What I do and why you should care

A brief introduction of my work might go something like this: I am a PhD student at the faculty of Aerospace Engineering at Delft University of Technology where I am studying the damage tolerance of adhesive bonds and composite materials. That might sound impressive, but what does it actually mean? Let’s break it down into chunks: PhD student, adhesive bonds and composites, and damage tolerance.

Being a PhD student means that you study a certain topic for four years (in the Dutch system) and at the end write a book summarising your contributions to science on that topic. If the book is deemed good enough by a committee of experts in that field you are awarded the academic degree of ‘doctor of philosophy’ (abbreviated to PhD, because Latin).

Adhesive bonds is a fancy way of saying ‘things that have been glued together’. Composite materials are technically any material composed of two or more distinct components, but in the context of aerospace by composites we usually mean fibre reinforced plastics (FRPs). FRPs are composed of laminated layers of carbon or glass fibres embedded in a resin. Apart from aircraft structures you might have encountered FRPs in sailing boats or high-end sporting gear such as tennis rackets or hockey sticks (both of the field and ice variety).

Now for the final piece of the puzzle: damage tolerance. When we build a structure and use it out in the world it is often subjected to repeating loads. For example in an aircraft every flight the fuselage is pressurised so that you can keep breathing normally when at cruise altitude. You might also have noticed how the wings flex up and down during flight, especially if you encounter turbulence (yes, they’re supposed to do that, there’s no need for worry). These repeated loads are called fatigue cycles, and they can cause small cracks to appear and grow in the structure. You have probably heard about ‘metal fatigue’, but this is something that can happen in nearly all materials (so we just call it ‘fatigue’). What we mean by damage tolerance is how and when these cracks appear, how fast they grow, and how big they can get before posing a threat to the integrity of the structure.

In my research I’m looking specifically at cracks between the different layers of an adhesive bond (so growing in the adhesive that is between the two things that are stuck together) or between the layers of a composite laminate. The ultimate goal is to take the properties of a fatigue cycle (e.g. the maximum and minimum force or deflection) and from that be able to predict how much further the crack will grow during that cycle.

So why should you care about this research? Well, as you might know an aircraft is built up of different parts. Tens or hundreds of thousands of parts in fact, and these need to be joined together somehow. The most common way of doing this currently is by using rivets. You can see a nice example of this in the picture.

IMG_4336
Rivets holding together the structure of a C130 Hercules transport plane.

The problem with using rivets is that in order to install them you have to drill holes in your structure. This introduces stress concentrations, which are areas where the forces in your structure are greatly intensified. To compensate for this we have to add reinforcing materials to riveted joints, which makes them a lot heavier. In contrast, if we join parts using adhesive bonding the stress concentrations that we introduce are much smaller, so we can get away with using less reinforcement, resulting in a lighter structure.

The amount of fuel a plane uses is directly related to how heavy it is. By using adhesive bonds we can build lighter planes, which therefore use less fuel. As a result the environmental impact of those planes is lower and it’s cheaper to fly on them. So not only can adhesive bonds reduce the price of your holiday or business trip, they can reduce the ecological footprint you’ll be leaving too, which sounds like a win-win to me (unless you’ve invested heavily in oil companies). Before we can use adhesive bonds effectively and safely however, we need to understand and be able to predict their crack growth behaviour, which is what I’m working on.

To sum all that up into a tl;dr version: I’m trying to understand the growth of cracks due to fatigue loads in adhesive bonds, well enough that we can predict how fast it will be. This will allow us to safely and efficiently use adhesive bonding to join aircraft parts together, resulting in planes that use less fuel.

If there’s anything in there that is unclear, let me know in the comments and I’ll do my best to explain it.

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