Theoretical and Computational Modeling of the Fracture Behavior of Composite Structures and Interfacial Problems

The recent design of advanced engineering components based on high-performance composites has increased the scientific attention towards accurate theoretical and computational models to simulate the mixed-mode delamination and fracturing phenomena, involving mechanical nonlinearities of materials and interfaces. In a context where fiber reinforced polymer (FRP) composites are largely adopted as reinforcements in masonry and concrete substrates, in the first part of this chapter, we study the bond behavior in FRP-to-concrete structural systems, even accounting thermal variations affecting the interfacial stresses and material properties of adhesives, by means of an extended contact algorithm in a classical finite element environment, capable to handle both closure and opening of bodies at their interfacial level. Among more recent sustainable and reversible retrofitting systems, the so-called composite-reinforced mortar (CRM) with inorganic binders within a matrix is a valid alternative to FRPs, since it ensures a higher compatibility with substrates because of the high vapor permeability of mortar. Thus, the second part of this chapter is devoted to the numerical study of the fracturing behavior in CRM single-lap shear tests, as provided by a cohesive zone (CZ) modeling and concrete damage plasticity (CDP) modeling, in a classical finite element setting. The selected specimens are characterized by the presence of mortar with mechanical properties approximated analytically, according to different polynomial or exponential functions. Different fracturing modes are explored numerically for such CRM specimens, in their matrix and reinforcement phases. A systematic investigation is, thus, performed numerically, discussing about the reliability of the proposed tool to predict the response of the overall reinforcement system, rather than costly experimental tests. The proposed results could represent valid numerical solutions for further experimental and/or analytical investigations in the field. The last part of this chapter proposes a theoretical study of the mixed-mode interfacial response of functionally graded coatings (FGCs) on different substrates, here modelled as asymmetric double cantilever beams, accordingly to experimental tests, while resorting to an enhanced beam theory (EBT), in which subcomponents are partially connected by a continuous arrangement of elastic-brittle interfacial springs, either in tangential or normal directions. Based on the Timoshenko beam theory, the differential equations of the problem are defined directly in terms of unknown interfacial modes I and II stresses, whereas analytical distribution laws account for different material functional graduations in the thickness direction of specimens. Such variation is verified to affect the local and global response in terms of interface stresses, internal actions, energy quantities, and load-displacement curves, with a high accuracy, as checked against classical single beam theories (SBTs).