei together and the energy of the chemical bonds holding the molecule together. olecules haverotational, vibrational and electronic #due to the electrons$ energy. !t is the vibrational and electronic energies

of the molecule that contribute to fluorescence. olecules, like atoms, will generally be in their ground state.

olecules can move to a greater energy state only when energy is added to their system. 0ne of the ways a

molecule can gain energy is by absorbing light. !f a molecule absorbs light, the energy of the light must beequal to the energy required to put the molecule in one of the higher energy states. 8hen a molecule reaches

an ecited state, it does not stay there for very long. :ather it quickly returns to a lower energy state either

by emitting light or colliding with another atomic particle. 8hen a molecule emits light, the energy of thatlight is equal to the energy difference between the quantum levels that molecules has moved between

Dimensional Threshold of Fluorescence

The dimensional threshold of fluorescence is a property that is not currently controlled by the specifications

but appears to largely determine the sensitivity of a fluorescent penetrant. A. L. 8alters and :. . caster

conducted an eperiment that led to the understanding of this condition. Two optically flat plates of glasswere clamped tightly together. A drop of fluorescent penetrant was placed at the interface of the plates. The

penetrant could be seen migrating in between the plates but when eposed to black light, no fluorescence

was seen. The phenomenon was not fully understood until '(>* when Alburger introduced the concept ofthin;film transition of fluorescent response.

The dimensional magnitudes of typical crack defects correspond to the dimensional thresholds of

fluorescence response which are characteristic of the available penetrant. Alternately stated, the degree of

fluorescence response, under a given intensity of ultraviolet radiation, is dependent on the absorption ofultraviolet radiation, which in turn depends on dye concentration and film thickness. Therefore, the ability of

a penetrant to yield an indication depends primarily on its ability to fluoresce as a very thin film. The

performance of penetrants based on the physical constraints of the dyes can be predicted using /eer+s Lawequation. This law states that the absorption of light by a solution changes eponentially with the

concentration of the solution. This equation does not hold true when very thin layers are involved but works

well to establish general relationships between variables.

I Ioxe'Ct

8here9

IC Transmitted light intensity

IoC !ncident light intensityeC /ase of natural log #2.?'


19/59

This equation states that the intensity of the transmitted energy is directly proportional to the intensity of the

incident light and varies eponentially with the thickness of the penetrant layer and its dye concentration.Therefore, when the dye concentration is increased, the brightness of the thin layer of penetrant generally

increases. 1owever, the dye concentration can only be increased so much before it starts to have a negative

effect on brightness. A eniscus;ethod Apparatus can be used to measure the dimensional threshold offluorescence.

:eferences9

Alburger, E.:., @otes on the 1istory of Testing Panels for !nspection Penetrants, Paper ummaries, @ations

pring onference, @ew 0rleans, LA, Published by A@T, April '(?).

Fram, /., echanisms ontributing to 4luorescence and Gisibility of Penetrants, Proceedings of the 4ifth

!nternational onference on @ondestructive Testing, ay '(>?, pp 22=;266.

)ltraviolet and Thermal #tability of Penetrant Indications

Hposure to intense ultraviolet light and elevated temperatures can have a negative effect on fluorescent

penetrant indications. 4luorescent materials can lose their brightness after a period of eposure to highintensity -G light. 0ne study measured the intensity of fluorescent penetrant indications on a sample that

was sub&ected to multiple -G eposure cycles. Hach cycle consisted of '= minutes of *o4$. !t

should be noted that the sensitivity of an 4P! inspection can be improved if a part is heated prior to applying


20/59

the penetrant material, but the temperature should be kept below ?' o #'>*o4$. ome high temperature

penetrants in use today are formulated with dyes with high melting points, greatly reducing heat relatedproblems. The penetrants also have high boiling points and the heat related problems are greatly reduced.

