T.G.F. Gray

Computational prediction of out-of-plane welding distortion and experimental investigation:

The main aim of the work was to investigate a simplified finite element simulation of the out-of-plane distortion caused by fusion butt welding. The thermal transient part of the simulation made use of a finite element analysis of the two-dimensional cross-section of the weld joint and the thermoelastic-plastic treatment was based on analytical algorithms describing transverse and longitudinal deformations, leading to predictions of transverse angular deformation and longitudinal contraction force. These results were then applied to a non-linear elastic finite element model to provide predictions of the final angular and overall deformations of the butt-welded plates. The validity of the simulation was investigated via full-scale tests on 4m x 1.4m x 5 mm steel plates, butt welded using a flux-cored Ar-CO2 metal-inert gas process. Thermography and thermocouple arrays were used to validate the thermal transient computations and out-of-plane deformations were measured using displacement transducers for transient deformations and a laser scanning system to measure the profiles of the whole plates before and after welding. The results of six full-scale tests are given and comparison with the simulations shows that the procedure provides good prediction of the angular and overall out-of-plane deformations. Prediction accuracy requires account to be taken of initial shape, gravity loading, and support conditions.

Computationally efficient welding distortion simulation techniques

The studys aim is to improve the applicability of finite element analysis to the prediction of welding distortions, with particular emphasis on out-of-plane deformations. Robust simulation strategies are established which take account of material properties and joint configurations, but are at the same time computationally efficient. This is achieved by reducing the full transient thermo-elastoplastic analysis to variants of an uncoupled thermal, elasto-plastic and structural treatment. A two-dimensional cross-section thermal model was used to establish thermal transients. The maximum temperatures, experienced at each node during the welding cycle, were then used to link the thermal welding strains to the elasto-plastic and structural response of the welded structures. Three efficient models have been identified that reduce the transient analysis to a simple multi-load-step analysis and these were applied to sample butt-welded plates. The simulation techniques are supported by full-scale welding tests on steel plates. The evolution of angular distortion can be treated simply through non-linear, stepwise, static analyses. This captures important effects of restraint and material properties. The longitudinal bending distortion can then be established via simple elastic-perfectly plastic algorithms, through a fictitious elastic thermal load analysis.

Thermal distortion of stiffened plate due to fillet welds : computational and experimental investigation

ABSTRACT Prediction and control of thermal distortion is particularly important for the design and manufacture of multiply stiffened welded structures. This study aimed to develop and experimentally validate a comprehensive simulation tool to predict distortion, with particular emphasis on out-of-plane deformation generated in double-sided fillet-welded attachments. Simulation was used to optimise the relative positions of a twin-arc configuration, to give minimum out-of-plane deformation consistent with reasonable production time for single stiffener, double-fillet attachments. The critical buckling load of the structure was approached and exceeded as the arcs were brought closer and simulation allowed the influence of this factor to be determined.

Alternative simulation techniques for distortion of thin plate due to fillet-welded stiffeners

This study aims to develop and validate a wide-ranging simulation tool to predict welding distortion in stiffened plates and shells, with particular emphasis on out-of-plane deformation. The approach adopted in this study uncouples the thermal, elasto-plastic and structural effects leading to distortion. The computational models and results are supported by realistic welding tests and appropriate measurements to validate the simulated thermal fields and out-of-plane distortions. The simplest and most computationally efficient model makes use of algorithms, instead of numerical computation, to link the thermal welding strains to the elasto-plastic and structural responses of the welded assembly, via a static, single-load-step analysis. Alternative, more computationally intensive models are explored which simulate the full transient thermal and elasto-plastic structural responses in an uncoupled fashion. These provide a cross-reference for the more rudimentary but computationally efficient models. The experiments and computational strategies are applied to welded assemblies incorporating double-fillet-welded stiffeners.

