as the primary structural design for the observation window in submersibles. The spherical fan-shaped design stands out as the most ideal form for withstanding external pressure in observation windows. [PERSON] conducted thorough theoretical and experimental research focusing on the performance and design of observation windows. Comprehensive long-term hydrostatic pressure tests were executed on conical frustum observation windows with varying cone angles and different window diameter-thickness ratios. The results provided valuable insights into the critical pressure and axial displacement of conical frustum windows under different working conditions [3; 4; 5]. Polymethyl methacrylate (PMMA) is a commonly selected material for deep-sea viewport windows due to its exceptional material properties [6]. PMMA possesses excellent properties such as superb colorability and transparency. In particular, the observation windows made of transparent PMMA are specially crafted for underwater observation and are connected to the titanium hull using O-ring sealing equipment. In order to ensure safety, the prescribed procedure requires the observation window to be tested in the pressure cylinder before being put into use to validate its reliability. PMMA demonstrates significant deformation under high-stress load conditions due to its relatively small elastic modulus. Furthermore, PMMA exhibits both elastic and viscous deformation characteristics. Creep is an inherent characteristic of polymers, and it can manifest even at room temperature [7; 8]. [PERSON] et al. [9] conducted creep tests under various stresses and temperatures, developing a UMAT (User Material Subroutine) program to simulate the creep behavior of PMMA. [PERSON] et al. [10] investigated through both experimental and numerical methods at different temperatures (30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C) and stress levels. Furthermore, [PERSON] et al. [11] investigated the non-monotonic creep behaviors exhibited by polymers. The aforementioned scholars conducted a series of creep tests and analyses using PMMA samples. However, the inherent creep issue in polymer observation windows represents a long-term damage mechanism. To ensure safety, observation windows must meet the necessary strength and stability requirements [12]. Therefore, it is crucial to study the behavior of observation windows under underwater pressure conditions. The observation window for a manned submersible typically has a conical shape. In the case of deep-sea submersibles, either 90\({}^{\circ}\) conical-frustum windows or spherical windows can be employed. However, for practical manufacturing considerations, a 90\({}^{\circ}\) conical-frustum window is generally preferred. During the submersible's service, the viscoelastic properties of the PMMA material may lead to the deformation of the viewport window over time, a phenomenon known as creep [13]. Hence, certain researchers have conducted stress analyses on frustum viewport windows, considering various thickness-to-diameter ratios and angles [14; 15]. [PERSON] and [PERSON] [16; 17] performed both theoretical and numerical calculations for various viewport conical angles, ranging from 60\({}^{\circ}\) to 120\({}^{\circ}\)  
Figure 1: The observation window of deep-sea manned cabin. They suggested that the preferred included conical angle falls within the range of 80\({}^{\circ}\) to 120\({}^{\circ}\). [PERSON] [12] examined the fatigue behavior of PMMA windows and diving shells using the rain flow counting method and cumulative damage principle. In the high-pressure seawater environment, the space between the inner window and the window seat within the metal spherical hull undergoes compression as a result of increased displacement. Increased displacement could potentially lead to the complete destruction of the observation window.