Discontinuity Interpretation and Evaluation Flashcards

An NDT technician evaluates a radiograph of a multi-pass weld and observes several dark, irregularly shaped indications with well-defined outlines, located between weld passes. Which type of discontinuity is most likely being observed?

Answer: Slag Inclusions

Slag inclusions are non-metallic materials trapped in the weld, often between passes. On a radiograph, they appear as dark, jagged, or irregularly shaped indications because they are less dense than the surrounding weld metal. Their location between weld beads is a key identifier.

In the context of Nondestructive Testing, what is the fundamental distinction between a "discontinuity" and a "defect"?

Answer: A "discontinuity" is an interruption in the material, while a "defect" is a discontinuity that does not meet specified acceptance criteria.

A discontinuity is any interruption in the normal physical structure of a material. It only becomes a defect when an evaluation determines that it compromises the part's integrity or fails to meet the acceptance criteria outlined in the relevant code, standard, or specification.

During a shear wave ultrasonic inspection of a weld, the technician observes a sharp, high-amplitude signal that moves across the A-scan display as the transducer is moved parallel to the weld. The signal's amplitude changes significantly with slight angulation of the transducer. This is most characteristic of:

Answer: A planar discontinuity, such as a crack or lack of fusion.

Planar discontinuities like cracks or lack of fusion act like mirrors to the ultrasonic beam. They produce sharp, high-amplitude signals but are very sensitive to the angle of incidence. The significant change in amplitude with slight transducer angulation (rocking) is a classic sign of a planar, crack-like flaw.

When evaluating a known discontinuity, which of the following characteristics is generally considered the MOST critical in determining its potential to cause a structural failure under tensile stress?

Answer: Its sharp, narrow tip and orientation perpendicular to the primary stress.

Sharp, crack-like discontinuities act as significant stress concentrators (or stress risers). When oriented perpendicular to applied tensile stress, the stress at the tip of the crack can be magnified many times, leading to crack propagation and eventual failure, even at stress levels below the material's nominal yield strength.

An inspector performs a liquid penetrant test on a new aluminum casting. After applying the developer, a series of very fine, intermittent linear indications appear. Upon closer visual examination, the inspector notes that these indications perfectly align with the machining marks from a previous manufacturing step. How should these indications be classified?

Answer: Non-relevant indications.

Non-relevant indications are true penetrant indications caused by intended design features or manufacturing processes, such as press fits, threads, or, in this case, machining marks. They are not due to unintended discontinuities like cracks or porosity and should be identified and documented as non-relevant.

An NDT Level II inspector identifies and measures a relevant indication that exceeds the maximum allowable length specified in the applicable acceptance standard. The final step in the evaluation process is to:

Answer: report the indication as "unacceptable" or "rejectable."

The core function of NDT evaluation is to compare the characteristics of a relevant indication against the acceptance criteria defined in a code or standard. If the indication exceeds the established limits, it is deemed a defect, and the part is formally reported as "unacceptable" or "rejectable."