**4. Strain rate of biocomposites**

creep at a given time, and rezero when it reaches the estimated time then deformation is recorded by another equal time. For example, for a biocomposite made of polyethylene aluminum fiber fique, we can observe the response to creep and creep recovery in **Figure 9**, a maximum stress of 1.2 MPa was applied in a bending test at three points at 25°C, 1.2 MPa effort is an attempt that was obtained after identifying the linear viscoelastic region, a biocomposite

**Figure 9.** Creep curves and creep recovery for biocomposite LDPE-Al-Fique 10% reinforcement of natural fibers.

As presented schematically in **Figure 6**. For an adjustment with a four-parameter interpretation is the same, it is important to note that the effect of reinforcement on a biocomposite can be studied by this method, especially when amending fiber volume, nanoreinforcements, fillers, filler, or some surface modification is made to the fibers or fillers in order to improve performance micromechanical, and default creep performance is used. In **Figure 10**, one can see a behavior of an LDPE manufactured biocomposite-Al-Fique 30% fiber volume. Being possible to observe the effect of increased volume is positive with respect to creep, decreasing the speed of decoration and to increase enforcement effort in the linear viscoelastic region by nearly 60%.

test strain sweeps at 25°C in a DMA (RSAIII).

322 Composites from Renewable and Sustainable Materials

**Figure 10.** Creep curves for biocomposite LDPE-Al-Fique 30%.

To calculate the strain rate of the biocomposites subjected to creep tests, one can be approximated using the four-parameter model represented by Eq. (6), or model parameters *n* represented by Eq. (11). By making adjustments mathematical models, is provided mainly for comparing quantitatively the effect of strain rate, and further study of the performance of the incorporation of fibers, surface treatment agents couplings for fibers or polymers, fillers, nanoreinforcements, and fillers to manufacture biocomposites, controlled conditions of temperature and constant effort. In **Figure 11**, one can observe an experimental curve creep, and the respective model adjustment four parameters and the model of *n* parameters, it is emphasized that in the two curves creep models over four parameters. It is better than the fourparameter fit, which facilitates the study of the incorporation of fillers, reinforcements, nanoreinforcements for biocomposites.

**Figure 11.** Adjustment of an experimental curve of a biocomposite creep, using the four-parameter model and model parameters *n*.

**Figure 12.** *n*1 parameter for 10, 20, and 30% biocomposites reinforced with natural fiber sisal, and unreinforced LDPE-Al.

In **Figure 12**, we can see the response of viscous parameter *n*1, which allows the calculation of the strain rate for creep tests; this exercise was carried out, varying the volume of natural fibers fique biocomposite, corroborating the possibility of deepening the study of the effect of reinforcements or fillers to biocomposites.

**Figure 13** shows the setting of the four-parameter model, two biocomposites, with different volumes of natural fiber reinforcement sisal. It is possible to corroborate and validate the model.

**Figure 13.** Setting the four -parameter model to LDPE-Al-Fique 10 and 30% incorporation biocomposites of fibers.

**Table 7** shows the adjustment of the different parameters for a model of six parameters, including error, which is the estimate of least squares adjustment parameters can be obtained using the weighted sum of squares (weighted sum of squares, WSS), using Eq. (12):


**Table 7.** Parameters of creep tests obtained by adjustment to a model of six parameter biocomposites LDPE-Al-Fique 10 and 30% incorporation biocomposites of fibers.

$$\hat{\mathbf{w}}\text{SSC} = \sum\_{i=1}^{n} \mathbf{w}\_{i} \left[ \boldsymbol{\varepsilon}(t\_{i}) - \hat{\boldsymbol{\varepsilon}}(t\_{i}) \right]^{2} \tag{12}$$

where *ε*( *ti*) represents the observed at time *ti* experimental deformation (*ti* ), *ε* is estimated by the model, and *wi* the difference between two samples of time deformation. A smaller value indicates a better fit WSS model to experimental data.

In **Figure 14**, we can see micrographs obtained by electronic scanning microscopy of an LDPE-Al-Fique biocomposite, where you can see that the fiber has a hydrophobic property, not adhere completely to the polymer, while aluminum has some adhesion but the manufacturing process of compression molding favors the adhesion between the faces, which can be reflected in a decrease of mechanical and viscoelastic performance of the biocomposites.

**Figure 14.** SEM micrographs of LDPE-Al-Fique biocomposite.

Currently, it is known that these defects can be corrected by surface treatments in the fibers, primarily using coupling agents or modifications to the polymers to achieve a greater adhesion and micromechanical relationship to reduce the strain rate.
