NONDESTRUCTIVE TESTING-LIQUIDPENETRANTS

Nondestructive evaluation (NDE) encompasses those physical and chemical tests that are used to
determine if a component or structure can perform its intended function without the test methods
impairing the component's performance. Until recently, NDE was relegated to detecting physical
flaws and estimating their dimensions. These data were used to determine if a component should be
scrapped or repaired, based on quality-acceptance criteria. Such traditional definitions are being expanded
as requirements for high-reliability, cost-effective NDE tests are increasing. In addition, NDE
techniques are changing as they become an integral part of the automated manufacturing process .The NDE methods reviewed here consist of the five classical techniques—penetrants, ultrasonic
methods, radiography, magnetic particle tests, and eddy current methods. Additionally, we have briefly
covered thermal-inspection methods.

LIQUIDPENETRANTS

Liquid penetrants are used to detect surface-connected discontinuities in solid, nonporous materials.
The method uses a brightly colored penetrating liquid that is applied to the surface of a clean part.
The liquid in time enters the discontinuity and is later withdrawn to provide a surface indication of
the flaw.

The Penetrant Process

Technical societies and military specifications have developed classification systems for penetrants.

Society documents (typically ASTM E165) categorize penetrants into two methods (visible and fluorescent)

and three types (water washable, post-emulsifiable, and solvent removable). Penetrants, then,

are classified by type of dye, rinse process, and sensitivity. 

The first step in penetrant testing (PT) or inspection is to clean the part .

Many times this critical step is the most neglected phase of the inspection. Since PT detects only

flaws that are open to the surface, the flaw and part surface must, prior to inspection, be free of dirt,

grease, oil, water, chemicals, and other foreign materials. Typical cleaning procedures use vapor

degreasers, ultrasonic cleaners, alkaline cleaners, or solvents.

After the surface is clean, a penetrant is applied to the part by dipping, spraying, or brushing.

 shows the penetrant on the part surface and in the flaw. In the case of tight

surface openings, such as fatigue cracks, the penetrant must be allowed to remain on the part for a

minimum of 30 minutes to enhance the probability of complete flaw filling. Fluorescent dye penetrants

are used for many inspections where high sensitivity is required.

At the conclusion of the minimum dwell time, the penetrant on the surface of the part is removed

by one of three processes, depending on the characteristics of the inspection penetrant. Ideally, only

the surface penetrant is removed and the penetrant in the flaw is left undisturbed 

The final step in a basic penetrant inspection is the application of a developer, wet or dry, to the

part surface. The developer aids in the withdrawal of penetrant from the flaw and provides a suitable

background for flaw detection. The part is then viewed under a suitable light source; either ultraviolet

or visible light. White light is used for visible penetrants while ultraviolet light is used for fluorescent

penetran

Categories of Penetrants

Once the penetrant material is applied to the surface of the part, it must be removed before an
inspection can be carried out. Penetrants are often categorized by their removal method. There are
generally three methods of removing the penetrant and thus three categories. Water-washable penetrants
contain an emulsifier that permits water to wet the penetrant and carry it from the part, much
as a detergent removes stains from clothing during washing. The penetrant is usually removed with
a water spray. Post-emulsifiable penetrants require that an emulsifier be applied to the part to permit
water to remove the excess penetrant. After a short dwell time, during which the emulsifier mixes
with the surface penetrant, a water spray cleans the part. For solvent-removable penetrants, the excess
material is usually removed with a solvent spray and wiping. This process is generally used in field
applications where water-removal techniques are not applicable.

 Reference Standards

Several types of reference standards are used to check the effectiveness of liquid-penetrant systems.
One of the oldest and most often-used methods involves chromium-cracked panels, which are available
in sets containing fine, medium, and coarse cracks. The panels are capable of classifying penetrant
materials by sensitivity and identifying changes in the penetrant process.

Limitations of Penetrant Inspections

The major limitation of liquid-penetrant inspection is that it can only detect flaws that are open to
the surface. Other methods are used for detecting subsurface flaws. Another factor that may inhibit
the effectiveness of liquid-penetrant inspection is the surface roughness of the part being inspected.
Very rough surfaces are likely to produce excessive background or false indications during inspection.
Although the liquid-penetrant method is used to inspect some porous parts, such as powder metallurgy
parts, the process generally is not well suited for the inspection of porous materials because the
background penetrant from pores obscures flaw indications.

figure Shows

1. Section of material with a surface-breaking crack that is not visible to the naked eye.
2. Penetrant is applied to the surface.
3. Excess penetrant is removed.
4. Developer is applied, rendering the crack visible.