Comprehensive Guide to Liquid Penetrant Testing (LPT): Techniques, Principles, and Applications

 Discover the essentials of Liquid Penetrant Testing (LPT)

Liquid Penetrant Testing (LPT) is a widely used nondestructive testing method for detecting surface discontinuities in materials, including welds. This blog covers everything you need to know about LPT, from basic principles and surface preparation to penetrant application, excess penetrant removal, developer application, and final cleaning. Whether you're a beginner or an experienced professional, this guide provides detailed procedures and best practices to ensure accurate detection and interpretation of surface flaws. Learn about different types of developers, the importance of observation and interpretation, and the necessary testing equipment for both field and stationary inspections. Enhance your understanding of LPT and ensure the integrity and reliability of your materials and structures with this in-depth resource.

LIQUID PENETRANT TESTING

 

1 INTRODUCTION

 

Penetrant examination is generally considered to be one of the easiest methods of surface inspection and locate discontinuities that are open to the surface. Directions for use are often simply mentioned in a few square inches on the spray can or container of the material required and consequently there is a tendency to underestimate the need for an accurate application and competent personnel. To obtain optimum results, the method should be applied with care and accuracy. LPT can be used on any material except those considered extremely porous.

2 PRINCIPLES

 

Surface discontinuities, such as cracks or other separations, as well as porosity open to the surface, can be detected by bleeding after the surface has been treated with a penetrant. A developer is generally used to increase the evaluation efficiency.

 

Following steps are involved in getting final results by penetrant inspection:

 

Surface preparation

Application of the penetrant

Controlling time for penetration

Removal of excess penetrant from the surface

Application of developer

Observation of the developing process

Interpretation

Final cleaning

 

2.1 Surface Preparation

 

The effectiveness of liquid penetrant testing is based upon the ability of the penetrant to enter surface discontinuities. The article to be tested must be clean and free from foreign matter that may cover or fill its discontinuities. All paint, carbon, oil, varnish, oxides, scale, rust, etc. must be removed from the article prior to the application of the penetrant. Detergent cleaning, vapor degreasing, steam cleaning, rust removers, pickling solutions, etc. are used, depending upon the composition of the article under test and the type of soil to be removed. Any cleaning process used should leave the surface clean and dry and should not harm the article under test. Sandblasting, grinding, and similar operations are not suitable since they will close up the surface discontinuities. In case the above operations are done, etching should be done prior to penetrant application.

 

2.2 Penetrant Application

 

Since the main feature of penetrant testing is the visibility of the indications, the liquid penetrant contains a colored dye easily seen in white light (usually red dye) or a fluorescent dye visible under black (ultraviolet) light. Hence, the penetrant can be classified as visible dye penetrant or fluorescent dye penetrant. Hence this is also called as Dye Penetrant Test.

 

Penetrants are applied by spraying, swabbing, brushing, and dipping; the method of application depends upon the size of the component, the type of component, and the type of equipment, etc.

 

The period of time during which the penetrant is permitted to remain on the specimen is a vital part of the test. The minimum time required for the penetrant to enter into the discontinuity is determined by the following factors:

 

Manufacturer's recommendation

Type of material tested

Type of discontinuity expected

Temperature of the specimen

Humidity of the environment

It may be pointed out here that tight crack-like discontinuities may require as high as 30 minutes while gross discontinuities may require only 3 to 5 minutes.

 

The temperature of the specimen is most important. Usually only at 50 to 110°F, the usual liquid penetrants will become dry and hence will not penetrate into discontinuities. Similarly, at temperatures below 50°F, the mobility of the particles will be very slow and hence, there will not be any penetrating action. For use at higher temperatures, special penetrants have been developed. However, in case of low temperature, the article has to be heated to a temperature of 50 to 100°F.

 

2.4 Excess Penetrant Removal

 

After sufficient dwell time, the excess penetrant on the specimen surface is to be removed. This has to be done without disturbing any penetrant which has entered into the discontinuity. Complete removal of surface penetrant is effected to ensure against formation of irrelevant indications.

 

The excess penetrant is first removed by a clean dry cloth or rag. The cloth or rag used should be lint-free. The cloth is moistened with the solvent recommended by the penetrant manufacturer and the penetrant gently wiped off from the specimen surface. In any case, the solvent should not be forced on the specimen surface or otherwise it might wash out or dilute the penetrant in the discontinuity. In the case of fluorescent penetrant, the excess penetrant removal is ascertained by viewing the specimen under black light. For visible dye penetrants, traces on the wiping material ensure complete penetrant removal.

 

2.5 Developer Applications

 

Since penetrant commences to bleed out of discontinuities immediately following the removal of excess surface penetrant, developer is applied to the specimen. The purpose of developer application is two-fold; on one hand by reverse capillary action, it seeps out penetrant from the discontinuities and spread it out to a greater area. On the other hand, it also serves as a color contrast background for dye-material.

 

Developers are classified into categories:

 Wet developer

Non-aqueous developer

Dry developer

2.5.1 Wet Developer

 

This contains a suspension of absorptive white powder in water. The mixture is prepared as per manufacturer's recommendations and is mildly agitated prior to application. Since the developer itself contains water, water-washable penetrants after washing of excess penetrant need not be dried. The developer is applied either by immersion or by spray method, to form a thin coating. After the application of developer, the specimen is allowed to dry either by natural drying or artificial drying with temperature.

