An engineer examines crane bridges periodically partly to make sure the structure is still sound. But new ways of measuring fatigue in crane bridges and other welded structures may help reduce the need for examinations.
Engineers are developing sensors that can predict cracking before it appears. If the sensors work as planned, they may reduce or simplify the need for periodic structural examination, claim the inventors.
A fatigue sensor called CrackFirst is less than a year from serial manufacture, claims John Davenport, programme manager at research organisation The Welding Institute of the UK, which is managing the commercialisation of the sensor.
The sensor has an inbuilt crack that grows in response to stresses in the structure when permanently attached during manufacture of the crane.
The initial implementation has no battery – it is not powered. It is up to users to check the crack growth. They would attach a portable computer that uses the thin-film technology in the sensor to measure the growth of the crack.
In other possible implementations, a 10-year battery powers sensor electrics, and when an inspector connects the computer the information is downloaded. Another concept includes a wireless transmitter that would send information automatically to a nearby computer.
‘This device is really in competition with the strain gauge market,’ says Peter Tubby, manager of the structural assessment group at TWI. ‘But there are some subtle differences between strain gauges and the sensor. With a strain gauge, you need it to be powered all the time, and you really need a computer looking at stress cycles doing damage calculation. The advantage of the CrackFirst sensor is that it does all that integration on board the sensor itself. You don’t need continuous power or external wire connections or computers.’
An enclosure protects the sensor from moisture and mechanical damage, Tubby says. But the sensor is not suitable for very high temperatures and has not been tested for cold weather. The enclosure is not currently certified for water-tightness.
The sensor is currently being field-tested by Caterpillar Peterlee, which makes dumper trucks. To date, it has not been tested on cranes, says Tubby, though he adds that testing partners are still being sought.
The current implementation is designed to match the fatigue strength of a typical fillet weld. Because fillet welds are both relatively weak and hard to avoid when building structures, they can become the weakest link in a structure. The sensor is permanently attached near a critical joint with an adhesive and threaded studs. It must be oriented in the direction of the principle stress direction. ‘If you don’t put it in the right direction, you won’t get the right answer,’ says Peter Tubby. It only responds to one-dimensional stresses. The sensor cracks in 12 steps.
Over the past two years, The Welding Institute has been testing the sensor in a laboratory, though Tubby did not wish to release data about the sensor’s reliability. TWI has been commercialising the invention, with collaboration from UK electronics firm Micro Circuit Engineering, which would manufacture the sensor in volume, and the UK’s University of Manchester Institute of Science and Technology (UMIST), the Fatigue Monitoring Bureau (FMB) and Caterpillar Peterlee.
The sensor design received a patent in 1991 (EP 0452573) by Nicholas John Prescott and Michael Burdekin. Burdekin told Hoist that Prescott worked for construction contractors Redpath Dorman Long and consultant Ove Arup, and came up with the design. The two met on an offshore engineering project. Burdekin was then a professor at UMIST. The two set up a partnership – the FMB. Following Prescott’s death in 1993, his widow Rhona stepped in. Burdekin and Rhona Prescott continue to collaborate with the Welding Institute to develop the CrackFirst product.
According to Burdekin, the total development project has cost about £1m ($1.9m) and has received about £400,000 of funding from the UK Government.
The new structural health monitoring
The CrackFirst sensor is just part of a new field of materials science. A peer-review journal, Structural Health Monitoring, was established in 2002. And a US researcher is developing the use of electrically-conductive paint to detect cracks and vibration. Yufeng Zhang, assistant professor at USA’s Lehigh University, has tested a formulation of paint impregnated with particles that generate an electric signal when subject to stress. A crack changes the signal that the paint gives out. Paint is a good carrier because it can cover difficult areas such as weld toes.
Zhang says that the CrackFirst sensor may be too big. ‘Stress concentrations happen in a very small area. By looking at the size shown in the picture, the size of the sensor might not be as small as required for accurately predicting the fatigue life time in critical joints.’ But he adds that he cannot make a proper judgement without real data.
