Quantifying stresses in materials is increasingly important to assess a plant's integrity, extend plant life, ensure safety and to improve component tolerance during manufacture.
The integrity and hence safe life of engineered structures and components can only be assessed by fracture mechanics if the severity of any defects present is known. Various non-destructive testing techniques are used routinely to detect, locate and size defects. However, defect severity depends not only on type, size and orientation, but also on the stress environment each defect experiences.
A recent innovation from the National Non Destructive Testing Centre, MAPS is an exciting development in stress measurement on ferromagnetic materials. It is a portable system, designed for ease of use, offering quick, non-destructive measurement with the minimum preparation in a wide variety of industrial plant structures and components.
MAPS enables operators to identify potential stress problems and help predict the location of future faults. This ability to predict the lifetime of components offers enormous benefits, as it allows operators to replace components at the optimum moment (not too early, incurring avoidable costs, nor too late, after catastrophic failure).
The measurement of stress is an invaluable tool for industry. For instance, MAPS can indicate whether welds have been stress-relieved (vital for the petrochemical industry with miles of pipelines to maintain), or monitor residual stresses from manufacture (important for the manufacturers of components, such as bearings, and of steel producers, who can use it in-line to check every production item). It is useful in the automotive and other industries, where rolled steel can be pressed into shape more precisely if the residual stresses are known.
The new system offers significant advantages over existing methods for applications requiring rapid and nondestructive measurements on structures in situ or on ferromagnetic components. A small motorised probe is placed on the surface of the material and rotated as a magnetic field is applied. Modelling software is used to examine the measurements taken to define the absolute stress levels at each location and at a range of depths (down to 5-6 mm). Accuracy is high (typically 10-40 MPa or 3- 10% of yield), and measurement is rapid (typically 2 minutes per measurement). The MAPS system includes a library of calibrations for a wide range of materials (such as ferritic steels, cast iron, some stainless steels and nickel super alloys), but special calibration for specific materials can be provided.
MAPS can be used both for product improvement and for integrity assurance. Its emphasis on the prevention of failure before it occurs, rather than recovery afterwards, offers operators the greatest benefit.
AEA Technology plc
Sonomatic Products and Services
RD9, Risley, Warrington, WA3 6AT, UK
Phone: +44 1925 253442
Fax: +44 1925 253676
Email: Sonomatic@aeat.co.uk