Journal of Composite Materials current issue

Normalization of Shear Test Data for Rate-independent Compressible Fabrics

Harrison, P., Wiggers, J., Long, A.C. — 2008-10-01

This article describes a method of using both picture frame (PF) and bias extension (BE) tests together to characterize accurately the trellis shearing resistance of engineering fabrics under low in-plane tension conditions. Automated image analysis software has been developed to reduce the amount of laborious manual analysis required to interpret BE data accurately. Normalization methods for both PF and BE tests on rate-independent compressible fabrics are presented. Normalization of PF test results is relatively straightforward while normalization of BE test results for direct comparison with PF data is more complicated. The normalization method uses a number of simple assumptions to account for the nonuniform shear strain field induced across BE samples during testing. Normalized results from BE tests on samples of different aspect ratios are compared and provide validation of the theory.

Characterizing Mechanical Properties of Particulate Nanocomposites Using Micromechanical Approach

Tsai, J.-L., Tzeng, S.-H. — 2008-10-01

This research aims to propose a continuous micromechanical model for characterizing the mechanical properties of the nanocomposites containing silica nanoparticles embedded in polyimide matrix. The molecular structures of the nanocomposites were established through molecular dynamic (MD) simulation, from which the non-bonded gap as well as the non-bonded energy between the nano-sized inclusion and the surrounding matrix were evaluated. It was postulated that the normalized non-bonded energy (non-bonded energy divided by surface area of the inclusion) is correlated with the degree of interfacial interaction. Subsequently, an effective interphase micromechanical model including inclusion, matrix and effective interphase was developed, in which the dimension of the effective interphase was assumed equal to the non-bonded gap and the corresponding elastic stiffness was calculated from the normalized non-bonded energy. Comparison of the results calculated from the micromechanical model and the MD simulation indicates that the effective interphase model is capable of describing Young's modulus of particulate nanocomposites with accuracy. In addition, it was revealed that when the particulate size decreases, the corresponding modulus of the nanocomposites increases.

High Energy Mill Processing of Polymer Based Nanocomposites

2008-10-01

Polymer matrix nanocomposites have attracted growing attention due to the potential for significantly improving the properties of the polymer by adding a very small amount of nanoparticles. However, the improvement in properties has been related to the degree of dispersion of the nanoparticles in the polymer matrix and, due to their enormous specific surface area, nanoparticles tend to agglomerate. Thus, processing techniques able to produce complete particle dispersion in polymer matrix are of great interest. The purpose of this work is to present a new processing technique for polymer matrix nanocomposites using a high energy mill as an effective approach to disperse ceramic nanoparticles in a polymer matrix. SiO₂/epoxy nanocomposites were processed with various SiO₂ contents using the proposed approach. Transmission electron microscopy (TEM) micrographs of the nanocomposites processed indicated good particle dispersion. In addition, agglomerates were not observed on the scanning electron microscopy (SEM) fractographs of the nanocomposites, up to 3wt% SiO₂. The processed nanocomposites were also characterized by dynamic mechanical analysis (DMA) to investigate the effect of nanoparticles content on the viscoelastic properties and on the glass transition temperature. In summary, the technique was found promising in achieving good levels of particle dispersion in a thermoset polymer matrix.

Stiffness of Aligned Wood Fiber Composites: Effect of Microstructure and Phase Properties

Marklund, E., Varna, J., Neagu, R. C., Gamstedt, E. K. — 2008-10-01

The effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness is analyzed. An analytical model for an N-phase composite with orthotropic properties of constituents is developed and used. This model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. It was found that most macro-properties are governed by only one property of the cell wall which is very important in attempts to back-calculate the fiber properties. The role of lumen (whether it filled by resin or not) has a very large effect on the composite shear properties. It is shown that several of the unknown anisotropic constants characterizing wood fiber are not affecting the stiffness significantly and rough assumptions regarding their value would suffice. The errors introduced by application of the Hashin's model and neglecting the orthotropic nature of the material behavior in cylindrical axes are evaluated. The effect of geometrical deviations from circular cross-section, representing, for example, collapsed fibers, is analyzed using the finite element method (FEM) and the observed trends are discussed.

Effect of Low Temperature on Mechanical Properties of Bidirectional Glass Fiber Composites

Shrivastava, A.K., Nazir Hussain, Md. — 2008-10-01

In this article, low temperature and its degradative influence on the material together with hygro-thermal effect are highlighted. Hygrothermal factor is a major environmental factor experienced by the composites. An attempt has been made to study the mechanical properties of bidirectional glass cloth specimens exposed to low temperature and humid condition.

Tensile strength records its highest value after 10 days exposure at -55°C, compressive strength at 0°C exposure increases but after 10 days exposure at -55°C decreases, shear strength of exposed specimen at 0°C increases, flexural strength of exposed specimen at -55°C increases and records its highest value of 10 days exposed specimen. Further, it has been observed that the strength of the composite is in the following order:

Compressive strength of specimen

In this article an attempt has been made to study effect of low temperature on mechanical properties of bidirectional glass fiber composites.