Liquid penetrant or penetration testing
The liquid penetrant test is one of the oldest and simplest methods of materials testing, used as far back as the 19th century. In the beginning, limestone milk was used to test mainly transport equipment, such as railway components. Subsequent mixtures made using kerosene and petroleum displaced lime milk, as its use was severely limited and no minor material defects could be detected. Methods using different dyes were experimented with in the 1940s.
The fluid penetrant test is based on the capillary effect. The tracer fluid penetrates the cracks in the workpiece under test by the effect of capillary action. Therefore, this method is only suitable for the detection of surface defects, which are open defects running to the surface, but with a very good efficiency. For porous or rough materials with a rough surface, the method is of limited use. The tracer fluid that penetrates into porous materials and then leaks back out covers the entire surface, making it impossible to detect defects. In the case of coarse, rough surfaces, one of the most delicate operations is the removal of excess marking paint.
The essence of the liquid penetrant test
The tracer fluid is applied to the surface of the test object, where it seeps into the continuity defects. After some time, the tracer is thoroughly wiped off and an absorbent developer fluid is applied to the surface. This acts as a drinker paper to 'suck' the tracer fluid collected in the defects onto the surface, so that the defect indications are clearly visible on the surface of the part under test.
In principle, all liquids can penetrate cracks of a certain size, but the purpose of material testing is to detect continuity defects that are not visible to the naked eye. Therefore, it is necessary to minimize the surface tension of the tracer fluid and to have a high contrast between the tracer fluid and the developer to ensure good visibility.
Indicating liquids can therefore be basically divided into two types:
- fluorescent tracers: they produce a visible indication when exposed to UV light
- colour-contrast (visible) tracers: they mostly contain a sudan red dye, which provides high contrast against a white developer liquid.
It can be argued that fluorescent penetrant testing provides more sensitive flaw detection than contrast, but the latter has the advantage of not requiring a darkened work area and UV lamp testing.
There is also magneto-fluorescence, which is a combination of magnetic crack detection and fluorescent tracer, and fluorescent colour contrast tracer - not discussed here.
The popularity of penetrant testing can be attributed to two main factors: its relative ease of use and flexibility. Almost any material can be tested using this method, provided that its surface is not extremely rough or porous, including metals, glass, ceramics, rubber and plastics.
Steps of the penetrant test
The exact procedure for fluid penetrant testing depends on a number of factors, such as the quality of the penetrantion materials used, the size and material of the defects to be detected, and the external conditions of the test, such as temperature. However, the general steps can be summarised as follows:
1. Surface preparation
In all cases, the test starts with the preparation of the surface of the object to be tested. The surface must be free of oil, grease and water. The test piece shall be cleaned of all dirt, paint, laitance, rust, etc. which may obscure defects or prevent the penetration of the tracer fluid. A wet surface also interferes with good flaw detection, so the surface must be completely dry. Various grit cleaning methods are not recommended as they can clog minor defects on the surface.
2. Application of the tracer
The tracer (penetrating fluid, penetrating chemical) can be applied to the surface by several methods. For large-scale industrial applications, for serial tests, for full surface tests, the products to be tested are immersed in tubs of penetrating agent or applied by spray gun. For targeted, localised testing, the test agent is typically applied by brush, less frequently by spray.
All manufacturers indicate the penetration time on their products, which of course depends on the material of the object to be tested, the size and type of defects. The average penetration time is at least 10 minutes, but can be up to an hour - waiting longer is not a bad idea for fault detection. The ideal penetration time is often determined by individual experimentation. The smaller the defect size, the longer the penetration time. During the penetration time, no operation on the test piece is required and therefore no supervision is necessary.
Penetrants are also classified according to sensitivity/detectability. The five sensitivity levels:
½ - Ultra low sensitivity level
1 - Low sensitivity level
2 - Medium sensitivity level
3 - High sensitivity level
4 - Ultra high sensitivity level
3. Removing excess tracer fluid
This is the most delicate step of the process, because excess tracer fluid must be removed from the surface without removing it from the defects. Depending on the penetrating system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water.
At Control Labor we use a solvent system. The marking fluid must be thoroughly removed from the surface with a solvent-soaked, lint-free, absorbent cloth. Repeat the process several times with a clean cloth - even though the marker is no longer visible, it is still there. Inexperienced users often make the mistake of not cleaning the surface thoroughly enough and generate worthless false error indications when they are called up. For complex shaped pieces, cleaning the indicator fluid is more difficult. An experienced materials inspector can distinguish a false indication from a true one in such cases.
Penetrants are also classified by the method used to remove excess penetrant:
A - water-soluble: removed by plain water rinsing
B - lipophilic emulsifier: removed by oil-based emulsifier
C - solvent-based
D - hydrophilic emulsifier: removed by water-soluble emulsifier
The developer is applied to the test specimen, which has been thoroughly cleaned of tracer fluid. These are also available in different forms, there are dry powders and wet developers. The most common and convenient solution is to use a penetration system containing the cleaner and developer in spray form. The developer spray should be applied evenly to the object from a distance of about 20-25 cm without dripping. Spray developers dry very quickly, in a maximum of one to two minutes. After a few more minutes, failure indications will be clearly visible. Here again, the smaller the defects you want to detect, the longer the development time you need.
It is up to the customer whether he wants the piece to be cleaned from the developer after penetration.
Penetrants are hazardous chemicals, so it is essential to follow the correct health and safety procedures and use protective equipment (e.g. a mask).
It is important to use identical (compatible) test equipment! Mixing different systems will reduce the efficiency of fault detection. As a standard, the tracer fluid and the agent to remove excess tracer fluid should come from the same manufacturer!
Developers are classified into six standard development categories according to their material:
a - dry powder
b - water-soluble
c - water-suspendable
d - non-aqueous type 1: fluorescent (solvent-based)
e - non-aqueous type 2: visible dye (solvent-based)
f - special applications
The temperature of the penetrant and the component under test is essential for fluid penetrant testing. The literature most often recommends temperatures between 5 and 50°C for optimal results. Too low a temperature is particularly critical, as water can condense or even freeze into the defects, rendering the defect detection ineffective. Special test equipment is used for conditions other than the recommended temperature.
Advantages of liquid penetrant testing
- High sensitivity (small material continuity errors can be detected)
- Testing a wide range of materials (metallic and non-metallic, magnetic and non-magnetic, conductive and non-conductive)
- Quick control of large surfaces and quantities
- Testing of complex shaped parts
- Fault indications are clearly visible to the naked eye on the surface of the piece
- Portable system (test agents are also available in aerosol sprays)
- Relatively low cost (materials and associated equipment are relatively cheap)
- Csak felületi (felületre kifutó) hibák észlelhetők
- Only suitable for testing materials with non-porous surfaces
- A lot of preparation is needed to clean the surface, as dirt can mask defects
- Surface roughness can affect the sensitivity of the test
- Penetrants are harmful chemicals and their use requires caution