Question 4 of 7: Primary Bonding in Solids; Materials Selection for an Aircraft Wing

Question 4: Primary Bonding in Solids; Materials Selection for an Aircraft Wing (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A conceptual question on atomic bonding and on materials selection criteria for an airframe component (essay, no numeric data).

Find. (a) The four bond types, discussed, with their effect on properties. (b) The key properties an aircraft-wing material/structure must satisfy.

(a) The four bond types. Solids are held together by three primary (strong, "chemical") bonds — ionic, covalent, and metallic — plus the much weaker secondary (van der Waals / hydrogen) bonding; the question groups all four together as the "primary" bond types worth discussing for their role in properties.

Ionic bonding arises from electron transfer between atoms of very different electronegativity, producing oppositely-charged ions held by simple electrostatic (Coulombic) attraction. The bond is non-directional, so ions arrange into whatever geometric packing maximizes attraction and minimizes like-charge contact. This gives materials that are hard, stiff, and brittle (a small relative shift of a close-packed ionic plane brings like charges into contact, fracturing the crystal rather than deforming it), with high melting points and, in the solid state, poor electrical conductivity because the electrons are localized on the ions rather than free to move.

Covalent bonding arises from shared electron pairs between atoms of similar electronegativity, and is strongly directional (fixed bond angles set by the shared orbitals). Materials with extended covalent networks (diamond, SiC) are extremely hard and high-melting because breaking the solid means breaking real, oriented chemical bonds throughout; covalent bonding is also the backbone bond of polymer chains, where it coexists with much weaker secondary bonding between chains.

Metallic bonding is a lattice of positive ion cores immersed in a delocalized "sea" of shared valence electrons. Because the bonding is non-directional and the electrons are free to redistribute as atomic planes slide past one another, metals can deform plastically by dislocation glide without breaking bonds in a fixed direction — the structural basis of metals' characteristic ductility. The same free electrons give metals their high electrical and thermal conductivity and their reflective lustre.

Secondary (van der Waals) bonding is a much weaker (roughly $1/10$–$1/100$ the energy of a primary bond) electrostatic attraction between permanent or induced dipoles — including the special, stronger case of hydrogen bonding. It governs the properties of molecular solids and is what holds adjacent polymer chains, or adjacent graphite sheets, together: it explains why polymers soften well below their covalent-bond-breaking temperature, and why graphite's basal planes shear easily (a good solid lubricant) even though the in-plane covalent bonding is extremely strong.

Role in properties. Bond type and strength set the elastic modulus and melting point directly (stronger/stiffer bonds $\Rightarrow$ higher $E$ and $T_m$); bond directionality sets whether a material can deform plastically (non-directional metallic bonding) or is inherently brittle (directional covalent/ionic bonding, where dislocation glide would have to break and reform specific bond angles); and the degree of electron localization sets electrical/thermal conductivity (free in metallic bonding, localized in ionic/covalent bonding).

(b) Aircraft wing design properties. A wing is a thin-walled, weight-critical, cyclically-loaded structure, so materials selection is governed by several properties simultaneously, not any single one:

• Specific strength and specific stiffness (strength/density and modulus/density) — minimizing structural weight for a required load capacity and required stiffness (to avoid excessive flexing/aeroelastic flutter) is the overriding aerospace design driver.
• Fatigue resistance — a wing sees millions of load cycles from gusts, maneuvers, and pressurization cycles over its service life, so the fatigue (endurance) limit, not just the static strength, governs the allowable design stress.
• Fracture toughness and damage tolerance — the structure must survive with a small crack or impact damage present (bird strike, tool drop, corrosion pit) without catastrophic, unstable fracture before the next scheduled inspection ("fail-safe"/damage-tolerant design philosophy).
• Buckling resistance — the wing's thin compression skin must resist elastic buckling under bending loads, which depends on stiffness ($E$) and geometry (stringer/rib spacing), not just strength.
• Corrosion and environmental resistance — aluminum alloys are susceptible to exfoliation and stress-corrosion cracking in humid/salt environments; any material choice must survive decades of environmental exposure with a defined inspection/maintenance program.
• Manufacturability and joinability — the material must be formable/machinable into thin, precise skins and joined (riveting, bonding) without introducing stress concentrations or, where dissimilar metals meet, galvanic corrosion at the joint.

These criteria explain the historical dominance of high-strength aluminum alloys (2024-T3, 7075-T6) for wing skins and the growing use of carbon-fibre-reinforced polymer composites, which offer superior specific stiffness/strength and fatigue resistance at the cost of more complex damage detection and galvanic-corrosion management at metal/composite joints.