1owever, a loss of brightness can still take place when the penetrant is eposed to elevated temperatures

over an etended period of time. 8hen one heat resistant formulation was tested, a 2* D reduction wasmeasured after the material was sub&ected to '>6o #62=o4$ for 2?6 hours. The various types of fluorescent

dyes commonly employed in today+s penetrant materials begin decomposition at ?'o #'>*o4$. 8hen the

temperature approaches ()

o

#2**

o

4$, there is almost total attenuation of fluorescent brightness of thecomposition and sublimation of the fluorescent dyestuffs.

:eferences9

/rittain, P.!., Assessment of Penetrant ystems by 4luorescent !ntensity, Proceedings of the )th Huropean

onference on @ondestructive Testing, Gol. ), Published by Perganon Press, '(


21/59

). ethod 59 Post;Hmulsifiable, 1ydrophilic

ethod relies on a solvent cleaner to remove the penetrant from the part being inspected. ethod A has

emulsifiers built into the penetrant liquid that makes it possible to remove the ecess penetrant with a simplewater wash. ethod / and 5 penetrants require an additional processing step where a separate

emulsification agent is applied to make the ecess penetrant more removable with a water wash. Lipophilic

emulsification systems are oil;based materials that are supplied in ready;to;use form. 1ydrophilic systems

are water;based and supplied as a concentrate that must be diluted with water prior to use .

Lipophilic emulsifiers #ethod /$ were introduced in the late '(=*+s and work with both a chemical and

mechanical action. After the emulsifier has coated the surface of the ob&ect, mechanical action starts to

remove some of the ecess penetrant as the miture drains from the part. 5uring the emulsification time, theemulsifier diffuses into the remaining penetrant and the resulting miture is easily removed with a water

spray.

1ydrophilic emulsifiers #ethod 5$ also remove the ecess penetrant with mechanical and chemical action

but the action is different because no diffusion takes place. 1ydrophilic emulsifiers are basically detergentsthat contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities

and prevents these pieces from recombining or reattaching to the surface of the part. The mechanical actionof the rinse water removes the displaced penetrant from the part and causes fresh remover to contact and liftnewly eposed penetrant from the surface.

The hydrophilic post;emulsifiable method #ethod 5$ was

introduced in the mid '(?*+s. ince it is more sensitive than the

lipophilic post emulsifiable method it has made the later methodvirtually obsolete. The ma&or advantage of hydrophilic emulsifiers is

that they are less sensitive to variation in the contact and removal

time. 8hile emulsification time should be controlled as closely aspossible, a variation of one minute or more in the contact time will

have little effect on flaw detectability when a hydrophilic emulsifier is

used. 1owever, a variation of as little as '= to 6* seconds can have asignificant effect when a lipophilic system is used.

:eferences9

;; /oisvert, /.8., 1ardy, F., 5organ, E.4., and elner, :.1., The 4luorescent Penetrant 1ydrophilic :emover

Process, aterials Hvaluation, 4ebruary '(


22/59

'. 4orm a ; 5ry Powder

2. 4orm b ; 8ater oluble

6. 4orm c ; 8ater uspendable

). 4orm d ; @onaqueous Type ' 4luorescent #olvent /ased$

=. 4orm e ; @onaqueous Type 2 Gisible 5ye #olvent /ased$

>. 4orm f ; pecial Applications

The developer classifications are based on the method that the developer is applied. The developer can be

applied as a dry powder, or dissolved or suspended in a liquid carrier. Hach of the developer forms hasadvantages and disadvantages.

Dry Powder

5ry powder developer is generally considered to be the

least sensitive but it is inepensive to use and easy to

apply. 5ry developers are white, fluffy powders that canbe applied to a thoroughly dry surface in a number of

ways. The developer can be applied by dipping parts in a

container of developer, or by using a puffer to dust partswith the developer. Parts can also be placed in a dust

cabinet where the developer is blown around and allowed

to settle on the part. Hlectrostatic powder spray guns arealso available to apply the developer. The goal is to allow

the developer to come in contact with the whole

inspection area.

