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 90 o. 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 to pull 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, nonporous 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 Penetrant Classifications 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 Form Dry 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: L= distance travelled L 2 = T cos. t / (2η) η= coefficient of viscosityl = 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. Where: I = I o x e -lct I = Transmitted light intensity I o = 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.
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 71 o C (160 o F). 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 71 o C (160 o F). 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 overwashing 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 Method Penetrants 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 waterwashable 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.