Steel pipelines are widely used in offshore oil/gas facilities. To achieve economically feasible designs, regulatory codes permit utilization of the pipelines well past their elastic response limit. This requires thorough integrity check of the pipeline subject to large scale yielding (LSY). Engineering criticality assessments (ECA) are used to justify the integrity of a cracked pipeline against fracture failure. The currently used ECA crack driving force equation was developed for load-controlled components subject to very limited crack-tip plasticity. Moreover, fracture toughness data are extracted from deeply-cracked laboratory specimens that produce the lowest margin of toughness values. Therefore, the current framework can be overly conservative (or include non-uniform inaccuracies) for ECA of modern pipelines that undergo LSY and ductile crack growth prior to failure. The two main goals of this thesis are: (i) Development of an alternative crack driving force estimation scheme, (ii) Justification of the use of use of shallow-cracked single edge notch tensile (SENT) specimens for the ECA. Strain concentration in concrete coated pipelines, and effect of Lüders plateau on the fracture response are also investigated. A new reference strain J-estimation scheme is proposed and calibrated to 300 nonlinear parametric FE models, which takes advantage of the linear evolution of the J with LSY bending strains. The scheme is hence strain-based and needless of limit load solutions, providing additional accuracy and robustness. The near-tip stress and strain fields of cracked pipelines were also investigated and compared to those obtained from a K-T type formulation. It is shown that the J-Q constraint theory can satisfactorily characterize these fields up to extreme plastic bending levels. Similar J-Q trajectories were also observed in the SENT and pipeline models. Subsequently, FE models utilizing a voided plasticity material were used to parametrically investigate ductile crack growth and subsequent failure of pipelines subject to a biaxial stress state. Plastic strain and stress triaxiality fields ahead of the propagating crack, along with R-curves, were compared among SENT and pipeline models. It is concluded that the SENT specimen could be a viable option for ECA of such pipes based on the observed crack tip constraint similarity.

In the last decade aluminium started to be considered as an alternative to steel to produce car body panels, especially considering the strict demands to decrease fuel consumption which require vehicle weight reduction. In order to keep their leading role, steel companies have to produce stronger materials to reduce the thickness of steel sheets used in cars and are now considering non-conventional steel making processes. The purpose of this PhD research was to investigate the possibility of strengthening thin sheets of interstitial free steel (IF steel) by using carbon rich films deposited on the steel surface using Physical Vapour Deposition (PVD). These films then act as a carbon reservoir which upon heat treatment release carbon in the IF steel and strengthen it. Coated tensile coupons 200 μm thick were annealed at different temperatures under high vacuum. Tensile tests show that a 100 MPa increase in yield stress can be obtained after annealing at 430 ˚C for 1h in high vacuum. The effects of annealing environment, film thickness and prestrain on carbon diffusion were also investigated. It was shown that carbon diffusion from the film to the IF steel substrate is limited by the film transformation into cementite at temperatures equal or higher than 530 ˚C. All tensile curves showed a plastic instability known as Lüders plateau, which is undesirable as it results in surface markings on the deformed part. FEM analyses were performed to find ways to suppress the Lüders plateau, proving that increasing strain-hardening or having a graded instead of uniform carbon content through thickness can suppress or limit Lüdering. The possibility of creating a through thickness gradient of microstructure was investigated as it could suppress Lüdering and result in higher strength. For these tests, FeC coated coupons were induction heated to 820 ˚C followed by water quenching. After only 2 minutes of heat treatment the yield stress was increased by 250 MPa and the ultimate tensile strength reached 400 MPa. With an annealing of 4 minutes, the Lüders plateau was fully suppressed and the microstructure consisted in ferrite grains and TiC nanocarbides. This work demonstrates that FeC films can be effectively used to diffuse carbon into steel and that a significant increase in mechanical properties can be obtained after a heat treatment of only a few minutes.