I have just returned from a deep vacation. If you are reading this then may be you are not, in which case I hope you have enjoyed one already or have one planned. If you are not sure what I am talking about then read my earlier posts entitled ‘Mind wandering‘ on September 3rd, 2014 and ‘Love an engineer‘ on September 24th, 2014. Meanwhile enjoy the picture – its better than the fracture surfaces from last week [see my post entitled ‘Forensic engineering‘ on July 22nd, 2015]!
The picture above shows the fracture surface of a thin bar of aluminium alloy that had a circular hole through the middle, like the peep-hole in a front door. The photograph was taken in a Scanning Electron Microscope (SEM) at x160 magnification. There is a scale bar in the bottom right corner showing a length of 100 microns. We are looking approximately in the longitudinal direction, which was the direction of loading, and across the photograph from left to right corresponds to the direction you would look through the hole. The lower one third of the picture shows the machined surface of the hole cut or machined by the drill. The top two-thirds shows the surface created by the fatigue crack as it extended incrementally with each cycle of load. The crack started from edge of the machined surface approximately on the vertical centre-line of the picture. I can tell this because all of the features in the texture of the fracture surface point towards this point because the failure radiated out from this location. The picture below shows the crack initiation area at x1000 magnification. It is a small area at interface with hole above the letters ‘SS40’ in the top photograph; this should be enough to let you identify the common features but the interpretation of these images requires significant skill.
Fractography is the forensic study of failure surfaces such as this to establish the cause of failure. In this example, the hole in aluminium bar ensured that it will always fail with cyclic loading through the growth of a crack from somewhere around the hole. The textured form of the fracture surface occurs because the material is not homogeneous at this scale but made up of small grains. The failure of each grain is influenced by its orientation to the loading which results in the multi-faceted surface in the photographs.
I made the photographs the focus of this post because I thought they are interesting, but may be that’s because I’m an engineer, and because they are a tiny part in a fundamental research programme on which I have been spending a significant portion of my time. A goal of programme is to understand how to use these materials to build more energy-efficient structures that are cheaper and last longer without failing by, for example, fatigue.
The bar was 1.6mm thick and 38mm wide in the transverse direction and made from 2024-T3 Aluminium alloy. The hole diameter was 6.36mm. A tension load was repeatedly applied and removed in the longitudinal direction which caused the initiation and growth of a fatigue crack from the hole that after many cycles of loading led to the bar breaking in half along a plane perpendicular to the load direction. The pictures were taken at the University of Plymouth by Khurram Amjad with the assistance of Peter Bond and Roy Moate using a JEOL JSM-6610LV.
Most of us are aware of the rising levels of anthropogenic carbon dioxide in the atmosphere and its impact on climate change but what about the potential loss of our oxygen supply? Far fewer of us are aware of what is sometimes referred to as the ‘other’ carbon dioxide problem, which is the acidification of the oceans. Carbon dioxide dissolves in the surface of the ocean when the concentration in the water is lower than in the atmosphere. Joanne Hopkins of the National Oceanography Centre in Liverpool describes this as the reverse of bubbles escaping when you open a fizzy drink, because the concentration of carbon dioxide in the air is less than in the drink. Carbon dioxide is also taken up in the ocean by tiny marine plants, known as phytoplankton, which convert it into organic matter and oxygen. Tiny marine animals, known as zooplankton, eat the phytoplankton and in turn are eaten and so on. Phytoplankton are important not just because they are the bottom of the food chain but also because they produce about half the oxygen that we breathe. The problem is that dissolved carbon dioxide is shifting the pH balance of the oceans which is beginning to cause demineralisation of microorganisms the ocean. At a recent Royal Society Regional Meeting in Bristol, Professor Daniela Schmidt described this as analogous to osteoporosis, a ‘brittle’ bone disease suffered by humans. Many years ago, my research group worked with a pathologist, Dr Dennis Cotton to examine whether it was possible that osteoporosis sufferers could break their leg and fall over rather than fall over and break their leg. In other words, could osteoporosis change the material properties of bone so dramatically that the structural integrity was insufficient for everyday activities such as getting out of bed or walking upstairs? Our answers at the time were inconclusive, at least in the generic case. Professor Schmidt is working with another team of engineers to examine the structural integrity of microorganisms in the oceans and the impact of demineralisation. The concern is that they could become structurally unstable and die and this could lead to a major reduction in our oxygen supply.
