Andris Chate

Calculation of elastic properties of natural fibers

This article deals with the calculation of the elastic properties of cellulose based natural fibers by using two different types of idealization and assumptions. One model (model A) bases on antisymmertrical laminated structure, while the second one (model B) bases on a thick laminated composite tube model. Model B is able to take into account the elliptic geometry, the hollow based structure of the cross section of the fiber cell. The calculated relationships between spiral angle and modulus in fiber axis by model A fits successful experimental data for holocellulose fibers which were published elsewhere. In general, modulus in fiber axis decreases with increasing spiral angle as well as the degree of anisotropy, while shear modulus reaches a maximum for a spiral angle of 45°. Fiber cell modulus increases linear with increasing cellulose content for both, the calculated (model A) and measured values. The correlation between experimental data and calculation ones was not as high as in the case of modulus versus spiral angle. The discrepancy between model A and a more real cross section is calculated (model B) with roughly 30%.

Finite element analysis of damping the vibrations of laminated composites

Abstract There are several ways of decreasing the vibration energy of structures, such as diminishing the source energy, designing structures with desired eigen-frequencies and excitation frequencies, using materials that have damping properties, etc. Special damping layers made of various viscoelastic materials are widely applied in structures subjected to dynamic loading, especially those used in ship building and aerospace technology. A typical structure is that whose basic layer is covered with a damping layer and a thin constraining layer. Such classical sandwich structures have been widely investigated, especially with reference to the analysis of elastic vibrations.

Natural-fibre-reinforced polyurethane microfoams

Polyurethane-based composites reinforced with woven flax and jute fabrics were prepared with an evenly distributed microvoid foam structure. The relationship between the resin-filled grade and the microvoid content and the density was described. The influence of the type of reinforcing fibre, fibre and microvoid content on the mechanical properties was studied. The investigation results for the static mechanical properties of the composites were described by approximate formulae. It was found that the specific data were only slightly dependent on microvoid content. Increasing the fibre content induces an increase in the shear modulus and impact strength. However, increasing the microvoid content in the matrix results in a decreased shear modulus and impact strength. The woven flax fibre results in composites with better mechanical strength than the woven jute fibre composites.

Method for identification of elastic properties of laminates based on experiment design

Abstract A numerical-experimental method for the identification of mechanical properties of laminated composites from the experimental results is developed. For the first time it is proposed to use the experiment design to solve the identification (inverse) problems. The basic idea of the proposed approach is that simple mathematical models (response surfaces) are determined only by using the finite element solutions in the reference points of the experiment design. Therefore, a significant reduction (about 50–100 times) in calculations of the identification functional can be achieved in comparison with the conventional methods of minimization. Numerical examples of identification of elastic properties of different laminates from the measured eigenfrequencies of plates are discussed.

Effect of interphase on the transverse Young's modulus of glass/epoxy composites

Abstract The interphase in fibre-reinforced composites depends on the fibre, matrix and surface treatment of fibres. Many investigations have been carried out for industrial fibres with unknown content of sizes or resins. In the present paper, fibres, sizes and resins with known ingredients are considered. A gradient in the epoxy to curing agent, ratio in the interphase is created by the surface treatment or chemical reactions. As a consequence, the mechanical properties in this region change. The Youngs modulus of the interphase is described with the help of a mathematical model. The Youngs moduli in composites with different-treated glass fibres and differently cured epoxy resins are determined for unidirectional composites under tension transverse to the fibre direction. A model for the Youngs modulus of composites taking into account the interphase is developed. The results of the experiment and the model are compared, and the average Youngs modulus of the interphase is identified.

Determination of elastic constants of glass/epoxy unidirectional laminates by the vibration testing of plates

Identification of elastic properties of unidirectional glass/epoxy laminates from the measured eigenfrequencies has been performed. The stiffness of the laminates has been investigated by a mixed numerical/experimental method employing the vibration test of plates. Elastic constants of laminates have been determined by using an identification procedure based on experiment design, the finite-element method and the response-surface approach. Elastic properties of laminates with two different fibre-surface treatments have been compared. It was found that only for the transverse elastic modulus is there a statistically significant difference between the composites with good and poor fibre/matrix adhesion.

