MPT works by applying a magnetic field to the material under test. The magnetic field causes magnetic flux lines to form within the material. If there is a discontinuity, such as a crack, on or near the surface, some of the flux lines will leak out of the material. These leakage fields can be detected by applying fine magnetic particles to the surface. The particles will be attracted to the areas where the flux lines are leaking, forming visible indications of the discontinuity.
There are two main techniques for MPT: dry and wet. Dry MPT uses dry magnetic particles, such as iron filings or powder, that are applied to the surface by dusting, blowing, or spraying. Wet MPT uses wet magnetic particles, such as iron oxide or fluorescent pigments, that are suspended in a liquid carrier, such as oil or water. Wet MPT can be further divided into visible and fluorescent methods. Visible MPT uses particles that can be seen under normal lighting conditions. Fluorescent MPT uses particles that emit visible light when exposed to ultraviolet (UV) light.
The ASTM E 709 standard is a guide for MPT that covers the general principles, techniques, equipment, and procedures for both dry and wet methods. The latest edition of ASTM E 709 was published in June 2021 and supersedes the previous edition from 2015. The standard provides recommendations and best practices for performing MPT on various types of materials, parts, and structures. It also includes information on safety, quality control, calibration, and documentation.
The ASTM E 709 standard is intended as a reference for developing specifications, standards, procedures, and techniques for MPT. It does not specify acceptance criteria or performance requirements for the parts or structures being tested. These should be defined by a separate code, specification, or agreement between the parties involved.
The ASTM E 709 standard can be purchased from the ASTM International website. It can also be accessed through online platforms such as SAI Global Store and IHS Markit. The standard is updated periodically to reflect the latest developments and innovations in MPT.
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