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INTERVIEW - Yves Gourinat, Professor ISAE-SUPAERO, University of Toulouse

News International-French

1 Jun 2020

The mission of ISAE-SUPAERO – the French Institute for Aeronautics and Space based in Toulouse – is to provide top quality higher education in aeronautics, space and related fields; to conduct scientific research and develop technologies; and to offer doctoral training programmes.

INTERVIEW - Yves Gourinat, Professor ISAE-SUPAERO, University of Toulouse  - Photo : ISAE-SUPAERO
Composite materials are undeniably at the heart of instruction. Today, among other things, ISAE-SUPAERO is working hard on perfecting the safety and performance of the airplane of the future. To find out more, JEC Composites Magazine got in touch with Professor Yves Gourinat, who heads up several projects there.

JEC Composites Magazine: How long have you been working in the field of aviation, and composite materials in particular?
Yves Gourinat:
I was lucky to begin my career in 1988 as a composite structures engineer, working specifically on the A340. I’ve been immersed in composites for more than 30 years now, and the evolution I’ve witnessed has been extraordinary. Thinking back to the situation at the time, the A320 had just completed its first flight, and was in the final phase of certification. I was employed by Aerospatiale, working in the Aircraft division. The company had taken over for Sud-Aviation and is known today as Airbus Commercial Aircraft. In those days, airplanes were composed essentially of light aluminium-based alloys and made up of around 15-20% composites. In 30 years, one could say that the proportion has practically reversed.

As an aside, I was part of the composites calculations division directed by Michèle Thomas at that time, where I stayed until 1994. Chantal Fualdes, now an international expert on the subject recognized by Airbus, Boeing and the FAA, was already a part of the group. In fact, Chantal replaced Ms. Thomas after she retired. When I switched to the space domain, then to structures in 2003 as a professor and research supervisor at SUPAERO, composites continued to be omnipresent. 

JEC Composites Magazine: As observed from your control tower, exactly how long has it been since we’ve known with certainty that composites have the capacity to age well? 
Yves Gourinat:
It goes back further than we sometimes believe (or hear). Thirty years ago, ageing was already being taken into account. It’s not new. For example, I can show you a paper from the NACA in 1957, that explicitly mentions materials ageing for their qualification. The National Advisory Committee for Aeronautics (NACA) created in 1915, is the institute that preceded NASA. It was dissolved in 1958 when the American space agency was created. And starting from the 60s, papers on ageing from NASA have abounded. It’s quite droll, actually, to see that the Americans recently created an agency focused on human, and biological, ageing that’s called... NACA (National Advisory Council on Aging). In fact, developments on ageing in aeronautics go all the way back to structural fatigue calculations initiated by the De Havilland Comet in the 50s. You’ll recall that the Caravelle took flight shortly after the Comet. From the very beginning, the French have been very closely involved in R&D on fatigue, and then in matters of ageing. Since the 70s, our labs and test centers have been at the forefront of the topic internationally. In particular, we can mention the Toulouse-based cluster with SUPAERO, ENSICA, and the Aeronautics Test Centre of Toulouse (CEAT), which is now known as the DGA-TA.

Also, we already had a fair amount of experience when we first used truly structural composites (on the A310 particularly), but we were not yet taking full advantage of what they offer in terms of performance. And actually, when something is certified, performance characteristics must be assured not only at the beginning of the service life but also at the end. In other words, a marathon is won at the finish line. Constant scientific and technological progress actually allows us to predict and implement these materials in an ever more efficient way, all while guaranteeing – and increasing – their safety over time. This is one of the big lessons in contemporary certification: performance and safety are not opposed, they work together.

