Avaliação do uso de placas tracionadas para simular o ensaio de tubos de parede fina pressurizados internamente

Abstract

High diameter-to-thickness ratio (D/t) cylinders under internal pressure are ideally subjected to the same stress state than a plate under traction. The objective of this work is to evaluate, through finite-element modeling, if it is adequate to use a flat plate to simulate tests of thin-walled cylinders with radial crack-like flaw internally pressurized. It is aimed to establish a frame of geometric and mechanical characteristics that could validate this hypothesis. Mechanical tests were performed to define the stress-strain behavior under traction and the crack-growth resistance curve of a steel frequently used by the oil & gas industry. The theory of elastic-plastic fracture mechanics was used to analyze the crack-growth resistance. The results showed that the use of a plate is not adequate to simulate thick-walled cylinders with D/t10. On the other hand, for D/t25, i.e., thin-walled cylinders, the hypothesis might be valid if fracture instability or plastic collapse is the fully dominant failure mechanism. If the former is dominant, the difference between the use of a plate and a cylinder was found to be less than 1% for crack lengths up to 15% of the wall thickness. For the latter, if the limit load is defined by the nominal cross section, the difference was 11.4%. It was showed that this difference is function of the yield-to-tensile stress ratio, being less than 5% for ratios above 0.9. If the limit load is defined by the ligament, the difference was found to be less than 1%. If both plastic collapse and fracture instability play an important role as failure mechanism, the elastic-plastic fracture mechanics loses validity. Thus, for this last case, this study was not conclusive.