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STRUCTURE OF COMPOSITES




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A composite material consists of two or more components. The components have different mechanical properties.

There are the following types of composites:
A. composites reinforced by particles;
B. composites reinforced by chopped strands;
C. unidirectional composites;
D. laminates;
E. fabric reinforced plastics;
F. honeycomb composite structure;


Composite materials are widely used in aerospace structures, passenger airplanes, cars, and sporting goods.
Use of aramid fiber reinforced plastics 1, carbon fiber reinforced plastics 2, hybrid fiber (aramid + carbon) reinforced plastics 3, and glass fiber reinforced plastics 4 decreases the weight of passenger airplanes.


Particle content is defined by studying the cross section of a specimen. The parameter is equal to the ratio of the total area of the particles (fibers) to the cross sectional area of the specimen.

Regarding particle and fiber reinforced matrices, the modulus of elasticity will increase with larger hard particle content.


Broken rigid fibers in a flexible matrix causes a stress concentration in the neighboring fibers. The stress concentration factor increases with the difference between the modulus of elasticity of the matrix and fiber. It can range from 1.2 to 1.5.


There are shear stress concentrations at the bond surface between components.


Stress concentration in laminate materials is higher than in isotropic materials. Hence, the strength of a notched composite specimen is rather high.

The notation [0o/90o]2S means the laminate is assembled with two layers oriented at 0o and 90o.


Tensile and shear stresses cause different failure scenarios for composite structures.


Honeycomb composite structure has high flexural strength. Mechanisms of stability loss under compression depends on many factors such as adhesive quality, size of honeycomb, fiber filament, etc.


There is a residual technological microcracking in composites reinforced by particles. If thermal expansion is high for particles, then the particles are under compression after cooling. Weak particles contain internal microcracks. If the matrix or bond border is weaker than particles, there are tangential microcracks in the matrix and at the border.



 

FIBERS

 

Fibers demonstrate unique mechanical properties: modulus of elasticity and strength.

The critical force for a fiber is equal to the production of critical stress (strength) by the fiber area.
Knotted aramid fiber keeps up to 50% of its original strength. Other fibrous material are more brittle.
There is small effect of temperature and deformation rate on strength of brittle fibers such as boron or SiC.


Fibers differ from other structural materials due to a larger scatter in experimental data.


Shear stress causes fracture of the fiber-matrix bond.

Usually fibers are round. Larger bond surfaces between matrix and fiber corresponds to higher crack resistance of a composite material.


Effect of friction between matrix and fiber after bond fracture is reflected in a В«force-displacementВ» diagram. Critical forces depend on bond length for small L only.

 

RIGIDITY

 

Modulus of elasticity (Young's modulus) is a measure of rigidity. Modulus of elasticity has units of stress - [GPa] or [MPa].


The modulus of elasticity of a composite material depends on the component's parameters, fiber content and structure of the composite.

Boron, aramid, carbon fibers are more rigid than an aluminum or epoxy matrix:

Ef >> Em

 

Both the matrix and fibers have the same strain under tension.
Stress is higher in the more rigid component.


The stress pattern in a composite beam demonstrates higher stress in rigid components and in external layers. Local delaminations and voids have a small effect on the flexural stiffness of the composite beam.


Poisson's ratio is a measure of transverse deformation under tension (compression).
An ordinary value of Poisson's ratio is 0.3 for steels.
There is lay-up scheme for which transverse deformation in a composite is higher in the longitudinal direction. Heres, Poisson's ratio is higher than 1.

 

STRENGTH

 

The mechanisms of fracture of a laminate are different under tension and compression. Surface layers can lose stability under compression.


Inner defects and edges are sources of fracture initiation. Transfibrous defects are the most dangerous. Delamination has no great effect on strength of the composite system.


Holes and cracks decrease the tensile strength of composites. Composite materials are less В«sensitiveВ» to small defects.


Cracks perpendicular to the applied load are more dangerous than a hole with the same maximum size.


Tensile strength reduces В«fasterВ» than shear strength in presence of voids and cracks in multi-layer composites.


Curved and inclined fibers decrease the compression strength of the composite. The first factor is more critical in reducing the strength.

 

CRACK RESISTANCE

 

Multi-component materials demonstrate high crack resistance. Mechanical properties of the components and their bonds define the crack resistance of the composite, such as tensile strength of matrix (A - the property is low), bond strength (B), tensile strength of fibers (C). A matrix with higher ductility and high-strength fibers is peculiar to high crack resistance.


Each fiber breakage is reflected in a peak in the В«force-displacementВ» diagram. The critical force for the first fiber is larger than for others.


Unidirectional composites have a high crack resistance if the maximum tensile stress acts along the fibers. Tension in the transverse direction demonstrates a crack resistance that is lower than the same parameter for a ductile matrix.


The damaged zone in the crack tip in a multi-directional laminate depends on the stress intensity factor (SIF). The SIF is the driving force in fatigue crack growth equations.


For [0o/90o]s lay-up, the transverse layer is the weakest. It starts to fracture by small microcracking, final failure occurs when the microcracks penetrate the longitudinal layer.


A specimen fractured in a fatigue test has a great deal of debonding and extracted fibers. Microdamage is less if a specimen was fractured at high deformation rate.


Voids in matrix decrease the strength of unidirectional composites. The effect of strength decrease is highest for transverse tension and shear, lower for tension along the fibers.

 

OPTIMIZATION

 

The angle between fiber direction and the tensile load will affect the stiffness of the composite layer. The stiffness and strength are higher if the maximum tensile load acts along the fibers.


Stiffness of a fabric depends upon the load angle. An angle of 45o corresponds to minimum rigidity. Stiffness of straight fibers is higher than that for curved ones.


Fracture of a multi-directional laminate is a multi-stage process. First, the microstructural interlayer damage will take place at the edge of plate.

Interlayer shear stress causes a coupling effect. The figure shows the cross sectional areas of original and deformed plates.

Multi-oriented laminate [0o/45o/90o/135o/...]s has better notch resistance than a laminate with a lower variety of fiber orientation [0o/90o]s .


Tangential stress in the pressure vessel is twice as large as radial stress. The stress ratio defines the optimal angle of two-directional lay-up : 55o.


The optimal lay-up direction is coincident with the direction of the maximum inner force.

An optimum honeycomb composite skin structure will have symmetrical stacking and its 0-oriented laminae will be placed along the direction of the maximum tensile stress at external surfaces.

 




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