LIQUID (DYE) PENETRANT

INSPECTION

(LPI)

Introduction This module is intended to provide an

introduction to the NDT method of penetrant testing.

Liquid penetrant inspection (LPI) or Penetrant Testing (PT), is a nondestructive testing method that builds on the principle of Visual Inspection.

PT increases the seeability of small discontinuities that the human eye might not be able to detect alone.

Outline General Introduction Penetrant Materials and Considerations Basic Steps in Penetrant Testing Common Equipment Advantages and Limitations Summary Glossary of Terms

Introduction

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

breaking flaws by bleed out 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 discontinuity or flaw by capillary action

Capillary action is the ability of a liquid to flow in narrow spaces

without the assistance of, or even in opposition to, external forces

like gravity.

The tendency of a liquid to penetrate or migrate into small openings

such as cracks is due to capillary action.

When a liquid wets a surface, the angle of contact or the wetting

angle (defined as the angle between the contact surface and the

tangent at the point of contact) is less than 90o.

Wettability reduces as the contact angle increase.

How Does PT Work? In penetrant testing, a liquid with high surface

wetting characteristics is applied to the surface of a component under test.

The penetrant penetrates into surface breaking discontinuities via capillary action and other mechanisms.

Excess penetrant is removed from the surface and a developer is applied topull trapped penetrant back the surface.

With good inspection technique, visual indications of any discontinuities present become apparent.

What Makes PT Work? Every step of the penetrant

process is done to promote capillary action.

This is the phenomenon of a liquid rising or climbing when confined to small openings due to surface wetting properties of the liquid.

Some examples: Plants and trees draw water up

from the ground to their branches and leaves to supply their nourishment.

The human body has miles of capillaries that carry life sustaining blood to our entire body.

Basic Process of PT

1) Clean & Dry Component 4) Apply Developer

2) Apply Penetrant

3) Remove Excess

5) Visual Inspection

6) Post Clean Component

What Can Be Inspected Via PT?

Almost any

material that

has a relatively

smooth, non-

porous surface

on which

discontinuities

or defects are

suspected.

What Can NOT be Inspected Via PT?Components with rough

surfaces, such as sand castings, that trap and hold penetrant.

Porous ceramics Wood and other fibrous

materials.

Plastic parts that absorb or react with the penetrant materials.

Components with coatings that prevent penetrants from entering defects.

Defect indications become less distinguishable as the background noise level increases.

What Types of Discontinuities

Can Be Detected Via PT?All defects that are open to the surface.

Rolled products-- cracks, seams, laminations.

Castings--cold shuts, hot tears, porosity, blow holes, shrinkage.

Forgings cracks, laps, external bursts.

Welds cracks, porosity, undercut, overlap, lack of fusion, lack of penetration.

Choices of Penetrant Materials

PenetrantClassifications of Penetrants

LPI is classified based on the indicating method into:

1. Dye Penetrants : solution of red dye which reveal a red

indication on white developer

2. Fluorescent penetrants: solution containing fluorescence which

emits green fluorescent indication when viewed in the dark

They are also classified based on the method used for the removal

of excess penetrant into:

1. Water washable

2. Solvent removable

3. Post emulsifier

Developer FormDry Powder

Wet, Water Soluble

Wet, Water Suspendable

Wet, Non-Aqueous

Penetrant Materials

What is a penetrant and what makes a good penetrant?

Basically, any liquid could be considered a penetrant.

However, the liquid penetrant must posses certain properties:

Penetration:

ability to enter fine surface defects in the component under test

Fluidity

ability to drain away from the component well

Stability (under temperature & humidity)

stable over a wide range of temperatures and humidity and should

not lose its volatile constituents while it is stored in open tanks or

drums

Washability

It must be possible to remove excess penetrant from

component surfaces easily without affecting the penetrant

within the defect

Drying characteristics

Must resist drying out during hot air drying of the component

after the wash has been completed

Wetting ability

Must have good wetting ability and be able to maintain a

surface film on the component, and hence continue to feed into

a defect over a considerable period of time

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

Not be harmful to the inspector or to the material being tested

Sensitivity Levels Penetrants are also formulated to produce a variety

of sensitivity levels. The higher the sensitivity level, the smaller the defect that the penetrant system is capable of detecting.

The five sensitivity levels are: Level 4 - Ultra-High Sensitivity

Level 3 - High Sensitivity

Level 2 - Medium Sensitivity

Level 1 - Low Sensitivity

As the sensitivity level increases, so does the number of nonrelevent indications. Therefore, a penetrant needs to be selected that will find the defects of interest but not produce too many nonrelevent indications.

Details on general requirements of

penetrant The industry and military specifications that control penetrant

materials and their use, all stipulate certain physical properties

of the penetrant materials that must be met.

