Stress-induced magnetization changes of steel pipes--Laboratory tests, Part II

Magnetometer surveys above gas pipelines have shown stress-induced magnetic anomalies at pipe bends. This suggests a technique for noninvasive monitoring of stress in pipelines and other ferromagnetic structures. Previous laboratory measurements of the external magnetic field changes due to single bending and internal pressure cycles on 110 mm diameter steel pipes demonstrated that stress and magnetic hysteresis effects were important. The stress-induced changes in magnetization produced during multiple cycles of elastic bending and internal pressure for pipes which have been demagnetized and then magnetized to simulate magnetic inspection of a real pipeline is reported. The stress-induced changes can be divided into relatively large irreversible changes occurring principally during initial stressing and smaller reversible changes occurring during any stress cycle. This is in agreement with Jiles´ and Atherton´s recent theory of the effect of stress on magnetization which shows that the application of stress to a ferromagnet shifts its magnetization towards the anhysteretic which is also slightly stress dependent. The significance of results to applications is indicated.