The following is an excerpt.
New knowledge on how to predict the behaviour of elements impossible to see with the naked eye.
Four years ago, Bjørn Håkon Frodal submerged into the microstructures of aluminium alloys. His mission was to predict the behaviour of elements impossible to see with the naked eye. To most of us, a mission impossible. All the more reason to send a loving thought to researchers like PhD candidate Bjørn Håkon Frodal, who embark on journeys into the universe of metallic materials with its grains, particles and atoms. Their motivation is to understand and predict the behaviour of these tiny and invisible inhabitants. And as a matter of fact, it is life-saving research.
Fundamental understanding makes us safer
«We must improve our understanding of what happens deep down in the interior when the materials are subjected to straining in compression and tension. Understanding the physical mechanisms at the microscopic scale and the role of the microstructure in metallic materials is fundamental», according to Frodal.
Simply because the more we know, the industry can create alloys that will make cars, ships, pipelines, buildings and other structures safer.
A solid step forward
Frodal´s work is a substantial step forward when it comes to understanding and describing the ductile fracture process of aluminium alloys. This is a type of fracture marked by extensive deformations, where the material is pulled apart instead of shattering.
His thesis is a comprehensive study that consists of experimental and numerical studies. The base is about 200 experiments on 3 different extruded aluminium alloys heat-treated to 3 different conditions. Thus, he could perform his compression-tension loading tests with 9 materials of varying yield strength, work hardening, texture – and behaviour.
Torturing materials
The tests are equal to extensive torture. First, the specimens go through extreme levels of compression. Then, they are continuously pulled to fracture in tension.
Why? Because in the real world, this is what happens, for instance, when a car collides. Bjørn Håkon Frodal
tells us to imagine the crash box (photo right), that lies behind the bumper beams in our car. It is made of aluminium, and its task is to absorb energy and protect our fragile human bodies. During impact, the material will first be compressed and fold. After, there is a tension-stage. The PhD candidate is motivated by what kind of influence the compression loading has on the damage evolution and the failure in the tension-stage that follows.
Inside the grains
To find out, he digs deep into the interior of this silvery-white, soft, light and ductile metal. It has a granular structure, and each grain measures 50-100 micrometre (0,05-0,1 mm).
Inside the grains, there are stacks of extremely well-organised atoms: They lie nicely in grids, all with the same spatial orientations. There are also precipitates, particles shaped like needles or disks that consist of magnesium and silicon. The temperature during processing is critical, but those precipitates are the guys that harden the metal and contributes to its strength.
«There is always this feeling that we need to explore more. Nothing is fulfilled or perfect. New issues pop up all the way».
Iron-men that collapse
«The deeper we go down in scale, the more information we get. But my primary interest is at the level of crystals and primary particles. Because it is here where damage and failure happens», the PhD candidate says.
Actually, he has contributed to a crystal plasticity model, that makes it possible to predict what goes on inside each of those tiny little grains when the material is loaded. As they consist of iron and silicon, primary particles are hard. When the material deform, they crack and form voids.
As strain increases, the voids grow and will eventually merge with other growing voids. When this happens, the material fractures.
«The primaryparticles are unwanted, but it is not realistic to process aluminium without them. It is possible, but totally clean aluminium would be too expensive», says Frodal.
Strengthens the link to nanoscale
One of the 4 articles in Frodal’s PhD thesis is a joint effort with fellow PhD candidate Emil Christiansen who also will defend his thesis this autumn. Christiansen works with nanoscale characterization at SFI CASA´s Lower scale programme.
The results of their experiments are said to be very exciting, and the aluminium industry pays close attention to their findings. Also, the work of the two researchers strengthen the link between the research programmes Lower scale and Metallic materials.
Another of Frodals achievements is the implementation of crystal plasticity modelling in SFI CASA´s Virtual Laboratory (VL) for the design of aluminium structures.
You can think of the VL as a modelling chain, with four main ingredients starting with nanostructure modelling. Next level is Bjørn Håkon Frodal’s home-ground, the crystal plasticity modelling. Then come unit cell modelling and localisation analysis. The latter is used to establish a failure criterion for the alloy.
Improving the VL´s Modelling Chain
According to his supervisor, professor Odd Sture Hopperstad, Frodal’s research has strengthened the modelling chain and improved the VL as a tool for the design of cars and protective structures. It enables the researchers to perform more robust and faster simulations of greater variety. His work means a significant step towards SIMLab’s long term objective:
That the virtual lab could minimise or even make physical testing superfluous in design of structures.
This is urgent because physically wrecking cars or components is expensive, time-consuming and harmful to the environment.
«The Virtual Lab will help to optimise the design process, save time and lower costs. In the long term, products and structural components become safer, as virtual tests are carried out before prototypes are made», Bjørn Håkon Frodal states.
From car mechanics to crystal plasticity
The now 30 year old researcher had a certificate of completed apprenticeship in car mechanics in hand when he started his studies in mechanical engineering at NTNU. The title of his Master´s thesis is «A Multi-Scale Approach for Modelling of Fracture in Aluminium Alloys under Impact Loading».
He reveals that his most-happy moments at work are when he runs finite element-model stuff and the curves that occur on his screen fit nice and precisely with the physical tests. Those are the moments of truth, when models and simulations correspond with what happens in reality.
«I enjoy going to work every day. I appreciate the feeling of having the time to immerse in thesis questions, explore things that you cannot in an ordinary job as an engineer».
For the next 2 years, he will work as a post-doc at the Toppforsk project FractAl.
«There is always this feeling that we need to explore more. Nothing is fulfilled or perfect. New issues pop up all the way».
Bjørn Håkon Frodal will defend his thesis, titled «Micromechanical modelling of ductile fracture in aluminium alloys», on October 8, 2019. His main supervisor is Professor Odd Sture Hopperstad. Professor Tore Børvik has been co-supervisor.
