Which ultrasonic test frequency would probably provide the best penetration in a 12 inch thick specimen of coarse-grained steel?

• A. 1 MHz
• B. 2.25 MHz
• C. 5 MHz
• D. 10 MHz

Answer: (A)

In coarse-grained steel, penetration through a thick section is best achieved with the lowest frequency among the options. Lower ultrasonic frequency has a longer wavelength, which reduces scattering from grain boundaries and lowers attenuation, allowing waves to travel deeper before their signal diminishes. For a 12-inch-thick specimen, you want that deeper reach, so the lowest frequency provided would be most effective. Among the given choices, that is one megahertz. Keep in mind, using a lower frequency trades off with reduced resolution and sensitivity to smaller flaws, but it maximizes penetration in this scenario.