In principle, the method is based on the fact that the penetrant penetrates into microfine cracks and pores due to low surface tension and high capillary action.

Surface defects can be detected in all metallic materials, many plastics, glass and ceramics.
Penetrant testing can only be performed on non-porous materials. Care must also be taken to ensure that the surfaces to be tested are free of any coatings or contaminants so that the penetrant can penetrate any surface defects that may be present.

The penetrant method can be used in daylight with the dye penetrant method or with fluorescent penetrants under UV light. The inspection can be carried out directly on the component using the simplest means or using "penetration equipment", e.g. for series parts.

The procedure consists of the following steps:

During pre-cleaning, the surface must be carefully freed from any coatings and contaminants.

Penetration procedure
The agent can be brushed on, sprayed on, applied electrostatically or in an immersion bath. The penetrant should wet the entire test surface and be allowed to act between 5°C and 50°C for 5 to 30 minutes.

During intermediate cleaning*, the excess penetrant is removed from the test surface; this is done with a system-specific special cleaner or with water. During intermediate cleaning, it is particularly important to ensure that any imperfections are not washed out.

The drying process can be carried out in air or in a suitable drying oven.

Developing process
The developer can also be applied in various ways (as a wet developer, as a powder or electrostatically). It is important to ensure that the developer layer is applied evenly and thinly. After the development time, which is usually the same as the penetration time, inspection of the test area begins.

In the case of the dye penetrant, the indications are colored (usually red) on a white background.
In the case of fluorescent dye penetrant, the defect indications are bright yellow-green on a dark purple background.

To avoid the risk of over-washing, "post-emulsifiable penetrants" are often used. This is a non-water-soluble penetrant.

The intermediate cleaning procedure is then carried out in three stages:

1 stage: pre-washing with water
2 stage: application of the emulsifier (makes the penetrant water-soluble)
3 stage: post-washing

*In fluorescent testing, both the intermediate cleaning and the evaluation are performed under UV light.

Magnetic particle testing is also called MT testing, magnetic particle crack testing, flux testing or fluxing.

The method is used to detect cracks on or near the surface of ferromagnetic materials.

For the test (depending on the size of the component), the workpiece can be magnetized completely or in partial areas.

Principle of the test

If a magnetic flux is generated in a ferromagnetic material, it forms a magnetic leakage flux on the surface of the workpiece at surface cracks and also at internal cracks close to the surface.

If ferromagnetic powder particles are brought into the area of the flux leakage, they are attracted and held. Since the magnetic flux leakage is larger than the crack, the magnetic powder accumulation is also displayed larger and is more visible to the human eye.

A prerequisite for flux leakage is that the magnetic flux is perpendicular to the crack or has a sufficiently strong component in this direction.

The workpiece under test forms a path for the magnetic flux from one pole to the other. Thus, the area of the workpiece surface lying between the two magnetic poles is strongly magnetized. All cracks that run approximately perpendicular to the field direction are displayed.

Cracks lying parallel to the field do not cause any leakage flux and are therefore not displayed.

Magnetization process

Various methods can be used to magnetize the components.

Magnetization by means of hand magnets

The hand magnet or also called yoke magnet works either with a direct field or an alternating field magnetization. For most inspection tasks, an alternating magnetic field is the optimum solution. An alternating magnetic field introduced into the workpiece primarily detects the areas near the surface (skin effect).

Magnetization by means of a magnet bank

In magnetization systems, the component is clamped and exposed to a magnetic field by means of current flow or field flow. These two magnetization processes can also be combined.

X-ray testing, also known as radiographic testing or RT testing, is a procedure in non-destructive materials testing for displaying material differences in the workpiece.

RT testing is a standard procedure for testing safety-relevant components.

It can be used (depending on the tube or radiation source used) to detect inclusions or cracks in the volume of the test specimen in various components such as castings or welds.

With the use of an X-ray tube or gamma source (e.g. Iridium 192 or Cobalt 60), the component (or test section) is projected onto an X-ray film.  

To produce an X-ray image, the test section is irradiated from one side and an X-ray film is attached to the opposite side of the test object.

The X-rays that penetrate the component and strike the X-ray film blacken the film in the process. Due to different material thickness or density of the test surface, the projection image is displayed in different blackenings. The higher the density or thickness of the test section, the less radiation can penetrate the component and the brighter these areas are displayed on the X-ray film.

The recognition of details on an X-ray film depends on the contrast and resolution of the image.

The image quality depends on various factors such as material thickness, material density, the type of radiation source and the resolving power of the X-ray film used.  

Image quality test specimens (e.g. DIN wire bars with seven wire bars of different widths) are often used to evaluate the image quality. Based on the thinnest wire that can still be detected, conclusions can then be drawn about the smallest detectable defect size.

Ultrasonic testing is a non-destructive testing method for detecting material defects using ultrasound. UT testing" allows various components to be inspected even when installed.

