NeT bead-on-plate round robin: Comparison of residual stress predictions and measurements
Introduction
NeT Task Group 1 (TG1) was formed in May 2002 to examine the benchmark problem of a single weld bead laid down on the top surface of an austenitic stainless steel plate. This weld geometry produces a strongly three-dimensional residual stress distribution, with similar characteristics to a weld repair, in a compact, portable specimen which is amenable to residual stress measurement using diverse methods. The single weld pass is relatively straightforward to model using finite element methods, and allows full moving heat source residual stress simulations to be performed in practical time scales. However, because the weld bead is laid onto a relatively thin plate, the predicted stresses are very sensitive to key modelling assumptions such as total heat input and the thermal transient time history, making the bead on plate a challenging benchmark problem. TG1 have organised and performed two parallel round robin activities centred around the single bead-on-plate specimen, covering residual stress measurement and prediction.
Four nominally identical bead-on-plate specimens (see Fig. 1) were manufactured under closely controlled and documented conditions, and appropriate materials characterisation testing was performed on both plate and weld material. Two protocol documents were written to define and control the experimental residual stress measurement and finite element residual stress prediction round robins, respectively. These are summarised elsewhere [1]. The specimens were then circulated to TG1 measurement round robin participants to perform residual stress measurements. In parallel, the finite element analysis protocol was circulated to TG1 analysis round robin participants to allow them independently to predict the transient welding temperature history and the weld residual stresses in advance of measurements being available. The results from this initial round robin are referred to as Phase 1 results. Predicted and measured temperatures and stresses were compared in detail in December 2004.
After completion of the phase 1 activities, two of the bead-on-plate specimens were destructively examined to confirm the weld fusion boundary geometry, and a number of parametric studies performed to confirm the importance of selected finite element solution variables. The understanding gained from the phase 1 round robin and subsequent parametric studies was then used to update the finite element analysis protocol and initiate a phase 2 analysis round robin, which is still underway.
This paper considers the phase 1 residual stress predictions made by Task Group 1, and has the following objectives:
- 1.
To assemble together the results of the Phase 1 mechanical finite element analyses and compare predicted residual stresses with measurements.
- 2.
To understand the influence of key thermal analysis variables on predicted residual stresses.
- 3.
To understand the cyclic thermo-mechanical loading history in the bead-on-plate specimens.
- 4.
To understand the influence of the assumed material constitutive behaviour on predicted residual stresses.
It makes use both of the phase 1 analyses submitted by December 2004, and of the parametric studies performed up to May 2005. Any analyses performed after that date are deemed to be phase 2 analyses, and are not considered here. A companion paper in this special issue describes and reviews the phase 1 transient temperature predictions [2], and should be read first.
Section snippets
Mechanical modelling assumptions
The participants in the phase 1 residual stress prediction round robin are listed in Table 1. The most important analysis variables are summarised in Table 2 for the main study and Table 3 for the parametric studies.
All the participants took advantage of symmetry to analyse half models, with a mesh size varying from 4850 to 25,522 elements, using both eight and twenty node solid elements. Three analysis codes were used, ABAQUS [3], ANSYS [4], and SYSWELD 2004 [5]. All but one analysis made use
Overview of predicted residual stress field
Fig. 2 presents typical contour plots of longitudinal and transverse stresses viewed from the top of the plate, with a cross-section along the centre of the weld bead.
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High tensile transverse stresses are predicted beneath the weld bead, peaking beneath the weld stop end, and also on the top surface of the plate alongside the weld bead, again peaking at the weld stop end. The tensile stresses are balanced by transverse compressive stresses beyond the ends of the bead.
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High tensile longitudinal
Discussion
The response on the mid-length through-wall line B–D provides the best insight into the impact of mechanical solution variables on the predicted stresses. Here, weld metal undergoes a simple tensile cool-down cycle, and the final stress is determined by the level of equivalent plastic strain and the assumed stress–strain behaviour. The choice of isotropic or kinematic hardening is not relevant. Transverse stresses in weld metal are generally not sensitive to the weld metal material properties,
Detailed conclusions
- 1.
The use of isotropic hardening leads to over-conservative predictions of stress in the single bead-on-plate specimens, particularly in the longitudinal direction.
- 2.
The use of non-linear kinematic hardening leads to the most accurate predictions of stress.
- 3.
Correlation between the “best” non-linear kinematic finite element prediction and the mean of the measurements is reasonable, but there is much room for improvement. No participants used mixed isotropic-kinematic hardening schemes, so their
Acknowledgements
We acknowledge the contributions of all members of NeT TG1, in particular the simulation round robin participants listed in Table 1. The specimens were manufactured and welded by Doosan Babcock. The support of British Energy Generation Ltd for the comparative analysis is also gratefully acknowledged. This paper is published by permission of British Energy Generation Ltd.
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