Aerospace Material Science and Fastener Technology

Posted on Nov 21, 2025 in Aerospace Engineering

Aircraft Fasteners and Mechanical Components

Turnbuckles and Tension Adjustment

Turnbuckles are mechanical connections used to adjust cable tension in aircraft control systems.

Bearing Strength Definition

Bearing strength is the ability of a joint to withstand any form of crushing or excessive compressive distortion.

Preload Indicating Washer (PLI) Operation

A PLI washer consists of concentric inner and outer rings and two high-strength steel washers. The outer ring is thinner than the inner ring and has a series of radial holes drilled through it. A stiff wire tool is inserted into the holes in the outer ring and is used to check whether the ring is free to rotate. As the nut is tightened, the inner ring is compressed until the outer ring is nipped between the washers. At this point, the outer ring can no longer be rotated, indicating the correct preload has been achieved.

Comparison of Flat and Hollow Saddle Keys

  • Flat Saddle Key: Requires a flat keyway to be cut in the shaft to fit the flat bottom surface of the key.
  • Hollow Saddle Key: Has one face hollowed to suit the surface of the shaft, requiring no keyway.

Advantages of Riveted Joints

Advantages of using a riveted joint over other types of fasteners include:

  • Low maintenance cost and high productivity (easy and cheap to produce).
  • Lightweight, strong, and fatigue resistant.
  • No moving parts to malfunction.

Solid vs. Blind Rivets

The main difference between solid and blind rivets is the required access for installation:

  • Solid Rivet: Requires access from both sides to deform the tail and form a second head. Generally stronger but requires more time and effort.
  • Blind Rivet: Requires access from one side only. A tool pulls the mandrel through the body, causing the creation of a second head before the mandrel is removed.

Countersunk Rivets on Aircraft

Countersunk rivets are used on the external structure of an aircraft because they create an aerodynamic profile which reduces drag and improves overall performance.

Components of a Jo-Bolt

Three components make up a Jo-Bolt:

  1. Alloy steel bolt.
  2. Alloy steel nut.
  3. Stainless steel sleeve.

Common Uses of Plain Washers on Aircraft

Four common uses of plain washers on aircraft are:

  1. Acts as a corrosion inhibitor.
  2. Provides a smooth bearing surface.
  3. Helps obtain the correct grip length.
  4. Distributes load.

Definition of a Nut

A nut is a type of fastener used to secure a bolt, creating a secure connection. Nuts can be classified by the thread size, pitch, and the material.

Common Quick Release Fasteners

The three most common quick release fasteners are: Dzus, Camloc fasteners, and Airlock.

Rivnut Installation and Use

A Rivnut is inserted into a pre-drilled hole and installed by pulling a mandrel or pin through the center, causing the back to expand. After removing the mandrel, another fastener (such as a screw or bolt) can be installed to secure the rivnut.

Decoding Metric Bolt M8x1.25

The bolt designated M8x1.25 means:

  • M: Metric thread.
  • 8: Nominal diameter of 8 mm.
  • 1.25: Thread pitch of 1.25 mm.

Pitch of a Screw Thread

The pitch of a screw thread is the distance between adjacent threads on the screw.

Low Temperature Self-Locking Nut Operation

This nut is designed to prevent a bolt from loosening due to vibration. It is often used with bolts that have a low coefficient of thermal expansion so that differential expansion does not cause the joint to fail.

Decoding Metric Bolt M30x2

The bolt designated M30x2 means it has a nominal diameter of 30 mm and a thread pitch of 2 mm.

Material Properties and Metallurgy

Kevlar Mechanical Properties

Kevlar is popular for land and air vehicles due to its:

  • Great strength and stiffness.
  • Greater resistance to impact.
  • Lighter weight (but more expensive than glass fiber).

Difficulties Drilling Kevlar

When drilling Kevlar, fibers catch on the drill bit and leave a fuzzy fringe around the hole.

Hardness vs. Toughness

  • Hardness: The ability to resist cutting, penetration, or abrasion.
  • Toughness: The ability to resist tearing or breaking when the material is bent or stretched.

Malleability Definition

Malleability is the ability to be bent, formed, or shaped without cracking.

Thermal Expansivity

Thermal expansivity is the tendency of a material to expand or contract in response to changes in temperature.

Properties of Pure Iron

Pure iron is a silvery-gray metal that has magnetic properties. Its relative density is 7.83 g/cm³.

Definition of Ferrous Metal

A ferrous metal is an alloy containing iron as its principal element.

Disadvantages of Magnesium in Aerospace

Two disadvantages of using magnesium in the aerospace industry are:

  1. Highly flammable.
  2. Highly prone to corrosion.

Titanium Protection from Heat

Titanium must be protected when exposed to heat because it can ignite and burn.

Stainless Steel Fabrication Challenges

Difficulties encountered when fabricating stainless steel include:

  • It is more difficult to cut and form.
  • It has a greater expansion coefficient, which makes welding more difficult.

Note: When using corrosion-resistant steels for aircraft, the technician must ensure the proper type is selected for the specific application.

Stainless Steel Alloying Elements

The two major elements added to steel to make stainless steel are:

  1. Chromium: Gives it the corrosion-resistant property.
  2. Nickel: Enhances the resistance to corrosion and heat.

Effect of Phosphorous on Steel Quality

Low quantities of phosphorous improve the quality of steel by having a positive effect on two properties: increased strength and improved toughness.

Alloying Manganese with Steel

The primary reasons for alloying manganese with steel are to increase strength, corrosion resistance, and resistance to abrasion.