Composite cryogenic fuel tank - Photo: NASA

JEC Composites Magazine: What significant advances have been made that have enabled us to overcome ageing and shock tolerance?
Yves Gourinat:
Scientific and technological advances have always been made, and always will be, as they are intricately linked. But I still haven’t understood whether it was the chicken or the egg that came first... In order to make progress, we must first determine where the barriers are, which stem mainly from the complexity of composites. We seek to draw the most from the performance of fibers (which are unidirectional, and therefore ultraspecialized, like a piano wire), the matrix (along with additives) and the assembly.

There are precedents, and in particular wood, which is a conglomerate of fibers – carbon, actually – and a matrix of resins. We can even say that our current composites are already bio-based structures! And if we add honeycomb between the skins, then we are drawing our inspiration twofold from living systems. While wood is a marvelous material, we are aware of how delicate, or susceptible to damage, it is, and even fragile. It’s very sensitive to moisture and impacts; this sensitivity is exploited when splitting logs. In addition, we notice that there are defects during the manufacturing process (natural additive manufacturing for wood-based materials) – cracks, knots, and delamination. Despite these defects, it’s a high-performing and safe material. We can appreciate the expertise of carpenters and coopers. From naval shipwrights to sailboats carpenters, for example, they are masters of beam structures; as for coopers, they’re masters of hull structures. 

Scientific progress is therefore based on physical modelling (mechanical and thermodynamic, including chemistry). Applied mathematics are able to represent, in an increasingly better way, what’s happening at the heart of a material. This may relate to failure, and the intimate contact between fiber and resin; the interface between plies; and thermodynamics, starting from manufacturing all the way to performance in operation, all the way up to the end of its service life.

Technological advances then adopt these models, improving the performance of bonding between fibers and matrix (adhesion), the assembly, and resistance to damage. Machining is actually a type of damage, since when you drill a hole, you’re cutting fibers! And that’s where the time factor comes into play. What wrinkles appear over time? What conditions in the environment (humidity, radiation, shocks/impacts, vibrations, noise, fuel) hamper performance and cause a material to become obsolete? 
The first three laws of thermodynamics are integrated into the models (and technology), in order to see things more clearly, and make the structures as safe as possible. The third law on entropy, a phenomenon said to be irreversible, is obviously the most important. The fourth is not yet explicitly used, but let’s wait and see... We could thus consider ourselves bounty hunters in hot pursuit of entropy, but we’ve got a license to kill it.

JEC Composites Magazine: What challenges lie ahead in this area? 
Yves Gourinat:
Our next challenge is to reduce the loss of performance caused by impacts and ageing. To do so, we'll have to rely on the interlinking of computing/calculations and technology. I'll explain. Up till now, we’ve integrated the extreme decline in performance that is due to everything that the material will experience during its service life, such as impacts and vibration, chemistry, radiation, and time. As the models did not do a very good job predicting this, we’ve made it a habit to overdimension the structures, which minimizes the advantage of using composites in the place of metal. In the future, we will surely reduce this overdimensioning. At that point, we will have achieved the third generation of composites. It’s the end of “black metal” in the sense that a composite is now considered a material in its own right, and not used simply to replace metals. 

Fracture surface of a fiber-reinforced ceramic composed of SiC fibers and SiC matrix. The fiber pull-out mechanism shown is the key to CMC properties - Photo: Wikipédia

JEC Composites Magazine: Your work focuses on modelling and simulation. Where do you stand at this time and what are your goals? 
Yves Gourinat:
Models are improving concurrently with the power of computers. Fundamental improvements have been made, which stem from having a better grasp of the microstructure of components. Dynamics, nonlinear methods, heat, and radiation are being explicitly integrated. Indeed, models are not only mechanical, they are also becoming multiphysical. What’s more, in terms of mathematics, a new trail is being blazed as an alternative to the finite element method (the certified method since the 1970s). And in a rather astonishing way, these new structural methods come from other fields, in particular from fluids, though a solid doesn’t quite behave the same way as a fluid. First, you have the particulate matter methods which are known as discrete element methods (DEM).

