Evaluation of pipeline girth welds using low-frequency horizontally polarized waves

The practical implementation of alternative acceptance criteria for pipeline girth welds requires the use of inspection tools capable of determining the principal dimensions and positions of planar flaws. A new ultrasonic inspection method is described that permits complete volumetric inspection of the girth welds. The new system uses noncontacting electromagnetic-acoustic transducers (EMATs) that operate at low ultrasonic frequencies (454 kHz). Theoretical models of the measurements are developed and verified experimentally. In addition, practical performance limits of the new system are established in terms of minimum flaw sizes that can be detected. The results are related to accept-reject curves based on a model of the failure processes. An inspection protocol for field applications is also described.     

Life prediction of butt welds containing welding defect

In the present study, the welding defects are divided into two types, i.e. the crack-like defect and the uncrack-like defect, based on the experimental and analytical results, and the criterion for distinguishing the defect type is tentatively proposed. The life predicting methods are given to accurately compute the fatigue life of butt welds containing different types of defect. The computation and the experiments show that the fatigue crack propagation life of the butt welds containing the crack-like defect is approximately equal to the total fatigue life and the fatigue crack initiation life can be neglected. Therefore, the fatigue life of this kind of welds is greatly decreased. On the other hand, the fatigue life of the butt welds containing uncrack-like defect consists of two phases, i.e. the fatigue crack initiation life and the fatigue crack propagation life and the fatigue crack initiation life occupies a greater portion, which cannot be neglected. In order to accurately predict the fatigue life of welded elements, not only the defect size but also the defect type should be determined. Consequently, the technology and the equipment for the nondestructive detection need to be highly developed.     

Numerical simulation of crack extension in aluminium welds

Experimental and numerical investigations on stable crack extension of aluminium-laser beam welds (6XXX-series) are presented. Experiments are carried out on C(T)-specimens. Initial cracks are assumed in the base material and in the fusion zone. Local mechanical quantities are determined by micro-flat tensile specimen, taken from the weld nugget, the heat affected zone and the base material. Finite element calculations together with an evolution strategy are used in order to determine parameters of the Gurson–Tvergaard–Needleman model (GTN-model) used for the simulations. Crack propagation in the base material as well as in the undermatched fusion zone is described in terms of fracture resistance curves. It is found that the GTN-model is able to simulate ductile crack growth in the base material and the fusion zone using non-standard model parameters.     

Simplified FE welding simulation of fillet welds – 3D effects on the formation residual stresses

In this study two- and three-dimensional finite element welding simulations have been carried out. The welded component studied is a T-type fillet weld which is frequently used in the heavy vehicle machine industry with plate thicknesses of eight and 20 mm, respectively. The software’s used for the welding simulations is MSC.Marc and ANSYS. The objective is to study the formation of the residual stresses due to 3D effect of the welding process. Moreover, welding simulations using solid models and contact models in the un-fused weld roots were carried out in order to investigate the possible effect with respect to the residual stresses. Residual stress measurements were carried out using X-ray diffraction technique on the manufactured T-welded structure. The 2D residual stress predictions shows good agreement with measurements, hence the 2D welding simulation procedure is suitable for residual stress predictions for incorporation in further fatigue crack growth analysis from weld defects emanating from the weld toe and the un-fused root.     

Finite-element simulation of transient heat response in ultrasonic transducers

The application of the finite-element method to a transient heat response problem in electrostrictive ultrasonic transducers during their pulsed operation is described. The temperature and thermal stress distribution are of practical importance for the design of the ultrasonic transducers when they are operated at intense levels. Mechanical vibratory loss is responsible for heat in the elastic parts, while dielectric loss is responsible in the ferroelectric parts. A finite-element computer model is proposed for the temperature change evaluation in the transducers with time. Natural and forced cooling convection and heat radiation from the transducers' boundaries are included. Simulation is made for Langevin-type transducer models, for which comparison is made with experimental data.     

