What term describes an interruption in the normal physical structure that produces an MT indication?

The term that describes an interruption in the normal physical structure producing a magnetic particle (MT) indication is "discontinuity." This term specifically refers to any break, crack, void, or other flaw in a material that can disrupt its continuity. In the context of Magnetic Particle Inspection, identifying these discontinuities is crucial because they can indicate potential structural weaknesses or failures in the material being evaluated.

Discontinuities can be either inherent to the material (like those created during manufacturing) or introduced through service conditions (such as fatigue cracks). When magnetic particles are applied in the inspection process, they tend to gather at these discontinuities, creating visible indications that can be assessed by the inspector.

The other options, while related to the context of material flaws, do not specifically define the interruption in the physical structure as accurately as discontinuity does. A defect may refer broadly to any fault or imperfection in the product, but it does not inherently describe the nature of the physical interruption. An indication refers to the visible evidence left by an interruption or defect, and deformation relates more to a change in the shape or structure of a material under stress rather than a specific interruption. Thus, "discontinuity" is the most precise term in this context.