Dye Penetrant Testing (Overview)

Thuy Thanh Nguyen, winter semester 2017/18

In the non-destructive testing process, the dye penetrant testing (PT) is a comparatively common, simple and low-cost examination method to detect surface breaking defects. It is based on the capillary action where the dye penetrant bleeds out and reveals the narrow crack opening through a significant difference in contrast to the remaining material surface.

Fundamentals

Material flaws, for example pin holes, cracks, lack of fusions, overlaps and wrinkles caused by milling, welding, casting or forging processes, can lead to a failure of the component in an alternating mechanical load. Big fissures can be detected with visual examination, but fine hairline cracks can be overlooked or need an additional work with magnifying glasses or microscopes to detect. PT is applied to almost any material that is not too coarse or porous to display material defects. For metallic materials the magnetic particle testing (MT) is preferred.

Physical principles

PT uses the effect of capillary action, that means the dye liquid soaks into the flaw because of the narrow cavity. If a capillary dives into a liquid, it will rise onto a certain height. This effect is based on the liquid’s surface tension and the interfacial tension between the liquid and the solid surface. In this case cohesion and adhesion will appear. Cohesion forces are holding the molecules in the liquid together while the adhesion forces ensure that the liquid molecules are sticking to the component’s solid surface.

If the gap width converges to zero, the height will rise to its maximum. But the narrower the gap width, the less amount of the liquid will be in the flaw. PT can detect defects within a penetration depth bigger than 0,01 mm. ^[1]

Dye penetrant types

The penetrant is defined by its characteristics for example colour, sensitivity and removability. There are three varieties of penetrants, fluorescent penetrants (type I), coloured penetrants (type II) and fluorescent coloured penetrants (type III). The advantages of an emulsifying penetrant is that it has a very high sensitivity, due to that flat cracks can be obviously detected. However, emulsified penetrants are difficult to apply to coarse surfaces.

The opportunities to apply the dye penetrant and the developer are various, generally brushes, aerosol cans or pipes are used or the parts are immersed into a penetrant bath.

Advantages and Disadvantages

The advantages of this common and widely applied examination method are that the examinee is flexible portable, low-cost and can be done with minimal effort because it is simple and fast. It can be applied to nearly any surface and material and on any geometric shape.

Nevertheless, components without a crack opening to the surface are not detectable. Furthermore, this examination method is still not automatable, in consequence the quality of the examination depends on the diligence of the operator.

PT is a standardised process according to the DIN EN ISO 571-1 which runs in several steps. ^[2]

Inspection steps

Surface Preparation

The requirement for PT is that the crack being detected needs to be on the surface and its geometry must be shaped so that the dye liquid can enter the flaw. First, the component needs to be cleaned from any kind of dirt that prevents the dye penetrant from soaking the flaw, shown in step 1 in figure 1. Water or solvents are common tools, after all there are also alternative cleaning techniques with electric or ultrasonic forces. If the crack opening is polluted with solid particles, special caustics are used. Before continuing with the next step, the sample must be dried first, shown in the second step in figure 1.

Penetrant Application

Afterwards, the dye liquid penetrant is applied to the component, shown in step 3 in figure 1. Depending on the viscosity of the dye penetrant and the geometry of the flaw, it takes about 5 to 240 minutes to soak in. ^[4] The so called “dwell time” does not only depend on its defect size but also on the dye penetrant that is being used and also the investigated sample material. The sensitivity of the penetrant and its resistance against the intermediate cleaning is classified into four different classes from low, medium, high to ultra-high sensitivity. The dwell time must not be exceeded, otherwise it will dry out.

Excess Penetrant Removal

The following step is the intermediate cleaning. The excess dye penetrant on the surface needs to be removed carefully to ensure the crack is still filled with the dye penetrant, shown in step 4 in figure 1. Flat cracks have the adverse effect that they can be easily washed out. The common chosen cleaning methods are either with water and/or with solvents or with lipophilic or hydrophilic emulsifiers, that depends on the used penetrant. Advantages of water-washable penetrants are that they can be easily applied to coarse surfaces and are well automatable.

Developer Application

Then in the following step the developer is applied to the sample, shown in step 5 in figure 1. It has to be applied very thinly that the dye penetrant can bleed out to the surface. It works like a blotting paper that absorbs the excess penetrant, only the crack openings become visible. The developer has to counteract against the capillary action or even the gravity. Due to this fact, the capillary of the developer has to have a smaller diameter than the width of the crack itself to produce a stronger capillary action. Developers come in two different forms, dry developer and wet developer.

