Dr. Rajil Saraswat and Dr. Daowu Zhou
TWI Ltd, UK
ESI UK Ltd, UK
Roger O' Brien
ThyssenKrupp Tallent Ltd, UK
Paper presented at NAFEMS 2010, Oxford, UK. 8-9 June 2010.
OPTWELD (Real-time virtual prototyping tools for OPTimising WELDed products) is a UK Technology Strategy Board (TSB) initiative to develop software for faster distortion prediction and fatigue life calculation. Hot-spot stress-based fatigue life prediction is used in the OPTWELD project.
The fatigue module in OPTWELD takes into account welding residual stresses and distortion in the structure. This is done by carrying out a thermal-metallurgical-elasto-plastic finite element analysis to predict the residual stress. This is followed by elastic analyses under unit loads on the distorted structure. A spectrum of applied loads is then used to calculate the applied stress range spectrum which is also modified to account for the residual stresses. The fatigue damage factor is finally evaluated using a Miner's summation rule.
The calculations have shown that the estimated fatigue life is longer when the benefit of any compressive residual stress is included. This is advantageous since it can reduce unnecessary replacement or repair of the structures.
Weld models are now more frequently being used by industry to optimise welded joint design to reduce distortion. However, it is of concern that the required thermal-metallurgical-elasto-plastic models are time consuming. This is a major bottleneck for large welded structures where a full scale 3D model is impossible to execute. In recent years the scientific community has developed approaches to minimise runtimes. OPTWELD is a recent initiative funded by TSB to develop a user-friendly interface for non-experts to a software platform which can be used to carry out analyses of large structures in reasonable time frames.
The software at present has distortion and fatigue modules. The fatigue module is used to predict the fatigue life for the welds in the structure. A survey was carried out amongst the partners in the OPTWELD consortium. This showed that stress based fatigue assessments are predominantly used for welds. The hot spot stress based fatigue life calculation method [Niemi et al 1997] is therefore being implemented in the OPTWELD fatigue module.
Welding residual stresses can have a substantial effect on the fatigue life of the weld. Residual stresses are usually assumed to be high and tensile in order to ensure a conservative life prediction [BS7608 1993]. The fatigue lives of welded structures are therefore assumed to be independent of the mean stress. However, this is sometimes over-conservative and therefore results in using thicker sections leading to higher costs. The OPTWELD fatigue module accounts for the effect of the welding residual stress which is calculated from the distortion simulation module using a thermal-metallurgical elasto-plastic stress analysis. The hot spot stress is then calculated based on the distorted structure instead of the original un-welded structure which enables the inclusion of the secondary stresses due to geometric change. The benefit of any compressive residual stress is also taken into account and it is expected that the predicted fatigue life will be longer in many cases. This paper gives details of the fatigue module via a case study of a simple T-joint.
The fatigue module requires predicted stresses very close to the weld toe. The conventional thermal-metallurgical-elasto-plastic analysis is carried out and the results are fed into the fatigue module. The fatigue module requires that certain element/node sets describing the weld are defined in the distortion module. The fatigue module only operates on these pre-defined regions instead of the whole model. The fatigue module is divided into several sub-modules to carry out various tasks as shown in Table 1.
Table 1: Sub-modules in the fatigue analysis software
| Mesh reader
|| Stress superposition
|| Structural hot spot stress
|| Cycle counting
|| Damage calculation
3: Case study
A 3-D transient, sequentially coupled thermo-metallurgical-mechanical model was developed using the commercial finite element software, SYSWELD, to predict the evolution of the temperature and stress/strain in a fillet joined steel plate as shown in Figure 1. A moving heat source with energy of 0.38kJ/mm was specified for the weld. Quadratic brick elements were used in the weld region. The temperature distribution was evaluated in the thermal analysis and used as thermal strains in the mechanical analysis to calculate displacements and stresses.
Figure 1: Details of the welded structure
The residual stress in the direction transverse to the weld is shown in Figure 2. The transverse stress is primarily tensile for most of the weld. In the regions away from the weld and close to the boundaries the transverse residual stress turns compressive to balance the stresses.
Figure 2: Transverse residual stress (MPa) contours
The fatigue damage factor was calculated for two types of loading on the structure. A unit normal and a shear force were applied on the top edge of the vertical plate. Elastic analyses were carried out to calculate the stresses due to these applied loads. A loading spectrum of these two load types was defined. The fatigue analysis program was used to predict the damage factor at the nodes on the weld toe using Miner's rule. The class 'D' S-N curve in BS7608 was used to calculate the cycles to failure. The output of the fatigue analysis program is shown in Figure 3. The damage factor is higher for Toe 1 compared with Toe 2 since the plate has a lower thickness here and consequently has a higher stress level. The life at Toe 1 is also calculated for the full stress range as if the residual stress was at yield magnitude (conservative).
Figure 3: Damage factor prediction for the nodes on the weld toe
Real-time virtual prototyping tools for OPTimising WELDed products - OPTWELD is a collaboration between the following organisations: TWI Limited, ESI UK Ltd, Newcastle University, University of Greenwich, BAE Systems Surface Ships Ltd, ThyssenKrupp Tallent Ltd, and BAE Systems Land System Ltd. The Project is managed by TWI Limited and is partly funded by the TSB under the Technology Programme ref: 'TP/8/ADM/6/I/Q2068B'. The present work is also funded by Industry Members of TWI, as part of the Core Research Programme.
BS7608 (1993) 'Code of practice for fatigue design and assessment of steel structures', British standard institution.
Niemi E, Fricke W and Maddox S J (1997) 'Fatigue analysis of welded components', International Welding Institute, IIW-1430-00.
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