BY Om | Page no 1 Topic 2: Military Jets BY Om | Page no 2 Introduction Supersonic fighter - jet wings face very tough conditions during flight. They must withstand high mechanical loads, aerodynamic heating and rapid vibrations that can damage materials. For the purposes of this Topic (Military Jets) this study targets tactical fighter - class aircraft that operate primarily in the supersonic envelope (typical operational speeds up to ≈ Mach 1 – 2). Accordingly, the material selection emphasises polymer composites that retain stiffness, damage tolerance and vibration damping for win g skins and primary structure at surface and skin temperatures up to roughly 200 – 260 °C Caveat (hypersonic): Hypersonic flight (Mach ≥5) produces very high local stagnation/leading - edge temperatures (reported up to ≈1600 K in computational studies); such local zones exceed polymer limits and therefore require ceramic/metallic thermal protection systems (e.g., C/ SiC or metallic TPS). Hypersonic leading - edge conditions are discussed separately below. Problem Statement Aircraft wings, especially those on supersonic fighter jets, operate under exceptionally demanding conditions that challenge conventional materials. These wings are subjected to intense mechanical stresses during aggressive maneuvers, severe thermal loads from aerodynamic heating at high spe eds, and persistent high - frequency vibrations from engine operation and aerodynamic buffeting. Collectively, these factors critically influence the structural integrity and longevity of fighter jet wings, necessitating advanced material solutions to sustai n performance and safety. Mechanical Stress Supersonic fighter - jet wings endure extreme mechanical loads during high - G maneuvers, causing unique stress patterns and fatigue characteristics that civilian aircraft do not encounter. In operational profiles of the F - 16 Falcon, wing structures are subjec ted to repeated bending and shear forces, leading to crack initiation at fastener holes and rib attachments. As noted in the Aircraft Structural Integrity Program’s crack database, “F - 16 wing structures experience specific failure pattern s under high - G military maneuvers distinct from civilian operations”. Twenty years of ASIP data reveal that the wing tip rib extensions and internal spar webs are particularly vulnerable, with fatigue life determined by the cyclic load spectrum in combat p rofiles. These localized high - stress regions necessitate advanced materials capable of withstanding repeated mechanical loading without significant strength degradation. BY Om | Page no 3 Thermal Loading In the supersonic fighter regime (typical tactical fighters up to ≈ Mach 1 – 2) aerodynamic heating produces moderate surface temperature increases (from tens to a few hundred °C depending on stagnation conditions and flight profile). In this regime the critical requirement for poly mer composites is modulus retention and damage tolerance up to ≈200 – 260 °C so that modal frequencies and aeroelastic margins remain acceptable under operational thermal bias. A practical polymer selection therefore targets high - performance thermoplastics ( e.g., PEEK/PEKK) that retain structural stiffness in this temperature band while offering improved fracture toughness and damping. Hypersonic caveat (separate): computational fluid – structure coupling studies show that hypersonic leading edges (Mach 5 – 6 cases) can experience local temperatures reaching ≈1600 K at stagnation points; these transient, highly localized temperatures are well above the service limits of any polymer matrix and therefore make polymers unsuitable at those local zones. Where hyperson ic leading - edge heating is relevant, those regions must be protected with ceramic or metallic thermal protection systems (for example C/SiC or metallic TPS) while polymer composites may be used in mid - chord and aft regions where temperatures remain within the polymer service window. Vibration Fatigue High - frequency vibrations from engine nacelles and aerodynamic buffeting impose additional fatigue challenges on composite wing panels. Experimental tests reveal that “carbon - epoxy wing panels exhibit dominant modal frequencies around 8 kHz under Mach 2 co nditions”, and fatigue failure occurs after 5×10^5 cycles when subjected to 150 °C thermal loads and 8 kHz vibration. These oscillations induce stress concentrations that propagate microcracks, diminishing