Abstract
This paper summarizes the principal features of composites’ responses when subjected to constant amplitude (CA) cyclic loadings. The stochastic nature of the responses; the absence of a detectable fatigue limit; the sudden drop of strength; the general validity of the strength-life equal-rank assumption (SLERA); and, ultimately, the residual strength-life equal-rank assumption (RSLERA) are discussed on the basis of the selected experimental data available in literature. The objective is defining a robust test in order to ascertain the reliability of the phenomenological models. A two-parameter phenomenological model accounting for the maximum cyclic stress, σmax, and the stress ratio, R = σmin/σmax, was used for guidance through the phenomenology of fatigue. It is concluded that the robustness of the models dealing with fatigue can be checked only when the characteristics of the composites’ responses are described simultaneously with fixed parameters.
Highlights
The failure of traditional isotropic materials is generally dominated by a single crack nucleation and propagation, while in composite materials, it is attributed to the diffuse damage accumulation resulting from the synergistic effects of different damage mechanisms, eventually occurring at different time and length scales
This observation can enlarge the concept of strength-life equal-rank assumption (SLERA), namely, the stronger sample should have a longer life expectancy, and a higher residual strength under given loading conditions, and this allows for the modification of SLERA into residual strength-life equal-rank assumption (RSLERA)
The results reported so far outline the principal characteristics of composites’ responses when subjected to constant amplitude loadings
Summary
The failure of traditional isotropic materials is generally dominated by a single crack nucleation and propagation, while in composite materials, it is attributed to the diffuse damage accumulation resulting from the synergistic effects of different damage mechanisms, eventually occurring at different time and length scales. Materials 2019, 12, 2586 loadings show common features, despite the wide materials’ diversity These features include, but are not limited to, the following: (i) the statistical nature of the responses, (ii) the absence of any detectable fatigue limit, and (iii) the validity of the strength/residual strength-life equal rank assumption (SLERA/RSLERA), stating that samples with a higher static strength have both longer life expectancies and higher residual strength prior to the final collapse. Phenomenological models are used in industrial environments [3,4,5,6,7] as a basis for the structural design of specific components, where the fatigue characteristics of the constituent material are obtained at the level of coupons This does not exclude that, when applicable, the fatigue models based on a deep understanding of the damage mechanisms correlated to the macro properties may drive more reliable design tools.
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