Predicting the Fatigue Life of Welds under Combined Bending and Torsion
by
J.-Y. Yung
F. V. Lawrence, Jr
Departments of Civil Engineering and Metallurgy
Abstract
Tube-to-plate weldments were fatigue tested under combined bending and torsion loadings. The initiation and early growth of cracks were restricted to the weld toe. Compressive residual stresses were measured at the weld toe using X-ray methods. Thermal stress relief lowered the weld-toe residual stresses and reduced the fatigue life. Two approaches were investigated to predict the fatigue crack initiation life.
The first approach used three different strain-based parameters: the maximum shearing strain amplitude, the Lohr-Ellison parameter and the Kandil-Brown-Miller parameter. These parameters were combined with the Coffin-Manson equation to predict the fatigue crack initiation life under multiaxial cyclic stresses. The required notch-root strains were calculated using an elastic-plastic finite element analysis.
In a second approach, the Basquin equation was modified to estimate the fatigue life of welds for lives greater than 105 cycles. The worst-case-notch condition was assumed to be valid for both torsion and bending. The predicted fatigue lives were within a factor of three of the observed fatigue lives of the tube-to-plate weldments.
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