Stress relaxation and structural performance analysis of conical viewport for deep-sea submersible

For its high transparency, polymethyl methacrylate (PMMA) is widely applied as observation windows or detector protective covers of various aircraft and submersibles. Firstly, a modified viscoelastic Prony series model with three parameters has been derivated and applied into triaxial stress state. Based on stress relaxation test data of compression specimens made of PMMA, the relaxation parameters of the modified Prony series model are determined. Then, the PMMA specimens and conical viewports were simulated on software ABAQUS, coupled with presented improved stress relaxation model, and verified its accuracy in stress relaxation analysis. Moreover based on the finite element method (FEM) with improved stress relaxation model, the relaxation behavior of PMMA conical viewport with multiple parameters and structural optimization was accomplished accordingly, which shows that there is a good consistency between the numerical and experimental results. Finally, the presented numerical model had been applied into relaxation behavior analysis of conical viewport under high external pressure. Results indicates that, the structure of the conical viewport with a cone angle of 70(degrees) is more reasonable and optimized, and the stress relaxation level performance of viewport is better, which reveals that the thickness increasing, bigger than delta/D-i = 1, will have a few response to its stress relaxation degree.