Material must be reasonably centered in the test coil of a flaw detection system because:

Material must be reasonably centered in the test coil of a flaw detection system primarily because parts of the material that are furthest away from the coil may receive a less sensitive inspection. In eddy current testing, the eddy currents induced in the material are strongest in the area closest to the coil. As the distance from the coil increases, the intensity of these currents diminishes, leading to a reduction in sensitivity. This means that flaws located farther from the center of the coil may not be detected or may be detected with lower sensitivity, which can result in missed defects. Proper centering ensures that the entire material being tested is subjected to the same effective level of inspection sensitivity, thereby improving the chances of detecting any potential flaws regardless of their location within the test specimen.