Distribution of residual welding stresses in thick plates with circular welds

We determine residual stresses in plates with circular welds by the method of conditional plastic strains. Numerical analysis of this problem is performed on the basis of equations of the theory of plates taking into account three-dimensional effects and equations of the plane stressed state. It is shown that the three-dimensional stressed state is formed in the vicinity of a weld even in relatively thin plates, which may result in the embrittlement of the material in this zone. State University “L'vivs'ka Politekhnika,” L'viv; Ternopil' Academy of National Economy, Ternopil'. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 3, pp. 31–35, May–June. 1998.     

Finite-element simulation of a new two-dissipative mechanisms model for bulk medium-density polyethylene

A two-dissipative mechanisms model, associating a Maxwell and an elastoplastic model in parallel, is discussed in order to account for the non-linear viscoelasticity of bulk medium-density polyethylene. On the one hand, the experimental determination of the constitutive equations coefficients is described from a tensile specimen machined from gas pipes. On the other hand, finite-element simulation of the stress relaxation experiment, proposed by Sweeney and Ward, is achieved, which yields a complete analysis of the dissipative mechanisms interaction during the test. The finite-element code built upon this modelling is finally used in a tentative simulation of a cyclic pressure test on a pipe specimen.     

Finite-element simulation of wave propagation in periodic piezoelectric SAW structures

Many surface acoustic wave (SAW) devices consist of quasiperiodic structures that are designed by successive repetition of a base cell. The precise numerical simulation of such devices, including all physical effects, is currently beyond the capacity of high-end computation. Therefore, we have to restrict the numerical analysis to the periodic substructure. By using the finite-element method (FEM), this can be done by introducing periodic boundary conditions (PBCs) at special artificial boundaries. To be able to describe the complete dispersion behavior of waves, including damping effects, the PBC has to be able to model each mode that can be excited within the periodic structure. Therefore, the condition used for the PBCs must hold for each phase and amplitude difference existing at periodic boundaries. Based on the Floquet theorem, our two newly developed PBC algorithms allow the calculation of both, the phase and the amplitude coefficients of the wave. In the first part of this paper we describe the basic theory of the PBCs. Based on the FEM, we develop two different methods that deliver the same results but have totally different numerical properties and, therefore, allow the use of problem-adapted solvers. Further on, we show how to compute the charge distribution of periodic SAW structures with the aid of the new PBCs. In the second part, we compare the measured and simulated dispersion behavior of waves propagating on periodic SAW structures for two different piezoelectric substrates. Then we compare measured and simulated input admittances of structures similar to SAW resonators.     

Analysis of non-loading carrying fillet welds joint using SGBEM

Fatigue failure is probably the most common type of failure in welded construction. It is usually initiates at a stress concentration area within the structure. The fatigue behavior of non-load carrying cruciform fillet welded joint has been studied extensively using boundary element method. The symmetric boundary element method for multiple cracks problem is derived using Betti's reciprocal theorem in auxiliary fictitious state. High order element is proposed to solve the double integrals. The analysis demonstrates that symmetrical Galerkin boundary element method (SGBEM) can be used effectively for analyzing non-load carrying fillet welded cruciform joints containing any number of surface and embedded cracks. The stress intensity factor and the magnification factors M _k are analyzed. General formulation for this kind of fatigue life estimation is derived for engineering design purpose. The fatigue life estimate results are consistent with the code of practice and those of other researchers. This can provide a good method for engineering design under fatigue loading conditions.     

Corrosion resistance of girth joints of pipes made by contact butt welding

We establish that welded joints made by contact butt welding of ferritic-austenitic steel are not susceptible to general, intercrystalline, and pitting corrosion, and the mechanical properties of the joints of pipes comply with the requirements of the ISO API-1104 standard. The base metal and welded joints are subjected to corrosion cracking in a boiling 42% MgCl₂ solution and NACE solution. E. O. Paton Institute of Electrical Welding, Ukrainian Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 1, pp. 107–108, January–February, 1999.     

