Tag Archives: strain

Third time lucky

At the end of last year my research group had articles published by the Royal Society’s journal  Open Science in two successive months [see ‘Press Release!‘ on November 15th, 2017 and ‘Slow moving nanoparticles‘ on December 13th, 2017].  I was excited about both publications because I had only had one article published before by the Royal Society and because the Royal Society issues a press release whenever it publishes a new piece of science.  However, neither press release generated any interest from anyone; probably because science does not sell newspapers (or attract viewers) unless it is bad news or potentially life-changing.  And our work on residual stress around manufactured holes in aircraft or on the motion of nanoparticles does not match either of these criteria.

Last month, we did it again with an article on ‘An experimental study on the manufacture and characterization of in-plane fibre-waviness defects in composites‘.  Third time lucky, because this time our University press office were interested enough to write a piece for the news page of the University website, entitled ‘Engineers develop new method to recreate fibre waviness defects in lab‘.  Fibre waviness is an issue in the manufacture of structural components of aircraft using carbon fibre reinforced composites because kinks or waves in the fibres can cause structural weaknesses.  As part of his PhD, supported by Airbus and the UK Engineering and Physical Sciences Research Council (EPSRC), Will Christian developed an innovative technique to generate defects in our lab so that we can gain a better understanding of them. Read the article or the press release to find out more!

Image shows fracture through a waviness-defect in the top-ply of a carbon-fibre laminate observed in a microscope following sectioning after failure.

Reference:

Christian WJR, DiazDelaO FA, Atherton K & Patterson EA, An experimental study on the manufacture and characterisation of in-plane fibre-waviness defects in composites, R. Soc. open sci. 5:180082, 2018.

Designing for damage

Eighteen months ago I wrote about an insight on high-speed photography that Clive Siviour shared during his 2016 JSA Young Investigator Lecture [see my post entitled ‘Popping balloons‘ on June 15th, 2016].  Clive is interested in high-speed photography because he studies the properties of materials when they are subject to very high rates of deformation, in particular polymers used in mobile phones and cycle helmets – the design requirements for these two applications are very different.  The polymer used in the case of your mobile phone needs to protect the electronics inside your phone by absorbing the kinetic energy when you drop the phone on a tiled floor and it needs to be able to do this repeatedly because you are unlikely to replace the case after each accidental drop. A cyclist’s helmet also needs to protect what is inside it but it only needs to do this once because you will replace your helmet after an accident.  So, the kinetic energy resulting from an impact can be dissipated through the propagation of damage in the helmut; but in the phone case, it has to be absorbed temporarily as strain energy and then released, like in a spring.

Of course there is at least an order of magnitude difference in the consequences associated with the design of a phone case and a cycle helmet.  We can step up the consequences, at least another order of magnitude, by considering the impact performance of the polycarbonate used in the cockpit windows of airplanes.  These need to able absorb the energy associated with impacts by birds, runway debris and other objects, as well as withstanding the cycles of pressurisation associated with take-off, cruising at altitude and landing.  They can be replaced after an event but only once the plane as landed safely.  Consequently, an in-depth understanding of the material behaviour under these different loading conditions is needed to produce a successful design.  Of course, we also need a detailed knowledge of the loading conditions, which are influenced not just by the conditions and events during flight but also the way in which the window is attached to the rest of the airplane.  A large and diverse team is needed to ensure that all of this knowledge and understanding is effectively integrated in the design of the cockpit window.  The team is likely to include experts in materials, damage mechanics, structural integrity, aerodynamic loading as well as manufacturing and finance, since the window has to be made and fitted into the aircraft at an acceptable cost.  A similar team will be needed to design the mobile phone casing with the addition of product design and marketing expertise because it is a consumer product.  In other words, engineering is team activity and engineers must be able to function as team members and leaders.

