NDT Process Design

Our NDT Process Design Methodology

NDT process design engagements follow a structured methodology that moves from damage mechanism characterisation through technique selection, detection demonstration, and procedure qualification to a deployable inspection process. The depth and duration of each stage depends on the novelty and complexity of the application.

Stage 1 — Inspection Requirement Definition.

The inspection process design begins with a precise definition of what the inspection is required to detect: the damage mechanism type (crack, corrosion, disbond, inclusion), the minimum defect size and aspect ratio that must be reliably detected, the location within the component cross-section where the damage mechanism initiates and grows, the required probability of detection and confidence level, and the applicable acceptance standard or fitness-for-service criterion against which detected indications will be evaluated.

This stage also captures the constraints that the inspection process must operate within — the access geometry, the surface condition, the permissible inspection media, the available equipment categories, the time available for inspection, and the environmental conditions (temperature, contamination, confined space) that limit technique options. A technique that is physically incapable of working within the deployment constraints is eliminated at this stage, not after equipment has been mobilised to site.

Stage 2 — Technique Selection and Feasibility Modelling.

Technique selection is performed against the detection requirement and the deployment constraints — identifying the method or methods physically capable of detecting the target defect population at the required probability, accessible to the surface condition and geometry of the component, and operable within the inspection environment. Where multiple techniques are feasible, comparative assessment of their detection capability, sizing accuracy, coverage rate, and qualification complexity determines which technique or technique combination is recommended.

For non-standard applications, technique feasibility is assessed through physical modelling or simulation — acoustic modelling for UT technique design, electromagnetic modelling for eddy current or ACFM technique development, radiographic exposure calculation for RT technique optimisation — before committing to physical trials on representative test pieces or qualification blocks. This reduces the number of physical trials required and structures them more efficiently.

Stage 3 — Probability of Detection (POD) Study Design.

Where the inspection application is safety-critical or where the required probability of detection must be formally demonstrated — aerospace, nuclear, high-consequence pipeline, fitness-for-service with reduced safety factor — the NDT process design includes the design of a POD study. The POD study defines the test piece population (material, geometry, defect type, defect size range), the inspection protocol, the analysis methodology (hit/miss or signal response), the statistical model, and the sample size required to demonstrate the target POD with the required confidence level.

POD study design at Altair Engineering Inspection is structured in accordance with the applicable POD methodology — MIL-HDBK-1823A, the NTIAC Nondestructive Evaluation (NDE) Capabilities Data Book, MAPOD methodology, or client/regulator-specified POD requirements. The study design is prepared before physical trials begin — ensuring that the trial data collected is structured to support the statistical analysis the POD claim requires.

Stage 4 — Procedure Writing and Qualification.

The designed technique is documented in a written NDT procedure covering all parameters required by the applicable code: equipment specification, calibration procedure and frequency, scanning index pitch and coverage mapping, sensitivity setting, acceptance criteria, indication characterisation protocol, and record format. The procedure is qualified on representative qualification test pieces — demonstrating that the technique, as documented, achieves the detection and sizing performance the design basis specifies.

Procedure qualification documentation — the technique design basis, the qualification trial records, the POD demonstration data where applicable, and the Level 3 approval authority — is structured to meet the requirements of the applicable standard (ASME Section V, EN ISO 17640, NAS 410, or regulator-specific procedure approval requirements) and to withstand independent technical review.

Stage 5 — Implementation Support.

NDT process design engagements conclude with implementation support — equipment specification for procurement, personnel qualification criteria definition, training requirement specification for Level 2 operators, first-article inspection oversight, and a post-deployment review of inspection results against the design basis. This closes the loop between the designed process and its actual performance in the field — and provides the evidence that the qualified process is being executed as specified.