**fibres, mechanical properties of**(rus. волокон, механические свойства) — set of characteristics that determine mechanical behaviour of fibres under the action of external forces.

### Description

Specific mechanical tests are performed to study mechanical properties and determine mechanical characteristics of fibres; those tests differ in deformation type and loading conditions. The choice of test method is determined by the objectives and the type of fibres under study. The most important mechanical properties of fibres are: Young's modulus in the direction of the fibre axis, strength, creep, and the corresponding specific values; the characteristics divided by the fibre density.

The Young's modulus of a fibre is determined by the nature of the material, crystallographic orientation (for single-crystal fibres) and texture of polycrystalline fibres. Young's modulus of carbon fibres, for example, varies widely (from 150 to 800 GPa), depending on the orientation of graphite planes relative to the fibre axis. Young's modulus is virtually independent of the interaction between the fibre and the matrix and is determined by tests on individual fibres, from the slope of the deformation curve of the sample under tension or from the natural frequency of the sample.

The strength of a fibre (usually brittle) σ is determined by defects in its structure and is described by Weibull statistics, so the probability of failure of a fibre with the length is equal to

where and

where ( · ) denotes the gamma function. The standard deviation D is determined from the second moment

The variation coefficient of strength

The effective strength of a fibre in a composite may differ significantly from the strength of individual fibres tested using one of the methods. Therefore, to determine the strength characteristics of the fibres it is recommended to perform tests on specially prepared samples of composites.

The creep resistance of fibres is determined over a given period of time (typically 10

The Young's modulus of a fibre is determined by the nature of the material, crystallographic orientation (for single-crystal fibres) and texture of polycrystalline fibres. Young's modulus of carbon fibres, for example, varies widely (from 150 to 800 GPa), depending on the orientation of graphite planes relative to the fibre axis. Young's modulus is virtually independent of the interaction between the fibre and the matrix and is determined by tests on individual fibres, from the slope of the deformation curve of the sample under tension or from the natural frequency of the sample.

The strength of a fibre (usually brittle) σ is determined by defects in its structure and is described by Weibull statistics, so the probability of failure of a fibre with the length is equal to

_{},where and

_{}are constants, and_{}is the parameter associated with the constant_{},where ( · ) denotes the gamma function. The standard deviation D is determined from the second moment

_{}.The variation coefficient of strength

_{}depends only on the value of β. The ratio of the average strengths of fibres of lengths_{}and_{}is_{}.The effective strength of a fibre in a composite may differ significantly from the strength of individual fibres tested using one of the methods. Therefore, to determine the strength characteristics of the fibres it is recommended to perform tests on specially prepared samples of composites.

The creep resistance of fibres is determined over a given period of time (typically 10

^{2}-10^{5}h) for a fixed value of creep strain (typically 0.5-1%). Creep testing of individual fibres is a long process; therefore, simplified methods are used to assess this characteristic. These include, for example, measuring the residual curvature of a fibre attached to a hard cylinder over a given time at a given temperature.#### Author

- Mileiko Sergey T.

#### Sources

- Concise Encyclopedia of Composite Materials / Ed. by A. Kelly. — Elsevier Science, 1994. — 378 p.
- Chawla K. K. Fibrous Materials. — Cambridge University Press, 1998. — 309 p.
- Mileiko S. T. Metal and Ceramic Based Composites. — Elsevier Science, 1997. — 704 p.
- Handbook of Composites. V. 1: Strong Fibers / Ed. by W. Watt, B.V. Petrov. — N.Y.: Elsevier, 1989.