Finite-element simulation of firearm injury to the human cranium

An advanced physics-based simulation of firearms injury to the human cranium is presented, modeling by finite elements the collision of a firearm projectile into a human parietal bone. The space-discretized equations of motion are explicitly integrated in time with Newmark's time-stepping algorithm. The impact of the projectile on the skull, as well as the collisions between flying fragments, are controlled through a nonsmooth contact algorithm. Cohesive theories of fracture, in conjunction with adaptive remeshing, control the nucleation and the propagation of fractures. The progressive opening of fracture surfaces is governed by a thermodynamically irreversible cohesive law embedded into cohesive-interface elements. Numerical results compare well with forensic data of actual firearm wounds to human crania.     

Effects of welding residual stresses on fatigue crack growth behaviour in butt welds of a pipeline steel

In the present test the fatigue crack growth rate in the parent plate, weld and cross-bond regions was measured and the results were correlated with the stress intensity range ΔK and the effective stress intensity range ΔK_eff. It is indicated that the welding residual stresses strongly affect the crack growth rate. For the weld metal and cross-bond compact tension specimens in which crack growth is along the weld line the fatigue crack growth rate increases as the crack grows. However, for the T compact tension specimen in which crack growth is perpendicular to the weld line at a constant value of applied ΔK the crack growth rate initially decreases as the crack grows. Particularly, at a low constant value of applied ΔK the crack growth rate obviously decreases and the crack fails to grow after short crack growth. When the crack grows to intersect the welded zone, the fatigue crack growth rate gradually increases as the crack grows further. It is clear that the effect of welding residual stresses on the crack growth rate is related to the position of the crack and its orientation with respect to the weld line. Finally, the models of welding residual stress redistribution in the compact tension specimens with the growing crack and its influence on the fatigue crack closure are discussed. It appears that for a butt-welded joint one of the crack closure mechanisms may be considered by the bend or rotation deformation of crack faces due to the welding residual stress redistribution as the fatigue crack grows in the welded joint.     

Softened zone formation and joint strength properties in dissimilar friction welds

The mechanical properties of dissimilar MMC/AISI 304 stainless steel friction welds with and without silver interlayers were examined. The notch tensile strengths of MMC/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds increased when high friction pressures were applied during the joining operation. The higher notch tensile strengths of dissimilar MMC/AISI and MMC/Ag/AISI 304 stainless steel friction welds resulted from the formation of narrow softened zones in MMC material immediately adjacent to the bondline. The influence of softened zone width and hardness (yield strength) on the notch tensile strengths of dissimilar welds was analysed using finite element modelling (FEM). FEM in combination with the assumption of a ductile failure criterion was used to calculate the notch tensile strengths of dissimilar joints. The key assumption in this work is that dissimilar weld failure wholly depended on the characteristics (mechanical properties and dimensions) of the softened zone formed in MMC material immediately adjacent to the bondline. The modelling results produced based on this assumption closely correspond with the actual notch tensile strengths of dissimilar MMC/Ag/AISI 304 stainless steel and MMC/Ag/AISI 304 stainless steel friction welds.     

Influence factors of X80 pipeline steel girth welding with self-shielded flux-cored wire

For girth weld of high-pressure oil and gas transmission pipeline, there is impact toughness values deviation phenomenon with self-shielded flux-cored semi-automatic welding technology. The macro-images, microstructure and mechanical performance of girth welding joint have been investigated by OM, SEM, TEM. The results show that there are several factors of impact toughness unqualified values of weld joints, such as welding heat input, coarse grain zone and a chain of M–A organisations, Al₂O₃ and Zr Precipitates particle sizes and distribution et al., which are the main unqualified reasons of welding impact toughness of the semi-automatic self-shielded core butt welding process of X80 pipeline steel.     

