Sofema Aviation Services (SAS) www.sofemaonline.com considers the type of defects which may be encountered during the inspection of Aircraft Composite Material.
Disbond
Disbond is the separation between laminates, e.g. a bonded joint, or the separation of a laminate skin from honeycomb core material.
- Early detection of disbond is important because it may provide an indication of imminent joint failure.
Note – This may be critical for any bonded Primary Structural joints that do not have secondary fastening.
- It should be noted that the progression from the initial areas of disbond to catastrophic failure may be very rapid.
- The progression may be accelerated by moisture ingress or fatigue loading at the joint.
Disbond may be evident as a gap in the adhesive line at the joint section edge,
or as ply peeling and/or paint damage along the joint edge when viewed perpendicular to the presented face.
Disbonds & Tap Test
Disbond may sometimes be detected by using a Tap Test. Again, as with delamination, a ‘duller’ sound will be produced at the damaged site than in the surrounding structure.
- However, interpretation may be difficult due to changes in a section at the joint, varied back-up structure, and the presence of other joints.
- Inspection of the disbond initiation surface, if visible, may help determine the potential severity of a problem.
Note – The disbond may be the result of poor bonding between the adhesive and the
adherend. Such a failure is potentially catastrophic because it may be the result of the adherend joint surfaces being contaminated. (Such a contamination is very likely to have affected most, if not all, of a joint.)
- Such a finding must be followed by repair action that will recover full strength to the whole of the joint unless it can be shown that the remaining joint is at full strength, e.g. a small disbonded area is the result of known local contamination.
Structural Intra-Ply Damage
Fibre Damage – Composite material fibres carry the laminate load via shear transfer from the matrix material.
- Therefore, the failure of any fibres may be significant to the strength of the part, particularly when tensile loads are parallel to the major fibre direction.
Fibre failure may be visibly evident as fibre breakout. Fibre breakout is often the result of impact damage, poor handling, or poor drilling, and is more likely to be evident on the back face of the structure.
Any fibre damage, particularly that to the primary structure, must be repaired immediately.
Matrix Damage
Matrix material allows the transfer of load to and between fibres.
- The failure of the matrix material may be particularly significant to the shear and compressive strengths, and stiffness, of a structure.
Heat Damage
Heat may soften the matrix such that shear and compressive strengths and stiffness are significantly reduced. If the heat is excessive then irreversible damage may occur as the matrix breaks down.
Note – While heat can cause discolouration, exposure to lesser heat may not discolour the matrix.
Lightning Strike Damage
Composite materials do not tend to be good conductors and consequently, the energy from a lightning strike may be dissipated via complete, or partial, destruction of the part.
Lightning strike damage is a particularly severe form of heat damage which may be evident, assuming that total destruction has not occurred.
- There may be evidence of ‘pin-hole’ burns at the lightning contact points
- Further clues may include damage to any attached conductive paths
– e.g. aluminium ‘window frames’, or missing static wicks.
– The extremities of an aircraft are particularly vulnerable to lightning damage, e.g. wing tips, fins, control surfaces etc.
Manufacturers tend to recommend the use of bond testers to check that the resistance of the system remains within limits.
Fluid Ingress
Fluid ingress may refer to both the uptake of fluid by the matrix or the uptake of free-standing fluid, the latter being a particular problem with sandwich structure.
Fluid ingress may degrade the matrix material resulting in strength reduction
- The extent of degradation will vary from one fluid to the next. Fluid, if not removed from the part, may make repair impossible due to part destruction if the repair cure temperature exceeds the fluid boiling point.
Porosity
Porosity may allow fluid ingress and result in material degradation. It may be evident as local surface pitting.
- Severe internal porosity may sometimes be located using a Tap Test. The existence of porosity may also indicate that the local structure has excess resin or that a local repair exists.
- Porosity should be dried and sealed. However, severe porosity requires a more substantial repair.
Fibre-Matrix Disbond
A laminate manufactured from predominately unidirectional plies in many orientations may split parallel to fibres in some of the plies as the fibre disbonds from the matrix.
- This is typical of unidirectional CFRP and may not be too significant if the splitting occurs in a small number of plies laying perpendicular to the load direction.
- However, splitting may have a significant effect on the stiffness and compressive strength in some designs, particularly those subject to bi-axial loading.
– Such damage may be evident as cracking in the damaged plies at the laminate edge or as fibre breakout from the presented face. ( many designs avoid the use of unidirectional materials in the outer plies, thus reducing the opportunity of detecting such damage.)
Sandwich Structure Damage
- The sandwich structure comprises of a honeycomb, or foam, core sandwiched between skins.
- Skin Damage
– The laminate skins may suffer similar damage to that already described, the limited thickness of the skins increases the likelihood of, and therefore the importance of, detecting skin penetration because these damages allow access for free-standing fluid to the cell structure of the core.
– Fluid ingress may sometimes be detected using a Tap Test if the mass of free-standing fluid is adequate to alter the sound transmission qualities of the structure.
– A Tap Test may also be used to detect near-side skin-core disbond away from the edges of the structure.
- Core Damage: Core damage is often obvious, e.g. Core Depression, Lateral Core Crushing, Skin Bulging, Dents etc. However, the skin may disbond from the crushed core and recover the original structural profile.
Structural Damage Causes
Overload
This occurs if any of the primary failure strengths e.g. tensile, compressive, shear etc., are exceeded.
Fatigue
Composite materials may suffer fatigue damage, particularly when damaged and exposed to the environment. Fatigue damage is often evident throughout the life of the structure.
Typical Damage includes
- Initial transverse fibre-matrix disbond
- Intra-ply matrix cracking
- Inter-ply matrix cracking
- Delamination, and
- Fibre failure.
- Reduction in stiffness and strength
- Fastener locations in composite structures are particularly vulnerable to fatigue damage.
- Loose fasteners result in delamination, hole deformation, and sometimes heat damage.
Wear and Tear
‘Wear and Tear’ refers to general degradation such as fastener hole bearing damage, often the result of repeated panel removals, erosion of leading edges, and minor ground handling damage, including abrasions, gouges, nicks and scratches etc.
The significance of the damage must be assessed on a case-by-case basis and repaired accordingly. Such damage is usually self-evident on a cleaned surface.
Ultra-Violet (UV) Radiation
Although not usually directly visible, UV damage may be evident through other damage types, e.g. matrix surface crazing, gel coat crazing etc. (note that some gel coats are partly intended to protect the composite from UV).
- UV damage reduces the engineering properties of the matrix material and makes it more vulnerable to load, the environment etc.
Existing Repairs
Repairs are structural discontinuities and will tend to provide unsymmetrical and unbalanced structures, thus making them a likely source of problems, e.g. repair ply peeling and delamination.
- Repairs tend to be more porous than the surrounding original structure.
- Repair locations are usually obvious on unpainted structures, whilst the location of repairs on painted structures may be possible by careful visual inspection at a shallow angle to the clean surface.
- The identification of repairs may be eased by consulting the manufacturer’s repair documentation which should provide some clues as to typical repair shapes
– e.g. square, circular, etc, and typical repair styles, e.g. flush, scab doublers etc.
Note – A Tap Test may help to confirm the presence of a repair. Laminate repairs tend to have overlapped joint areas, additional plies, e.g. doublers etc, and increased porosity.
- Repaired sandwich structure tends to have extensive potting material around the repair boundary. These factors will alter the sonic response of the structure.
Next Steps
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Aircraft Composite Inspection, Composite Materials, Disbond, Heat Damage, Lightning Strike Damage, Matrix Damage, SAS blogs, Structural Damage, TAP Testing
