Material’s Response Due to External Loading

Experiments were conducted in the mechanical workshop where a height gauge meter (digital) is used to identify material’s Young’s Modulus. Besides, each material is placed to the indenter base and then load is applied. When load is applied to the material, real time data are collected by the data acquisition software. In this section, response of different types of the viscoelastic materials are shown under external loading conditions.

• VC60135

During the experimental setup, the sensor was placed underneath the material and, it was found that force does not transmit to the sensor instantly due to the viscoelastic property of the materials. Also it was found that the load becomes constant after a period of time. The amount of load applied to the material was found different from the force transmitted to the sensor. When the external load was applied to the wooden disc (Figure 4.1), the indenter starts deforming the viscoelastic material and the sensor reads the deformation values. But due to the thickness of the material and sensor characteristics, the load transmitted to the material is different from the load applied to it.

The variation of applied force and transmitted force is described by the term called

Transmissibility Factor. Transmissibility Factor is defined as the change of percentage in transmitted load to the sensor. It is characterized by the following equation 5.2 below.

ζ = Ft

Fa

(4.2)

Whereζ is the transmissibility factor of the surface material and Fais the total force placed

on the top of the indenter and Ft is the transmitted force to the sensor.

Transmissibility factor is considered as an important parameter for materials. In a real life scenario, when someone is lying or sitting on a surface then the actual force applied by the person would not be the same at the sensor placed underneath the surface.

This difference will occur due to transmissibility. Especially for Pressure ulcer research, it is important to know the material’s transmissibility factor. If the factor is known then it can be simply multiplied with actual force and then transmitted force can be obtained. For example, a load of 4.5N is applied to the material but due to thickness of the material, the load transmits to the sensor is 0.69 N. So only 15.3% of applied is transmitted. So the transmissibility factor of the material is 0.153. It has been found from all other experiments, that change in thickness and change in load have an impact on transmissibility. This is why transmissibility values are considered for individual material. Also it has been found that if the material is not very thick then after some variation in loading, the transmissibility reaches 100% but same loading gives different transmissibility when thickness is changed. Finally, after conducting experiments, the transmissibility factor is identified experimentally for all the different types of materials with different thicknesses.

First, the load was varying from 4.5 N to 21.26 N for VC60135 and the thickness remains same. After this experiment, transmissibility is calculated for different thickness of the material. In figure 4.3, transmissibility is found proportional with the load. Also, it depends on the thickness of the material. When the thickness starts increasing then transmissibility of the material starts decreasing. Next, the thickness of the material is varying and the

Fig. 4.3 Applied Load vs. Transmissibility (VC60135)

transmissibility was measured. In this stage the applied load remains same. Figure 4.4 shows the change of transmissibility due to thickness. When the applied load was 21.26 N, maximum transmission (72%) occurs for 0.02 m thickness but for same amount of load, transmissibility becomes 9.41% when thickness increases from 0. 02 to 0.07 m.

Figure 4.5 shows applied force vs. transmissibility for all five different materials. Thickness

Fig. 4.4 Change of transmissibility due to thickness (VC60135)

of all the materials remain same (0.015 m) and load varied from 4.5-21.26 N. Now due to load variation from a very small amount to a higher amount, material’s transmissibility changes significantly e.g. VASCO40, VASCO50 and VASCO60 materials transmit 100% of applied load when 21.26 N applied. In order to observe the transmissibility at higher thickness for different materials, thickness vs. transmissibility is plotted.

Figure 4.6 shows thickness vs. transmissibility graph.

Here, thickness of all five materials varies from 0.015 m to 0.07 m and the applied load is 4.5 N. For VC60135 material, maximum transmissibility is found 20.5% at 0.015 m thickness and minimum transmissibility (2.69%) occurs at 0.07 m thickness. VASCO40, VASCO50 and VASCO60 provide higher transmissibility compare to VC60135 and VL75075. This is identified by comparing experimental data. As discussed earlier, even at a higher load, these materials provide high transmissibility (almost 100%). Figure 4.7 shows the transmissibility

Fig. 4.6 Thickness vs. Transmissibility (for five materials at 4.5 N).

Fig. 4.7 Thickness vs. Transmissibility (for five materials at 21.26 N)

factor for all five materials at a load of 16.35 N. After conducting experiments for individual materials, a comparison is done between all the materials.

Also applied force vs. transmissibility, thickness vs. transmissibility graphs are plotted to show different transmissibility factor for different materials. Figure 4.5 and 4.6 show the comparison between all different types of viscoelastic materials. When applied load increases to 4.5 N to 21.26 N transmissibility for VL75075, VASCO40, VASCO50 and VASCO60 become 100%. But for VC60135 it is 71.2% which is 30% less compare to other four materials. In this stage, transmissibility factor is considered as an important factor because it can provide information about the load applied and load transmitted (when the sensor is placed underneath the surface). But this factor becomes less important when the sensor is placed at the top of the surface. To identify the behaviour of viscoelastic material, the sensor is placed at the bottom of the surface and then load was applied to the material (Figure 4.1).