Application of welding simulation to block joints in shipbuilding and assessment of welding-induced residual stresses and distortions

International Journal of Naval Architecture and Ocean Engineering.
2014.
Jun,
6(2):
459-470

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

- Published : June 30, 2014

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Ship structure, Block joint, Welding-induced residual stress, Distortion
;
Numerical welding simulation

INTRODUCTION

The welding simulation is today a wide-spread method to predict the welding-induced distortions and residual stresses. Not the process is modelled here, but the heat-input into the weld.
First attempts to explain the effects of the weld process in a scientific way started with the development of heat conduction models. Already in 1941,
Rosenthal (1941)
published a set of formulae for the calculation of the temperature field due to a moving heat source. 1952 appeared one of the detailed books by
Rykalin (1952)
for the calculation of the temperature development during welding. In the 1980’s, these theories were further developed by
Lancaster (1986)
. Different partial phenomena such as the formation of the arc with plasma flux, the vaporization at the surface of the molten weld pool, the melting and dropping of the electrode as well as the forming of the weld bead surface were increasingly included in the investigations (
Karlsson and Lindgren, 1990
;
Lowke et al., 1992
;
Choo and Szekely, 1994
;
Hoffmeister, 1986
;
Ruyter, 1993
;
Böllinghaus, 1995
). Up to then the individual models were only partially combined to models for engineering purposes (
Haibach, 2002
;
Radaj, 2003
;
Rörup, 2003
;
Radaj et al., 2006
). The accuracy of the simulations was determined in the past mainly by the limited computer capacity. With the increasing development in this area, new possibilities exist to increase the accuracy of the models.
Generally, different levels of accuracy of such simulations exist, starting from the modelling, followed by the implementation of the temperature-dependent material data until the consideration of the phase transformation. The more parameters are considered in the computation, the larger will be the time required for the solution. For this reason, the considered geometry is far idealized in many cases, however, it has been seldom verified up to now if the original geometry behaves in the same way, i.e. the results are comparable and transferable. Therefore, the main objective of the investigation presented here is to develop a method which allows the computation of relatively large geometries. A block joint in the forward part of a RO/RO vessel is considered. Local regions, idealized with small models, are compared with the results of the original geometry (block joint) so that conclusions regarding the comparability and transferability of the results from small models can be drawn and the simplified models can be used during fatigue design. All computations were performed with the finite element program ANSYS.
PROCEDURE

In addition to the computing time mentioned above, another problem exists during the investigation of large structures: The validation of the results is frequently impossible. Therefore it is very important to verify the algorithm chosen. The application of the welding simulation to the block joint considered requires simplifications because otherwise computation times of several months would have to be expected. A stepwise reduction of the mesh density is performed, starting point for the calibration of the different mesh density is a reference model which was thoroughly investigated and validated in an extensive research project (
Zacke and Fricke, 2011
). As shown in
Fig. 1
, a comparison model is calibrated against this reference model, the former containing the necessary simplifications in order to transfer the process to the block joint (original geometry).
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REFERENCE MODEL

The reference model consists of a plate with a 250
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Heat input.

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COMPARISON MODEL

A great number of approaches exist to simplify the algorithm for calculating residual stresses. The objective of the calculation has to be taken into account and determines or limits admissible measures; therefore, these have to be defined at first: the residual stress state of the original geometry (block joint) shall be determined. It is not wanted to compute the exact residual stress distribution in the local weld region where very high temperature and stress gradients occur which would require a very fine mesh. This local area is restricted to about 2.5 times the weld seam width acc. to
Osawa et al. (2007)
.
Furthermore, it is assumed that phase transformation and the associated changes in material data do not have a significant effect on the development of residual stresses. The effects of phase transformation are, of course, known and can be considered in specialized simulation software, however, the welding simulations of the reference model (
Zacke and Fricke, 2011
) have shown that the distortions and the reaction forces can be well determined when neglecting phase transformation.
Apart from the knowledge of the input parameters, decisive is the necessary mesh fineness when performing a welding simulation. Normally, element lengths of a few millimetres are used which lead to an enormous number of nodal points. The requirement of small elements is due to the high temperature gradients. If these are not correctly reproduced, the resulting strains and stresses and also the reaction forces cannot correspond to reality. Therefore, the temperature field with its input parameters and mesh density are calibrated with measurements. The temperature distribution is created by a heat source which represents the welding nozzle respectively the feed wire at a definite instant of time. As the nozzle is moving continuously along the weld, the question has to be answered how the increments of the heat source are defined during the simulation.
Fig. 7
illustrates the situation.
From an engineering point of view, the essential problem is the correct heat input of the total energy and the formation of the three temperature gradients in space. As the focus is only on global residual stresses, it seems to be logical to calibrate the model on the basis of the measured reaction forces which result from the shrinking due to the thermal load. This has the advantage that a coarser mesh is possible as the final stage is compared and not the temperature field resulting from the first step of the analysis.
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- • Reduction of the multi-pass to a single-pass weld
- • Increase of the element length (and hence, the increment of the heat source)

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ORIGINAL GEOMETRY (BLOCK JOINT)

The original geometry is a block joint in the forebody of a 193
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VALIDATION OF THE SIMPLIFICATIONS

The procedure is validated by comparing the results of the original geometry (block joint) with those obtained for the comparison model containing the same simplifications and the reference model for which extensive measurements have been performed. These are the basis for the validation, see also
Fig. 1
.
As illustrated in
Fig. 14
, the reference model, which represents a section of the block joint due to its constraining conditions, corresponds to a section in the first plate field just above the inner bottom. The calculated residual stresses are compared with those in the reference model in the following.
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ASSESSMENT OF THE RESULTS

The welding simulation was performed as shown in
Fig. 12
in three sections. The computation time of clustered workstations was six days. The results presented in the following refer only to longitudinal membrane stresses in the components.
Fig. 18
shows these stresses in the shell after the weld in the third section has cooled down. The stresses represent the global stress state acting transverse to the weld line.
The first impression is that the welding of the block joint affects a large region when looking at the resulting residual stresses. The longitudinal stresses reach values between 11 and 33
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SUMMARY AND CONCLUSIONS

The paper describes the application of the numerical welding simulation to a block joint in shipbuilding. The main objective is the determination of the residual stresses occurring due to the constraint by the surrounding structure. Its effect was at first studied by 250
Acknowledgements

The investigations were performed within the project “Strength of block joints welded with wide gaps in shipbuilding” which was funded with public means within the programme “Industrial Cooperative Research” by the German Federal Ministry of Economics and Technology via the AiF and was coordinated by the Center of Maritime Technologies (CMT) in Hamburg.

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Citing 'Application of welding simulation to block joints in shipbuilding and assessment of welding-induced residual stresses and distortions
'

@article{ E1JSE6_2014_v6n2_459}
,title={Application of welding simulation to block joints in shipbuilding and assessment of welding-induced residual stresses and distortions}
,volume={2}
, url={http://dx.doi.org/10.2478/IJNAOE-2013-0192}, DOI={10.2478/IJNAOE-2013-0192}
, number= {2}
, journal={International Journal of Naval Architecture and Ocean Engineering}
, publisher={The Society of Naval Architects of Korea}
, author={Fricke, Wolfgang
and
Zacke, Sonja}
, year={2014}
, month={Jun}