The following is an excerpt.
The weakest adhesive John Fredrick Berntsen has studied in his doctoral dissertation is strong enough to lift a 1 500 kilos car with a bonded area of only 3×3 centimetres.
The strongest one would do the same job with only 2×2 centimetres.
«Rough estimates, and in ideal conditions, obviously. But under no means your standard glue from the kindergarten», Berntsen says.
In any case, the main task of the adhesives he has studied is not to lift cars off the ground. Their job is to connect the range of fundamentally different materials found in modern car bodies. They improve crash performance, too. Plus, superglues possess several other beneficial properties, that explain why their use rise in the design of modern cars.
ADHESIVES WITH ATTRACTIVE PROPERTIES
The trends of lightweight designs in the automotive industry are pushing towards more optimised structures. Which again, lead to more complex structures with more and more multi-material joints.
Traditionally, car frames of steel extensively used spot-welding as the primary joining technique. Adhesives have become a vital component of the joints as they are much more flexible concerning the materials than typical fasteners such as spot welds.New generations of car bodies require new joining techniques – and this is where the beauty of adhesives comes in. For, they do not only join fundamentally different materials. They also increase the overall stiffness of the vehicles in addition to improving crash performance.
Further, adhesives are beneficial concerning noise and vibration. Besides, they also act as corrosion barriers.
FROM EQUATIONS TO SIMULATIONS
According to the PhD candidate John Fredrick Berntsen, there is generally a lack of understanding for both the performance and the modelling of adhesives. That explains why he took upon the task to figure out how the sticky substances behave when exposed to extreme loading until failure.
During the last years, he has performed numerous material tests to investigate the strength and ductility of a variety of adhesives and bonded combinations. Based on observations and measurements, he describes their behaviour with a bunch of mathematical equations. After that, he writes the equations into computer code which is used to generate virtual versions of the adhesive.
«In turn, these can be used to predict how the adhesive behaves, for instance in a full crash simulation», Berntsen explains.
A NEW FOCUS ON LARGE-SCALE ANALYSIS
Multiple modelling scales are relevant to the topic of Berntsen’s thesis. The monograph’s title is «Testing and modelling of multi-material joints». He has investigated models for detailed local analysis of the adhesive behaviour and then linked these to large-scale modelling techniques. His supervisor, Associate professor David Morin, emphasises that the goal is not as much to provide detailed analysis methods. Instead, the main interest is to include the effect of adhesives in a large-scale analysis, without using too much calculation time.
Berntsen’s studies on large-scale analyzes show promising results. According to Morin, his work is in line with the goals of CASA, namely to deliver techniques that are useful to the industry.
GOOD NEWS FOR THE INDUSTRY
To support the work on reliable large-scale analysis, the PhD candidate has proposed a «partial virtual lab». This is doctoral work that brings good news to the industry. The partial virtual calibration procedure has a considerable potential to reduce costs and time linked to calibration.
«Based on the demonstration shown in my thesis, the calibration procedure for large-scale models of an adhesive in various configuration could be reduced to just a few hours. That is, given an already calibrated lower-scale model», Berntsen explains.
David Morin states that «Berntsen’ work will certainly contribute to the needs of the automotive industry. It could also have application in other business sectors – such as physical security».
THE FLEXIBLE BETAFORCE AND THE STIFF SIKAPOWER
Two selected adhesives play the main characters in Berntsen’s PhD-thesis. The first is the «semi-structural two-component adhesive Betaforce 2816». Despite the flexibility of this adhesive and its great applicability to a variety of materials, it has so far not received much attention in scientific studies.
The second, «SikaPower 498», is a structurally hardened epoxy adhesive which has received much more attention and figures in many studies. Sikapower is known for its high stiffness and strength and is often referred to as a crash-stable adhesive. This is, by the way, the glue that can lift a car with a glued area measuring 2×2 centimetres. Berntsen found significant differences between the two adhesives, which he concludes would entail different modelling approaches for each of them.
REPLACE THE EXPENSIVE AND TIME-CONSUMING TESTS
When asked what kind of needs that urge this kind of research, he says:
«Since the performance of the joints could have a significant impact on the overall behaviour of a component, it is critical to have sufficiently accurate modelling strategies for adhesives». Besides, there is a cost aspect. It would be very beneficial to replace the expensive and time-consuming tests with virtual experiments.
In the future, the goal is to avoid having to perform expensive and time-consuming experiments typically used to determine large-scale models for the adhesive.
CHALLENGING TO MODEL
John Fredrick Berntsen refuses to have had a particular fascination for glues that brought him to do a PhD thesis on the topic.
He says that, honestly, he found it interesting because of the potential challenge involved. Adhesives are known for not being simple to model.
Scientists agree there is currently a lack of understanding of the behaviour of adhesively bonded joints, as well as how to model them with confidence.
Berntsen says his work contributes on different levels. Some specific observations concerning the specific adhesives, and some more general.
«For the semi-structural adhesives, I think the contribution would be one part of showing an experimental regime sufficient for characterising and determining which phenomena governing mechanical behaviour. Additionally, my work shows how to model these phenomena with an acceptable degree of accuracy on the lower scale. Which is considerably different from how adhesives typically are modelled».
As an MSc student, John Fredrick Berntsen spent half a year as an intern with Toyota in Brussels. This was back in 2015, and his task was to verify the qualities of the polymer model in SIMLab’s Tool Box.
Berntsen reveals that he still has a genuine interest in working with the automotive industry in the future. In fact, partly, he already does so. After handing in his thesis, he was hired as a post-doc at CASA for roughly 2 more years. Now he works on industrial implementation with David Morin, CASA’s extended arm towards the Centre’s automotive partners.
John Fredrick Berntsen will defend his PhD thesis at NTNU 3 December.