Breaking Through the Bottleneck of High-Speed Impact: Innovative Application of Discontinuous Thermoplastic Composites in Aero-engine Field

In the wave of the aviation industry's pursuit of efficiency and sustainability, weight reduction and energy consumption reduction have become core objectives. Composites have demonstrated remarkable effectiveness in replacing heavy metal components in aero-engines, and discontinuous long fiber (DLF) thermoplastic composites show even greater potential. Greene Tweed's DLF materials have achieved an average weight reduction of 35% in over 500 types of aerospace components, far outperforming aluminum. However, to apply them to the key parts at the front end of turbofan engines that are vulnerable to hail impact, it is necessary to overcome the challenge of high-speed impact performance, clarify the influence of composite material composition, microstructure, and performance after hybridization with continuous fibers, and ultimately establish a set of design methods and solutions that balance impact resistance.

DLF Materials: Basis and Challenges for Aviation Applications

DLF thermoplastic composites (TPCs), using aviation-grade carbon fiber/PEEK unidirectional prepreg tapes as raw materials, are cut into thin sheets and then molded. They have successfully replaced metals in complex-shaped aerospace components for many years. These materials exhibit excellent chemical resistance, stiffness, high-temperature stability, and anti-creep properties above the glass transition temperature. They also enhance design freedom through integrated molding, enabling the integration of multiple parts into a single structure. Greene Tweed has completed the quasi-static mechanical property characterization of its DLF materials, accumulating key data on high-temperature strength, fatigue, and creep, which supports an analysis-based design approach.

The fan platform at the front end of the engine is an important target application component for DLF materials, enabling a weight reduction of over 8 pounds per engine compared to metal. These components have complex geometries. Traditional continuous fiber materials (such as unidirectional tapes and woven fabrics) require complex lay-up or resin transfer molding (RTM)-like processes for shaping. In contrast, DLF materials, through the Xycomp series of molding compounds, can rapidly manufacture complex components using automated, repeatable near-net-shape molding processes. Additionally, the thermoplastic matrix endows the material with high recyclability from raw material to finished product and meets the smoothness requirements of aerodynamic surfaces.

Although DLF materials have been installed in nearly 500,000 parts across 12 commercial aircraft (including structural brackets, housings, and cover plates), their application at the front end of engines still faces the severe challenge of high-speed hail impact. Previously, although DLF prototype fan platforms could meet conventional requirements such as maximum overspeed strength and dynamic performance, their performance in high-speed hail impact tests fell short of expectations, becoming a key bottleneck restricting their broader application.

Plate Impact Testing: Revealing Damage Mechanisms and Optimization Directions

To systematically characterize the impact performance of Xycomp DLF materials, Greene Tweed adopted a "building block pyramid" research approach, starting with plate specimen testing and then progressing to validation with molded demonstration parts. The testing used 2-inch-diameter spherical transparent hailstones launched by a self-made impact testing device, with a high-speed camera recording at 10,000 frames per second to capture the damage process. The test plates, measuring 6×12 inches with a thickness of 0.15 inches, were fixed with their long edges tilted at 30° horizontally, focusing on studying the impact resistance of different material combinations.

(Image description placeholder for the 6×12×0.15-inch plate impact testing setup)

Impact Testing Results of 6×12×0.15-inch Plates: Anti-impact Performance of Different DLF Compositions under 2-inch Hail Impact (with Impact Velocity as the Variable)

The test samples covered a variety of material configurations: including UD tape-cut thin sheets of AS4 and IM7 carbon fibers combined with PA6, PEEK, and PEKK matrices, two types of AS4/PEEK materials with consistent quasi-static performance (for comparing the influence of prepreg tape structure), S2 glass fiber-reinforced PEEK material, and carbon fiber/PEEK material using a new "2.0" thin sheet shape (which had previously increased tensile strength by over 50% in quasi-static and fatigue tests). Additionally, the tests included quasi-isotropic continuous fiber laminates, cross-ply fabrics, and hybrid structures with "continuous fibers on the impact surface + DLF on the back," while also studying the influence of plate thickness on impact performance.

 and aero-engine demonstration platform (bottom))

The test results revealed the core damage mechanism of DLF plates: under high-speed impact, tensile cracks initiate from the impact point on the back of the plate and propagate towards the top. This is because the tensile strength of DLF materials is significantly lower than that under other load forms. High-speed impact generates local bending stress below the impact area, and failure occurs when the stress exceeds the tensile strength. High-speed camera footage taken from the back showed local "tearing" of the material on the impact side, while the front showed no visible damage, a phenomenon confirmed by computed tomography (CT) scans.

