To assess the safety and reliability of a structure under working conditions, integrity assessment is a major requirement. In case of welded structures, the presence of imperfections is an impediment for the fitness-for-service. The existence of local strength variations in the weld region and the softening or hardening of heat affected zones gives rise to complexities for the assessment of weld defects, which are not yet fully understood. This paper is concerned with the plasticity analysis defected welded connections loaded under uniaxial tension, using the concept of slip line field theory. In a joint research project between Ghent University and University of Maribor, an attempt is made to understand the development and trajectory of slip lines derived from deformation bands. This paper reports on the results of an experimental program, supported by finite element analysis. Single Edge notched Tension (SE(T)) specimens were extracted from butt-welded steel (S690 QL) plates. Deformation bands were derived using Digital Image Correlation (DIC) and characterized by devoted post-processing. The attained results are a basis for the development of an analytical crack driving force estimation scheme that considers weld heterogeneity. Such scheme is expected to assist in the improvement of standardized defect assessment procedures.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 20779542Several well established fracture mechanics based Engineering Critical Assessment methods are widely used for Fitness-for-Service analysis of engineering structures. These methods provide procedures to evaluate structural integrity of strength matched and mismatched homogeneous welds by simplification of local strength variations in weld and heat affected zone. A major research question is how the referred methods can be applied to the structural integrity evaluation of fusion welds with pronounced strength heterogeneity. In order to provide reference data, fracture toughness investigations of homogeneous undermatched and overmatched welded joints have been conducted in a joint project between the authors. Single edge notched bend (SENB) specimens have been utilized and the results are presented by means of J-R resistance curves. Additionally, detailed investigations of strain distributions in front of the crack tip have been conducted by application of Digital Image Correlation.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 20741142Standardized integrity assessment techniques simplify a weld by ignoring local strength variations within weld and heat affected zones (HAZ). The effect of weld simplification on crack driving force predictions is analyzed for single-edge notched tension specimens containing an HAZ notch. Two techniques of simplifying a weld are compared, i.e. – (a) using All Weld Metal Tensile Tests (AWMTT) and (b) integrating properties along slip lines originating from crack tip. Two heterogeneous welds are considered, one of which has a hardened HAZ, the other one a softened HAZ. Limit loads (obtained using the ASME twice elastic slope method) are compared with reference simulations that include the full complexity of weld heterogeneity. Results show overestimations of Limit loads by simplified welds, leading to non-conservative results.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 20741654According to the standard BS 7448: Part 2 for determining fracture toughness of welded joints, the mechanical tensile properties at the test temperature of the region in which the crack tip lies are relevant. These properties are difficult to assess or estimate in the case of heterogeneous microstructures of fusion welded joints. Arc welds are heterogeneous due to thermal cycling and material mixing, which leads to local variations in the microstructure. When the welded joint with a crack is stressed, the heterogeneity influences the crack driving force and growth. Both heterogeneous microstructure and the crack driving force have a strong impact on the resultant structural integrity of the welded joint. The goal of this paper is to analyse the influence of heterogeneity microstructure on the global mechanical properties of mismatched welded joints. The final result is the lower boundary of mechanical properties to be taken into account for assessing structural integrity. The analysis was made using the concept of the weakest link and the Weibull distribution.
B.04 Guest lecture
COBISS.SI-ID: 19061782Flaw assessments assume homogeneous material properties, but welds are heterogeneous. A procedure to account for heterogeneity effects on crack driving force (developed at Ghent University and Limerick University) requires strength properties along slip lines originating from the defect tip. For uniaxial tension loading, these lines are assumed straight and oriented 45° to the axis of loading. The presented paper investigates the soundness of this assumption. Clamped single-edge notched tension tests and simulations have been performed on base metal and welded specimens. Extensive deformation analyses reveal the evolution of slip line trajectories as the specimen deforms. Observed slip lines in base metal specimens are initially close to the assumed 45° trajectories, but deviate towards the notch section as deformation is increased. The change in angle is roughly linear as a function of crack tip opening displacement. Deformation analyses of the welded specimens reveal that hard regions can serve as barriers, causing slip lines to deviate from linearity and split into different branches. The implications of these observations on the flaw assessment of heterogeneous welds are investigated.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 19723286