Papers by Keyword: Welding Defects

Abstract: This paper presents the development of numerical models which were used to simulate the behaviour of welded joints containing different combinations of multiple defects under tensile loads. Four representative combinations of defects were selected (including undercuts, incomplete root penetration, misalignments...), based on practical experience. In order to create accurate and functional models, this research involve a number of stages. This paper will focus on the various improvements made to the models, which started in relatively simple form. For this purpose, initial experimental and numerical analyses were carried out on specimens made of low-alloyed low-carbon steel S235, and after their accuracy was verified, the same methodology was applied to specimens made of higher quality material, steel S275. Improvements made to the models involved geometry, different combinations of boundary conditions and loads, and some were based on stress-strain states obtained by a combination of tensile testing and digital image correlation. The final result was a set of detailed numerical models which accurately simulated the behaviour of welded joints with multiple defects in them.

Abstract: The detection of welding defects is becoming an important operation in the industry and the field of non-destructive testing. Among the most used techniques in the detection of weld defects, it is radiography. The radiographic images acquired are generally of low contrast, poor quality, and uneven lighting. Therefore, the detection of welding defects becomes a difficult task. In this work, a new hybrid approach based on the combination of several techniques is proposed. It consists of three stages: firstly, we define the region of interest (ROI). Secondly, a preprocessing operation based on an improved version of denoising by soft thresholding of wavelet coefficients and an optimized threshold is applied to improve the image quality (noise reduction, contrast enhancement). Thirdly, an enhanced Chan-Vese model is proposed to segment the denoised ROI region. This enhanced model is based on the choice of a cluster obtained by the Fuzzy C-Mean algorithm (FCM) as the initial contour. The proposed approach is applied to the various radiographic welding images from the GDxray database to extract the characteristics of the welding defects. The results obtained clearly show the effectiveness of the proposed approach compared to conventional techniques.

Abstract: The analysis of official data shows that the average service life of a ship in Russia exceeds 25 years, but it is no secret that a significant part of all ships is operated beyond the standard period. The ability of ship structures to withstand fatigue damage (cracks) from variable loads and extreme load damage (cracks, plastic deformations and loss of stability due to violation of their shape) is a complex concept that determines the strength of the entire welded hull. When repairing a ship, hull work accounts for up to 30% of the total repair and is carried out in difficult working conditions. The cost of hulls repairing is high not only because of the basic material costs, but also because the ship is decommissioned for a long time. Additional work on the repair of welds defective sections increases the time and complexity of the construction as well as ships repair. In this regard, the authors of the article consider it necessary to establish dependencies showing the mutual influence of defects in welded steel joints, the parameters of cyclic loading and the duration of the stages of fatigue failure. For this purpose, the authors analyze the cases of destruction of welded structures from welding. The choice of research methodology and materials for the study is justified. To study the effect of internal weld defects on the fatigue strength, the optimal number of test samples is taken. In connection with the specificity of the tasks to be solved, the authors were tasked to develop a scheme and design as well as to manufacture an experimental installation for conducting fatigue tests.

Abstract: – Welding defects can create a major threat in industrial equipments and pressure vessels can during operation. Detecting and identifying of existing welding defects has a great role in the assessment of the probability of failure in different situations. Acoustic emission (AE) is only a nondestructive technique that can be used as in-service testing and used on large structures. This paper examines the changes in the properties of AE source generated from Pencil Lead Break (PLB). AE is recorded across different seam welds with pre-identified welding defects. Series of experiments are carried out on three certified standard Carbon Steel plates. Each plate has different pre-identified defect such as lack of fusion, porosity and slag. Different AE parameters such Energy, Amplitude and number of counts have been used to identify quantitatively the type of defects. The results are showing that AE technique is capable to identifying the different defects types which can be upgraded for higher productivity and accuracy for welding inspection.

Abstract: Nondestructive inspection and evaluation of welds is very important to ensure the safety of industrial products. In this study, the metal magnetic memory (MMM) and alternating current field measurement (ACFM) methods are utilized to characterize welding defects of a butt welded tubular specimen respectively. The results show that the MMM method has potentials in evaluating both early stress concentration and macro-defects but only qualitatively, while the ACFM method can provide crack depth sizing information quantitatively, suggesting that the two methods should be combined for nondestructive evaluation welding defects to improve the detection efficiency and accuracy.

Abstract: The current work focuses on the particular case of dissimilar 6082 Al alloy/pure copper butt-friction stirred joints. It takes advantage of voluntarily non optimized welding conditions in order to test the potential of an original approach of identification of the welding defects by means of a single tensile test. The sequence and mechanism of the fracture events arise from their localization on the fracture surfaces thanks to strain maps obtained by digital image correlation. This technique of flaws identification is proved to be particularly efficient at least with the present highly damaged welds.

Abstract: Comparing with traditional welding technology, there are many advantages of magnesium laser welding, for example, high welding speed, low line energy, small welding distortion, easy to achieve automate production et al. It is widely used in the area of magnesium welding. Fine microstructure and excellent weld shape can be obtained by choosing reasonable technology parameters and the mechanical properties of welding joint can also be promoted. But there are some defects in the process of welding magnesium alloys with laser, such as, stomas, cracks, impurities and so on. Some actions can be taken to improve or prevent those defects. For example, choosing the appropriate process parameters can solve the problem of cracks, impurities. In this paper, the status of laser welding magnesium alloy is reviewed in recent years, and some problems in magnesium laser welding were analyzed and some suggestions of solving the problems were put forward.

Abstract: Friction stir welding (FSW) is a quite recent welding method which takes advantage of being performed in the solid state. Compared with the usual welding processes, it therefore presents many benefits such as a lower heat-input, a reduction of residual stresses and an elimination of the solidification defects etc.. Up to now, it has essentially been applied to aluminium alloys and far more recently to a small number of bimaterials. The present study deals with three kinds of beads between pure copper and a 6082 aluminium alloy. Both materials were butt joined by FSW. The welds differ by the location of the tool which was placed either at the interface between the two metals or on the copper or the 6082 side of this surface. Their structure was characterized at a multi-scale level by using a number of techniques. Tensile and microhardness tests were also performed. The tool place is shown to govern the microstructure and the ensuing mechanical behaviour of the weld. Its influence on the plastic flow with its repercussions on i) welding defects and ii) mechanical properties is going to be demonstrated. Some ways of improvement of the welding process will finally be suggested.

Abstract: In this study, taking die-cast magnesium alloy AZ91D as investigation objects, the process characteristics of electron beam welding (EBW) was studied. The microstructure and familiar defects in the welded joints were analyzed. The main reasons of these defects were discussed. The experimental results showed that boundary surface between fusion zone and heat affected zone in the joint of AZ91D magnesium alloy by electron beam welding is obvious, the fusion zone (FZ) consisted of fine-equiaxed grain. Pore and crack are the main defects in AZ91D magnesium alloy joints. Magnesium has higher hydrogen solubility in liquid than in solid. The difference in the solubility between the solid and liquid phase is the major reason of the pore of magnesium alloys during welding. The crack was caused because of α + β-Mg17Al12 phase with low melting point and pores in the fusion zone. There were other defects found in the joints such as incomplete fusion and penetration.