-nless the part is electrostatically charged, the powderwill only adhere to areas where trapped penetrant has wet the surface of the part. The penetrant will try to

wet the surface of the penetrant particle and fill the voids between the particles, which brings more penetrant

to the surface of the part where it can be seen. ince dry powder developers only stick to the area wherepenetrant is present, the dry developer does not provide a uniform white background as the other forms of

developers do. 1aving a uniform light background is very important for a visible inspection to be effective

and since dry developers do not provide one, they are seldom used for visible inspections. 8hen a dry

developer is used, indications tend to stay bright and sharp since the penetrant has a limited amount of roomto spread.

/ater Solu-le

As the name implies, water soluble developers consist of a group of

chemicals that are dissolved in water and form a developer layerwhen the water is evaporated away. The best method for applying

water soluble developers is by spraying it on the part. The part can be

wet or dry. 5ipping, pouring, or brushing the solution on to thesurface is sometimes used but these methods are less desirable.

Aqueous developers contain wetting agents that cause the solution to

function much like dilute hydrophilic emulsifier and can lead to

additional removal of entrapped penetrant. 5rying is achieved byplacing the wet but well drained part in a recirculating, warm air dryer

with the temperature held between ?* and ?=4. !f the parts are not

dried quickly, the indications will will be blurred and indistinct. Properly developed parts will have an even,pale white coating over the entire surface.

/ater Suspenda-le


23/59

8ater suspendable developers consist of insoluble developer particles suspended in water. 8ater suspendable

developers require frequent stirring or agitation to keep the particles from settling out of suspension. 8atersuspendable developers are applied to parts in the same manner as water soluble developers. Parts coated

with a water suspendable developer must be forced dried &ust as parts coated with a water soluble developer

are forced dried. The surface of a part coated with a water suspendable developer will have a slightlytranslucent white coating.

0ona1ueous

@onaqueous developers suspend the developer in a volatile solvent

and are typically applied with a spray gun. @onaqueous developers

are commonly distributed in aerosol spray cans for portability. Thesolvent tends to pull penetrant from the indications by solvent action.

ince the solvent is highly volatile, forced drying is not required. A

nonaqueous developer should be applied to a thoroughly dried part toform a slightly translucent white coating.

Special Applications

Plastic or lacquer developers are special developers that are primarily

used when a permanent record of the inspection is required.

Preparation of Part

0ne of the most critical steps in the penetrant inspection process is preparingthe part for inspection. All coatings, such as paints, varnishes, plating, and

heavy oides must be removed to ensure that defects are open to the surface

of the part. !f the parts have been machined, sanded, or blasted prior to thepenetrant inspection, it is possible that a thin layer of metal may have

smeared across the surface and closed off defects. !t is even possible for metal

smearing to occur as a result of cleaning operations such as grit or vaporblasting. This layer of metal smearing must be removed before inspection.

Contaminants

oatings, such as paint, are much more elastic than metal and will not

fracture even though a large defect may be present &ust below the coating. The

part must be thoroughly cleaned as surface contaminates can prevent the

penetrant from entering a defect. urface contaminants can also lead to ahigher level of background noise since the ecess penetrant may be more

difficult to remove.

ommon coatings and contaminates that must be removed include9 paint, dirt, flu, scale, varnish, oil,

etchant, smut, plating, grease, oide, wa, decals, machining fluid, rust, and residue from previous penetrant

inspections.

ome of these contaminants would obviously prevent penetrant from entering defects, so it is clear they mustbe removed. 1owever, the impact of other contaminants such as the residue from previous penetrant

inspections is less clear, but they can have a disastrous effect on the inspection. Take the link below to review

some of the research that has been done to evaluate the effects of contaminants on LP! sensitivity.

lick here to learn more about possible problems with leaning Practices.
http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/MethodsTech/researchoncleaning.htmhttp://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/MethodsTech/researchoncleaning.htm


24/59

A good cleaning procedure will remove all contamination from the

part and not leave any residue that may interfere with the inspectionprocess. !t has been found that some alkaline cleaners can be

detrimental to the penetrant inspection process if they have silicates in

concentrations above *.= percent. odium metasilicate, sodiumsilicate, and related compounds can adhere to the surface of parts and

form a coating that prevents penetrant entry into cracks. :esearchers

in :ussia have also found that some domestic soaps and commercialdetergents can clog flaw cavities and reduce the wettability of themetal surface, thus reducing the sensitivity of the penetrant. onrad

and audill found that media from plastic media blasting was

partially responsible for loss of LP! indication strength.icrophotographs of cracks after plastic media blasting showed

media entrapment in addition to metal smearing.

