Liquid Penetrant Testing Paper Example

Introduction

Liquid penetrant testing which is also referred to as liquid penetrant inspection or dye penetrant inspection (DP) is extensively used, and it is a low-cost inspection technique used for checking surface-breaking discontinuities in the non-porous materials such as ceramics, plastics, or metals. The application of the penetrant is on ferrous or non-ferrous materials. However, magnetic-particle inspection is used for ferrous materials. Liquid penetration inspection is utilized for detecting forging, detecting and welding surface discontinuities such as leaks, surface porosity, fatigue cracks, and hairline cracks. Liquid penetrant testing works under the principle of capillary action. There are specific steps that should be followed to examine surface discontinuities of a material. The adherence to these steps ensures that accurate results are obtained. The paper examines the principle behind liquid penetrant inspection and its method of application.

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The Principle of Liquid Penetrant Testing

Liquid penetrant inspection is founded on the principle of capillary action. This means that the fluid which is a low surface tension penetrate a surface thereby breaking discontinuities. The penetrant moves into the material surface using capillary action. Capillary action can be described as-as the process by which a fluid surface is depressed or elevated after coming into contact with a solid (Smith, 2015). It means that liquid penetrant testing materials, procedures and processes are developed to ensure capillary action and enable visibility of such actions and ultimately appropriate interpretation. An example in which forces of capillary actions are visible when a glass straw is put in a class filled with colored water. The insertion shows the water molecules entering the straw and proceeding to pull other nearby molecules up the straw in a concept called cohesion. This continues as water molecules higher in the glass straw. It is important to understand the movement of the water up the straw stops when the pull of surface tension is at equilibrium (Smith, 2015). Also, cohesive and adhesive collectively works to prevent water from moving down the straw. Capillary action in liquid penetrant testing is complex since there are additional surface conditions that prevent or help the action. Some properties of the liquid penetrant are that it has low surface tension and heightened capillary action.

It is also important to note the concept of contact angle in which a liquid exhibiting large contact angles have reduced wettability. This means that such a liquid would have less penetrability in comparison to liquid exhibiting small contact angle. Another factor to consider during liquid penetrant testing is that of the contrast ratio ("Nondestructive Examination" n.d.). The primary aim of LPT evaluation is the increment of the visible contrast ratio that exists between flaws and its background. It is accomplished through the application of a penetrant liquid with characteristics of increased penetrability and mobility to the surface of the material and causing a bleed out which is visible ("Nondestructive Examination" n.d.). The "bleed-out" from the discontinuity can be seen under daylight conditions when using visible dye penetrant. It can be visible after exposure from black light when using fluorescent penetrants.

As earlier explained, the penetrant moves into the surface discontinuities using the principle of capillary action. Then, there is the removal of the excess liquid penetrant from the component surface, followed by the application of a developer which is usually a dry powder. The objective is drawing out the liquid penetrant in the discontinuities and unveiling an indication. This technique can detect very narrow discontinuities (Smith, 2015). Since the indication is broader in comparison to the actual crack, the visibility is enhanced. The application of this technique is appropriate for metallic and non-metallic materials which are not porous. Also, it is important that these surfaces are cleaned before initiating the test. It is important to understand that the inspection procedure can be manual, semi-automatic or fully automatic.

The Procedure of Liquid Penetrant Testing

Different factors determine the procedure for liquid penetrant testing. These include used penetrant system, the condition and environment of the test, the expected type of discontinuities, and the size and material of the system or component being examined (Hijazi, n.d.). However, there exists a general process for liquid penetrant inspection. These steps include pre-cleaning; penetrant application, penetrant dwell time; elimination of excess penetrant; introduction of the developer; developer dwell; inspection; and lastly final cleaning.

Pre-cleaning is the first step of liquid penetrant testing, and it is one of the most critical. It free the surface being evaluated of grease, water, oil and other deferent contaminants. The objective is to eliminate any obstacle that can prevent the entrant of the penetrant into the discontinuities. For this step, a solvent is often used. However, the component may necessitate cleaning using chemicals, vapor, steam or ultrasonic means. In case the component has been machined, sanded or grit-blasted, it is essential that etching is performed to eliminate substances that can potentially prevent the entrance of the penetrant (Smith, 2015). The second step is penetration application which entails using visible or fluorescent dye penetrants. The penetrants are applied to the component being examined using a variety of methods. These can be brushing, spraying or event having the component immersed in a penetrant bath. In most cases, the decision on application is often by preference. It can either by size and shape of the component under examination (Hijazi, n.d.). Other factors that influence application include the equipment available for the test and the test requirements.

The third step is penetrant dwell time which involves leaving the penetrant on the surface for an appropriate amount of time. This permits the penetrant to seep into any surface discontinuities. It is important to note that the dwell time varies with different penetrants. It is affected by different factors such as test specifications, surface finish, type of material and temperature. Figure 1 shows the penetrant moving into surface discontinuities. The fourth step is penetrant removal which involves taking away the excess liquid penetrant from the component surface as shown in Figure 2. The removal of liquid penetrant is dependent on the penetrant used (Shyamji & Prasad, 2017). For instance, the excess visible dye is removed by a solvent; the excess fluorescent dye is removed by water or by the post-emulsifiable method. Irrespective of the excess penetrant method used, it is essential to ensure the tested component is placed in the low-temperature oven to dry before the application of a developer.

The fifth step is the application of the developer. In this step, there is an application of a thin layer of the developer. The objective is drawing the penetrant that had been sipped into the discontinuities back into the surface. This ensures that the penetrant is visible as indications (Shyamji & Prasad, 2017). The application of the developer can be accomplished by spraying a wet developer of dusting with a dry powder. The sixth step is usually the developer dwell time in which the developer is left for a specific amount of time so that the indications and fully develop (Smith, 2015). This is followed by inspection which is accomplished by a trained and certified inspector. When the test is by the visible dye, sufficient white light is necessary. When using a fluorescent dye, on the other hand, it is important to ensure darkened conditions with a black light. The last step is post-cleaning in which all penetrant testing residues are removed.

References

Hijazi, A. (n.d.). Liquid Penetrant Testing. Retrieved from https://eis.hu.edu.jo/ACUploads/10526/Liquid%20Penetrant%20Testing.pdf

Nondestructive Examination (NDE) Technology and Codes Student Manual. Retrieved from https://www.nrc.gov/docs/ML1214/ML12146A180.pdf

Shyamji, S., & Prasad, S. (2017). Non Destructive Method by Penetrant Testing. International Journal of Advance Research, Ideas and Innovations in Technology, 3(4), 308-310. Retrieved from https://www.ijariit.com/manuscripts/v3i4/V3I4-1219.pdf

Smith, R. (2015). Guidance Document: An Introduction to NDT Common Methods. Retrieved from http://www.bindt.org/admin/Downloads/Apprenticeship-Guidance-Document.pdf