The discovery of the neutral currents

Apart from the barrel, Beumler also initiated other test work. In the full-scale fatigue test on the A320, cracks were found in the circumferential strips (‘butt straps’) that connected the fuselage sections. Since this strip is located inside the aircraft, it is difficult to inspect for cracks. Because the design of the aircraft was already finished, a material was needed that could replace the original strip without necessitating changes in the rest of the structure. Glare was an excellent candidate for this, although titanium was also an option. Since Glare still was not qualified as a material, a special part qualification programme was needed for the butt strap that would cost 2000 man-hours. The programme management of the A320 was not sufficiently interested to spend the money, although successful tests were done by an undergraduate student in Hamburg and in Delft on realistic specimens with a butt strap connecting two pieces of skin. Another Delft student did computer calculations on this detail with Glare. Delft’s formability expert Jos Sinke

developed methods allowing Glare to be able to produce the required complex shape of the connections of the stiffeners over the butt strap. Sinke continued to work on the manufacturing aspects of Glare, which would become essential for the later application of the material on the A380. Since this problem also affected the French section of the fuselage, Aérospatiale was also considering Glare. The French were still not very enthusiastic about the material and finally it was decided to stick to the original design in aluminium and just to inspect the butt-straps more frequently to ensure that possible cracks would be found in time.

In the early 1990s a large reorganisation aimed at cost reduction was taking place in Germany at DASA under the name ‘Dolores’, and a lot of employees were forced into early retirement. As a consequence of ‘Dolores’, the budget for new development was cut and the work on Glare slowed down considerably. Suddenly there was no money available for the qualification of the Glare material. However, the good results from the Glare parts of the barrel were now available and Beumler managed to keep the development alive by getting a Glare part on the full-scale fatigue tests of the

A340. For this aircraft, a bulkhead section at the rear of the fuselage was designed and manufactured in Glare. Koshorst financed this project from Airbus Industrie via SLC, which had to produce the necessary parts. The rear pressure bulkhead is a curved shell, which closes the pressure vessel of the fuselage and is vital to the aircraft. Yet the rear bulkhead is difficult to inspect because it is hidden by installations, toilets and galleys.

Damage tolerance of this part was high on the agenda since a couple of years earlier a bomb had exploded in an A300 toilet situated next to the bulkhead, damaging it considerably. Although the aircraft had been able to withstand this damage, the incident had created an awareness of the essential level of damage tolerance of this aircraft part. Secondly, possible ways to save weight are an issue for aircraft engineers at any time.

The Glare section produced for the bulkhead was the first Glare part that was manufactured and tested with a curvature in two directions. The three separate 0.3 mm aluminium sheets were stretch formed together in Deutsche Airbus’ Nordenham plant before bonding them together by placing the intermediate prepreg layers in between the formed aluminium parts. This procedure was unavoidable, since the minimum radius of the bulkhead is no larger than 800 mm. Later, in 1996, double-curved Glare was produced by laying up aluminium sheets in a double-curved mould without preforming and by using splices to prevent excessive wrinkling of the sheets and autoclave pressure to press the sheets in the right contour. Student Van Oostrom did the first production trials in this way in 1996. The stretch-forming process is very expensive. Bonding of the pre-formed aluminium sheets into Glare took place at a specialised company in Los Angeles. Although highly contoured, a splice had to be included, which allowed the part size to be increased by 50% compared to the baseline. At another location in the bulkhead, three radial crack stoppers in titanium were replaced by stoppers made from Glare. Although the tests again proved the excellent properties that had already been found in Delft on small coupons to be valid for realistic structures, the chicken-and-egg situation of no application without qualification and vice versa was not broken. Enthusiasm was still limited to the engineers, while the management remained sceptical. However, important experience was obtained in this way.

Sometimes results were not immediately favourable for Glare. For example, it proved extremely difficult to produce a bulkhead attachment. It took a lot of belief to trust that some real aircraft parts could be constructed out of the aluminium sheets that came out of the Glare forming process

looking like wrinkled handkerchiefs once more experience was obtained with the production process. Without the shielding environment of the university, where it was always possible to look for innovative solutions, a new technology like Glare would never have grown up. Had Glare been developed in the highly competitive environment of a big aircraft company like Airbus, the idea would have been discarded as impractical long before.

Competing groups are always willing to attack the technology on the basis of unexpected results. The fight was certainly not an easy one and the Glare technology needed some protection, especially in the early days. A new development is never without disappointments that have to be solved. The basis of the Glare support in terms of the engineers that were familiar with its properties and who backed the material remained small and the support of higher management levels would be needed. In DASA, Beumler managed to neutralise all internal attacks against Glare, but in fact he was just a member of the fatigue and damage tolerance department. It was not his task to develop a new aircraft material but to do fatigue analysis on existing ones.

Yet the freedom and flexibility of the culture of the department made it possible that someone could take up this challenge. One of the problems with Glare was that there was actually no department that could take Glare under its wings. The aircraft materials groups were divided in two camps:

one that promoted carbon composites, and an aluminium group which was busy with the new aluminium alloys that were developed as a response to carbon. As we mentioned in the previous chapter, ALCOA had developed a new alloy and was expending a lot of effort to make it a commercial success.

The Glare material had no clear position in this competition between aluminium and carbon. For the aluminium fans it was a composite and for the carbon promoters it was an unclear compromise. The result was also that the development work on Glare in Germany and in Delft did not fully cover the required scope of different aspects.