Vibration and damping analysis of laminated composite plates by the finite element method

The present investigation is concerned with the utilisation of the finite element technique for predicting the natural frequencies and the modal damping factor (also called the loss factor) of anisotropic fibre‐reinforced composite laminated plates. The simple definition of the modal damping factor is defined as the ratio of the strain energy dissipated per radian of vibration, in the mode of interest, to the total strain energy of the entire laminate at maximum displacement during the same cycle. Results for the vibration and damping analysis of multi‐layered plates obtained by the present methods are compared with the results obtained by other authors and with the results of experiments.

Free Damped Vibrations of Sandwich Shells of Revolution

Using a zig-zag model the free damped vibrations of sandwich shells of revolution are investigated. As special cases the vibration analysis under consideration of damping of cylindrical, conical and spherical sandwich shells is performed. A specific sandwich shell finite element with 54 degrees of freedom is employed. Starting from the energy method the damping model is developed. Numerical examples for the free vibration analysis with damping based on the proposed finite element approach are discussed. Results for sandwich shells show a satisfactory agreement with various reference solutions.

Correlation between injection moulding processing parameters and mechanical properties of microcellular polycarbonate

Since many decades, microcellular foamed materials have been produced basically to reduce the density of the materials in order to get lightweight parts. Meanwhile, it is well known that microcellular foaming by injection moulding offers many more advantages compared to compact injection moulding. Those are, e.g. lower shrinkage and warpage, shorter cycle times, lower clamp forces, reduced viscosity but improved properties of the foamed material in contrast to the compact material. These arguments are all known, but to improve the properties of the material, it is necessary to understand the interrelationship between the morphology and the mechanical properties. Furthermore, it is important to know how the processing parameters influence the morphology and the properties of the produced part. By understanding the relation between processing parameters and the consequential properties, it has become possible to create microcellular foamed parts with exactly defined properties. Through the variation of different processing parameters such as blowing agent concentration, injection velocity, mass temperature, mould temperature, weight reduction and different moulding processes like gas counter pressure injection moulded test, samples were produced to characterise the morphology and the mechanical properties. The experiments were performed with a polycarbonate type from Bayer MaterialScience. The cell size, thickness of the skin layer and distance between the cells were correlated to the processing parameters by means of nonlinear regression equations. Based on these equation, 3D graphs were created by variation of two parameters by fixing the remaining parameters to illustrate the relationships. Furthermore, the relation between the morphology and the mechanical properties was correlated, which makes it possible to produce parts through injection moulding with a well-defined Young’s modulus or flexural strength.

Correlation Between Morphology and Notched Impact Strength of Microcellular Foamed Polycarbonate

Polycarbonate has the reputation of having a tough breaking behavior, but it is widely unknown that this applies only to special conditions. The impact strength of polycarbonate depends on the temperature, thickness (with a tough brittle transition as thickness increases), contribution of notch tip radius, impact speed, physical blowing agent, molecular weight of the polymer, and processing parameters. Research results indicate that microcellular foams produced by injection molding with physical blowing agent (MuCell TM Technology by Trexel) show a significantly higher notched impact strength than compact polycarbonate if the compact material is brittle under the same testing parameters. However, if the compact polycarbonate breaks toughly, the notched impact strength of the foamed material is always lower. Therefore, it is highly important to pay attention to the testing parameters and conditions when comparing the toughness of the foam with that of the compact material. The toughness of microcellular foams has similar properties like PC/ABS and PC/PP blend systems, which provides the possibility to combine the higher impact strength with the advantages of microcellular foaming such as weight reduction, lower shrinkage, shorter cycle times, lower clamp forces, and reduced melt viscosity. In order to use technologies and conditions, which are applied in the polymer industry as well, all materials were produced by an injection molding process. Special processing technologies such as gas counter pressure and precision mold opening were used in order to reach microcellular foam structures with cell diameters around 10 μm. These technologies yield exactly adjustable foam morphologies. Special morphologies are required to improve the notched impact strength of the foamed material. Two different equivalent models were extracted from the analyses, which indicate a significantly higher notched impact strength than the compact material under the same testing conditions. The knowledge of the ideal foam morphologies enables the industry to produce foamed materials with improved mechanical properties.