Ok, there is a lot of uncertainty about the series of interactions described above, about the magnitude of the effects and about the ability of ecosystems to adapt to the new conditions. However, the potential consequences are so catastrophic that we should not ignore them. Urgent action is needed to reduce our production of carbon dioxide, and since our governments appear incapable of action we have to take individual responsibilty as advocated by Kofi Annan and reported in my post entitled ‘New Year Resolution’ on December 31st, 2014.
By the way, look out for the announcement of the $2M Wendy Schmidt Ocean Health XPrize on July 20th to one of five teams of scientists for the best sensor for making real-time measurements of ocean acidity.
Cotton DWK, Whitehead CL, Vyas S, Cooper C & Patterson EA, Are hip fractures caused by falling and breaking or breaking and falling? Photoelastic stress analysis, Forensic Science Int. 65: 105-112, 1994.
It’s not often that someone presents you with a completely new way of looking at the world around us but that’s what Dr Gregory Sutton did a few weeks ago at a Royal Society Regional Networking Event in Bristol where he is a University Research Fellow funded by the Royal Society. He told us that every flower is a conductor sticking out of the ground which on a sunny day has an electric field around it of the order of 100 volts per metre. Bees can identify the type of flower that they are approaching based on the interaction between this field and the electrostatic field generated around them as they fly. Bees are covered in tiny hairs and he believes that they use these to sense the electric field around them. The next research question that he is tackling is how bees are affected by the anthropogenic electric fields from power lines, mobile phones etc.
The plots of the electric field around a flower really caught my attention. You can see one in the thumbnail photo. I walked across Brandon Hill in Bristol after the talk to meet a former PhD student for dinner. I kept stopping on the way to try to detect this field with the hairs on the back of my hand. It was a beautiful sunny day but I was not sensitive enough to feel anything. Or maybe I was sensing it but my brain is not programmed to recognise the sensation. We discussed it over dinner and marvelled at the bees’ ability to process the information from its multiple sensors in the light of our knowledge of the computing power required to handle what it is fashionable to call ‘Big Data’ from man-made sensors.
Once again Nature humbles us with its ingenuity and makes our efforts look clumsy if not feeble. Dr Sutton’s insights have given me a whole new way to attempt to connect with Nature while I am on deep vacation.
Sorry about the pun in the title. I couldn’t resist it.
Clarke D, Whitney H, Sutton G & Robert D, Detection and Learning of Floral Electric Fields by Bumblebee, Science, 5 April 2013: 66-69. [DOI:10.1126/science.1230883].
The Sun supplies approximately 100,000 TeraWatts (TW) of energy to the Earth continuously. To put this into perspective the entire generating capacity of China is 1TW and the global population as a whole uses 15TW. Plants use about 100TW via photosynthesis. Most our energy consumption is derived from biomass created millions of years ago by photosynthesis and stored as coal, gas or oil when the plant died and was crushed by geological processes.
I am stealing and paraphrasing from Professor Neil Hunter’s presentation at the Royal Society’s Scientific Discussion Meeting on Bio-inspiration for New Technologies. Of course, as Neil pointed out, the energy from the Sun arrives across a range of wavelengths some of which are damaging to our health. So fortunately for us the Earth’s atmosphere filters out a number of wavelengths but nevertheless a broad band of wavelengths still arrives at the Earth’s surface. Photosynthesis only makes use of two relatively narrowbands of light….
Mankind’s efforts to use solar energy look pathetic alongside Nature’s performance and should be humbling to any engineer or scientist. But it is also an inspiration to do better. We need cheap clean energy for everyone. It is being delivered everyday but we don’t know how to use it.