In terms of performance influencing factors, several key conclusions were drawn from the tests: Each 1-millimeter increase in plate thickness improved the impact resistance velocity by approximately 40 m/s; replacing AS4 with IM7 carbon fibers did not improve impact resistance; hybrid structures with continuous fibers on the impact surface did not enhance performance, further confirming the tensile failure mechanism; while replacing carbon fibers with S2 glass fibers, using polymer matrices with lower crystallinity, or reinforcing the tensile side (back) of the specimen with continuous fibers (with DLF on the front) all improved impact resistance.

The most significant performance breakthrough came from the application of the new "DLF 2.0" thin sheet shape. This patented design reduces stress concentration at the ends of fiber thin sheets by optimizing their geometry, significantly improving the material's apparent toughness. Without changing other parameters, DLF 2.0 specimens demonstrated superior anti-hail impact performance compared to continuous fiber laminates, including quasi-isotropic laminates made of the same base material. The research team believes that the high-speed impact performance of DLF components is not dominated by a single compositional parameter but depends on the apparent toughness of the composite material—a metric determined by the composition combination, interface bonding state, and prepreg tape microstructure. The optimization of the microstructure through thin sheet geometry is the key to the performance leap.

(Image description placeholder for a rendering showing the five impact locations tested on the demonstration platform, with the most critical location highlighted at the top. This shows the test results of 2-inch hail impacts on platforms made of various materials, where "MM" (far right on the x-axis) indicates changes in molding parameters to better utilize the new DLF 2.0 sheet material.)

Fan Platform Demonstration Part Testing: Validating Practical Application Feasibility

Based on the key findings from plate testing, Greene Tweed designed and manufactured a fan platform demonstration part with a structure highly similar to actual components in engines with thrust ratings of 25,000-35,000 pounds. The core design principle was to control impact deflection through geometric structure optimization: center ribs were set inside the component, reinforcing gussets were arranged along the sides, and the unsupported range of the aerodynamic surface impact area was reduced to ensure that deflection under hail impact remained at a safe level.

Five impact locations were selected for testing, with the top right corner identified as the most dangerous due to its largest unsupported area. The team tested key hybrid materials screened from plate studies at this location. The results showed that even with standard DLF materials, the impact resistance target could be achieved through design optimization. Although DLF 2.0 materials offered better performance, the达标 (meeting the standard) of standard DLF materials meant there was no need to recertify new materials, significantly lowering the application threshold. Meanwhile, the design criteria refined from plate studies provided crucial support for improving the impact resistance of the demonstration part, with DLF 2.0 remaining an alternative for scenarios with higher performance requirements.

Micro-CT scan results after impact were consistent with plate testing: under the local bending-induced tensile failure mode, components without visible surface damage showed no internal damage. This is because the maximum strain under such loads is concentrated on the outer surface of the component, making key damage areas easily observable and detectable. Ultimately, the DLF fan platform demonstration part manufactured using the certified AS4/PEEK carbon fiber material (Xycomp 5175) successfully passed high-speed hail impact tests, validating its commercial application feasibility.

(Image description placeholder for Greene Tweed's aero-engine fan platform demonstration made of Xycomp discontinuous long fiber (DLF) thermoplastic composite material.)

Technological Breakthroughs Empower Composite Material Application Upgrades

Through this targeted research and development on the high-speed impact performance of DLF materials, Greene Tweed has expanded several core capabilities based on its existing production foundation, including hybrid material combination design, geometric structure optimization, non-destructive testing/micro-CT applications, and continuous iteration of DLF 2.0 materials, driving discontinuous thermoplastic composites towards more demanding aviation application scenarios.

The success of this research not only validates the application potential of DLF materials in key parts such as the front-end fan platforms of aero-engines but also solidifies their comprehensive advantages over machined aluminum in terms of weight reduction, performance, and cost. As the technology continues to improve, discontinuous thermoplastic composites are expected to achieve wider applications in the aviation industry, injecting new impetus into the efficiency enhancement and sustainable development of aviation equipment.