The non-destructive tests

One of the most important phases in the quality control of materials are the non-destructive tests also called NDT, or NDE (Non-Destructive Evaluation).

By definition, NDE tests verifies the presence of defects without interfering with the material's integrity or its aptness to perform its designated function.

There are different types of NDEs. Those utilised by Cividac are:

  • Radiography
  • Magnetic particles
  • Dye penetrant inspection
  • Ultrasounds

Each one of these has advantages and disadvantages, let's look at them in detail.

Radiography

What it is. This test is applied to ferritic and non-ferritic materials, and uses X and Gamma rays, which penetrate in the material, forming an image on a film. The resulting image can then be examined like a picture.

Advantages. With a radiographic test, it's possible to document the results and examine them even after a long time. It's also particularly suitable for thin materials of any type.

Disadvantages. It's not suitable to detect surface defects or for thick materials. Also, the possibility of operators being exposed to X and Gamma rays makes it potentially hazardous. The resulting analysis is bi-dimensional and in case a defect is detected, it won't indicate its depth.

Magnetic particles

What it is. In this type of test, a magnetic flux is generated, which deviates its course whenever it encounters an anomaly in the material, highlighting the presence of a defect.

Advantages. This type of test is relatively easy to carry out, and can be automated.

Disadvantages. Its application is limited to surface defects of ferromagnetic materials, such as iron, nickel and cobalt. In spite of its simplicity, a faulty execution can leave some defects undetected.

Dye penetrant inspection

What it is. This test is carried out on ferromagnetic and non-ferromagnetic materials, by applying a penetrating liquid (normally a bright red oil) that highlights the presence of any defect. In the part to examine, any trace of other materials, grease or any other contaminant will have to be thoroughly removed.

After the liquid is removed, a thin layer of chalk is applied, which attracts to the surface any liquid that has penetrated through a crack. The result is a visual representation of the defect.

Advantages. This type of test is relatively easy to perform, and is particularly suitable for ferritic and non-ferritic materials.

Disadvantages. It's limited to surface defects and requires a high level of cleanness.

Ultrasounds

What it is. This type of test detects surface and subsurface defects. It’s carried out by generating ultrasound impulses through an electric charge applied to a piezoelectric crystal. Depending on the amount of time it takes the wave to get through to the other side of the material, it's possible to have a representation of the defect and its depth.

Advantages. This test can be used for very thick materials and it's very precise, as it indicates position, size, and type of defect. The equipment is portable, it's quite sensitive and doesn't require the use of other materials.

Disadvantages. The use of ultrasounds won't generate any documentation that can be examined afterwards, unless the latest systems are used. Although it's not a difficult test to carry out, it still requires a particular knowledge to interpret the results.

All operators carrying out a NDE are required to have a technical, theoretical and practical knowledge, and must have passed certification exams according to EU regulations EN ISO 9712, or US regulations SNT-TC-1A.