Images of the grains in the alloys AA6061 (left) and AA6063 found from EBSD measurements. Each grain is assigned a colour to ease the separation of each grain from the others.
The following is an excerpt.
By extensive use of experiments, Susanne Thomesen uncovers the secrets of aluminium. The inside information fills in the big picture under construction by Toppforsk project FractAl.
Susanne Thomesen is the first of five PhD candidates in Toppforsk project FractAl to defend her thesis. She has investigated the microstructures and what happens inside aluminium alloys when subjected to severe loading conditions.
«I explore the basics. Knowledge about the smallest parts helps us to explain how the material behaves under large deformations and fracture», she states.
How to Model Aluminium that Breaks
The 5-year Toppforsk project FractAl started in 2016. The full name of the project is Microstructure-based Modelling of Ductile Fracture in Aluminium Alloys. It is a concurrent project to CASA, which reaps benefits from the research.
Where CASA focuses on structural analysis of steel, aluminium, polymers and glass, FractAl-researchers like Susanne Thomesen makes a concentrated effort in a specific area.
These people understand more than most what happens when aluminium fractures. They search to know the properties of different alloys down to the nano-level, and how the atoms react to different kinds of deformation. They aim to build a multiscale modelling framework that gives a fundamental understanding of ductile fracture in the material.
A New Way to Design Aluminium Structures
Thus, they will pave the way for an entirely new way to design aluminium structures. Hopefully, the new framework will enable designers and engineers to select the most suitable alloy for a given structure. In the future, this will also reduce the need for time-consuming and costly mechanical tests.
«Think of a carmaker, that might run several hundred crashes on car prototypes every year. If you could simulate some of those crashes instead, it would save the environment and the industry of large sums of money and a lot of time», Thomesen states.
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Strong Force, Slow Speed, Lack of Action
She kind of excuses the lack of action in her experiments. Especially when compared with colleagues at CASA who expose concrete or glass to blast loadings or maltreat aluminium profiles in the powerful Kicking Machine. The PhD candidate subjects her tiny specimens to a combination of slow speed and tension.
She laughs when explaining that it takes about fifteen minutes before anything exciting happens, that is, when the specimens give in and fracture. During loading, she uses different techniques to measure the deformation of the samples. This work involves the use of extensometers, laser-based measuring, digital image correlation (DIC) and edge tracing techniques.
Climate-friendly, Safe and Green
Thomesen explains: «The distribution of grains and
particles, their size and their orientation to one another. It all matters for the behaviour of the material when subjected to large deformations».
Of course, now and then, people ask her about what on earth is she doing and what is the greater good of her research. She states calmly: «No, I am not doing anything that makes neither yours nor my everyday life in the kitchen or elsewhere at home easier. But I like to think that I contribute to the bigger picture. Like saving resources. A more climate-friendly production. Safer constructions, greener cars or the likewise».
Research for safer solutions
Thomesen has studied three different aluminium alloys under various loads. The alloys are those most commonly used in protective car components as crash boxes and bumpers. During a collision, these are the parts of a car that absorbs the most energy.
(The figure shows fracture surfaces of representative tension tests for alloy AA6110. The images of each fracture surface show three different magnifications).
Her work includes hundreds of experiments and simulations and divides into three categories:
- She studies the microstructural features of the alloys by the use of optical and scanning electron microscopes.
- To better understand the influence of these microstructural features on the work hardening and fracture behaviour of the alloy during deformation, she does experimental tests.
- The knowledge of the microstructure and their impact on mechanical behaviour can then be put to use in microstructure-based modelling and simulations. In combination with finite element simulations, this can improve existing models on various scales.
Supplier of FractAls extensive database
«I enjoy doing tests, and the satisfaction that comes when my expectations fulfil, and I find the answers I search. I am not the kind of person that could mainly sit and break long equations. I thrive the best when putting theory into practice».
The PhD candidate describes her work as a kind of supportive activity. FractAl has established an extensive database for ductile fracture of different types of aluminium alloys. Her thorough descriptions of the microstructures will supply the base and implement into FractAl´s microstructure-based modelling sequence.
«The idea is that the information from this level can be transferred and the models validated on larger scales».
One Piece in a Big Puzzle
She shows some of the tiny specimens that she subjected to various loading conditions. Now they are broken and maltreated, stored in transparent and carefully marked plastic bags in the shelves of her office. Thomesen has spent countless of hours zooming in on the fracture surface of those specimens and the microstructure in the surrounding material.
«The experiments and the characterization I have done on these materials can now be put to use by other researchers. Through my studies, I contribute to data useful in the further development of the framework, and hopefully, the models involved can be improved. My work is one piece of this puzzle, and I do feel that what we do is important».
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The Right Place to Be
Susanne Thomesen has worked under supervision from Professor Odd Sture Hopperstad and Professor Tore Børvik. She defends her thesis on November 14. The title is «Plastic flow and fracture of isotropic and anisotropic 6000-series aluminium alloys: experiments and numerical simulations».
When the PhD party is over, a new chapter in life opens. She has signed up for a post-doc-position in SFI CASA. According to her descriptions, SIMLab seems the right place to be, both professionally and socially.
«I appreciate being part of this group. The spirit is good, and people are nice and very inclusive. We help each other, and I feel that we all join forces and pull in the same direction. People enjoy being here, and I enjoy being part of it».
