Technique Design and Qualification

Learning outcomes

By the end of this chapter, the learner should be able to translate an inspection objective into essential variables; select probes, angles, frequencies and scan patterns logically; distinguish procedure qualification from routine verification; design representative demonstrations; and control changes and technical departures.

7.1 Start with the discontinuity-mechanism pair

Technique design begins with what can occur and how ultrasound could reveal it. The Level 3 defines likely discontinuity type, position, orientation, size range and morphology from the manufacturing or service mechanism. The design then selects beam directions, modes and response features that address those conditions.

A fatigue crack growing from a shaft fillet, lack of sidewall fusion in a weld, shrinkage in a casting and a disbond in a laminate require different acoustic questions. Selecting a 60-degree probe because "that is what we use" reverses the design process.

7.2 Essential variables

Potential essential variables include material family and product form, thickness and curvature range, weld preparation, access surface, wave mode, nominal angle or focal-law range, frequency, aperture, probe and wedge model, scan pattern, index step, sensitivity method, reference reflector, transfer method, sizing method, data processing and personnel competence.

A change is essential when it can materially affect demonstrated capability or required results. Essential-variable limits should be supported by engineering reasoning, governing documents and qualification evidence.

7.3 Technique selection matrix

Technique selection begins by demonstrating that the beam can reach the required volume. That demonstration may use a geometry model, drawing review or measured component profile. The selected direction must also interact favourably with the expected discontinuity orientation rather than merely intersect its nominal location on a ray diagram.

Penetration is supported by reference response and signal-to-noise evidence at the longest required path. Where close spacing or near-surface position matters, representative reflectors are used to demonstrate resolution. Calibration, index and angle verification, together with a defined coordinate system, support positioning accuracy. Finally, the design must provide a controlled sensitivity basis, authorised sizing method and identified acceptance source so that the result can actually be evaluated.

7.4 Qualification and demonstration

Qualification establishes that a technique can achieve defined performance under a bounded set of conditions. A representative demonstration may use mock-ups containing known reflectors. The population should challenge important locations, orientations, sizes and masking conditions. Blind trials reduce confirmation bias when detection and sizing performance are being assessed.

Success criteria must be defined before the trial. They may include probability or proportion of detection, false-call controls, location tolerance, sizing error, coverage and repeatability. Passing one obvious reflector is a demonstration of optimism, not capability.

7.5 Change control

When a job condition falls outside qualified limits, the Level 3 records the difference, assesses technical effect and determines whether analysis, supplementary trial, requalification or a new procedure is required. A deviation must identify authority, duration, affected items and restoration conditions. Temporary changes have an odd habit of becoming permanent when nobody gives them an expiry date.

Failure modes and decision control

Weak practice commonly includes beginning with available probes rather than the inspection objective; defining essential variables after the trial; using mock-ups that contain only easy reflectors; changing several variables at once and losing causality; accepting one successful scan as repeatability evidence; and allowing unrecorded field adaptations.