## 4 Parametric Analysis on Creep Deformation of PMMA Frustum Observation Windows
### Effect of Friction Coefficient on Creep of Frustum Observation Window  
Under the influence of deep-sea pressure, variations in the friction coefficient at the contact surface between the observation window and the window seat can impact the stress and deformation of the observation window. Six groups were taken between the friction coefficient of [0.001-0.3], which were 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3, respectively. The finite element model parameters for the observation window in this section, based on the analysis of the aforementioned loading rate, are specified as follows: the observation window's mesh size is 5 mm, the window seat's mesh size is 15 mm, the temperature is set at 30 \({}^{\circ}\)C, and the loading rate is 4.5 MPa/min. The calculation employs the improved time-hardening creep model subroutine. Figure 19 reveals notable variations in the maximum stress at the center of the inner surface of the observation window during the loading stage prior to creep. Specifically, when the friction coefficient is 0.05, the maximum stress is 17.0423 MPa, whereas it increases to 55.5423 MPa with a friction coefficient of 0.3. Following creep, the maximum stress in the observation window decreases with an increase in the friction coefficient, and the stress gradually stabilizes over time. It can be seen from Figure 20 that before and after the creep of the observation window, the \(y\)-axis displacement at the center of the lower surface decreases with an increase in the friction coefficient of the contact surface, and the axial displacement of the observation window after creep is greater than that before creep. The maximum axial displacement is 7.7638 mm when the friction coefficient is 0.05, and the minimum axial displacement is 6.0814 mm when the friction coefficient is 0.3. As illustrated in Figure 21, the strain at the center of the inner surface of the observation window progressively increases with an elevation in the friction coefficient. When the friction coefficient is 0.05, the maximum strain is 0.0053; when the friction coefficient is 0.1, the maximum strain is 0.0071; when the friction coefficient is 0.15, the maximum strain is 0.0099; when the friction coefficient is 0.2, the maximum strain is 0.0131; when the friction coefficient is 0.25, the maximum strain is 0.0157; and when the friction coefficient is 0.3, the maximum strain is 0.0174. In summary, an elevation in the friction coefficient on the contact surface of the observation window leads to increased adhesion, resulting in a reduction in the creep displacement of the viewport window. Notably, both stress and strain at the center of the inner surface of the observation window show an upward trend with an increase in the friction coefficient, eventually stabilizing. Consequently, for the finite element simulation of the observation window, a friction coefficient of 0.1 is recommended. It is advisable to appropriately lubricate the contact surface during installation to mitigate potential safety hazards.  
Figure 19: The variation curve of observation window stress with friction coefficient.