B-06a Aircraft Metals & Corrosion

Metal Loading Forces

• Tensile strength is the ability of a metal to withstand stress in tension.
• There are three definitions of tensile strength:
  • Yield strength: the stress at which material strain changes from elastic deformation to plastic deformation.
  • Ultimate strength: the maximum stress a material can withstand when subjected to tension, compression, or shearing.
  • Breaking strength: the stress coordinate on the stress-strain curve at the point of rupture.

Stress-Strain Curve

• Reference numbers on the stress vs strain curve for structural steel include:
  • Ultimate strength
  • Yield strength (elastic limit)
  • Rupture (or fracture)
  • Strain-hardening region
  • Necking region

Fundamental Loading Forces

• The remaining four fundamental loading forces (excluding tension) are:
  • Compressive strength: a metal's ability to withstand being pressed or squeezed.
  • Shear strength: a metal's ability to withstand shear stress.
  • Torsional strength: a metal's ability to resist rotational shear.
  • Bending strength: a metal's bending strength.

Additional Types of Strength

• Fatigue strength (or endurance strength): a metal's ability to resist repeated loading.
• Impact strength (toughness): a metal's ability to resist shock.

Tensile Testing

• Tensile testing is a method to determine the tensile strength of a material.

Fatigue Strength Testing

• Fatigue strength testing establishes the stress level at which structural failure will occur.
• A specially shaped test piece is gripped at one end, while at the other end a ball race is fitted.
• The test piece is then rotated from the end at which it is held by an electric motor, stressing it in tension and compression once every revolution.
• The number of cycles completed is measured to determine fatigue failure.

Airframe Fatigue Testing

• Aircraft are subjected to fatigue tests to provide key data that help design engineers identify the likelihood and causes of premature fatigue damage or wear on the airplane's structure and structural components.
• Fatigue strength tests are used to determine the fatigue life of components, which helps with the development of maintenance programs.

Materials and Hardware

• Elasticity: a metal's tendency to return to its original shape after normal stretching and bending
• Toughness: a material's ability to resist tearing or breaking when bent or stretched
• Conductivity: a metal's ability to carry heat or electricity
  • Thermal conductivity: a metal's ability to conduct heat away from its source
  • Electrical conductivity: a metal's ability to allow electron flow
• Thermal Expansion: a metal's ability to expand when heated and shrink when cooled
  • Coefficient of expansion: a predictable amount of expansion or contraction at specific temperatures

Alloying Agents in Steel

• Purpose of ferrous metal alloys: adding small amounts of other materials to molten iron to change its properties
• Carbon: the most common alloying element found in steel
  • Forms compounds of iron carbides called cementite
  • Allows steel to be heat-treated to obtain varying degrees of hardness, strength, and toughness
  • Higher carbon content decreases malleability and weldability
• Classification of ferrous materials based on carbon content:
  • Low-carbon (mild) steel: primarily used in non-structural areas, easily welded, machinable, and does not accept heat treatment
  • Medium-carbon steel: accepts heat treatment, adaptable for machining or forging, and used where surface hardness is desirable
  • High-carbon steel: very hard, primarily used in springs, files, and some cutting tools

Heat Treatment of Steel

• Purpose: to increase strength, improve machinability, and alter certain manufacturability objectives
• Types of heat treatment processes:
  • Annealing: softens steel and relieves internal stress
    • Involves heating, soaking, and cooling slowly
  • Normalising: relieves stresses within steel caused by forging, welding, and machining
    • Involves heating, soaking, and cooling in still air

Reinforcing Fibres in Composites

• Types of fibres:
  • Fibreglass (glass cloth): made from small strands of molten silica glass, woven into cloth
    • Widespread availability, low cost, and less strength
  • Aramid (Kevlar): an organic aromatic-polyamide polymer, exhibits high tensile strength, flexibility, and toughness
    • Non-conductive, produces no galvanic reaction with metals, and has an outstanding strength-to-weight ratio
  • Carbon: high compressive strength, degree of stiffness, and high tensile strength
    • Cathodic, promotes galvanic corrosion with aluminium and steel, and requires special corrosion-control techniques
  • Ceramic: used in specific applications

Fibreglass

• Weaves: many different weaves available, depending on the application
• Properties: weighs more than most other composite fibres, but has less strength
• Recent developments: newly developed matrix formulas have increased the benefits of using fibreglass

Aramid (Kevlar)

• Properties: high tensile strength, flexibility, high tensile stiffness, low compressive properties, and excellent toughness
• Advantages: non-conductive, produces no galvanic reaction with metals, outstanding strength-to-weight ratio, and excellent vibration-damping characteristics
• Disadvantages: stretches, causing problems when cut, and drilling can be difficult

Metal Loading Forces

• Tensile strength is the ability of a metal to withstand stress in tension and has three definitions: yield strength, ultimate strength, and breaking strength.

Tungsten Steels

• Tungsten steels have high density, retain hardness at elevated temperatures, and are used for control surface balance weights and breaker contacts in magnetos.

Titanium

• Titanium steel alloys have high tensile strength, toughness, and corrosion resistance, and are lightweight, making them suitable for high-temperature applications.

Stainless Steel

• Stainless steel is a classification of corrosion-resistant steels containing large amounts of chromium and nickel, making them suitable for high-temperature applications.

Heat Treatment of Steel

• The purpose of heat treatment is to change the physical and mechanical properties of steel without changing its original shape and size.
• Heat treatment involves heating and cooling carbon steel to control the distribution of carbon.

Solution Heat Treatment

• Solution heat treatment involves heating certain aluminium alloys to allow alloying elements to mix with the base metal.

Reheat Treatment

• Reheat treatment is possible for materials that have been previously heat-treated, but clad materials are limited to no more than three heat treatments.

Non-Heat-Treatable Aluminium

• Non-heat-treatable aluminium alloys are designated in the 1000, 3000, 4000, 5000 Series and can be strengthened through cold work or strain-hardening.

Strain-Hardening

• Strain-hardening, also referred to as cold-working or work-hardening, strengthens and hardens metal through mechanical working at a temperature below its critical range.

Hardness Designations

• Hardness, or temper, is indicated by a letter designation separated from the alloy designation by a dash, with optional numbers following the letter designation for more specific definition.

Other Alloy Types

• Magnesium alloys are used for castings and are available in sheets, bars, tubing, and extrusions, and have a lower density than aluminium.
• Drawbacks to using magnesium include high susceptibility to corrosion, overcome by treating the surface with chemicals that form an oxide film and exclude oxygen.