ADVANCED WAYS OF IMPROVING THE WELDED STRUCTURES

A package of work is presented on development of highly effective welded structures, currently conducted by the E. O. Paton Electric Welding Institute. New types of cost - effective and reliable welded struc- tures have been developed, including light - weight building structures, bridge frameworks, TV tow- ers, heavy - duty structures from high - strength steels, large-sized stringer panels and shells, unique structures of a transformable volume. A series of methods of investigations of welding stresses and strains have been created, they being characterized by a high accuracy and information content. The physical and mathematical fundamentals of optical simulation of welded stresses have been estab- lished and the theory of their similarity has been constructed. Holographic methods of investigation of the quality and stressed - strained state of welded joints have been advised. Portable holographic sys- tems for been been developed which allow investigations to be conducted without the vibration - insulation facilities. A non-destructive testing method of residual stresses determination has been elaborated based on ultrasonic probing of the appropriate sections of the welded structure. A new trend has been formed, namely the distortion - free welding of structures, which is based on regulation of the thermal processes during welding with the aim of reduction of the heat input and inducing the preliminary stressed - strained states optimized in terms of welding stresses and strains. Specialized procedures and equipment based on the acoustic emission effect have been developed for diagnostics of welded structures. The technology and equipment have been for ultrasonic impact treatment of welded joints in or- der to improve their fatigue resistance, as well as computerized methods for design of welded joints and structures.

The reliability analysis of welded structure with embedded cracks by using three dimensional stochastic FEM

In this paper, the reliability of welded structure with an embedded elliptical crack was calculated by using three dimensional stochastic finite element method. This method has overcome the shortcomings of conservative results in safety assessment with deterministic fracture mechanics method. The calculation of reliability was based on three dimensional perturbation stochastic finite element program which was developed by ourselves. The effects of variables (such as fracture toughness K IC , load P, the ratio of minor axis to major axis a/c) on the structure reliability were also discussed. The calculation results indicate that among the three variables (K IC ,P, a/c) , the fracture toughness K IC has the greatest effect on the structure reliability. The next is external load P. The ratio of the a/c has lower effect on structure reliability than both K IC and P . The method has put forward a new way for safety assessment of welded structure with embedded elliptical cracks.

Calculation of the Strengths of Jointswith Fillet Welds in Welded Structure

In the paper, the finite element method (FEM) , engineering calculationmethod (ECM) and empirical fromular method (EFM) were used to calculatethe strengths of fillet joints in an Al-alloy vehicle. The results obtained withdifferent calculation methods were analysed, and then each method wasreviewed and assessed. FEM is accurate, but complicated, and its results haveno strength reserve; ECM and EFM are simple and convenient, butconservative.

FATIGUE DAMAGE AND LIFETIME PREDICTION OF AERONAUTIC WELDED STRUCTURES UNDER HIGH TEMPERATURE

The fatigue damage evolution equations and the relation of fatigue damage parameter with maximum cyclic stress of superalloy GH150 and its welded structures are established. The fatigue damage evolution equations in a multiaxial stress state are also given. By use of cyclic thermal elastoplastic damage constitutive relations, the fatigue damage and lifetime predictions are carried out for the welded combustion chamber of aeroengine.

STUDY ON DISLOCATION STRUCTURES IN WELDED METALS UNDER FATIGUE LOAD

With transmission electron microscope, it has been observed and analyzed that the dislocation arrangements and the fatigue damage of eutectoid present in welded metals which are loaded at different fatigue loads and load cycles. The results indicated that the character of fatigue damage in ferrites of welded metals is the dislocation arrangement change and eutectoids of welded metals are the broken cementites. This may produce microcracks.

AN IMPROVED FOSM METHOD FOR CALCULATING FAILURE PROBABILITY OF WELDED PIPES WITH FLAWS

The R F first order second moment method will produce more error for calculating the reliability of welded engineering pipe structures when the failure function is seriously nonlinear and the random variables don′t serve as normal distribution. In order to increase the computing accuracy of reliability, an improved FOSM method is used for calculating the failure probability of welded pipes with flaws in this paper. Because of solving the problems of the linear expansion of failure function at the failure point and constructing equivalent normal variables, the new algorithm can greatly improve the calculating accuracy of probability of the welded pipes with cracks. The examples show that this method is simple, efficient and accurate for reliability safety assessment of the welded pipes with cracks. It can save more time than the Monte Carlo method does, so that the improved FOSM method is recommended for engineering reliability safety assessment of the welded pipes with flaws.

