DETECTING DEFECTS IN COMPOSITE MATERIALS

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8 DETECTING DEFECTS IN COMPOSITE MATERIALS

While composites do not suffer the corrosion and cracking problems, associated with metals and also have good fatigue characteristics, they do, however, require regular inspection for the defects to which they are particularly prone.

The areas to be inspected are, usually well known and they will be detailed in the relevant chapter (51-57 for Airframe topics, 61-61 for Propellers) of the Aircraft Maintenance Manual (AMM). The inspection methods to be used will be found in the Non-destructive Testing Manual (NTM) and the approved repair procedures will be outlined in the Structural Repair Manual (SRM).

Repairs in unexpected areas, or damage, which is not covered in the SRM, will necessitate the request of specific repair drawings from the aircraft manufacturer.

8.1 CAUSES OF DAMAGE

If a sharp object strikes a thermosetting plastic, the plastic is liable to crack and shatter, like glass, with straight sharp edges. The reason for this is that, once a crack starts in the plastic, it travels very easily and quickly in a straight line.

Damage of this kind would be disastrous in a load-bearing component.

The damage appears as a ‘star’ in the composite, providing it has no surface finish applied to it. An important point about this type of damage is that there is little loss of strength in the overall material, in addition to the absence of the shattering that occurs without fibre reinforcement.

The majority of damage to composite structures occurs during ground handling (such as from dropped tools), and damage from ground equipment. Bird-strike damage can also require extensive repairs. Damage to composite structures may result from a number of other causes such as:

• Erosion caused by rain, hail, dust etc.

• Fire

• Overload caused by heavy landings, flight through turbulent air and excessive ‘g’ loading.

• Lightning strikes and static discharge.

• Chafing against internal fittings such as pipes and cables.

8.2 TYPES OF DAMAGE

The types of damage, which may affect fibre-reinforced structures are:

• Cracks which may simply affect the outer lamination or may penetrate through the skin.

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Fibre reinforced plastics however, apart from being much stronger than normal plastics, have different failure modes. Each strand of fibre acts as a trap, to stop cracks travelling through the plastic (refer to Fig. 19). A travelling crack quickly reaches a fibre, which is difficult to break so, instead, the crack travels along the fibre. Eventually the crack reaches another fibre and is deflected again. This process continues until the failure is divided into many small cracks, which will not have propagated far from the initial damage.

(a) (b) (c)

Crack travelling Crack travelling Cracks around fibre towards fibre along fibre

Crack Propagation within a Composite Fig. 19

• Delamination - which involves separation of the fibreglass layers and may affect single or multiple layers.

• Debonding - when honeycomb sandwich structures are damaged, the effect usually entails separation of the honeycomb from the skin. The reason for this is that the bonding of the skin to the honeycomb walls is along very fine lines, and this bond is fairly easily broken.

Once there is separation, the strength of the whole structure is reduced by a significant amount. Greater damage can be due to the crushing of the honeycomb core itself, which may require extensive repair or even replacement of the complete component.

• Blisters - which usually indicate a breakdown in the bond within the outer laminations and may be caused by moisture penetration through a small hole, or by poor initial bonding

• Holes - these may range from small pits, affecting one or two outer layers, to holes, which completely penetrate the component. Holes may be caused by lightning strikes or by static discharge.

Fibre

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Areas on the aircraft that are likely to be damaged, should be inspected regularly, and complete removal of the component may be required at overhaul.

8.3.1 VISUAL INSPECTION

Visual inspection is used to detect cracks, surface irregularities (from an internal flaw) and surface defects such as delamination and blistering. A lamp and a magnifying glass are useful in detecting cracked or broken fibres. A small microscope or a x20 magnifier may be helpful in determining whether the fibres in a cracked surface are broken, or if the cracks affect only the resin.

Delamination may sometimes be found by visual inspection. If the area is examined at an angle, with a bright light illuminating the surface, the delaminated area may appear to be a bubble, or an indentation in the surface. When viewed from the inside, a change of colour could indicate delamination because of a change in light reflection.

A visual inspection can also find several manufacturing defects such as resin-rich or resin-starved areas, pinholes, blisters and air bubbles.

8.3.2 RING OR PERCUSSION TEST

To detect internal flaws, or areas suspected of delaminations, a ring or percussion, test can be used. In some instances a properly designed miniature hammer is used for the test while, in other procedures, a length of an appropriate hardwood, or a particular size of coin is employed to tap against the surface of the suspect area.

Variations in the tapping sound will provide clues as to the quality of the bond. A sharp solid sound indicates a good bond, whilst a dull thud indicates bond separation. Care must be taken to make allowances for changes in material thickness, fasteners and earlier repairs, all of which can give false indications.

Whenever damage is found visually, then a percussion test should be done around the area. In the majority of instances, if there is a hole, crack or other damage, there is, often, also delamination.

8.3.3 ULTRASONIC INSPECTION

To detect internal damage, an ultrasonic test may be done by authorised, specialist, personnel. This procedure involves the directing of a low-frequency ultrasonic beam through the structure and viewing the pattern of the resulting sound echo on an oscilloscope.

8.3.4 RADIOGRAPHY

Radiography can, sometimes, be used to detect cracks in the surface in addition to being able locate internal faults that cannot be visually detected. Radiographic procedures may also be employed to detect water ingress within honeycomb core

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8.4 ASSESSMENT OF DAMAGE

One of the greatest problems, caused by replacing aluminium alloy with composite structures (especially honeycomb sandwich), is the inspection for damage. It is unfortunate that when a composite of any kind is struck, the majority of the damage occurs internally and, often, there is little or no visible damage showing at the surface

It is vital that ANY damage to a composite structure be thoroughly inspected, not only for damage to its surfaces, but also (in a sandwich structure) for possible damage to its core, which is usually softer than the skins. Damage that gives little clue to its depth or significance is often referred to as Barely Visible Damage (BVD).

As with metal structures, the damage occurring to GFRP or CFRP structures may be classified as negligible (or allowable), repairable by cover patch, repairable by insertion or repairable by replacement.

These classifications may only be determined by reference to the appropriate aircraft SRM. Signs of secondary damage (i.e. damage occurring remote from the primary damage) must not be overlooked. This is particularly important in the case of impact damage where the shock may be transmitted through the structure, to cause damage away from the point of impact. In some instances secondary damage may be more serious than the primary damage.

Sometimes damage may be difficult to detect, due to the natural flexibility of the material which may cause it to spring back into shape. Any evidence of cracking, straining, crazing or scuffing of the gel coat should be regarded with suspicion, as it may indicate the presence of damage.

Where delamination is known, or suspected to exist, the area surrounding the visible damage should be checked to determine the extent of the damage and integrity of the laminations.

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