Digital Imaging of Pipeline Mechanical Damage and Residual Stress
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Digital Imaging of Pipeline Mechanical Damage and Residual Stress

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    Final Report
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    The purpose of this program was to enhance characterization of mechanical damage in pipelines through application of digital eddy current imaging. Lift-off maps can be used to develop quantitative representations of mechanical damage and magnetic permeability maps can be used to determine residual stress patterns around mechanical damage sites. Note that magnetic permeability is also affected by microstructure variations due to plastic deformation and plowing. High resolution digital images provide an opportunity for automated analysis of both size and shape of damage and a permanent archival record that can be compared against future measurements to detect changes in size or shape of the damage. Also, multiple frequency measurements will enable volumetric and even through-wall imaging at mechanical damage sites to support further risk assessment efforts. The two major accomplishments of this Phase I SBIR were the development of methods to measure the geometry of pipeline mechanical damage using lift-off measurements with modified grids to correct for large scale geometry changes and the mapping of permeability around pipeline mechanical damage sites to characterize the residual stress state at these damage sites. To accomplish these objectives required the development of experimental techniques as well as selected experimental verifications of these techniques. In this project, JENTEK, through laboratory testing, demonstrated techniques that can make accurate measurements of the pipeline mechanical damage geometry. To accomplish this, the sensor was mounted on a hard-backed plate for lift-off measurements. A finite element analysis was used to scale the measured MWM-Array lift-off measurements to provide accurate depth measurements. The scale factor was shown to be a function of dent diameter and dent depth. In the future, this correction factor could be built into the GridStation software so that the corrected lift-off values would be calculated and displayed. The mapping of residual stress around a pipeline mechanical damage site requires that the sensor is mounted on soft-backed foam so as to conform to the pipeline surface. In addition, the measurement of the permeability at perpendicular orientations and the subsequent averaging of these two sets of measurements provide an improved mapping of the permeability changes around a pipeline mechanical damage site. In follow-on efforts, it is anticipated that a sensor that measures permeability at these two orthogonal directions and software that averages these measurements automatically could be developed to provide enhanced permeability mapping around pipeline mechanical damages sites. The electro-magnetic properties of several steel plates were also measured for a variety of conditions. The measurements showed permeability increased with tensile strain and decreased with compressive strain. This was consistent with the permeability measurements made around mechanical damage sites such as dents. In the dented plates the center of the dent is in compression and the permeability was decreased from the baseline value. In the area adjacent to where the plates are supported during the denting process, the stress is tensile and the permeability is increased from the baseline value. We demonstrated that if these dented plates are annealed at a temperature of 650°C for 1-½ hours, the permeability approaches the values of the baseline un-dented specimen. However, there are still some geometric effects on the permeability values that are probably associated with the approach used to convert the sensor responses into effective properties. Future work could develop a procedure for compensating for these geometric effects on the permeability values.
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