With a shared interest in the structural fatigue life of the ‘legacy’ F/A-18 Hornet strike fighter, Finland is collaborating with Australia in a comprehensive test program to validate a Finnish Air Force (FINAF) modification designed to prevent cracking in one of the more highly stressed locations in the aircraft.
Finland’s Patria Aviation, in conjunction with the FINAF, has developed a boron doubler, which is currently being applied to the FINAF Hornet’s center fuselage structure, also known as the center barrel section, with access via the main landing gear bay.
Australia’s Defence Science and Technology Organization (DSTO) in Melbourne has an extensive history of both Hornet fatigue testing and international cooperation, having performed a full-scale test on a Hornet aft fuselage section in collaboration with Canada, and is currently involved in ongoing work on center barrel fatigue testing for the Royal Australian Air Force (RAAF) and other clients. DSTO has conducted fatigue tests on 18 retired center barrels, provided by the RAAF, US Navy and Royal Canadian Air Force, courtesy of a major structural modification program to extend the operating life of the aircraft, which involves a new-for-old replacement of the center barrel.
With full-scale center barrel test articles and structural test expertise available at DSTO, an agreement between Australia and Finland has resulted in the recent application of a boron doubler to one test article and subsequent fatigue testing to assess its performance. At the time of writing, the results of this work were being analyzed, with a paper on the subject to be presented to the International Committee on Aeronautical Fatigue and Structural Integrity (ICAF) biennial conference in Helsinki in early June by Geoff Swanton, DSTO’s F/A-18 center barrel test manager from DSTO’s aerospace division.
DSTO can trace its aeronautical research history back to the 1940s, when structural test work was performed on Commonwealth Aircraft Corporation (CAC) Boomerang and De Havilland Australia Mosquito wings.
Its location at Fishermans Bend in Melbourne was no accident, being located close to Australia’s two major aircraft manufacturing facilities at the time – CAC and the Commonwealth Department of Aircraft Production (DAP, later Government Aircraft Factories). In 1947 DSTO’s Arthur Wills pioneered research into aircraft structural fatigue and two years later presented a paper titled The Life of Aircraft Structures, which was regarded as a seminal work on the subject. Today, a specially constructed fatigue test facility at DSTO bears his name.
In 1950 what ultimately became a 12-year program into the fatigue behavior of aircraft structures began. This program involved the testing of 222 Mustang wings under a complex series of repeated loads and was the most extensive series of fatigue testing of a single type of aircraft structure ever undertaken.
During the 1970s full-scale fatigue test programs included work performed on Nomad and Mirage IIIO aircraft, as well as research into the improvement of the wing carry-through structure of the General Dynamics F-111C, prior to its introduction into service with the RAAF. During this time DSTO also pioneered fatigue life extension by the application of boron patch repairs to aircraft structures, local applications for which included RAAF Lockheed C-130E Hercules, Mirages and F-111C wings, and this technology was also used by the US Air Force in its Lockheed C-141 Starlifter life-extension program.
Discussions regarding international collaboration on Hornet fatigue testing between Australia and Canada began in 1988, which resulted in the International Follow On Structural Test Project (IFOSTP).
Testing the Hornet
Initial structural testing of the legacy Hornet by McDonnell Douglas, the OEM, was predicated on the operational profile and configuration of its major customer, the US Navy.
The flight profiles of land-based customers of the aircraft, such as Australia and Canada, were not the same, however, as well as there being some structural configuration differences, so as a result it soon became understood that further testing would be required.
Under the IFOSTP agreement, Canada would be responsible for the testing of the Hornet center and forward fuselage and wings, while Australia assumed responsibility for aft fuselage and empennage testing, using a representative flight spectrum common to both countries.
However, during certain flight regimes the Hornet is subject to
high-frequency dynamic buffet loading, including to the tail surfaces, which until the IFOSTP testing began, could not be applied to the test article at the same time as the typical maneuver loads.
Conventional test rigs use ‘whiffle trees’ – a series of large beams that apply distributed loads over a wing or fuselage of a full-scale test specimen – however the mass and stiffness associated with this system has historically precluded the application of simultaneous vibratory loads.
In response DSTO developed a unique system using airbags to apply maneuver loads and electromagnetic shakers to apply buffet loads. The airbags didn’t add a significant amount of structural stiffness to the test article, permitting the simultaneous application of dynamic loading via the shakers.
These tests formed the basis of the structural certification and set the life limits for the Hornet structure under RAAF operating conditions, and the work won the prestigious International Council of Aeronautical Sciences von Karman award in 2002, recognizing “outstanding examples of international cooperation in the field of aeronautics”.
Since then delays to the introduction into service of the Lockheed Martin F-35A, the successor to the Hornet in RAAF service, has resulted in further DSTO analysis of the center barrel structure beyond the IFOSTP to support life-extension activities.
