During the last few decades, one of the most prevalent issues associated with the performance of aerospace composite materials has revolved around low interlaminar mechanical properties. The innovations described in this work outline a number of new and improved techniques for advancing the state of the art in composite design and manufacturing, and are focused primarily on enhancements in interlaminar tensile (ILT) and interlaminar shear (ILS) strengths. All concepts are introduced from a technical perspective, but are intended to provide a high degree of flexibility and feasibility in a practical shop environment.
From a measurement standpoint, ply-to-ply adhesion deficiencies can often be attributed to a combination of debilitating forces or stresses associated with “out-of-plane” ply-to-ply tensile contributions (Mode I), and “in-plane” ply-to-ply shearing effects (Mode II). Weak interlaminar interactions or “weak planes” can be ascribed directly to the specific fabrication methods employed and the material properties of the constituents. Mechanisms that influence the chemical bonding effects between constituents, mechanical interlocking, differential moduli, thermal mismatch, matrix porosity, and inter- and intra-bundle porosity are a few of the factors that control the interlaminar quality of a composite article. Such factors are governed by the type of matrix resin chosen, how it is formulated and introduced into the reinforcement network, the specific laminate design and assembly process executed, and the specific time-temperature-pressure profiles applied during the cure and post-cure processes.
The need to resolve, or at least substantially improve interlaminar weaknesses is a worldwide concern as laminated carbon fiber-reinforced composites are finding more applications throughout industry every year. This project effort will address some of these issues by introducing a few topics with two primary objectives in mind:
To enhance and advance the state of the art by refining and improving current methodologies, and developing new and alternative approaches to many of the interlaminar strengthening techniques that have already been attempted or are currently in practice to varying degrees of success; and
To explore new, alternative, novel, and radical ways to improve z-directional ILS/ILT attributes and ply-to-ply interactions with minimal loss in 2D mechanical properties.
Most advanced composites consist of a reinforcement phase (typically carbonized or ceramicized fibers and fabrics) and a matrix phase, which could consist of polymers, ceramics, carbon, metals, or possibly a hybrid of these. Polymer matrix composites (PMC) are the most prevalent today. The matrix phase binds the reinforcement together while the reinforcement phase may consist of fibers, tow, yarns, and/or particles that are continuous throughout the composite article, or are discontinuous by utilizing entities such as chopped fibers, whiskers, or nanoparticles. Perhaps a combination of continuous, discontinuous, and discrete reinforcements strategically portioned, positioned, and oriented within the composite body provides the desired enhancements while maintaining constituent-to-constituent compatibility and a low closed porosity fraction.
There is a third constituent in all composite platforms that is often overlooked. The interphase zone is the functional boundary or transition region along the fiber-matrix interfaces where all the chemical and mechanical bonding interactions occur, joining the two phases together. This could be called the “slip-and-grip” zone, as it is the most critical region in the composite network. It should be a major topic in any study that seeks to improve the mechanical and thermomechanical performance properties of fiber/fabric-reinforced polymers, ceramics, carbon, or metal matrices. Interfacial chemical binding is particularly important in PMC systems, while mechanical interactions become the principal binding mechanisms in CC and CMC platforms.