Modeling the Effect of Helical Fiber Structure on Wood Fiber Composite Elastic Properties

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.

Modelling time-dependent mechanical behaviour of softwood using deformation kinetics

“…It is clear from the previous that the rotational freedom of the fibre is seen to have a major influence on the relaxation behaviour. This has also been found to be the case in the elastic domain, where freedom of rotation decreases the MOE of the fibre (Marklund and Varna 2009). In the time-dependent case, larger freedom of rotation results in faster stress relaxation.…”

Weibull analysis for the diameter dependence of the elastic modulus of curaua fibers

“…Based on the 3D hollow model, applying laminated composite tube theory [23], Placet et al [22] concluded that in hemp fibers the fraction of crystalline cellulose, the microfibril angle, the amorphous cellulose shear modulus and the crystalline cellulose elastic modulus are the main factor for the E vs. d correlation. The authors also emphasized the importance of the fiber lumen, which is also a kind of defect, although with lower sensitivity in their analysis.…”

Rotational 3D printing of damage-tolerant composites with programmable mechanics

“…Wood cell walls possess cellulose microfibrils that are arranged in layers and helically oriented with respect to the long axis of the cells. The helical angle of the cellulose microfibrils affects the elastic properties ( 3 , 4 ) and the extensibility ( 5 ) of the wood, and varies spatially from the core wood to the outer mature wood as well as in response to environmental stresses that occur during growth ( 6 , 7 ). Helical fiber arrangements have also been observed in other natural composites, such as the hammer-like stomatopod dactyl club ( 8 ), where extreme damage tolerance is required.…”