Non-Destructive Testing of Materials

Non-Destructive Testing of Materials

Introduction

Non-destructive testing defects could either be inherent, primary or secondary. Inherent defects include: hot tears, shrinkages and inclusions. Primary defects occurs during shaping process such as forging, extrusion and welding. Secondary or in-service defects deals with machine defects during operation. The processes of carrying out the procedure includes: visual observation and mechanical testing. These tests can be carried out on metals, plastics, ceramics, cement, etc. in order to evaluate cracks, internal voids, surface cavities, incomplete defective welds and any type of flaw that could lead to premature failure. To evaluate all these flaws, different methods can be used, they include: visual observation, magnetic method, liquid penetration method, radiographic method, ultrasonic method and mechanical method which is not so good because during this procedure, the process involves having more contact with the material which might actually cause more discontinuity.

The procedure described here involves the use of liquid penetrant method to evaluate the discontinuity. Liquid penetrant method is the most widely used method of evaluating discontinuities in the industry. The reasons are as a results of the fact that it is very easy to use and it is very flexible. The scope behind the method is the ability of a liquid to be able to penetrate into a clean surface and its flow being broken by capillary action. But it is used for surface inspection only.

Some of the flaws liquid penetrant can be used for includes: 
1. Fatigue cracks
2. Pin holes in welds
3. Porosity
4. Overload and impact fractures
5. Lap scams
6. Lack of fusion or braising along the bond line
7. Quench cracks. 

For a liquid to be used as penetrant, it has to have the following characteristics which include: 
1. They must have a very high viscosity.
2. They must have the ability to remain in coarse opening.
3. They must be able to be applied and removed easily.
4. They must have a permanent colour when exposed to heat.
5. They must have the ability to remove discontinuities after developer must have being applied.

Objective of the Experiment

This procedure is carried out to test a material for errors without having to destroy the material. It involves looking for defects and imperfections in a material.

Equipments Needed

Ethanol and Aerosol spray
Black spray paint penetrant

Procedures

1. The pre-cleaning process: The surface of the material to be evaluated has to be free from all contaminations. So the aim of the pre-cleaning process is to remove all the contaminations such as: oil, grease, dirt, dust. From the surface of the material. The pre-cleaning process has to be done carefully in order for the discontinuities not to be compounded. Ethanol and aerosol spray is used as the pre-cleaning agent, and after the application of the aerosol spray and ethanol, the surface was left to dry before carrying out the next step

2. The application of the liquid penetrant: The liquid penetrant used in the experiment is a black spray paint. The black spray paint is applied after the conclusion of the pre-cleaning process and after the application of the penetrant, the specimen should be left for 15 minutes. The reason for the waiting time is so the penetrant would be able to penetrate well into the capillaries and the discontinuities. 

3. The application of the developer: The function of the developer is to absorb the penetrant that infiltrated into the flaws and then react with it to form a geometric shape which is the negative of the shape which the surface features have, that the penetrant was sucked out from.

4. Analysis of the discontinuities: the diagram of the material will be drawn to scale and then the discontinuities will be located one by one on the diagram or the evaluated material will be snapped and evaluated accordingly. 

5. The post cleaning process: The same material used for the pre-cleaning process is used for the post cleaning process. The purpose of the post cleaning process is to remove the applied developer and penetrant so the discontinuities can be corrected.


Results

After the application of the developer, it will be found out that the reaction of the developer and the penetrant will bring out a geometrical shape over the entire welded joint. The discontinuities on the welded joint may be circular in shape or streak-like or any other geometrical shape.

Discussion and Conclusion

It can be concluded that the liquid penetrant method works effectively in exposing the discontinuities of the parts welded together.

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