Menna, Costantino (2013) Multiscale damage modeling of advanced composite materials. [Tesi di dottorato]

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The use of composite materials has spread over the years throughout the engineering areas of structures. The technological progress in this field has recently expanded, resulting in the design of new composite configurations, including multilayered composite materials and multifunctional nanostructured materials. Even though traditional and emerging composite materials offer wide potentialities for engineering, a significant challenge is still open with respect to damage phenomena. Driven by safety requirements and cost-effective optimization needs, damage modeling has gained a fundamental role for composite engineering. It represents a strong motivation to support design procedures by means of numerical methods, such as finite element analyses. Recently, multiscale computational analyses effectively gained a major role within the challenging task of damage prediction. Particularly, by bridging physical phenomena occurring at different scales, i.e. macro, meso, micro and even nano, damage evolution can be accurately predicted. The present work is collocated within this scenario with the aim of exploring and addressing different critical issues related to the failure mechanisms acting at different length scales of different composite systems. The multiscale procedures, proposed to evaluate the damage behavior of such materials, involved experimental, analytical and numerical tools. In detail, damage modeling has been performed for different case studies: i) GFRP composite laminates, ii) phenolic impregnated skins/honeycomb Nomex core sandwich structures, iii) Carbon Nanotube/Nanofiber modified S2-Glass/epoxy composites. For the case study i), the activity concerned the damage occurred in case of low-velocity impact tests, carried out on glass fabric/epoxy laminates. In this case, the multiscale modeling was implemented to account for both intralaminar and interlaminar levels of damage occurring within the composite laminate. This allowed to characterize the critical parameters acting at the smaller (interlaminar) scale which affect the macroscopic impact response of the composite laminate. With reference to honeycomb sandwich structures of case ii), due to their hierarchical structure, a multiscale approach was necessary in order to suitably capture damage mechanisms occurring to the composite skins and honeycomb core. The study was firstly aimed at accurately addressing the out-of-plane compressive response; particularly, in order to evaluate the influence of imperfection variability on the buckling and crushing behavior, a statistics-based approach was proposed and applied to a detailed finite element model of a single representative honeycomb cell. Furthermore, the impact was also investigated. Finite element numerical models, based on the sandwich assembly structure, were progressively validated through experimental tests, both static and dynamic, performed from the coupon to the sandwich assembly length scale. In the case study iii) the multiscale damage modeling procedure was focused on some issues related to the Carbon Nanotube/Nanofiber length scale, including nanotube length and orientation characterization, stress transfer to the matrix and nanotube toughening mechanisms. A micromechanical model, taking into account CNT length and orientation distribution, was implemented in order to model mode I interlaminar fracture toughness of multiscale CNT/CNF S2-Glass/epoxy composites. For all the investigated case studies the adopted multiscale based strategies revealed to be mostly effective in capturing the most significant damage-related parameters at the lower scales, influencing the structural mechanisms, acting at the structure/component scale.

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