2. Principles of defect detection. Different NDT techniques

2.1 Heterogeneities and defects

The term "defect" is ambiguous, relative and imprecise, but its negative connotation is a good illustration of the role played by non-destructive testing in the quest for quality. In fact, to detect a defect in a part is, in physical terms, to highlight a heterogeneity of material, a local variation in physical or chemical properties detrimental to its proper use. Having said that, we usually classify defects into two main categories according to their location: surface defects and internal defects.

• Surface defects, accessible to direct observation but not always visible to the naked eye, can be classified into two distinct categories: point defects and appearance defects.

  The first...