Testing technology - Requirements and overview of methods

As part of quality assurance and functional testing, testing technology plays a crucial role in the production environment. A variety of material testing methods are used, ranging from destructive tests, such as tensile testing, to non-destructive techniques, such as ultrasonic testing. This article provides a comprehensive overview of the requirements for testing technology as well as various testing methods.

Basics of Testing Technology

Testing technology is part of quality assurance, but it also serves to verify the functionality of components and products. It ensures that materials and components meet requirements and that defects are detected and corrected early in the production chain. The terms "measurement technology" and "testing technology" are often used interchangeably, but in fact, measurement technology is only one part of testing technology. You can find more information on measurement technology in our article Measurement Technology - Quality Control Through Measurement Methods. Testing technology encompasses many different methods, such as visual or material testing. The following overview provides a more detailed insight into various testing techniques.

Measurement refers to the quantitative determination of physical or otherwise numerically quantifiable properties of a component, such as length or diameter. Inspection refers to an evaluative process in which, for example, measurement results are compared with one another under specific conditions and requirements. During testing, for example, compliance with standards is determined. This category also includes gage-based testing. Gage-based testing is function-oriented and aims to test the component under examination in its operational environment. For this purpose, so-called gages are constructed. You can learn more about this in our article Testing Technology – Gage Construction.

The quantitative and qualitative approach

Quantitative and qualitative testing methods describe two fundamental approaches to quality control in industrial manufacturing. Quantitative testing methods measure quantifiable, continuous values—such as the length, hardness, or surface roughness of a component. These tests provide precise data that can be statistically analyzed and used for process monitoring. In contrast, qualitative methods record non-measurable characteristics, often in the form of attributive inspections, in which test objects are classified as “good” or “bad”—for example, through visual inspection for scratches or discoloration. Attributive inspections are used in particular when qualitative characteristics cannot be measured or can only be measured with disproportionate effort. Evaluation is often performed visually, through sound testing, or using gauges. Examples include assessing the surface finish of products, sound testing of ceramic items, or visual inspection for weld seam irregularities. One problem with attributive testing is subjectivity. Since the assessment is made individually by the inspector, the evaluation of the same component by a different inspector may differ. The lack of differentiation is also problematic. There is no gradation in the quality assessment. The only relevant information is whether a characteristic is present or not.

The advantage of qualitative methods lies in their simplicity and speed of execution; their disadvantage is greater subjectivity and lower statistical significance. Quantitative inspections, on the other hand, are objective and reproducible, but often require more complex measurement technology and staff training. In practice, both methods are frequently combined to reliably inspect both measurable and visually assessable quality aspects.

Testing Requirements

Testing technology in general must meet certain requirements so that test results can be meaningfully processed and add value to the overall process:

• Reproducibility: Test results must be consistent under identical conditions, regardless of the operator. This is the only way to ensure comparability, especially in high-throughput series production.
• Accuracy: Even the smallest defects must be detected. At the same time, the false positive rate must not be too high.
• Automation capability: Modern inspection systems should integrate seamlessly into existing production lines.
• Data acquisition and traceability: Complete documentation of inspection results is essential to enable traceability and process optimization.
• Validation of inspection methods: The inspection method must be demonstrably suitable for reliably testing the required quality characteristics under real-world conditions.

Various testing methods

Testing methods can generally be divided into destructive and non-destructive methods. The tensile test, for example, is a method of material testing in which the mechanical properties of the material are evaluated by applying a tensile load. This allows the tensile strength, yield strength, and elongation to be determined. However, the tested specimen is completely destroyed in the process. The focus of this article, however, is on non-destructive testing methods (also known as NDT methods), in which the test object is not destroyed.

Non-destructive testing (NDT) enables the early detection of material defects without compromising the component. It is used in fields such as mechanical engineering, aerospace, the automotive industry, and plant and pipeline construction.

Typical targets of the testing include cracks, pores, bonding defects, or delaminations. Modern NDT methods can be integrated into production lines, thereby enabling testing directly during the ongoing production process. Surface crack testing, in particular, is one of the most common non-destructive testing methods in metalworking and is an integral part of many material testing procedures for metals.

