The proportional and nonproportional high-cycle fatigue behavior of mild-steel, tube-to-plate welded specimens was studied. In-phase bending and torsion, constant-amplitude fatigue tests were used to define the weldment baseline material properties and also to serve as a comparison with out-of-phase tests. In-phase, combined bending-torsion experiments were conducted on the tube-to-plate specimen to simulate the common laboratory tests involving stationary loads on a beam. Out-of-phase combined bending-torsion loading paths were generated for a tube-to-plate specimen to simulate cyclic stress states typical of highway bridge beams subjected to a single passage of a vehicle. ; The fatigue life of the tube-to-plate weldments was found to be load-path dependent for a bending-to-shear-stress ratio $\lambda$ = 2.39. For the load history considered, in-phase loading was found to give an order of magnitude longer fatigue life than an out-of-phase loading having the same principal stress amplitudes. Approximately 80% of the total fatigue life of the tube-to-plate weldments was spent in initiating fatigue cracks in the life regime of 10$\sp5$ to 10$\sp7$ cycles for all loading paths studied. ; The fatigue test results were correlated with various multiaxial fatigue damage parameters based on the local, notch-root stresses from finite element analyses of the weld-toe notches. The tests results for both the in-phase and out-of-phase load histories could be correlated using Findley's equivalent shear stress model. The in-phase and out-of-phase test data could not be correlated using a maximum principal stress model, but a model based on a von Mises effective stress did better than the former but not as well as Findley's model. The ability of the current fatigue design codes to deal with nonproportional loading cases was evaluated. ; Made available in DSpace on 2011-05-07T13:44:54Z (GMT). No. of bitstreams: 2 license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5) 9136736.pdf: 2886300 bytes, checksum: ...