Primary Mechanical Properties of a Long-Fibre Composite
There is a large set of properties of a long fibre composite that one needs in design and analysis. However, it is generally agreed that the minimum requirement to assess the three main properties – modulus, strength and ductility, are the parameters listed below:
- Tensile modulus
- Compressive modulus (uniaxial)
- Flexural modulus
- Shear modulus (in plane)
- Lateral contraction ratios
- Tensile strength
- Compressive strength (uniaxial)
- Flexural strength
- Apparent interlaminar shear strength
- Fracture toughness (various modes)
The requirement of property evaluation listed above is based upon the tests for an isotropic homogeneous sample. However, this minimum requirement is not sufficient to completely quantify the strength and stiffness tensors. Further, it neglects the viscoelastic behaviour aspect of the composites.
All the tests included in the minimum requirement of the property evaluation are not carried out by most of the industries working in composites. Their objective can be different and carry out some of the tests. For example, most of the fibres manufacturing companies give the properties of composite which are fibre properties dominant. In such cases, the properties like axial tensile and flexure are given more significance. However, the resin manufacturers tend to give more significance to compression and shear properties of the composite. In this case, these properties are dominated by matrix properties.
The call for ‘open hole’ tests reflects reservations about the reliability of the theories of failure and about the relevance and relative paucity of the empirical evidence from conventional fracture toughness tests. The protagonists of such tests sometimes seem to be preoccupied with a search for authentic and/or definitive data which is perpetually frustrated by a preponderance of mixed-mode failures in their experiments.
The following are the primary engineering properties for preliminary selection of composite materials in a commercial aeroplane industry:
- Tensile strength at room temperature
- Uniaxial compression at room temperature
- Interlaminar shear at room temperature
- Open hole tension at room temperature
- Open hole compression at
- Hot/wet compression strength
- Edge-plate compression strength after impact at room temperature
The tests mentioned above are the essential tests in the initial testing phase.
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