Elongation enlargement

The increase in elongation is a phenomenon that occurs in fiber-reinforced plastics . It is characterized by the fact that when loaded across the fiber, the matrix takes on a greater part of the stretch than the fiber.

The increase in elongation is an important problem of fiber-reinforced plastics (e.g. glass fiber-reinforced plastics ), since it reduces the transverse tensile strength of the composite. The increase in elongation affects the strength of a fiber-reinforced plastic and not its stiffness . The increase in elongation increases with the proportion of fiber volume .

Micromechanical explanation

Micromechanical disc model for calculating the expansion of elongation

In the direction transverse to the fiber there is a mechanical series connection of the stiffnesses . The reinforcing fibers also generally have a higher modulus of elasticity across the fiber than the surrounding matrix.

In the case of an impressed external stretch , the proportion of stretching of the fiber is significantly less than that of the matrix . This is due to the unfavorable ratio of the moduli of elasticity. ${\ displaystyle \ varepsilon _ {\ rm {total}}}$ ${\ displaystyle \ varepsilon _ {\ rm {fiber}}}$ ${\ displaystyle \ varepsilon _ {\ rm {Matrix}}}$

The increase in strain in the matrix results in a local increase in stress. It is the reason for the low strength of fiber-reinforced plastics across the grain. The expansion increases with the fiber volume fraction and with the ratio . ${\ displaystyle E _ {\ rm {Fiber}} / E _ {\ rm {Matrix}}}$

Effects

Reinforcing fibers, which are particularly stiff in the transverse direction , produce a strong increase in elongation. In particular, the isotropic glass fiber should be mentioned here. Has z. If, for example, pure epoxy resin still has a tensile strength of about , the transverse tensile strength in the composite with glass fibers drops to about half. ${\ displaystyle R _ {\ text {resin}} = 90 \, \ mathrm {N} / \ mathrm {mm} ^ {2}}$

Fibers with a low module across the fiber, such as carbon fiber , have a significantly lower increase in elongation, which is expressed in a higher transverse tensile strength in the composite.

literature

U. Knaust: For the analysis and optimization of fiber composite lightweight components . VDI progress reports, series 20, no.11. VDI-Verlag, Düsseldorf 1989.
GW Ehrenstein: Fiber composite plastics - materials - processing - properties . Hanser, 1992.