Environmental Ageing of Adhesively-Bonded Joints. II. Mechanical Studies

The effects of exposure to moisture on the mechanical properties of a series of adhesively-bonded structures are reported. Changes observed in the maximum load, shear modulus, strain at maximum load, fracture energy, fracture toughness and stress are discussed and correlated with variation of the dielectric parameters. An initial increase in fracture toughness observed in the joints correlates well with the uptake of moisture having led to a lowering of the glass-rubber transition temperature. Differences in the ultimate strength and energy to failure for different surface pretreatments are observed. Loss in mechanical properties observed over the period of the study are paralleled by changes in the dielectric properties of the joints.

Computational methods and experimental validation of welding distortion models

Multiply-stiffened, thin plate, welded fabrications are used in a wide variety of transport fields, however the resulting out-of-plane distortion associated with welding exacts a severe design penalty. Depending on the information required, the size of the structure under investigation and the computer power at hand, three computational strategies may be considered to predict welding distortion. If prediction of the localized residual stresses from welding is of major importance, then a full transient, uncoupled thermo-elastoplastic analysis is preferred. This method is not readily applicable to predict welding distortions in industrial-scale welded structures. More computationally efficient models are required and two other models are suggested in the current study. A series of experimental tests of a realistic nature were performed to validate the proposed computational strategies. Computational and experimental study of butt and fillet welding of small and industrial size fabrications is considered.

Investigation of diesel generator shaft and bearing failures

Abstract A shaft failure in a 634 kW diesel generator after 4000 hours of operation required investigation, especially when similar sets began to show signs of excessive bearing housing wear. The failure was found to be due to torsional fatigue caused by operation at an unforeseen resonant condition. The damage to the bearing housing of this type of set was also attributed to this behaviour. Replacement of the bearings and housings and redesign of the flexible coupling has been undertaken to prevent further failures. Modifications to design procedures are now being adopted to ensure similar problems do not occur in other existing or new machines.

Experimental investigation and finite element analysis of welding induced residual stresses

Welding is one of the most important metal joining techniques due to its advantages in relation to speed and versatility. One of the unwanted effects of gas metal arc welding is residual stress, that arises due to the inherent temperature loadings and can lead to structural integrity or assembly problems. Experimental investigation of these effects is highly desirable but often costly, especially in large-scale construction such as shipbuilding. It is therefore of interest to exploit the potential of computational analysis techniques, such as finite element analysis, for the assessment of these effects. The use of finite element analysis in this context is however not straightforward, due to the complex nature of the problem. The work presented here deals with an experimentally validated modelling strategy applied to two weld joint configurations: butt and fillet welded plates. Simulation techniques are presented which are aimed at predicting residual stresses. These are validated against measurements carried out using the hole drilling method for residual stress determination. Results show a good qualitative and quantitative match in simulated and experimentally measured values, which back up the modelling techniques adopted.

Integration of Case Study Technical Investigations in Honours/Masters Engineering Courses

The main aim of the development described in this paper is to establish a practice whereby undergraduate engineering students exercise their growing technical knowledge and analytical capability in the context of real engineering problems. This is done by setting ‘technical investigations’, which are based on actual industrial or consulting problems. In the present case, these case studies are set within a relatively conventional, ‘engineering science’ course on the theme of structural integrity, but the principle and the benefits to learning would be equally valid if applied to other engineering themes. The background needs which have prompted the introduction of this element and the strategies used to integrate and assess such work in a conventional lecture course are discussed and concrete examples of suitable case studies are given. Technical investigations have now been developing over three academic sessions in the authors department and information from feedback questionnaires in the first and most recent periods is included. The conclusion is that the integration of such exercises has several benefits, in terms of mastery of the subject area and in relation to the development of professional skills.

Understanding welding distortion: thermal fields thermo-mechanical effects

A first introduction to welding mechanics is given, based at this stage on uncoupled analytical solutions for thermal and thermomechanical behaviour. Rosenthal’s well-known quasi-static solution for the temperature field caused by a fast-moving, concentrated heat source is used to demonstrate the influence of different material properties and welding process conditions on the temperature fields. An approximate thermo-mechanical solution due to Okerblom is used to demonstrate the roles of the thermal dilatation of the material being welded and the heat input on welding deformations. This solution is also used to illustrate the development of residual stress fields and the potential for buckling.