E. Wiesel

Study of the interface in Kevlar 49-epoxy composites by means of microbond and fragmentation tests: effects of materials and testing variables

The work presented in the present paper focuses on the interface in Kevlar 49-epoxy composites. Experimental results for the interfacial shear strength obtained using the microbond test by means of different loading configurations (parallel plate loading, parallel conical plate loading, circular loading), are presented and compared. Contrasting with recent finite-element predictions proposed in the literature, the interfacial shear strength is found to be altogether insensitive to the type of microbond loading configuration. A comparison of the results obtained using two micromechanical tests (microbond and fragmentation) is performed. The interfacial shear strength results obtained by means of the fragmentation test are found to be higher by a factor of about 50% than those obtained by means of the microbond test. A possible explanation for this difference is proposed and discussed, and the value of the “true” interfacial shear strength is conjectured to fall between the values measured by these two tests. The effect of fibre surface chemistry modification (surface desizing) is probed by surface-sensitive techniques (XPS-ESCA and contact angle measurements from droplets) and by micromechanical testing techniques. Surface-sensitive techniques and micromechanical testing provide compatible information for the Kevlar-epoxy system studied here, and the knowledge of the chemical characteristics of the fibre surface can therefore be used as a means of predicting the interfacial shear strength.

Structural origins of morphing in plant tissues

Plant tissues are able to generate complex movements via shape modifications. These effects are tightly related to distinctive multi-scale composite architectures of the plant material, and can therefore largely be interpreted by composite mechanics principles. Here, we propose a generic framework for the analysis and prediction of the shape morphing of intricate biological composite materials, arising from changes in humidity. We have examined in depth the hierarchical structures of three types of seed pods for which we propose a theoretical scheme that is able to accurately simulate the relevant shape deformations. The validity and generality of this approach are confirmed by means of laboratory scale synthetic models with similar architectures leading to equivalent morphing patterns. Such synthetic configurations could pave the way to future morphing architectures of advanced materials and structures.

Crack deflection at a transcrystalline junction

Abstract Using a model polypropylene-based composite, we report the observation of longitudinal damage in a transcrystalline interphase under the effect of a transverse notch, propagating in a direction perpendicular to the axis of a Kevlar fiber. We find that a Cook–Gordon-like damage mechanism is active. The damage growth rate in the transcrystalline layer, parallel to the fiber axis, is larger in thinner layers. A schematic model of the microstructural aspects of damage, which reflects the anisotropic microstructure of the transcrystalline layer, is proposed and briefly discussed.

Interfacial studies of carbon fiber/epoxy composites using single fiber fragmentation test

Single fiber fragmentation tests were carried out to measure the properties of the fiber-matrix interface in several carbon fiber (CFs)/epoxy composite systems. Four kinds of CFs were studied: (1) primary CFs (used as received); (2) desized CFs (sizing removed through thermal treatment); (3) resized CFs (deposited with epoxy sizing by solution); (4) carbon nanotube (CNT)-grafted CFs (grown with CNTs using a chemical vapour deposition method). The interfacial shear strength decreased by around 30% for the desized CFs and the CNT-grafted CFs compared with the pristine CFs. The value of interfacial shear strength for the resized CFs was 20% larger than that of the desized CFs. There is a good agreement between the results of single fiber fragmentation tests and that of contact angle tests.

Evidence of dynamic effects in the fragmentation of optical fibers in epoxy

Single-fiber composite experiments designed to assess the stress transfer ability of the interface between a commercial epoxy resin and optical fibers have been conducted. Unconventional fragmentation patterns were observed, which demonstrate the presence of dynamic energy release processes. Such dynamic contributions, which are not easily seen (if at all) in fragmentation tests with thinner fibers (such as carbon, glass, and kevlar), may have to be accounted for in the theoretical modeling of the test.