Vijay Babbar

Mechanical damage detection with magnetic flux leakage tools: modeling the effect of localized residual stresses

We used three-dimensional (3-D) magnetic finite-element analysis (FEA) to simulate the effect of localized residual stresses on magnetic flux leakage (MFL) signals from a steel plate in the absence of a geometrical defect. We derived the local residual stress patterns from finite-element structural modeling of simulated dents. The magnetic FEA model simulates these localized residual stresses by assigning appropriate directional permeability values to the magnetically anisotropic materials. Considering the necessary simplifications required for magnetic FEA modeling, the simulated MFL patterns are in good agreement with the experimentally observed patterns associated with the stresses around a dent.

Understanding Magnetic Flux Leakage Signals From Dents

The Magnetic Flux Leakage (MFL) technique is sensitive both to pipe wall geometry and pipe wall stresses, therefore MFL inspection tools have the potential to locate and characterize mechanical damage in pipelines. However, the combined influence of stress and geometry make MFL signal interpretation difficult for a number of reasons: 1) the MFL signal from mechanical damage is a superposition of geometrical and stress effects, 2) the stress distribution around a mechanically damaged region is very complex, consisting of plastic deformation and residual (elastic) stresses, 3) the effect of stress on magnetic behaviour is not well understood. Accurate magnetic models that can incorporate both stress and geometry effects are essential in order to understand MFL signals from dents. This paper reports on work where FEA magnetic modeling is combined with experimental studies to better understand dents from MFL signals. In experimental studies, mechanical damage was simulated using a tool and die press to produce dents of varying aspect ratios (1:1, 2:1, 4:1), orientations (axial, circumferential) and depths (3–8 mm) in plate samples. MFL measurements were made before and after selective stress-relieving heat treatments. These annealing treatments enabled the stress and geometry components of the MFL signal to be separated. Geometry and stress ‘peaks’ tend in most cases to overlap — however stress features are most prominent in the dent rim region and geometry peaks over central region. In general the geometry signal scales directly with depth. The stress scales less significantly with depth. As a result deep dents will display a ‘geometry’ signature while in shallow dents the stress signature will dominate. In the finite element analysis work, stress was incorporated by modifying the magnetic permeability in the residual stress regions of the modeled dent. Both stress and geometry contributions to the MFL signal were examined separately. Despite using a number of simplifying assumptions, the modeled results matched the experimental results very closely, and were used to aid in interpretation of the MFL signals.Copyright

Understanding Magnetic Flux Leakage Signals from Gouges

The Magnetic Flux Leakage (MFL) technique is sensitive both to pipe wall geometry and pipe wall strain, therefore MFL inspection tools have the potential to locate and characterize mechanical damage in pipelines. The present work is the first stage of a study focused on developing an understanding of how MFL signals arise from pipeline gouges. A defect set of 10 gouges were introduced into sections of 12″ diameter, 5m long, end capped and pressurized X60 grade pipe sections. The gouging tool displacement ranged (before tool removal) between 2.5 to 12.5mm. Gouges were approximately 50mm in length. The shallowest indentation created only a very slight scratch on the pipe surface, the deepest created a very significant gouge. All gouges were axially oriented. Experimental MFL measurements were made on the external pipe wall surface (pressurized) as well as the internal surface (unpressurized). The early results of the experimental MFL studies, and a hypothesis for the origin of the MFLaxial signal “dipole” are discussed in this paper.© 2010 ASME

Modelling Magnetic Flux Leakage Signals From Dents

The Magnetic Flux Leakage (MFL) technique is sensitive both to pipe wall geometry and pipe wall strain, therefore MFL inspection tools have the potential to locate and characterize mechanical damage in pipelines. However, the combined influence of strain and geometry makes MFL signals from dents and gouges difficult to interpret for a number of reasons: 1) the MFL signal from mechanical damage is a superposition of geometrical and strain effects, 2) the strain distribution around a mechanically damaged region can be very complex, often consisting of plastic deformation and residual (elastic) strain, 3) the effect of strain on magnetic behaviour is not well understood. Accurate magnetic models that can incorporate both strain and geometry effects are essential in order to understand MFL signals from mechanical damage. This paper reviews work conducted over the past few years involving magnetic finite element analysis (FEA) modeling of MFL dent signals and comparison with experimental results obtained both from laboratory-dented samples and dented pipe sections.Copyright