fatigue life. Effective vibration damping — quantifi ed by loss factors exceeding 2% — is essential to attenuate high - frequency modes and extend service life. A polymer composite tailored for combined thermal and vibrational resilience must therefore deliver both high damping capacity and retention of mechanic al strength under cyclic loading. Collectively, these intertwined mechanical, thermal, and vibrational challenges underscore the need for an advanced polymer composite — such as a matrix - toughened CFRP/SiC hybrid — that sustains high stiffness at elevated temperatures, exhibits superior dampin g behavior, and endures rigorous fatigue spectra. BY Om | Page no 4 Objective of the Assignment • Characterize the bending, thermal, and vibration spectra experienced by supersonic fighter - jet wings. • Compare the performance of high - temperature CFRP, SiC - reinforced composites, and matrix - toughened hybrid composites under these spectra. • Tabulate and analyze key properties for each candidate material, including strength, stiffness, damping (loss factor), and thermal stability. • Recommend an optimized polymer composite formulation for fighter - wing service based on the comparative analysis. Literature Review / Background Study This Literature review / Background study shows us the polymer Composites for Mechanical Stress, Thermal, and Vibration Issues in Aircraft Wings Mechanical Stress in Aircraft Wings • Aircraft wings endure complex mechanical stresses during flight, including bending, torsion, and aerodynamic flutter. These loadings require materials with high strength - to - weight ratios and excellent fatigue resistance to ensure structural integrity and f light stability. Conventional materials struggle to balance these mechanical properties with manufacturability and durability especially under thermal variations. • A common structural solution is the use of composite sandwich panels made of lightweight foam cores bonded with stiff carbon fiber reinforced polymer (CFRP) skins. This design offers exceptional bending stiffness while keeping weight low. Experimental and numerical studies indicate natural frequencies and mode shapes of these panels are very sensitive to geometric variations (e.g., thickness distribution), impacting vibration behavior and accuracy in identifying material properties. BY Om | Page no 5 • Research demonstrates that thermoplastic matrix composites reinforced with carbon fibers like PEEK, PEKK, and LM - PAEK provide higher fracture toughness and damage tolerance compared to thermoset composites. These properties improve resistance to microcrack ing, delamination, and fatigue under mechanical stress. • Precise manufacturing techniques, including automated fiber placement (AFP) and fusion bonding, contribute to consistent material quality, reducing mechanical variability and enhancing fatigue life. Thermal Challenges and Management • Wings on aircraft experience thermal gradients caused by fluctuating flight temperatures leading to expansion and contraction, which induce thermal stresses critical to avoid premature failure. • Semi - crystalline thermoplastics develop residual stresses during manufacturing due to crystallization and thermal contraction, requiring optimized thermal cycles for manufacturing. • Advanced process modeling and simulations are employed to predict crystallinity evolution and residual stress distributions, which help in improving dimensional stability. • Thermoplastic composites provide excellent thermal stability and resistance to hydrolytic and chemical degradation, making them well - suited for fluctuating flight temperature environments. • Welding and fusion bonding techniques used in assembling thermoplastic composites require precise thermal control to maintain joint quality and reduce stress concentrations. • Incorporation of carbon - based nanomaterials like CNTs and graphene into composites aids in thermal dissipation, mitigating local thermal stresses contributing to damage. BY Om | Page no 6 Vibrational Problems and Mitigations • Aircraft wings face dynamic vibrations such as those from aerodynamic flutter and engine - induced vibrations. These vibrations can induce fatigue cracks and damage over time, threatening structural integrity. • Composite sandwich panels show sensitivity to vibration behavior that depends strongly on panel geometry and manufacturing accuracy, necessitating advanced numerical methods