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Life prediction methodologies based on low-cycle fatigue concepts are well established for the evaluation of component fatigue behavior. A critical assumption made in applying these concepts is that the fatigue damage process in the critical region of the component is similar to that which occurs in the smooth specimen used to characterize the material fatigue behavior. This approach has been verified experimentally for uniaxial loading conditions but methods for applying these concepts to multiaxial fatigue problems are not yet established.
Multiaxial fatigue theories that relate to the physical damage processes have shown the most promise for reliable design criteria. Two test programs are reported and discussed to further the understanding of the physical damage processes. First, thin-wall tube specimens have been tested in combined tension-torsion strain controlled loading. Second, notched shaft specimens, designed to represent a typical component, have been tested in combined moment control torsion-bending. For both test series, the development and growth of fatigue damage from initiation to failure is reported.
The thin-wall tube geometry is considered the smooth specimen for multi-axial fatigue. Tests on this geometry establish the expected damage behavior for the normalized SAE 1045 steel used in this investigation. Damage initiates on planes of maximum shear strain. At long lives a single crack develops and eventually grows perpendicular to the maximum principal strain. At short lives extensive damage develops on shear planes and failure occurs rapidly by a crack linking process. For equivalent local strain states similar behavior is expected for the notched shaft. However, initiation and early damage development in the notched shaft occurs in the circumferential direction of the notch, rather than on maximum shear planes. At long lives crack growth resulting in failure occurs on planes perpendicular to the maximum principal strain. At short lives extensive damage occurs in the notch and failure results by crack linking perpendicular to the primary bending stress.
Specimen geometry and stress gradients have a significant influence on damage development during multiaxial loading of components. The damage processes in the notched shaft are not represented by the thin-wall tube tests. Increased understanding of the detailed micromechanisms of fatigue represents an important direction for future improvements in life prediction methods.
Biaxial fatigue, multiaxial fatigue, notch effects in fatigue, stress gradients, crack initiation, crack growth, damage, Stage I-Stage II cracking
Research Engineer, General Electric Company, Cincinnati, OH
Research Scientist, G.K.N. Technology Limited, Wolverhampton,
Engineering Science Engineer, Garrett Turbine Engine Company, Phoenix, AZ
Professor, University of Illinois at Urbana-Champaign, Urbana, IL