Effects of fiber/matrix interactions on the properties of graphite/epoxy composites

A state-of-the-art literature review of the interactions between fibers and resin within graphite epoxy composite materials was performed. Emphasis centered on: adhesion theory; wetting characteristics of carbon fiber; load transfer mechanisms; methods to evaluate and measure interfacial bond strengths; environmental influence at the interface; and the effect of the interface/interphase on composite performance, with particular attention to impact toughness. In conjunction with the literature review, efforts were made to design experiments to study the wetting behavior of carbon fibers with various finish variants and their effect on adhesion joint strength. The properties of composites with various fiber finishes were measured and compared to the base-line properties of a control. It was shown that by tailoring the interphase properties, a 30% increase in impact toughness was achieved without loss of mechanical properties at both room and elevated temperatures

Anisotropy of reinforcement fibres and its influence on the apparent interfacial shear strength in thermoplastic composites

Optimization of the stress transfer capability of the fibre-matrix interphase region is critical to achieving the required performance level in thermoplastic matrix composites. Despite the ever increasing diversity of the reinforcements available for polymer composites, glass fibres still account for 95% of fibre reinforcements used in the composites industry, primarily due to of their highly attractive performance/price ratio. Due to its initial location on the glass fibre surface, the sizing layer is an important component in the formation and properties of the composite interphase. A large proportion of the research published on interphase optimization in these materials has focussed on the role of the organosilane coupling agents which are almost universally present in glass fibre sizing. Perhaps due to their common name of "coupling agents", perhaps due to their reactive nature or even perhaps due to the early focus of composites research on chemically reactive thermosetting matrices, there exists a dominant mindset in the composites research community to approach the interphase from a chemical bonding viewpoint. While this approach may very well be justified in thermosetting matrix composites it is not at all clear that this is also the case for reinforced thermoplastics, in particular for the commercially important polyolefin based composites

Computational simulation of high temperature metal matrix composites cyclic behavior

A procedure was developed and is described which can be used to computationally simulate the cyclic behavior of high temperature metal matrix composites (HTMMC) and its degradation effects on the structural response. This procedure consists of HTMMC mechanics coupled with a multifactor interaction constituent material relationship and with an incremental iterative nonlinear analysis. The procedure is implemented in a computer code that can be used to computationally simulate the thermomechanical behavior of HTMMC starting from the fabrication process and proceeding through thermomechanical cycling, accounting for the interface/interphase region. Results show that combined thermal/mechanical cycling, the interphase, and in situ matrix properties have significant effects on the structural integrity of HTMMC

Interfacial properties of fibre reinforced thermo-plastics

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Interface-controlled creep in metallic glass composites

In this work we present molecular dynamics simulations on the creep behavior of $\rm Cu_{64}Zr_{36}$ metallic glass composites. Surprisingly, all composites exhibit much higher creep rates than the homogeneous glass. The glass-crystal interface can be viewed as a weak interphase, where the activation barrier of shear transformation zones is lower than in the surrounding glass. We observe that the creep behavior of the composites does not only depend on the interface area but also on the orientation of the interface with respect to the loading axis. We propose an explanation in terms of different mean Schmid factors of the interfaces, with the amorphous interface regions acting as preferential slip sites.Comment: 11 pages, 13 figure

Are silanes the primary driver of interface strength in glass fiber composites? An exploration of the relationship of chemical and physical parameters in the micromechanical characterisation of the apparent interfacial strength in glass fiber composites

It is probably not an overstatement to say that organosilanes are the most important class of chemicals used in the glass fiber, and consequently the composites, industry. One of the best-known assertions about these multifunctional silane molecules is that they promote chemical bonding across the fiber-matrix interface. However, the development of (non-reactive) thermoplastic matrix composites raises questions about the simplistic chemical bridging model of silanes at the interface. Moreover, despite the high level of attention commonly focused on the chemical influences on interfacial adhesion, a growing number of researchers have also commented on the role of residual stress contributing to the stress transfer capability at the fiber-matrix interface. We will review data on the temperature dependence of the apparent interfacial shear strength (IFSS) in (unsized) glass fiber-polypropylene, a system where there is no a priori reasoning to expect any chemical bonding at the interface. The results indicate that the apparent IFSS in thermoplastic composites can be largely explained by the residual thermal stresses. This phenomenon is characterised by a large drop in the measured IFSS when the test temperature is raised above the matrix Tg. We will also present data to show that the same phenomenon is present in the IFSS of glass fiber-epoxy composites, although the magnitude of the measured values of IFSS for epoxy systems cannot be explained by residual thermal stress alone. However, by further considering the possible contribution of the thermoset phenomenon of cure shrinkage we will demonstrate that it is also possible to explain the level of IFSS in this chemically reactive system by physical residual stresses alone. The state of the interface/interphase in epoxy systems is somewhat more complex than for (relatively) non-reactive thermoplastics. This presentation will review our results on the investigation of this complex experimental challenge. Many of the properties required in the modelling of residual stress in these systems vary with the curing agent to epoxy resin ratio near the interface. Since fibers are often coated with sizings containing reactive groups found in both curing agents and epoxy resins it is likely that the polymerised matrix near the fiber surface will have a different ratio of reactive groups than was mixed in the original liquid resin system. To fully explore this concept it is therefore necessary to characterise both the IFSS and the epoxy matrix properties (such as Tg, modulus, and thermal expansion coefficient) as a function of temperature and stoichiometry

Temperature dependence of the interfacial shear strength in glass reinforced polypropylene and epoxy composites

We have recently reported the development of a method which allows the measurement of IFSS over a wide temperature range [6,7]. In this paper we present data obtained using the microbond test in the temperature controlled environment of a thermo-mechanical analyser (TMA). IFSS in glass fibre–polypropylene and glass fibre-epoxy systems in the temperature range -40°C to 150°C are presented and discussed