What is the purpose of Non-Destructive Testing, otherwise known as NDT? It is a process wherein the material, specimen, or a part undergoes a test to inspect for defects that could fail in service without impairing material usefulness or serviceability. NDT is a critical inspection procedure that will define the examined material to be deemed safe for use, to be discarded or repaired. There are several inspection methods used in NDT with the most important being Visual Inspection (VT).
Magnetic Particle (MT) and Liquid Penetrant (PT) help detect defects on surfaces or slightly sub-surface that cannot be seen unaided. Technologies such as Radiography (RT), Ultrasonics (UT) and Eddy Current (ET) can be used to detect under surface coatings or beneath the surface. Combining these NDT methods and complying critical parts clear of defects allows engineers and asset owners to operate or use materials with confidence.
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The Magnetic Particle inspection method is effective at detecting defects on both surface braking and slightly sub-surface in ferromagnetic materials. ASTM E1444 prepared by the American Society for Testing and Materials (ASTM) give guidelines on Magnetic Particle Testing use methods. Direct Current (DC) magnetization is recommended for detection of slightly sub-surface discontinuities, while Alternating Current (AC) magnetization is commonly used in the detection of surface breaking defects as AC fields are known to agitate particles more effectively on part surfaces.
Several auxiliary tools are used when performing Magnetic Particle Testing such as magnetic flux density gauges, weighted metal blocks are used to demonstrate equipment magnetic strength performance, calibrated test pieces and specimens with known defects. To determine how well a stationary magnetic particle system is working an AS5282 Tool Steel Ring or Ketos 01 Tool Steel Ring is used. Often the terminology between these two test pieces is inter-changed by MT practitioners.
The use of an AS5282 Tool Steel Ring and the Ketos 01 Tool Steel Ring are described in ASTM E1444. It states in section 7.1.2 that, …fabricated test or production parts with artificial discontinuities or a ring specimen similar to the ring of Annex A4 may be used. The material used for both types of rings are essentially similar to the steel chemistry and steel hardness being the same. To identify the difference between a Ketos and the AS5282 ring is by locating AS5282 stamped or engraved on its side.
ASTM E1444 Annex A4, Table A4.1 and A4.2 define the expected responses on what each of the rings should detect based on the type of suspension fluid and current (amperage) used. Depending on the current applied the response through the number of detectable lines on an AS5282 Ring is higher than the Ketos Ring. It is imperative that when performing checks on stationary magnetic particle equipment the type of ring used is clearly identified so that correct system verification results are compared with requirements given in ASTM E1444.
Using the correct verification tools to ensure the NDT method is working correctly is paramount to the inspection success in detecting defects before they fail in service.
Written by Atila R. Ruiz & Paul Trigg