Introduction

On this page you can find some additional information on common aircraft materials and how to select the appropriate material for a specific application. To start off, a short introduction into material properties is given to create a basis for understanding the subsequent discussion.

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Basic material properties

 

Dr. Parlevliet mentioned that for proper material selection during the design phase it is important to know the material properties. But, what properties are interesting? Here, we will introduce basic concepts that will aid in the understanding of material properties.

 

Strength and stiffness

Strength and stiffness are important characteristics of a material that describe the behavior under load. Strength can be described as the ability of a material to withstand an applied load without failure or plastic deformation (e.g. an irreversible change in the materials size or shape). Stiffness can be described as the resistance to deformation under an applied load.

These properties are generally determined using test, for example a tensile or compression test. During the test the material response (e.g. deformation) is recorded for a given load. These loads and deformations are expressed as stresses and strains. After the test, a so called stress-strain curve is obtained, from which both of these parameters can be determined. An example of an engineering stress-strain curve of a ductile material is seen below.

Nicoguaro, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Note the bubbles corresponding to different material groups. In the figure, the ultimate strength of a material is found as the highest stress reached in the curve. The stiffness can be determined from the slope of the elastic region. This slope is generally referred to as the Young’s modulus, which can be calculated as

Where

and

The yield strength is the strength after which plastic deformation starts occurring.

 

Density

In the lectures it was explained that weight is a very important parameter for a material when designing for sustainable aviation. The material property that captures weight is the density. Density, or specific mass, of a solid depends on the atomic weight of its atoms or ions and the way they are packed together. Metal are generally made up by heavy atoms that are closely packed together, hence a higher density. Polymers generally consists of low weight atoms packed less dense, hence a lower density.

Material selection for specific components

With the basic properties of materials known, the next step is to look into which material is most suitable for a specific component.

Generally, this is done by first investigating the component and the loads it will experience. This determines the type of problem to be solved: it is a bending problem, a tensile problem or perhaps a torsional problem. The type of problem determines the applicable equations and which material properties are important for that specific problem.

As an example, imagine the material for a tie rod on a bi-plane needs to be chosen. This component is loaded in tension, and therefore it is a tension problem. Since it’s being used on an airplane, we would like the wire to be as light as possible. It has to withstand a stress of σ_f.

The mass of the wire is given by the mass multiplied with the cross-sectional area and the length of the wire, or:

Using the equation for σ above, we can substitute the for the area resulting in:

The parameter  consists fully of material properties. The other elements are a free variable (the force) and a design variable (the length). Therefore, this parameter is the most important when we want to choose a material. In our case, we want to minimize the weight, which means minimizing  or maximizing . Such a parameter to be maximized is known as a performance or material index. Therefore, when selecting a tie rod material, we want a material with the highest strength over density ratio.

Ashby plots

Ashby plots, or bubble diagrams, are a convenient way to visualize two different properties in one graph. This also allows for plotting different material together in one graph, such as the material strength and density discussed above. The bubble refers to the groups created by similar materials, like metals or wood. By choosing the material properties in a smart way, different performance indices as discussed above can be visualized. An example of an Ashby plot can be seen in the figure below.

 

Nicoguaro, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

In this plot, the performance index of  is shown as well as the index  are used. These indices are for a tensile and bending problem. Suitable materials are located above the performance indices in the Ashby plot.

In the above image using the performance lines indicated, only materials above both lines are suitable for that application. Therefore, only technical ceramics are suitable materials in this example.