Which of the following test frequencies would generally provide the best penetration in a 12-inch thick specimen of coarse-grained steel?

When considering the best frequency for ultrasonic testing, particularly for a 12-inch thick specimen of coarse-grained steel, it's important to understand how frequency affects penetration and resolution in ultrasonic testing.

Lower frequencies, such as 1.0 MHz, have longer wavelengths, which enables them to penetrate thicker and denser materials more effectively than higher frequencies. In the case of a 12-inch thick specimen, a lower frequency is advantageous because it can travel deeper into the material before being attenuated, allowing for better detection of flaws or features within the steel.

Higher frequencies, such as 2.25 MHz, 5 MHz, and 10 MHz, while they provide higher resolution and can detect smaller defects, are subject to greater attenuation when traveling through thick or coarse-grained materials. This means they may not reach the desired depths effectively, limiting their usefulness for testing a specimen of this thickness. Thus, the 1.0 MHz frequency is generally more suitable for ensuring adequate penetration in a coarse-grained steel application.