Fatigue of Titanium Welds: Models for Assessment of Defect Sensitivity

Welds produced by the Tungsten Inert Gas (TIG) process in riser grade titanium may contain pores, typically of sub-millimetre dimensions. Fatigue testing has proved that fatigue crack growth may be initiated from even very small pores. The size and existence of pores thus may be a governing factor for fatigue strength. For assessment of inspection and design criteria, tools for calculation of fatigue life from pre-existing pores are required. Numerical models for calculation of fatigue crack growth from internal defects were developed. The models were based on parametric formulae for the stress intensity factor for elliptic cracks in finite thickness plates. Coupons from TIG-welded titanium pipe were fatigue tested, and initial defects were identified and sized by Scanning Electron Microscopy. The test data was used for evaluation of the fracture mechanics models. Inherent uncertainties and limitations for fracture mechanics calculation of fatigue life were identified and discussed. By choosing appropriate values for crack growth threshold the accuracy of the calculated fatigue life was within a factor of two compared to the experimental data.