to model uncertainties for improved vibration assessment. • Carbon - based nanomaterials, including carbon nanotubes (CNTs) and graphene, offer significant improvements in vibration damping, stiffness, and fatigue life, reducing crack initiation and propagation under cyclic loading. • These nanomaterials also add multi - functional capabilities, such as enhanced electrical conductivity useful for structural health monitoring, besides improving vibration management. • The combination of high - performance thermoplastic composites with nanomaterial reinforcement enables effective vibration resistance and long - term durability. • Controlling process precision, including thickness and crystallinity, is vital to reducing vibration impact and residual stresses during manufacturing. Key recommendations for Aircrafts Wings Composites • Use carbon fiber reinforced thermoplastic composites with high - performance polymers (PEEK, PEKK, LM - PAEK) for structural applications. • Enhance composites with carbon nanotubes or graphene to improve structural stiffness, vibration damping, and thermal conductivity. • Emphasize process precision and monitoring (thickness control, crystallinity optimization) to minimize vibration effects and residual stress during production. • Develop robust numerical models that incorporate geometric uncertainties and random fields to ensure accurate design and performance verification. • Employ fast, reliable assembly techniques such as ultrasonic, induction, and resistance welding to reduce production variability and improve joint quality. BY Om | Page no 7 Methodology / Approach Adopted In this project we tackle how an aircraft wing copes with three things at once — mechanical stress, heat, and vibration — and then pick a polymer composite that can handle them all. 1. Literature and Data Review a. Read key studies on thermoplastic composites and aircraft wings (from NASA reports, Scientific Reports, Procedia, and AIAA Journal). b. Note each material’s toughness, temperature rating, and vibration behavior. 2. Material Testing Framework a. At the coupon level, measure basic properties like tensile strength and fracture toughness using Double Cantilever Beam (DCB) tests for Mode I and End - Notched Flexure (ENF) tests for Mode II. b. At the sub - element and element levels, run curved - beam and pull - off tests to see how small parts behave under load. 3. Thermal – Structural Analysis a. Model heat flow in the wing using the heat equation ρ c ∂T/∂t = ∇ ·(λ ∇ T) where ρ is density, c is specific heat, T is temperature, and λ is thermal conductivity. b. Calculate resulting thermal stresses by adding material stiffness Ke^T and geometric stiffness Ke^S so total stiffness Ke = Ke^T + Ke^S. 4. Structural Integrity Assessment a. Use ASIP crack data to find where wings usually crack under mechanical and thermal cycling. b. Calculate margins of safety for upper skin (compression), lower skin (tension), spars, and ribs (shear). 5. Vibration Analysis a. Write the wing’s motion as a sum of vibration modes: u₃(x,t) = Σ ηₖ(t) U₃ₖ(x) b. Solve for each mode’s response ηₖ(t) using η̈ ₖ + 2 ζ ₖ ω ₖ η̇ ₖ + ω ₖ² η ₖ = 0 where ζₖ is damping and ωₖ is the natural frequency. 6. Composite Selection Criteria Choose a material that combines: – High fracture toughness (GIc > 1.5 kJ/m²) – Thermal stability above 200 °C – Good vibration damping – Easy processing and repair BY Om | Page no 8 BY Om | Page no 9 Analysis and Discussion This analysis examines how modern military - jet wings endure and mitigate combined mechanical, thermal, and vibrational loads, drawing on experimental data and physics - based modeling from the provided sources. 1. Mechanical Stress and Fracture Resistance Military - jet wings face cyclic bending, shear, and torsional loads during maneuvers and gust encounters. Critical structural elements include: • Upper skin, loaded in compression (buckling), lower skin, loaded in tension (inter - fastener shear) • Internal spars and ribs, loaded in shear flow Margin - of - safety calculations ensure these elements remain below failure criteria. Fracture toughness tests quantify delamination resistance: Table 1: Fracture Toughness (Mode I and II) Material GIc (kJ/m²) GIIc (kJ/m²) Source Corbon - Fiber/PEEK (TPC) 0.7 - 2.1 0.7 - 2.1 NASA TM – 20240005376 Epoxy - based TSC 0.08 - 0.31 0.08 - 0.31 NASA TM – 20240005376 Thermoplastic composites (TPC) deliver up to an order - of - magnitude higher GIc and GIIc versus thermosets, improving damage tolerance and reducing the risk of catastrophic delamination under high bending and shear loads. 