What type of test coil arrangement is best for detecting large surface defects?

The arrangement using encircling coils is optimal for detecting large surface defects due to its design, which allows for a more comprehensive coverage of the surface being tested. Encircling coils surround the test material, enabling the magnetic field to penetrate effectively and uniformly across a larger area. This design is particularly beneficial for identifying flaws that may be spread out or extensive on the surface, as it can generate a stronger signal response from defects, facilitating better detection.

The other arrangements, such as probe coils and single absolute coils, typically focus on specific points or linear paths rather than encompassing an entire surface. Wide coils may provide some benefits in terms of coverage, but they are less effective in creating the necessary magnetic coupling with surface flaws compared to encircling coils. Thus, the ability of encircling coils to create a larger and more effective electromagnetic field around the test material contributes significantly to their effectiveness in detecting large surface defects.