What method should be used to check for fatigue cracking in a vessel operating in cyclic temperature service?

Fatigue cracking in a vessel that operates in cyclic temperature service often develops as a result of variations in stress and temperature. When inspecting for such cracks, the sensitivity of the detection method to surface and near-surface defects is critical.

Magnetic particle testing and dye penetrant inspection are both surface examination techniques that are particularly effective for identifying fine cracks on the surface of the material, which is where fatigue cracks typically initiate. Magnetic particle testing works well on ferromagnetic materials and can reveal minute cracks that may not be detectable by more invasive methods. Dye penetrant inspection also enhances visibility of cracks by using a dye that seeps into the cracks and is revealed during the inspection process.

While ultrasonic testing and radiographic testing can detect subsurface cracks as well as other types of discontinuities, they may not be as effective for identifying surface-breaking fatigue cracks, which are more commonly of concern in cyclic service applications. Subsurface detection is less crucial in this specific case because fatigue cracks tend to initiate at or near the surface due to repeated loading.

Therefore, using magnetic particle or penetrant testing methods is the most suitable approach for effectively checking for fatigue cracking in vessels operating under these conditions.