2. Thermal Loading and Aero - Thermo - Elastic Effects Supersonic regime (fighter - class): In typical supersonic tactical flight, aerodynamic heating raises wing skin temperatures moderately; transient heat conduction is governed by ρ cₚ (∂T/∂t) = ∇ ·(λ ∇ T) where ρ is density, cₚ specific heat and λ thermal conductivity. These temperature gradients modify material stiffness (elastic modulus reduction of the matrix) and therefore alter modal properties; the resulting thermal stresses augment geometric stiffne ss so that the total structural stiffness matrix can b e written as Ke = Ke ᵀ + Keˢ For polymer composite selection we therefore require materials that retain sufficient modulus and damping across the expected supersonic skin temperature range (≈RT to ≈200 – 260 °C). BY Om | Page no 10 BY Om | Page no 11 Hypersonic caveat (separate region): By contrast, hypersonic regimes (Mach ≥5) can create highly localized stagnation/leading - edge temperatures that are far higher (computational studies report leading - edge temperatures up to ≈1600 K). These transient extreme temperatures exceed polymer matr ix service limits; where such leading - edge heating may occur the structural solution must include ceramic/metallic thermal protection systems rather than exposed polymeric skins. Where Ke ᵀ = ∫ B ᵀ D ᵀ B d Ω, Keˢ = ∫ N ᵀ S ᵀ N d Ω capture material softening (elastic modulus reduction) and geometric stiffening from thermal stress, respectively. Polyetheretherketone (PEEK) - based composites (k≈0.25 – 0.35 W/m·K) sustain continuous service temperatures >200 °C while retaining necessary structural stiffness. Thermoset composites with lower thermal conductivities develop larger gradients and greater dis tortion under the same heat flux. 3. Vibration and Modal Damping Wing structures vibrate under aerodynamic excitation and shock loads. The modal expansion u(x, t) = Σ₍ ₖ₌₁₎ ᵐ η ₖ(t) φ ₖ(x) yields modal coordinates ηₖ governed by η̈ ₖ + 2 ζ ₖ ω ₖ η̇ ₖ + ω ₖ ² η ₖ = 0 with ζₖ damping ratio and ωₖ natural frequency. Thermoset composites exhibit low inherent damping (ζ≈0.005 – 0.01), risking prolonged oscillations and fatigue. Conversely, TPCs typically achieve ζ≈0.02 – 0.05 from viscoelastic polymer behavior, dissipating vib rational energy more rapidly and reducing fatigue accumulation. For example, in cantilever - beam vibration control with multiple flexoelectric actuators, harnessing viscoelastic damping accelerates decay of the first three modes by 50 – 70% versus a purely el astic baseline. BY Om | Page no 12 4. Integrated Material Trade - offs Military jets demand a balance of weight, cost, high - temperature performance, damage tolerance, and vibration damping. Table 2 summarizes key material attributes: Table 2: Material Characteristics and Trade - offs Property Carbon - fiber/PEEK TPC Epoxy - based TSC Notes Fracture Toughness GIc=0.7 – 2.1 kJ/m² GIc=0.08 – 0.31 kJ/m² TPCs → ↑damage tolerance, ↓delamination risk Thermal Conductivity 0.25 – 0.35 W/m·K 0.15 – 0.20 W/m·K TPCs → smaller thermal gradients, ↓distortion Service Temperature >200 °C ≈120 °C TPCs → high - speed hypersonic applications Inherent Damping ζ 0.02 – 0.05 0.005 – 0.01 TPCs → faster vibration decay, ↑fatigue life Weight Penalty +5 – 10% (resin content) nominal TPCs mildly heavier for toughness and thermal benefits Cost Factor +20 – 30% base Advanced polymers → higher material expense Despite moderate increases in resin mass (+5 – 10%) and cost (+20 – 30%), carbon - fiber/PEEK composites yield superior thermal - structural performance and vibration damping, justifying their adoption for critical high - performance military - jet wing structures. This enables extended fatigue life, enhanced safety margins, and sustained structural integrity under the combined effects of mechanical, thermal, and vibrational loading. BY Om | Page no 13 Result / Findings Combined Mechanical, Thermal and Vibrational Challenges in Supersonic Fighter - Jet Wings Supersonic fighter - jet wings concurrently endure: • High‐cycle bending, torsion and shear in aggressive maneuvers (e.g. F - 16 4 - g pullouts at Mach 1.1) that initiate fatigue cracks at fasteners, ribs and spars. • Aerodynamic heating to >1 600 K at the leading edge during sustained Mach 5 – 6 flight, reducing elastic modulus and shifting natural frequencies. • High - frequency (1 – 10 kHz) engine - and buffeting - induced vibrations that accelerate microcracking and fatigue. Key Material Requirements 1. Fracture Toughness: Mode I and II GIc, GIIc ≥1.0 kJ/m² to resist wing‐tip and spar crack growth. 