Last month I was at the Photomechanics 2018 conference in Toulouse in France. Photomechanics is the science of using photons to measure deformation and displacements in anything, from biological cells to whole engineering structures, such as bridges or powerstations [see for example: ‘Counting photons to measure stress‘ posted on November 18th, 2015]. I am interested in the challenges created by the extremes of scale and environmental conditions; although on this occasion we presented our research on addressing the challenges of industrial applications, in the EU projects INSTRUCTIVE [see ‘Instructive update‘ on October 4th, 2017] and MOTIVATE [see ‘Brave New World‘ posted on January 10th, 2018].
It was a small conference without parallel sessions and the organisers were more imaginative than usual in providing us with opportunities for interaction. At the end of first day of talks, we went on a guided walking tour of old Toulouse. At the end of second day, we went to the Toulouse Aerospace Museum and had the chance to go onboard Concorde.
I stayed an extra day for an organised tour of the Airbus A380 assembly line. Only the engine pylons are made in Toulouse. The rest of the 575-seater plane is manufactured around Europe and arrives in monthly road convoys after travelling by sea to local ports. The cockpit, centre, tail sections of the double-deck fuselage travel separately on specially-made trucks with each 45m long wing section following on its own transporter. It takes about a month to assemble these massive sections. This is engineering on a huge scale performed with laser precision (laser systems are used to align the sections). The engines are also manufactured elsewhere and transported to Toulouse to be hung on the wings. The maximum diameter of the Rolls-Royce Trent 900 engines, being attached to the plane we saw, is approximately same as the fuselage diameter of an A320 airplane.
Once the A380 is assembled and its systems tested, then it is flown to another Airbus factory in Germany to be painted and for the cabin to be fitted out to the customer’s specification. In total, 11 Airbus factories in France, Germany, Spain and the United Kingdom are involved in producing the A380; this does not include the extensive supply chain supporting these factories. As I toured the assembly line and our guide assailed us with facts and figures about the scale of the operation, I was thinking about why the nuclear power industry across Europe could not collaborate on this scale to produce affordable, identical power stations. Airbus originated from a political decision in the 1970s to create a globally-competitive European aerospace industry that led to a collaboration between national manufacturers which evolved into the Airbus company. One vision for fusion energy is a globally dispersed manufacturing venture that would evolve from the consortium that is currently building the ITER experiment and planning the DEMO plant. However, there does not appear to be any hint that the nuclear fission industry is likely to follow the example of the European aerospace industry to create a globally-competitive industry producing massive pieces of engineering within a strictly regulated environment.
There was no photography allowed at Airbus so today’s photograph is of Basilique Notre-Dame de la Daurade in Toulouse.
Some research has a very long incubation time. Last month, we published a short paper that describes the initial results of research that started just after I arrived in Liverpool in 2011. There are various reasons for our slow progress, including our caution about the validity of the original idea and the challenges of working across discipline boundaries. However, we were induced to rush to publication by the realization that others were catching up with us [see blog post and conference paper]. Our title does not give much away: ‘Characterisation of metal fatigue by optical second harmonic generation‘.
Second harmonic generation or frequency doubling occurs when photons interact with a non-linear material and are combined to produce new photons with twice the energy, and hence, twice the frequency and half the wavelength of the original photons. Photons are discrete packets of energy that, in our case, are supplied in pulses of 2 picoseconds from a laser operating at a wavelength of 800 nanometres (nm). The photons strike the surface, are reflected, and then collected in a spectrograph to allow us to evaluate the wavelength of the reflected photons. We look for ones at 400 nm, i.e. a shift from red to blue.
The key finding of our research is that the second harmonic generation from material in the plastic zone ahead of a propagating fatigue crack is different to virgin material that has experienced no plastic deformation. This is significant because the shape and size of the crack tip plastic zone determines the rate and direction of crack propagation; so, information about the plastic zone can be used to predict the life of a component. At first sight, this capability appears similar to thermoelastic stress analysis that I have described in Instructive Update on October 4th, 2017; however, the significant potential advantage of second harmonic generation is that the component does not have to be subject to a cyclic load during the measurement, which implies we could study behaviour during a load cycle as well as conduct forensic investigations. We have some work to do to realise this potential including developing an instrument for routine measurements in an engineering laboratory, rather than an optics lab.
Last October I cited John Hull’s audio diary in which he said ‘Cognition is beautiful. It is beautiful to know.’ [See my post entitled ‘Cognition is beautiful‘ on October 19th, 2016] Last week, I watched the film ‘Notes on Blindness‘ based on his book ‘Touching the Rock‘. We found it a moving and life-enriching experience. At one point, John Hull, after he has lost all of his sight, opens his front door during a rain storm and describes the beauty of the rain. “Rain has a way of bringing out the contours of everything; it throws a coloured blanket over previously invisible things; instead of an intermittent and thus fragmented world, the steadily falling rain creates continuity of acoustic experience.” You can read more of this extract at www.johnmhull.biz/Touching the Rock.html in which John wishes that rain could fall inside a room to give him a sense of the things in the room. This seemed particularly poignant to me, a sighted person, who benefits from photons raining down on everything around us during daylight or when the light is switched on. The photons cause light waves to radiate from every surface in a similar way that the rain drops cause sound waves to radiate from everything as John experienced. Our eyes are amazing with 137 million separate ‘seeing’ elements on the retina, or in digital camera terms, that’s 137 megapixels. But to quote the Roman poet, Lucretius who in his poem ‘De Rerum Natura’ wrote “Nothing in the body is made that we may use it. What happens to exist is the cause of its use.” In other words, we do not have eyes so that we can see but we see because we have eyes. John Hull discovered new ways to experience the world using what was available to him although he struggled with what he had lost. It is difficult to imagine losing one’s sight but his diary and the film bring us considerably closer to an appreciation of the loss.
Yes, I know I switched from a particle to wave description of light but I wanted to emphasize that the photons don’t just bounce off surfaces, otherwise all surfaces would look the colour of the illuminating light.