An engineer uses ultrasound to check a metal pipeline for internal cracks without cutting it open. Explain how ultrasound waves can be used to detect a crack inside the pipeline and how the position of the crack can be determined.

OCR A-Level Physics A (H556) — 5.2 Circular motion and gravitational fields · Explain · 5 marks · View as Markdown

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

Ultrasound pulses are sent into the metal pipeline from one end. A receiver placed at the same end detects any reflected pulses. The speed of ultrasound through the metal is known.

Model answer (5 marks)

Ultrasound waves are transmitted from the source into the metal pipeline. They propagate through the metal until they reach a discontinuity such as a crack or the end of the pipe. At the interface between metal and the crack (or the air inside the crack) part of the wave is reflected back towards the source. The receiver, placed at the same end as the transmitter, detects this reflected pulse. Because a crack is closer to the source than the far end of the pipe, the reflected pulse from the crack returns after a shorter time than the pulse reflected from the far end. By measuring the time interval between the emission of the pulse and the detection of the reflected pulse, the distance to the crack can be calculated using the relation distance = (speed of ultrasound × time)/2, where the speed of ultrasound in the metal is known.

Examiner tips

  • Use the term ‘partial reflection at a discontinuity’ to show understanding of wave behaviour.
  • Show the time–distance calculation explicitly and include the factor ½ for round‑trip time.
  • Mention that the crack causes an earlier return of the pulse compared with the far end.
  • Use the word ‘discontinuity’ or ‘boundary’ to link to the mark scheme.”

Mark scheme (5 marks)

  1. Ultrasound waves travel through the metal pipeline until they reach a boundary
  2. At the boundary between the metal and the crack (or air gap), the ultrasound waves are partially reflected back
  3. The reflected waves are detected by the receiver / detector
  4. A crack causes reflections to return earlier / after a shorter time than reflections from the far end of the pipeline
  5. The distance to the crack can be found because the speed of ultrasound is constant and the time between emission and detection of the reflected pulse is measured (distance = speed × half the time)

Key terms in this question

ultrasound waves

Related

More Circular motion and gravitational fields questions

▶ Try answering this question with AI marking (free) →