Norway: Micro-Level Physics Life-Saving Research for Better Materials

When accidents happen, the difference between life and death may come down to the materials of the car, boat or building that you find yourself in. The best possible protection requires understanding as much as possible about how different materials behave under stress. Norwegian scientists examine this issue at the nano level.

Trondheim/Norway — Imagine building blocks so small that billions of them fit into a piece of aluminium the size of a sugar cube. Imagine modelling how those building blocks behave when exposed to crashes, shocks or other external stresses. Here, in this universe of atoms and particles, is where scientists Emil Christiansen and Bjørn Håkon Frodal are working at SFI Casa to understand how aluminium behaves under extreme loads.

The scientists are trying to understand how the smallest building blocks behave when we bend, stretch and deform the material. Their knowledge is to provide better quality building materials. Frodal concurs that understanding the micro-level physics inside metallic materials is fundamental.

The researchers offer three examples of industry that needs this knowledge, and why:

• In order for aluminium manufacturers to develop, update and improve their alloys, they need to know as much as possible about the properties of the materials they make.

• The automotive industry has a goal of drastically cutting down on physical crash testing of cars. In order to do that, it needs data models that can simulate the behaviour of the materials instead.

• The energy and construction industries need to understand the behaviour and limitations of the materials they use, in order to prevent accidents and damage as effectively as possible.

Frodal and Christiansen are the first of six PhD candidates to defend their theses at Casa this fall. Casa (Centre for Advanced Structural Analysis) is a Centre for Research-Driven Innovation, or SFI, in NTNU’s Department of Structural Engineering. The centre hosts the SIM-Lab research group. This group is a world leader in the field of how materials and structures behave under extreme stress. A major focus area for CASA is its virtual laboratory (VL) for designing aluminium structures. One of its objectives is to help reduce, or ideally eliminate, physical crash tests of cars. In other words: test crashes on PCs can save the industry tremendous amounts of time and money. Virtual testing also offers significant environmental benefits.

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Blueprint Lies at the Nanoscale

Christiansen believes that it is necessary to take a look at the nanoscale to see what’s really happening in a material. To this end, he worked on one of the world’s most sophisticated electron microscopes to see what’s moving in the materials at the atomic level: the large transmission electron microscope (TEM) at NTNU. There, he can zoom into the aluminium universe populated by various atoms. They are inconceivably small, but they behave in ways one can recognize: When the material is subjected to strain or pressure, the atoms get shoved into each other’s intimate spheres. It’s like being invaded by troublesome neighbours from all sides. Sparks fly, and at some point when the atoms have had enough, they make their escape, so to speak. The moment they give in, move away or dislocate, the material fails or becomes unstable.

Aluminium is a silvery-white and lightweight material. In its pure form it is so malleable that it can easily be bent by hand. The metal has a granular structure, and inside the grains are stacks of extremely well-organized atoms lying in grid formation. Each grain measures 50-100 microns (0.05-0.1 mm). To increase the strength of the material, tiny amounts of magnesium and silicon are added. These substances form precipitates, particles that are shaped like needles or discs, and function in the same way as reinforcement bars in concrete.

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Taking a Peak at Nanoclusters

Cars and Vulnerable Human Bodies

Frodal has helped develop a model for what is called crystal plasticity. The model makes it possible to predict what happens in each of the tiny grains when the material is subjected to a crash, impact or other extreme loading. He has done about 200 experiments on three different extruded aluminium alloys. These are in turn heat-treated in three different ways. The tests can be compared to violent torture. The materials are subjected to extreme pressure before being stretched until they break.

Frodal’s research is a step forward in understanding and describing the ductile fracture process for aluminium alloys. Ductility describes the material’s moldability and ability to deform without breaking. The primary particles are hard since they contain iron and silicon. As the material deforms, the particles crack and form voids. As the strain increases, the voids grow and merge with other voids. When this happens, the material fractures.

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