A proposal to Investigate Stitched Composites Undergoing Delamination Using Multiscale Modeling Approach
Carbon fiber-reinforced composite (CFRC) materials are extensively used in the aerospace industry to enable significant weight savings due to their high in-plane specific strength and stiffness. However, this benefit is countered by their low out-of-plane properties, such as interlaminar strength, that make CFRC structures susceptible to delamination. To prevent delamination, through-the-thickness stitching has been shown experimentally alleviate the damage propagation due to impact in CFRCs. Material optimization of stitched composites is required to reduce delamination at a macroscale. Atomistic to macroscale structure-property relationships need to be established and quantified to reduce delamination behavior of stitched composites. This proposal presents a pathway to develop hierarchical multiscale modeling approach from all length scales to reduce delamination.
Multiscale Modeling Approach
The multiscale modeling approach will be performed at all individual length scales for both the epoxy and carbon fiber constituents. These length scales are the structural, macro, meso, micro, atomistic, and electronic length scales. At the atomistic level, atomistic potientals are required to study the molecular behavior of epoxy chains and carbon-fiber crystalline structure under deformation. These atomistic potentials can be calculated from Density Functional Theory and the Modified Embedded Atom Theory (MEAM). MEAM has been previously used to calculate the interatomic potential for saturated hydrocarbons. However, MEAM theory has not yet been extended for cross-linked epoxy polymers that are not hydrocarbons. Therefore, a part of this research will be used to develop interatomic potentials using MEAM for highly cross-linked epoxies.
Using the interatomic potentials from MEAM, molecular dynamic (MD) simulations will be performed to understand polymer chain mobility and the crystalline structure of the carbon fiber. The strain rate mechanisms at the atomistic level will be evaluated and upscaled to a macroscale continuum model. Additionally, course-graining MD can be used to reach higher length scales to study the void nucleation behavior that results from cavitation, crazing, and chain scission at the atomistic level. Interaction studies of the carbon fiber will also need to be performed to evaluate the interfacial shear strength and interfacial stiffness between the carbon fiber and epoxy. Recent studies have shown that the interfacial stiffness can vary near the graphite atoms with different surface chemical groups to promote adhesion.
Information regarding void nucleation can be incorporated into a micromechancs finite element model (FEM) to investigate void and crack interaction. Void and crack propagation can be studied due to their interaction in polymer stitched composites at a macroscale continuum level. Surrogate optimization techniques such as design of experiments and ensemble weighted method can be subsequently employed to minimize the delamination behavior at the structural scale.
Experimental Research for Upscaling Length Scale Behavior
Experimental research is needed to understand and statistically quantify significant length scale behavior in order to include their effects at higher length scales. Therefore, a design of experiments approach will be used to evaluate the effect of each length scale factors on the subsequently higher length scales. For instance, Changwoon et al. reported that cross-link density and chain mobility can affect macroscale properties of polymer thermosets. Different levels of cross-linking and chain mobility will be evaluated to understand their significance at higher a macrolength scale. This research will provide validation of the models being used are appropriate with respect to experimental data.
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