Neutron Diffraction Studies of Residual Stresses Around Gouges and Gouged Dents

The residual stress pattern surrounding gouges is complex and, to date, has not been accurately modeled using stress modeling software. Thus measurement of these stress distributions is necessary. Neutron diffraction is the only experimental method with the capability of directly evaluating residual strain throughout the entire thickness of a pipe wall, in and around dent or gouged regions.Neutron diffraction measurements were conducted at the NIST reactor on three gouged dents in X52 pipeline sections. These were part of a larger sample set examined as part of the comprehensive MD4-1 PRCI/DOT PHMSA project. Gouges contained in pipeline sections were termed BEA161 (primarily a gouge with little denting), and BEA178 (mild gouging, very large dent). Measurements were also conducted on a coupon sample – P22, that was created as part of an earlier study.For the moderate gouges with little or no associated denting (BEA161 and P22) the residual stress field was highly localized around the immediate gouge vicinity (except where there was some denting present). The through wall stress distributions were similar at most locations — characterized by neutral or moderate hoop and axial stresses (50–100MPa) at the outer wall surface (i.e. at the gouge itself) gradually becoming highly compressive (up to −600MPa) at the inner wall surface. The other sample (BEA178) exhibited a very mild gouge with significant denting, and the results were very different. The denting process associated with this kind of gouge+dent dominated the residual stresses, making the residual stress distribution very complex. In addition, rather than having a residual stress field that is localized in the immediate gouge vicinity, the varying stress distribution extends to the edge of the dented region..Copyright

Effects of Detector Dynamics on Magnetic Flux Leakage Signals From Dents and Gouges

The current study was designed to model the dynamic effects of detector ride and magnet liftoff on Magnetic Flux Leakage (MFL) signals from dents as well as gouges that have significant denting. The MFL tools have long been used for the detection and sizing of corrosion defects. This is comparatively straightforward for a number of reasons, one of which is that the MFL detector assembly can ride relatively smoothly along the inner pipe wall surface. This is not the case when significant denting is present, since the dent presents a perturbation in the pipe wall that can cause liftoff of the detector or magnet system. Since the tool travels at relatively high speeds down the pipe, the dent itself can cause the detector to lose contact with the trailing half of the dent. In addition, the magnet pole piece may experience partial liftoff as it traverses the dent, thus causing a change in the local flux density.In this study results from ‘static’ measurements are compared with a dynamic case in which detector liftoff is simulated through modeling and experiment. Results are discussed regarding the severity of MFL signal loss at the trailing edge of the defect as a result of detector liftoff.The effect of partial liftoff of the magnet as it passes over the dent is also examined. Magnet liftoff is found to increase the local magnetic flux near the liftoff region, causing the MFL signal from the dent wall to increase rather than decrease in the vicinity of magnet liftoff region.Copyright

FINITE ELEMENT MODELING OF MAGNETIC FLUX LEAKAGE SIGNALS FROM MECHANICAL DAMAGE CONTAINING CORROSION PITS

Steel pipeline defects such as mechanical damage (dents) and corrosion pits have been studied using the magnetic flux leakage (MFL) technique and finite element modeling (FEM). The nature of MFL signals from these defects is generally known. However, since a dented region is more susceptible to corrosion, a dent‐pit defect combination may be formed, which adds complexity to signal interpretation. The present work employs FEM to investigate the change in dent signal in the presence of a pit of varying dimensions. It helps to infer if a pit located inside a dent is detectable.

Modeling the Effects of Pit Corner Geometry on Magnetic Flux Leakage Signals

Three-dimensional magnetic finite element analysis (FEA) has been used to simulate the effects of different pit corner geometries on magnetic flux leakage (MFL) signals. The pit corner geometries studied fall into three main categories: right angle, smooth triangular, and smooth round. These geometries are simulated using rectangular grooves and circular pits in steel plate. The axial and radial components of the MFL signal are obtained on the topside as well as the underside of the steel plate. Both the peak position and the peak height/depth of the MFL signal were found to be influenced by the sharpness of the pit corner. In general, the underside axial MFL peaks shifted toward the center of the pit, and their height increased with decreasing sharpness of the corner. For the underside radial MFL peaks, there was an appreciable peak shift; however, the change in peak height was insignificant. However, the peak height depends strongly on pit depth. The topside results were only slightly different from the underside. Despite some changes in MFL signal position and form, it was concluded that in practical inspection situations these changes would be relatively small and difficult to quantify.