Flatness is a tricky term to define. Technically, it is the deviation, or lack of deviation, from a plane. However, something that appears flat to human eye often turns out not to be at all flat when looked at closely and measured with a high resolution instrument. It’s a bit like how the ocean might appear flat and smooth to a passenger sitting comfortably in a window seat of an aeroplane and looking down at the surface of the water below but feels like a roller-coaster to a sailor in a small yacht. Of course, if the passenger looks at the horizon instead of down at the yacht below then they will realise the surface of the ocean is curved but this is unlikely to be apparent to the sailor who can only see the next line of waves advancing towards them. Of course, the Earth is not flat and the waves are better described as surface roughness. Some months ago I wrote about our struggles to build a thin flat metallic plate using additive manufacturing [see ‘If you don’t succeed, try and try again…’ on September 29th, 2021]. At the time, we were building our rectangular plates in landscape orientation and using buttresses to support them during the manufacturing process; however, when we removed the plates from the machine and detached the buttresses they deformed into a dome-shape. I am pleased to say that our perseverance has paid off and recently we have been much more successful by building our plates orientated in portrait mode, i.e., with the short side of the rectangle horizontal, and using a more sophisticated design of buttresses. Viewed from the right perspective our recent plates could be considered flat though in reality they deviate from a plane by less than 3% of their in-plane dimensions and also have a surface roughness of several tens of micrometres (that’s the average deviation from the surface). The funding organisations for our research expect us to publish our results in a peer-reviewed journal that will only accept novel unpublished results so I am not going to say anything more about our flat plates. Instead let me return to the ocean analogy and try to make you seasick by recalling an earlier career in which I was on duty on the bridge of an aircraft carrier ploughing through seas so rough, or not flat, that waves were breaking over the flight deck and the ship felt like it was still rolling and pitching when we sailed serenely into port some days later.
Most manufactured things break when you subject them to 90% strain; however Professor Xiaoyu Rayne Zheng of the Department of Mechanical Engineering at Virginia Tech has developed additively-manufactured metamaterials that completely recover from being deformed to this level. Strains are usually defined as the change in length divided by the original length and is limited in most engineering structures to less than 2%, which is the level at which steel experiences permanent deformation. Professor Zheng has developed a microstructure with a recurring architecture over seven orders of magnitude that allows an extraordinary level of elastic recovery; and then his team manufactures the material using microstereolithography. Stereolithography is a form of three-dimensional printing. Professor Zheng presented some of his research at the USAF research review that I attended last month [see ‘When an upgrade is downgrading‘ on August 21st, 2019 and ‘Coverts inspire adaptive wing design’ on September 11th, 2019]. He explained that, when these metamaterials are made out of a piezoelectric nanocomposite, they can be deployed as tactile sensors with directional sensitivity, or smart energy-absorbing materials.
Rayne Zheng and Aimy Wissa [‘Coverts inspire adaptive wing design’ on September 11th, 2019] both made Compelling Presentations [see post on March 21st, 2018] that captured my attention and imagination; and kept my phone in my pocket!