Some of these requirements address the safe use of the

materials, such as toxicity, flash point, and corrosiveness, and

other requirements address storage and contamination issues.

Still others delineate properties that are thought to be primarily

responsible for the performance or sensitivity of the penetrants.

The properties of penetrant materials that are controlled by AMS

2644 and MIL-I-25135E include flash point, surface wetting

capability, viscosity, color, brightness, ultraviolet stability,

thermal stability, water tolerance, and removability.

Properties can affect penetrant testing

Surface Energy

Density or Specific Gravity

Viscosity

Color and Fluorescence Brightness

Dimensional Threshold of Fluorescence

Ultraviolet and Thermal Stability

Removability

Surface Energy (Surface Wetting

Capability)

one of the important characteristics of a liquid penetrant material is its

ability to freely wet the surface of the object 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 adhesive forces 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 cohesive forces are

stronger than the adhesive forces), the liquid beads-up and does not

wet the surface of the part.

The contact angle is the angle formed by the solid/liquid interface and

the liquid/vapor interface measured from the side of the liquid. Liquids

wet surfaces when the contact angle is less than 90 degrees.

For a penetrant material to be effective, the contact angle should be as

small as possible. In fact, the contact angle for most liquid penetrants is

very close to zero degrees.

Capillary Rise

An approximate expression for the distance

travelled by the liquid in a uniform capillary

cross section is given by:

L2 = T cos . t / (2)

L= distance travelled

= coefficient of viscosity

= contact angle

T= surface tension force

t = time taken

L

Good penetrant, must having high wettability

(low contact angle), low viscosity

Density or Specific Gravity

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.

Increasing the specific gravity by decreasing the

percent of solvent (by volume) in the solution will

increase the penetration speed.

Penetrant Color and Fluorescence

The color of the penetrant material is of obvious

importance in a visible dye penetrant inspection, as

the dye must provide good contrast against the

developer or part being inspected.

Sensitivity: the higher the contrast, the easier objects

are to see. The dye used in visible dye penetrant is

usually vibrant red but other colors can be purchased

for special applications.

Fluorescent penetrant When fluorescent materials are involved, the effect of color and

fluorescence is not so straightforward.

PT materials fluoresce because they contain one or more dyes

that absorb electromagnetic radiation over a particular

wavelength and the absorption of photons leads to changes in

the electronic configuration of the molecules.

The radiation absorption and emission could take place a

number of times until the desired color and brightness is

achieved. Two different fluorescent colors can be mixed to

interact by a mechanism called cascading.

The emission of visible light by this process involves one dye

absorbing ultraviolet radiation to emit a band of radiation that

makes a second dye glow. Since the human eye is the most

commonly used sensing device, most penetrants are designed

to fluoresce as close as possible to the eyes' peak response.

Visible Vs Fluorescent PT Inspection can be performed

using visible (or red dye) or fluorescent penetrant materials.

Visible Pt is performed under white light while fluorescent PT must be performed using an ultraviolet light in a darkened area. All are all in the level 1 sensitivity range.

Fluorescent PT is more sensitive than visible PT because the eye is more sensitive to a bright indication on a dark background. Sensitivity ranges from 1 to 4.

Photo Courtesy of Contesco

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.

The degree of fluorescence response, under a given intensity of

ultraviolet radiation, is dependent on the absorption of ultraviolet

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.

Where:

I = Transmitted light intensity

Io = Incident light intensity

e = Base of natural log (2.71828)

l = Absorption coefficient per unit of concentration

C = Dye concentration

t = Thickness of the absorbing layer trolled to a certain degree by the

concentration of the fluorescent tracer dye in the penetrant.

I = Io x e-lCt

The intensity of the transmitted energy is directly

proportional to the intensity of the incident light and

varies exponentially with the thickness of the

penetrant layer and its dye concentration.

When the dye concentration is increased, the

brightness of the thin layer of penetrant generally

increases. However, the dye concentration can

only be increased so much before it starts to have

a negative effect on brightness.

Ultraviolet and Thermal Stability of

Penetrant Indications

Exposure to intense ultraviolet light and elevated temperatures

can have a negative effect on fluorescent penetrant indications.

Fluorescent materials can lose their brightness after a period of

exposure to high intensity UV light.

At an elevated temperature, penetrants can experience heat

degradation or "heat fade." Excessive heat:

1. evaporates the more volatile constituents which increases

viscosity and adversely affects the rate of penetration.

2. alters wash characteristics.

3. "boils off" chemicals that prevent separation.

4. kills the fluorescence of tracer dyes.

Generally, thermal damage occurs when fluorescent

penetrant materials are heated above 71oC (160oF).

