Why is reliability important

Why reliability is important

There is no clear definition of the term “reliability”. For this reason, the requirements for a NDT system and the distinction between a reliable and an unreliable system should first be clarified. When trying to define the reliability of a NDT system, the general evaluation of a measuring system is sometimes used. The focus here is on the accuracy of a measuring system. The accuracy is divided into precision and correctness. The correctness describes the systematic error in the measurement. The precision is referred to as the proximity of the measuring points to one another.

At the same time, it has been shown that the determination of a measurement error or the accuracy sometimes does not meet the requirements and the desired benefit of a customer with regard to the capability of a system. This is especially true for a NDT system, as its failure does not lead to clear consequences. So is for example

An undetected material defect cannot be equated with the failure of the tested specimen.


In the second American-European workshop on NDT reliability, the following was defined about the reliability of NDT systems: The reliability of NDT systems is the degree of capability according to which the NDT system fulfills its requirements with regard to detection, characterization and false alarms. The requirements are defined as follows:


  • Detection describes the finding of an existing defect;
  • Characterization is divided into:

- Position determination, size display;

- classification; i.e. the naming and evaluation of the display with regard to the type of defect;

  • False alarm expresses the incorrect assessment or misinterpretation of a defect-free or non-critical component.


The detection of defects is an essential step of a reliable NDT system, since otherwise no further characterization is possible. This is why many evaluations of NDT reliability focus on defect detection. A related definition approach for the reliability of NDT systems considers the sub-areas that affect the result of the test:

  • inherent factors (intrinsic capability) that describe the physical properties of the test system (test sensors, test aids, manipulator, etc.)
  • Application parameters that affect the test (coupling, other sources of noise, environmental influences, etc.)
  • human factors that have an influence on the test (e.g. factors from the environment of the person, the organization, the work as well as human or individual characteristics that have a safety-relevant influence on the work behavior.)
  • internal and external organizational influencing factors (organizational context)
  • Processes of test planning and test execution, cultural influences, etc.)


From the point of view of the ability to detect defects, a NDT system reliably fulfills its task if a truthful answer is reported back after being asked whether there is a defect in the component.

There are two different approaches to evaluating a NDT system. The standards-based approach, which is mainly used in Europe, and a performance demonstration, which is predominant in the United States. In the optimal NDT management concept, both the minimum requirements from the point of view of the standards and the probabilistic requirements must be comprehensively examined. In addition, the process should be embedded as an overall context in the real test environment:

The standards and guidelines stipulate the minimum requirements for the procedure in the regulated area. The probabilistic requirements realistically estimate the actual ability of the system with regard to possible wrong decisions. If a system has gone through the qualification process, further influences have to be considered for the actual use (e.g. in the production test): environmental influences, human factors or organizational influences.