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Ultrasonic NDT

Rail Bolt Hole Inspection

Published on 10th December 2020

Rail Bolt Hole Inspection

Brief Description

Inspection of bolt holes in rail is a common application that detects cracking caused by expansion and vibration. These cracks originate from the bolt hole and can spread out, this causes the rail ends to eventually come apart. The most commonly used method for this inspection is conventional ultrasound, the inspector must use a variety of transducers and a different offset point for each hole.

This application note highlights the use of phased array ultrasonics to reliably perform the bolt hole inspection in a more time efficient manner.

Beamtool scan plan and an example of rail bolts through holes

Prisma with X5 Probe

Typical signal response


Figure 1 - No Defect


Figure 2 - Several Defects in the First Skip (Direct)

Figure 3 - No Defects only Geometrical Reflections
Figure 4: Defects Detected in the Second Skip

The four images above show the bolt hole with and without defects, Fig 1 shows the response going direct onto the bolt hole, this produces one reflection from the hole. Fig 2 shows the same direct scan, this time hitting 3 or 4 defects around the hole, it is possible to differentiate the top response as a defect rather then geometry because its two separate reflections close together.

Figures 3 and 4 show the scan images of the bottom side of the bolt hole, this time skipping is needed, when the sound is reflected up from the bottom of the track in order to get full coverage. Fig 3 shows the response with no defect, whereas fig 4 shows with two defects, the zoomed in image shows the two reflections from the right had signal.


  • Rail
  • Manufacturing

Typical Parts

  • Steel Rail Tracks

Inspection Techniques

  • PA Sectorial Scan
  • Pulse-Echo PAUT

Features & Benefits

  • Faster than current UT method
  • Ability to save data for future reference
  • Greater accuracy of detection
  • Only one transducer is required
  • Field upgradable units
  • Lond battery life
  • Remote access from multiple locations via Xpair software
  • Fast reporting

Features & Benefits

  • Prisma or Veo+
  • X5 PA Probe
  • Quicktrace encoder

For further information or support, please contact the Sonatest Applications Team:


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Application Notes