LIQUID (DYE) PENETRANT INSPECTION (LPI) - … filea component under test. •The penetrant...

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  • 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


    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


    or defects are


  • 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


    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:


    ability to enter fine surface defects in the component under test


    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


  • 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


    Color and Fluorescence Brightness

    Dimensional Threshold of Fluorescence

    Ultraviolet and Thermal Stability


  • Surface Energy (Surface Wetting


    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


    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.


    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


    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


    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.