What Composite Materials Can Do

- extraordinary adaptability to structural needs

- materials properties often not attainable with classical materials:

- low or very high stiffness

- high strength

- very high fatigue resistance

- low density

- specific mechanical properties by many times higher than conventional materials

- high vibration damping

- very low or slightly negative temperature expansion

- corrosion resistance

- radio transparency (some of them)

Typical fibrous composites with polymer (epoxy) matrix and most common lay-ups (fiber pattern) have densities from an interval <1.343; 2.09> Mg/m³, moduli <20; 557> GPa, ultimate tensile static strengths <140; 2580> MPa, resulting into specific moduli <7; 309> GPa/Mg/m³ and specific ultimate tensile strengths <70; 1562> MPa/Mg/m³ as is apparent from the following figure:

The classical engineering materials have much lower specific properties:

On the other hand there is one other natural property of composites - larger scatter of all materials properties and their relative uncertainty.

Since each composite material originates till during the component manufacturing, the actual material properties, strongly affected by the used constituents, lay-up and by the manufacturing process, can be determined ex-post only. It brings difficulties into the  structural design, which shall shall manage the task with uncertain materials properties.

The structural design with composites can be performed by professionals, being aware of all the contexts.

Attainable Component Properties

- low total weight being only 30 % up to 60 % of the original weight of the component made from traditional materials

- simplicity, reduced count of component structural elements, reduced machining

- minimal joint count

- optimal shaping with low stress concentrations for attaining max.strength 

- tailoring the component properties exactly according to the assigned requirements

- optimal internal structure (fiber pattern configuration) for attaining max.strength & demanded stiffnes at lowest weight

- very high fundamental frequency of vibrations with high damping

Composites: Logical Trend in Engineering

"Nihil novum sub sole", used to say the ancient Romans. 

Nature always designs with composites. Trees, bones, teeth, etc. are excellent examples of the most advanced structures. All nature structures are recyclable and environmental-friendly!

Also boats are traditionally constructed from composites: Plain log, monoxylon, log-raft, reed-raft, boats made from wooden skeletal framework and leather, boats and ships from wooden beams and boards, boats made from plywood, sometimes even from reinforced concrete, from glass laminate and sandwiches (often with balsa core), from carbon, Kevlar or hybrid laminates. Since ancient times, wood was the predominant structural material, due mainly to its availability in combination with processability, relatively good stiffness, strength and low weight.

Wide application of metals appeared during the industrial revolution, due to high strength & toughness despite high specific mass and lower availability. Along with mass production of metals the classical engineering design had been developed. Development progressed from pure empiric and intuitive design, over simple empirical calculations with considering just static loads, to mechanics (theory of elasticity and strength) of isotropic materials, considering also effects of time, temperature, environment, i.e. impact, fatigue, creep, corrosion, and their combinations.

  Today's technology is occuring in a period of transition from applications of several few traditional structural materials (e.g. wood, steel, light alloys) for individual applications of very individual special materials, tailored exactly for the concerned cases. Using a suitable selection of materials constituents, together with proper shaping and dimensioning, the material of a designed component can be tailored exactly according to the requirements of the application. By this way enormous spectrum of the mechanical, physical and other properties of composite materials can be achieved. 

Good, efficient and cost effective design with composites is significantly different from designing with conventional engineering materials. The fundamental differences consist of following key points:

- material and component are designed simultaneously and also manufactured simultaneously

- useful structural material properties only exist in lengthwise direction of fibers, thus for optimum performance the fibre orientations must be coincident with the load paths

- influence of the low transverse properties on a design must be checked, not considered to be a secondary feature

Ccomposite structures design requires a thorough understanding of material fundamentals, including the possibilities for tailoring a composite to meet particular requirements. Design of structures for various environments requires additionally a knowledge of the environment, its influence on material properties and the special forms of loading and structural response with which it is associated.

A good composite component shall reveal:

 - no rapid changes of shape (so called shape discontinuities) i.e. no sharp notches, just very smooth stress concentrators (if any)

 - the least count of stress concentrators

 - no rapid changes in stiffness

 - thin plies

 - direct loading path into reinforcing fibers

 - the least amount of free edges

 - compensation (at least partial) of loading stresses by residual thermal expansion stresses and by moisture expansion  stresses 

Engineering Design Concepts

In a classical engineering design concept the structure is resolved onto simply loaded (at best uniaxially stressed) elements, where behaviour can be relatively simply described and calculated. The structure is characterised by low specific load. Each of its elements have just one function. Therefore the classical structural design is relatively easy. Materials, dimensioning, shape, processing are considered separately. Performance, efficiency, safety & durability are checked subsequently. The structure is designed just for time of its service (or warranty). 

The resulting structure is relatively complicated and heavy, with many joints and many stress concentrators. 

A modern structural design concept requires a multidisciplinary approach. It shall take into account most of the mutual effects between the structure and environment acting during a whole life cycle of the product. 

The structural elements from the individually tailored materials are loaded complexly and have many simultaneous functions. The structures or components are characterised by high specific load, it is light, effective and efficient. It contains few joints (often adhesively bonded) and no or few stress concentrators. This structural design is difficult, needs high professional qualification, and can be performed only with help of the advanced mathematical instruments, modern computer technology and sophisticated experiments. Materials selection, dimensioning, shaping, processing, performance, durability and safety are considered together as the unseparable aspects of the one issue.

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