These are developing quite well at the moment, for instance through the work of my colleague Prof. Christine Espinosa. She’s the eminent specialist in modelling fracture phenomena, which is essential for composites. Then you have the “integral” methods (also known as the singularity method) which manage to achieve extreme condensation. I personally place great hope on their performance for modelling shell-fluids interactions, complex materials (including composites) and also biological systems.

Finally, I’d like to tell you about Virtual Testing. There will always be models and tests, but the cursor can shift a bit towards computing. This involves increasing reliability still more – compatible with conditions on board, meaning guaranteeing a risk that’s substantially lower than 10-6 per flying hour – all while optimizing the processes (cost and lead times). Math helps us, and we are all working towards this goal. And actually, it appears that a new approach is emerging, one on two levels, combining both analytical and numerical calculations in perfect harmony. For that matter, as a research supervisor, I’d like to say that the link to academics is essential to this topic and gives us assistance on the more advanced research. This link is particularly evident at SUPAERO, and I may have the opportunity to come back to this.

JEC Composites Magazine: We hear a lot of talk about thermoplastics. Do you think that they will make a place for themselves in aeronautics? What main benefits do you see here, if any?
Yves Gourinat:
Indeed, this distinction relates to the matrix, the strategic element that binds the fibers together. Up until now, most structural composites used a thermoset matrix. This means that the matrix is made from a resin that hardens during manufacturing, where curing is induced by heat, forming a compact molecular network. This is a very powerful and effective phenomenon, but one that is totally irreversible, resulting in a rigid, breakable, unit.

A thermoplastic matrix uses materials that are very different, that soften when heated, which makes forming a much simpler process. It also facilitates recycling at the end of service life. As you know, standard composites are complex to drape. It’s basically like attempting to mould plywood to a surface that’s not developable. Until now, these matrices still had performance problems. Progress made in chemistry (and additives) now allow us to envision a broader use of thermoplastics for a growing number of structural parts, in harmony with thermoset composites. Little by little, the progress made on both sides will allow specific performance characteristics to be improved, and to use “the right material in the right place”.

JEC Composites Magazine: In an ideal world, could an airplane be made entirely out of composites?
Yves Gourinat:
No, quite frankly not. An airplane made solely out of composites could become a “nightmare” liner. Perhaps my answer surprises you? I show students the “The Plastic Inventor”, a cartoon from 1944, featuring Donald Duck. It imagines the idea in a really funny way. Even if composites will continue to advance, composing up to 90% of the structural mass of an airplane, some elements will still have to be in metal. Or they will use other materials – at least in the near future. Not to mention that such other materials, including metallic, are also making strides. 
I’ll give you two simple examples. To be perfectly transparent with you, the window glass used in the cockpit and in the cabin, vital elements, will still be made of vitreous materials. The landing gear (we say “landers”, now) will remain metallic for a long time, for reasons involving resilience, geometry and technology. The advent of a hydrogen-powered flying wing may change this, but that’s a whole different story.

JEC Composites Magazine: Your institute is recognized worldwide in the field of aviation. What are your strengths?
Yves Gourinat:
It’s our architectural vision. It’s not just about excellency in a specific field (and actually, on staff we have specialists in the fields in question) but also about having a system-based approach. In fact, to certify an aerospace curriculum, a deep understanding of the interactions between disciplines (the non-diagonal terms of the tensor!) and complex systems is required. We are also certified for structures. These are two very strong points here at ISAE.
With respect to structures, durability, nonlinear dynamics and interactions with the environment represent a major contribution made by ISAE-SUPAERO. This has led to ten accreditations to supervise research on the subject, through the Mechanics, Structures & Materials department, and in conjunction with the Clément Ader Institute (ICA), in charge of solid mechanics. The ICA is a joint research unit, formed by the CNRS and the Federal University Toulouse Midi-Pyrénées. Moreover, these scientists work in a close, cooperative way with the other departments at ISAE-SUPAERO. Their presence allows us to develop research that is quite specific to (passively and actively) controlled structures, to interactions between fluids-structures, radiation and lightning, and with life sciences.

