The following is an excerpt.
Kristin Qvale is an expert in energy absorption and fractures in aluminium components, such as the crash boxes in cars. The key to knowledge lies in the microstructures. Any change here affects the whole component’s behaviour.
«The essence of my work is to perform various studies on ductile fracture in 6000-series aluminium alloys. I look at how certain changes in the microstructure affect the fracture behaviour at higher scales and how fracture affects the crash behaviour of aluminium components», says Kristin Qvale.
A PART OF TOPPFORSK PROJECT FRACTAL
She is the 4th of 5 PhD candidates to defend her thesis within the framework of the Toppforsk project FractAl. It is a concurrent project to SFI CASA, led by Professor Odd Sture Hopperstad. Its full name is «Microstructure-based Modelling of Ductile Fracture in Aluminium Alloys». The prime objective is to develop and validate a multi-scale framework for modelling plastic deformation and ductile fracture of age-hardening aluminium alloys.
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VIRTUAL DESIGN AND TAILORMADE ALLOYS
The 5-year project was completed in May 2022. It opens completely new possibilities in the design and use of aluminium.
«By introducing such a modelling procedure, we can -potentially- develop crash components almost completely virtually. One may tailor an alloy to optimise a component’s behaviour to failure», Kristin Qvale says.
Even though Fractal focuses on modelling 6000-series (Al-Mg-Si) aluminium alloys, parts of the modelling framework can also be used for other structural metals. The goal is to reduce the need for expensive physical tests by partially replacing them with numerical simulations.
«My research is most relevant for those who develop alloys for specific purposes. Simulating crushing profiles could be relevant for the automotive industry in a quest to reduce physical testing».
BRIDGING THE GAP
Her doctoral work spans from the nanoscale to full-size components. She has entitled it «Energy absorption and failure in aluminium alloys: An experimental and numerical study». If you imagine the FractAl framework as a somewhat intricate puzzle, her work will consist of pieces that fit in several places.
«My project is not the most complete in itself», she says.
«But it follows the work of others, while others can use parts of my work. I emphasise the macroscopic scales, with fracture included. Thus, I help build a bridge between the microstructure and the component behaviour».
WANTED: RELIABLE NUMERICAL TOOLS
Due to their attractive features, the automotive industry increasingly turns to aluminium alloys when designing crash components. Many alloys provide a good balance of strength-to-weight ratio and ductility, which is the ability to change physical shape without breaking. Fracture of a crash component should be avoided to perform optimally in a crash situation. For the car makers, numerical simulation of the behaviour becomes an ever more important tool in the design process.
«Accurate numerical predictions of deformation and fracture can enable developers to optimise and utilise the material’s energy absorption capacity to its full potential. In addition, it may reduce the need for expensive test programs», according to PhD candidate Qvale.
READ ALSO: A Tailormade Approach to the Design of Aluminium Alloys
INVESTIGATING HOW CHANGES AFFECT FRACTURE
Qvale investigates how changes in the alloy and the materials processing affect ductile fracture and the mechanical behaviour of the components. A crucial part of her work is to perform physical tests and numerical simulations of alloys. She does so by varying compositions and processing at various scales and stress states. Thus, she extends the experimental behaviour knowledge, enabling validation and modelling activities’ support.
«As a researcher it is important to think of the end goal, but at the same time, we are supposed to explore the unknown. Sometimes we do not exactly know where we will end up – which can be both fun and frustrating. That is why it is crucial turning every little stone. You never know what may be useful in the future».
VALIDATING THE FRAMEWORK 
Her validation tests represent a fracture mode relevant to typical aluminium structures. Additionally, she has performed component tests that represent a typical application of aluminium alloys. For the latter, she also performed tensile tests to determine the alloy’s mechanical properties.
«We chose these tests for their relevance for application of the 6000-series, and the results will form a good selection of validation data», she says.
A PHD-STORY IN THREE PARTS
This PhD thesis is a solid contribution to the FractAl project’s experimental database. The research focuses on the higher physical scales of the modelling framework. The experiments include various features, and the work forms a basis for further research. The complete work consists of 3 different studies.
In Part 1, she investigates how changes in the constituent particle content affect one specific fracture behaviour in three different alloys. She studies one commercial version and one tailormade version of each alloy. The latter had approximately three times higher content of constituent particles iron and silicon. While the fracture mode and mechanisms remained the same, the elevated content significantly affected the tear resistance.
Strain field at 2% of maximum force for all materials: (a) 6061A, (b) 6063A, (c) 6110A, (d) 6061B, (e) 6063B and (f) 6110B. Note that the colour legend is scaled differently for each material.
CRASH BOXES SUBJECTED TO CRUSHING
In Part 2, Qvale investigates the behaviour of two double-chamber aluminium profiles subjected to quasi-static and dynamic axial crushing. Visual inspection and X-ray Computed Tomography scans of the tested profiles show that dynamic loading causes a more significant amount of fracture than quasi-static loading.
SIMULATIONS VS COMPUTED-TOMOGRAPHY SCANS
In the 3rd part, the aim is to see how solid element simulations with coupled plasticity and damage in the material model may predict the behaviour of the crash boxes from Part 2.
This study aims to predict the component’s crash behaviour using explicit finite element simulations. How well the simulations represented the experimental behaviour was evaluated through comparison to computed tomography (CT) scans throughout the entire volume of the profiles.
Numerical simulation is becoming a more and more important tool in developing crash components. Accurate numerical predictions of deformation and fracture can enable developers to optimise and utilise the energy absorption capacity of the material to its full potential.
The figure shows Alloy AA6063, dynamic crushing. Figs. (a-c) show renders of the CT scan, Figs. (d-e) show renders of the deformation from the simulation, and Fig. (f) shows the fracture in the simulation on the undeformed profile.
THE VERSATILE ALUMINIUM
One of Kristin Qvale’s crash boxes left a prominent mark on the cover of the SFI CASA annual report in 2019. The motif, an X-ray CT scan of a crushed double-chamber aluminium crash box, created excitement. It did not only resemble the famous, ghost-like Hattifatteners from the Moomin valley books. Also, Quale’s simulations captured the folding patterns of a quasi-static crushing of an AA6063 with high accuracy.
STIFFNESS, STRENGTH, LIGHTWEIGHT, CORROSION-RESISTANT
«Aluminium is intriguing because of its versatility. The metal is a favourite in many structural applications. The reason is its stiffness and strength, lightweight properties, and corrosion resistance. At the same time, it is a material with high potential as an energy absorber through plastic deformation. Additionally, the ability to tune the mechanical properties through ageing treatments is an interesting feature», says Kristin Qvale.
A CONTRIBUTION TO GREENER, SAFER, MORE COST-EFFECTIVE SOLUTIONS
When we ask her if her work helps improve the industry’s ability to design greener, safer and more cost-effective components and structures, she says:
«Everything we learn about the behaviour of materials will contribute to the ability to design greener, safer, and more cost-effective. Additionally, with the increasing computational power, detailed numerical simulations should be able to replace physical testing more and more».
«Although my simulations are computationally heavy, they show great potential for predicting the behaviour of components. With reliable numerical tools, effective design optimisation should be possible».
Kristin Qvale now lives in Italy. Her supervisors are Professors Tore Børvik (main) and Odd Sture Hopperstad. She defended her thesis online on 28 June.
