Duncan Camilleri

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.

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.

Shakedown of a Thick Cylinder With a Radial Crosshole

The shakedown behavior of a thin cylinder subject to constant pressure and cyclic thermal loading is described by the well known Bree diagram. In this paper, the shakedown and ratchetting behavior of a thin cylinder, a thick cylinder, and a thick cylinder with a radial crosshole is investigated by inelastic finite element analysis. Load interaction diagrams identifying regions of elastic shakedown, plastic shakedown, and ratchetting are presented. The interaction diagrams for the plain cylinders are shown to be similar to the Bree diagram. Incorporating a radial crossbore, Rc/Ri=0.1 or less, in the thick cylinder significantly reduces the plastic shakedown boundary on the interaction diagram but does not significantly affect the ratchet boundary. The radial crosshole, for the geometry considered in this study, can be regarded as a local structural discontinuity and neglected when determining the maximum shakedown or (primary plus secondary stress) load in design by analysis. This may not be apparent to the design engineer, and no obvious guidance, for determining whether a crosshole is a local or global discontinuity, is given in the codes.

A Progressive Failure Analysis Applied to Fiber-Reinforced Composite Plates Subject to Out-of-Plane Bending

The ability to predict the structural response of composites offers a significant advantage to design engineers and provides the possibility of identifying structurally efficient composite assemblies. Various analytical and numerical models are possible, but care has to be taken to ensure that the appropriate structural performance and failure criteria are used. In particular, modeling the progressive failure of composite laminas requires robust and validated failure algorithms that are not only computationally efficient, but are also able to predict the load–deformation characteristics and to ultimately establish the failure load appropriately. This study looks into different progressive failure macromechanical algorithms applied to e-glass-fiber-reinforced composite plates subject to out-of-plane bending. The influence of different boundary conditions of the plates, ranging from fully clamped to simply supported ones, on their ultimate failure load is also investigated. The results are validated by experimental data found in the literature and show that boundary conditions have a significant influence on the predicted ultimate failure load. The study also shows that, in this case, the predominant failure mechanism is the failure of matrix, and after the redistribution of stresses, no consecutive failure due to fiber or fiber-matrix failure occurs in the lamina, therefore a sudden-degradation progressive ply failure algorithm based on the failure mode is sufficient to model the structural performance of composite plates subject to out-of-plane bending.

Local heat generation and material flow in friction stir welding of mild steel assemblies

In friction stir welding, assemblies are joined by means of plasticising, shearing and stirring non-molten material. The heat generation is directly related to the viscous behaviour of plasticised material, through coupled Navier–Stokes thermo-fluid flow stress equations. A significant amount of research has been conducted on aluminium friction stir welding but studies on mild steel assemblies are limited. The aim of this work is to understand the influence of the tool rotational and traverse speed on the resulting material stir zone shape and the heat power generated in friction stir welding of mild steel assemblies. A numerical and experimental approach is adopted in this study. Material visco-plastic properties are primarily established experimentally and are then applied to a computational fluid dynamics model through user-defined material flow stress constitutive laws. The model was further validated through a series of thermocouple and macrograph measurements and later on used to fulfil the aims of this work. This study identifies that the total heat generated for different welding parameters follows a non-linear variation with radial and angular tool position. These results provide a platform for the accurate definition of heat flux inputs and thermal strains to global thermo-elasto-plastic models, replacing more simplified linear specifications currently used in the literature.

Evaluating plastic loads in torispherical heads using a new criterion of collapse

In ASME Design by Analysis, the plastic load of pressure vessels is established using the Twice Elastic Slope criterion of plastic collapse. This is based on a characteristic load-deformation plot obtained by inelastic analysis. This study investigates an alternative plastic criteria based on plastic work dissipation where the ratio of plastic to total work is monitored. Two sample analyses of medium thickness torispherical pressure vessels are presented. Elastic-perfectly plastic and strain hardening material models are considered in both small and large deformation analyses. The calculated plastic loads are assessed in comparison with experimental results from the literature.Copyright

Thermal Stresses and Distortion Developed in Mild Steel DH36 Friction Stir-Welded Plates: An Experimental and Numerical Assessment

Welding processes involve localized heating which in turn give rise to thermal stresses and distortion. Friction stir welding (FSW) is a solid state joining process where temperatures below melting are experienced. Nonetheless, some degree of thermal heating and consequently thermal stresses develop at the joint. This study aims to quantify the stresses developed in friction stir welding of mild steel DH36 plates, through an experimental and numerical investigation. The temperatures and transient strains developed during FSW, are experimentally measured and used to validate thermo-elastoplastic numerical models. These models are used to investigate the evolution of thermal stresses and distortion for different welding parameters.