ISAE-SUPAERO - Credit photo : Alain Félix Masaï

JEC Composites Magazine: Is the school planning to go global? 
Yves Gourinat:
We already have! We’re totally global, every day. My colleague Prof. Christophe Bouvet is an internationally renowned specialist on composites damage tolerance. My colleague, Prof. Joseph Morlier, is the European specialist on Multidisciplinary Structural Optimization, and frequent host to guest professors from foreign universities. As for Prof. Guilhem Michon, he’s a reference on nonlinear methods, and also hosts colleagues from abroad, in particular from the US. We regularly participate in courses and seminars at American universities. Personally, my connection is with the University of Texas. I conduct research on composites (3D methods) in fact, along with my colleague, Prof. Frédéric Lachaud.

Research is international by nature, and so it’s for good reason that we’re planning to host specialists in composites from Boeing, Airbus, NIAR, FAA and EASA this summer as part of an international workshop dedicated to the Virtual Testing of composites. We also regularly take part in review panels for international journals, and in conferences. To conclude, I’ll add that 40% of our students are foreign.

JEC Composites Magazine: What is your view on composites education in general? Do you feel that we’re training enough specialists on the topic?
Yves Gourinat:
We have all the tools and specialists at our disposal, but to account for the explosion in needs, I think that we’ll have to increase the number of engineers trained in composite methods. Given the number of airplanes in service, and the certifications done throughout its life cycle (and even after being removed from service, since composites recycling poses a major challenge), engineering and design offices, production, and in-flight operations will require a greater number of composite structures engineers. Instruction on composites is included in the mechanics of solids, materials and structures program, and is one of ISAE-SUPAERO’s core strengths. New technologies for training and innovation such as MOOC (Massive Open Online Course), SPOC (Small Private Online Course) and seminars relating to current R&D will help us expand this mission.

JEC Composites Magazine: Could we consider exchanges with schools in other industries that are less advanced than aviation, in order to promote the use of composites? And actually, how do you account for the fact that some sectors are lagging?
Yves Gourinat:
I would qualify this question by saying that other sectors are simply other sectors, but that to make progress, composites are actually a remarkable example of synergy. I’ll take, for example, the sports world, where carbon arrows, snowshoes in composite materials, skis, and bicycles are all very advanced examples of the use of these materials. These have been developing precisely at the same time as in aviation. For ground transport, you’ve got the problem of having to manufacture in very large series (e.g. the automotive industry), but as it happens, thermoplastics will improve this. And for ageing in humid environments, shipbuilding is on the cutting edge, though we still have something to show them about gusts and wind dynamics.

JEC Composites Magazine: What would you tell people from the markets and sectors that hesitate to use composites? And what would you say about their ability to help us meet the environmental challenges we’re facing?
Yves Gourinat:
By means of their performance characteristics, composites enable us to lighten the weight of vehicles. This, in itself, is a step forwards in terms of ecology, as fuel consumption is noticeably reduced. On some primary structures, the gain in comparison to solutions in aluminium or steel becomes extremely high (in 30 years, we’ve summarily passed from -20% to -40%, which substantial). Furthermore, the fact that composite materials draw inspiration from nature is a sign of optimization, which is appealing in terms of fabrication, operating and overhead costs (in any case, for third-generation composites). Here again, they enable us to meet environmental targets. The durability of these composites and their potential reuse constitutes an additional argument. 

If I may, I’d like to finish with an example based, yet again, on nature. We mentioned the shells skin-honeycomb combination, which results in high-performing structures. We can also mention spiral-wound tanks inspired by the silkworm, which allow us to create shells with optimized shapes. Winding software programs from the world of textiles are now fully operational for use with composites and facilitate manufacturing of extremely lightweight pressurized tanks.