Engineers use ultrasound to check for cracks inside large metal components, such as aircraft engine parts, without cutting them open. Explain how ultrasound waves are used to detect a crack inside a metal component, and why ultrasound is suitable for this purpose rather than audible sound.

OCR A-Level Physics B: Advancing Physics (H557) — 5.3 Astrophysics and cosmology · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Quality-control engineers at an aircraft maintenance facility direct ultrasound pulses into solid metal engine parts to check for internal cracks. The component does not need to be damaged or cut open during the process.

Model answer (5 marks)

Ultrasound waves have a frequency above 20 kHz, which is beyond the upper limit of human hearing. A pulse is sent into the metal part; when it reaches the interface between the metal and a crack (an air gap) part of the wave is reflected back. A detector next to the emitter receives this reflected pulse. Because the pulse reflected from the crack returns sooner than a pulse reflected from the far surface, the time difference between emission and detection can be measured. The speed of ultrasound in the metal is essentially constant, so the distance to the crack is calculated from the time delay. Ultrasound is used instead of audible sound because its short wavelength gives much higher resolution, allowing small cracks to be detected.

Examiner tips

  • Use the word "ultrasound" and state its frequency >20 kHz; mention reflection at the crack interface; explain time‑delay measurement; justify short wavelength for resolution.
  • Show the calculation idea: distance = ½ speed × time delay – even if not required, it demonstrates understanding.

Common mistakes

  • Confusing audible sound with ultrasound; not mentioning the reflection at the crack; failing to explain why the wavelength matters for detecting small cracks.

Mark scheme (5 marks)

  1. Ultrasound waves have a frequency above 20 kHz / above the upper limit of human hearing
  2. When the ultrasound pulse reaches the boundary between the metal and the crack (air gap), it is partially reflected back
  3. A receiver / detector next to the emitter detects the reflected pulse earlier than a pulse reflected from the far surface
  4. The speed of ultrasound in the metal is constant, so the time between emission and detection is used to calculate the position / depth of the crack
  5. Ultrasound (rather than audible sound) is used because its short wavelength allows it to detect small cracks / gives greater resolution / detail

Key terms in this question

ultrasound waves

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