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The paper reports the analysis of an accident that occurred in a petroleum refinery. During the start‐up maneuver of the topping column, the anchoring of the riser pipe to metal guide support on the topping column suddenly failed. This caused a rip in the riser pipe, and gasoline squirted out, followed by the ignition of a jet fire. The investigations showed an unusual differential thermal expansion between the topping column and riser. The riser guide to the column was wrongly installed, preventing adequate relative displacements when thermal expansion occurred. Metallographic and fractographic analyses did not discover any inadequacy in the steel of the failed parts. But the lack of penetration of the welding seam resulted in a weak bond between the support and the riser. The high stress induced by the badly installed guide and the weakness of the welding was the cause of the failure and the jet fire. The paper also analyses the application of existing codes to the riser support installation and shows that the code recommendations do not satisfactorily cover the case of topping column risers. The issue is raised to alert operators in the plant, stimulate the improvement of the codes and standards, and avoid future accidents. [ABSTRACT FROM AUTHOR]

In this study, we investigated defect detection in dissimilar welded joints by the acoustic emission (AE) method. The study objects were carbide and decarburized interlayers, which are formed at the fusion boundary between austenitic and pearlitic steels. Diffusion interlayers, as a structural defect, usually have microscopic dimensions and cannot be detected using conventional non-destructive testing (NDT) methods. In this regard, the AE method is a promising approach to diagnose metal objects with a complex structure and to detect microscopic defects. In this paper, the AE signatures obtained from testing defect-free specimens and specimens with diffusion interlayers are analyzed. We found that the AE signature for defective and defect-free welded joints has significant differences, which makes it possible to identify descriptors corresponding to the presence of diffusion interlayers in dissimilar welded joints. [ABSTRACT FROM AUTHOR]

In this paper, the stress intensity factor (SIF) expression for defected butt welds containing undercut and inclined lack of penetration (LOP) subject to far-field tensile stress is derived. Some of the standards such as ISO 5817 and BS EN 25817 have specified allowable limits for the length of the undercut and LOP defects and for the height of the weld. In addition, EN 29692 standard has determined an acceptable range for the groove angle. In this paper, the effect of these acceptable geometries on stress intensity factor (SIF) of butt welded joint is investigated through following steps: i) elastic analyses to predict crack tip stress intensity (KI, KII) and shape factors, ii) approximation of shape factors by Response Surface Method (RSM). These expressions provide design guidelines for welded butt joint containing unavoidable undercut and inclined lack of penetration (LOP) defects.

• Predicting fatigue strength of details with weld imperfections by FEM. • Limit function for failure location at the cruciform joint. • Calculation of stress increase due to external weld imperfections. The paper presents an investigation into the fatigue resistance of load-carrying cruciform joints that exhibit weld imperfections. To accomplish this, a finite-element model of the joint is validated by a comparison to experimental investigations. The model includes both a global and a submodel that integrates the effective notch stress concept into its design. This enables the simulation of various types of weld imperfections, such as lack of penetration, lack of root fusion, undercut, excessive convexity, incorrect weld toe, excessive asymmetry of fillet welds and linear misalignment, to assess their impact on the fatigue strength of the cruciform joint. The analysis reveals that these imperfections lead to an increase in stress at the notches of the cruciform joint, which can be characterized as a function of the joint's geometrical properties. Global geometry influences on the fatigue strength of different cruciform join geometries can be regarded independently from the influence of the weld imperfections that is given by stress concentration factors. The normalised stress concentration factor enables a comparison between imperfection influences and shows, that the notch-softer design with full penetration welding at the cruciform joint is in a relative relation more strongly influenced by imperfections than the design with fillet welds. Also, the undercut represents the imperfection with the strongest effect in a relative relation within the sizes considered. Being able to evaluate the fatigue strength values of steel components with weld imperfections eliminates the need for costly repairs and new fabrications, which would otherwise result in a loss of resources and economic efficiency. [ABSTRACT FROM AUTHOR]

The aim of this study is to determine the stress intensity factor (SIF) and fatigue lifecycle of load-carrying 6 mm-thick fillet-welded cruciform joints subjected to fatigue loading conditions by means of finite element analysis (FEA). These joints are typical of automotive structures such as the mid-series rear axle of motor trucks which are sensitive to fatigue loading because of their construction and loading conditions. Finite element software was used to develop various cruciform joint models with varying geometrical dimensions, namely the depth of penetration and weld throat length, and simulation and analysis of the crack propagation were performed with 2D and 3D crack simulation software. The effect of the variations in the weld geometry with an induced crack at the weld root and weld toe on fatigue life was determined from the simulation results. The stress intensity factor values and lifecycles determined by the fracture mechanics approach were compared with the simulation results. It was shown that an increase in the depth of weld penetration and the weld size in isosceles triangles fillet weld shape for crack initiated in the weld root can decrease the stress intensity factor (SIF) and intensify the fatigue lifecycle. It was also found that linear misalignment had no significant effect on the SIF and fatigue life of cracks originating from the weld toe.

