The title of this post comes from two lines in ‘In Memoriam A.H.H.‘ by Alfred, Lord Tennyson. The theory of plate tectonics evolved about fifty years ago so it is very unlikely that Tennyson was thinking about the hills as waves of rock flowing across the landscape. However, we now understand that Earth’s crust is divided into plates that are moving as a result of currents in the liquid magna beneath them. For example, the African plate is moving northwards crashing into the Eurasian plate causing the edges of the plate to buckle and flow forming the Alps and Pyrenees along the edge of the Eurasian plate. At the same time, the Eurasian plate is moving eastwards very slowly at a speed of about 2.5 cm per year, or about 2 metres in an average human lifetime. So, nothing stands still. Everything is a process. It’s just that some processes are quicker than others [see ‘Everything is in flux but it’s not always been recognised‘ on April 28th, 2021].
You would not think it was difficult to build a thin flat metallic plate using a digital description of the plate and a Laser Powder Bed Fusion (L-PBF) machine which can build complex components, such as hip prostheses. But it is. As we have discovered since we started our research project on the thermoacoustic response of additively manufactured parts (see ‘Slow start to an exciting new project on thermoacoustic response of AM metals‘ on September 9th, 2020). L-PBF involves using a laser beam to melt selected regions of a thin layer of metal powder spread over a flat bed. The selected regions represent a cross-section of the desired three-dimensional component and repeating the process for each successive cross-section results in the additive building of the component as each layer solidifies. And there in those last four words lies the problem because ‘as each layer solidifies’ the temperature distribution between the layers causes different levels of thermal expansion that results in strains being locked into our thin plates. Our plates are too thin to build with their plane surfaces horizontal or perpendicular to the laser beam so instead we build them with their plane surface parallel to the laser beam, or vertical like a street sign. In our early attempts, the residual stresses induced by the locked-in strains caused the plate to buckle into an S-shape before it was complete (see image). We solved this problem by building buttresses at the edges of the plate. However, when we remove the buttresses and detach the plate from the build platform, it buckles into a dome-shape. Actually, you can press the centre of the plate and make it snap back and forth noisily. While we are making progress in understanding the mechanisms at work, we have some way to go before we can confidently produce flat plates using additive manufacturing that we can use in comparisons with our earlier work on the performance of conventionally, or subtractively, manufactured plates subject to the thermoacoustic loading experienced by the skin of a hypersonic vehicle [see ‘Potential dynamic buckling in hypersonic vehicle skin‘ on July 1st 2020) or the containment walls in a fusion reactor. Sometimes research is painfully slow but no one ever talks about it. Maybe because we quickly forget the painful parts once we have a successful outcome to brag about. But it is often precisely the painful repetitions of “try and try again” that allow us to reach the bragging stage of a successful outcome.