Stress effects on magnetic Barkhausen noise measurements from magnetized pipeline steel

The magnetic flux leakage (MFL) technique is used to determine nondestructively the maximum allowable operating pressure of steel pipelines for oil and gas [1,2]. This method involves magnetically saturating the pipe wall and measuring the leakage flux near the pipe wall surface with Hall probes or induction coils. The goal of the nondestructive testing (NDT) industry is to estimate the losses in the pipe wall to better than 5% precision. Pipelines are essentially pressure vessels that operate up to 70% of their yield strength [3]. The magnitude of leakage flux depends on several parameters including the magnetic anisotropy of pipeline steel which is stress dependent [4, 5, 6, 7, 8, 9].; Magnetic Barkhausen noise (MBN), the irreversible motion of 180-degree walls, is sensitive to residual and applied stress in a ferromagnetic material, such as steel. Thus MBN has been proposed as a viable nondestructive evaluation technique for monitoring magnetic anisotropy [10] and inhomogeneity in magnetic materials [11, 12, 13, 14]. MBN occurs at the greatest rate of change in magnetization, essentially where B ∼ 0 T [15]. This is not the same condition under which MFL is performed on pipeline steel when it is almost magnetically saturated. In this work MBN signals from magnetized (∼1.6 T) pipeline steel are acquired and the effects of stress are also studied. A coercive field or pinning model was developed for the MBN anisotropy data acquired from the magnetized but unstressed steel pipe that has proven to yield the magnetic easy axis of the sample prior to magnetization.; Finally, both the MBN and MFL techniques were used on a magnetized (1.8 T) and stressed (up to 270 MPa) pipe sample such that differences in the permeability of regions of magnetic inhomogeneity could be compared.