Last week I spent a relaxing day painting the old railings in front of our house. Since I am not a painter and decorator by trade the end result is not perfect but they look much better in shiny black than two-tone rust and matt black. One of the fleurs de lis on our railings had been knocked off when either we moved in or the previous occupiers moved out. It’s a way of life being an engineer, so the shape of the failure surface on the broken railing was bugging me while I was painting the rest. You would expect wrought iron railings to be ductile, i.e. to deform significantly prior to fracture, and to have a high tensile strength. Wrought iron’s properties are derived from its very low carbon content (less than 0.25%) and the presence of fibrous slag impurities (typically about 2%), which almost make it a composite material. It was historically used for railings and gates. However, my broken railing had exhibited almost no deformation prior to fracture, i.e. it was a brittle failure, and the fleur de lis had broken in half on impact with the stone flags. So on one of the rainy days last week, when I couldn’t paint outside, I did a little bit of historical research and discovered that in the late 1790s and early 1800s, which is when our house was built, cast iron started to be used for railings. Cast iron has a high carbon content, typically 2 to 4%, and also contains silicon at between 1 and 3% by weight. Cast iron is brittle, i.e. it shows almost no deformation prior to fracture, so the failure surface tends be to flat and smooth just like in my fleur de lis.
This seems like a nice interdisciplinary, if not everyday, engineering example. It would be vandalism to go around breaking iron railings in front of old buildings. So, if you want Everyday Engineering Examples of ductile and brittle behaviour, then visit a junk shop and buy an old china dinner plate and a set of cutlery. The ceramic of the china plate is brittle and will fracture without deformation – have some fun and break one! The stainless steel of the fork and spoon is ductile and can be easily bent, i.e. it is easy to introduce large deformation, in this case permanent or plastic deformation, prior to failure. In fact you will probably have to bend the fork back and forth repeatedly before it will snap with each bending action introducing additional damage.
The more curious will be wondering why some materials are ductile and others brittle. The answer is associated with their microstructures, which in turn is dependent on their constituents, as hinted above. However, I am not going to venture into material science to explain the details. I have probably already given materials scientists enough to complain about because my Everyday Engineering Examples are not directly analogous at the microstructural level to wrought iron and cast iron but they are more fun.
Sources: http://www3.westminster.gov.uk/spgs/publications/Railings.pdf
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