Imad Barsoum

Load Carrying Capacities of Butt Welded Joints in High Strength Steels

The aim of this study is to investigate the influence of yield strength of the filler material and weld metal penetration on the load carrying capacity of butt welded joints in high-strength steels (HSS) (i.e., grade S700 and S960). These joints are manufactured with three different filler materials (under-matching, matching, and over-matching) and full and partial weld metal penetrations. The load carrying capacities of these mentioned joints are evaluated with experiments and compared with the estimations by finite element analysis (FEA), and design rules in Eurocode3 and American Welding Society Code AWS D1.1. The results show that load carrying estimations by FEA, Eurocode3, and AWS D1.1 are in good agreement with the experiments. It is observed that the global load carrying capacity and ductility of the joints are affected by weld metal penetration and yield strengths of the base and filler materials. This influence is more pronounced in joints in S960 steel welded with under-matched filler material. Furthermore, the base plate material strength can be utilized in under-matched butt welded joints provided appropriate weld metal penetration and width is assured. Moreover, it is also found that the design rules in Eurocode3 (valid for design of welded joints in steels of grade up to S700) can be extended to designing of welds in S960 steels by the use of correlation factor of one.

Ductile Failure Modelling of Expanded Aluminium Tubes With Embedded Circular Holes

A common technology used for well completion in the oil and gas industry is the solid expandable tubular technology, where a metal pipe is expanded radially towards the well bore. A challenge in this technology is to assess the mechanical integrity of the pipes during the expansion process. In this paper the ductile failure behavior of mechanically expanded aluminum tubes was studied experimentally and numerically. The expansion of the tubes was performed mechanically by using a conical mandrel with the objective to study the failure mode that governs the expansion process of this material. To localize the failure the tubes were drilled with circular holes. The fractured surfaces of failed expanded tubes were examined and revealed a flat ductile dimple rupture characteristic. A finite element model, which is based on continuum damage mechanics, is developed to mimic the experiments. The model also predicts ductile crack propagation and failure in the expanded tubes with embedded holes very well making it a suitable tool for studying the tubular expansion process and for optimizing the expansion tools used in this process.Copyright

Micromechanics of Rupture in Combined Tension and Shear

A micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice [1] is developed. The model employed consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The micromechanics model is applied to analyze the rupture mechanisms associated with mixed mode ductile fracture. The stress state is characterized by the stress triaxiality T and the Lode parameter μ, which adequately describe the stress state ahead of a crack tip under mixed mode loading of an isotropic elasto-plastic material. The main focus is the influence of μ on void growth and coalescence behavior. It is shown that the Lode parameter exerts a strong influence upon this behavior.

Micromechanical Analysis of Rupture Mechanisms in Mixed Mode Ductile Fracture

The fracture toughness of ductile materials may differ considerably under mode I and mode II/III loading. The main reason for this is that the fracture mechanisms differ in important aspects between the opening loading mode (mode I) and the shear loading mode (mode II/III). Experimental studies show that the mode I failure mechanism involves void nucleation, growth and coalescence. In mode II/III however, intense plastic straining ahead of the crack tip promotes nucleation of micro voids which typically experience limited growth before linking up under intense shear deformation. Hence, under general mixed mode loading, there will be at least two failure mechanisms that may co-operate or even compete. At lower stress triaxialities (mode II/III), the shear dimple rupture mode will be favoured, whereas at higher stress trixailities (mode I) the flat dimple rupture mode will be favoured. In the flat dimple rupture mode the final link up between voids occurs through necking of void ligaments until impingement. By contrast, in the shear dimple rupture mode, final link up between voids occurs through shear localization of plastic strain in the ligaments between voids. In the latter case, voids rarely grow until impingement. In this study we are focusing on the conditions that govern the transition between the two mechanisms. This is carried out by micromechanical modelling using finite element analysis.