*3.1. Mechanical*

Polymer chemical and physical properties are dependent on their molecular weight, chemical composition, and physical structures [70]. One of the properties of polymeric material is strong, which is the stress required to break the sample [6]. These strength properties are as follows: tensile, compressional, flexural, impact [71]. Factors affecting the strength of polymers are molecular weight, cross-linking, and crystallinity [71].

Young's modulus or tensile modulus is the induced stress divided by strain in the elastic region [71]. Ultimate elongation is defined as its ability to undergo deformation. Toughness measures the energy absorbed by the material to deform without fracturing.

The mechanical properties of the polymer are affected by temperature. Figure 4 shows a typical graph of stress versus strain and the effect of temperature, with an increase in the temperature, the elastic modulus, and tensile strength decrease, but the ductility increases [71].

**Figure 4.** Effect of temperature on the mechanical properties of the polymer. Adapted from [71].

Viscoelasticity, two types of deformations exist, namely elastic and viscous. Elastic deformation is recoverable, while viscous deformation is a plastic deformation where the deformation is permanent upon removing the applied stress [71].

HV insulating systems' fabrication involves utilizing composite polymer; its application could be seen in electrical appliances. They are subjected to quivering wear and tear due to the rate of occurrence of magnetic force and shearing stress. To improve the mechanical performance of polymers, inorganic fillers are added to the polymers [72]. To enhance the strength and toughness is the need for polymer nanocomposites [73]. The composites' mechanical properties are strongly influenced by the filler's size and shape, the matrix properties, and the interfacial adhesion between the filler and matrix [74]. The basic principles of mechanical properties comprise tensile, compressive, bending, shear, and impact behavior is of importance [75]. The polymeric material is tested with standardized tests such as tensile strength, Izod impact strength, and softening point, but its application in a critical situation increases. Thus, the choice of material used depends upon a balance of stiffness, toughness, processability, and price in applying polymer [76].

## 3.1.1. Impact Behavior

Impact energy is known to reduce the static strength, reliability and improving tensile properties that simultaneously impact property. The basic stuff is impact toughness, which measures the needed energy to split a particular specimen. Figure 5a,b show supported beam load and cantilever beam load, respectively [77]. The determination of the Charpy impact strength of an unnotched (*acU*) (KJ/m2) specimen is given in Equation (1).

$$a\_{cII} = \frac{\mathcal{W}\_c}{b \times h} \tag{1}$$

**Figure 5.** (**a**) Charpy (beam load) and (**b**) Izod (cantilever beam load). Adapted from [77].

Notched Charpy impact strength (*acN*) (KJ/m2) can be determined by Equation (2), where *Wc* is the absorbed energy, *b* (m) is the with, and *h* (m) is the height.

$$a\_{cN} = \frac{W\_c}{b\_N \times h} \tag{2}$$

The difference between Charpy impact strength acU and notched Charpy impact strength *acN* indicates how sensitive a material is to external notches, i.e., takes the problematic notch effect for the Charpy impact test into consideration and shows how effective fillers are. Thus, notch sensitivity can be calculated from the quotients of *acN* and *acU* indicated in Equation (3):

$$k\_z = \frac{a\_{cN}}{a\_{cII}} \times 100\% \tag{3}$$

3.1.2. Tensile Test

Tensile tests involve the application of tensile force causing the elongation until the specimen breaks, and various loading conditions are essential and cannot be overemphasized [78]. The young modulus (*E*) (kN/m2) can be estimated with Equation (4):

$$E = \frac{\sigma\_2 - \sigma\_1}{\varepsilon\_2 - \varepsilon\_1} \times 100\% \tag{4}$$

where *σ*2 − *σ*1 is a change in stress (kN/m2) and *ε*2 − *ε*1 is a change in a strain which is dimensionless. A typical stress–strain curve is shown in Figure 6. Non-flexible polymers have high Young's modulus, while ductile polymers are elastic modulus with the ability to withstand extended elongation without fracturing [71]. In contrast, elastomers have a low stress–strain relationship with tough elastic texture [71].

**Figure 6.** Stress–strain curves of various polymeric materials. Adapted from [77].