I wrote this post shortly after Clive’s lecture but since then it is has languished in my drafts folder – in part because I thought it was too long and boring.  However, my editor encourages me to write about engineering more often and so, I have dusted it off and shortened it (slightly!).

Image: https://commons.wikimedia.org/wiki/File:Airbus_A350_cockpit_windows_(14274972354).jpg

Instructive report and Brexit

Even though this blog is read in more than 100 countries, surely nobody can be unaware of the furore about Brexit – the UK Government’s plan to leave the European Union.  The European Commission has been funding my research for more than twenty years and I am a frequent visitor to their Joint Research Centre in Ispra, Italy.  During the last decade, I have led consortia of industry, national labs and universities that rejoice in names such as SPOTS, VANESSA and, most recently MOTIVATE.  These are acronyms based loosely on the title of the research project.  Currently, there is no sign that these pan-European research programmes will exclude scientists and engineers from the UK, but then the process of leaving the EU has not yet started, so who knows…

At the moment, I am working with a small UK company, Strain Solutions Ltd, on a EU project called INSTRUCTIVE.  I said these were loose acronyms and this one is very loose: Infrared STRUctural monitoring of Cracks using Thermoelastic analysis in production enVironmEnts.  We are working with Airbus in France, Germany, Spain and the UK to transition a technology from the laboratory to the industrial test environment.  Airbus conducts full-scale fatigue tests on airframe structures to ensure that they have the appropriate life-cycle performance and the INSTRUCTIVE project will deliver a new tool for monitoring the development of damage, in the form of cracks, during these tests.  The technology is thermoelastic stress analysis, which is well-established as a laboratory-based technique [1] for structural analysis [2], fracture mechanics [3] and damage mechanics [4], that I described in a post on November 18th, 2015 [see ‘Counting photons to measure stress’].  It’s exciting to be evolving it into an industrial technique but also to be looking at the potential to apply it using cheap infrared cameras instead of the current laboratory instruments that cost tens of thousands of any currency.  It’s a three-year project and we’ve just completed our first year so we should finish before any Brexit consequences!  Anyway, the image gives you a taster and I plan to share more results with you shortly…

BTW – You might get the impression from my recent posts that teaching MOOCs [see ‘Slowing down time to think [about strain energy]’ on March 8th, 2017] and leadership [see ‘Inspirational leadership’ on March 22nd, 2018] were foremost amongst my activities.  I only write about my research occasionally.  This would not be an accurate impression because the majority of my working life is spent supervising and writing about research.  Perhaps, it’s because I spend so much time writing about research in my ‘day job’ that last year I only blogged about it three times on: digital twins [see ‘Can you trust your digital twin?’ on November 23rd, 2016], model credibility [see ‘Credibility is in the Eye of the Beholder’ on April 20th, 2016] and model validation [see Models as fables on March 16th, 2016].  This list gives another false impression – that my research is focussed on digital modelling and simulation.  It is just the trendiest part of my research activity.  So, I thought that I should correct this imbalance with some INSTRUCTIVE posts.

References:

[1] Greene, R.J., Patterson, E.A., Rowlands, R.E., 2008, ‘Thermoelastic stress analysis’, in Handbook of Experimental Mechanics edited by W.N. Sharpe Jr., Springer, New York.

[2] Rowlands, R.E., Patterson, E.A., 2008, ‘Determining principal stresses thermoelastically’, J. Strain Analysis, 43(6):519-527.

[3] Diaz, F.A., Patterson, E.A., Yates, J.R., 2009, ‘Assessment of effective stress intensity factors using thermoelastic stress analysis’, J. Strain Analysis, 44 (7), 621-632.

[4] Fruehmann RK, Dulieu-Barton JM, Quinn S, Thermoelastic stress and damage analysis using transient loading, Experimental Mechanics, 50:1075-1086, 2010.

