Numerical and Experimental Investigations of Fracture Behaviour of Welded Joints with Multiple Defects

Abstract

Current standards related to welded joint defects (EN ISO 5817) only consider individual cases (i.e., single defect in a welded joint). The question remains about the behaviour of a welded joint in the simultaneous presence of several different types of defects, so-called multiple defects, which is the topic of this research. The main focus is on defects most commonly encountered in practice, such as linear misalignments, undercuts, incomplete root penetration, and excess weld metal. The welding procedure used in this case was metal active gas welding, a common technique when it comes to welding low-alloy low-carbon steels, including those used for pressure equipment. Different combinations of these defects were deliberately made in welded plates and tested in a standard way on a tensile machine, along with numerical simulations using the finite element method (FEM), based on real geometries. The goal was to predict the behaviour in terms of stress concentrations caused by geometry and affected by multiple defects and material heterogeneity. Numerical and experimental results were in good agreement, but only after some modifications of numerical models. The obtained stress values in the models ranged from noticeably lower than the yield stress of the used materials to slightly higher than it, suggesting that some defect combinations resulted in plastic strain, whereas other models remained in the elastic area. The stress–strain diagram obtained for the first group (misalignment, undercut, and excess root penetration) shows significantly less plasticity. Its yield stress is very close to its ultimate tensile strength, which in turn is noticeably lower compared with the other three groups. This suggests that welded joints with misalignment and incomplete root penetration are indeed the weakest of the four groups either due to the combination of the present defects or perhaps because of an additional unseen internal defect. From the other three diagrams, it can be concluded that the test specimens show very similar behaviour with nearly identical ultimate tensile strengths and considerable plasticity. The diagrams shows the most prominent yielding, with an easily distinguishable difference between the elastic and plastic regions. The diagrams are the most similar, having the same strain of around 9% and with a less obvious yield stress limit.

An Experimental Study of Damage Detection on Typical Joints of Jackets Platform Based on Electro-Mechanical Impedance Technique

“…The results show that the presence of the internal defects in the welded Q345B steel joints depends on the stir–pin rotation speed, weld speed, and cooling intensity during welding. Mihajlo et al [ 52 ] performed both numerical and experimental studies in order to understand the fracture behaviour of welded joints with multiple defects. The results suggest that the weakness of the welded joints are due to the misalignment and incomplete root penetration and also due to the unseen internal defects.…”

Formability Prediction of Laser-Welded Stainless Steel AISI 304 and AISI 430

“…The true-stress-true-strain curves show a better prediction of the formability of the material than the agreed engineering characteristics of materials Re, tensile strength Rm, uniform tensibility, and tensibility. The actual stress can be calculated according to Equation (9) [33,38]:…”

“…The results presented in [32,33] show that the widely used standards ISO 5817 and ISO 5178 have room for innovation to meet the needs of numerical simulations. In order to take into account the material inhomogeneity of welded joints in the formability analysis in the numerical simulation of laser welded blanks, using the finite element method (FEM), the basic mechanical properties and values of the material properties necessary for the simulation of the sheet-metal forming processes were determined (strength constant K, strain hardening exponent n, and plastic strain ratio r) on specimens with a longitudinal laser weld.…”

Comparison between Fractal and Statistical Approaches to Model Size Effects in VHCF

“…From this experimental campaign, it was found that the larger specimens showed lower fatigue resistance due to the appearance of larger inclusions in the fish-eye fracture origin. In fact, it is well-known that crack nucleation in the VHCF domain is originated from the largest inclusion embedded in the risk-volume, which represents the weakest site within the highly-stressed volume [15][16][17]. Hence, since the VHCF fracture mechanism is governed by the defect population, it follows that the gigacycle fatigue strength can be related to the size of the largest defect within the risk-volume according to Murakami's model [18,19].…”