Prediction of welding buckling distortion in a thin wall aluminum T joint

In this paper, local and global welding buckling distortion of a thin wall aluminum T joint is investigated. A thermo-elastic–viscoplastic model is employed to determine longitudinal residual stresses; analysis of thermal model and elastic–viscoplastic (Anand) model are uncoupled. Molten puddle motion (speed of welding) is modeled by using time dependent birth and death element method. Three dimensional nonlinear-transient heat flow analysis has been used to obtain the temperature distribution, and then by applying thermal results and using three dimensional Anand elastic–viscoplastic model, stress and deformation distributions are obtained during welding and after cooling. Local buckling is investigated by analyzing the history of stress and strain relations. Local buckling is assumed to occur at a point if a small change in the magnitude of stress causes large deformation during of the welding process. By applying residual stresses on a structural model and using eigenvalue methods, global buckling instability of the welded structure is determined.     

Characterisation of dissimilar P91 and P92 steel welds joint

In the present study, dissimilar weld joint was prepared using the P91 and P92 steel plate of 8-mm thickness, using the multi-pass gas tungsten arc (GTA) welding with filler (weld 1) and autogenous tungsten inert gas welding (A-TIG) process (weld 2). Evolution of δ-ferrite patches was studied in weld zone and heat affected zone (HAZ) for both weld 1 and weld 2. Effect of varying post weld heat treatment (PWHT) duration was also studied on δ-ferrite patches and mechanical properties of the dissimilar weld joint. PWHT was carried out at 760°C. For weld 2, weld zone showed poor impact toughness and higher peak hardness as compared to weld 1. After the PWHT, a considerable reduction in hardness was obtained for both weld 1 and weld 2,while impact toughness of weld zone showed a continuous increment with PWHT duration. For weldments characterisation, optical microscope, scanning electron microscope (SEM) and microhardness tester were utilised.     

HAZ microstructure simulation in welding of a ultra fine grain steel

In the present work the evolution of grain structure in the weld HAZ (heat affected zone) under welding thermal cycle was simulated. Especially the grain growth in the HAZ of a SS400 ultra fine grain steel was investigated. An integrated 3-D Monte Carlo (MC) simulation system for grain growth of the weld HAZ was developed based on Microsoft Windows. The results indicate that MC simulation is an effective way to investigate the grain growth in weld HAZ. The method not only simulates the non-isothermal dynamics process of the grain growth in the weld HAZ, but also visualizes the austenite grains realistically. Moreover, the thermal pinning effect can be easily included in the simulation process. The grain sizes of the CGHAZ (coarse grain heat affected zone) obtained from MC simulation are basically in agreement with the experimental measurement of the real welded joints under different heat input. Furthermore, the simulation indicates that the grain growth degree is higher for the SS400 ultra fine grain steel compared to conventional steel. With the increase in the heat input, the grain growth of the CGHAZ rapidly increases. Because the activation energy of the grain growth is lower for the SS400 ultra fine grain steel, austenite grains can grow at a relatively lower temperature, hence the range of the CGHAZ becomes wider.     

The safe-end length effect on welding residual stresses in dissimilar metal welds of surge nozzles

The effects of safe-end length and kinematic boundary conditions during welding on weld residual stresses in dissimilar metal welds are systematically investigated using finite element analyses for typical pressurized water reactor surge nozzle configurations. The study is generalized by idealizing complex nozzle geometries as straight pipes with two welds. The impact of these variables on axial residual stresses at inner surface is examined, since these are of most concern for primary water stress corrosion cracking. Possible mechanisms controlling the development of welding residual stresses are used to qualitatively explain the predicted behavior.     

Numerical simulation of welding distortion in thin plates

A three-dimensional model based on the finite-element method is developed to simulate the temperature field and stress distribution in the welding and heat-affected zones during fusion welding of thin plates. The governing equations are solved using the SYSWELD program commercial code. The model’s predictions are tested and verified against the experiments. Angular distortion and longitudinal bending are measured, the results are compared with those obtained from the mathematical model, and a relatively good agreement between them is found. The verified model is used to evaluate the effects of various parameters on the temperature and stress distributions in the welding and heat-affected zones of a thin austenitic stainless steel plate.