Do NDTs provide actual measurements of defects?
A common question that we usually receive at the Non-Destructive Testing area of AZTERLAN is whether conventional NDT provide real measurements of defects, whether they are internal or external defects.
The answer to this question is no: conventional non-destructive methods do not provide the real measurements of the defects. On the contrary, they only allow sizing the indications caused by discontinuities in the parts.
Below we shortly review the main non-destructive testing methods and the characteristics of the measurements that each of them offers:
- Penetrating liquids tests: the measurement refers to the sizes of the “spots” that depend on the applied procedure, the products and the part under inspection.
- In magnetic particles tests: the measurements will be determined by the type of defect, the sensitivity of the product and the type of current applied. That is, in these cases, a segregation or a crack, for example, is not sized the same.
- In ultrasound tests: indications are measured based on a certain reference threshold. In this case, it also happens that the inclusions look different than air cavities. This is due to the different impedance of both elements and their different reflection coefficient.
- In X-ray testing: On the one hand, radiographs show 2D projected areas of a volumetric defect. For its part, the computed axial tomography delimits the discontinuities that differ by 20% in shades of gray with respect to the adjacent area.
In conclusion:
If to all these characteristics specific to each of the non-destructive testing modalities we add the environmental conditions for carrying out the test and the surface state of the pieces, it is easy to understand that the real size of the defects and the measurements are not the same concept.