!t is very important that the material being inspected has not been

smeared across its own surface during machining or cleaningoperations. !t is well recogni%ed that machining, honing, lapping,

hand sanding, hand scraping, grit blasting, tumble deburring, and

peening operations can cause some materials to smear. !t is perhaps less recogni%ed that some cleaningoperations, such as steam cleaning, can also cause metal smearing in the softer materials. Take the link below

to learn more about metal smearing and its affects on LP!

lick here to learn more about metal smearing.

:eferences9

:obinson, am E., 1ere Today, Fone TomorrowI :eplacing ethyl hloroform in the Penetrant Process,aterials Hvaluation, Gol. =*, @o. , @o. =, August '((,

@o. 6, @ew ork, @ Plenum7onsultants /ureau, Eanuary '((*, pp. 6>';6>?.

Fla%kov, u . A., /ruevich, H.P., and amokhin, @.L, pecial 4eatures of Application of Aqueous olutions

of ommercial 5etergents in apillary 4law !nspection, 5efektoskopiya ; The oviet Eournal of@ondestructive Testing, Gol. '(, @o.


25/59

8hen sensitivity is the primary consideration for choosing a penetrant system, the first decision that must be

made is whether to use fluorescent penetrant or visible dye penetrant. 4luorescent penetrants are generallymore capable of producing a detectable indication from a small defect. Also, the human eye is more sensitive

to a light indication on a dark background and the eye is naturally drawn to a fluorescent indication.

The graph below presents a series of curves that show the contrast ratio required for a spot of a certain

diameter to be seen. The ordinate is the spot diameter, which was viewed from one foot. The abscissa is the

contrast ratio between the spot brightness and the background brightness. To the left of the contrast ratio ofone, the spot is darker than the background #representative of visible dye penetrant testing$Q and to the right

of one, the spot is brighter than the background #representative of fluorescent penetrant inspection$. Hach ofthe three curves right or left of the contrast ratio of one are for different background brightness #in foot;

Lamberts$, but simply consider the general trend of each group of curves right or left of the contrast ratio of

one. The curves show that for indication larger than *.*?> mm #*.**6 inch$ in diameter, it does not reallymatter if it is a dark spot on a light background or a light spot on a dark background. 1owever, when a dark

indication on a light background is further reduced in si%e, it is no longer detectable even though contrast is

increased. 4urthermore, with a light indication on a dark background, indications down to *.**6 mm #*.***'

inch$ were detectable when the contrast between the flaw and the background was high.

4rom this data, it can be seen why a fluorescent penetrant offers an advantage over a visible penetrant forfinding very small defects. 5ata presented by 5e Fraaf and 5e :i&k supports this statement. They inspected

"identical" fatigue cracked specimens using a red dye penetrant and a fluorescent dye penetrant. The

fluorescent penetrant found >* defects while the visible dye was only able to find 6( of the defects.

:ef9 5e Fraaf, H. and 5e :i&k, P., omparison /etween :eliability, ensitivity, and Accuracy of

@ondestructive !nspection ethods, '6th ymposium on @ondestructive Hvaluation Proceedings, an

Antonio, TB, published by @T!A, outhwest :esearch !nstitute, an Antonio, TB, April '(6 Lester 1onor Lecture,@ondestructive Testing, Gol. 2', @o. >, @ov.;5ec. '(>6, pp. 6=);6>


26/59

sensitivity and are very convenient to use. 1owever, they are usually not practical for large area inspection

or in high;volume production settings.

