Discontinuity Origins and Ultrasonic Response

Learning outcomes

By the end of this chapter, the learner should be able to distinguish discontinuity, indication and defect; relate manufacturing and service mechanisms to morphology; predict orientation effects on ultrasonic response; avoid over-classification from amplitude alone; and build a cause-response evidence chain.

12.1 Terminology

A discontinuity is an interruption in material or geometry. An indication is evidence produced by the examination. Interpretation assigns a likely source to the indication. Evaluation compares a relevant indication with specified rules. A defect is a condition that fails applicable acceptance criteria. These terms protect the logic of the decision.

12.2 Product mechanisms

Cast products may contain shrinkage, porosity, inclusions, cold shuts and hot tears. Wrought products may contain laps, seams, bursts, laminations, flakes and segregation. Welds may contain lack of fusion, incomplete penetration, slag, porosity, undercut and several crack forms. Service can produce fatigue cracks, creep damage, stress-corrosion cracking, hydrogen-related damage, erosion and corrosion loss.

Mechanism predicts morphology. Fatigue often initiates at a stress concentrator and grows approximately normal to principal cyclic stress. Lack of sidewall fusion tends to follow a fusion face. Laminations tend to lie parallel to the rolling surface. These tendencies guide beam direction, but individual conditions vary.

12.3 Specular and scattered response

Smooth planar reflectors can behave specularly: strong response occurs when the beam is near normal to the plane, and a small orientation change can reduce amplitude sharply. Rough cracks scatter and diffract energy over more directions. Volumetric clusters can produce distributed echoes and noise-like response. Crack tips may produce small diffracted signals.

Amplitude depends on orientation, surface roughness, size relative to wavelength, beam position, sound path, shadowing and calibration. A large unfavourably oriented crack may respond less strongly than a small drilled hole. Amplitude is evidence, not identity.

12.4 Dynamic response

Probe translation, rotation and orbital movement reveal response shape. A long planar condition may show a sharp maximum and predictable positional movement. Porosity may produce multiple short-lived peaks. Geometry often repeats along a known profile. Dynamic patterns should be documented and combined with position and manufacturing knowledge.

12.5 Mechanism-based case

An in-service shaft shows a sharp response at a fillet, strongest when the beam is directed normal to the high-stress plane. The response is absent on the original manufacturing record and has tip-like satellites. Fatigue cracking is plausible, but the Level 3 should require confirmation by alternate angles or another suitable surface method, assess sizing uncertainty, and escalate disposition using the governing integrity process. A plausible mechanism is not permission to skip confirmation.

Failure modes and decision control

Weak practice commonly includes calling every indication a flaw; identifying discontinuity type from amplitude alone; ignoring manufacturing direction and service stress; assuming a weak response means a small condition; using one probe movement as complete characterisation; and assigning acceptance before confirming relevance.