Quality Control

Nondestructive testing

Detecting defective components at an early stage with industrial computed tomography

What is nondestructive testing?

Nondestructive testing (NDT) is a technique for checking components, workpieces or assemblies for defects. Various methods such as X-ray, ultrasound, computer tomography or magnetic powder can be used in NDT.

The aim of NDT is to detect defects in materials at an early stage and remove the defective workpieces from production. In order to detect these defects, the test pieces are subjected to thermal, mechanical and chemical stress during nondestructive testing. Faults or defects include pores, air pockets, cracks or surface corrosion. The test must be carried out without destroying or damaging the test piece. After the test, the workpiece or material must be as suitable for use as it was before the test. This makes NDT an important method for quality control and safety testing of components and systems, since defective materials and structural flaws can, in extreme cases, even compromise the safety of the component or building.

What is the difference to destructive testing methods?

Unlike nondestructive testing, destructive testing damages the material to the point where it can no longer be used. Standardized test pieces specially manufactured for this purpose are used. Furthermore, it cannot be said with certainty that the next component produced will be free of defects. In industry, it is therefore extremely important to carry out nondestructive testing in order to ensure the quality, safety and cost-effectiveness of production.

The purpose of destructive testing lies in another detail: The destruction of the test piece provides important information about which material is suitable and how the geometry and dimensions of the part to be manufactured should be.

Destructive testing is carried out to determine material characteristics. This is not the case with nondestructive testing. Afterwards, all that is known is whether the examined device needs to be replaced or whether it is technically safe.

The different methods of nondestructive testing

There are many methods of nondestructive testing. In addition to the testing methods already mentioned, such as ultrasound or X-ray inspection, there are other techniques such as acoustic testing, thermography or ground-penetrating radar. Laser inspection is also used in NDT. The choice of method depends on various factors, such as the type of material, the size of the test piece or the type of defect to be detected.

The "classic" nondestructive testing methods are the most important methods in NDT, which differ between surface inspection and the inspection of internal structures.

Visual inspection is an optical inspection to check parts and devices. It is particularly suitable for the quality control of surfaces and during the assembly of components. Various aids can be used during the procedure, such as magnifying glasses, mirrors, microscopes, endoscopes, cameras, scanners or, of course, the naked eye. However, one disadvantage of nondestructive visual inspection is that only surface faults or defects can be detected, while faults inside the test piece initially remain undetected. This requires further nondestructive testing methods.
Dye penetrant testing involves spraying or dipping a colored or fluorescent contrast agent onto a component. The agent settles in cracks, pores or holes on the surface and makes them visible. Dye penetrant testing is often used for welds in particular. A disadvantage of this method is that the environmental aspects of the contrast agents must be considered and the staining does not give an accurate indication of the depth of the cracks or holes. In addition, rough surfaces can display pseudo defects that are not actual defects. Despite these limitations, dye penetrant testing remains an important method in the quality control of components and systems.
Magnetic particle testing, also known as magnetic flux leakage testing, is a nondestructive testing method that is used for magnetizable materials and workpieces. With this method, the test piece is first magnetized. Fluorescent magnetic particles are then applied to the test piece using a liquid or powder. Defects become visible because they generate a different magnetic field. Magnetic flux leakage testing is a very fast test method and enables the detection of even the smallest cracks, which can be up to four times thinner than a human hair.

Magnetic particle testing therefore offers high sensitivity in the detection of surface defects and is frequently used in metal processing and the automotive industry. Another advantage of the method is that it is easy to perform. Despite its advantages, magnetic flux leakage testing has limitations because it can only be used with magnetizable materials and the results can be affected by the magnetic properties of the material.
Eddy current testing is a nondestructive test that can only be used for electrically conductive materials. The surface of the test piece is examined for defects. A magnetic field is generated around the workpiece, creating eddy currents in the test piece itself, which in turn create their own magnetic field. Changes in the material such as cracks, air pockets or similar defects, but also impurities, generate a different magnetic field than the actual material, as they have a different electrical conductivity.

The advantage of this nondestructive component testing is that it requires only low resource consumption and maintenance costs compared to other methods. Eddy current testing is therefore one of the most economical and environmentally friendly nondestructive testing methods. It is frequently used in the automotive and aerospace industries. However, eddy current testing can only be used for electrically conductive materials and is limited to surface inspection, as the eddy currents are only induced in the upper layers of the material.

