Composite Materials for Hybrid Aerospace Gears
Open Access
- Author:
- Waller, Matthew David
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 10, 2021
- Committee Members:
- Kevin Koudela, Major Field Member & Dissertation Advisor
Namiko Yamamoto, Outside Unit Member
Charles Bakis, Chair of Committee
Robert Campbell, Outside Field Member
Sean McIntyre, Special Member
Albert Segall, Program Head/Chair - Keywords:
- hybrid gear
composite materials
pitch-based carbon fiber
BMI-2
BMI
bismaleimide
fatigue
mechanical testing - Abstract:
- Hybrid gears, which combine steel and fiber-reinforced polymer (FRP) composite materials, are an emergent technology for weight reduction of rotorcraft drivetrains. Research has shown feasibility of the hybrid gear concept in small-scale (8.9 cm/3.5" pitch diameter) spur gears under controlled, low-temperature (48°C/120°F) test conditions, and preliminary hybrid gear designs are approximately 20% lighter than all-steel gears. The high operating temperatures of rotorcraft gearboxes (approximately 93°C/200°F), combined with oil-off performance requirements and the high-cycle nature of gear loading, create a unique set of requirements for the composite material. In this context, thermal stability (for maximum operating limit of at least 288°C/550°F), thermal conductivity of approximately 50 W/(m-K), and resistance to fatigue damage are identified as the most important material characteristics. This work aimed to extend the capability of hybrid gears to meet aircraft gearbox performance expectations, including oil-off capability, by developing composite materials tailored to meet these characteristics. This study consisted of several inter-related research tasks, starting with materials selection. Based on a review of the literature, promising constituent materials were identified in Chapter 2. These included various carbon fiber types for strength, stiffness, and thermal conductivity. Numerous polymer resins were considered, with modified low-temperature-cure bismaleimides (BMI) selected as the most appropriate. Test panels were fabricated by a variety of methods including autoclave prepreg, channel-flow resin transfer molding (RTM), and vacuum-assisted resin transfer molding (VARTM). In Chapter 3, finite element analysis was employed to predict stress and strain in a hybrid spur gear geometry at the ply-level. These results were useful for narrowing the mechanical property requirements. In Chapter 4, overall performance of the composites was evaluated in terms of static mechanical and thermal properties. Notably, Chapter 5 contains the first study of fatigue performance of multi-directional, pitch-based carbon fiber composites, and Chapter 6 contains the most comprehensive study of the properties of low-temperature cure carbon/BMI composites to date. Major contributions from this work include new insights into fatigue of ultra-high-modulus, pitch-based carbon fiber composites. Specifically, it was shown that the fiber modulus greatly affects fatigue mechanisms and failure modes, presumably by its effect on matrix strain and consequent matrix crack development or lack thereof. Compared to similar laminates made with standard-modulus, PAN-based carbon fiber, the pitch-based carbon fiber composites exhibited higher normalized fatigue strength, a flatter S-N curve (slope of -6.5 MPa/decade versus -44.6 MPa/decade), less matrix damage, and a more fiber-dominated failure mode. Despite their relatively low quasi-static tensile strength (48% lower than equivalent PAN-based carbon laminates), the extrapolated S-N curves of ultra-high-modulus, pitch-based carbon/epoxy and standard-modulus, PAN-based carbon specimens were shown to intersect at approximately N = 20.4*10^6 cycles. Other contributions include thorough testing of the novel bismaleimide resin known as BMI-2, which is unique and appealing to many applications due to its ability to attain a high degree of cure (96%) from a low cure temperature (163°C/315°F), out-of-autoclave infusion processing characteristics, and high glass transition temperature (onset at 366°C/691°F). It was found that, compared to other commercially available BMI resins, BMI-2 attains a significantly higher Tg from a lower cure temperature, enabling production of high-temperature composites with lower residual stress. Other testing included quasi-static tension, compression (at a range of temperatures, and after environmental conditioning), and shear; DSC, TGA, and moisture absorption. In quasi-static tension, it was found that multi-directional carbon/BMI-2 laminates failed by delamination, which is an unusual failure mode for this loading condition, and may be indicative of a particularly brittle matrix or poor fiber/matrix interface.