TOFD

In TOFD, the size of a flaw is computed from the time difference between signals produced when sound waves travelling between an opposed transmitter and receiver are diffracted at the flaw tips. Its distinguishing feature is that it relies on the arrival time of the diffracted signal, not on echo amplitude.

The aim is to detect flaws in a weld and, above all, to measure flaw HEIGHT with high accuracy. That measurement is the input to fracture-mechanics based engineering critical assessment (ECA).

• Butt welds of pressure equipment
• Pipeline girth welds
• Thick-walled weld seams
• Storage tank shell welds
• Monitoring of crack growth during service

NDT has no interval of its own; how often it is applied follows the inspection programme it serves. In manufacturing the governing factor is the coverage required by the product standard and the specification (e.g. a given percentage of welds); in operation it is the asset integrity or risk-based inspection programme.

• Volumetric and planar defects in the weld seam
• Measurement of through-wall defect height
• Defect length and depth location
• Root and surface defects (outside the dead zone)
• Monitoring of crack growth over time

• ISO/IEC 17020
• ISO 10863
• ISO 15626
• EN ISO 17640
• ISO 9712

• TOFD-capable ultrasonic instrument
• Transmitter and receiver probe pair and wedges
• Encoder and scanning rig
• Reference calibration blocks
• Couplant
• Data analysis software

1. 1Selection of Method and TechniqueThe type and location of the discontinuity sought and the material properties are assessed to select a suitable method and technique; a written procedure is prepared.
2. 2Verification of Personnel and EquipmentThe level and industrial-sector qualification of the operator under EN ISO 9712 and the calibration records of the equipment are checked.
3. 3Surface PreparationThe inspection surface is prepared as the method requires; coating, rust and contamination directly affect the sensitivity of the method.
4. 4Calibration and Sensitivity VerificationA reference block or control specimen is used to demonstrate that the system can detect the discontinuity sought.
5. 5Performing the InspectionScanning is carried out according to the procedure; indications are recorded and located.
6. 6AssessmentIndications are evaluated against the applicable acceptance standard to determine whether they are defects; they are sized.
7. 7ReportingMethod, technique, equipment, calibration, findings and acceptance criteria are reported together.

Defects are located on the D-scan image, and their height is calculated from the time difference between diffraction signals. Acceptance is based on a standard such as ISO 15626 or on a fracture-mechanics-based assessment.

• The inspection body and the personnel's EN ISO 9712 level and sector
• Method and technique applied and the written procedure number
• Part and weld examined, with a location sketch
• Equipment, consumables and calibration records
• Surface condition and inspection conditions
• Indications recorded, with their location and size
• Acceptance standard applied and the result of the evaluation
• Accept / reject decision

Non-destructive testing is carried out as an accredited inspection activity under ISO/IEC 17020. The second decisive factor in NDT is personnel qualification: the operator must be certified under EN ISO 9712 for the method, level and industrial sector concerned. An examination performed with a method outside the scope of that certificate is not valid. EUROLAB holds internationally recognised accreditation under ISO/IEC 17020, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement.

• VT Visual
• PT Penetrant
• MT Magnetic Particle
• UT Ultrasonic
• RT Radiographic
• ET Eddy Current
• PAUT
• Leak Testing