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Demonstrating defect-free removal of weld face and weld toe undercuts

Weld geometry improvement methods have been widely investigated and have in most cases been found to give substantial increases in fatigue strength.[1] By removing the weld face and any weld toe undercuts, typically by manual belt or disc grinding, a more continuous weld geometry may be created, and the presence of stress concentrations in structures may be significantly reduced as shown in this formular by Niu og Glinka [2]:

Kw = 1 + 0,5121 · θ0,572 · (t/p)0,469

Where           Kw                 is the notch stress concentration factor due to the weld profile;

θ                   is the weld flank angle in radians;

p                   is the weld toe radius;

t                    is the plate thickness.

The International Institute of Welding has, however, pointed out that there are large variations in the actual improvements achieved, and one reason for this variation is that the effectiveness of the process depends heavily on the skill of the operator. Smith and Hirt [3] points out that grinding defects introduced may be just as serious as those removed, and that manual weld grinding may not remove all weld toe undercuts.

Together with RTOs FORCE Technology and Danish Technological Institute, we aim to demonstrate that our MAX TM401 mobile CNC robot will deliver fatigue strength improvement in wind turbine support structures at high rate and in a continuous and uniform quality and without introducing or compounding weld defects.

The robot is designed to remove weld face and weld toe undercuts from both inside and outside circumferential welds in monopiles, transition pieces and tower sections in a single-cycle operation and at a rate of 72 meters per productive machine hour.

Thank you to MADE for supporting this work and partnership with a demonstration project grant.

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[1]: P. J. Haagensen and S J. Maddox, IIW Recommendations on Post Weld Improvement of Steel and Aluminium Structures, Revised 16 February 2004

[2]: X. NIU and G. GLINKA, The weld profile effect on stress intensity factors in weldments in International Journal of Fracture 35:3-20 (1987)

[3] Ian F. C. Smith and Manfred A. Hirt, A review of fatigue strength improvement methods in Canadian Journal of Civil Engineering, February 2011

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