Metal pipes are sometimes swaged by a metal cone to enlarge them, which increases the strain in the material. The amount of strain is important because it affects the burst and collapse strength. Burst strength is the amount of internal pressure that a pipe can withstand before failure, while collapse strength is the amount of external pressure that a pipe can withstand before failure. If the burst or collapse strengths are exceeded, the pipe may fracture, causing critical failure. Such an event could cost the owners and their customers millions of dollars in clean up, repair, and lost time, in addition to the potential environmental damage. Therefore, a reliable way of estimating the burst and collapse strength of strained pipe is desired and valuable. The sponsor currently rates strained pipes using the properties of raw steel, because those properties are easily measured (for example, yield strength). In the past, the engineers assumed that the metal would be work-hardened when swaged, so that yield strength would increase. However, swaging introduces anisotropic strain, which may decrease the yield strength. This study measured the yield strength of strained material in the transverse and axial direction and compared them to raw material, to determine the amount of anisotropy. This information will be used to more accurately determine burst and collapse ratings for strained pipes. More accurate ratings mean safer products, which will minimize risk for the sponsor’s customers. Since the strained metal has a higher yield strength than the raw material, using the raw yield strength to calculate burst and collapse ratings is a conservative method. The metal has even higher yield strength after strain aging, which indicates that the stresses are relieved. Even with the 12% anisotropy in the strained and 9% anisotropy in the strain aged specimens, the raw yield strengths are lower and therefore more conservative. I recommend that the sponsor continue using the raw yield strength to calculate these ratings. I set out to characterize the anisotropic nature of swaged metal. As expected, the tensile tests showed a difference between the axial and transverse tensile strength. The correlation was 12% difference in yield strength in the axial and transverse directions for strained material and 9% in strained and aged material. This means that the strength of the metal in the hoop (transverse) direction is approximately 10% stronger than in the axial direction, because the metal was work hardened during the swaging process. Therefore, the metal is more likely to fail in axial tension than in burst or collapse. I presented the findings from the microstructure examination, standard tensile tests, and SEM data. All of this data supported the findings of the mini-tensile tests. This information will help engineers set burst and collapse ratings and allow material scientists to predict the anisotropic characteristics of swaged steel tubes.

Lorsqu’on découpe un feuillard à l’aide d’un outil, ou lorsqu’on découpe un tube selon son axe, au fur et à mesure que l’on propage la fissure qui traduit la découpe il arrive que des ondulations de flambage perturbent les deux bords libres générés par la propagation de la fissure. Cette étude vise à analyser les origines de ces ondulations. Nous avons mené une campagne expérimentale, dans laquelle des tubes en acier inox avec différentes géométries (rayon/épaisseur) sont « découpés » selon une génératrice. Une instrumentation adéquate, couplant des mesures ponctuelles, à l’aide de jauges de déformation, et une méthode champ par corrélation d’image, nous a permis de correctement mettre en exergue la phénoménologie, en particulier les cinématiques induites à l’échelle géométrique de la fissure (front de fissure) ainsi qu’à l’échelle du tube, avec les longueurs d’onde de flambage observées à l’aval de la fissure. La modélisation numérique menée en non linéaire géométrique (flambage), matériau (déchirure ductile), et conditions aux limites (contact) est aussi abordée à l’aide du code de calcul Abaqus/Standard. Pour la gestion de la propagation de la fissure, deux modèles de rupture sont proposés. Le premier modèle dit zone cohésive est développé et implanté dans le code Abaqus via la subroutine UEL. Pour la deuxième modélisation, nous avons utilisé le modèle dit « d’endommagement ductile » du code Abaqus. La modélisation via des éléments massifs ou des éléments coques volumiques ainsi que l’utilisation de ces modèles de rupture permettent de corroborer les observations expérimentales. Ces travaux montrent que l’augmentation de la charge inhérente au déplacement de l’outil de « découpe », induit une extension dans la direction circonférentielle et donc une striction dans la direction radiale amenant finalement la rupture. Lors de la rupture, un « sillage plastique » apparait, relativement large, près et parallèle aux bords de la fissure. « Confiné » par les autres parties du tube qui restent élastiques, des contraintes de compression axiale résiduelles apparaissent dans ce sillage plastique, à l’aval de la fissure, leur intensité est suffisante pour produire les ondulations des bords libres qui traduisent un flambage local. Les contraintes résiduelles liées à l’opération de découpe induisent donc le flambage. / When one uses a tool to cut a sheet metal, or a tube according to his axis, as one propagates the crack which translates cutting it arrives that undulations of buckling disturb the two free edges generated by the propagation of the crack. This study aims at analyzing the origins of this behavior. We conducted an experimental campaign, in which stainless steel tubes with various geometries (radius/thickness) are « cut out » according to a generator. An adequate instrumentation, coupling of specific measurements, using strain gauges, and a field method, by digital image correlation, allowed us accurately to put forward phenomenology, in particular the kinematics at the scale of the crack (ahead of crack tip) and at the level of tube, with the wavelengths of buckling observed at the downstream of the crack tip. The numerical modeling taking into account nonlinearities of material (ductile tear), geometry (buckling) and boundary conditions (contact) is also approached using the code Abaqus/Standard. For the management of the crack propagation, two rupture models are proposed. The first model called cohesive zone is developed and implemented in the Abaqus code via the user routine UEL. For the second modeling, we used the model called “ductile damage model” in the Abaqus code. Modeling via solid elements or shell continuum elements as well as the use of these rupture models make it possible to corroborate the experimental observations. These studies show that the increase of the load inherent in the displacement of the tool of « cutting » induced a circumferential extension of the tube that leads to a local necking in the radial direction bringing the rupture finally. During the failure, a “plastic wake” appears, relatively wide, close and parallel to crack lips. Constrained by other parts of the tube which remain elastic, sufficient axial residual compressive stresses produced in this plastic wake produce the undulations which represents a local buckling. The residual stresses related to the operation of cutting thus induce buckling.