Concrete fatigue strain and fatigue modulus evolution play a vital role in the evaluation of the material properties. In this paper, by\nanalyzing the advantages and disadvantages of existing concrete strain analysis methods, the level-S nonlinear fatigue strain model\nwas proposed.The parameters� physical meaning, the ranges, and the impact on the shape of the curve were all discussed.Then,\nthe evolution model of fatigue modulus was established based on the fatigue strain evolution model and the hypothesis of fatigue\nmodulus inversely related fatigue strain amplitude. The results indicate that the level-S model covered all types of fatigue strain\nevolution. It is very suitable for the description of strain evolution of concrete for its strong adaptability and high accuracy. It was\nfound that the fitting curves coincided with the experimental curves very well, and the correlation coefficients were all above 0.98.\nTheevolution curves of fatigue strain modulus both have three stages, namely, variation phase, linear change stage, and convergence\nstage. The difference is that the fatigue strain evolution curve is from the lower left corner to the upper right corner, but the fatigue\nmodulus evolution curve is from the upper left corner to the right lower corner.
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