An anisotropic damage mechanics model is presented to describe the behavior and failure of concrete\nunder biaxial fatigue loading. Utilizing the approach of bounding surfaces, the limit surface\nbecomes a special case when the number of loading cycles is set to one. By increasing the number\nof loading cycles, the strength of concrete gradually decreases and the limit surface is allowed to\ncontract and form new curves representing residual strengths. The magnitude of loading, load\nrange, and the load path are known to influence the fatigue life and hence are addressed in this\nformulation. In this paper, a strength softening function is proposed in order to address the reduction\nin the strength of concrete due to fatigue. Separate softening functions are also proposed\nto account for the deformation characteristics in concrete under cyclic loading. Numerical simulations\npredicted by the model in both uniaxial and biaxial stress paths show a good correlation\nwith the experimental data available in the literature
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