Center Barrel cracking
The Hornet center barrel structure comprises three major bulkheads, each of which has two large lugs at its upper corner, to which the wings are attached. Flight loads are transferred from the wings to the center barrel structure via these lugs, and the bulkheads also have cut-outs to accommodate the engine air inlet ducts, a bladder fuel tank, fuel and hydraulic lines, etc.
Geoff Swanton explains the nature of the center barrel bulkhead failures experienced during testing: “One of the locations that initially failed in the OEM fatigue test was addressed by retrofitting a metallic doubler to fleet aircraft to reduce the stress in that location,” he says. “This doubler was subsequently fitted to the IFOSTP Canadian center fuselage test, and this also failed, but at the end of the doubler instead. While the OEM doubler successfully addressed the initial cracking, a by-product was that it shifted the stress concentration and potential for cracking to the end of the doubler,” he explains.
The RAAF Hornet fleet has two configurations of center fuselage bulkhead: the early configuration that included the metallic doubler retrofit, and a later production version that featured a thicker section in lieu of the doubler where the OEM failure occurred.
“The bulkheads are optimized structures designed to withstand high loads without compromising weight and strength, and this means that there are numerous regions that also experience high stresses,” continues Swanton. “Over the past 10 years our center barrel testing program has demonstrated that those other areas do fail as well.”
It is important to note, however, that scatter factors and conservative assumptions are applied to the demonstrated test lives to reduce the risk of these failures ever occurring during the nominal lifetime of the aircraft. “The FINAF boron doubler is considerably longer than the original OEM doubler and was designed to address several of these areas, not just the original one that failed in the OEM’s test,” says Swanton.
The Finnish solution
The boron doubler developed by Patria in collaboration with the FINAF is applied to the bulkhead through the Hornet’s main landing gear wheel wells of the aircraft. This easy access means that installation can be accomplished during programmed maintenance. The only disassembly required is the temporary removal of some hydraulic tubing in the wheel well.
“The FINAF is concerned that, under its own operating conditions, the fatigue life of its F/A-18C/D Hornets might not be adequate to get the aircraft out to their retirement date,” explains Swanton. “The doubler testing at DSTO while the FINAF is modifying its fleet is a great example of concurrent engineering. The opportunity to take advantage of DSTO’s accelerated test program means that any potential issues would become apparent years before they ever manifested in the fleet.”
As a result of the collaborative agreement, four Patria technicians traveled to Fishermans Bend during 2013 and over a four-day period installed the boron doubler on an ex-US center barrel test article, before handing it back over to DSTO for fatigue testing and analysis.
Testing the boron solution
The center barrels are cycled in a purpose built test rig, which require the barrels to be rotated 90° before connecting them to the loading beams via the wing attachment lugs on each bulkhead, which are in turn connected to opposing pairs of hydraulic jacks. The rig is operated by a bespoke closed-loop control system designed by DSTO.
The jacks impart a load to the beams, which is transferred to the center barrel structure as a wing root bending moment. The cycling of the jacks simulate representative flight loads, which during flight would otherwise be fed into the center barrel via the wings.
The FINAF boron doubler and surrounding bulkhead structure
were instrumented with conventional foil strain gauges so that the local strains could be accurately measured and compared. Testing began in late 2013 and continued through to April this year. “We applied a representative RAAF load spectrum to the test article and cycled until a failure occurred. We didn’t know what part of the bulkhead was going to fail or when – the whole point was to find out,” explains Swanton.
“The bulkhead failed just outboard of the doubler, in one of the areas known to be susceptible to cracking. Previous DSTO center barrel tests had also failed there, without the presence of the Finnish boron doubler, so it wasn’t a surprise in that regard. However, the strain gauges indicated that the stresses were reduced considerably in the areas covered by the doubler, and certainly in the other areas prone to cracking there was no failure.”
Because an RAAF spectrum was applied to the test, a correlation exercise will be required to determine what this means in terms of the equivalent FINAF fatigue life, and whether the results will give the FINAF the margin it is seeking to operate its Hornet fleet out to its planned retirement date.
“The correlation activity will close the loop on this program,” continues Swanton. “We should be able to estimate the number of flight hours for FINAF operations based on the results of testing under an RAAF spectrum. We’ve basically got to do a retrospective comparison to see how the test result translates to a FINAF spectrum. Hopefully that will give the fatigue life it’s after, but we can’t answer that question yet because we haven’t done the correlation.”
The correlation will be carried out at DSTO via a coupon test program, cycling some coupons with the FINAF spectrum and others with the RAAF spectrum, and comparing the respective crack growth rates as determined by quantitative fractography of the fracture surfaces.
“We could do it analytically and complete the work in a couple of weeks if it were using software simulations alone. However, we prefer to generate real crack growth data from which to perform the analysis,” concludes Swanton. “The preferred approach of generating and using real crack growth data could take up to a couple of months to complete, but it is believed that the data will be more robust, ultimately resulting in a more accurate estimate for the management of the FINAF fleet center barrel.”