Main Alloying Elements in Aluminum Series

The main alloying elements in the standard aluminum alloy series are:

  • 2xxx Series: Copper (Cu)
  • 3xxx Series: Manganese (Mn)
  • 4xxx Series: Silicon (Si)
  • 5xxx Series: Magnesium (Mg)
  • 6xxx Series: Magnesium (Mg) and Silicon (Si)
  • 7xxx Series: Zinc (Zn)

Heat Treatment and Classification

SAE Steel Alloy Classification System

SAE classifies steel alloys using a four-digit numerical index system:

  • The first digit denotes the principal alloying element in the steel.
  • The last two digits represent the percentage of carbon within the steel (in hundredths of a percent).

Steel Normalization Process

The steel is heated up to 56 degrees or above its critical temperature until the metal is evenly heat-soaked. It is then cooled in still air. This process relieves stress created by welding and machining.

Objectives of Tempering

The three main objectives of tempering are:

  1. Reduces hardness.
  2. Relieves stress.
  3. Improves ductility and toughness.

Aluminum Alloy Annealing Process

The metal is heated to a specific temperature and held there until the material is fully softened, then it is cooled very slowly.

Decoding Aluminum Alloy Code 7075-T6

Code 7075-T6 refers to a hard, strong, zinc-based aluminum alloy:

  • The first digit (7) represents Zinc as the major alloying element.
  • The second digit (0) indicates no special control on individual impurities.
  • T6 means that the alloy has been solution heat-treated and artificially aged.

Corrosion and Testing

Corrosion Types and Accelerating Factors

The types of corrosion illustrated are Creep Corrosion and Dendritic Growth.

Two factors that may increase the corrosion rate are:

  1. Water (moisture/humidity).
  2. Voltage bias (electrical potential difference).

Exfoliation Corrosion

Exfoliation corrosion occurs in aluminum alloys and is most prone to occur in areas where the alloy is under high stress.

Factors Influencing Metal Corrosion

Eight factors that influence metal corrosion are:

  1. Temperature
  2. Corrosive agents
  3. pH level
  4. Mechanical stresses
  5. Alloy composition
  6. Coatings
  7. Water (moisture)
  8. Voltage bias

Purpose of Hardness Test

The purpose of a hardness test is to measure the resistance of a material to permanent indentation or deformation.

Brinell vs. Rockwell Hardness Test

The difference between the Brinell and Rockwell hardness tests is the measurement method:

  • Brinell: Measures the diameter of the indentation hole made during the experiment.
  • Rockwell: Measures the depth of the indentation.

Charpy Impact Test Procedure

The Charpy test measures the resistance and toughness of a material and its sensitivity to impact. A pendulum is allowed to swing freely from a fixed height and strike the test piece at the bottom of its swing. The tougher the material, the more energy will be lost by the pendulum in breaking the test piece.

Objectives of Fatigue Resistance Testing

The two primary objectives when carrying out fatigue resistance testing of materials are:

  1. To test the highest maximum stress that will permit the material to undergo an infinite number of cycles (Endurance Limit).
  2. To determine the highest maximum stress that will permit a given life in terms of stress cycles.

Composites and Woven Materials

Disadvantages of Using Composites on Aircraft

Four disadvantages of using composites on aircraft are:

  1. They do not conduct electricity (requiring special lightning protection).
  2. Damage is difficult to detect (requiring specialized NDT).
  3. Moisture absorption can occur.
  4. They can be corrosive to aluminum alloys (galvanic corrosion).

Advantages of Using Composites

Six advantages of using composites are:

  1. Corrosion resistance.
  2. Durability.
  3. Fire resistant.
  4. Easy to repair.
  5. High impact resistance.
  6. Design flexibility.

Woven Cloth Threads and Weaves

The names given to threads that run along the length and across woven cloth are Warp (length) and Weft (across).

The four most common weaves of woven products are: Plain, Twill, Satin, and Unidirectional.

Aircraft Procedures and Specifications

Wire Locking Procedure

Wire locking connects one or more rotating fasteners together so that they do not vibrate loose. A stainless steel wire is drawn through a small hole in the fastener and secured to another fastener or directly to the structure.

Gear Shaft Speed Calculation

Given Input (Driving) Gear Shaft Speed = 4800 RPM.

Gear Ratio = (39/42) x (14/104) = 1/8.

Speed of Driven Gear = 4800 RPM x 1/8 = 600 RPM.

Decoding Rivet MS20470AD4-5

MS20470AD4-5 is decoded as:

  • MS: Military Standard.
  • 20470: Specification number of the rivet.
  • AD: Material code (2117-T4 Aluminum Alloy) and Universal Head style.
  • 4: Diameter of the rivet (4/32″ or 1/8″).
  • -5: Length of the rivet (5/16″).

Decoding Rivet MS204704D4-5 (Original Input Interpretation)

Based on the provided interpretation:

  • MS: Manufactured to Military Standards.
  • 204: Rivet falls under the category of fasteners.
  • 704: Aluminum alloy designation.
  • D4: Diameter and grip of the rivet (D indicates 2017-T4 material).
  • 5: Length of the rivet (5/16 inch).

Decoding Rivet AN426AD6-12

AN426AD6-12 is decoded as:

  • AN: Manufactured for Army/Navy standards.
  • 426: Series number of the rivet (Countersunk head style).
  • AD: Material of the rivet (2117-T4 Aluminum Alloy).
  • 6: Diameter of the rivet (6/32″ or 3/16″).
  • -12: Length of the rivet (12/16″ or 3/4″).