DIN EN ISO 9712 certifies personnel to perform non-destructive material testing. There are three qualification levels, with Level 1 qualifying personnel to perform NDT under supervision. Certification can be obtained for various methods, such as ultrasonic testing, visual inspection, or magnetic particle testing. In the following section, we will introduce you to several types of non-destructive material testing.

Visual inspection

Visual inspection is the simplest method of non-destructive material testing. It is performed either with the naked eye or with the aid of measuring instruments such as magnifying glasses, endoscopes, or camera systems. Penetrant testing is also a type of visual inspection. In this method, penetrants—which are often dyed—are applied to the material being tested. Due to its low surface tension and high capillary action, the penetrant can easily penetrate openings and cracks in the surface of the test piece. This makes defects more easily detectable. In some cases, fluorescent penetrants are also used, making even the smallest defects visible under UV light.

The purpose of visual inspection is to detect surface defects such as cracks, corrosion, or imperfections. While it is relatively inexpensive, it requires specially trained personnel to accurately assess surface irregularities. Another issue with subjective inspection methods is that they also depend on the inspector’s concentration and daily performance. This can lead to variations in assessment, resulting in false positives or false negatives. Furthermore, human inspectors become fatigued during long shifts and may overlook defects that are less clearly visible—especially during monotonous tasks or in low-contrast conditions. Visual inspection should therefore always be combined with other methods to ensure a comprehensive assessment.

Eddy current testing

Eddy current testing is an electromagnetic testing method used to detect defects in electrically conductive materials. An excitation coil in the test probe generates a high-frequency alternating magnetic field, which induces eddy currents in the test object. Changes on the surface result in measurable signal changes. The test probe must be configured accordingly, which varies depending on the material and the type of defect.

Magnetic particle inspection

Magnetic particle testing is a method for detecting surface and near-surface defects in ferromagnetic materials. In this process, the test object is magnetized and then coated with fine magnetic particles; these particles accumulate at defects, thereby making cracks or inclusions visible.

Magnetic particle testing can be used to detect fine surface and weld defects in ferromagnetic materials. However, just as with visual inspection, the decision as to whether a crack is considered too large rests solely with the inspector. This means that misjudgments can occur depending on the inspector’s level of experience. In addition, the cleaning effort required after the test is quite significant. Magnetic particle testing is therefore particularly suitable for small batch sizes and experienced inspectors.

Automated optical inspection

Using camera systems and software, products are automatically analyzed and evaluated for defects. Modern AOI (Automated Optical Inspection) systems detect faulty structures, dimensional deviations, or surface defects. Through the use of high-speed cameras and multispectral lighting systems, even low-contrast defects, such as dull spots or microcracks, can be made visible. Modern AOI systems increasingly utilize AI algorithms and deep learning to automatically adapt to new defect patterns and minimize false positives. Applications range from printed circuit boards to complex injection-molded parts. The advantage of AOI inspection is the fully automated and rapid detection of surface defects. The disadvantage is that it is limited to visually detectable defects.

Ultrasonic testing

Ultrasonic testing uses high-frequency sound waves to detect internal irregularities in materials. The test object is fitted with a transducer containing an ultrasonic transmitter and receiver, and the ultrasonic waves are then transmitted into the test object via a coupling medium. By analyzing the reflected sound waves, defects both near the surface and deep within the material can be identified. Typically, the ultrasound is reflected only once, for example, off the back surface of the test object. However, if there are defects in the test object, they will reflect an additional echo. Ultrasonic testing is suitable for all types of materials.

While methods such as eddy current testing quickly reach their limits once a certain penetration depth is exceeded, ultrasonic testing can be used to inspect particularly thick or hard-to-reach components. The disadvantage of ultrasonic testing is that the results must be interpreted, which requires experienced personnel. In addition, calibration is quite complex.

Testing of electronic components

During manufacturing, electronic components and circuit boards are tested as part of in-circuit tests or functional tests. For example, reliable signal transmission, insulation, and defined resistance values are checked. So-called spring contact pins are used for testing. Due to the spring-loaded tip of the spring contact pins, they can flexibly adapt to the surface and, when the contact pressure is correctly selected, do not damage the surface. You can find more information on spring contact pins in our article Testing technology - Testing of printed circuit boards (spring contact pins).