Quasi-isotropic laminate

A quasi-isotropic laminate is a fiber plastic composite whose elastic properties in the laminate plane are invariant with respect to the rotation around the laminate normal. The quasi-isotropic laminate is not completely isotropic ; different properties can exist perpendicular to the plane of the laminate. The properties outside the laminate plane can even be anisotropic .

application

The quasi-isotropic laminate is mainly used for the substitution of metallic materials or for stresses with changing load directions.

The material substitution of metals makes use of the advantage that the quasi-isotropic laminate behaves like a metallic material as a disk. The stiffness of the pane is adjusted via the laminate thickness. The cost of the redesign is therefore low.

Specialized laminates are unsuitable for heavily directional loads. They usually only show good properties in their preferred directions (e.g. balanced angle connection and unidirectional layer ).

Laminates made from random fiber mats or fleeces also behave quasi-isotropically. The individual short or long fiber bundles are orthotropic or transversely isotropic. The orthotropic effects cancel each other out due to the statistical distribution of the fiber angles. Quasi-isotropic laminates made from random fibers have poorer strength and rigidity properties than quasi-isotropic structures made from woven fabrics.

Layered structure

Polar diagram of the moduli of elasticity of the quasi-isotropic laminate

Quasi-isotropic laminates can be created from an infinite number of material combinations and fiber angles. The classic laminate theory shows whether a selected combination is quasi-isotropic. There are a number of commonly used, quasi-isotropic laminates made from fiber-reinforced plastic . The quasi-isotropic laminates differ in the number of layers ( unidirectional layers ) from which they are built. A quasi-isotropic laminate with fewer than three layers is not possible.

Three grain directions

The fibers of the composite intersect at an angle . Each unidirectional layer must have the same layer thickness. The network is thus an AWV60 with an additional, unidirectional layer. ${\ displaystyle {\ frac {360 ^ {\ circ}} {6}} = 60 ^ {\ circ}}$

Four grain directions

The fibers of the composite intersect at an angle . Each unidirectional layer must have the same layer thickness. The network therefore consists of two AWV45s that have been rotated 45 ° to each other. This quasi-isotropic structure can be produced particularly easily with two layers of fabric or cross- ties. To do this, a fabric layer is rotated by 45 °. ${\ displaystyle {\ frac {360 ^ {\ circ}} {8}} = 45 ^ {\ circ}}$

Five grain directions

The fibers of the composite intersect at an angle . Each unidirectional layer must have the same layer thickness. The following options are available for production: ${\ displaystyle {\ frac {360 ^ {\ circ}} {10}} = 36 ^ {\ circ}}$

A UD layer, an AWV36 and an AWV72
One UD layer and, two AWV18
A UD layer, an AWV108 and an AWV18

Structure with random fibers

Laminates made of random fibers, such as. B. fleeces and mats are quasi-isotropic. Due to the statistical distribution of the fibers in the plane, they have no preferred direction. In terms of their strength and rigidity properties, due to the shorter fiber lengths, they are inferior to the layered, quasi-isotropic laminates made of woven fabrics.

Injection-molded components made of short-fiber reinforced thermoplastic or components made of sheet molding compound or bulk molding compound are only quasi-isotropic to a first approximation. During the injection or pressing process, a fiber orientation is established along the flow front. The strength and type of fiber orientation depends on a large number of parameters.

properties

The quasi-isotropic laminate is isotropic as a pane and therefore also orthotropic. However, it can have a disc-plate coupling (see: classical laminate theory ). Since the disk-plate coupling is generally undesirable, only symmetrically layered, quasi-isotropic laminates are used in practice. According to its elastic properties, the quasi-isotropic laminate behaves like a metallic material.

The strengths of a quasi-isotropic laminate are usually not directional. A check with the help of break criteria for fiber reinforced plastics is therefore essential.

Web links

Fiber composite materials (accessed on August 13, 2020)
Laminate structure (accessed on August 13, 2020)
Load-bearing capacity of CFRP-metal laminates under mechanical and thermal loads (accessed on August 13, 2020)
Modeling of the three-dimensional damage behavior of fiber composite materials (accessed on August 13, 2020)
Experimental characterization and continuum mechanical simulation of the failure behavior of structurally sewn fiber-plastic composites (accessed on August 13, 2020)