Thread by @realscientists

Good morning - it's @bmatb here.

Time for a brief introduction to some of our work on materials for jet engines.

@BMatB A jet engine is a marvel of modern technology - and it is only made (economically) possible to the invention of new materials.

@BMatB Here's a picture of a modern (large) jet and turbofan engine - the basic principle is "suck, squeeze, bang, blow".

(high res and source careers.rolls-royce.com/~/media/Files/…)

@BMatB Suck - at the front of the jet turbofan engine - we suck air in, and to push air past. Most of the air (70%) goes over the engine and provides direct (fan-based) thrust. 30% is directed inwards to for the next steps in the jet. The fan is made of titanium and/or composite.

Squeeze - next the air is passed through the compressor set. It heats up, it is compressed to ~200% (depending on engine). Here we are trying to get it hyper compressed so we can burn fuel efficiency. We use fancy titanium alloys here.

Bang - the compressed air is mixed with jet fuel. We want a clean burn - which is why we want a good ratio of fuel to air mix. The jet fuel is burnt very hot through combustion nozzles, and directed over turbine blades.

In this turbine set - here we extract work from the jet - converting chemical energy to mechanical. The hotter we can burn the fuel, the more efficient our engine can be. We use a class of materials called superalloys, which are typically nickle based.

The superalloys are a modern marvel of materials science - we literally created a new alloys system here. They need to run hot for a long time, so we want them to be resistant to creep (time dependant plastic deformation below the yield point).

The structures in the nickel based superalloys are shown nicely in this video - where they are probed with electron microscopy at different lengthscales

The mechanical work is extracted through blowing the hot gasses over the turbine set - and work is transferred back up the drive shaft of the engine to turn the turbine at the front (which pulls in more air, and bypasses more air to generate lift).

The turbine blades work effectively, as we have a system in place. We coat the blades with an oxide, to support a high temperature gradient. We blow a thin layer of air over them to protect them (like your skin holds an air blanket using bodyhair).

More: phase-trans.msm.cam.ac.uk/2003/Superallo…

In @expmicromech - we work to understand how you make the titanium materials, and their performance, at the front and in the compressor, and the performance of the Ni-materials in the turbine set. Using a mixture of experiments and simulations.

@ExpMicroMech One major aspect of our recent work was contributing to @EPSRC funded programme #HexMat - where we worked to understand "cold dwell fatigue" of the titanium disks.

@ExpMicroMech @EPSRC As with any engineering system - there is a risk that it breaks. In most cases we push this risk from our minds (e.g. we don't ~mind when vacuum cleaner motor dies) - but in an aeroengine system - engineers (rightly) spend lots of time thinking about this - so you don't have to.

@ExpMicroMech @EPSRC One engineering challenge in titanium systems is that the fan blades towards the front of the engine are held in place with a fan disk. If a single blade fails - it can be contained by the engine cover. If the disk fails, it's more problematic.

@ExpMicroMech @EPSRC This means we generate models - using lots of experiments (e.g. spinning a whole disk assembly, as well as stuff we do in my lab at the nm scale) to understand the performance limits of the materials and to estimate component life.

@ExpMicroMech @EPSRC This means that an engine manufacturer can put limits in place for when they inspect the engine for issues, and when parts are rotated out and replaced with new ones.

Engine failure, & maintenance, are both disruptive and cost money - so the more we know about life the better.

@ExpMicroMech @EPSRC "Cold dwell fatigue" is an issue - where each flight cycle (take off, cruise, landing) is one 'cycle' of deformation.

You've probably broken a spoon by bending it back & forth, never bending it to fracture in each cycle, but exhausting plasticity incrementally with each bend. This is fatigue. It affects all cyclic parts. We engineer systems to outlive the fatigue life of the materials.

"Cold dwell fatigue" is a surprising issue, found out in the ~1970s, where our methods of predicting life in the lab didn't match the disk tests, as titanium creeps at room temperature (very unusual) and the life is reduced during the load-hold of the cruise.

I stress carefully here - cold dwell fatigue is a "watching brief" in the aerospace industry - we know it can affect components, so we have changed how we manage parts to manage it. But this management is very conservative, and thus expensive. If we knew more - that would be fab.

In #HexMat we started at the nm lengthscale - using tests conducted inside and outside of an electron microscope - to build up a picture of materials performance. We passed that information into the computer & build a virtual material system. We used this model to emulate service

The #HexMat programme was £5m sponsored by @EPSRC and we have contributed towards saving of (probably) ~£100m per year for aerospace sectors, and we are working with US firms now too to help this become industry wide knowledge.

@EPSRC If you are interested in this story - you can see a video we produced on #HexMat - to share how 'blue skies' R&D can interface with industry, to motivate our studies and provide tangible gains for society.

Website: imperial.ac.uk/hexmat

@EPSRC Other cool fact - twin engine planes can fly with just one engine (even wide bodies - with two aisles), that's how we build in the safety margin. It's not comfortable if one goes, but you can still fly it to the next runway.

@EPSRC This process of flying with one engine is one reason why when flying from the UK to the US we fly via Canada (and not following the shortest 'great circle' around the globe. We have a safe flying window, e.g. if an engine fails.

This is all within ETOPS.

en.wikipedia.org/wiki/ETOPS

@EPSRC As with all of this - most of the engineering we do is to improve performance (which includes safety) but when I talk about it - everyone leaps on the safety story.

Humans, in their day to day lives, are terrible at judging risk.

You are ~300x times more likely to get hurt walking than flying in a commercial plane. I also spend SIGNIFICANTLY more time walking than I do flying.

tjpalanca.com/2014/03/on-pro…

Though I should also say that a significant fraction of the safety improvement in flying is due to the wonders of the next generation of materials that my colleagues (across the world) and I have been working on.