Post-cleaning and inspection

The last steps are the post-cleaning and the inspection of the sample. The inspection is made after about 10 to 30 minutes, longer development times are necessary for tighter cracks. ^[5] The post-cleaning process uses the same methods as described in step 4, furthermore it will be cancelled, if the sample is not used anymore. The flaw is not only indicated through a significant difference in contrast to the remaining material surface but also magnifies the crack opening optically, shown in step 6 in figure 1. The analysis and evaluation of fluorescent penetrants takes place under UV light, alternatively also under day light or with additional visible light. The documentation comes after a successful inspection

PT in applied practice

PT can be analysed in a testing chamber or the sample is arbitrary and flexible portable. This testing method is automatable to limited extent because the analysis and evaluation still has to be done by an operator, that is the reason for there currently being only semi-automatic machines. The choice of which dye penetrant or developer or the dwell time depends on the sample and the crack. PT can also be used as a leakage test. A leakage will exist, if the penetrant quits both sides of the sample.

Comparison between PT and MT

PT is primarily preferred because it can be applied to any material. It is low in cost and has a smaller investigation setup compared to MT.

Literature

• Deutsch, V.; Wagner, R.: Prüfung auf Oberflächenrisse nach dem Eindring-Verfahren. Informationsschriften zur zerstörungsfreien Prüfung ZfP kompakt und verständlich. Band 9, Castell Verlag, (1999), p. 33
• DIN EN 571-1: Zerstörungsfreie Prüfung - Eindringprüfung - Teil 1: Allgemeine Grundlagen, Beuth (1997), S.1-11
• DIN EN 2002-16: Luft- und Raumfahrt - Metallische Werkstoffe; Prüfverfahren - Teil 16: Zerstörungsfreie Prüfung, Eindringprüfung, Beuth (2000), S.1-16
• DIN EN 10228-2: Zerstörungsfreie Prüfung von Schmiedestücken aus Stahl - Teil 2: Eindringprüfung. Beuth (2012), S.1-12
• DIN EN ISO 3059: Zerstörungsfreie Prüfung – Eindringprüfung und Magnetpulverprüfung -Betrachtungsbedingungen. Beuth (2013), S. 1-12
• DIN EN ISO 3452-2: Zerstörungsfreie Prüfung – Eindringprüfung – Teil 2: Prüfung von Eindringprüfmitteln. Beuth (2012), S. 1-31
• DIN EN ISO 3452-3: Zerstörungsfreie Prüfung – Eindringprüfung – Teil 3: Kontrollkörper. Beuth (2012), S. 1-11
• DIN EN ISO 3452-4: Zerstörungsfreie Prüfung – Eindringprüfung – Teil 4: Geräte. Beuth (1999), S. 1-8
• DIN EN ISO 12706: Zerstörungsfreie Prüfung – Eindringprüfung – Begriffe. Beuth (2010), S. 1-13
• DIN EN ISO 23277: Zerstörungsfreie Prüfung von Schweißverbindungen – Eindringprüfung von Schweißverbindungen – Zulässigkeitsgrenzen. Beuth (2013), S. 1-9
• Erhardt, A.: Verfahren der Zerstörungsfreien Materialprüfung – Grundlagen. 2014; Hrsg. DGZfP; DVS Media GmbH.
• Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Eindringprüfung. 1. Auflage, Springer Vieweg (2014)
• Tracy, Noey (Tech.Ed.); Moore, Patrick (Ed.): Liquid Penetrant Testing, Nondestructive Testing Handbook, Volume 2, American Society for Nondestructive Testing, Columbus, OH, (1999), ISBN 1-57117-028-6

References

1. Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Eindringprüfung. 1. Auflage, Springer Vieweg (2014), S. 144
2. Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Eindringprüfung. 1. Auflage, Springer Vieweg (2014), S. 148
3. Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Eindringprüfung. 1. Auflage, Springer Vieweg (2014), S. 6
4. Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Eindringprüfung. 1. Auflage, Springer Vieweg (2014), S. 47
5. Schiebold, K.: Zerstörungsfreie Werkstoffprüfung – Eindringprüfung. 1. Auflage, Springer Vieweg (2014), S. 122