| Non-destructive testing /NDT/ plays an important role in the system of quality management in the manufacturing process because it allows early detection of internal defects in a finished or semi-finished product that could hinder its effective use or may cause breakdown of a structure after a certain time of operation. For detection and quantitative evaluation of detected defects, a large number of methods using various physical principles are used in the field of non-destructive testing.
The advantage of non-destructive methods over destructive is that the tested product remains unchanged for further use. Methods of NDT for individual types of semi-finished and finished products are prescribed in the relevant standards and rules. In this respect, it is generally the customer who decides which standards or rules should be followed when evaluating the product using NDT.
Characteristics of basic NDT methods
From a viewpoint of practical application, non-destructive methods of testing can be divided into two basic groups:
a) Methods for detecting surface or near-surface defects in components or semi-finished products (cracks, laps, notches, cold shuts, etc.)
b) Methods for detecting defects in the whole volume (bubbles, inclusions, cold joints, etc.)
The first group includes the following methods:
- visual testing /VT/
- magnetic testing /MT/
- penetrant testing /PT/
- electromagnetic testing /ET/
In the second group, the following methods are of crucial importance:
- radiographic testing /RT/
- ultrasonic testing /UT/
Besides these basic methods, there are tens of other NDT methods and their combinations. The following of them have the most practical application:
- leak testing /LT/
- acoustic testing /AT/
- infrared and thermal testing /IRT/
Visual testing - a basic method for detecting surface defects and deviations in shape. Visual control and evaluation of welds may include, for example, evaluation of acceptable convexity, the extent of cold shuts, misalignment, etc. Surface defects in other types of semi-finished or finished products are evaluated in a similar manner. Various types of measuring and visual instruments are used for this purpose.
Magnetic testing - with this method, surface defects are detected and indicated by changes in magnetic field at the area of a crack or other surface discontinuity. Thus, it can only be used on ferromagnetic materials.
Penetrant testing - this method can be used also with other types of materials, with the exception of highly porous ones. Detection involves the application of dye liquid penetrant and capillary action in surface cracks.
Electromagnetic testing - requires electrically conductive material. That’s why it is very useful, for example, in detection of surface cracks in components made of aluminium alloys.
Radiographic testing - allows projecting internal discontinuities or defects onto an X-ray film, where they are seen as darker objects as a result of lowered absorption. X-ray equipment or certain types of isotopes are used as a source of radiation. The range of thickness that can be tested using this method is limited. The method is most frequently used for inspection of welds and castings.
Ultrasonic testing - the detection is based upon reflections of ultrasonic waves from discontinuities or defects. It can be used for inspection of both metal and non-metal materials. The advantage of this method is that it allows testing of semi-finished products of large thickness (up to several metres in forgings).
However, it needs to be stressed that none of NDT methods allows exact determination of the actual extent of a defect. Even MT, PT, and RT methods give us only a two-dimensional projection of a defect. It is possible, in some cases, to determine the third dimension using special procedures.
Defect detection is, however, not the only field of application of NDT methods. Some of the methods referred to above may be applied in evaluation of structural changes and changes in physical properties of materials.
Practical application of all NDT methods is considerably demanding for operators, both physically and in terms of professional knowledge and experience. This concerns in particular those methods (UT, ET), where the presence of defects is evaluated “indirectly”, based on an indication (signal) displayed on a screen in real time. Similarly, proper evaluation of radiographs and indications when using magnetic or penetrant testing method is not simple and requires considerable amount of experience.
Therefore, technicians in the field of NDT must complete special training and become certified for one or more methods. That’s why training of every technician is very time-consuming and costly.
Non-destructive testing of products makes a substantial contribution to the overall cost of production; however, its inclusion in the system of control has lately become a necessity in order to ensure the competitiveness of our products.