Materials and constructions of the future

New knowledge on materials is essential to optimise and tailor them for specific applications and improve the many products that surround us in everyday life.

Part of the research at DTU Mechanical Engineering focuses on materials, including their related manu-facturing processes and their performance in constructions and products. The work includes basic research on understanding, describing and develop-ing materials, as well as more application-oriented research, which departs from meeting performance challenges. The challenges can relate to many dif-ferent performance parameters, but typically focus on combinations of strength, fracture, toughness, corrosion and wear of materials.

“The continuously increasing size of wind turbines is a good example of this. The enormous forces that wind gusts impose on the gears and bearings in the drivetrain can be detrimental and lead to premature  failure. As researchers, we can contribute to solving this problem by examining where and how the  materials fail, in order to provide options for material optimisation. In other words, we take the challenge into the laboratory, where we look into the material and try to find out why the damage occurs and how it can be avoided. For example by proposing another material or a specific treatment that leads to im-proved performance,” says Professor Marcel Somers.

3D printing could kick start a renaissance
Marcel Somers’ research team also works with material  technologies in relation to 3D printing, which can significantly change the use of a material. Titanium is a splendid example of this. The application of titanium has previously been very costly due to the manufac-turing, which includes both its synthesis and shaping by conventional cutting and deformation processes. 

“Titanium is an interesting material because it combines high strength with low weight and is abundant in the earth’s crust. With the help of 3D printing, it could experience a renaissance and become relevant for many more applications than is the case today.  Titanium is therefore a material that we are intensively researching into,” says Marcel Somers.

Surface microstructure of selective laser melted      Microstructure in over-heated and partly melted
titanium (3D print), showing the result of an island      ductile cast iron, showing solidified regions with 
scan strategy with different patterns in the islands.     dendrites, spheroidal graphite (black), lenticular .
Each island has dimensions 0.5 x 0.5 mm.      mar-tensite (leaflets) and retained austenite (yellow).
Image taken with digital reflected light microscopy.     Image taken with reflected light microscopy

The properties of a 3D printed metal are not the same as those of the metal we otherwise know. In the 3D printing process, metal powder is melted together layer-by-layer. The repeated heat impact and subsequent rapid cooling means that the 3D printed titanium must have post heat-treatment to ensure the same strength as conventional titanium. Furthermore, a surface treatment is necessary to improve the wear resistance of titanium to widen the application potential. 

New technology gives researchers new opportunities
New methods and technologies also make it  possible for researchers to challenge and improve the knowledge gained over the years on different types of metals and their properties.

“Today, we have powerful electron microscopes, which makes it possible to study the microstructures of the metals in two dimensions in high resolution and in much detail. With the help of x-ray and neutron tech-nologies, we can now also look into the material and characterise the microstructures in 3D and see what happens when the metal is used. This provides great insight into how the metal reacts when it is exposed to e.g. stress or heat,” explains Professor Dorte Juul Jensen, who is leading an ERC (European Research Council) Advanced Grant project.

With this new insight in 3D, it has proved neces-sary to revise many of the previous theories on the behaviour of metals.
“Whereas we have previously based the analysis mainly on mean values, we now have the opportunity to look into the metal at the crystal level and see its internal structures and strains and thus the local variations in 3D. This gives us new knowledge to  understand for example the properties of the metals. In the long run, this new knowledge will help reduce the very large safety margins that many engineers have worked with in their constructions and thus contribute to resource savings, says Dorte Juul Jensen.

The new insight into the properties of metals is not only generated with the use of advanced characterisa-tion techniques, it is also related to new possibilities for handling big data. Machine Learning may be used to assist the researchers in revealing trends in the measured data caused by inhomogeneity in the ma-terial, and which the researchers have not identified on the basis of the usual analysis methods.

“This is important, as even small metal improvements can be of great importance and lead to much better properties, saving raw material and increasing safety due to lower risk of fracture, for example,” emphasis-es Dorte Juul Jensen.

Composite materials for means of transport
So-called ‘composite materials’ is another group of materials that the researchers at DTU Mechanical Engineering conduct extensive research into. These are very light materials that are typically used in transportation means such as aircraft, ships and land vehicles, as well as in other weight-critical construc-tions as wind turbine blades, where both strength and stiffness are essential.

“Our research focuses among other things, on the so-called ‘sandwich materials’. A light core material of, for example, structural foams or honeycomb struc-tures is placed between two thin layers of fibre composites, to give the desired strength and stiffness necessary in a component of a ship hull, the tailplane of an aircraft, the blade of a wind turbine or in the underbody of a car. And it does so without consider-ably increasing the total weight of the construction,” says Associate Professor Christian Berggreen, who heads the research on lightweight constructions at DTU Mechanical Engineering.

Composite materials are used in 
weight-critical constructions as e.g. 
wind turbine blades where both 
strength and stiffness are essential.

Research in thisarea focuses on the damages that can occur in composite materials and components, and on the speed with which the size of cracks in-crease during use. The goal is to provide users of the construction with the knowledge to assess whether a damage is harmless or in need of repair.

“Part of our work is to develop simulation models based on laboratory testing of the materials and com-ponents from which the constructions consist. During testing, the materials and components are exposed to real-life conditions such as low temperatures or large pressure differences encountered by ships and aircrafts, respectively. The simulation models are subsequently used as supportive decision tools when deciding if a particular damage requires repair,” says Christian Berggreen.

The researchers at DTU Mechanical Engineering are at the forefront, internationally, within lightweight con-structions, collaborating with e.g. NASA, Airbus and the US Navy on advanced sandwich materials for aircraft and ships. Other partners include the Danish Defence, which needs lightweight vehicles for international missions that are both easy to transport and strong enough to withstand roadside bombs, for example.

World-class research facilities

DTU hosts world-class facilities for the analysis and testing of materials and constructions. Several advanced microscopes, both electron and x-ray microscopes, can be found in the university’s lab-oratories, where the microstructures and internal stresses of the materials can be studied.

In addition, the university has established a multi-scale testing facility, CASMaT, Villum Center for Advanced Structural and Material Testing, where large constructions of composite, concrete and metallic materials can be tested in newly es-tablished and advanced facilities. Part of CASMaT consists of the laboratory unit DTU Structural Lab, which is used both for teaching and research in the field of mechanical testing of materials and constructions in addition to offering test services to companies on commercial terms.

Structural optimisation

DTU Mechanical Engineering has been a world leader in structural optimisation for more than half a century. This strong tradition has led to a continuous interaction with leading scientists in the field around the world. The research devel-ops methods for size and shape optimisation providing stronger, and weight-saving structures, as well as structures with optimised dynamical properties. Topology optimisation was spun out of the traditional optimisation methods in the late 1980s, providing mathematical methods for structural designs that are unconstrained by human imagination, thus providing designs with freely varying topologies.

The department has many research projects in collaboration with both academia and industries,  where the focus is on improving structural designs,  for various applications. Furthermore, our re-searchers build on the experience in optimisation of structural systems, to extend the methods to other fields including fluid flow as well as optical and electromagnetic devices.