Applied Composite Materials (v.18, #2)

Properties of Wood Fibre-Polypropylene Composites: Effect of Wood Fibre Source by Svetlana Butylina; Ossi Martikka; Timo Kärki (101-111).
This study examined the effect of type of wood fibre source on the physical and mechanical properties of wood fibre-polypropylene composites. Wood flour, fibres of heat-treated wood and pellets were used as sources of wood fibres in the manufacturing process. All studied wood fibre-polypropylene composites were made from 75% wood, 22% recycled polypropylene (PP) and 3% maleated polypropylene (MAPP). Wood fibre-polypropylene composites were compounded in a conical twin-screw extruder. Water absorption and thickness swelling were studied. Mechanical properties of the composites were characterised by tensile, flexural, and impact testing. Micromechanical deformation processes were investigated using scanning electron microscopy done on the fractured surfaces of broken samples. The durability of composites exposed to three accelerated cycles of water immersion, freezing and thawing was examined. The results showed that the density of the composites was a key factor governing water absorption and thickness swelling. A significant improvement in tensile strength, flexural strength, and Charpy impact strength was observed for composites reinforced with heat-treated fibre compared to composites reinforced with pellets and especially to wood flour reinforced composites. The flexural strength and dimensional stability performance reduced after exposure to freeze-thaw cycling for all composites, but the degree of these changes was dependent on the wood fibre source.
Keywords: Wood fibre-polypropylene composites; Thickness swelling; Charpy impact strength; Freeze-thaw cycling

This paper addresses the prediction of intralaminar and interlaminar damage onset and evolution in composite structures through the use of a finite element based procedure. This procedure joins methodologies whose credibility has been already assessed in literature such as the Virtual Crack Closure Technique (for delamination) and the ply discount approach (for matrix/fiber failures). In order to establish the reliability of the procedure developed, comparisons with literature experimental results on a stiffened panel with an embedded delamination are illustrated. The methodology proposed, implemented in ANSYS © as post-processing routines, is combined with a finite element model of the panel, built by adopting both shell and solid elements within the frame of an embedded global/local approach to connect differently modelled substructures.
Keywords: Layered structures; Delamination; Buckling; Damage mechanics; Finite element analysis

The effects of interphase between fibers and matrix on the micro-and macro-mechanical behaviors of fiber-reinforced composite lamina subjected to transverse shear load at remote distance have been studied. The interphase has been modeled by the compliant spring-layers that are linearly related to the normal and tangential tractions. Numerical analyses on composite basic cells have been carried out using the boundary element method. For undamaged composites the micro-level stresses at the matrix side of the interphase and effective shear modulus have been calculated as a function of the fiber volume fraction and the interphase stiffness. Results are presented for various interphase stiffnesses from perfect bonding to total debonding. For a square array composite results show that for a high interphase stiffness k > 10, an increase in a fiber volume fraction results in a higher effective transverse shear modulus. For a relatively low interphase stiffness k < 1, it is shown that an increase in the fiber volume fraction causes a decrease in the effective transverse shear modulus. For perfect bonding, the effective shear modulus for a hexagonal array composite is slightly larger than that for a square array composite. Also for the damaged composite with partially debonded interphase, local stress fields and effective shear moduli are calculated and a decrease in the effective shear modulus has been observed.
Keywords: Metal-matrix composites (MMCs); Debonding; Interface/interphase; Micro-mechanics; Boundary element analysis (BEA); Effective shear modulus

An Investigation of the Tensile Strength of a Composite-To-Metal Adhesive Joint by Nicholas G. Tsouvalis; Vassilios A. Karatzas (149-163).
The present study examines the feasibility of a simple concept composite-to-metal butt joint through the performance of both numerical and experimental studies. The composite part is made of glass/epoxy unidirectional layers made with the vacuum bag method. The geometry of the joint is typical for marine applications and corresponds to a low stiffness ratio. Two major parameters are investigated, namely the overlap length and the surface preparation of the steel adherent. Manufacturing of specimens and the procedure of the tensile tests are described in detail, giving hints for obtaining a better quality joint. Axial elongation and strains at various places of the joint were monitored and also numerically calculated. The tests revealed that the joint is quite effective, irrespectively of the steel surface preparation method. The failure loads are comparable and in some cases superior to other corresponding values found in the literature. The numerical models proved to adequately predict the structural response of the joint up to the loading where debonding starts.
Keywords: Adhesive joints; Debonding; Finite element analysis; Welding/Joining; Surface roughness

The damage evolutions of C/SiC composites fabricated by chemical vapor infiltration with two different sizes of yarns were compared by cyclic tensile load/unload tests accompanied with the acoustic emission (AE) monitoring. The results show that an inflexion was observed in both the evolution of hysteresis characteristics and the corresponding AE curve for composite with fine yarns, which is, however, absent for composite with coarse yarns, suggesting that the former is much closer to the tough material than the latter one. Felicity effect was observed for both composites, which is more pronounced for the composite with coarse yarns due to the large yarn sizes leading to more matrix debris inside the yarns.
Keywords: Ceramic matrix composites; Acoustic emission; Damage evolution; Yarn sizes