Papers by Author: B. Medjo

Abstract: Seam casing pipe used in an oil drilling rig, manufactured by high frequency (HF) contact welding of API J55 steel, is tested. The influence of an initial defect (machined surface crack) is analysed, by performing pressure test of a pipe segment closed at both ends. Besides the damages at the internal surface, casing pipes are exposed to damage at the external surface, which is why such configuration is analysed here. Measurement of strains and crack mouth opening displacement (CMOD) enabled the application of direct method for J integral evaluation. This procedure is based on the path independence of the J integral and can be applied both in laboratory conditions (on specimens) and on structures. However, it requires a demanding experimental - computational procedure, which is accomplished here using the developed routine. Additionally, the behaviour of the pipe under internal pressure, including fracture mechanics parameters determination, is modelled numerically (by finite element method) in software package Abaqus. The pressure is applied as distributed load acting on the inner surface of the three-dimensional model, and axial tension is applied at the end of the pipe to simulate the closed end. J integral values determined numerically and using direct method are used for estimation of the critical pressure corresponding to the crack growth initiation. Additionally, plastic limit load, i.e. pressure which causes yielding of the ligament, is determined. Based on the results, criteria for pipe integrity assessment are discussed

Abstract: Among various micromechanical models for ductile fracture analysis using local approach, those based on the Gurson plastic flow criterion have been intensively developed in the past three decades. However, their application is still subject of many researches, with the aim to improve the assessment of ductile fracture in various, mainly metallic, materials. In this work, Gurson criterion is applied to welded joints produced with one or two different weld metals (one of these is overmatched, while the other is undermatched). Welded single-edge notched bend (SENB) specimens are examined, with an initial pre-crack located in the symmetry plane of each joint. Local approach is chosen for assessment of behaviour of the joints under the external loading, to capture the effect of material inhomogeneity and initial crack length on deformation and ductile fracture initiation in analysed specimens.