Local Stress Effects on MFL Signals

Abstract

Of these various factors that influence the Magnetic Flux Leakage (MFL) signal, stress is the most complex and the least understood. Pipe wall operating stresses may exceed 70% of the yield strength, but much higher local stress levels are present around defects because of stress concentrations. Understanding how these stresses affect MFL signals is crucial to accurate defect depth predictions. We have conducted a number of studies that investigate the effects of bulk and local stresses on magnetic behaviour and MFL signals. Both single defects and interacting (2-hole) defects have been investigated. In addition to MFL studies, we have used a Magnetic Barkhausen Noise (MBN) measurement technique to characterize magnetic behaviour of the sample in response both to local and bulk stresses. In general we find that, for typical inspection geometry (axial magnetizing field and hoop stress) the MFL signal decreases as stress increases. At high flux densities stress effects diminish, but are still significant. Although the general trends described above are common to all defects, different types of defects (through wall hole, blind hole, interacting defects) all display uniquely different MFL stress responses. We attribute this to the difference in local stress distributions (stress concentrations) around these defects.