Internal component inspection methods

X-ray inspection

X-ray inspection, also known as radiographic testing, is an important nondestructive testing method that allows you to look inside a component. Unlike ultrasonic testing, radiographic testing can also determine the type and exact location of defects, which is an advantage in many applications. Digital X-ray inspection, such as computed tomography, opens up many other advantages, as the test results can be stored and evaluated digitally.

X-ray inspection is very important: NDT 4.0, i.e. the combination of the digitalization of radiographic testing and the use of AI, delivers decisive advantages.

More about NDT 4.0

Ultrasonic testing

In ultrasonic testing, ultrasonic waves are sent from a probe through the test piece. Defective areas in the material reflect the waves and can be tracked on a screen. By measuring the transit time of ultrasonic wave transmission and reflection, the type and location of a defect can be precisely determined. Modern ultrasonic testing even measures the size of a defect.

Ultrasonic testing is suitable for nondestructive testing of flat and voluminous components, for example for measuring wall thickness. Ultrasonic testing provides better results than X-ray inspection for flat defects. One disadvantage of the method, however, is that the material is not only subjected to acoustic strain during the test, but also to thermal strain. Ultrasonic testing is also more difficult with coarse surfaces.

CT scans

In NDT with computed tomography (CT), the object to be tested is X-rayed, similar to radiographic testing. Numerous 2D images are captured from different perspectives. These images are then converted into a three-dimensional model using a computer. This model then provides insight into the interior of the object, showing possible defects or variations in the material. A unique feature of CT-NDT is that it provides a higher resolution than other nondestructive testing methods, which means that even very small or difficult-to-detect defects can be detected.

With industrial computed tomography, even small, complex parts can be inspected with maximum precision. Device control and measurement data evaluation are carried out using special software. As a result, scans can be generated that meet the high metrological requirements of the industry, detecting core breaks and even the smallest defects, pores and voids in the part. The CT scans even capture components that older measurement technology could not test due to the part's inaccessibility.

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Horst Lang
Head of Corporate Quality Operations Support, Festo

Why is nondestructive testing so important in the industry?

The modern technology of NDT creates great added value for the industry. Early detection of defective components allows them to be removed from the production process. If possible, a repair can be carried out. In the case of irreparable defects, the material is scrapped and returned to the material cycle.

This results in the following striking advantages:

Saving resources, time and money
Fewer complaints
Increased quality and safety of components
Reduced risk to people and the environment due to faults in components

Who may carry out an NDT?

Nondestructive testing is subject to a standard, DIN EN ISO 9712. In accordance with this standard, only certified individuals may carry out nondestructive testing procedures. The certification follows strict directives to ensure the quality and safety of the tests.

Depending on their experience and training, certified individuals can achieve three levels in their respective test procedure. A separate certification is required for each test method. For example, a person certified only to perform ultrasonic testing may not perform nondestructive radiography or eddy current testing.

The three levels of certification include the following:

Stage 1: Authorization to carry out test procedures and document the test results
Level 2: Authorization to additionally evaluate test results (according to standards and regulations)
Level 3: The person conducting the audit may decide on the appropriate audit procedure, specify the testing method and is also responsible for the facility in which the audit is conducted

Certification is issued by various certification bodies such as the German Society for Nondestructive Testing (DGZfP), the American Society for Nondestructive Testing (ASNT) and other bodies. These in turn require confirmation from the German Accreditation Body (DAkkS).

Certificates must be renewed every 5 years to ensure that testers have current knowledge and expertise in their testing procedures.

AI: The advantages of NDT 4.0

Industrial computed tomography has become one of the most important methods in NDT. Complex internal structures are checked for defects non-destructively and in 3D. As a result, the process parameters are coordinated and the manufacturing process is continuously improved.

The digitalization and automation of nondestructive testing heralds progress in the industry and is referred to as NDT 4.0. Since factors such as fatigue affect human testing, the use of AI is particularly beneficial. These factors do not play a role in an automated NDT. What's more: Test results or evaluations of errors can differ from person to person. Tests taken by the same person can also vary due to fatigue, for example. AI-based automated defect detection eliminates human error and maximizes the reproducibility of test results. Thanks to machine learning, the AI can also learn new test parameters at any time and continuously improve the testing process. The goal of NDT 4.0 is to take nondestructive testing to the next level, combining human expertise and experience with the efficiency of artificial intelligence to create synergy.

AI-based automated defect detection eliminates human error and maximizes the reproducibility of test results. Thanks to machine learning, the AI can also learn new test parameters at any time and continuously improve the testing process. The goal of NDT 4.0 is to take nondestructive testing to the next level, combining human expertise and experience with the efficiency of artificial intelligence to create a synergy.

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