Different Types of Defects Detectable Through Visual Inspection and by TAP testing Part 1

Sofema Online (SOL) www.sofemaonline.com considers the type of defects that may be encountered during the Inspection of Aircraft Composite Material.

Introduction

The significance of composite damage is dependent upon the function of the structure and the type and extent of the damage.

>> The most likely cause of damage, approximately 80%, is impact, often the result of ground handling. This may produce many of the damage types discussed here.
>> The most significant damage types are

o Delamination,
o Disbond and
o Material penetration.

>> The significance of the extent of damage is typically a function of the design.

Types of Damage:

Damage may be categorized as cosmetic or structural.

Cosmetic Damage:

>> Cosmetic damage is that which is of no immediate structural concern.
>> However, cosmetic damage that could allow fluid ingress should be repaired, i.e. dried and sealed, to prevent it from progressing to become structural damage.

Wrinkling and Dimpling:

>> Minor skin wrinkles and dimpling (sandwich panel skin wrinkling that adapts to the shape of the honeycomb cells, which should not be present if the part has been correctly designed with small honeycomb cells),
>> Defects present from manufacture may be distinguished from damage by careful inspection for other evidence of degradation,

o e.g. surface crazing, fibre breakout, loose resin material, delamination etc.

Note: If in doubt, such structure should be Tap Tested for delamination to ensure that any apparent 'wrinkling and dimpling' is not the result of skin buckling, i.e. damage which is of structural interest.

Resin Rich and Resin Starved Areas:

>> Porosity in resin-rich areas, or exposed fibre in resin-starved areas, should be obvious and must be dried and sealed to prevent long-term degradation due to fluid ingress.

Surface Damage:

Many structures use a sacrificial outer ply layer, e.g. a woven glass ply, to resist 'wear and tear' or to help prevent unidirectional fibre breakout.

>> The manufacturer’s drawings should be referenced to avoid the incorrect determination of the significance of any surface ply damage.

o If any doubt exists regarding the function of the outer ply, then it should be treated as a structural ply and repaired accordingly.
o Damage to a sacrificial layer, although of no immediate concern, may allow fluid access to the main structure. Again, such damage should be dried and sealed.

Structural Damage:

>> Structural damage is that which threatens the function of the structure, whether it be damage to the Primary or lesser structure.
>> Composite structural damage may be further categorized as being

o Penetration damage, damage between plies (inter-ply damage), or
o Damage to the constituent materials (intra-ply damage), e.g. matrix or fibre damage.

>> Sandwich structure adds further possibilities, e.g. honeycomb damage or core-to-skin interface damage.

Penetration Damage:

>> Any laminate penetration, e.g. holes resulting from impact damage etc, is a concern because it represents damage to both fibre and matrix material.
>> Such damage is often self-evident, although smaller holes may be missed and could allow long term moisture ingress to occur.

o This is particularly true for sandwich structure. Any penetrations should be repaired.

Structural Inter- Ply Damage:

Damage between plies.

Delamination:

Delamination is the separation between plies in a laminate,

i.e. in the plane of the laminate. It may run across the whole laminate, or it may run to the laminate edges, and/or it may occur between many plies in any single laminate.

>> Composite structure is often compression critical.
>> Delamination may further reduce the compressive strength, both at the local fibre level and at the component buckling level.
>> Furthermore, delamination is often BVD, or NVD, on the external face of the structure.
>> Therefore, a thorough inspection of a clean structure and access to the internal face of the structure is essential if the chances of detecting potentially significant damage are to be maximised.

Note: Delamination may sometimes be visible as cracking parallel to the fibres at the laminate edges.

>> Any cracked paint and debris at laminate edges should be removed, using non-metallic scrappers etc., prior to inspection of the edge.
>> Delamination that is not visible may sometimes also be located by using a Tap Test.

Further Comment

The chances of finding hidden delamination are greatly increased by knowledge of an event, e.g. an impact, or by the presence of other damage.

>> Any fibre break-out on the internal face of the structure is likely to be associated with delamination.
>> Any clues that indicate the presence of delamination, e.g. dents, paint damage, deformations, should be followed by a Tap Test.
>> Inspection for delamination should include:

o All laminate edges,
o Cutouts, and
o Any opened fastener holes.
o Delamination resulting from poor fastener hole fit,
o Wear and tear,
o Poor drilling, or
o Excessive fastener pull-up load is common.

Next Steps

Sofema Aviation Services (www.sassofia.com) offers the Composite Material TAP Testing for Aviation Inspectors training – please see the following link