The shell section of an AO Smith Layered Pressure Vessel (LPV) built in the 1960’s and in prior service at the NASA Marshall Space Flight Center is evaluated for crack instability, leakage rates and related growth rates. The objectives of this project were: (i), to determine limiting flaw sizes for various crack sizes in the shell layer, (ii), to apply an analytical model which approximates leakage rates due to various cracks with and without interlayer gaps and (iii), to calculate related crack growth rates for typical in service cycles of the AO Smith LPV. Limiting flaw sizes for axial and circumferential cracks in the shell layer were first determined in accordance with the API 579-1 standard. Leakage rates were then calculated incorporating compressibility and frictional effects. A first approximation analytical method was also developed in order to model the effect of interlayer gaps on leakage rates. This model applied the isentropic compressible flow relationships while accounting for friction using the Darcy‑Weisbach friction factor. A fatigue crack growth analysis was also performed using the Barsom equation for crack growth. The aspect ratios of the circumferential surface cracks at their limiting flaw size were found to be approximately 20% of the aspect ratio of axial surface cracks at comparable sizes. Leakage rates for axial through cracks were higher than those for circumferential through cracks of the same aspect ratio. The interlayer gap length was found to be inversely proportional to leakage rate, however frictional effects decreased with increasing crack size. Fatigue crack growth analysis indicated that neither axial nor circumferential cracks will be unstable for typical service pressure cycles, for sizes below their respective LFS. Recommendations for methods to monitor crack sizes in the shell layer of the LPV while in service are also provided.
AEROSPACE SMART STRUCTURES
Drone applications are becoming ubiquitous in both civilian and military environments. Research on the development of a drone’s ability to monitor its structural health is relatively sparse; additionally the application of non-traditional flight concepts to drone performance (ex. flapping wings, smart aerodynamic surfaces) is limited. I am interested in developing dynamic systems that can accurately model the aforementioned cases. Some questions would include: how many and what type of sensors would allow an intelligent system to monitor the extent of damage to its structure? How is damage charaterized? Where and in what configuration should they be placed? Can the aircraft skin temporarily repair itself?
The material and geometric properties of implants and fixation devices remain essentially constant over time. As these systems directly interface with bone (with time and loading dependent properties), the time and loading dependency mismatch results in unnatural stress distributions within the bone. This may lead to early arthritis, osteoporosis, and loss of flexibility or mobility. Additionally, there are no implants available that allow patients to perform athletic activities that involve repetitive impact such as football, martial arts or running. My objective is to develop solutions for smart robust implants with dynamic properties that are able to accommodate changing bone properties and have impact properties that allow participation in repetitive high impact activities. These solutions involve dynamic and static analysis, mathematical modeling, simulations and experimental work involving 3D printed bioresorbable composites.
When dynamic compression plates are used for long bone fracture repair, the healing process, while generally successful, is not optimal. The bone modulus gradually increases while the plate modulus remains constant. This non-response by the plate to the changing modulus of the healing bone results in undesirable localized stress shielding during healing and even after healing, when the plates are retained. A composite plate with a modulus that reduces as the fracture heals is therefore proposed. The hybrid internal fixation device design is comprised of Poly-L-lactide and Hydroxyapatite (HA/PLLA) and Titanium. A preliminary mathematical model is developed that predicts the rate of reduction of the elastic modulus for this composite internal fixation plate. This first level model is based on composite theory while ignoring viscoelastic effects. The Finite Element Method (FEM) is used to establish relationships between geometrical constraints (material interfaces) while a theoretical elastic modulus is determined using the Reuss model. The geometrical relationships established by the FE static model are combined with the dynamic math model to predict the rate of decrease of the elastic modulus of the hybrid device. This first level model predicts that the composite plate modulus decreases at a rate that appropriately compensates for the increasing modulus of the healing bone during fracture healing. This suggests that the proposed composite fixation plate has a strong potential for improved fracture healing through the reduction of stress shielding while the fracture heals, and the elimination of stress shielding after fracture healing.