Knowledge / Shipyards and production

Non-destructive testing of ship welds: method selection, coverage and acceptance

A technical guide to surface and volumetric examination, discontinuity orientation, ultrasonic sound paths and weld acceptance.

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Examining a weld does not mean that every possible discontinuity has been sought with equal reliability. Non-destructive testing is a measurement arrangement that can reveal particular features in a particular material and geometry. For ship construction, method selection starts with the joint's structural function, expected discontinuities, access and acceptance criteria. The sophistication of an instrument is only one part of that decision.

Define the examination question first

Searching for embedded lack of fusion in a new butt weld differs from searching for a fatigue crack at an in-service weld toe. The former emphasises the weld volume and fusion faces; the latter emphasises the initiation location and whether a crack reaches the surface. Material, thickness, preparation geometry, welding process, examination timing and coating condition belong in the same definition.

Visual examination can reveal misalignment, profile, surface openings and access conditions. It cannot by itself demonstrate the absence of embedded discontinuities. Coverage is also more than a percentage of weld length: the report should identify the volume examined, the access side and the orientations to which the arrangement is sensitive. A region that cannot physically be scanned is a coverage limitation, even if the accessible weld length has been examined completely.

Different physical limits of surface methods

TWI's explanation of surface examination distinguishes penetrant examination of surface-breaking discontinuities from magnetic-particle examination of suitable magnetic materials. Magnetic-field direction relative to a discontinuity affects sensitivity; examination in one direction does not cover every crack orientation equally. Penetrant enters an open surface discontinuity by capillary action; magnetic particles gather around flux leakage. Suitable magnetic techniques can also have limited near-surface sensitivity, without a universal detection depth.

A closed discontinuity that penetrant cannot enter can remain present despite a clean surface result. Paint, oil or metal smeared by surface preparation can change the examination problem. A magnetic method used for ferromagnetic steel cannot simply be transferred to an austenitic stainless joint. Temperature, dwell time and chemical compatibility come from the qualified procedure and product instructions; one generic application time is insufficient for all materials.

Read ultrasonic and radiographic methods as complementary

An ultrasonic echo depends on an interface encountered by the sound beam. If a planar discontinuity is not insonified at a suitable angle, a strong reflection may fail to return to the receiver. Geometry, grain structure, attenuation and probe access therefore matter. TWI's lack-of-fusion note identifies the frequent advantage of ultrasonics for inter-run lack of fusion, while noting that material affects performance.

A radiographic image reflects differences in attenuation along a radiation path. A volumetric feature such as porosity and a thin planar opening do not have identical visibility. A two-dimensional projection also does not inherently provide depth. TWI's radiography overview explains that physical basis. Ionising radiation requires authorised personnel and work-specific area controls; a production schedule cannot substitute for those arrangements.

An example separating sound path from depth

In the pulse-echo principle, measured time includes the outward and return journeys. For an illustrative calculation, assume a shear-wave velocity of 3,250 m/s in homogeneous steel and an echo time of 20 µs after removing instrument and wedge delays. The one-way sound path is s = c t / 2 = 32.5 mm. The velocity is an example assumption; the actual component requires calibration.

If the beam is at 60° to the surface normal and travels along its first straight leg, reflector depth is s cos 60° = 16.25 mm, while surface distance is s sin 60° ≈ 28.15 mm. Reporting 32.5 mm directly as depth would be a geometric error. This calculation excludes back-wall skips, curved surfaces, uncertainty in refracted angle and reflector sizing. A calculated location does not establish whether the reflector is acceptable.

Calibration, qualification and representativeness

Reading the correct distance on a reference block helps check the instrument time base. It does not prove that every target discontinuity will be detected in the actual weld. Probe angle, wedge, couplant, surface condition, material and scan path form one examination arrangement. Differences between a demonstration specimen and the production joint become limits on what the demonstration establishes.

A colourful digital scan is not itself evidence of adequate qualification. Uncovered volume, recording resolution, saturated signals and evaluation settings should be identifiable. Where automated classification is used, the relevant error types and validation specimens matter. Personnel qualification and the suitability of a procedure for a particular joint are separate pieces of evidence, even when both are required for the same examination.

From indication to acceptance decision

An indication is a response produced by the instrument or examination medium. A discontinuity is a physical feature in the material. An unacceptable defect is the result of evaluation against the applicable criteria. Combining these terms can make every response look like damage requiring repair, or make an unrecorded response look like proof of absence.

A useful report links joint identity, coverage, procedure version, settings, location and acceptance reference. After repair, examining only the repair metal's surface can leave the surrounding affected region unresolved. Re-examination extent follows the accepted repair and inspection plan. No general ranking of methods automatically changes the scope required by class or contract.

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