Magnetism measures stress

Measuring residual stress accurately has been something of a challenge in the past. Yet, by applying an alternating magnetic field to a piece of ferromagnetic material – and then relating the alternating field resulting from that magnetisation to the material – this can be achieved. The crucial underlying effect is the way that stress affects magnetic hysteresis loops. Engineers have been trying to use this as a method to measure residual stress in steel for many years, mostly with a marked lack of success. The new method, which seems to work well, has been developed for the oil and gas industry. However, after prolonged trials, its transferability means it can now be offered to the rail industry. Maps, or Magnetic Anisotropic Prediction of Stress has been developed by ESR Technology in Abingdon, Oxfordshire. The probe has two poles, with a sensing coil between them. It is normally placed against the pipe or rail of interest and rotated through 360 deg. The analysing electronics establishes the relationship between the in phase and quadrature components of the detected magnetisation and performs what MAPS-SFT product manager Alan Hayes describes as a “twelve frequency deconvolution”. Understandably, the company is not inclined to give away too many of the details, though Hayes says the technique has taken 10 years and cost £3.5 million to develop. “We are now in a position to take it to many industries,” he will confirm, “since it works with all ferromagnetic alloys, including the nickel superalloys. Accuracy has been demonstrated to 5MPa. The technique measures stress at depths from 0.015mm to 7mm, and, as Hayes explains: “We can also make measurements while moving. We have performed demonstrations at up to 7m/s. We don’t need a laboratory surface. We can work on most surfaces, even when they are rusted, as long as the rust is not too thick.” On the rails The method has been “validated against other strain measurement techniques, such as neutron diffraction, hole drilling and with strain gauges”. In its rail application mode, it has been tested against samples of rail originally installed in 45 sections of track at various times from 1975 to 2005 and five rails that had not been installed at all. It can distinguish residual stresses resulting from manufacture and those induced by temperature. During the course of its development, the probe designs became much smaller and less temperamental, says Hayes, with the probe system now battery operated. “It normally takes 12 minutes per location.” Its importance to the rail industry is likely to grow, in response to pressures to improve safety . In oil and gas, it could detect residual stresses from welding and cutting in structures that must not fail, such as those supporting offshore platforms. In power stations, the technique can map stress changes caused by in-service damage and degradation, while in the chemical industries, it could lead to the anticipation and avoidance of stress corrosion cracking. In manufacturing, it should show up stresses in critical components, especially for aerospace, that result from machining and finishing. In many cases, it is the goal of surface treatments to put surfaces into compressive stress to combat the onset of fatigue cracking. In these cases, the absence of surface stress must be avoided.