IIW White Paper

As another most important item, the prevailing stiffness (restraint intensity) in the repair area due to surrounding assembly groups and the associated strain constraint are usually much higher than during fabrication welding and have to be assessed before a repair welding procedure is carried out. For crack- resistant repair welding, knowledge of the stresses introduced by welding is of major importance, especially when the repair welding procedure has to be carried out at a high shrinkage restraint. Since the research efforts in welding have been concentrated on the respective materials and the development of new welding procedures, investigation of the design aspects of repair welding probably represents a major target for the near future. This involves correct and quantitative evaluation of shrinkage restraints for repair welds, including the effects of the various joint types and reinforcements. With this respect, it has to be anticipated that the intensity of restraint as a quantitative parameter to evaluate structural stiffness in the near and far field of a joint will gain increasing importance. Additionally, a precise knowledge of the thermo-mechanical effects of the repair welding procedure on the residual lifetime has to be elaborated. It is also essential to determine the welding sequences with lowest possible likely stress-strain distribution during and after the repair of steel structures. This will allow it to enlarge the available load spectrum for later service. In this context, it has to be mentioned that interaction of concurrent repair welds has not been understood up to the present time. The failure resistanceof a repairweld is alsodependent on the appliedfillermaterials and their transformation behaviour depending on metallurgical, welding and heat treatment parameters. With respect to the repair of steel structures and components, high-strength filler materials with correspondingly lowered martensite transformation temperatures have to be developed further to achieve lower residual stresses in the repair welding at respectively higher strength, i.e. service load capacities. It has also to be mentioned that a series of downstream methods are available for reducing welding-specific loads in repair joints or even for producing compressive residual stresses at the surface. Such technological procedures, like stress relieving, shot peening, peening, ultrasonic treatment etc., are generally very time- consuming and costly and should be developed further regarding better applicability to repair welding. It can only be emphasised that repair welding requires decent component weld tests, rational residual stress evaluation and respective numerical calculations to achieve an actual increase in the life time of a component or structure and to avoid further failure origins in the repaired parts. 4.4 Advanced design and structural integrity rules Recent advances in joining technologies together with newmaterials bring increased attention to the damage tolerance design, long service life and improved structural performance together with the developments in structural integrity assessment rules (e.g. BS 7910, API 579, R6, FITNET FFS) for the load-bearing structures. Recently, IIW Commission X has taken the task to develop IIW recommendations for the assessment of structural integrity of welded structures by taking into account recent developments in this field. IIW FFS Recommendations for Fracture Assessment of Weld Flaws (Doc. X-1637-08/Rev.3, Vol. I Procedure, Vol. II Annex) document is now in its 3 rd revision and available as a working document. For example, in the field of aircraft manufacturing of new welded integral airframe structures, specific “Local Engineering” considerations in design and fabrication have potential for further improvements in local laser weld joint properties. Established damage tolerance assessment rules for conventional (riveted) structures may need to be further developed for welded integral airframe structures. Fatigue and fracture assessments can be over-conservative with current methods and this in turn may act as a limiting factor for successful implementation of advanced joining technologies in airframemanufacturing. Therefore, R&D efforts for better understanding of failure mechanisms of joints, development and validation of testing, structural integrity rules and hence an overall roadmap for LBW and FSW welded integral airframe structures are needed.

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