 

2.5.2 Non-Aqueous Developer

 

In this type of developer, the white powder is suspended in a solvent vehicle. It is usually applied by spraying from a pressurized spray can or other spraying devices. Prior to application, it is necessary to see the powder is thoroughly mixed with the solvent. It should be applied to form a thin, white coating on the specimen which must be completely dry.

 

2.5.3 Dry Developer

 

This type of developer contains a loose, fluffy talcum powder with high absorbent properties. This is applied on the specimen by dusting, blowing, or dipping the specimen. This is used only with fluorescent dye penetrants and is usually accomplished in a booth with a blower or fan arrangement.

 

The specimen is coated with the powder uniformly and left for a reasonable period for the penetrant to seep out and form indications. For small parts, the dry developer may be applied by dipping them in a container filled with the developer powder. The developer-coated parts are then agitated to ensure uniform coverage. For larger parts, the developer powder is applied using a hand duster or a blower.

 

 

 

2.6 Observation of Developing Process

 

Observation of the developing process is crucial. Characteristics of indications should be observed immediately as they appear. Flaws might bleed out to larger spots, making it difficult to derive accurate information later. Blotting out effect is more pronounced for color penetrants than fluorescent types.

Since the penetrant seeps out immediately after the application of the developer, the specimen should be observed for the formation of indications. The indications should be observed as they appear, as this will provide more accurate information regarding the location, size, and shape of the discontinuities. If observation is delayed, the indications may become diffused, making it difficult to interpret the results accurately. The observation can be carried out under appropriate lighting conditions, white light for visible dye penetrants and ultraviolet light for fluorescent dye penetrants.

 

 

2.7 Interpretation

 

For correct interpretation, it's essential to observe the characteristics of the indications immediately. Interpretation also depends on the size and complexity of the surface examined, and often supplementary optical examination with a magnifying glass or microscope is needed. Knowledge of the material and its possible imperfections is crucial.

After observing the indications, the next step is to interpret the results. The interpretation process involves determining the type, size, location, and extent of the discontinuities based on the indications observed. Interpretation requires skill and experience, as some indications may be false or irrelevant. False indications are usually caused by improper cleaning or incomplete removal of the excess penetrant, while irrelevant indications may result from surface irregularities that are not actual discontinuities.

 

2.8 Final Cleaning

 

After inspection and recording of findings, the developer should be completely removed from the parts, using water or solvents recommended by the manufacturer.

After the interpretation of the results, the final step is to clean the specimen. This involves removing the developer and any remaining penetrant from the surface of the specimen. Cleaning is usually done using water or a solvent recommended by the penetrant manufacturer. The specimen should be thoroughly cleaned to ensure no residues are left, which could affect subsequent processing or use of the specimen.

 

3 TESTING EQUIPMENT

 

Various equipment from test stations to portable kits are available depending on the type of material, frequency of testing, and size of the component. Portable kits are common, containing cleaners, penetrants, developer, wiping cloths, brushes, and spraying equipment. For fluorescent penetrants, portable black light equipment is provided.

Various types of equipment are available for liquid penetrant testing, ranging from simple, portable kits to more complex, stationary systems. Portable kits are often used for field inspections and typically include cleaners, penetrants, developers, applicators, and inspection aids such as brushes and cloths. For fluorescent penetrant inspections, portable ultraviolet light equipment is also provided. Stationary systems are used for high-volume or automated inspections and may include equipment for pre-cleaning, penetrant application, excess penetrant removal, developer application, and inspection under appropriate lighting conditions.

 

4 EVALUATION OF TEST RESULTS

 

Interpretations are classified as:

 

False indications

Irrelevant indications

True indications

False indications result from improper penetrant removal, irrelevant indications from surface discontinuities, and true indications from actual discontinuities in the specimen. The type, extent, and effect of the discontinuity on the service life must be evaluated.

The evaluation of test results involves classifying the indications observed during the inspection. Indications are classified into three categories: false indications, irrelevant indications, and true indications. False indications are usually caused by improper cleaning or incomplete removal of excess penetrant. Irrelevant indications may result from surface irregularities that are not actual discontinuities. True indications are actual discontinuities that are open to the surface and have been detected by the penetrant. The type, extent, and effect of the discontinuity on the service life of the specimen must be evaluated based on the applicable codes, standards, or specifications. The evaluation process requires knowledge of the material being inspected, the type of discontinuities expected, and the service conditions of the specimen.

 

5 APPLICATION IN WELDING

 

Liquid penetrant inspection is crucial in repairing weldments by ensuring any flaws are removed before refilling.

Liquid penetrant inspection is widely used in the field of welding for detecting surface discontinuities in welds. It is particularly useful for inspecting non-magnetic materials, such as Monel and stainless steel, where magnetic particle inspection is not applicable. Liquid penetrant inspection can be used at various stages of the welding process, including in-process inspection of multi-layer welds, final inspection of completed welds, and inspection of weld repairs. It is also used for detecting heat treatment cracks and fatigue cracks in welded components. The use of liquid penetrant inspection in welding ensures that surface discontinuities are detected and corrected, improving the quality and reliability of welded structures.