What type of magnetic field is often used for large and thick specimens in magnetic particle testing?

In magnetic particle testing, continuous direct currents are generally favored for large and thick specimens due to their ability to create a more homogeneous and stronger magnetic field. This stability in the magnetic field ensures effective magnetization, which is vital for detecting subsurface defects and discontinuities within the material.

When using continuous direct currents, the magnetization is consistent and can penetrate deeper into thicker materials, allowing for more effective testing compared to alternatives. This direct current method does not fluctuate, keeping the particle alignment stable, which is necessary for true representation of potential flaws.

High-frequency alternating fields and low-frequency alternating currents tend to induce changing magnetic fields, which can lead to inconsistent magnetization and challenges in holding magnetic particles in place around defects. Conversely, static magnetic fields, while useful in certain applications, are not as effective in providing the depth of penetration required for thick specimens and may not produce the strong field necessary for adequate testing.