Avionic Structural Materials & Material Properties - Lecture 7
1. Introduction to Avionic System Structural Materials
Aluminium alloys are supplied in a very wide range of tempers, classified mainly into two principal groups:
Non-heat treatable alloys - These alloys achieve strength/mechanical properties through cold working methods (e.g., rolling, extruding) and are sometimes referred to as work hardening alloys. The temper designation for these is denoted by the letter H.
Heat treatable alloys - Strength and mechanical properties are acquired through heat treatment followed by cooling. Two categories exist:
Solution Heat Treatment: Heating aluminium to a specified temperature for a designated time, followed by rapid cooling, typically by quenching in water.
Natural Aging (T1, T2, T3, T4): Spontaneous hardening at room temperature until the metal stabilizes after solution heat treatment.
Artificial Aging (T5, T6, T9): Involves heating the metal for 2 to 30 hours at temperatures ranging from 100ºC to 200ºC for enhanced properties.
1.1 Heat-Treatable Alloys
Solution heat treating increases strength through subsequent cooling.
Natural aging solidifies the alloy at room temperature, while artificial aging hastens this process at lower temperatures and increases strength.
1.2 Non Heat-Treatable Alloys
Work Hardening (H14): Increases strength while reducing ductility through processes like rolling and drawing, often termed strain-hardening.
Partial Annealing (H24): Softening the alloy post-work hardening, improving ductility.
Stabilizing (H34): A thermal treatment used to improve ductility and stabilize other mechanical properties through heat introduction.
2. Temper Designation for Al-Alloys
Temper codes for aluminium alloys are a standardized method to describe the treatment and properties of aluminium alloys, reflecting any heat treatment or working processes they have undergone.
2.1 Temper Codes for Al-Alloys
A comprehensive list includes various temper identifiers.
Code F: As fabricated, no property limits specified
H14: Work hardened to half hard, not annealed after rolling
H24: Work hardened then partially annealed to half hard
Other codes vary for specific temper designations, emphasizing the treatment and mechanical characteristics of the alloy.
3. 6061 Aluminium Alloy
6061 alloy is utilized in diverse applications due to its strength and toughness, especially in avionics products.
Common applications include:
Electronic box chassis
Covers
Segregation plates
Components in structural, pilot-applied, load paths in Active Inceptor systems
3.1 6061 Aluminium Alloy Properties
Detailed properties include:
Diameter and thickness considerations are essential for the characteristics obtained during quenching.
Variations in the mechanical properties are influenced significantly by the extrusion thickness and quenching process.
3.2 Design Mechanical and Physical Properties of 6061
Mechanical Properties Tables
Tables provide specific properties such as:
Fy (ksi): Yield stress in pounds per square inch
Fu (ksi): Ultimate tensile strength
E (10³ ksi): Modulus of elasticity
G (10³ ksi): Shear modulus
μ: Poisson's ratio
w (lb/in³): Density
3.3 Fatigue Properties of 6061 Aluminium Alloy
Demonstrated fatigue life curves help establish the expected behavior under cyclic loading conditions, crucial for design considerations. Important fatigue life equations include
Log N = 20.68 - 9.84 log (Soq)
where Seq = Smax (1-R)^{0.63}.
The models provide insights into the longevity of the material under specified loads.
4. 7075 Aluminium Alloy
This alloy features high strength due to the influence of zinc as the primary alloying element.
4.1 7075 Aluminium Alloy Properties
The alloy's properties provide insights into the tensile strength and fatigue characteristics under various conditions.
4.2 Fatigue Properties of 7075 Aluminium Alloy
Best-fit S/N curves help predict the lives of materials subjected to cyclical stresses. Common equations include those for estimating the fatigue life as influenced by applied stress ranges.
5. Titanium Alloy
Titanium is known for its lightweight and corrosion resistance, which can be improved through alloying and heat treatments. Advantages include:
Good strength-to-weight ratio
Low thermal expansion
High oxidation resistance at intermediate temperatures
5.1 Titanium Alloy – 6Al-4v Product Forms
This titanium alloy is extensively used in high-temperature and critical applications, displaying excellent toughness and resistance.
5.2 Ti-6Al-4V Properties
Mechanical properties are influenced by the product form and heat treatment condition.
6. Steels
Steel selection relies on diverse factors affecting mechanical properties which can be significantly influenced by heat treatment methods.
6.1 Steel Material Properties
The robustness of steels can be adjusted through various methods of heat treatment, such as martensitic hardening and age hardening, which refine mechanical properties over a range of applications.
7. Tutorial Question on Fatigue
A practical exercise using Miner’s Rule to determine the cumulative fatigue damage of a structural component, given specific stress ranges and the presumed number of applied cycles. An equation provides a framework for calculations, linking stress to life predictions.
Miner's Rule indicates cumulative damage from fatigue failure:
D = rac{N1}{N{a1}} + rac{N2}{N{a2}} + … + rac{Nn}{N{a_n}} where D indicates cumulative damage and N denotes applied cycles against individual cycle lives.