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There is also the potential for interfacial bonds for intbetween components to fail as the adhesive asdegrades over time. Many composite airframes ny comin smaller sport and light GA aircraft use bonded t GA airjoints. A structure which contains bonded joints h containpresents some potential long-term structural g-durability issues.

Repair of an entirely bonded structure is also challenging—you can’t just unbolt a damaged panel. The damaged area usually has to be cut away and replaced, potentially inducing secondary delamination. Also, determining the extent of the damage in composites requires a competent person, usually trained in non-destructive testing (NDT), to assess the damage. This is not the case in a metal structure where the extent of the damage is usually apparent to anyone, even those without NDT training.

Fatigue is not an issue in ‘all-composite’ aircraft, and material strains are low, but in-service skin repairs, residual strength with hidden damage, and lightning protection are concerns. Additionally, there are some regulatory and industry capability issues which should be considered.

Composite materials are not the ‘new technology’ they once were.Many basic sandwich/honeycomb panel composites have been in useon civilian aircraft for decades and on military aircraft for even longer.A common perception of the layperson, and even some in the industry,is that composite materials do not age, or age at a much slower ratethan the equivalent metal structure. While it is true that fi bre laminatestructures are less susceptible to fatigue than their metal counterparts,composites have their unique ageing problems.

AN AGEING ‘ALL-COMPOSITE’ AIRCRAFT?

While many would think ‘all-composite’ aircraft are immune toageing, some issues are beginning to emerge. These include, but arenot limited to:

hidden damage (delamination and disbond);

environmental susceptibility (i.e. trapped moisture freezing andcausing delamination); as well as

UV- and adhesive degradation. Many adhesives were not testedfor long-term durability in service. Many were tested only forstatic and peel strength when newly applied, so there are manyunknowns here.

Cracking of fi ller materials, normally non-structural and in manycases aesthetic in nature, but in some cases indicating a moreserious structural problem underneath.

Richard Castles, one of CASA’s senior airworthiness engineers, with a special interest in composite materials, looks at ageing and composites.

Maintaining

aageing composite-material aircraft

. . . with a composite structure, even a severe impact may leave no discernible marks on the outside of the structure.

COMPOSITE NDT CAPABILITY

The capability of industry NDT practitioners to detect and assess damage to advancedcarbon fi bre composite structure is emerging as an area which needs to be addressed. Are there any training or knowledge gaps? Are all industry NDT practitioners up-to-speed with advanced composite structural inspection, so that they can competentlydetect hidden damage to primary and secondary structures?

New structural technologies employed in modern aircraft include carbon fi bre wrapped or laminated structure with carbon fi bre stiffeners; monolithic 3D composite structures; and extensive structure in which hidden disbonding ordelamination may not be immediately apparent. New equipment and inspection techniques will be required to inspect this type of advanced structure. Are there any training requirements which need to be identifi ed? Do local and international regulatory NDT qualifi cation standards refl ect the level of expertise required toinspect primary composite structures?

INDUSTRY REPAIR CAPABILITY

Some maintenance organisations in Australia possess very limited capability to undertake advanced repairs of composite primary structure. Current capability is generally confi ned to standard structural repairmanual (SRM)-based repairs on primarily secondary structure, andlimited to aluminium/Nomex honeycomb panels and simple bonded oneycomb panels and simple bonded and laminate structures.

Modern composite aircraft contain a signifi cant portion of monolithichsignifi cant portion of monolithiccarbon/resin composite structure with integral stiffening, which will make repairs more complex. Heat control during curing will require advanced equipment, not just the traditional ‘layup and heatblanket’ approach. Major repairs will require advanced autoclavecapability, or advanced ‘in-situ’ heat blanket repairs.

GROUND HANDLING DAMAGE

The characteristics of the structure of modern composite aircraft are so different that it is not simply the maintenance technicians and engineers who may need specialist training. With aconventional metal structure, any collision between the aircraftand ground support equipment is immediately apparent - there will be dings, dents, gouges, scratches, etc.

You can assess the severity of such damage, often simply bylooking at it. However, with a composite structure, even a severe impact may leave no discernible marks on the outside of the structure. But it may be a different story inside—withconsiderable structural damage: delamination, disbonds,broken fi bres, rovings and so on. Research has examined mthe effects of high energy/blunt impact damage scenarios, asuch as those involving ground support equipment.

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Carbon and aluminium/titanium are ‘dissimilar materials’, and

as such, corrosion can occur in these interface areas, so we need to change the mindset that corrosion only occurs in and between different metals.

. . . Continued from page 32

To summarise:

Signifi cant damage can occur without visible surface indications.

Damage cannot be allowed to grow to degrade strength below design ultimate. Residual strength is the criterion, NOT damage growth rate.

Damage must be detectable BEFORE residual strength falls below design ultimate. This is damage tolerance.

LIGHTNING DAMAGE

The higher resistance of composites when compared to a traditional metallic structure means that when a lightning strike occurs, more heating of the composite matrix will occur, resulting in more instances of melting or charring of the matrix resin. Repairs are likely to be more complex. As with ground blunt impact damage, not all lightning damage is easily detectable from the outside of the aircraft.

CORROSION PREVENTION AND CONTROL

You shouldn’t assume corrosion prevention and control is not necessary just because there is a high composite content in the aircraft. The interface between carbon components and titanium/aluminium parts and fasteners is the very place where ‘dissimilar materials’ corrosion can occur. Carbon and aluminium/titanium are ‘dissimilar materials’, and as such, corrosion can occur in these interface areas, so we need to change the mindset that corrosion only occurs in and between different metals.

REPAIRS OUTSIDE STRUCTURAL REPAIR MANUAL

LIMITS

There may be a need to assist industry engineers to acquire the requisite knowledge to assess and provide repair schemes for damage repair outside structural repair manual (SRM) limits. It is worthwhile therefore to ask questions such as:

Are there any training or knowledge gaps?

Are engineers up-to-speed with advanced composite structures, so that they can confi dently and competently approve and perform a repair outside SRM limits? Many are highly competent, but some may require assistance. Generally, large manufacturers will provide comprehensive formal training, but what about smaller manufacturers?

IN SUMMARY

These are some of the maintenance challenges to be faced as our ‘all-composite’ aircraft age, but being forewarned is being forearmed. If cwe can anticipate the training and capability requirements in advance, wthen we will be better placed to implement strategies to deal with the tthunique nature of modern composite aircraft.un

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