Modeling the Effect of Helical Fiber Structure on Wood Fiber Composite Elastic Properties
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Modeling the Effect of Helical Fiber Structure on Wood Fiber Composite Elastic Properties Erik Marklund & Janis Varna
Received: 24 June 2009 / Accepted: 30 June 2009 / Published online: 16 July 2009 # Springer Science + Business Media B.V. 2009
Abstract The effect of the helical wood fiber structure on in-plane composite properties has been analyzed. The used analytical concentric cylinder model is valid for an arbitrary number of phases with monoclinic material properties in a global coordinate system. The wood fiber was modeled as a three concentric cylinder assembly with lumen in the middle followed by the S3, S2 and S1 layers. Due to its helical structure the fiber tends to rotate upon loading in axial direction. In most studies on the mechanical behavior of wood fiber composites this extension-twist coupling is overlooked since it is assumed that the fiber will be restricted from rotation within the composite. Therefore, two extreme cases, first modeling fiber then modeling composite were examined: (i) free rotation and (ii) no rotation of the cylinder assembly. It was found that longitudinal fiber modulus depending on the microfibril angle in S2 layer is very sensitive with respect to restrictions for fiber rotation. In-plane Poisson’s ratio was also shown to be greatly influenced. The results were compared to a model representing the fiber by its cell wall and using classical laminate theory to model the fiber. It was found that longitudinal fiber modulus correlates quite well with results obtained with the concentric cylinder model, whereas Poisson’s ratio gave unsatisfactory matching. Finally using typical thermoset resin properties the longitudinal modulus and Poisson’s ratio of an aligned softwood fiber composite with varying fiber content were calculated for various microfibril angles in the S2 layer. Keywords Wood fiber composite . Ultrastructure . Microfibril angle . Helical . Cell wall
E. Marklund Swerea SICOMP AB, Box 104, SE-431 22 Mölndal, Sweden J. Varna (*) Dept. Appl. Physics and Mech. Eng., Lulea University of Technology, SE-97187 Lulea, Sweden e-mail: [email protected]
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Appl Compos Mater (2009) 16:245–262
1 Introduction Cellulosic fiber reinforced polymers, such as flax- and wood fiber reinforced composites, have potential as structural materials due to the high specific stiffness- and strength of the fibers [1, 2]. In fact, cellulosic fiber composites are extensively used already today in the automotive industry, but with the fibers acting mainly as filler material in non-structural interior panels. Cellulosic fiber composites for structural purposes do exist, but are limited in use due to some major drawbacks still associated with these materials. The fibers generally show lack of well defined mechanical properties, highly variable dimensions, shapes and low ability to adhere to non-polar matrix materials, which in turn has negative effect on efficient stress transfer. Another problem is the fibers dimensional stability under moist conditions. At present day many researchers are wo
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