Hydrostatic stress effects in low cycle fatigue

Classical metal plasticity theory assumes that hydrostatic stress has negligible effect on the yield and postyield behavior of metals. Recent reexaminations of classical theory have revealed a significant effect of hydrostatic stress on the yield behavior of notched geometries. New experiments and nonlinear finite element analyses (FEA) of 2024-T851 and Inconel 100 (IN100) test specimens have revealed the effect of internal hydrostatic tensile stresses on yielding. Nonlinear FEA using the von Mises (yielding is independent of hydrostatic stress) and the Drucker-Prager (yielding is linearly dependent on hydrostatic stress) yield functions were performed.; Mechanical tests were performed to characterize the material properties of two metals, IN100 and 2024-T851. In addition, monotonic and low cycle fatigue tests were performed on several notched round bar (NRB) geometries to use for comparison with the finite element results.; To perform the cyclic finite element analyses, a pressure-dependent constitutive model was developed as an ABAQUS user subroutine (UMAT). This UMAT incorporates the Drucker-Prager yield theory with combined multilinear kinematic and isotropic hardening. Finite element models (FEM's) of a variety of test specimens were created including: smooth tensile, smooth compression, NRB, and equal-arm bend geometries. For all cases, load-displacement or load-microstrain test data was compared to von Mises and Drucker-Prager finite element solutions.; For the monotonic tensile loading, the Von Mises solutions overestimated experimental load-displacement curves, while the Drucker-Prager solutions essentially matched the test data. For the low cycle fatigue tests, using a yield function that is dependent on hydrostatic stress significantly altered the predicted hysteresis response of notched specimens, particularly for the first few cycles. Specifically, for the 2024-T851 and IN100 test specimens, the Drucker-Prager solutions more accurately predicted the specimen's behavior for first few cycles compared to the von Mises solutions. However, once the stable material response was reached, neither the Drucker-Prager nor von Mises results were entirely satisfactory. Also, neither solution truly captured the shapes of the hysteresis loops.