What are the criteria that govern the choice between titanium, composites, and more common materials, like aluminum?

Question

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?

Answer 1

What are the criteria that govern the choice between titanium, composites, and more common materials, like aluminum?

Three main criteria: cost, strength-to-weight ratio, and fatigue resistance.

• Cost. Of the three, aluminium used to be the clear winner, with composites having made large advances due to improved manufacturing processes. Titanium is the most expensive and is difficult to machine.
• Strength-to-weight.
  • Buckling. Titanium alloys have higher strength-to-weight than aluminium (this answer, more on temperature below), but aluminium is lighter and that gives it an advantage in structures loaded with compression stresses: buckling resistance is a function of cross sectional dimension as well. For the wing upper surface aluminium would be lighter than titanium, despite the lower specific strength. Composites have the highest specific strength of all.
  • Temperature. The graph in the linked answer also shows the influence of temperature on the specific strength of materials: aluminium is first to drop, and at higher temps titanium is the best next choice, as from the comment by @PeterKämpf.

• Fatigue resistance. Titanium alloys, like steel allows, have a fatigue endurance limit. If stresses remain below this limit, the construction can endure an endless number of cycles. Aluminium does not have an endurance limit, and will eventually fail even from small stress amplitude cycles: aluminium constructions require careful monitoring and maintenance for prevention of fatigue failure.

  

  Composites do not have an endurance limit, but fibre orientation and material choice can improve fatigue life.

Image source

Answer 2

"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.

Answer 3

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.