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

The selection of frequency in ultrasonic testing is crucial in determining the ability of the sound waves to penetrate materials effectively. Lower frequencies typically have longer wavelengths, which enhances their ability to penetrate thicker and denser materials. In the case of a 12-inch thick specimen of coarse-grained steel, a lower frequency would be advantageous because it is better able to travel through the material without excessive attenuation.

Using a frequency of 1.0 MHz would allow for greater penetration compared to higher frequencies, such as 2.25 MHz, 5 MHz, or 10 MHz. Higher frequencies are more sensitive to small defects and provide better resolution but are limited in penetration power, especially in thick or coarse-grained materials. Thus, for testing a specimen as thick as 12 inches, the lower frequency of 1.0 MHz is the most suitable choice for effective ultrasonic penetration and assessment of the material's integrity.