Literature Review on the Potential Use of Non-Destructive Testing Techniques for Residual Stress Measurement
By James Kern and Jazeel Chukkan
Residual stress is imparted during a manufacturing process and is generally known to have a detrimental effect on the structural integrity of a component. Without knowledge of the residual stress field, modern structural integrity codes consider highly conservative residual stress levels in welded components. A more precise method for residual stress field measurement will allow for a more accurate structural assessment, that could potentially increase the loading limit or life assessment of a structure.
There are many techniques available for residual stress measurement which are categorised as destructive, semi-destructive or non-destructive. Mechanical methods such as centre-hole drilling (CHD) have been extensively used but can only be conducted at a limited number of points on the surface of a structure as the technique is semi-destructive and requires material removal.
Non-destructive testing (NDT) technology such as X-ray and neutron diffraction (ND) are able to provide a more detailed residual stress map. However, neutron diffraction cannot be conducted out on site and X-ray diffraction (XRD) is often prohibitively expensive. There is a need for a relatively inexpensive NDT for site applications where it is not possible for components to be removed for testing e.g. in power plants.
This literature review summarises the latest research in the application of ultrasonic inspection and electromagnetic acoustic transducers (EMAT) to measure residual stress, including a review of commercial off the shelf (COTS) equipment available for residual stress measurement.
- Multiple NDT techniques are available that can accurately measure residual stresses. Of the techniques available there are currently five commercially available systems for site based residual stress testing.
- No published work is available that compares the different COTS equipment and critically assesses their site capability.
- TWI have in-house techniques, namely critically refracted longitudinal (LCR) and birefringence, that can be developed into capable on-site inspection techniques.
- An accurate and cost effective on site deployable non-destructive measurement technique could be very beneficial in residual stress redistribution studies.
- Current state of LCR and birefringence techniques are not suitable for a theoretical investigation into residual stress redistribution under cyclic load.
- Three low TRL techniques have been identified: magnetic anisotropy and permeability system (MAPS), Magnetic Barkhausen noise (MBN) and Hall effect sensors, which may be suitable for further investigation/development.