Regular readers of this blog will know that I spent a relaxing day painting railings a few weeks ago [see post entitled ‘Engineering archaeology‘ on July 23rd, 2014]. A day or so later, I went out with my pail of whitewash to paint the walls of the light-well that the railings protect. ‘The summer world was bright and fresh, and brimming with life’ but unlike Tom Sawyer I was not looking for Jim to do my white-washing for me. I was looking forward to another therapeutic session painting the walls at the front of our house. It was an interesting standing in the light-well facing the wall, un-noticed by most passers-by. We live on a city street close to tourist attractions and there is a constant stream of coaches and taxis stopping to drop-off and pick-up tourists. I have written about the noise insulation in our house before [see Noise Transfer on April 13th, 2013] which means that we don’t notice the constant growl of diesel engines outside but I did while I was painting. However, there were other sounds in the city. The voices of pedestrians deep in conversation as they passed by on the pavement just above my head. I recognised Chinese, French, Italian and English but there were many different languages that I didn’t recognise. There were young children asking parents questions as they walked down the street. For a while I could hear cathedral bells. When there was a pause in the traffic then it was possible to hear the cooing of pigeons, a neighbour’s radio or television and an ever-present idling diesel engine which I discovered was an ice-cream van dispensing a constant trickle of black soot and an occasional ice-cream. It is curious that as a society we tolerant high levels of noise pollution at tourist attractions, especially ones that are meant to be places of calm and contemplation. Most tourists are, almost by definition, on holiday seeking relaxation and a lowering of stress levels – how much more pleasant would it be to glide to your destination in a silent electric coach or taxi?
We have the technology to provide such a service [see Are electric cars back? on May 28th, 2014]. Yes, it requires some investment by tour operators and taxi firms in hybrid or electric vehicles and by the city council in re-charging facilities. Induction charging stations at tourist attractions would allow vehicles to recharge while dropping off and picking up passengers. The technology is available and has been used by buses in Genoa and Turin for more than a decade. So a little bit a regulatory pressure and investment from city councils acting together could create a calmer, quieter and cleaner environment for everyone.
Can we look forward to solar-powered ice-cream vans?
Sources: Thank you to Richard for reminding me about Tom Sawyer.
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.