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In the newest large commercial jets, composite materials seem to be all the rage. The first thing I wanted to know is why composites were chosen instead of titanium? If I'm not mistaken, titanium has even more specific tensile strength.

Let's also be clear about what exactly composites are. "Composite" just means a mix of two or more materials, and in aerospace, it is predominantly autoclave-cured carbon-epoxy.

Ultimately, it would be good to understand the criteria that govern any choice among titanium, composites, and aluminum. Specifically, which areas of the plane are candidates for an advanced material? and why? What factors dictate the decision?

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  • $\begingroup$ Aerospace composites include fiberglass, resins, epoxies, and phenolics, in addition to carbon. $\endgroup$ – J Walters Jan 20 '16 at 2:49
  • $\begingroup$ The choice between all of those materials is extremely broad... is there a certain application you are interested in? $\endgroup$ – fooot Jan 20 '16 at 3:29
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    $\begingroup$ The cost of a titanium airliner would be ridiculous... $\endgroup$ – Ron Beyer Jan 20 '16 at 3:53
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    $\begingroup$ Titanium is roughly 4.20 USD/kg, whereas aluminum is roughly 1.20 USD/kg. Composites are even cheaper, not to mention lighter weight. I wouldn't base the issue on tensile strength alone, cost, tooling, repairability, availability, and weight are big factors. The reason I gave a close vote as too broad is that really this question would require a course on material sciences along with aircraft design, and can't be succinctly answered in a few paragraphs. $\endgroup$ – Ron Beyer Jan 20 '16 at 4:15
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    $\begingroup$ @RonBeyer I don't think it is too broad. How do we know it can't be answered succintly unless we give someone the chance of trying? Closing the question would close that chance. $\endgroup$ – curious_cat Jan 20 '16 at 7:27
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The first question is decently answerable.

Composites have the nice property that they are not homogeneous at a mesoscopic scale. It's almost unavoidable that small cracks form in materials under stress. This happens in aluminium, titanium and composites alike. This is not dangerous if there's something to stop them from growing. Within aerospace composites, the local material boundaries stop crack growth. Practically speaking, for a laminar composite this means that one layer may develop a crack. The two adjacent layers will stay glued to both sides of the crack, and keep the panel together.

Composites are also easier to engineer for some specific purposes, such as having higher tensile strengths in critical directions. This isn't impossible for titanium, but it is hugely expensive. You have to grow and cut a single crystal of titanium. In comparison, with a carbon composite it's just a matter of orienting the fiber layers.

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    $\begingroup$ Can you explain exactly what "grow and cut a single crystal of titanium" means? Can't titanium just be welded? $\endgroup$ – DrZ214 Apr 2 '16 at 1:39
  • $\begingroup$ @DrZ214: You can do so, but the resulting structure will be a hodgepodge of lots of little crystals oriented in all different directions. Many to most, if not all, crystalline materials are strongest against forces applied parallel to one of their crystal axes. If your structural panel is machined from a single huge crystal, or from multiple crystals all with the same orientation, then the panel will be strongest in one particular direction, and you can orient the panel such that the direction in which the panel experiences the most stress also happens to be the direction in which (1/3) $\endgroup$ – Sean Jun 5 at 23:28
  • $\begingroup$ the panel is strongest. If, on the other hoof, the panel is made up of lots of little crystals with random orientations, the crystals' strongest axes will point in all different directions, and the panel will have the same strength in every direction - which is less than the strength of the monocrystalline (or polycrystalline with aligned crystals) panel in its strongest direction. A monocrystalline panel, or a polycrystalline panel with all the crystals pointing the same way, can be made so that it is strongest in the direction in which it needs to be the strongest, (2/4) $\endgroup$ – Sean Jun 5 at 23:32
  • $\begingroup$ rather than wasting strength in directions in which it isn't needed. In contrast, a polycrystalline panel with isotropically-oriented crystals will be the same strength in every direction - which is stronger in some directions than it needs to be, yet not as strong as it could be in the directions that it does need to be strongest in. Finally, when we compare a monocrystalline panel to a polycrystalline panel with identically-aligned crystals, the monocrystalline panel is stronger (and thus superior) to the polycrystalline panel, as it's made from a single strong crystal, while (3/4) $\endgroup$ – Sean Jun 5 at 23:36
  • $\begingroup$ the polycrystalline panel is made from a bunch of separate crystals stuck together at the edges, creating weak points at the boundaries between the crystals. (4/4) $\endgroup$ – Sean Jun 5 at 23:37
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"The application of aircraft" is extremely vague. Modern aircraft use all of these materials in many different places, for many different reasons. Each component will have different tradeoffs that depend on many factors. The following is an extremely general overview.

Aluminum is a popular material in aircraft because it is relatively cheap and light, and has alloys with good material properties. It is fairly easy to work with but must be protected from corrosion. Light weight and low cost mean that it is used in large areas like fuselage and wing skin, and for a lot of the underlying structure.

Titanium is useful for its ability to withstand higher temperatures, while being stronger than aluminum but also heavier. However, it is much more expensive than aluminum.

Composites are a large family of materials, with many different types and combinations possible. Composites can be strong and light, but don't withstand high temperatures as well. Although composites don't corrode like some metals, some situations such as carbon fiber contacting aluminum need to be avoided. Exposure to UV light or moisture can also be an issue. Manufacturing composites can become very expensive depending on the materials used. Since composites are typically manufactured from multiple layers sandwiched together, they lend themselves more easily to applications with large and thin sections. Larger and more complex parts are more difficult to make from composites. Another important factor with aircraft is electrical conductivity. While metal parts will naturally conduct electric charge between each other, composites will need special treatment to ensure conductivity for protection from lightning and static charges.

There are also many other considerations. Besides yield strength, many materials on aircraft need to have good fatigue properties to withstand cyclic loading over time. Material properties at high and/or low temperatures may also be important. While metals tend to bend and dissipate energy before breaking, composites tend to suddenly snap. Metals are also easier to inspect and repair, while composites can be much more complicated. While a metal can be categorized fairly well by its type and dimensions, composites are more complex with their multiple plies. This makes definition and analysis more complicated.

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