Since a co-ordinated effort to adopt the material was lacking, the test work that was done was not always defined by the specialist who had to be convinced. Some aspects did not receive the proper attention that was required because not all the necessary questions were asked. Not all aspects could be covered by Delft; the strength of the Delft staff was that they were generalists who were able to bridge the various disciplines required for the application of a new material. A specialist would not be able to accomplish this. However, for the material to become a success, the various specialists at Airbus also needed to become involved. Within Airbus,

the number of people that were involved was too small to ask all the relevant questions. In Hamburg, it was expected that Delft would take the lead and vice versa. This resulted in the fact that after all the initial work in Delft, Hamburg and Toulouse had been completed, a large number of uncertainties remained to be covered to close all the technological gaps. The main emphasis was, however, to keep the Glare development alive and to strengthen the case for the material in such a way that it could be seriously addressed. This would gradually happen in the course of Glare’s application in the Airbus A380, which is outlined in the next chapter. The durability of the material was just one example of a problem that was not completely treated in this period, since it was not possible to determine the best way to get a handle on the subject. The long-term behaviour under influence of moisture and changing temperatures and under realistic loading conditions over 25 years of aircraft use is difficult to simulate in a lab. However, a convincing set of evidence was needed to create confidence that Glare would withstand realistic conditions. It took negotiations with all the specialists to develop a test programme that could be considered realistic.

In 1994/1995 Roebroeks carried out tests which created a lot of anxiety in Delft. Significant strength reductions – of the order of thirty percent – were found after exposure of Glare to a high humidity environment.

Roebroeks discovered that this effect is characteristic for small coupon specimens, where the moisture can penetrate through the whole width. In contrast he found that the US Navy’s experience with large glass fibre-reinforced structures, when examined after twenty years of service, showed that no degradation had taken place. Roebroeks reached the conclusion that in real structures only the edges of the laminate are affected and that not only periods of high humidity will occur but also dry periods in which the moisture evaporates out of the structure. This indicated that the accelerated lab tests on small pieces of material were not relevant. However, these tests are standard for composites. It was the first serious drawback for Glare, and although the story was convincing it certainly required a positive, realistic mind set.

The flexibility in the department in Hamburg certainly played a crucial role in this stage of the development of Glare. The positive results gradually created trust in the technical soundness of the material. Many smaller tests were done in Hamburg and a lot of experience was gained with the new technology. In this period, from 1992 to 1996, a second project involving a number of aircraft companies in Europe got under way, sponsored by the

European Union under the name Brite Euram project BE 2040. Participants in this project were: Fokker, Deutsche Aerospace Airbus, British Aerospace, Aérospatiale, Dassault, Dornier, DASA, Structural Laminates-bv, NLR, DLR, TU Delft, TU Braunschweig, Alenia and CASA. In this way, other European manufacturers also gained experience with the material. Although the project did not really develop Glare further and repeated much of the work that had already been done in Delft and Hamburg, it did help to spread knowledge and experience with the material across Europe. The contact and exchange between the different companies in such a European project was valuable, although it did not lead to real co-operation because every partner tended to pursue its own interests and expertise.

U.S.

Although Boeing discontinued its association with Glare, the connection between Delft and the U.S. always continued. It is hard to imagine any development in aviation without involvement from the United States. On the one hand, Airbus feared any flow of knowledge across the ocean, while on the other hand the company would be surprised if no interest was shown. The technology for Glare applications had to be guarded, but Airbus realised that a technology can only be successful if widely applied. Rob Fredell, a US Air Force officer who had already worked on Arall and was present on the first Arall conference in 1987, started his PhD-study in Delft in 1991. This was quite unique, since American officers usually did their PhD-work in the U.S. In Delft, Fredell started a project on repair and managed to gather a large group of undergraduate students for his team. His research focussed on the possibility of repairing existing aluminium aircraft structures with bonded Glare patches, comparing the results with the conventional composite patches that are applied for these purposes.

He discovered that because of the smaller thermal mismatch between aluminium and Glare compared to the combination of aluminium and composite, the Glare patches behave better. This initiated a lot of work on bonded patches for several years in Delft and when Fredell returned to the US Air Force Academy he continued his research there by hiring two Dutch graduates, Kees Guijt and Stephan Verhoeven. They remained the Glare bridgehead in the U.S. and brought a special Dutch flavour to the

highly bureaucratic US Air Force Academy by introducing the characteristic Delft culture and humour. This repair project even led to the fact that the first Glare application to take to the air as part of a primary structure was achieved in the U.S.; in October 1995 a bonded Glare repair patch was installed over cracks in the fuselage of a gigantic C-5A Galaxy transport aircraft. The repair project drew a lot of international attention, which helped to keep the world’s specialists interested. The repair project was also continued in Delft and later another PhD-student, Arjan Woerden, started to work in this area. Another American project was for the US Navy, in co-operation with Drexel University. From 1992 to 1994, research was done on special high temperature laminates for supersonic transport, which suffer from aerodynamic heating. On the Dutch side Coen Vermeeren did the work on this type of fibre-metal laminates. As a follow-up to this project, the NIVR sponsored a project, carried out by Kees Guijt, on high temperature fibre-metal laminates for space applications consisting of thin layers of titanium and special adhesives reinforced by carbon fibres. This high temperature variant of fibre-metal laminates is currently still under development.