Types of Materials

Fix a long, thin piece of material on one end, and hang a load at the other end. For very small extensions, most materials would obey Hooke's law. This means that the extension would be proportional to the load (force). For very large extensions, most materials would break.

For loads that are in between, materials can extend in different ways. Materials can often be grouped into three different types, according to how their extensions behave: ductile, brittle and polymeric.

A brittle material could extend by a small amount only, and then break. It would usually obey Hooke's law for most of its extension. Imagine fixing a bucket at one end of the material and adding stones of the same weight one by one. Each time, the material extension would increase by the same amount - until it breaks. If we plot a graph of load versus extension until its break point, it would be mostly straight. Examples of such materials are glass and porcelain.

A ductile material would initially follow Hooke's law when a load is applied. Imagine fixing a bucket at one end of the material and adding stones of the same weight one by one. Each time, the material extension would increase by the same amount. At some point, the extension would start to increase by a larger and larger amount each time. Then it reaches a point when it breaks. Examples of such materials are metals like iron and copper. If we plot a graph of load versus extension, it would at first be straight with a certain slope. Then the slope becomes less steep, and the graph bends until it is horizontal.

Examples of polymeric materials are rubber and plastic. Imagine fixing a bucket at one end of the material and adding stones of the same weight one by one. Each time, the material extension would increase by the same amount. At some point, the extension would start to increase by a larger and larger amount each time. After that, it may increase by a smaller amount. Such materials tend extend much further than brittle and ductile materials before they break. If we plot a graph of load versus extension, the gradient of the slope may change a few times.


Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 11 October 2010.