Abstract
Fiber reinforced composite materials are typically comprised of two phases, i.e., the reinforcing fibers and a surrounding matrix. At a high volume fraction of reinforcing fibers, the matrix is confined to a microscale region in between the fibers (1–200 µm). Although these regions are interconnected, their behavior is likely dominated by their micro-scale. Nevertheless, the characterization of the matrix material (without reinforcing fibers) is usually performed on macroscopic bulk specimens and little is known about the micro-mechanical behavior of polymer matrix materials. Here, we show that the microscale behavior of an epoxy resin typically used in composite production is clearly different from its macroscale behavior. Microscale polymer specimens were produced by drawing microfibers from vitrifying epoxy resin. After curing, tensile tests were performed on a large set of pure epoxy microfiber specimens with diameters ranging from 30 to 400 µm. An extreme ductility was observed for microscale epoxy specimens, while bulk scale epoxy specimens showed brittle behavior. The microsized epoxy specimens had a plastic deformation behavior resulting in a substantially higher ultimate tensile strength (up to 380 MPa) and strain at break (up to 130 %) compared to their bulk counterpart (68 MPa and 8%). Polarized light microscopy confirmed a rearrangement of the internal epoxy network structure during loading, resulting in the plastic deformation of the microscale epoxy. This was further accompanied by in-situ electron microscopy to further determine the deformation behavior of the micro-specimens during tensile loading and make accurate strain measurements using video-extensometry. This work thus provides novel insights on the epoxy material behavior at the confined microscale as present in fiber reinforced composite materials.
Highlights
Fiber-reinforced polymer composites are one of the go-to materials for high-end applications with excellent mechanical properties at low mass, used in the aerospace, wind energy, and automotive sectors [1,2]
The characterization of matrix materials such as epoxy resins is typically performed at the bulk scale, using the prescribed standardized test methods such as ASTM D638 [16] or ISO 527 [17]
The epoxy resin used was EpikoteTM Resin MGS RIMR 135, which is based on diglycidyl ether of bisphenol A (DGEBA), combined with a liquid diamine hardener, EpicureTM Curing Agent MGS
Summary
Fiber-reinforced polymer composites are one of the go-to materials for high-end applications with excellent mechanical properties at low mass, used in the aerospace, wind energy, and automotive sectors [1,2]. Their inner structure consists of individual fibers, with a typical diameter of 5 μm (carbon fibers) to 20 μm (glass fibers), surrounded by a matrix material such as an epoxy resin. This results in zones of interconnected matrix materials that have a dimension in the (sub)micron range such as the resin rich zones between fibers (1–25 μm, “intralayer”), as well as resin-rich zones in between reinforcement layers (10–200 μm, “interlayer”). The research into microscale (epoxy) matrix properties remains limited [3,4,5,7,18,19] these microscale matrix properties
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