FATIGUE AND TEMPERATURE EFFECT
Composites demonstrate a high resistance to fatigue crack growth. The fatigue strength of many composite materials is higher than for aluminum alloys.
Zone of non-linear deformation in the crack tip - zone of plasticity occurs in the matrix only.
RELIABILITY
A specific feature of composites is high scatter in strength of strong fibers. Composites usually have lower average strength and scatter in comparison to fibers.
Probability of failure of a series structure composed of similar elements (fibers) is higher than that for a parallel structure: P1 > P2.
The average strength of a composite is less than the average strength of its fiber. At low stress, the probability of fracture of a composite (series-parallel system) is lower than for fibers.
JOINTS
There is high stress concentration in the lap joint. It can be decreased if the Young's modulus of the adhesive decreases at the edges.
A. Matrix cure is caused by incorrect mixing or thermal exposure.
A. Core damage.
MATERIAL SELECTION
Advanced composite materials have unique mechanical properties in comparison with steel or aluminum alloys: fatigue strength, specific strength (ratio of strength to weight), specific rigidity (ratio of modulus of elasticity to weight), strength redundancy and high resistance of damaged structures to external loads. Contrary to metals, crack resistance of modern composite materials increase as strength increases. Crack resistance of composite materials depends on fiber tensile strength, its scatter, matrix tensile strength, and bond shear strength. Splitting of the bond between matrix and fiber helps material to stop macrocracks.
Use of composite materials decreases cost of many engineering structures. Fiber content increase does not excessively raise the cost of the composite.
A. Ultrasonic testing can reveal porosity, voids, incorrect volume fraction, foreign inclusions, translaminar cracks, delaminations, disbonds in adhesive, poor fillet in honeycomb construction. Ultrasonic testing is the most widely used technique for many composite materials. Group fiber breakage is the most dangerous defect in a composite. It can be revealed by one of the nondestructive methods from the above list.
REFERENCES Handbook of Composites Ed. by G. Lubin, Van Nostrand Reinhold Company, 1982. Composite Materials Ed. by L.J. Broutman and R.H.Krock, Academic Press, Volumes 1-8, 1973-1976. Kelly A., Macmillan N.H. Strong Solids, Oxford Science Publications, 1986. Damage in Composite Materials Ed. by G.Z. Voyaljis, Elsevier, New York, 1993. Design With Advanced Composite Materials Ed. by L.N.Phillips, Design Council, Springer, 1989. Chou, Tsu-Wei Microstructural Design of Fiber Composites, Cambridge University Press, 1992.
THEMES
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