Slowing down time to think [about strain energy]

161-6167_imgLet me take you bungee jumping.  I should declare that I am not qualified to do so, unless you count an instructor’s certificate for rock-climbing and abseiling, obtained about forty years ago.  For our imaginary jump, pick a bridge with a good view and a big drop to the water below and I’ll meet you there with the ropes and safety gear.

It’s a clear early morning and the air is crisp and fresh – ideal for throwing yourself off a bridge attached to a rope.  The rope is the star of this event.  It’s brand new, which is reassuring, and arrived coiled over my shoulder.  A few days ago, I asked you how much you weigh – that’s your real weight fully clothed, at least I hope that’s the number you gave me otherwise my calculations will be wrong and you’ll get wet this morning!  I have calculated how much the rope will stretch when it arrests your free-fall from the bridge parapet; so, now I am measuring out enough rope to give you an exciting fall but to stop you short of the water.  I’m a professor of structural materials and mechanics so I feel confident of getting this bit right; but it’s a long time since I worked as an abseiling instructor so I suggest you check those knots and that harness that we’ve just tightened around you.

You’ve swung yourself over the parapet and you’re standing on the ledge that the civil engineers conveniently left for bridge jumpers.  The rope is loosely coiled ready with its end secured to a solid chunk of parapet.  As you alternate between soaking up the beautiful view and contemplating the chasm at your feet, you wonder why you agreed to come with me.  At this moment, you have a lot of potential energy due to your height above the sparkling water [potential energy is your mass multiplied by your height and gravitational acceleration], but no kinetic energy because you are standing motionless.  The rope is relaxed or undeformed and has zero strain energy.

Finally, you jump and time seems to stand still for you as the fall appears to be happening in slow motion.  The air begins to rush past your ears in a whoosh as you build up speed and gain kinetic energy [equal to one half your mass multiplied by your velocity squared].  The bridge disappeared quickly but the water below seems only to be approaching slowly as you lose height and potential energy.  In reality, your brain is playing tricks on you because you are being accelerated towards the water by gravity [at about 10 metres per second squared] but your total energy is constant [potential plus kinetic energy unchanged].  Suddenly, your speed becomes very apparent.  The water seems very close and you cry out in surprise.  But the rope is beginning to stretch converting your kinetic energy into strain energy stored by stretching its fibres [at a molecular level work is being done to move molecules apart and away from their equilibrium position].  Suddenly, you stop moving downwards and just before you hit the water surface, the rope hurls you upwards – your potential energy reached a minimum and you ran out of kinetic energy to give the rope; so now it’s giving you back that stored strain energy [and the molecules are relaxing to their equilibrium position].  You are gaining height and speed so both your kinetic and potential energy are rising with that squeal that just escaped from you.

Now, you’ve noticed that the rope has gone slack and you’re passing a loop of it as you continue upwards but more slowly.  The rope ran out of strain energy and you’re converting kinetic energy into potential energy.  Just as you work out that’s happening, you run out of kinetic energy and you start to free-fall again.

Time no longer appears to stationary and your brain is working more normally.  You begin to wonder how many times you’ll bounce [quite a lot because the energy losses due to frictional heating in the rope and drag on your body are relatively small] and why you didn’t ask me what happens at the end.  You probably didn’t ask because you were more worried about jumping and were confident that I knew what I was doing, which was foolish because, didn’t I tell you, I’ve never been bungee jumping and I have no idea how to get you back up onto the bridge.  How good were you at rope-climbing in the gym at school?

When eventually you stop oscillating, the rope will still be stretched due to the force on it generated by your weight.  However, we can show mathematically that the strain energy and deformation under this static load will be half the values experienced under the dynamic loading caused by your fall from the bridge parapet.  That means you’ll have a little less distance to climb to the parapet!

Today’s post is a preview for my new MOOC on ‘Understanding Super Structures’, which is scheduled to start on May 22nd, 2017.  This is the script for a step in week 2 of the five-week course, unless the director decides it’s too dangerous.  By the way, don’t try this home or on a bridge anywhere.