Principle of the test

Before testing, a coupling agent (coupling gel, water or oil) is applied to the surface of the workpiece. By means of a test probe (also called test head) the surface to be tested is traversed. This process can be manual, mechanized or automated.

Acoustic changes in the test specimen, caused by interfaces at e.g. inclusions or cracks in the volume of the component, lead to the reflection of the acoustic pulse and send it back to the transducer in the probe.

Based on a time-of-flight measurement, the position of a possible defect (also called incompleteness) can be determined.

By means of a comparison with the reflection behavior of a substitute reflector such as cross holes or flat bottom holes, a rough estimate of the size of an incompleteness can be made.

With handheld digital instruments and automated inspection, this information can be stored and documented.

In ultrasonic wall thickness measurement, the ultrasonic signals are in many cases converted directly in the wall thickness gauge. The corresponding wall thicknesses can then be read directly.

All materials capable of conducting sound can be tested, including most metals.

Basically, a distinction is made between two methods:

The pulse-echo method or reflection sound method and the through-sound method.

With the pulse-echo method, hardly any material distances greater than 5 m are usually sonicated. Here, it must be taken into account that the sound path must cover the material distance twice, which corresponds to a sound path of 10 m. The sound path of the reflection sound method is then transmitted through the material.

In the through-transmission method, two probes are connected to one ultrasonic device. One probe transmits an ultrasonic pulse, which is received by the other probe on the exact opposite side of the specimen.

Special inspection procedures

There are also numerous special testing methods in which the sound effect in the material is not evaluated according to the classical method, such as the phased array technique or the TOFD method.

Visual testing (VT) is a recognized quality assurance procedure with a reliable test result. The exact procedure can be found in the DIN EN 13018 standard.

Experience has shown that visual testing is a useful addition to surface testing such as magnetic particle testing and penetrant testing.

Visual inspection is a test method that is used before all other tests. Here, initial findings are obtained that are an important prerequisite for all subsequent test procedures.

Visual inspections are also performed on parts in operation.

Visual inspection is used to locate and evaluate surface-related quality features such as shape deviations, imperfections and surface condition with the human eye or using optical aids (e.g. magnifying glass, microscope, endoscope, etc.).

A distinction is made between:

- Direct visual inspection without aids using the naked eye.

- Direct visual inspection with aids such as magnifying glasses, endoscopes, mirrors, etc.

- Indirect visual inspection with the use of cameras, endoscopes, videoscopes, etc.
 

The name TOFD stands for time of flight diffraction and is an ultrasonic testing method that was developed several decades ago. In contrast to conventional ultrasonic testing, it is primarily the diffraction signals that are evaluated.

The inspection is performed with two angle beam probes using V-transmission. The test results are documented directly and traceably on a notebook.

TOFD has a very high reliability and can be used especially on thick-walled components.
Furthermore, defects can not only be detected but also measured with TOFD.

The goal of weld seam inspection is to find various flaws or inclusions in the weld seam, which leads to a negative strength of the weld seam. Due to the various flaws, such as binding defects, cracks, pores and shrinkage cavities, a decision should first be made on suitable testing methods.

Radiographic testing can reliably detect the majority of inclusions during weld inspection. However, radiographic testing also has its limits. While voluminous defects such as pores or blowholes can be clearly detected on the X-ray film, the detection of cracks or weld line defects is very strongly dependent on the angle of incidence. In the case of weld line defects, the X-ray tube or isotope can be aligned according to the weld preparation to ensure an optimum irradiation angle. The alignment of cracks is usually unknown and thus difficult to detect with X-ray inspection.

Ultrasonic weld inspection can usually detect cracks better than radiographic inspection. Bond defects can also be detected well with ultrasonic testing. For further information on radiographic and ultrasonic testing, please refer to our procedural notes on RT testing for radiographic testing and procedural notes on UT testing for ultrasonic testing.

Test equipment and accessories for weld seam testing can be found under: X-ray testing and ultrasonic testing.

(Surface) crack inspection

Fine surface cracks are very difficult to detect with the naked eye and often cannot be detected at all. Very often these surface cracks are just the beginning of a large crack. Major damage to components is usually the result if these cracks are not found and repaired in time.

Dye penetrant testing and magnetic particle testing are most commonly used for surface crack detection. These two testing methods are procedurally very different and cannot be applied equally on all materials. On the other hand, the results are similar. The narrow cracks are visually displayed wider either with high-contrast color or fluorescent under UV light.
Both surface crack tests can be performed manually or automatically. The decisive factor in selecting the appropriate crack detection method is primarily the properties of the material. For example, only ferromagnetic materials can be tested in magnetic particle testing. In penetrant testing, the flaws to be found must be open at the top. Porous materials, on the other hand, cannot be tested.

For further information on crack detection, please refer to our PT testing procedure notes for penetrant testing and MT testing procedure notes for magnetic particle testing.

Test equipment and test devices for crack detection can be found under: Dye penetrant testing and magnetic particle testing