Numerical simulation and control of welding distortion for double floor structure of high speed train

The welding heat source models and the plastic tension zone sizes of a typical weld joint involved in the double floor structure of high speed train under different welding parameters were calculated by a thermal-elastic-plastic FEM analysis based on SYSWELD code.Then,the welding distortion of floor structure was predicted using a linear elastic FEM and shrinkage method based on Weld Planner software.The effects of welding sequence,clamping configuration and reverse deformation on welding distortion of floor structure were examined numerically.The results indicate that the established elastic FEM model for floor structure is reliable for predicting the distribution of welding distortion in view of the good agreement between the calculated results and the measured distortion for real double floor structure.Compared with the welding sequence,the clamping configuration and the reverse deformation have a significant influence on the welding distortion of floor structure.In the case of30 mm reverse deformation,the maximum deformation can be reduced about 70%in comparison to an actual welding process.

Fatigue Test of Homemade Z Direction Steel Welded T Tubular Joints

This paper introduces the process and result of fatigue test of steel (Z direction steel) welded T tubular joints used in offshore engineering. Detailed measurement of stress concentration factor, stress distribution, fatigue life and crack development has been performed. Through analysis, an empirical formula of stress concentration factor for T tubular joints, fatigue S-N curve and crack propagation rule are obtained.

Feature-based and objectoriented product information model for welding structure

Product information model for welding structure plays an important role for the integration of welding CAD/CAPP/CAM. However, existing CAD modeling systems are not capable of providing enough information for subsequent manufacturing activities such as CAPP and CAM. A new design approach using feature technique and object oriented programming method is put forward in this paper in order to create the product information model of welding structure. With this approach, the product information model is able to effectively support computer aided welding process planning, fixturing, assembling, path planning of welding robot and other manufacturing activities. The feature classification and representing scheme of welding structure are discussed. A prototype system is developed based on feature and object oriented programming. Its structure and functions are given in detail.

Numerical stress analysis of crack at welded beam-to-column connection

In the past, brittle fracture of steel structure was reported rarely under earthquake. However, recent earthquakes, especially Northridge Earthquake (USA) and Hyogoken Nanbu earthquake (Japan), astonished engineers in the field of construction. The experience from recent earthquakes of USA and Japan shows that brittle fracture of welded steel structure always starts from high stress zone with welded crack [1～5] . As backing bar for grooved weld on beam flange exists, artificial crack is formed because of lack of fusion at the root of flange weld. In this paper stress distribution of connection is computed with FEM, and stress concentration at the root of flange weld is also analyzed. Stress intensity factors (SIFs), K I, at the root of flange weld are computed in the method of fracture mechanics. The computation shows that stress intensity factor on bottom flange weld is obviously higher than that on top flange weld. It is proved by the fact that brittle fracture is liable to start at the root of bottom flange weld on actual earthquake [1,4] . Finally measures are brought forward to avoid fracture of weld structure against earthquake.

Feasibility Study on Welding Structure of the HT-7U Toroidal Field Coil Case

The Toroidal Field (TF) coil case of the HT-7U superconducting tokamak device is made of austenitic stainless steel 316LN and is designed to operate at cryogenic temperature (4 K). 316LN can retain high strength and fracture toughness at 4 K. Feasibility study on technical process of welding has been experimentally considered as a hopeful joint method for suppression of post-welding deformation and reduction of over-heating. Meanwhile the final range of stress intensity and the stress intensity factor (K) for pre-cracks of welding structure have been determined by using J-integral. These related results are optimistic and have shown that there's no problem in strength and fracture toughness at the vicinity of the pre-crack tip. This paper introduces the welding structure of TF coil case in detail.

