A new theory for multiaxial fatigue is presented that is based on a physical interpretation of the mechanisms of fatigue crack growth. It may be represented graphically by contours of constant life, which are expressed mathematically by where ε1, ε2 and ε3 are the principal strains, •ε1 ≥ ε2 ≥ ε3. This equation underlines the importance of strain parameters in correlating fatigue data. It illustrates the effect of both the shear strain and the tensile strain normal to the plane of maximum shear. The theory is compared with several classical and recent theories, which are briefly reviewed. It is shown that classical theories of fatigue failure cannot correlate experimental data, and may be dangerous if used for design purposes.

A new theory for multiaxial fatigue is presented that is based on a physical interpretation of the mechanisms of fatigue crack growth. It may be represented graphically by contours of constant life, which are expressed mathematically by where q, 6% and E~ are the principal strains,This equation underlines the importance of strain parameters in correlating fatigue data. I t illustrates the effect of both the shear strain and the tensile strain normal to the plane of maximum shear. The theory is compared with several classical and recent theories, which are briefly reviewed. I t is shown that classical theories of fatigue failure cannot correlate experimental data, and may be dangerous if used for design purposes.

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the problem associated with short crack growth, defined as situations in which the intensity of thc crack tip field is underestimated by linear elastic fracture mechanics unalyses, is briefly reviewed.Two cases are identified. cracks growing in plastically strained materials. such as occurs in high strain fatigue studies and at notch roots. and small cracks growing in single grains as occurs close to the fatigue limit in plain specimens.Important mechanical and metallurgical features of shor: cracks are discussed with particular reference to the upper and lower bound definition of a short crack.

Two limiting thresholds to fatigue crack propagation are discussed. The first threshold is related to the microstructural texture and this threshold may therefore be deemed a material-based threshold. The second threshold is mechanically-based, and is related only to the stress state at the tip of a substantial defect.The material-based threshold is characterized in terms of Microstructural Fracture Mechanics (MFM) and the mechanically-based threshold is characterized in terms of Linear Elastic Fracture Mechanics (LEFM). The former condition is important when considering the fatigue limit of materials and components, while the latter is more applicable to the fatigue limit of structures.The different factors which affect the two threshold conditions are briefly presented. Finally, this paper discusses aspects of MFM relevant to the fatigue resistance of metals and components.