Gamma Test NDT

The radiographic inspection method is one of the most widely used non-destructive inspection methods in the industry, as it is a very sensitive inspection method and the inspection results can be recorded permanently.

The test piece is irradiated with a beam of radiation (x or gamma rays) from a source. As the radiation passes through the material, it is absorbed and lost at a certain rate depending on the property of the material, and then reaches the film placed on the back surface of the part and affects the film. Since the discontinuities weaken the radiation differently, the intensity of the radiation passing through the regions with the discontinuities and the darkening it will create on the film will also be different. After the development of the film, the darkening of the film becomes visible as a sign of discontinuity.

This method is applied to ferromagnetic and non-ferromagnetic metals and all other materials. X-rays are widely used in non-destructive testing, as they provide the opportunity to examine the internal structures without damaging the materials.

Electrically generated x-rays and gamma rays emitted from radioactive isotopes are absorbed by the material they pass through. As the thickness increases, the amount absorbed increases. Hence, more radiation is absorbed in denser material. X and gamma rays are electromagnetic waves and the difference between them is that they have different wavelengths. Since the wavelengths of X and gamma rays are very small, they cannot be seen with the naked eye and can pass through materials. X and gamma rays have similar properties to light and affect the silver bromide crystals on the film. They form an image according to the ratio of radiation intensity reaching the film. The most basic rule in industrial radiography is to have a light source on one side of the material and a detector on the other. X or gamma-ray source is used as a radiation source and the film is used as a detector. The energy of the radiation source should be chosen at a higher power than the material will absorb. The parameter that determines the ability of the energy to pass through is the wavelength of the light. The smaller the wavelength, the greater the penetrating power. In X-ray radiography, the penetration power of X-rays is adjusted by the voltage applied to the X-ray tube. The film, which detects the rays reaching the other side through the material, is usually placed in an opaque envelope and placed on the back of the material being tested. The rule to be considered here is that the front surface of the envelope is made of material that can easily pass the rays. The image created by X-rays on the film is similar to the shadow cast by a normal light source. Unlike the shadow, the density of the image formed on the film changes depending on the thickness and density of the material. The clarity and size of the image depend on the size of the radiation source, the distance from the radiation source to the film, and the distance of the material from the film. After the film in the cassette is placed behind the test piece, it is exposed to X-rays for a certain period. After the exposed film is developed, the amount of blackening is checked. The darkening of the film is called density for short. The presence of different densities in the film indicates that there are different structures in the tested piece. The parts of the film that receive more radiation darken more. This means that the film density is high in this region. For example, if there is a gap in the examined region of the material, the beam will pass through this gap without loss, and therefore this area will appear blacker on the film. For the film to be read and evaluated properly, illuminated film reading devices should be used, in three types perforated, wired, and stepped to determine whether the applied inspection method is sufficient and the image quality level (sensitivity).

Penetrant (interference) inspection – Relates to the phenomenon of a rising or escalating liquid as it is confined to a small opening due to the characteristics of capillary action or the surface-wetting properties of the liquid. Penetrant inspection is used to find surface-open discontinuities on relatively smooth, non-porous surfaces. Types of defects that can be found in penetrant inspection: cracks and porosity.

The basic procedure to perform a liquid penetrant inspection consists of the following:

• Rolled Products: Identifies penetrant anomalies (cracks, seams, or laminations)
• Castings: Cast discontinuities, hot tears, porosity, air holes, or shrinkage
• Tattoos: Descriptive cracks, thrusts, or external crevices
• Welds: To identify cracks, porosity, combustion trough, cold thrust, lack of fusion, or insufficient penetration

There are two basic types of Penetrants available: Fluorescent or Visible. Within each method, there are many methods including water-washable, post-emulsifiable-lipophilic, solvent-removable, and post-emulsifiable-extreme hydrophilic methods. The type and penetrant method are chosen based on sensitivity levels 1 to 4 and depend on site conditions and other variables.

It is the examination of conditions affecting the quality such as discontinuities, structural defects, and surface condition on the surface of a product or metal parts, with or without the use of an optical aid (such as a magnifying glass).

Visual inspection can be seen as a very simple method, but it is one of the most important inspection methods. It is mostly work that needs to be done before the application of another non-destructive testing method. Most of the application standards prepared for other non-destructive testing methods require first visual inspection and recording of the findings.

This method can be applied to all metallic or non-metallic materials. Depending on the accessibility to the examination surfaces, it can be applied by using auxiliary devices such as endoscopes when necessary. Surface cleaning is generally not desirable as a preparation for the inspection surface. More precisely, the surface should be such that the expected defects are best visible. The examination is carried out under sufficient light conditions and according to appropriate viewing angles.

The magnetic particle method is widely used in ferromagnetic materials due to its simple, fast, and low-cost applicability, which is used in the detection and location of surface and near-surface defects.

In this method, the determination of surface defects depends on the size of the defect and its proximity to the surface, and it is applied only to ferromagnetic, that is, magnetizable materials. The basic principle of the method is based on the magnetization of the examined material. The magnetization process is carried out by passing an electric current or a direct magnetic flux through the part. Ferromagnetic materials do not show any resistance to this magnetic flux, on the contrary, they contribute to the passage of this magnetic flux.

Magnetic behavior of metallic materials:

1. Non-magnetic material
2. Magnetic material

If a magnetic field is applied to the surface of a material with faults, if ferromagnetic powders are sprinkled on the surface, these dust are attracted by the leakage fluxes formed in the areas where the faults are located and collect on these discontinuities and form a bridge for the passage of the leakage flux. Thus, the locations of the existing discontinuities are determined.

Not applicable to non-ferromagnetic materials. If the discontinuity is not positioned at an appropriate angle to the direction of the applied magnetic field, it cannot be determined. Large parts may require very high magnetizing currents. If the inspection surface is too rough, it affects the result negatively. If there is paint or coating on the inspection surface, its thickness directly affects the inspection result. If the magnetic powders are dry, they are in the form of flour-like powder. Powders are of two types, wet and dry, depending on where they are used. In addition, these powders are chosen in a color that will contrast with the ground on which they will be used, or with fluorescence content.

Application stages of the magnetic particle method:

• Pre-cleaning of the inspection surface
• Demagnetization if necessary
• Application of magnetizing current
• Spraying of ferromagnetic powders
• Interruption of the magnetizing current
• Examination
• Evaluation and report preparation
• Demagnetization and final cleaning