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Due to the typical large variability in the measured mechanical properties of flax fibres, they are often employed only in low grade composite applications. The present study aims to investigate the reasons for the variability in tensile properties of flax fibres. It is found that an inaccuracy in the determination of the cross-sectional area of the fibres is one major reason for the variability in properties. By applying a typical circular fibre area assumption, a considerable error is introduced into the calculated mechanical properties. Experimental data, together with a simple analytical model, are presented to show that the error is increased when the aspect ratio of the fibre cross-sectional shape is increased. A variability in properties due to the flax fibres themselves is found to originate from the distribution of defects along the fibres. Two distinctive types of stress-strain behaviours (linear and nonlinear) of the fibres are found to be correlated with the amount of defects. The linear stress-strain curves tend to show a higher tensile strength, a higher Young's modulus, and a lower strain to failure than the nonlinear curves. Finally, the fibres are found to fracture by a complex microscale failure mechanism. Large fracture zones are governed by both surface and internal defects; and these cause cracks to propagate in the transverse and longitudinal directions. [ABSTRACT FROM AUTHOR]

Cellular materials from hollow bronze spheres were prepared and investigated. The main aim was to determine the effective Young’s modulus of cellular materials by means of the dynamic resonant method, and the next goal was to demonstrate the possibility of using sandwich samples for such determination. For this purpose, two kinds of samples were prepared: quasihomogeneous rods consisting entirely of a cellular material, and sandwich rods containing the cellular material only in a core. Resonant frequencies of the fundamental flexural modes of all these samples were measured, and corresponding flexural rigidities were evaluated. Obtained values of flexural rigidities were subsequently utilized for calculation of the effective Young’s modulus of cellular material. A reasonable agreement between the modulus values determined by means of quasihomogeneous and sandwich samples was obtained. This indicates that the sandwich samples could be used for measuring the material properties of particular layers. [ABSTRACT FROM AUTHOR]

Most of adhesively bonded joints are under complicatedly distributed triaxial stress in the adhesive layer. For the estimating of the strength of adhesively bonded joints, it is crucial to clarify behavior of yield and failure of the adhesives layer under triaxial stress conditions. Two types of the adhesively bonded joints were used in this study: One is the scarf joint which is under considerably uniform normal and shear stresses in the adhesive layer, where their combination ratio can be varied with scarf angle. The other is the butt joint with thin wall tube in which considerably uniform pure shear can be realized in the adhesive layer under torsional load conditions. These joints can cover the stress triaxiality in adhesive layers of most joints in industrial application. The effect of stress triaxiality on the yield and fracture stresses in the adhesive layer were investigated using the joints bonded by three kinds of adhesives in heterogeneous and homogeneous systems. The results showed that both the yield and failure criterion depend on the stress triaxiality and that the fracture mechanism of the homogeneous adhesive is different from that of the heterogeneous one. From these experimental results, a method of estimating the yield and failure stresses was proposed in terms of a stress triaxiality parameter. [ABSTRACT FROM AUTHOR]

Analysis of a specific idealised model demonstrates the limitations of the equivalent elastic crack procedure when it is applied to internally pressurised crack problems. [ABSTRACT FROM AUTHOR]