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A sensor system to assist in structural health monitoring

The Crackfirst TM sensor

Connect, no.135, March/April 2005, p.2

At the heart of the CrackFirst system is a fatigue sensor which, when installed on a welded steel structure, indicates the portion of the design life that's been consumed and enables engineers to estimate its remaining life. The sensors, when suitably located, are subjected to the same loading history as the structure, thus provide an accurate record of cumulative weld fatigue damage.

The CrackFirst sensor system is ideal for use by all industries in which fatigue of steel welded structures presents a structural performance concern. The sensors will form an important part of many structural Health and Usage Monitoring Systems (HUMS).

Asset owners and maintainers will use the system to reduce the cost of safe operation. Designers and those involved in prototype testing of fatigue limited structures will value the insight that comes from the CrackFirst sensors. The following industry sectors will benefit from the availability of the CrackFirst sensor:

  • Transportation - off-road vehicles, trucks, rail, ships
  • Civil engineering - bridges, masts, towers and cranes
  • Energy - wind towers
  • Process plant subjected to cyclic loading
  • Off-shore installations - risers, drilling rigs and production platforms

The sensor comprises a steel coupon attached adjacent to a critical joint. Stress cycles cause fatigue crack growth in the coupon that is detected electrically. For a typical fillet welded joint the sensor output gives the proportion of the fatigue design life that has been used.

TWI is currently involved in the final stages of development and application of the system.

The CrackFirst TM system was developed through the collaboration of TWI Ltd., FMB, Micro Circuit Engineering Ltd., UMIST and Caterpillar Peterlee (A division of Caterpillar (UK) Limited) in a project funded by the DTI's LINKSensor and Sensor Systems for Industrial Applications Programme.

Enquiries from organisations interested in using the CrackFirst sensor system are welcome. Please contact Peter Tubby or John Davenport. E-mail: or

For more information please email: