Reference: Abbott, Richard. Analysis and Design of Composite and Metallic Flight Vehicle Structures 3 Edition, 2019

Composite materials comprise of more than one material. They are a mixture. This book covers laminated composite structures. Reinforcements arranged in layers (usually Carbon or Glass) set in a rigid matrix (usually epoxy resin).

Laminate composite structure is not isotropic (the same stiffness in all directions) and can usually be described as orthotropic.

Figure 4.1.1‑1: Difference between an Isotropic and an Orthotropic Panel (NASA-RP-1351, 1994)

The design and substantiation of composite structure has to be integrated considering the specific compliance methods for the aircraft project. i.e. the analysis methods used are validated for specific laminates and design features that can be unique for the aircraft in question. It is important for the engineer to realize that there is no reliable universal general solution for the analysis of carbon fiber composite structures in the context of airworthiness regulations (although accepted common methodologies are evolving) and that companies develop, qualify and validate analysis methods for design features that they wish to use.

There are design conventions for composite structure and these will be covered in this book. However, this book is not to be regarded as a comprehensive composite structural design manual.

Because of the unique nature of composite laminate structures – both physical and regulatory – novel or unusual features (those that are new to the company or particular aircraft) must first be discussed with the chief engineer before incorporation into the design of components and assemblies.

In this manner, composite structures are no different to metallic structures. It is worth pointing out the obvious, that metallic structures have been around for much longer than composite structures and conventions are better established and widely known. There is a wealth of information regarding the analysis of metallic (isotropic, elasto-plastic) structure – Timoshenko, NACA, NASA and many others.

The methods and philosophies discussed in this document are generally acceptable at the time of writing to North American certification agencies.

The substantiation methods covered in this document are targeted at primary structure. They would also be acceptable for the substantiation of secondary structure but if those methods are used for the sizing of secondary structure it may result in excess weight being built into the design.

General guidance for substantiation of secondary composite structures can be found in (FAA-PS-ACE100-2004-10030, 2005), while specifically applicable to FAA part 23 aircraft, the standards in this document are generally applicable to all aircraft.

For clarity the definition of secondary structures from  (FAA-PS-ACE100-2004-10030, 2005) is repeated here:

Secondary structures are those that are not primary load carrying members, and their failure would not reduce the structural integrity of the airframe or prevent the airplane from continuing safe flight and landing. This is the same definition used in AC 23-19, issued by the Small Airplane Directorate. For clarification, the secondary structure definition implies that a hazard assessment of the partial or complete failure of the structure has been performed and there is no reasonable threat to safety of flight or landing. Such an assessment should include consideration for flight stability and control. Also consider subsequent failures that are the logical result of the initial failure.

Secondary structures must be designed, fabricated, and maintained such that they will not depart the aircraft and/or cause other safety hazards. Those exterior components that meet the definition of secondary structures may include fairings, cowlings, and radomes. Non-structural components, including many interior parts, whose failure would be inconsequential, may also fit the definition of secondary structures. Clearly, engineering judgment, based on the location, design, and function of a particular secondary structure, will help determine the level of material and process evaluation needed in type certification and subsequent production controls.

Ambiguity also exists between secondary and primary structures. For differentiation purposes, we define primary structure in this policy as, “The structure that carries flight, ground, crash or pressurization loads, and whose failure would reduce the structural integrity of the airplane or may result in injury or death to passengers or crew.” Interior structures that carry crash loads, as required by 14 CFR part 23, §§ 23.561 and 23.562, are primary structure. Some structures may not satisfy the definitions of secondary or primary structure as provided in this policy. This may include structure that does not carry primary loads, but its failure may impact primary structure and prevent the continued safe flight of the airplane. Further coordination with the certification engineer may be required for these structures.

Composites may be susceptible to lightning damage. Lightning protection may be needed for secondary composite structure, such as engine cowls, where the effect of strike may be detrimental to engine operation. Demonstrate that the composite structure can dissipate P-static electrical charges, provides electromagnetic protection where required, and provides an acceptable means of diverting the lightning electrical current so as not to endanger the aircraft. Consider possible deterioration and undetected damage to the lighting protection system.Flammability and fire protection requirements also need to be substantiated for aircraft components. The use of composite structures/components should not decrease the level of safety prescribed by the existing requirements for flammability and fire protection. These components may include some of the composite airframe structures and non-structural interior components. For certification convenience, divide the latter into two classifications: (1) non-structural components/parts that are not subject to compartment interior fire protection requirements (e.g., knobs, handles, pulleys, etc.), and (2) non-structural components/parts that are subject to compartment interior fire protection requirements.

4.1.1.1. Some Important General Notes on Composite Laminates and Aircraft

1. Certifiable laminate composites rarely offer any real-world weight saving when compared to an analogous aluminum part or assembly. However, if composite parts and assemblies are well designed, and during the design process manufacturing are consulted on a regular basis, they can offer significant parts reduction and manufacturing cost reduction.
2. Composite structure, if well designed and manufactured, in theory, should have a greater service life than metallic equivalents. However, at the time of writing, there is not enough data on aging composite aircraft in the civil or commercial sector and only time will tell if this is correct or not. These issues are not well understood and unless these important issues are acknowledged at the start of the design process the results of the design process can be disappointing.
3. The composite section of this book is aimed almost exclusively at carbon fiber and epoxy resin fiber laminates as this combination is the most commonly used for critical aircraft structures applications. The methods and philosophies in the manual may be applicable to other matrix and reinforcement materials but the onus is on the reader to fully characterize whatever material system they choose and check the applicability of these methods to their chosen material system.
4. Almost all FAR part 25 aircraft adopt a ‘black metal’ approach to composite aircraft design i.e sandwich structure and adhesives are not used in primary structure and the resulting structure looks, to the lay person, like an aluminum aircraft design that has been built out of carbon fiber. This is due to both manufacturing process control and damage tolerance issues of sandwich structure and adhesive joints. However, cored structure and adhesive joints are commonly used in FAR part 23 aircraft primary structure and the analyst should be aware of their unique advantages and limitations and we will try to give some insight into these issues in the relevant chapters herein.
5. When composite materials are used the aircraft developer takes on a greater responsibility for material processing and quality control. The engineer must work closely with manufacturing and quality to ensure that the impact of material and part variability are considered in the substantiation and compliance work.

The following sections on composite laminate nomenclature, physical characteristics, strength and durability are no more than a high level ‘whistle-stop’ tour of the result of decades of work done by industry, government agencies and universities. It is highly recommended that the reader follows the links to the all the cited sources and read them in full.

This will take a significant amount of time, but it is important to realize the breadth and complexity of the subject and the issues that have driven the aircraft industry to adopt the current design conventions and analysis methods.

As previously mentioned, we are all standing on the shoulders of giants. We would be helpless without the work done by those who have preceded us.