Which principle is the basis for ultrasonic inspection?

• A. Any fault within a material will change the material's resonant frequency.
• B. Ultrasonic inspection detects color changes on the surface.
• C. Ultrasonic inspection relies on infrared radiation to detect cracks.
• D. Ultrasonic inspection measures density by weight.

Answer: (A)

Ultrasonic inspection is based on how high-frequency sound waves travel through a material and how defects disturb that travel. When you send a pulse into metal or composite, the waves propagate until they meet boundaries between different materials or a flaw inside. At those interfaces, part of the energy is reflected back to the transducer. The presence of a flaw changes the way the material supports vibration at ultrasonic frequencies, effectively altering its resonant behavior. That change shows up as altered echoes, different travel times, or changes in the signal’s frequency content, allowing you to detect and characterize defects. So the fundamental idea is that a fault changes the material’s resonant response to ultrasonic energy, making it detectable.

Ultrasonic testing isn’t about color changes on the surface, infrared radiation, or weighing the material to infer density. Those methods are unrelated to how ultrasound probes defects—the principle here is the interaction of ultrasonic waves with discontinuities inside the material.