The static strength of mechanically heterogeneous welded joints with a lack of fusion defect in the centre of the welded joint is calculated. The stress state of mechanically heterogeneous inclined butt-welded joints with a lack of fusion defect is investigated. A method is proposed for evaluating the static strength and ductility of the welded joints with a mechanical heterogeneity which can be used to determine the load-carrying capacity by adding the mechanical heterogeneity coefficient, the size of the lack of fusion defects and the angle of inclination to the calculation equations. [ABSTRACT FROM AUTHOR]

• Special fatigue tests on the impact of undercuts and lack of penetration. • Detailed measurements of weld imperfections using 3D laser scan. • Extensive data base of published literature on fatigue tests. The assumption of the detail categories in EN1993-1-9 is the presence of a certain weld quality level. In order to find out the sensitivity of the resulting fatigue strength for cases where a worse or better quality level compared to the selected level of EN ISO 5817 is achieved, in this paper, special fatigue experiments are presented. The experiments refer to the load-carrying cruciform joint with lack of penetration and root fusion as well as undercuts. The performed fatigue investigations on cruciform joints are based on and accompanied by further fatigue test results of literature, documented in a detailed fatigue database containing 147 test series. Detailed information on mechanical properties, fabrication and metallurgical characteristics of the cruciform specimens are given. Extensive 3D laser scans of the specimens and ultrasonic testing as well as magnetic particle testing have been performed in order to analyse the imperfection sizes and scatters. The fatigue test results of the cruciform joints show, that undercuts lead to lower fatigue strength at the weld toe, as was to be expected. When reducing the fatigue resistance, undercuts could however be tolerated at secondary structural elements or marginally fatigue stressed components. A lack of penetration and/or root fusion can be tolerated, if the fatigue verification at the weld root is performed. A verification at the weld root can only be neglected in case of lack of penetration and/or root fusion, if the weld toe is decisive due to undercuts or large enough weld angels, here shown for 45°. [ABSTRACT FROM AUTHOR]

A method for solving the problems of plasticity theory is used to calculate the static strength of welded joints in plates with X-shaped edge preparation and a welded joint with strength lower than that of the parent metal. The stress state of butt welded joints with non-symmetric mechanical heterogeneity is investigated. It is shown that the method can be used to determine the optimum geometrical dimensions of the weld metal and lack of fusion defects. [ABSTRACT FROM PUBLISHER]

Highlights: [•] Cylindrical shafts connected by differently-shaped fillet welds are analyzed. [•] The mode-I, II and III stress intensity factors (SIFs) are calculated as a function of the lack of penetration ratio. [•] Normalized SIFs decrease with decreasing lack of penetration size for all modes. [•] The higher the fillet weld throat cross section, the lower the SIFs of the LOP cracks. [ABSTRACT FROM AUTHOR]

The investigations of defects of structure of rail joints, made by flash-butt welding, were carried out. The defects were detected at the fracture surface of joints during static bending tests and also after fracture under the operation conditions. The analysis of microstructure and chemical heterogeneity of the fracture surface was performed using Auger-microprobe JAMP 9500F of the JEOL company (Japan). The defects formed as a result of deviation from the standard welding conditions include lacks of penetration and inclusions of iron-manganese silicates which considerably decrease the values at mechanical tests of welded joints. Their presence in the welded joints is not admissible. Clusters of inclusions of aluminium silicates, the so-called dead spots, and oxide films of the more complicated composition are formed in the joint on the basis of non-uniformly distributed non-metallic inclusions of rail metal. The dead spots of small area do not influence the values at mechanical tests of welded joints. Their total area on the fracture should not exceed 15 mm2. On the basis of carried out investigations the criteria of quality evaluation of the high-strength steel joints, made by flash-butt welding using modern control systems, were determined. [ABSTRACT FROM AUTHOR]

This paper describes the application of Cascade Back Propagation Network for automating the TOFD A Scan interpretation. Time of Flight Diffraction Technique, which is one of the NDE methods, which is commonly used for identifying weld defects. Austenitic stainless steel weldments with Commonly occurring weld defects like Lack of Fusion and Lack of Penetration are undergone TOFD experiment. TOFD A scans are extracted from these weldments and Pre processing of the TOFD A scan signals is achieved using discrete wavelet transform based denoising methods. Single scale features are extracted from the denoised signals. Scaled conjugate gradient back propagation network has been designed for classification of the signals. The number of hidden layers in the network is also varied to form an optimum network for the classification of weld defects. The percentage of defect detection is also found out for trained and untrained inputs with respect to various performance functions. The effectiveness of this approach is analyzed with the classification accuracy. [ABSTRACT FROM AUTHOR]