What Is Inspection?
Eddy current testing relies on an alternating magnetic field from a coil inducing eddy currents in a conductive material; a discontinuity disturbs those currents and changes the coil impedance. The method is contactless and allows high-speed scanning.
Why Is It Done?
The aim is to scan conductive materials rapidly for surface cracks and to detect internal corrosion and thinning in heat exchanger tubes.
Which Equipment Is Covered?
- Heat exchanger and condenser tubes
- Aluminium and stainless steel components
- Aerospace structural parts and rivet holes
- Surface cracks under coating
- Conductive-material sorting and thickness measurement
Legal Basis and Interval
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.
Scope of Inspection
- Surface and near-surface cracks
- Corrosion and thinning in tube walls
- Fatigue cracks around rivet and bolt holes
- Material mix-up and heat-treatment differences
- Non-conductive coating thickness
Related Standards
- ISO/IEC 17020
- ISO 15549
- ISO 15548 series
- EN ISO 17643
- ISO 12718
- ISO 9712
Method and Equipment
- Eddy current test instrument
- Surface probe, coil probe and rotating probe
- Reference calibration standards (with known notches)
- Coating-thickness measurement probe
- Data recording and analysis software
Inspection Process
- 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.
- 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.
- 3Surface PreparationThe inspection surface is prepared as the method requires; coating, rust and contamination directly affect the sensitivity of the method.
- 4Calibration and Sensitivity VerificationA reference block or control specimen is used to demonstrate that the system can detect the discontinuity sought.
- 5Performing the InspectionScanning is carried out according to the procedure; indications are recorded and located.
- 6AssessmentIndications are evaluated against the applicable acceptance standard to determine whether they are defects; they are sized.
- 7ReportingMethod, technique, equipment, calibration, findings and acceptance criteria are reported together.
Nonconformities
Indications are assessed by phase angle and amplitude, compared with signals obtained from reference notches. The acceptance criterion is defined in the specification; in tube inspection, the result is usually reported as a percentage of wall loss.
Inspection Report
- 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
Accreditation
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.
Scope and authorisationsRelated Inspections
Frequently Asked Questions
On which materials is ET applied?
It can only be applied to electrically conductive materials. It cannot be used on plastics, ceramics or composites. On ferromagnetic materials, variations in magnetic permeability distort the signal; in this case, saturation techniques or other methods are required.
How deep is the penetration depth?
Eddy currents concentrate at the surface and decrease exponentially with depth. Effective depth depends on frequency, material conductivity and magnetic permeability; as frequency decreases, depth increases but sensitivity decreases. For this reason, the method is not suitable for deep internal defects.
Can inspection be performed without removing the coating?
Yes; this is one of ET's greatest strengths. Surface cracks under paint and non-conductive coatings can be detected without removing the coating. However, coating thickness (lift-off) must be taken into account in calibration.
Who may carry out the inspection?
NDT personnel are certified under EN ISO 9712 by method, level (1, 2, 3) and industrial sector. Level 1 performs the examination, Level 2 evaluates it and writes the report, Level 3 prepares procedures and selects the method. An examination performed in a method or sector outside the certificate is not valid.