Carbon Materials

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First, What is Carbon Fiber

Carbon Fiber, not surprisingly, is made of carbon crystals aligned in a long axis. These honeycomb shaped crystals organize themselves in long flattened ribbons. This crystal alignment makes the ribbon strong in the long axis. In turn these ribbons align themselves within fibers. The fiber shape is the original shape of the material (its precursor) used to produce the Carbon Fiber.  These fibers (containing flat ribbons of carbon crystals) in turn are bundled by the manufacturer in thicker fibres and are woven into carbon cloth, made into felt, twisted or bundled without twisting. This is referred to as Roving. Carbon fiber is also offered as chopped strands and powder.


Carbon Fiber Properties, What's Not to Love!!

High Strength to weight ratio: Strength of a material is the force per unit area at failure, divided by its density. Any material that is strong AND light has a favourable Strength/Weight ratio. Materials such as Aluminium, titanium, magnesium, Carbon. Note that strength and rigidity are different properties, strength is resistance to breaking, rigidity is resistance to bending or stretching.
Because of the way the crystals of carbon fibre orient in long flat ribbon or narrow sheets of honeycomb crystals, the strength is higher running lengthwise than across the fibre. That is why designers of carbon fibre objects specify the direction the fibre should be laid to maximize strength and rigidity in a specific direction. The fibre being aligned with the direction of greatest stress.

Pan based precursor carbon fibre has higher strength than pitch based carbon fibre which has higher stiffness.
Rigidity: Rigidity or stiffness of a material is measured by its Young Modulus and measures how much a material deflects under stress. Carbon fiber reinforced plastic is over 4 times stiffer than Glass reinforced plastic, almost 20 times more than pine, 2.5 times greater than aluminium.

Remember stress is force, strain is deflection such as bending or stretching 


Corrosion resistance: carbon fiber itself do not deteriorate.

Fatigue Resistance: Resistance to Fatigue in Carbon Fiber Composites is good. Damage in tensile fatigue is seen as reduction in stiffness with larger numbers of stress cycles, (unless the temperature is high).
Test have shown that failure is unlikely to be a problem when cyclic stresses coincide with the fiber orientation. Carbon fiber is superior to E glass in fatigue and static strength as well as stiffness.


Good tensile strength but Brittle: Tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, or failing. Necking is when the sample cross-section starts to significantly contract. If you take a strip of plastic bag, it will stretch and at one point will start getting narrow. This is necking. Tensile Strength is measured in Force per Unit area. Brittle materials such as carbon fiber does not always fail at the same stress level because of internal flaws. They fail at small strains. (in other words there is not a lot of bending or stretching before catastrophic failure)


Weibull modulus of brittle materials

Testing involves taking a sample with a fixed cross-section area, and then pulling it gradually increasing the force until the sample changes shape or breaks. Fibers, such as carbon fibers, being only 2/10,000th of an inch in diameter, are made into composites of appropriate shapes in order to test.


(Units are MPa)

High density polyethylene (HDPE)     37
Polypropylene                          19.7-80
High density polyethylene                 37
Stainless steel AISI 302                  860
E-Glass alone                               3450
Carbon fiber alone                        4127
Carbon fiber in a laminate             1600
Kevlar                                         2757


Requires specialized experience and equipment to use: In order to maximize Carbon Fiber Characteristics, a relatively high level of technical excellence must be achieved. Imperfections and air bubbles can significantly affect performance.

Typically, autoclaves, or vacuum equipment is required. Moulds and mandrels are major expenses as well. The success of any amateur carbon fiber construction will be closely linked to the skill and care taken.