Test flight into the uncharted

Composites make aircraft lighter and more fuel efficient. But a simple way to test the new materials between flights has proved elusive. Julia Pierce reports.

The age of the composite airliner is just a few years away. But engineers are in a race against time to solve one glaring problem: how to test the new generation of materials for microscopic flaws between flights.

Tested exhaustively in laboratories, composites will allow manufacturers such as Boeing and Airbus to build stronger and lighter planes. However, without the performance data from years of actual flying time to study, some concern inevitably remains. And the RAF is taking the issue seriously enough to launch its own hunt for a technology able to carry out routine tests on composite structures.

Parts made from composites typically consist of strands of Kevlar, glass fibre or graphite, shaped, baked and pressure treated in an epoxy resin. They offer the strength of a metal but without the susceptibility to corrosion and are less prone to fatigue-related problems (if single fibres break there are millions of others present to take the load).

But as airlines struggle to operate with increasingly small profit margins it is the reduction in weight and therefore fuel use that composites can offer that is most attractive. The weight of the Airbus A300, which uses composites in areas such as floors, wing panels and landing-gear doors, has been reduced by almost 1,500kg compared to a metal-only design.

Increasing the amount of the material used and eventually developing an all-composite design are clearly desirable. However, while composites are more resistant to fatigue from vibration, they do have their flaws.

Regular use of aircraft made from composite parts in hot and humid climates can cause the carbon-fibre layers to separate as water penetrates between layers, a process that has sometimes been a problem with racing boats (see sidebar). Even the smallest flaws caused by impact from dropped tools, stones bouncing up on take-off and hailstones can cause fatal weakening of the finished material, causing layers to peel away from each other during stress.

At present, engineers rely on visual inspections and tap tests to assess whether composite parts have been damaged during routine use. However, this is susceptible to human error caused by factors such as environmental light, variations in materials’ stiffness and the training and alertness of personnel.

In any case, ply separation within the material may not be visible to the naked eye. While parts may be dismantled and subjected to ultrasound examinations to view deep within the material, the method is not practical for use in the short turnaround between battle missions or commercial flights.

Manufacturers such as Airbus are keen to stress that the materials are thoroughly tested before they are installed to judge their strength, performance and expected life. A spokesman for the company said that composite parts used in its aircraft were also designed to be inspected visually, removing the need to dismantle them for testing. Meanwhile, thousands of hours of testing meant that materials’ performance could be predicted, meaning alternative measures were only necessary if the material experienced an abnormal event such as being struck by an object.

Though performance of composites has been thoroughly tested within the laboratory and in field tests, unlike aluminium bodies, they have not undergone years of performance analysis while in use, causing concern.

‘We are ultimately not as sure of how they will behave and what stress can and can’t be detected,’ said Prof Phil Irving, holder of the civil aviation authority chair in damage tolerance at Cranfield University. ‘With aluminium, people shoved it on to aircraft and just hoped for the best. Lessons were learned, sometimes painfully. But now we are more safety conscious and must depend on calculations, not bitter experience.’

Despite reassurances from manufacturers these calculations may not be enough for those using composite aircraft. In the military sector, where majority composite aircraft are already being delivered, engineers are looking for their own solutions.

The Eurofighter Typhoon, delivered to several European countries this year, has an airframe surface area made up of only 15 per cent metal, consisting mostly of carbon-fibre composites. This makes the plane 30 per cent lighter than it would otherwise have been while reducing its radar signature.

‘The materials used for the Eurofighter Typhoon have been designed to a damage-tolerance specification, in that a low-energy impact to the carbon-fibre composite (CFC) structure shall not reduce the overall residual strength to less than the design’s ultimate load throughout the remainder of the forecast fatigue life. The structure has been subjected to extensive box section testing to prove this concept,’ said Andy Murphy, chief technician with the RAF’s NDT Squadron, which is currently assessing in-service non-destructive testing (NDT) equipment requirements for the Eurofighter Typhoon.

However, the possibility of damage occurring in the grey area between minor and significant events is being closely investigated. ‘Significant events above the low-energy impact threshold should be noticed by air and ground crew and will be subject to directed NDT,’ said Murphy. ‘Our main concern will be impacts from runway debris and the effect of repeated low-energy impacts in a localised area. We will be periodically inspecting a representative sample of the fleet during service to monitor the performance of the material.’

Developing a tool that can be taken to the aircraft to give an instant assessment of possible problems has therefore been the goal of both research groups and manufacturers (see sidebar). However, none has yet to reach widespread use, allowing the goal of a safe all-composite aircraft to be achieved.

The RAF is drafting a specification for equipment capable of carrying out the sample inspection of Typhoon aircraft and an invitation to tender is expected to be published by the end of this year.

‘A vast amount of money has been spent in the area of composite testing in the past 10 years,’ said Prof Irving. ‘It probably runs to tens of millions but nothing has really come into service. It is a major problem as small dents that may not be that easily seen can hide an array of delamination.’

Any tool must be highly sensitive to avoid false-positive results. ‘If we could develop a system that would, for instance, make all flaws show up as bright green when the aircraft landed then people would be very happy,’ said Irving. ‘However, in reality the entire body would be 99 per cent coloured. As composites can perform with no loss of strength when they are less than perfect the technique would still have to be able to differentiate between significant and insignificant damage or you would be facing a huge maintenance bill.’

