Question

Please discuss.

Let’s assume that the Chief Engineer has asked you to recommend a material for the design of a critical load-bearing structural element. It is critical since if it fails it can have a catastrophic impact on the performance of the system. For example, if a turbine blade fails in a jet engine, it can destroy it. The Chief has given you a choice between a low density, high stiffness but brittle ceramic material or more ductile, higher density, low stiffness metal material. Your task is to recommend which material is best suited for the design requirement.

Start by drawing a generic stress-strain curve for a brittle material and for a ductile material. Describe the curves and defend which one you would rather use to design a critical element of your structure.

Answer 1

Solution-

So, the choices available to us are either to choose a brittle material or a ductile material. First let’s have a look at the stress strain curves of both of the materials.

First let’s talk about brittle materials, as shown in the curve above brittle materials have a pretty linear relationship between stresses and strains. They are almost directly proportional to each other for most of the part. As we keep on increasing the stresses we will get higher strains upto a certain point after which if will break without any warning.

They have very less amount of ductility and hardly gives any warning about the rupture.

The stress-strain curve for brittle materials comprises of majorly two areas, in the first part stresses are directly proportional to strains and in the second part rupture strength can be easily assessed.

Whereas, if we look at the stress-strain curve of ductile materials they have more elastic nature. Even after the proportionality limit is achieved by the material it still has plenty of strength left and can function for longer period of time. Having more ductility allows it to give more warning about their rupture.

The stress-strain curve for ductile materials are more complex in nature. They comprise of many different limits as shown in the figure.

Proportional limits – under this limit stresses are directly proportional to strains

Elastic limit – up to this limit is the loads or stresses are removed the material will regain it’s original shape

Yield limit – at this limit the material will start to yield and is taken as the working stresses.

Ultimate strength – this is the maximum stresses or load that a structure can endure. Please note that the structure won’t fail at this point.

Rupture strength – due to ductility the structure still has some reserved strength after ultimate strength. It will fail at rupture strength as shown in figure.

As the structure that we are designing is very important and failure of which is catastrophic and might lead to the loss of lives so we want to use a material that has more strength and less chances of failure.

Ductile materials should be used in such cases as first the margin of factor of safety is more in ductile materials and even after failure it will give us enough time to replace it or plenty of time as a warning which will avoid or reduce the damages that might take place by the failure of it.

_{a feedback is highly appreciated.}