Experimentally Verified Thermodynamic Framework for Fatigue in Composite Materials

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The application of traditional fiber-reinforced composites in engineering products and structures is growing exponentially. While composite materials have many desirable properties, they are significantly susceptible to fatigue failure when subjected to cyclic or fluctuating loads. Complex failure mechanisms arise because of the elaborate makeup or meso-structure of composites, which are composed of layers, each with aligned or randomly distributed fibers embedded in host material. The presence of several material components and their distinct behaviors give rise to favorable as well as unfavorable interactions, especially at material interfaces. Existing fatigue failure modeling methods rely on empirical or observation-specific assumptions to account for the full spectrum of emergent behaviors, which has not proven to be a viable strategy except in special cases. This grant will support the development of an experimentally verified thermodynamic framework for rigorous, rapid, and accurate evaluation of fatigue performance and life of composite materials. The outcome will impact composite applications in industries such as automobile, aerospace, construction, transportation, and renewable power generation. The award will also support training of the next generation of skilled workforce at the graduate, undergraduate, and K-12 student levels as well as professionals via research participation, outreach at local high schools, and organization of industry days and development of a short course on fatigue, respectively.Inherent complexities associated with matrix cracking, delamination, and fiber breakage are among the intricacies that render the treatment of traditional composite fatigue very difficult. The objective of this research is to account for these failure mechanisms through a generalized thermodynamic entropy-based framework based on thermographic measurements. The hypothesis is that entropy, being tied to internal friction and energy dissipation, provides a natural time scale of performance degradation and aging of composites. That is, fatigue progression and failure can be described in terms of cumulative entropy generation during cyclic loading. The approach consists of in-situ thermal imaging of composite specimens during loading to quantify the entropy-based measures of fatigue life. Extensive characterization of the influence of ply layout, fiber distribution, constituent materials, and cyclic loading parameters will form the basis of the thermodynamics-based damage model, which will be used to predict remaining useful life as well as to investigate internal damage mechanisms with the help of experimental observations and validation. This project is jointly funded by the Division of Civil, Mechanical and Manufacturing Innovation (CMMI) and the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.