Manufacturers in the commercial sector have so far avoided concerns over materials by relegating their use to areas where strain is minimised. The main wings and fuselage, where most stress occurs, have remained metallic. Minimising fuel use and costs by reducing aircraft weight further means the amount of lightweight materials used must be increased. The design of the Airbus A380 has proved this, incorporating composites in the carry-through box connecting the wings through the fuselage. However, these are made from a hybrid metallic material, gaining some but not all of the advantages of a carbon-based material.

Constant monitoring of stressed areas using sensors has also been investigated, but use in the commercial sector is a long way off, despite the appearance of concepts such as Boeing’s 7E7, a mainly composite super-efficient plane announced at his year’s Paris Airshow and planned for 2008. A spokeswoman for Boeing’s research and technology centre in Madrid said that the group was investigating the possibilities of structural health monitoring using piezoelectric and fibre-optic sensors for its next-generation composite aircraft and helicopters. However, this research was at a very early stage.

But even when a useful portable tool arises, getting maintenance firms to accept new structural monitoring technologies also poses a challenge, said Dr Reza Zoughi, professor of electrical and computer engineering at the University of Missouri-Rolla in the US. He has been developing a technique known as near-field microwave non-destructive evaluation (NDE) to test composite materials used on NASA’s space shuttles. However, he believes testers’ familiarity with ultrasound may cause them to be wary of new testing methods, even if these are better, causing their adoption to be delayed.

‘Many engineers have used ultrasound for a long time and are comfortable with it,’ he said. ‘To go back to the beginning and try to learn a new technique may seem harder for them than trying to force ultrasound to work.’

British firm NDT Solutions believes it has come up with an answer. Research director Richard Freemantle said the company has developed an ultrasound-based rapid scanning technique using a roller that can be passed over an aircraft’s skin to identify deep flaws.

‘While key areas of metallic aircraft are inspected between flights, there is a need for 100 per cent inspection of a new material,’ he said. ‘Firms would prefer a tool that can scan the whole structure, hence the interest in systems such as embedded sensors. However, our tool can be taken to the plane and can provide an image of suspicious areas. Ultrasound is an established technology. The announcement of funding for the Airbus A400M (a military freighter that will have all-composite wings) will be a driver for the market in portable testing.’

Aircraft manufacturers may be keen to rely on materials testing calculations to prove that composites are suitable for widespread use, but users have not been convinced. However, the millions spent on developing a portable tool for monitoring materials is finally paying off. This, together with data gathered from military tests, should be enough to convince pilots and the public that the structures are safe. The goal of producing a safe, all-composite commercial airliner may finally be within reach.

Safety concerns

Advanced composites are already being used across the airline, maritime and construction industries. However, there has been a degree of suspicion over their safety and effectiveness.

Doubts were cast on their safety in aircraft construction following the destruction of American Airlines’ Flight 587, which crashed into the New York suburb of Queens in December 2001 with the loss of 265 lives. Shortly after taking off from JFK Airport, the aircraft’s vertical composite tail fin ripped off, sending it out of control.

Though the materials were exonerated from any blame, suspicion immediately afterwards fell on possible structural flaws in the fibre-reinforced plastic fin.

As investigations began, more than 60 of the airline’s pilots demanded the grounding of its entire fleet of part-composite A300-600s.

Union officials called for ultrasound inspections of the fleet’s composite parts, a move that would have required the dismantling of each aircraft’s tail. However, the official response to this called into question the ability of maintenance staff to monitor the health of non-metal parts.

An American Airlines spokesman admitted that the tails were not designed to be removed, while the FAA voiced concern that doing so could damage them, raising questions over how the materials’ health could actually be monitored while in use.

In the competitive sailing industry, though composites are now making hulls and structures lighter and stiffer than ever, the latest materials place a premium on expert construction.

A dangerous mixture of demanding new materials and sometimes less than highly skilled building techniques has certainly led to the industry suffering more than its fair share of high-profile failures over the past 15 years, including delamination occurring at critical times in races.

Sidebar:On-the-spot maintenance checks

Four techniques have shown particular promise in achieving the elusive goal of allowing non-destructive testing of composite materials in the windows between flights.

Acoustic emission

If a material suffers a fracture the area releases high-frequency sound waves, otherwise known as acoustic emissions. Their size is related to the extent of any impact on the area. An array of piezoelectric sensors placed on the material’s surface or embedded within it can detect and measure both the size and number of these and so determine the extent of any damage. Groups developing this technology include Airbus.

Local strain measurement

Optical-fibre sensors can be threaded through the composite material during manufacture. These can pick up vibrations and so measure abnormal strains on the material that can indicate the presence of a flaw. The technology is being used by NASA to measure strain on shuttle parts.

Electrical potential

A composite’s integrity can be assessed by measuring an electrical current passed across it for anomalies. Carbon fibres used in the construction of composites are good conductors of electricity. If fibres are broken during an impact this causes changes in the material’s electrical resistance, indicating the presence of flaws. The technique is being developed by Cranfield University.

Thermography

Delamination can be identified by thermally excited areas using a high-energy source such as a quartz lamp or high-intensity flash. As the material cools, emitted infrared radiation can be analysed with a thermal-imaging camera.

Different materials absorb and release IR energy at different rates as heat propagates through, thereby creating a thermal image that can progressively penetrate deeper layers of the material. Damaged areas contain air-filled pockets that act as insulators, creating a slightly altered thermal image.