Another consideration in the selection of a penetrant system is whether water washable, post;emulsifiable orsolvent removable penetrants will be used. Post;emulsifiable systems are designed to reduce the possibility

of over;washing, which is one of the factors known to reduce sensitivity. 1owever, these systems add

another step, and thus cost, to the inspection process.

Penetrants are evaluated by the - Air 4orce according to the requirements in !L;!;2='6= and each

penetrant system is classified into one of five sensitivity levels. This procedure uses titanium and !nconel

specimens with small surface cracks produced in low cycle fatigue bending to classify penetrant systems.

The brightness of the indications produced after processing a set of specimens with a particular penetrantsystem is measured using a photometer. A procedure for producing and evaluating the penetrant qualification

specimens was reported on by oore and Larson at the '((? A@T 4all onference. ost commercially

available penetrant materials are listed in the Jualified Products List of !L;!;2='6= according to their type,method and sensitivity level. Gisible dye and dual;purpose penetrants are not classified into sensitivity levels

as fluorescent penetrants are. The sensitivity of a visible dye penetrant is regarded as level ' and largely

dependent on obtaining good contrast between the indication and the background.

Penetrant !pplication and Dwell Time

The penetrant material can be applied in a number of different ways,including spraying, brushing, or immersing the parts in a penetrant

bath. The method of penetrant application has little effect on the

inspection sensitivity but an electrostatic spraying method is reportedto produce slightly better results than other methods. 0nce the part is

covered in penetrant it must be allowed to dwell so the penetrant has

time to enter any defect present.

There are basically two dwell mode options, immersion;dwell#keeping the part immersed in the penetrant during the dwell period$

and drain;dwell #letting the part drain during the dwell period$. Prior

to a study by herwin, the immersion;dwell mode was generallyconsidered to be more sensitive but recogni%ed to be less economical

because more penetrant was washed away and emulsifiers were

contaminated more rapidly. The reasoning for thinking this method was

more sensitive was that the penetrant was more migratory and more likelyto fill flaws when kept completely fluid and not allowed to lose volatile

constituents by evaporation. 1owever, herwin showed that if the

specimens are allowed to drain;dwell, the sensitivity is higher because the

evaporation increases the dyestuff concentration of the penetrant on thespecimen. As pointed;out in the section on penetrant materials, sensitivity

increases as the dyestuff concentration increases. herwin also cautions thatthe samples being inspected should be placed outside the penetrant tank

wall so that vapors from the tank do not accumulate and dilute the dyestuff

concentration of the penetrant on the specimen.

;; Gaerman, E., 4luorescent Penetrant !nspection, Juantified Hvolution ofthe ensitivity Gersus Process 5eviations, Proceedings of the )th Huropean

onference on @ondestructive Testing, Pergamon Press, awell 1ouse,

4airview Park, Hlmsford, @ew ork, Golume ), eptember '(


27/59

Penetrant Dwell 2ime

Penetrant dwell time is the total time that the penetrant is in contact with the part surface. The dwell time is

important because it allows the penetrant the time necessary to seep or be drawn into a defect. 5well timesare usually recommended by the penetrant producers or required by the specification being followed. The

time required to fill a flaw depends on a number of variables which include the following9

The surface tension of the penetrant. The contact angle of the penetrant.

The dynamic shear viscosity of the penetrant, which can vary with the diameter of the capillary. The

viscosity of a penetrant in microcapillary flaws is higher than its viscosity in bulk, which slows theinfiltration of the tight flaws.

The atmospheric pressure at the flaw opening.

The capillary pressure at the flaw opening.

The pressure of the gas trapped in the flaw by the penetrant.

The radius of the flaw or the distance between the flaw walls.

The density or specific gravity of the penetrant.

icrostructural properties of the penetrant.

The ideal dwell time is often determined by eperimentation and is often very specific to a particular

application. 4or eample, A 2>)?A requires that the dwell time for all aircraft and engine parts be at least2* minutes, while AT H'2*( only requires a five minute dwell time for parts made of titanium and other

heat resistant alloys. Fenerally, there is no harm in using a longer penetrant dwell time as long as the

penetrant is not allowed to dry.

