Description

Most plastics in use are pure, but when additional strength is needed, plastics are reinforced with fibres and become composite, also known as reinforced plastics. The reinforcing fibres provide strength and rigidity to the composite, while the plastic resin provides cohesive properties, stability and environmental resistance.

Definition and requirements

In today’s aerospace industry, most applications use carbon as reinforcing fibres, so they are called carbon fibre reinforced plastics (CFRP). CFRPs are made in layers added on top of each other until the piece has the properties necessary to support the loads it will carry.

Composite materials and manufacturing processes are qualified through trials and tests to demonstrate reliable design. The degree of care in the sourcing and processing of composite materials is one of the important characteristics of construction. Special care must be taken to check both the materials supplied and the way the material is processed once delivered to the manufacturing plant.

Given the rapid expansion of the use of composite materials in transport aircraft, damage tolerance maintenance practices must be standardised. Composites have different characteristics compared to metals and therefore require dedicated procedures.

Advantages

The use of composites provides significant benefits to air operators consisting of weight reduction, which leads to fuel savings, fatigue and corrosion resistance, which results in extended in-service life. Composite aircraft can be designed to respond as well as and, in some cases (like fatigue and corrosion) better than traditional metallic aeroplanes to operational threats. Composites provide some additional benefits in terms of fire behaviour: CFRPs are auto extinguishable and have more burn through resistant than aluminium.

Unique factors

There are factors unique to the specific composite materials and processes used for a given application. For example, the environmental sensitivity, anisotropic properties (i.e. having mechanical and/or physical properties which vary with direction relative to natural reference axes inherent in the material), and heterogeneous nature of composites can make the determination of structural failure loads, modes, and locations difficult. In addition, the reliability of such evaluation depends on repeatable structural details created by scaled manufacturing or repair processes.

There are currently few industry standards that outline critical damage threats for composite structural applications. Some factors to consider in developing a damage threat assessment for a particular composite structure include the function of the part, location on the aircraft, past service data, threats of accidental damage, environmental exposure, resistance to impact damage, durability of assembled structural details (e.g., long-term durability of bolted and glued joints), adjacent system interface (e.g., potential overheating or other threats associated with system failure), and abnormal management or maintenance events which can overload or damage the part.

Damage detection

Whilst scale damage, such as that caused by engine disintegration, bird strike or major collision with ground equipment, is readily detectable and no maintenance task assessment is required, low-velocity large-mass impact (e.g. by ground vehicle) may lead to large internal damage in composite structures (delamination) without much indication on the surface of the structure.

Low-energy impact usually causes small scale damage, i.e., non-visible impact damage (NVID) or barely visible impact damage (BVID). The design of composite aircraft structures often uses a BVID threshold. Structures containing BVID must sustain ultimate load (UL) for the life of the aircraft. The dent depth is normally used as the damage metric to define BVID.