Reference: Abbott, Richard. Analysis and Design of Composite and Metallic Flight Vehicle Structures 3 Edition, 2019
Composite materials comprise of more thanone material. They are a mixture. This book covers laminated compositestructures. Reinforcements arranged in layers (usually Carbon or Glass) set ina 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 methodsfor the aircraft project. i.e. the analysis methods used are validated forspecific laminates and design features that can be unique for the aircraft inquestion. It is important for the engineer to realize that there is no reliableuniversal general solution for the analysis of carbon fiber compositestructures in the context of airworthiness regulations (although acceptedcommon methodologies are evolving) and that companies develop, qualify andvalidate analysis methods for design features that they wish to use.
There are design conventions forcomposite structure and these will be covered in this book. However, this bookis not to be regarded as a comprehensive composite structural design manual.
Because of the unique nature ofcomposite laminate structures – both physical and regulatory – novel or unusualfeatures (those that are new to the company or particular aircraft) must firstbe discussed with the chief engineer before incorporation into the design ofcomponents and assemblies.
In this manner, composite structures are no different to metallicstructures. It is worth pointing out the obvious, that metallic structures havebeen around for much longer than composite structures and conventions arebetter established and widely known. There is a wealth of information regardingthe 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 coveredin this document are targeted at primarystructure. They would also be acceptablefor the substantiation of secondary structure but if those methods are used forthe sizing of secondary structure it may result in excess weight being builtinto 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 notprimary load carrying members, and their failure would not reduce thestructural integrity of the airframe or prevent the airplane from continuingsafe flight and landing. This is the same definition used in AC 23-19, issuedby the Small Airplane Directorate. For clarification, the secondary structuredefinition implies that a hazard assessment of the partial or complete failureof the structure has been performed and there is no reasonable threat to safety of flight or landing. Such an assessmentshould include consideration for flightstability and control. Also considersubsequent 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/orcause other safety hazards. Those exterior components that meet the definitionof secondary structures may include fairings, cowlings, and radomes.Non-structural components, including many interior parts, whose failure wouldbe inconsequential, may also fit the definition of secondary structures.Clearly, engineering judgment, based on the location, design, and function of aparticular secondary structure, will help determine the level of material andprocess evaluation needed in type certification and subsequent productioncontrols.
Ambiguity also exists between secondary andprimary structures. For differentiation purposes, we define primary structure in this policy as, “Thestructure that carries flight, ground, crash or pressurization loads, and whosefailure would reduce the structural integrity of the airplane or may result ininjury or death to passengers or crew.” Interior structures that carrycrash loads, as required by 14 CFR part 23, §§ 23.561 and 23.562, are primarystructure. Some structures may not satisfy the definitions of secondary or primary structure as provided inthis policy. This may include structure that does not carry primary loads, butit* failure may impact primary structure and prevent the continuedsafe flight of the airplane. Further coordination with the certificationengineer 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
- Certifiable laminate composites rarely offer any real-world weightsaving when compared to an analogous aluminum part or assembly. However, ifcomposite parts and assemblies are well designed, and during the design process manufacturing are consulted on a regular basis, they canoffer significant parts reduction and manufacturing cost reduction.
- Composite structure, ifwell designed and manufactured, in theory, should have a greater service lifethan metallic equivalents. However, at the time of writing, there is not enough data on aging composite aircraft in the civil or commercial sector and onlytime will tell if this is correct or not. These issues are not well understoodand unless these important issues are acknowledged at the start of the designprocess the results of the design process can be disappointing.
- The composite section of this book is aimed almost exclusively atcarbon fiber and epoxy resin fiber laminates as this combination is the most commonly used for critical aircraftstructures applications. The methods and philosophies in the manual may beapplicable to other matrix and reinforcement materials but the onus is on thereader to fully characterize whatever material system they choose and check theapplicability of these methods to their chosen material system.
- Almost all FAR part 25 aircraft adopt a ‘black metal’ approach to compositeaircraft design i.e sandwich structure and adhesives are not used in primary structure and the resulting structurelooks, to the lay person, like an aluminum aircraft design that has been builtout of carbon fiber. This is due to both manufacturing process control anddamage tolerance issues of sandwich structure and adhesive joints. However, coredstructure and adhesive joints are commonly used in FAR part 23 aircraft primarystructure and the analyst should be aware of their unique advantages andlimitations and we will try to give some insight into these issues in the relevant chapters herein.
- When composite materials are used the aircraft developer takes on agreater responsibility for material processing and quality control. Theengineer must work closely with manufacturing and quality to ensure that theimpact of material and part variability are considered in the substantiationand compliance work.
The following sections on compositelaminate 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. Itis highly recommended that the reader follows the links to the all the citedsources 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 allstanding on the shoulders of giants. We would be helpless without the work doneby those who have preceded us.