The following tables summari%e the dwell time requirements of several commonly used specifications. Theinformation provided below is intended for general reference and no guarantee is made about its correctness.

Please consult the specifications for the actual dwell time requirements.


28/59


29/59

Some Researc( Results on Dwell 2ime

An interesting point that 5eutsch makes about dwell time is that if the elliptical flaw has a length to widthratio of '**, it will take the penetrant nearly ten times longer to fill than it will a cylindrical flaw with the

same volume.


30/59

;; 5eutsch, . A, Preliminary tudy of the 4luid echanics of Liquid Penetrant Testing, Eournal of :esearch

of the @ational /ureau of tandards, Gol.


31/59

emulsifier as an integral part of the formulation. The ecess penetrant may be removed from the ob&ect

surface with a simple water rinse. These materials have the property of forming relatively viscous gels uponcontact with water, which results in the formation of gel;like plugs in surface openings. 8hile they are

completely soluble in water, given enough contact time, the plugs offer a brief period of protection against

rapid wash removal. Thus, water;washable penetrant systems provide ease of use and a high level ofsensitivity.

8hen 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. Post;emulsifiable penetrants require a separate emulsifier to

breakdown the penetrant and make it water washable. The part is usually immersed in the emulsifier buthydrophilic emulsifiers may also be sprayed on the ob&ect. pray application is not recommended for

lipophilic emulsifiers because it can result in non;uniform emulsification if not properly applied. /rushing

the emulsifier on to the part is not recommended either because the bristles of the brush may force emulsifierinto discontinuities, causing the entrapped penetrant to be removed. The emulsifier is allowed 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. The penetrant that has reacted with the emulsifier is easily cleaned away.

ontrolling the reaction time is of essential importance when using a post;emulsifiable system. !f theemulsification time is too short, an ecessive amount of penetrant will be left on the surface, leading to high

background levels. !f the emulsification time is too long, the emulsifier will react with the penetrant

entrapped in discontinuities, making it possible to deplete the amount needed to form an indication.

The hydrophilic post;emulsifiable method #ethod 5$ is more sensitive than the lipophilic post;emulsifiablemethod #ethod /$. ince these methods are generally only used when very high sensitivity is needed, the

hydrophilic method renders the lipophilic method virtually obsolete. The ma&or advantage of hydrophilic

emulsifiers is that they are less sensitive to variation in the contact and removal time. 8hile emulsificationtime should be controlled as closely as possible, a variation of one minute or more in the contact time will

have little effect on flaw detectability when a hydrophilic emulsifier is used. 0n the contrary, a variation of

as little as '= to 6* seconds can have a significant effect when a lipophilic system is used. -sing anemulsifier involves adding a couple of steps to the penetrant process, slightly increases the cost of an

inspection. 8hen using an emulsifier, the penetrant process includes the following steps #etra steps in bold$9

'. pre;clean part, 2. apply penetrant and allow to dwell, 6. pre'rinse to remove first layer of penetrant456

apply (ydrop(ilic emulsifier and allow contact for specified time4=. rinse to remove ecess penetrant, >.

dry part, ?. apply developer and allow part to develop, and


32/59

two pieces of acrylic with two thin sheets of vinyl clamped between as spaces. The plates were clamped in

the corners and all but one of the edges sealed. The unsealed edge acted as the flaw. The clearance betweenthe plates varied from '= microns #*.*=( inch$ at the clamping points to 6* microns #*.''< inch$ at the

midpoint between the clamps. The distance between the clamping points was believed to be 6* mm #'.'th !nternational onference,

3urich, wit%erland, @ovember ; 5ecember '(?

Developer Form

@onaqueous, 8et olvent

Plastic 4ilm

8ater;oluble

8ater;uspendable8ater;oluble

8ater;uspendable5ry

3et(od of Application

pray

pray

pray

pray!mmersion

!mmersion5ust loud #Hlectrostatic$


33/59

Penetrant Testing-Course Material.doc

Documents

Transcript of Penetrant Testing-Course Material.doc