In general, metallic materials can be regarded as isotropic and ductile. Whereas lamina composites can be generally regarded as anisotropic and brittle.
The analysis and certification methods for metallic components and assemblies are well established. The purpose of this text is not to redefine all of the established theory and practice of metallic structure but to give a practical overview of the most useful knowledge for the structural analyst.
It is useful to highlight the difference between composite and metallic materials. In my experience the three major differences between composite and metallic structure are as follows: (note that these are not the only differences, but I believe that that are the ones that have the most impact on the development and life cycle of the aircraft product.)
Composite materials are generally linear up to failure, metallic materials generally show plastic behavior
This single aspect drives most of the design and certification difference between composite and metallic structures. It is also ironic that metals displace clearly plastic behavior – especially the common aluminum alloys – whereas the most common composite primary structure material, carbon fiber, displays linear behavior up to failure.
This difference in the behavior of the two different material types fundamentally changes the analysis approach.
Composite materials are anisotropic – they exhibit different strengths and stiffness’s in different directions. Metallic materials can be generally considered to be isotropic (differences between L, LT and ST properties not withstanding).
This has most impact when considering the strength through the thickness of a laminate compared to the strength of a metal component loaded through the thickness of the part.
When manufacturing composite materials, the manufacturer takes responsibility for a large part of the materials and process quality assurance. With metallic materials, the material can be procured using a public domain and universally accepted specification from qualified vendors.
This aspect is probably the least understood by the aircraft developer. Common aircraft materials have been available to acceptable specifications from multiple vendors for decades. These metals have established and widely accepted characteristics and the processing of the metals to achieve these characteristics is all done by the material supplier. When an aircraft developer chooses to use composite materials, the material is bought in a more ‘raw’ form and the airframe developer takes on the responsibility for much of the material processing. This additional work includes a new quality system and new company processes to define for storing, handling, forming and curing. The engineer who develops composite structure must be aware of these new responsibilities and the new downside aspects of their ownership by the aircraft OEM.