An Empirical Life Assessment Framework for PMMA Pressure Hulls

Abstract

The lifecycle management of transparent poly(methyl methacrylate) (PMMA) pressure hulls in manned submersibles is currently dictated by rigid, calendar-based retirement schedules mandated by classification societies. While these empirical rules ensure absolute safety, they often enforce the premature disposal of highly engineered structures regardless of their actual physical condition. Transitioning toward a condition-based lifecycle assessment requires a reliable method to non-destructively quantify the internal viscoelastic degradation of the polymer matrix.

This thesis presents an integrated empirical framework coupling ultrasonic Structural Health Monitoring (SHM) with analytical Continuum Damage Mechanics (CDM). Low-frequency ultrasonic through-transmission was used to evaluate PMMA pressure hulls exhibiting a spectrum of operational fatigue, ranging from zero to 929 deep-ocean dives. By analyzing acoustic time-of-flight and signal attenuation, the dynamic storage modulus and loss modulus of the polymer network were extracted. An inverse optimization algorithm then translated these empirical measurements into fleet-specific material degradation constants.

Regression analysis of the operational histories demonstrated that chronological time-in-service, rather than cumulative hydrostatic loading or extreme pressure dives, is the dominant driver of macroscopic structural degradation. The PMMA matrix undergoes a continuous thermodynamic process of structural relaxation, dictating a logarithmic decay in elastic stiffness and an exponential increase in internal friction over time.

By applying Dynamic Mechanical Analysis (DMA) principles to an extrapolated fifty-year operational timeline, a critical structural inflection point was mathematically identified, establishing a condition-based failure threshold at a 2.50 percent reduction in pure elastic stiffness. The predictive envelope confirms that the structural stiffness of the PMMA fleet stabilizes safely above this limit, indicating that current calendar-based retirement schedules are conservative. This framework provides the scientific foundation necessary to track ongoing material health, enabling the maritime industry to safely transition toward condition-based lifecycle management.