CrackFirst™is a fatigue sensor system for welded joints in steel structures, capable of providing advance warning of the rate at which the design life is being consumed.
The CrackFirst™ sensor was designed for welded steel structures and patented in 1990. It consists of a thin shim of material attached to the target structure close to a critical joint. Under the action of cyclic stress in the structure, a fatigue pre-crack at the center of the shim, introduced during manufacture, extends by fatigue crack growth. The sensor design is such that the extent of crack growth in the shim is proportional to the cumulative fatigue damage for a welded joint subjected to the same loading. In other words, the condition of the sensor indicates how much of the design life of the adjacent weld has expired and how much remains.
- Figure - 1
- Figure - 2
Advantages of its use include:
- Provides an accurate record of cumulative fatigue damage
- Indicates the portion of design life consumed
- Provides design and development engineers with valuable information
- Improves maintenance scheduling
- Optimises operational efficiency
It is not necessary to install sensors on a structure from new. Installation part-way through life is feasible, provided the prior service is taken into account.
The principal advantage of the sensor is that it provides valuable information on the rate at which the fatigue life is being expended, and can therefore be used to set inspection intervals according to usage, rather than elapsed time. It is of most benefit for structures in which fatigue is the primary limit state, especially in situations where inspection and repair is difficult or impossible, or where structural failure would have significant consequences with respect to safety and/or financial loss.
Typical applications therefore include:
- earth moving machines,
- offshore structures, and
- railway bogies.
There are several methods of powering and interrogating the sensor, which are based on the clients requirements.
The simplest method is to interrogate the sensor using a multimeter. There is a known relationship between electrical resistance and crack length.
For remote location or more frequent checking, an on-board electronics unit regularly checks the sensor status and records in memory the date/time of each crack increment.
- Sensor data can be downloaded to a laptop PC via a wireless link or to a datalogger.
- The power can be obtained from an on-board battery, a remote battery pack, the vehicles power or from a mains supply.
The sensor comprises a steel shim 0.25mm thick, with openings at each end, and a central slit which acts as a starter notch for fatigue pre-cracking. It can be installed 10mm from the toe of a weld and is attached to a structure by a combination of threaded studs and adhesive bonding. In order to protect the sensor and the on-board electronics from mechanical damage and corrosion, a sealed enclosure is fitted over the entire installation area.
The sensor is prestressed after installation using the rig shown in Figure -2.
Data is expressed as a percentage of the design life for the appropriate joint class according to BS 7608.
The CrackFirst™ system was developed through the collaboration of TWI Ltd, FBM, Micro Circuit Engineering Ltd, UMIST and Caterpillar Peterlee (a division of Catapillar (UK) Ltd) in a project funded by the DTI's LINK Sensor and Sensor Systems for Industrial Applications Programme.