User:Eas4200c.f08.carbon.orear/HW1/Lecture Notes

When designing Aerospace Structures it is important that the structures are light weight, but also strong and stiff. Some commonly used aircraft materials are aluminum, steel, titanium, and carbon fiber composites.

Material Characteristics
Stiffness is defined by the slope of the plotted curve representing the stress and strain relationship. Stress is equal to the product of this slope (designated the elastic modulus, E) and strain. $$\displaystyle \sigma = E \epsilon$$

Strength can be characterized in many ways. Two common measurements important in engineering applications are the yield stress and the ultimate stress. Yield stress is the stress level that a material begins to deform and stretch. Ultimate stress is the stress level that the material ruptures.

Toughness is the ability to resist fracture. A material is said to possess "high fracture toughness" if it resists fracture well and "low fracture toughness" if it is less capable of resisting fracture.

The graph (drawn by Jeff Wright) depicts the general graph of the relationship between stresses and strains in metals.



Material Examples
We are interested in aerospace materials beyond just the cost and manufacurability of aircraft components. The nature of the materials' engineering properties (like strength, stiffness, and toughness) help us choose materials appropriate to a specific application.

The table (drawn by Jeff Wright) shows the mechanical properties of metals at room temperature in aircraft structures, and depicts the correlations between density, tensile ultimate stress, tensile yield stress, and stiffness.

Some material examples are given below:

HIGH stiffness and HIGH strength:
 * Steel Alloy
 * Titanium Alloy

Steel alloys have very high stiffness and strength. Titanium alloys have less high stiffness and strength, and Aluminum alloys still less. Modern aluminum lithium alloys are 10% stiffer and 10% lighter than conventional aluminum alloys.

-Aluminum alloys are resistant to fatigue and cyclic tensile stress, and thus are used on the fuselage and the lower wing skins. Aluminum has historically played a major roll as material used in aircraft structures because it is cheap to buy, manufacture and is abundant, as opposed to Titanium which is expensive.

-Titanium is heavier than aluminum and lighter than steel, and double the yield and ultimate stress of aluminum. It is popular for its corrosion resistance and ability to withstand extreme temperatures up to 1000°F.

-Steel Alloys have extremely high stiffness and strength, but is expensive and has poor resistance to corrosion. Thus, despite its unique material property, is general reserved for landing gear and fittings with high loads.

Reference Sun [2006], pg. 14-15

HIGH stiffness and LOW toughness:
 * Glass

Because of the very low toughness of glass, it has a very limited use on aircraft. Bulk glass is avoided on aircraft, and other forms of glass are preferred such as glass fiber for its improved mechanical properties. Because of the extreme forces acting on the fuselage in-flight, windows are designed to be very small and sturdy. Recent discoveries of new glass-like materials allow for larger windows. Also, the study of metallic glass has become increasingly significant and could prove to have major importance on aircraft in the future. The article below describes some of the issues and discoveries of metallic glass:

Metallic Glass


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!CORRECTION to HW1 submitted error: Properties with low stiffness and high toughness, from team Carbon
 * LOW stiffness and HIGH toughness:
 * Nylon
 * Plastic
 * Aluminum
 * Aluminum

Aluminum has good fracture toughness and fatigue life, and is therefore used on the fuselage skin.
 * }

This table (drawn by Jeff Wright) shows the material properties of aluminum and steel alloys

HIGH stiffness and HIGH toughness:
 * Composites

Fiber-reinforced composites allow for complex loads by introducing fibers embedded in a matrix (polymers, metals, ceramics, etc.). They are stiff, strong, and light, and are ideal to withstand high loads experienced during flight while still minimizing weight to increase the aerodynamic properties. The composite laminates "have excellent fatigue life, damage tolerance, and corrosion resistance". Fiber composites are becoming very popular for their unique material properties and compose 50% of the new Boeing 787. More facts on the 787 can be found at:

Boeing 787 Fact Sheet

Reference Sun [2006], pg. 15-16

Aspects of Structure
It is key to look at both geometry and material in aerospace structure design. Some geometry examples are mentioned below.

Monocoque structure is that of a single shell design or thin-shell structures. The word has the French base 'coque' meaning, literally, shell. Semi-monocoque, or stiffened shell structures can also be worth exploring. It is important to note that stiffness and brittleness in aerospace structures can be either highly valuable or highly problematic. Normally, shell structures are chosen for their appealing strength for a very limited weight.

Goals with geometry and material
In considering geometry and material in an aerospace structure, one should try to optimize and integrate the two to minimize cost and weight. Sometimes geometry is limited by design goals and aerodynamic properties such as lift and drag. Careful consideration must be taken, therefore, not only to provide sufficient use of the materials for one purpose but to look at the integration of the entire system and how to maximize the utility of the available materials and possible geometric configurations.

The following website contains content not created by Team Carbon. The link was obtained in a google image search. It shows the material breakdown of an F/A-18 Hornet. F/A-18 Materials Full credit is due to http://www.military.cz/usa/air/in_service/aircraft/f18/f18pics/f18materials.jpg