Determination of true stress-strain data behond the necking point and fracture loci of vintage steel pipes, using digital image correlation and finite element analysis

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Manufacturing, transportation, power generation and many other industries strongly rely on a high supply of oil and gas products. Pipelines are the main means of transportation and delivery of these vital products. However, like any structural system, they are vulnerable to through life damage and degradation. Depending on many factors, pipeline industry has defined and standardized a wide variety of assessment and inspection techniques to consider the integrity of their systems. Generally, the significance of features (e.g. dents, cracks, wrinkle and buckles) and metal loss (e.g. corrosion) determine the severity of the maintenance period to assure a safe integrity for the system. In recent years, particular interest was aimed at vintage (manufactured before 1970) steel pipe materials to investigate their micro- and macro-structural behaviors to large deformations, under different loading conditions (such as uniaxial tension, compression and bending). The consideration of large deformations/displacements in a pipeline system could result in the generation of large strains in the pipe material. Normally, very large strains (beyond the uniaxial necking point) exist in regions with features such as wrinkle or local buckling. These high levels of induced strains can lead to damage accumulation in the pipe and endanger the integrity of the pipeline system. In order to obtain better pipeline deformation predictions, feeding more reliable material properties to an FE model is of great importance. The main purpose of this work is to conduct uniaxial tensile testing, using a Digital Image Correlation (DIC) technique, to obtain Lankford coefficients, stress-strain curves over a large range of strains (beyond the necking point), and the fracture strains for different stress-triaxialities (fracture locus). In order to consider the material anisotropy, tests will be conducted for a variety of vintage steel pipe materials at two different pipe orientations, axial and circumferential. The effect of sample size on the resulting mechanical behavior will also be investigated.