The following is an excerpt.
They are ultralight, excellent energy absorbers, and they may save your life. This brilliant mix of properties explains why polymer foams play the leading role in Daniel Thor Morton’s doctoral dissertation.
PhD candidate Daniel Thor Morton has investigated foams used in the bumpers of modern cars. The aim is to understand, describe, and model how these materials respond to impact. Both under different loading rates and different climatic conditions and temperatures. To fulfil the task, he has delved deep down into the foam’s interior. He says that the internal structures bear a close resemblance to soapy foam. Morton’s efforts have been to develop a micromechanical model that resembles the foam structure and describes its behaviour.
MODELLING OF LARGE DEFORMATIONS
«The purpose of our research is to enable the industry to more efficiently design and develop components by utilizing virtual models and engineering software. The design process, and ultimately the product, benefits from improvements in efficiency and accuracy of these tools», Morton says.
Daniel Morton has tested several tiny, cubic samples from bumper components.
Generally, micromechanical modelling is well developed and widely used, also on foam. The framework developed as part of Morton’s research uses what he calls periodic boundary conditions. It allows for the modelling of large deformations. It accepts a wide range of settings, such as distributing material between the cell walls and cell corners. Most other models focus on one or two of these areas. Mortons model attempts to combine them. Thus, it makes it possible to explore a broader range of configurations.
HIGHLY ATTRACTIVE TO THE INDUSTRY
«Combining them has, as far as I know, not been done before. Hopefully, my work will provide a more complete picture of the behaviour of foams, subjected complex deformation states», he says
The soon-to-be-doctor hopes that his work has laid the foundation for further research and model development. Also, it allows for evaluation of existing models so that the best currently available model can be chosen.
«The idea is to capture as much of the response as possible under different conditions. This improves our understanding of responses in different climates and crash scenarios»
«Hopefully, the information, testing and modelling here will be a good starting point for others to better understand and capture the complex mechanical response of foam», he says.
The combination of low weight and excellent energy absorption makes polymer foams highly attractive to the industry. When applied in cars and bumpers, they serve as pedestrian impact protection. You will also find them in personal protective equipment like helmets. Besides, they can be tailored for specific use in the packing industry, electronic, aerospace, building construction, bedding, and medical applications.
READ MORE: Polymer Foams and Their Role in Pedestrian Impact Protection
READ MORE: The Art of Modelling Aluminium Atoms
FOAMS UNDER COMPRESSION, TENSION, SHEAR..
Morton has tested several tiny, cubic samples from bumper components. In the test facilities of SIMLab, he has subjected them to compression, tension, and shear – and he has studied their behaviour under different temperatures and deformation rates.
«The idea is to capture as much of the response as possible under different conditions. This improves our understanding of responses in different climates and crash scenarios», the researcher says.
In the automotive industry, his work gives engineers a better opportunity to consider different design scenarios.
«A relevant example would be the effect of different temperatures of foam behaviour. Ideally, a pedestrian impact case should yield similar results in northern Norway as in Spain. But, this is difficult to predict without a comprehensive understanding of mechanical behaviour. Hopefully, my work will provide a more complete picture of foams, subjected to complex deformation states», he says.
Samples tested in tension at different temperatures. Sample 8 illustrates failure within the material, while sample 10 is an example of bonding failure.
The research has so far shown the material to be strongly temperature-dependent. Thus, testing different loading cases exhibits the behaviour, which is difficult to represent with the currently available engineering tools. However, according to Morton, it is hard to say if these effects are significant enough to have a substantial impact on the outcome in a crash.
TAILORMADE AND DESIRABLE
The materials are challenging to describe with existing material models. The complexity comes from the broad range of possible deformations and the foam’s internal geometry. According to Morton, uniaxial compression testing is the easiest testing method. It is sufficient for simple applications, such as a square cube impacting a square piece of foam.
«If the foam component and impact scenario are more complex, for example, where there is a shear load combined with a compression load, we need a better understanding of the response. On such occasions, many of the currently available material models fall short. The consequence of the internal cells, their buckling and deformation, is complicated to realistically represent», he explains.
