# Thermo-Mechanical Properties of PLA/Short Flax Fiber Biocomposites
## 4 Results and Discussion
### Mechanical Properties and Modeling  
Figure 1 shows the typical stress-strain curves of the PLA/flax fiber composites. As shown, PLA and all the composites show a fragile break without yielding. The main tensile properties are summarized in Figure 2.  
As expected, the stiffness increased with increasing amount of flax fibers. This behavior is very common to other similar bio-composite systems encountered in literature [6]. The stiffness increase was reflected in a decrease in elongation at break. The tensile strength slightly increased with the fiber content up to 20 wt.% of fibers and, then, at higher fiber loading it resulted similar to that of pure PLA. This trend can be attributed to the well-known reinforcement effect of the flax fibers [30] up to 20 wt.%, while at 40 wt.% poor wetting of the fibers with the matrix may be present leading to reduced stress transfer across the fiber-matrix interface with consequent reduction of the tensile strength.  
The values of the tensile strength are in agreement with those reported by [PERSON] et al. [30] and [PERSON] and [PERSON] [31] that obtained tensile strengths of 53-54 MPa with 30 wt.% of fibers without fiber treatments and additives. In addition, the value for the Young's moduli of the PF20 is close to that found by [PERSON] and [PERSON] [31] with 20 wt.% fibers (5.1 GPa).  
Using the mechanical properties of the matrix (heat PLA) and the composites, the Pukanszky's plot can be obtained (Figure 3) and the B parameter can be evaluated.  
Figure 1: Stress-strain representative curves for pure poly(lactic acid) (PLA) and for PLA/flax fiber composites.  
Figure 2: Mechanical properties for pure PLA and PLA/flax composites: (**a**) Elastic modulus; (**b**) Stress at break; (**c**) Elongation at break.  
The interaction B parameter value of about 3.2 indicates a good adhesion between the fibers and the matrix. In fact, in the case of weak interfacial bonding, the fibers do not carry the load and, consequently, B is almost zero [41]. This result is coherent with the general increase of the tensile strength of the composites indicating a good load transfer between the fibers and the matrix.  
Interesting results were achieved using the analytical models for predicting the elastic modulus of the composites. In fact, as shown in Figure 4, the Cox and Halpin-Tsai models, typically used for aligned long fiber composites, did not fit the experimental data, especially at high fiber loading. On the other hand, the models for particulate filled systems ([PERSON] and [PERSON] models) provided a good fitting. This confirms that when the fibers are very short the mechanical behavior of their composites can be assimilated, as expected, to that of the particulate filled composites.  
Furthermore, it can be confirmed the good adhesion between the fibers and the matrix since the [PERSON] model presupposes a perfect adhesion between the filler and the matrix and a good filler dispersion [46].  
Figure 4: Comparison between experimental elastic modulus of the PLA/flax fiber composites and some predictive analytical models.  
Figure 3: Pukánszky’s plot for PLA/flax fiber composites.