It should be noted that the sensitivity of an FPI

inspection can be improved if a part is heated prior to

applying the penetrant material, but the temperature

should be kept below 71oC (160oF).

Some high temperature penetrants in use today are

formulated with dyes with high melting points, greatly

reducing heat related problems.

The penetrants also have high boiling points and the

heat related problems are greatly reduced. However,

a loss of brightness can still take place when the

penetrant is exposed to elevated temperatures over

an extended period of time.

Removability (Washability)

Removing the penetrant from the surface of the sample, without

removing it from the flaw, is one of the most critical operations of the

penetrant inspection process.

The penetrant must be removed from the sample surface as completely

as possible to limit background fluorescence.

In order for this to happen, the adhesive forces of the penetrant must

be weak enough that they can be broken by the removal methods

used.

However, in order for the penetrant to have good surface wetting

characteristics, the adhesive forces (forces of attraction between the

penetrant and the solid surface being inspected) must be stronger than

the cohesive forces (forces holding the liquid together). Proper

formulation of the penetrant materials provides the correct balancing of

these forces.

Another consideration in the formulation of the penetrant liquid is that it

should not easily commingle and become diluted by the cleaning

solution. Dilution of the penetrant liquid will affect the concentration of

the dye and reduce the dimensional threshold of fluorescence.

Emulsifiers

When removal of the penetrant from a 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

break the penetrant down and make it water-washable.

Most penetrant inspection specifications classify penetrant

systems into four methods of excess penetrant removal.

Method A: Water-Washable

Method B: Post-Emulsifiable, Lipophilic

Method C: Solvent Removable

Method D: Post-Emulsifiable, Hydrophilic

Method C relies on a solvent cleaner to remove the penetrant

from the part being inspected.

Method A has emulsifiers built into the penetrant liquid that

makes it possible to remove the excess penetrant with a simple

water wash.

Method B and D penetrants require an additional processing

step where a separate emulsification agent is applied to make

the excess penetrant more removable with a water wash.

Lipophilic emulsification systems are oil-based materials that are

supplied in ready-to-use form.

Hydrophilic systems are water-based and supplied as a

concentrate that must be diluted with water prior to use .

Lipophilic emulsifiers (Method B)

were introduced in the late 1950's and work with both

a chemical and mechanical action.

After the emulsifier has coated the surface of the

object, mechanical action starts to remove some of

the excess penetrant as the mixture drains from the

part.

During the emulsification time, the emulsifier diffuses

into the remaining penetrant and the resulting mixture

is easily removed with a water spray.

Hydrophilic emulsifiers (Method D)

also remove the excess penetrant with mechanical and

chemical action but the action is different because no

diffusion takes place.

Hydrophilic emulsifiers are basically detergents that

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 action of the rinse water removes the

displaced penetrant from the part and causes fresh

remover to contact and lift newly exposed penetrant from

the surface.

The hydrophilic post-emulsifiable is more sensitive than

the lipophilic post emulsifiable method.

The major advantage of hydrophilic emulsifiers is that

they are less sensitive to variation in the contact and

removal time. While emulsification time 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. However, a variation of as little as

15 to 30 seconds can have a significant effect when a

lipophilic system is used.

Penetrant Removal MethodPenetrants are also classified by the method of removing the excess penetrant.

Solvent Removable penetrants are removed by wiping with a cloth dampened with solvent. They are supplied in aerosol cans for portability and are primarily used for spot checks.

Water Washable penetrants are removed with a course spray of water. They are the easiest to employ and most cost effective when inspecting large areas.

Post-Emulsifiable penetrants are water-washable only after they have reacted with an emulsifier solution. A post-emulsifiable system is used when washing the penetrant out of the defect is a concern. The emulsifier is given time to reacts with the penetrant on the surface but not the penetrant trapped in the flaw.

Developers The role of the developer is to pull trapped

penetrant out of defects and to spread it out on the surface so that it can be seen. Also provides a light background to increase contrast when visible penetrant is used.

Developer materials are available in several different forms Dry Powder is a mix of light fluffy powder that clumps together where

penetrant bleeds back to the surface to produces very defined indications.

Wet, Water Suspendable is a powder that is suspended in a water that covers the surface with a relatively uniform layer of developer when the water is evaporated. The solution is somewhat difficult to maintain as the powder settles out over time.

Wet, Water Soluble is a crystalline powder that forms a clear solution when mixed with water. The solution recrystallizes on the surface when the water is driven off. Indications sometimes lack definition and look milky. Not recommended for use with water-washable penetrants.

Wet, Non-Aqueous - is supplied in a spray can and is the most sensitive developer for inspecting small areas. It is too costly and difficult to apply to large areas.