Recent progress related with ultra-steels at NIMS

Ultra - steel Project performed at National Institute for Materials Science(NIMS) 1997 -2005 proposed "Breakthroughs for innovation by steel science".The main concepts were composed of decreasing the environmental load,no or less special alloying elements and safer infrastructure with reduced the total life cost.Four "double strength and double life" steels were developed as follows:①800 MPa-class ecologically beneficial steels for welded structures,②1 500 MPa steels with high resistance to fracture,③advanced ferritic steels for 650C-USC boilers and④Ultra-steels for structures in marine and offshore environments.World steel society is still growing with threatening limits of resources and energy.Application of the ultra-steel ideas would be increasingly helpful for the global sustainability.The talk will introduce the recent progress in steel research at NIMS,based on the project accomplishments.

NON-EQUILIBRIUM STATIONARY STATE IN CHEMICAL REACTION OF SiO_2/Fe AT INTERFACE OF SLAG/METAL AND ITS STABILITY DURING ARC WELDING

For characteristics of open and far from thermodynamic equilibrium in welding chemical reaction, a new kind of quantitative method, which is used to analyze direction and extent for chemical reaction of SiO2/Fe during quasi-steady state period, is introduced with the concept of non-equilibrium stationary state. The main idea is based on thermodynamic driving forces, which result in non-zero thermodynamic fluxes and lead to chemical reaction far away from thermodynamic equilibrium. There exists certain dynamic equilibrium relationship between rates of diffusion fluxes in liquid phase of reactants or products and the rate equation of chemical reaction when welding is in quasi-steady state. As result of this, a group of non-linear equations containing concentrations of all substances at interface of slag/liquid-metal may be established. Moreover the stability of this non-equilibrium stationary state is discussed using dissipative structure theory and it is concluded theoretically that this non-equilibrium stationary state for welding chemical reaction is of stability.

The Review of the Welding Robot’s Precision

Welding is an significant link in industrial production,and it is known as the"industrial tailor".However,welding fume,arc light,and metal spatter cause a harsh welding working environment,and the quality of welding has a decisive influence on product quality.Low production capacity,difficult recruitment,and low profits have become drawbacks for the development of the welding field.Combining traditional welding with robots can solve these drawbacks and increase the precision of welding.Welding robots are industrial robots engaged in welding and are mostly used in large-scale manufacturing fields such as automobile manufacturing.Welding robots are divided into spot welding robots,arc welding robots and laser welding robots.In addition,body accuracy and control accuracy are the two main factors that affect the robot welding accuracy.In this case,the sensing technology of welding robot is also essential.

Prediction of Large Structure Welding Residual Stress by Similitude Principles

On basis of the similitude principles, the conception of virtual simulative component and the auxiliary value of welding residual stress, which is deduced by the welding conduction theory, the relation of the welding residual stress between the simulative component and the practical component was attained. In order to verify the correctness of the relation, the investigation was done from the view of the welding experiment and the numerical simulation about the simulative component and the practical component. The results show that the distribution of welding residual stress of the simulative component is the same as that of the practical component. The ratio of welding residual stress attained by the experiment or the simulation method between the practical runner and the simulative component was compared with the ratio obtained by the similitude principles. Moreover, the error is less than 10%. This provides a new idea to predict the welding stress distribution of large practical structure by the contractible physical model, which is important for the welding experiment and the numerical simulation.

Residual stress determination in friction stir butt welded joints using a digital image correlation-aided slitting technique

This paper presents an incremental cutting method for evaluating the longitudinal residual stresses in a butt welded thin plate via combining the traditional residual stress measurement methods and the advanced optical technique.The proposed approach,which can be called digital image correlation（DIC）-aided slitting technique,introduces a successive extension slot to a specimen and employs the DIC technique to measure the released displacement profiles of the cutting sections after each cutting increment.Then the displacement profiles are used to directly calculate the residual stress distributions up to the slot tip and hence,a stress distribution can be obtained after a cutting increment.Finally,all of the stress distributions are averaged to ultimately determine the original residual stress field.This method does not include any complex experimental operations or tedious derivation,and the resolution of stress variation is greatly improved by the continuous measurement of the released displacements.The presented method has been preliminarily verified by a specimen with residual stress introduced by a four-point bending test.The results show that residual stresses determined by the DIC-aided slitting technique agree well with those from finite element（FE） prediction.The residual stress in a friction stir welded aluminum specimen obtained by the presented technique is also consistent with the evaluations given by X-ray diffraction.Furthermore,the residual stresses obtained by the DIC-aided slitting technique demonstrate higher accuracy and stability than the evaluations derived by the DIC-aided contour method.