3.1.3. Bending Test

One of the most significant mechanical properties is the flexural strength and varies with specimen depth, temperature, and test span length [71]. The flexural strength ( *<sup>σ</sup>bh*), (MPa), known as the maximum stress at break, can be quantified using Equation (5):

$$
\sigma\_{\rm bh} = \frac{\sigma\_{f2} - \sigma\_{f1}}{\varepsilon\_{f2} - \varepsilon\_{f1}} \times 100\% \tag{5}
$$

The flexural or bending test is typically used in the quantification of the flexural strength. The flexural modulus (*Eh*), (MPa) can be estimated with Equation (6):

$$E\_h = \frac{1}{4} \frac{L^3}{bh^3} \frac{\Delta F}{\Delta f} \tag{6}$$

where *F* is the breaking force in (N), *L* is the support distance in (mm), *b* is the width (mm) of the specimen in (mm), *h* is the thickness of the specimen in (mm), Δ*F/* Δ*f* is the slope of the force-deflection curve. The load is applied at the specimen's center under standardized bending speed, temperature, and humidity. It is upon the most common type of loading encountered and essential in determining the polymer's characteristic values. The initial flexural modulus can be calculated using a slope of the line measuring Δ*f* and Δ*F* as presented in Figure 7, depicting a typical force bending flexural test.

**Figure 7.** The initial slope of the bending force-deflection diagram. Adapted from [77].

Lim et al. [79] use a combination of the different fillers of aluminum oxide (Al2O3), Zinc oxide (ZnO), and organoclay. However, it was discovered that with 1.5% aluminum oxide, improved mechanical, burning rate, and dielectric breakdown compared to Zinc oxide and organoclay. Hedir et al. [80] irradiated the crosslinked polyethylene (XLDPE) with fluorescent lamps for 240 h. The results show a decline in resistivity, mechanical properties, and contact angle, increasing water retention and weight loss. Yasmin and Daniel [81] and Yang et al. [82] attributed that the tensile strength and elastic modulus can be increased by adding fillers at optimum content. Ray et al. [83] research on filler effect by using up to the upper limit and postulated that the stiffness and rigidity increase up to a certain level. Afterward, it harms the mechanical properties.
