aircraft design goals: want aircraft to be light, yet strong
aircraft materials: high stiffness, high strength, light weight
|what is stiffness ?|
Young's modulus (slope) in linear relation between stress and strain , i.e.,
|what is strength ?|
|yield stress and ultimate (rupture) stress|
|what is toughness ?|
|ability to resist fracture, damage. usually called fracture toughness.|
|example of a material with high stiffness and high strength|
|steel alloys. titanium alloys have lower stiffness and lower strength than steel alloys. aluminum has yet lower stiffness and lower strength than titanium alloys; book, Table.1.1, p.14.|
|example of a material with high stiffness and low toughness|
|example of a material with low stiffness and high toughness|
|plastic, nylon. an example of a metal with good fracture toughness is aluminum, which is used in aircraft skin. but aluminum has lower stiffness, lower strength, and lower fracture toughness than steel alloys; book, Table.6.1, p.212.|
|what materials provide high stiffness and high toughness ?|
|composite materials: fiber reinforced composites in which a material (e.g., epoxy), used as the base material called the matrix, is reinforced with fibers to increase both stiffness and strength. carbon fiber reinforced plastic.
see also fiber metal composite in Further reading.
two aspects of a structure: geometry and material
- geometry of aircraft construction: monocoque (single-shell design, thin-shell structures), semi-monocoque (stiffened shell structures)
"Monocoque (French for "single" (mono) and "shell" (coque)) is a construction technique that supports structural load using an object's external skin. This stands in contrast with using an internal framework (or truss) that is then covered with a non-load-bearing skin. Monocoque construction was first widely used in aircraft, starting in the 1930s." from monocoque in wikipedia
"When these designs started appearing it was realized that the skin itself had significant structural properties of its own. With a sufficient thickness, one could do away with all of the internal structure. However this would be even heavier than the framing would have been. At thinner gauges the skin could easily provide the structure for tension and shear loads (metal resists being pulled apart quite well), and if bent into a curve or pipe, it became quite strong against bending loads as well. The only loading it could not handle on its own — at least for thin "skins" — was compression. Combining this sort of structural skin with a greatly reduced internal stiffening to provide strength against buckling in compression led to what is known as "semi-monocoque"." from monocoque in wikipedia
|caution about using wikipedia, contributions to wikipedia|
|The links to wikipedia articles that I provide in the course lecture notes are intended to be good starting points; many wikipedia articles are excellent and highly informative.
while any document can contain errors, wikipedia articles are particularly vulnerable to having errors since anyone, regardless of credentials, can anonymously edit these articles; see, e.g., Criticism of Wikipedia. in many instances, errors (pranks, or simply vandalism) appeared to be immediately discussed and corrected; sometimes errors may go for years without being corrected as experts don't usually read Wikipedia articles in their own field to correct these errors. I found a wikipedia article whose original text was based on the book titled The Complete Idiot's Guide to Chemistry; this reference was later deleted, but the erroneous text has remained in the article ever since. it is therefore a good idea to verify with standard textbooks often provided as references, or with other articles that can be found online. many articles simply list books as references without giving the page numbers, making it sometimes more time consuming to locate the source for verification (if at all possible).
wikipedia is generally useful as a good starting point to learn something new or to review something known. it is usually one of the first places to start searching for something. I would encourage you to make useful contributions to wikipedia to return the favor to the community.
- design of aircraft geometry also limited by aerodynamic consideration, such as lift and drag of airfoil
- want aircraft materials be used to full capacity (to avoid having more weight than is useful)
- optimize aircraft materials: replace aluminum and titanium by fiber reinforced composites, save 30% to 40% in weight
suggested contribution: the above figure is not clear; it is not easy to distinguish various gray shades. find a better figure showing the structure of an aircraft and the material used in each part.
- 50% of Boeing 787 is made of composites.
suggested contribution: find a picture of this aircraft.
Further reading 
Sun (2006), pp.1-2.
T. C. Wittenberg, T. J. van Baten and A. de Boer, Design of fiber metal laminate shear panels for ultra-high capacity aircraft, Aircraft Design Volume 4, Issues 2-3, June-September 2001, Pages 99-113.
suggested contribution: write a short wiki article on fiber metal composite. Discuss the difference with Metal matrix composite.