2. Thermal Stability: Continuous service temperature ≥200 °C with minimal elastic‐modulus loss (<10%). 3. Vibration Damping: Loss factor ζ ≥0.02 for 1 – 10 kHz modes to attenuate fatigue‐inducing oscillations. 4. Stiffness & Strength: Density <1.5 g/cm³, tensile modulus ≥120 GPa, yield ≥400 MPa. 5. Manufacturability & Repair: Out‐of‐autoclave in - situ consolidation, fusion - weldable joints, recyclable. Recommended Solution: Carbon - Fiber/PEEK Composite Reinforced with Carbon Nanotubes • Matrix: Polyetheretherketone (PEEK) – semi - crystalline thermoplastic with T<sub>g</sub>≈150 °C, continuous use to 260 °C, inherent damping ζ≈0.03. • Reinforcement: Unidirectional carbon fiber (e.g. IM7) for tensile modulus ≥140 GPa, density ≈1.5 g/cm³. • Nano - augmentation: 1 – 3 wt% multi - walled carbon nanotubes (MWCNT) – raises thermal conductivity to ≥1 W/m·K, enhances modal damping to ζ≈0.05. • Structure & Processing: Automated tape placement with in - situ consolidation; fusion welding of lap joints via ultrasonic or induction methods. Justification – Fracture Toughness: GIc≈1.0 – 2.0 kJ/m² (vs. 0.1 – 0.3 kJ/m² for epoxies) resists high‐cycle wing loads and spar‐web fatigue. – Thermal Conductivity: ≈1 W/m·K (vs. 0.2 W/m·K) reduces local temperature gradients, limiting thermal‐stress softening. BY Om | Page no 14 – Vibration Damping: ζ≈0.03 – 0.05 (vs. 0.005 – 0.01) dissipates high - frequency vibrations, extending fatigue life by ≥30%. – Modulus Retention: >90% of room - temperature stiffness up to 200 °C, ensuring stable aeroelastic behavior. – Manufacturability: Unlimited shelf - life, out - of - autoclave processing and welded assembly lower cycle time and part count. Conclusion Supersonic fighter - jet wings operate in a tri - modal environment where mechanical stress, aerodynamic heating, and high - frequency vibrations converge to challenge conventional materials and threaten long - term structural integrity. This investigation shows that wing structures demand three key properties: fracture toughness to resist delamination under high - G manoeuvres, thermal stability above 200 °C to preserve aeroelastic margins, and damping ratios ≥0.02 to mitigate fatigue - inducing oscillations in the 1 – 10 kHz range. Among the candidates, carbon - fiber/PEEK thermoplastic composite reinforced with 1 – 3 wt% carbon nanotubes provides the best balance. It achieves Mode I fracture toughness up to 2.1 kJ/m² (vs. 0.08 – 0.31 kJ/m² for epoxies), retains >90% stiffness at 260 °C, and offers inherent damping ζ≈0.03 – 0.05 that extends fatigue life by ≥30%. Its semi - crystalline matrix en ables unlimited shelf - life, out - of - autoclave processing, and fusion - weldable joints — clear advantages for military manufacturing and repair. Limitation: For hypersonic leading - edge regions (Mach ≥5, local T≈1600 K), polymer composites cannot survive, requiring ceramic or metallic thermal protection. Nevertheless, in the supersonic fighter regime, this composite system delivers a robust and forward - looking solution that directly addresses the triple challenge of stress, heat, and vibration for next - generation military aircraft. Solution: Nevertheless, in the supersonic fighter regime, this carbon - fiber/PEEK + CNT composite delivers a robust and forward - looking solution that directly addresses the triple challenge of mechanical stress, thermal load, and vibration fatigue for next - generation military aircraft wings. BY Om | Page no 15 Application / Future Scope The recommended carbon - fiber/PEEK thermoplastic composite with CNT augmentation can be applied directly in supersonic fighter - jet wings , particularly for skins, spars, and ribs that demand simultaneous resistance to stress, heat, and vibration. Its inherent damping and damage tolerance make it equally suited for unmanned aerial vehicles (UAVs) and next - generation drones where lightweight construction and high fatigue life are critical. A key advantage is its out - of - autoclave processing and fusion - weldable joints , which simplify field - level repair and reduce maintenance downtime in military operations. 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