To give you an idea of the challenges, we have to make a detour into some of the technicalities of the PhD candidate’s work. Foams are defined as cellular materials, and they can be tailored to specific applications by choosing an appropriate bulk material and manufacturing process. Desirable qualities include low density and good capability to absorb energy. Foams with polymer bulk material can be classified as either open or closed cell, depending on how the material is distributed within the microstructure.
LIKE SOAPY FOAM AND FLAT FACES
Imagine the previously mentioned resemblance to soapy foam, and visualize that you step inside a closed-cell foam. Morton describes it as you would be surrounded by flat faces. Each edge connects two, and each corner connects three faces. One can imagine a hexagon, but in three dimensions, to get a rudimentary idea of the structure.
«The wall of this structure would be composed of the base material, in our case, a polymer, and the edges connecting the faces would be relatively thin. An open foam, on the other hand, would see most of the material located in the edges, while the faces would be open».
THE MAIN BENEFITS OF FOAM
If the foam is subjected to compression, the thin walls and edges will start to deform and, or buckle, allowing the material to decrease in volume without too much effort. This is the main benefit of foams compared to a solid material, which does not easily change the volume.
«Most foams, irrespective of the material it is made from, has this quality. However, the base material determines some of the other behaviour. For example, how much it springs back after being deformed».
SEM images of 2 types of foams. Since the two materials have similar nominal density, but appear to have different mechanical response, it is of interest to compare the internal microstructure of the two foams. At the lowest magnification (×15), it is possible to see the individual beads which comprise the foam component. The internal cellular structure of the beads appears more clearly at the two higher magnifications (×45 and ×300).
A MODEL COMBINING A WIDE RANGE OF SETTINGS
Generally, micromechanical modelling is well developed and widely used, also on foam. The framework developed as part of Morton’s research uses what he calls periodic boundary conditions. It allows for the modelling of large deformations. It accepts a wide range of settings, such as distributing material between the cell walls and cell corners. Most other models focus on one or two of these areas. Mortons model attempts to combine them. Thus, it makes it possible to explore a broader range of configurations.
A FOUNDATION FOR FURTHER RESEARCH
«Combining them has, as far as I know, not been done before. Hopefully, my work will provide a more complete picture of the behaviour of foams, subjected complex deformation states», he says.
The soon-to-be-doctor hopes that his work has laid the foundation for further research and model development. Also, it allows for evaluation of existing models so that the best currently available model can be chosen.
«Hopefully, the information, testing and modelling here will be a good starting point for others to better understand and capture the complex mechanical response of foam», he says.
ROBUST MODELS, EASY TO LEARN AND USE
«Hopefully, my work will provide a more complete picture of the behaviour of foams, subjected complex deformation states», Daniel T. Morton says.
In general, there is a high threshold for adapting new research for industrial applications. Daniel Thor Morton emphasizes that CASA’s software and material models need to be robust and convenient to learn and use.
This is a crucial component of the research which cannot receive enough attention and effort», he says.
The PhD candidate finds the polymer foams inherently inspiring due to their often safety-related uses.
«Our type of work is part of an ongoing effort of improvement, both in productivity and safety for companies utilizing foams. The faster the groundbreaking research of CASA can be shipped to industry partners, the sooner we will have tangible gains in, for example, pedestrian impact safety».
«The currently available tools have also been evaluated based on the gathered data, such that our partners can make the most efficient choice of material model», Morton states.
FUTURE WORK
He will defend his thesis «Characterization and modeling of the mechanical behavior of polymer foam» on 10 June 2021. Professor Aase Gavina R. Reyes, OsloMet, has been Morton’s main supervisor. His co-supervisors are Professor Arild Holm Clausen and Professor Odd Sture Hopperstad, SIMLab, Department of Structural Engineering.
«For me, the “journey has become the goal”. As I started out, I had a vague sense of where the research should end but not what it would take to get there. Some things have gone well, while others have gone wrong. I have increased appreciation for the process more than any result. Such as learning new tools and skills required to make progress. In fact, these have, in some ways, become more motivating than the outcome of the final product».
About his future plans, Daniel Thor Morton says:
«So much of our work focuses on using code to make it as efficient and streamlined as possible. My background is from mechanical engineering. I see that there is still so much more to learn about using coding for engineering purposes. This is something which will be an important part of my future career».
Watch: SIMLab on YouTube
Follow: Extreme Dynamics NTNU on Twitter
