Elevated temperature fatigue crack growth in alloy 718. I. Effects of mechanical variables: Ghonem, H., Nicholas, T. and Pineau, A. Fatigue Fract Eng mater Struct (May 1993) 16 (5), 565–576

Abstract

The experiments reported here were conducted using the framework of the strain-space formulation of plasticity. A servohydraulic multiaxial testing machine was used to perform combined axial-torsional experiments on a thin-walled cylindrical aluminum specimen. The specimen was machined from extruded aluminum 1100 tubing, and annealed at 650 °F for 1 h. Conical epoxy sections were molded on the ends of the specimen for gripping. The specimen was loaded at strain rates under 20×10−6 per minute. An offset strain of 5×10−6 was used to define yield. A novel technique for identifying yield surfaces is introduced, which makes use of an experimentally measured focal point through which all yield probes are made to pass. Yield surfaces were obtained both in stress space and in strain space at various stages of plastic deformation. Although all strains were under 1%, the inelastic states included situations in which the origin in stress space lies outside the elastic region. The yield surfaces in both spaces were found to change shape considerably with deformation, usually developing a region of high curvature near the preload point and a region of low curvature on the opposite side of the yield surface. Experiments were also designed and conducted to test the validity of a prescription for identifying plastic strain in the context of the strain-space formulation. Changes in plastic strain were induced under incremental loading conditions and measured using the prescription. To within experimental error, the measured plastic strain increments satisfy the normality condition. The plastic strain increments are consistent with a flow rule of the type postulated in the strain-space formulation of the theory. The experimental results lend support to the constitutive theory and to the prescription for identifying plastic strain.