Diagnosis of discontinuities
Introduction
Non-destructive testing (END; in English, NDT, from nondestructive testing) is called any type of test carried out on a material that does not permanently alter its physical, chemical, mechanical or dimensional properties). Non-destructive testing is opposed to destructive testing in that it involves imperceptible or no damage. The different methods are based on the application of physical phenomena such as electromagnetic, acoustic, elastic waves, emission "Emission (physics)") of subatomic particles, capillarity, absorption "Absorption (chemistry)") and any type of test that does not involve considerable damage to the sample examined.
The terms non-destructive inspection, non-destructive testing, non-destructive examination and non-destructive evaluation are commonly used synonymously.
In general, nondestructive testing provides less accurate data about the state of the variable to be measured than destructive testing. However, they are usually cheaper for the owner of the piece to be examined, since they do not involve its destruction. Sometimes, non-destructive tests only seek to verify the homogeneity and continuity of the analyzed material, which is why they are complemented with data from destructive tests.
The wide application of non-destructive testing methods on materials is summarized in the following three groups:
• - Defectology. It allows the detection of discontinuities, evaluation of corrosion and deterioration due to environmental agents, determination of stresses, detection of leaks.
• - Characterization. Evaluation of the chemical, structural, mechanical and technological characteristics of the materials; physical properties (elastic, electrical and electromagnetic); heat transfers and isotherm tracing.
• - Metrology. Thickness control; single-sided thickness measurements, coating thickness measurements; filling levels.
Background
Nondestructive testing has been practiced for many decades. There has been a record since 1868 when work with magnetic fields began. One of the most used methods was the detection of superficial cracks in railway wheels and axles. The pieces were immersed in oil, and then cleaned and sprinkled with a powder. When a crack was present, the oil that had leaked into the discontinuity wetted the dust that had spread, indicating that the component was damaged. This led to the formulation of new oils that would be used specifically to perform these and other inspections, and this inspection technique is now called penetrant testing (PT).