Issue 35

A. Mehmanparast et alii, Frattura ed Integrità Strutturale, 35 (2016) 125-131; DOI: 10.3221/IGF-ESIS.35.15 130 values in Fig. 6(b) compared to Tab. 1 might be associated with uneven V-grooves in ref [12], different filler metal properties (e.g. over-matched, under-matched) employed in reference [11] and [12], etc. The peak transverse stresses in thick welded plates used in offshore monopiles (see Fig. 1) are expected to be closer to the values predicted in [12] and summarized in Tab. 1, though more finite element simulations need to be performed to investigate this further. The residual stress measurement and prediction results for multi-pass welded plates available in the literature suggest that to improve the structural integrity of the offshore wind turbine monopile structures, suitable welding sequences which lead to lower values of damaging (i.e. tensile) residual stresses need to be employed in the fabrication processes. This will provide more robust and cost efficient offshore wind turbine structures. Moreover, by measuring the residual stress profiles in welded plates, the time required to inspect damage/cracks initiated in operating structures can be significantly optimized by prioritizing the inspection to be carried out on parts of the structure (e.g. inner surface or outer surface of an offshore monopile) containing tensile residual stresses as opposed to compressive stresses. In other words, in the parts of the offshore monopile welded structures that a tensile residual stress profile exists in a direction parallel to that of the dominant environmental loading axis, frequent inspection will be required since the chance of damage/crack initiation in this region is higher than anywhere else. The fatigue crack growth results from the tests performed on HAZ C(T) specimens in [3] suggest that the initial part of the FCG behavior with a smaller slope, which can be observed in the bi-linear trend, may be associated with the compressive residual stress effects whereas the latter part of the bi-linear trend, which shows a higher slope, may be related to tensile residual stress profiles remained in the C(T) weld specimens. In order to interpret the fatigue crack growth results performed on specimens extracted from the welded plates employed in offshore monopiles, and also to provide reliable remaining lifetime estimates of the welded structures operating in offshore environments, neutron diffraction residual stress measurements need to be performed on thick welded plates employed in fabrication of offshore monopiles (e.g. Fig. 1). These measurements need to be performed along the HAZ path (through thickness direction) to examine the significance of residual stress effects in the bi-linear fatigue crack growth behavior of C(T) specimens with the crack path located in the HAZ region. C ONCLUSIONS elding residual stress profiles have been found severely sensitive to the yield strength of the filler metal, heat input, plate thickness, fusion zone shape and welding sequence. Numerical studies of multi-pass butt welding in double V-groove thick plates have shown that the highest and the lowest tensile peak stresses are expected to appear when multi-pass welding is performed unevenly and evenly, respectively. The finite element prediction results have shown that the welding sequence influences the residual stress trends and peak values and the changes in residual stresses may be more pronounced in transverse (along X axis) direction compared to the through thickness (along Y axis) direction. However, higher values of damaging residual stresses are generally observed in the through thickness residual stress direction. Suitable welding sequences which result in lower peak values of tensile residual stresses in thick welded plates need to be employed in manufacturing of offshore wind monopiles. Residual stress profiles in offshore structures need to be measured to provide accurate interpretation of the crack growth behavior in offshore welded components. This information also helps to optimize the inspection time required to investigate damage/crack initiation and propagation in offshore wind turbine structures. R EFERENCES [1] Jata, K. V., Sankaran, K. K., and Ruschau, J. J., Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-T7451, Metallurgical and Materials Transactions A., 31 (2000) 2181-2192. [2] Akita, M., Nakajima, M., Tokaji, K., and Shimizu, T., Fatigue crack propagation of 444 stainless steel welded joints in air and in 3%NaCl aqueous solution, Materials & Design., 27 (2006) 92-99. [3] Adedipe, O., Brennan, F., and Kolios, A., Corrosion fatigue crack growth in offshore wind monopile steel HAZ material, in: C.G. Soares, R.A. Shenoi (Eds), Analysis and Design of Marine Structures V, CRC Press., (2015) 207-212. [4] Jang, C., Cho, P.-Y., Kim, M., Oh, S.-J., and Yang, J.-S., Effects of microstructure and residual stress on fatigue crack growth of stainless steel narrow gap welds, Materials & Design., 31 (2010) 1862-1870. W