When examining a casting for critical service, which non-destructive testing technique is preferable if cracks may be present?

The selection of a non-destructive testing (NDT) technique for examining a casting, particularly when there is a concern about the presence of cracks, must prioritize the capability to detect surface and near-surface discontinuities effectively.

Magnetic particle testing and liquid penetrant testing are highly effective methods for revealing surface-breaking cracks. In the case of magnetic particle testing, the technique can easily identify flaws by using magnetic fields to detect discontinuities on or near the surface of ferromagnetic materials. It provides quick results and is well-suited for large, complex shapes, making it ideal for castings.

Liquid penetrant testing, on the other hand, involves applying a dye or fluorescent liquid that penetrates cracks and surface openings. After a dwell time, excess penetrant is removed, and a developer is applied, which draws the penetrant out of the flaws to reveal their presence visually. This technique is also highly sensitive to cracks and surface defects, providing a clear indication of their presence.

In this case, options such as eddy current testing, radiographic testing, and ultrasonic testing have their strengths but may not be as effective for detecting surface cracks. Eddy current testing is typically used for conductive materials and is more effective for subsurface defects than surface flaws